A couple of us are looking at upgrading our brakes on our v-storms as they are currently next to useless with the fronts doing all the work and the backs doing nothing.

Current set up is 255 vented discs with AP 4 pot calipers up front, vw mk 4 rear calipers with 253/273 solid discs with a remote tandem master cylinder 60/40 split f/r

The car is 33/66 weight distribution some research shows that atom go large and equal front and back along with the new Vühl 05 which has a very similar weight (740kg) and weight disribution to the v-storm. So we are looking to go same size front and back with 280 or there abouts discs and 4 pots all round with some sort of handbrake mechanism also potentially loosing the tandem master cylinder and replacing it with a 50/50 split along with a brake proporting/bias valve.

We have cortina/caterham uorights up front and sierra XR4 type bearing carriers with mounting points for calipers

Had a look at Wilwood and Hispec, bit of a minefield, looking for any thoughts on a creative cost effective solution.

If the backs are doing nothing then I would think that you should try a bias bar or different master cylinder sizes. Hi-spec do make a caliper which should be a drop-in replacement for the rear, which might be an improvement, but personally I would start with getting the balance right.

Doesn't braking balance depend on the dynamic weight distribution of the vehicle?

Don't waste your money changing rear brakes until you've tried a bias valve or similar method of reducing pressure to the fronts. This will mean you will need more pedal presure, but if you bias the brakes more to the rear then you will get more overall braking effort - it is obviously very important NOT to push the braking bias too much to the rear as the last thing you want is a rear end lock up.

Thanks all, Thanks Russ, hope all is well in your neck of the woods. The master cylinder unfortunatley is biased 2:1 which can't be adjusted so the plan is to find a suitable replacement and add in a bias valve of some sort. Ideally it would be great to add separate mastercylinders and a bias bar but a lack of installation room makes this difficult. I'm not 100% convinced about the efficacy of the rear callipers, only just scrapping through the IVA test so this may be the next items to go, possibly for powerlite handbrake callipers.

If anyone has any ideas on tandem master cylinder piston sizes etc as a starting point that would be helpful along with a recommendation for a bias valve. (I'm presuming a bias valve will only reduce the pressure to the back)

Would it not be possible to change the master cylinder to an equal one then add a wilwood bias valve in the rear line to reduce the rear braking effort?

Sounds like the cheapest/easiest route. The mk4 Golf handbrake calipers are pretty good and the handbrake element is a lot better than the Wilwood handbrake caliper.

Would think the Hi-Spec option will be much easier to fit to the rear than the Wilwood caliper, as you won't need to make a mounting bracket or anything, it should just bolt straight on. Golf rears should be OK though.

We've found a Wilwood 50/50 master cylinder and Wilwood proportioning valve(s). Next step is to spend more money (!!) and fit them.

I was interested to read about the proportioning valve having a 'knee' in the pressure graph - so at low braking pressures, the front and rears will get 50% each. Harder braking means more pressure will go to the front as the weight of the car shifts towards the front.

As Mike said, we'd love dual cylinders and a balance bar but it would take some serious surgery of the pedal box to fit.

I guess we're down to experimenting....

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Originally posted by russbost
Don't waste your money changing rear brakes until you've tried a bias valve or similar method of reducing pressure to the fronts. This will mean you will need more pedal presure, but if you bias the brakes more to the rear then you will get more overall braking effort - it is obviously very important NOT to push the braking bias too much to the rear as the last thing you want is a rear end lock up.

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You can of course adjust brake balance by selecting a caliper with suitable caliper piston area (similar to changing master cylinder area), but obviously this is expensive if you go through several iterations and caliper choice may be limited.

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Originally posted by britishtrident

Fitting a "bias valve" in the front brake circuit is a definite No NO! any valve that cuts off/reduces/proportions the hydraulic pressure is for the rear circuit only.

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Originally posted by rodgling
You can of course adjust brake balance by selecting a caliper with suitable caliper piston area (similar to changing master cylinder area), but obviously this is expensive if you go through several iterations and caliper choice may be limited.

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Originally posted by rodgling
You can of course adjust brake balance by selecting a caliper with suitable caliper piston area (similar to changing master cylinder area), but obviously this is expensive if you go through several iterations and caliper choice may be limited.

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Originally posted by rodgling

quote:

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Originally posted by rodgling
You can of course adjust brake balance by selecting a caliper with suitable caliper piston area (similar to changing master cylinder area), but obviously this is expensive if you go through several iterations and caliper choice may be limited.

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Indeed, and when we're talking about the rears, there is also the issue of a handbrake solution every time.

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I would also suggest getting the balance setup before swapping calipers and discs. Front calipers should be fine items, but are they available with different piston areas? I would not worry about the VW calipers, they should be up for the job.

First stop would be a proper MC with a (adjustable) pressure reduction at the rear.

"Fitting a "bias valve" in the front brake circuit is a definite No NO! any valve that cuts off/reduces/proportions the hydraulic pressure is for the rear circuit only"

Ok, I've never tried it, but how is fitting a bias valve into the front circuit in any way different to fitting it in the rear circuit? In either instance you'd only be doing it because there is inadequate braking getting to the circuit at the opposite end - I'm not saying you're not right, but I'd like to hear why it should be such a no no? So far as I can see the only down side is that you'd need more pedal pressure to acheive the same braking effort, providing you can still get the fronts to lock b4 the rear I can't see how that's a problem???

Yes the car needs about a 50-50 split I would estimate with the current setup has only about 20% the braking done by the rear.

This will be a quick post as I don't have time right at this moment to do Brake balance 101 but almost everything thing is wrong with this setup even just switching to Siarra 4x4 or MG TF rear callipers would give 20%+ more braking on the rear axle at the expense of a lot of unsprung weight.

Off the top of my head ISTR golf GTI rear callipers have a 37.5mm dia piston Sierra 2 litre 4x4 rear calliper pistons are about 43mm dia.

Again off the top of my head this gives an effective hydraulic area of 1,100 mm^2 for the Golf calliper. and about 1400 mm^2 for a Sierra 4x4 rear

In contrast a standard Cortina front calliper has an effective hydraulic area of 2,200 mm^2




I don't know any other calliper options for the rear that will give a bigger hydraulic area and have an effective handbrake mechanism.

The master cylinder also needs replaced, provided the front callipers are not enormous the Fiat or early Fiesta or early Polo items used by many Locosters should do the job if a balance bar is to awkward to fit in the space available.

But still more braking has to be removed from the front unless a balance bar can be fitted that leaves the only option is to fit smaller brakes to the front not as drastic as it sounds as even without knowing the piston diameters in the front callipers it is obvious the the front is way over braked both in terms of disc size and effective hydraulic area. I don't the diameters of the pistons in the curent calliper but I would suggest standard diameter Cortina solid discs (as vented discs are over kill and will result the front brakes being over cooled when the rears are warmed up also this will save on unsprung weight) and callipers

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Originally posted by britishtrident
Yes the car needs about a 50-50 split I would estimate with the current setup has only about 20% the braking done by the rear.

This will be a quick post as I don't have time right at this moment to do Brake balance 101 but almost everything thing is wrong with this setup even just switching to Siarra 4x4 or MG TF rear callipers would give 20%+ more braking on the rear axle at the expense of a lot of unsprung weight.

Off the top of my head ISTR golf GTI rear callipers have a 37.5mm dia piston Sierra 2 litre 4x4 rear calliper pistons are about 43mm dia.

Again off the top of my head this gives an effective hydraulic area of 1,100 mm^2 for the Golf calliper. and about 1400 mm^2 for a Sierra 4x4 rear

In contrast a standard Cortina front calliper has an effective hydraulic area of 2,200 mm^2




I don't know any other calliper options for the rear that will give a bigger hydraulic area and have an effective handbrake mechanism.

The master cylinder also needs replaced, provided the front callipers are not enormous the Fiat or early Fiesta or early Polo items used by many Locosters should do the job if a balance bar is to awkward to fit in the space available.

But still more braking has to be removed from the front unless a balance bar can be fitted that leaves the only option is to fit smaller brakes to the front not as drastic as it sounds as even without knowing the piston diameters in the front callipers it is obvious the the front is way over braked both in terms of disc size and effective hydraulic area. I don't the diameters of the pistons in the curent calliper but I would suggest standard diameter Cortina solid discs (as vented discs are over kill and will result the front brakes being over cooled when the rears are warmed up also this will save on unsprung weight) and callipers

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Originally posted by russbost

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Originally posted by britishtrident
Yes the car needs about a 50-50 split I would estimate with the current setup has only about 20% the braking done by the rear.

This will be a quick post as I don't have time right at this moment to do Brake balance 101 but almost everything thing is wrong with this setup even just switching to Siarra 4x4 or MG TF rear callipers would give 20%+ more braking on the rear axle at the expense of a lot of unsprung weight.

Off the top of my head ISTR golf GTI rear callipers have a 37.5mm dia piston Sierra 2 litre 4x4 rear calliper pistons are about 43mm dia.

Again off the top of my head this gives an effective hydraulic area of 1,100 mm^2 for the Golf calliper. and about 1400 mm^2 for a Sierra 4x4 rear

In contrast a standard Cortina front calliper has an effective hydraulic area of 2,200 mm^2




I don't know any other calliper options for the rear that will give a bigger hydraulic area and have an effective handbrake mechanism.

The master cylinder also needs replaced, provided the front callipers are not enormous the Fiat or early Fiesta or early Polo items used by many Locosters should do the job if a balance bar is to awkward to fit in the space available.

But still more braking has to be removed from the front unless a balance bar can be fitted that leaves the only option is to fit smaller brakes to the front not as drastic as it sounds as even without knowing the piston diameters in the front callipers it is obvious the the front is way over braked both in terms of disc size and effective hydraulic area. I don't the diameters of the pistons in the curent calliper but I would suggest standard diameter Cortina solid discs (as vented discs are over kill and will result the front brakes being over cooled when the rears are warmed up also this will save on unsprung weight) and callipers

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So, particularly with reference to your last paragraph "more braking has to be removed from the front" so why is a bias valve such a no no? I can't see the logic???

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I'm talking about an adjustable hydraulic brake bias valve, where is the difference between mechanically adjusting brake balance as with twin pedal box, or mechanically/hydraulically by playing with the piston sizes of calipers/master cyl etc or adjusting with a bias valve reducing pressure to the rears as is frequently fitted to 7 type cars, or, as we are talking about in this instance, a bias valve to reduce pressure to the front brakes (which is surely exactly what you are doing when you increase front master cyl dia?)

I'm obviously being exceptionally thick here, but what has weight transfer got to do with it? An adjustable system is just that, adjustable, it can't be 100% perfect for all road conditions, weight transfer conditions etc. I wasn't suggesting adjusting the front pressure back to the point where you're locking rears, tho' that could easily be accidentally done with a mechanical system, particularly when braking into a downhill corner

Jeremy and I know exactly where you are coming from Russ, even with my limited knowledge, and I think the course of action is decided, and the posts from Russ, Marc, Rdodger supoort the case to change out the master cylinder and fit a bias valve to the rears.

As rodgling states there are other options which are easy said in a sentence but costly in the real world (I think its fair to say both Jeremy and I have had quite a bit of that during the build of these cars!!) The master cylinder option will get us a step in the right direction is cost effective and there is scope to improve other componenets as required.

I also spoke to the garage that are repairing my chassis today, anything other than an oem handbrake mechanism is sh!te and the golf calipers should be up to the job and they are running Hispec handbrake calipers on their kit car!!!! so are in a good position to make that call in my book.

I think one of my rear calipers is faulty which is not helping

I'm not into forum debates and arguing the finer technical details of insignificant aspects, especially just to get my post count up and would rather the thread didn't go in that direction, so we'll leave the thread there, unless Jeremy or Russ have anything further they wish to add.

Thanks all.

Mike

BTW nice avatar Jeremy, saw mine today and it looks a total mess

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Originally posted by russbost
I'm talking about an adjustable hydraulic brake bias valve, where is the difference between mechanically adjusting brake balance as with twin pedal box, or mechanically/hydraulically by playing with the piston sizes of calipers/master cyl etc or adjusting with a bias valve reducing pressure to the rears as is frequently fitted to 7 type cars, or, as we are talking about in this instance, a bias valve to reduce pressure to the front brakes (which is surely exactly what you are doing when you increase front master cyl dia?)

I'm obviously being exceptionally thick here, but what has weight transfer got to do with it? An adjustable system is just that, adjustable, it can't be 100% perfect for all road conditions, weight transfer conditions etc. I wasn't suggesting adjusting the front pressure back to the point where you're locking rears, tho' that could easily be accidentally done with a mechanical system, particularly when braking into a downhill corner

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Trident as much as you think you are being helpful I find your posts quite arrogant and insulting to others that try and post in a positive light. If there was an ignore button for you I would have pressed it long before I posted the initial post in this thread. I try and refrain from posting within this forum spefically to avoid posts like yours which are unhelpful at best and insulting at worst. I posted knowing that there are some very helpful and valid views out there that would assist Jeremy and myself in making an informed choice, though also mindful also of responses like yours which I had hoped would not materialise.

Jeremy's first experience of this forum is this thread, whilst not wanting to break confidentiality, I can tell you that it is not a wholly positive experience thus far.

I ask that you refrain from knocking Russ, he is one of the most helpful guys in the game which he does with sincerity and integrity two qualities that your posts are lacking.

I'll also bring this thread to Chris's attention before it get's out of hand

Thank you

Mike

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Originally posted by Mr C
Trident as much as you think you are being helpful I find your posts quite arrogant and insulting to others that try and post in a positive light. If there was an ignore button for you I would have pressed it long before I posted the initial post in this thread. I try and refrain from posting within this forum spefically to avoid posts like yours which are unhelpful at best and insulting at worst. I posted knowing that there are some very helpful and valid views out there that would assist Jeremy and myself in making an informed choice, though also mindful also of responses like yours which I had hoped would not materialise.

Jeremy's first experience of this forum is this thread, whilst not wanting to break confidentiality, I can tell you that it is not a wholly positive experience thus far.

I ask that you refrain from knocking Russ, he is one of the most helpful guys in the game which he does with sincerity and integrity two qualities that your posts are lacking.

I'll also bring this thread to Chris's attention before it get's out of hand

Thank you

Mike

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I have no wish to get into a childish slanging match, I did say I've never tried this, but I've certainly never heard that a pressure reducing valve is banned from use on the front brakes, if so by what, construction and use? I'm not aware of it being mentioned in IVA, tho' quite happy to be corrected on that. It obviously isn't the ideal solution, selecting the right size of cylinder & caliper bores in the first place is the best starting point.

As you were adamant it was complete no no, I was interested to know the reason why, which I don't believe you have offered an adequate explanation of, I would have expected a civil reply, not a tirade of abuse. I do only have an HND in engineering & I did my training a long time ago, but I've also spent over 20 years at the sharp end of the motor trade & fixed more brake probs than you've had hot dinners. I have of course also designed a car from scratch which does handle & stop properly, so presumably I'm doing something right. I would still love to hear a proper explanation of how a pressure reduction by one method is so very different to a pressure reduction by any other method, I can imagine that the quality & accuracy of the valve could play a part but my gut feeling is still that if setup correctly in damp conditions it would still work in dry conditions, albeit requiring increasd pedal pressure.

I am not suggesting to anyone they should dive into any braking problem without due diligence, however I cannot see the harm in trying something providing it is undertaken in a careful, sensible & constructive way. Obviously if you are correct that it is for any reason banned then I wholehearthedly agree it is a complete no no!

Incidentally, I see no reason to bother Chris re this thread, it takes a great deal more to properly annoy me!

The problem using a proportioning valve on the front circuit is that it changes the linearity of the system and introduces a "knee" point where increased pedal effort does not translate into an equivalent braking effort.

To use a pressure device on the fronts is, in my opinion, not correct and potentially dangerous although 10 out of 10 for lateral thinking .

One thing that has been stated here, and I will reiterate, cars are designed to have natural understeer - it is more stable for 99.99% of drivers who do not want the car swapping ends on them. It is the same with braking.

I would suggest that if there is a suspicion that the front brakes are doing "too much" then the entire braking system should be looked at. To not look at it holistically is missing the point that it is an "entire car system".
As is it a kit car, it should be relatively straight forward to go from pedal to pad noting the system component details and their connections, ratios etc.

Of course you can swap and change components but you have to be sure what you are trying to achieve?
If you want more rear braking and you have a fixed range of line pressure, you have to either increase caliper piston area (more force but requires more fluid) or increase the effective radius of the pad (more torque).

Again, I will suggest that you need to decide what you want the extra braking for?
If it is just to have more efficient brakes then I stand by my statement - you have to look at the entire system.
If it is to "have a play", say on track then go for it - swap/change to your hearts content but tread carefully.
In either case, beware the consequences - a supposedly simple change can result in brakes that no longer function as intended - increased/decreased pedal effort/travel, brake fade, overheating etc.

Just to "make it similar to a similar car" should not be the only reason unless you know exactly how you cars compare in all the relevant operating conditions.

My one final piece of advice - if you make a change, for your own safety and the safety of others, make sure that you thoroughly test the results in a safe manner and location. Attempting a 70-0mph stop would not be the greatest idea for a first test

quote:

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Originally posted by onenastyviper
The problem using a proportioning valve on the front circuit is that it changes the linearity of the system and introduces a "knee" point where increased pedal effort does not translate into an equivalent braking effort.

To use a pressure device on the fronts is, in my opinion, not correct and potentially dangerous although 10 out of 10 for lateral thinking .

One thing that has been stated here, and I will reiterate, cars are designed to have natural understeer - it is more stable for 99.99% of drivers who do not want the car swapping ends on them. It is the same with braking.

I would suggest that if there is a suspicion that the front brakes are doing "too much" then the entire braking system should be looked at. To not look at it holistically is missing the point that it is an "entire car system".
As is it a kit car, it should be relatively straight forward to go from pedal to pad noting the system component details and their connections, ratios etc.

Of course you can swap and change components but you have to be sure what you are trying to achieve?
If you want more rear braking and you have a fixed range of line pressure, you have to either increase caliper piston area (more force but requires more fluid) or increase the effective radius of the pad (more torque).

Again, I will suggest that you need to decide what you want the extra braking for?
If it is just to have more efficient brakes then I stand by my statement - you have to look at the entire system.
If it is to "have a play", say on track then go for it - swap/change to your hearts content but tread carefully.
In either case, beware the consequences - a supposedly simple change can result in brakes that no longer function as intended - increased/decreased pedal effort/travel, brake fade, overheating etc.

Just to "make it similar to a similar car" should not be the only reason unless you know exactly how you cars compare in all the relevant operating conditions.

My one final piece of advice - if you make a change, for your own safety and the safety of others, make sure that you thoroughly test the results in a safe manner and location. Attempting a 70-0mph stop would not be the greatest idea for a first test

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quote:

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Originally posted by onenastyviper
The problem using a proportioning valve on the front circuit is that it changes the linearity of the system and introduces a "knee" point where increased pedal effort does not translate into an equivalent braking effort.

To use a pressure device on the fronts is, in my opinion, not correct and potentially dangerous although 10 out of 10 for lateral thinking .

One thing that has been stated here, and I will reiterate, cars are designed to have natural understeer - it is more stable for 99.99% of drivers who do not want the car swapping ends on them. It is the same with braking.

I would suggest that if there is a suspicion that the front brakes are doing "too much" then the entire braking system should be looked at. To not look at it holistically is missing the point that it is an "entire car system".
As is it a kit car, it should be relatively straight forward to go from pedal to pad noting the system component details and their connections, ratios etc.

Of course you can swap and change components but you have to be sure what you are trying to achieve?
If you want more rear braking and you have a fixed range of line pressure, you have to either increase caliper piston area (more force but requires more fluid) or increase the effective radius of the pad (more torque).

Again, I will suggest that you need to decide what you want the extra braking for?
If it is just to have more efficient brakes then I stand by my statement - you have to look at the entire system.
If it is to "have a play", say on track then go for it - swap/change to your hearts content but tread carefully.
In either case, beware the consequences - a supposedly simple change can result in brakes that no longer function as intended - increased/decreased pedal effort/travel, brake fade, overheating etc.

Just to "make it similar to a similar car" should not be the only reason unless you know exactly how you cars compare in all the relevant operating conditions.

My one final piece of advice - if you make a change, for your own safety and the safety of others, make sure that you thoroughly test the results in a safe manner and location. Attempting a 70-0mph stop would not be the greatest idea for a first test

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Sorry, no idea why that came up twice!

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Originally posted by russbost
I agree all the above, but had a thought overnight (see, us FOOLS never know when to stop thinking!) re Tridents statement that pressure reduction on the front system is illegal - so what's ABS then? I believe you'll find that it sometimes reduces pressure to the front system, sometimes one wheel at a time, or perhaps you believe it works on the rear brakes only ..............????????????

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What we are trying to achieve is balanced braking. The system on our cars is based on a Caterham which has very different weight distribution to ours.
At the same time, there isn't a bottomless pit of money to start swapping everything out.

The fronts lock up too quickly and there is minimal braking being done by the rears.

Rather than 'downgrade' the fronts, we are looking to even the bias. One option is to put bigger calipers on the rear.
A cheaper, simpler option is to change the master cylinder. The current one is 2:1 biased to the front which, with the difference in calipers is clearly giving too much front bias. As Mike stated, we are looking to use a 50/50 master cylinder. It's has also been suggested that we swap front and rear on the current MC to give 67% to the rears and 33% to the fronts but I am nervous about that.
Of course, experimentation is possible but these cars are being used as much as possible at the moment so any time off the road is lost time. Also, as has been observed, these things have to be done carefully and so testing is tricky on public roads.

There are no plans to limit the pressure to the front with a proportioning valve, as has been said these have a 'knee' in the pressure graph which means under hard braking, the force drops off proportionally which wouldn't be a great situation for the fronts.
I'm not aware of other valves which limit the pressure linearly.

It's great to get advice and opinion - that's what forums are all about. If we can do it without being personal or offensive, that would be ideal.

Just to add to Jeremy and Viper's comments and Vipers observations, we basically want brakes that do what they say on the tin. Our starting point is as described previously though I doubt that much science has been applied to get them to where they are. I'm certainly spent out on the car and need something that will get us in the right direction relatively cheaply and will be future proof or upgradable as such. I would very much want to avoid the suck it and see methodology of improvement because of cost.

My day job is a mental health nurse which is far removed from mechanics as you can get, so in the absense of expert knowledge, making comparisions is an easy logical process in the hope that the comparison has a system that is well thought through and works well.

I carry a touch more weight that Jeremy... and must admitt I have to apply considerablke force for the fronts to lock up except at lowish speeds, whilst the front wheels are caked in brake dust the rears are just...eerr...dusty I want brakes that are fit for hard road use and regular trackday use.

Not sure if I was explicit in describing the front setup, this is identical to a caterham R400 setup, (AP 4 pots vented 255 discs) though my view is they struggle because of the lack of rear braking and/or the extra weight over and above the caterhan setup

Once again guys, thanks for your constructive input, it is much appreciated.

The good thing about brakes (besides them stopping you) is that they are unlike rockets. Actually they are quite straightforward and can be calculated.

I see many options being mentioned, but it starts to seem to me that 'back to the drawing board' would be the correct approach. Not in the sense of unbolting it all and starting over fresh, but more in a sense to see what would be required to get it in the right ballpark and how to leave room for final adjustment.

So required info to analyze the current situation;
- Weight distribution and wheelbase
- height of COG (center of gravity) if known (can be measured)
- intended use / deceleration (street about 1G, track somewhat higher on race tyres)

- Piston area and braking radius front and rear (center of brake pistons to center of wheel)
- MC area front and rear
- Pedal ratio if known (not needed for balance, but will give idea of required effort)

Please excuse the slightly confusion by Jeremy's and Mr C posts.
Are we talking about the same car or the type of car?

Either way, my question to you both would be: Does the pedal move a large distance in order for it to stop effectively?

As a guide, something more than the travel you would expect on a normal road car.

Edit: the poster above asks more technical, back-to-basics questions which is good and correct. I just want to get a feel for the machines.

[Edited on 17/7/13 by onenastyviper]

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The good thing about brakes (besides them stopping you) is that they are unlike rockets. Actually they are quite straightforward and can be calculated.

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Originally posted by onenastyviper
Please excuse the slightly confusion by Jeremy's and Mr C posts.
Are we talking about the same car or the type of car?

Either way, my question to you both would be: Does the pedal move a large distance in order for it to stop effectively?

As a guide, something more than the travel you would expect on a normal road car.

Edit: the poster above asks more technical, back-to-basics questions which is good and correct. I just want to get a feel for the machines.

[Edited on 17/7/13 by onenastyviper]

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I do understand that making calculations to determine the optimum brake set up would be worth while, so you can then spec the correct size disc, caliper, M/C, vented solid disc etc. No one wants to spend more money than you have to buying 350mm vented discs with 8pot calipers.

In this case though I think we are maybe starting to go a little OTT. The guys have brakes that are safe enough as they have passed IVA. They just want to make them a bit better without spending fortunes and redesigning the whole system.

This was why I suggested the change of M/C to something with a 50/50 split then an inline valve to reduce the REAR effort to give a safe balance.

IMHO that is a safe, cost effective solution. The front calipers may well be overkill at the moment, but as many kit cars have pedal ratios of around 5:1 due to packaging issues the systems aren't ideal to start with.

AP calipers look cool so that's enough reason to have them anyway!

You say you don't want to downgrade front performance but to increase rear, but by changing the master cylinder to one with a different balance you may well be downgrading front performance, unless of course the front m/cyl dia is to stay the same & the rear decrease - this is not necessarily a bad thing, it all comes down to how much pedal pressure you need to stop the car in the first place, it's obviously going to be a lot more than your average tin top as I assume it's unservoed, but as long as you're not having to absolutely stand on the pedal to lock the fronts up you shouldn't have a problem.

I would suggest as a cheap/simple option, why not try reversing the existing m/cyl as you've suggested being a possibility & if this gives adequate front performance but too much rear then a bias valve could be introduced at that point on the rear brakes, which I do agree with everyone else is a much better situatiion than having a bias valve on the front. Obviously care needs to be taken with initial testing, but you can check brakes from 5mph upwards in careful stages, I would NOT suggest aiming it at the nearest corner at 90mph then standing on the pedal to see what locks (it's always embarrassing climbing out of a hedge!)

BTW the FOOL comment was only aimed at one person & it wasn't the one who responded to it!

I wasn't suggesting the calculations to redesign the whole setup from scratch. I do however feel that simple calculations (which anyone can put into a little excel sheet) based on the info indicated above will only cost you a few minutes and from that you can easily conclude where the biggest error is and focus on that.

For example changing the MC could be a good option, but it is nice to see that from the equations before putting more time, money and effort in.

Brakes are easy things to calculate. It is only when considering things like temperature during use and the effect of this temperature on brake balance etc. when things tend to be difficult. But that is fine tuning for racing, not basic setup for road use.

Thanks Marc

I will try and get those measurements over the weekend - would you be able to plug them into the spreadsheet?

Sure, no problem at all. I think I have the info on the VW calipers at hand (to be used on my car as well). Which AP calipers are used up front?

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Originally posted by russbost
You say you don't want to downgrade front performance but to increase rear, but by changing the master cylinder to one with a different balance you may well be downgrading front performance, unless of course the front m/cyl dia is to stay the same & the rear decrease - this is not necessarily a bad thing, it all comes down to how much pedal pressure you need to stop the car in the first place, it's obviously going to be a lot more than your average tin top as I assume it's unservoed, but as long as you're not having to absolutely stand on the pedal to lock the fronts up you shouldn't have a problem.

I would suggest as a cheap/simple option, why not try reversing the existing m/cyl as you've suggested being a possibility & if this gives adequate front performance but too much rear then a bias valve could be introduced at that point on the rear brakes, which I do agree with everyone else is a much better situatiion than having a bias valve on the front. Obviously care needs to be taken with initial testing, but you can check brakes from 5mph upwards in careful stages, I would NOT suggest aiming it at the nearest corner at 90mph then standing on the pedal to see what locks (it's always embarrassing climbing out of a hedge!)

BTW the FOOL comment was only aimed at one person & it wasn't the one who responded to it!

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I think my approach would be to try and get your setup similar to what you would find on a similar car that is known to work well (e.g., atom) and then fine-tune from there, if you have the details of their setup available. Ideally I think you would want the brakes roughly right, then fine-tune with a bias bar if that's possible.

You haven't said (?) but I assume you have the same, decent, pads each end?

It sounds like you have most of the weight at the rear, better calipers at the front and a lot of brake bias towards the front... so I guess it's not surprising that the rears don't do much (fronts lock before rears can do much?).

Better calipers at the rear can't hurt but I bet that's not the root of the problem.

As a cheap and easy starter, I would try swapping the master cylinders and VERY CAREFULLY testing somewhere where it is safe if the car spins (it will swap ends very very fast if the bias is too far rear). If that locks the rears first then you could add a bias valve to the rears to fine-tune? Would think a bias bar would be better though?

Marc

Finally, here are the stats:

- Weight distribution and wheelbase
Weight (with driver):
Front: 330KG
Rear: 540KG
Wheelbase: 260.5cm

- height of COG
40cm

- intended use / deceleration
street > some track

- Piston area and braking radius front and rear
Front. AP 4 pistons
piston diameter: Ø38.1
piston area - 45.6cm2
braking radius: 10.5cm
(Brake disc diameter = 260mm)

Rear VW 1 piston
piston diameter: Ø37 : piston area - 10.75cm2
(maybe 37.5 : area - 11.04cm2)
(maybe 38 : area - 11.34cm2)
braking radius: 11.2cm
Brake disc diameter = 253mm

- MC area front and rear
Tandem Master cylinder
1-1/16" bore size
Area (in) 0.88
Stroke 1.35
Primary Piston Stroke: 0.90 inches
Secondary Piston Stroke: 0.45 inches


- Pedal ratio if known (not needed for balance, but will give idea of required effort)
5:1

Does this give you all you need?

Thanks

Jeremy

One observation from that info...

When calculating effective piston area only add up the pistons acting on one side. So for a 4 pot caliper only add 2 piston areas together not all 4. Sounds strange but true.

I'm no expert on brake design (I just fix whats there usually) but I'd be looking for a smaller bore master cylinder and a proportioning valve for the rear circuit. Smaller bore will give better feel and more presure/braking effort.

quote:

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Originally posted by adithorp
One observation from that info...

When calculating effective piston area only add up the pistons acting on one side. So for a 4 pot caliper only add 2 piston areas together not all 4. Sounds strange but true.

I'm no expert on brake design (I just fix whats there usually) but I'd be looking for a smaller bore master cylinder and a proportioning valve for the rear circuit. Smaller bore will give better feel and more presure/braking effort.

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I think that's wrong. Wilwood say: ."What is Caliper piston area?"

It is the total surface area of all the pistons in one half of the caliper.

The piston area can be determined using the formula: Area = pi x the piston radius squared x the number of pistons. For an example, lets use a six piston caliper and for ease of math, let's say that all the pistons are equal in diameter at 1.5 inches: 3.14 x .5625 x 3 = 5.29 square inches.

quote:

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Originally posted by rodgling
I think that's wrong. Wilwood say: ."What is Caliper piston area?"

It is the total surface area of all the pistons in one half of the caliper.

The piston area can be determined using the formula: Area = pi x the piston radius squared x the number of pistons. For an example, lets use a six piston caliper and for ease of math, let's say that all the pistons are equal in diameter at 1.5 inches: 3.14 x .5625 x 3 = 5.29 square inches.

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Incidentally, if I put the statistics for a Caterham in the spread sheet (1268lbs and 51% front weight split). The braking system is completely balanced.

ok read this

http://www.stoptech.com/technical-support/technical-white-papers/proportioning-valves

then download this

Brake Design Spreadsheet

the spreadsheet is not perfect but it will get you a reasonable setup for your application make sure you start on introduction on the bottom tabs. if you get stuck send mike round

p.s. haven't seen this many handbags out for ages.

Read that article a long time ago

I'm using an updated version of that very spreadsheet!

Well done Jeremy for persevering with this. Hi again Ash two nights in a row ..get on and get that car fixed....

How easy would it be to do the calcs with willwood parts, maybe powerlite calipers, or the other option is another AP caliper on the back...?

My calculations suggest the car needs 50/50 bias on the brakes. So two possible options are:

1) Replace the tandem master cylinder with dual cylinders: 0.688" bore front and 0.5" bore rear. However, I don't think we'll find a 0.5" bore and serious modification would be needed on the pedal box.
2) Replace the rears with the same callipers and discs as the front. This presents the issue of a separate handbrake calliper (and the AP callipers aren't the cheapest). (And the master cylinder would need changing for a 50/50 bias version).

There is also the option of replacing all the callipers!

From my basic calcs along with a few assumption I get a brake pressure distribution of 38% front and 62% rear. This is for 1.1g deceleration which gives 55% front load and 45% rear load.

Also, the 1 1/16" MC will give a very high pedal effort. In the starting post however I noticed a 60/40 MC with unknown diameter.

So this 60/40 split seems ideal (with 40 at front and 60 in the back), but I am quite sure that the diameter will be too large (in general for boosted brake MCs) and probably the available stroke is the wrong way around.

With 0.625" being the smallest MC around (well available), you'd need about a 0.8125" for the front. If my calcs are correct this will balance out your brakes and seriously reduce pedal effort (less than half of what it is now).

Is the rear brake radius correct? I'd assume a 126.5mm disc radius - 18.5mm piston radius = 108mm brake radius, maybe a bit less if the brake pad overlaps the piston.

I would think that the calipers aren't too far off and going to separate MCs would be well worth the effort. Probably quite a bit cheaper than changing calipers.

quote:

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Originally posted by MarcV
From my basic calcs along with a few assumption I get a brake pressure distribution of 38% front and 62% rear. This is for 1.1g deceleration which gives 55% front load and 45% rear load.

Also, the 1 1/16" MC will give a very high pedal effort. In the starting post however I noticed a 60/40 MC with unknown diameter.

So this 60/40 split seems ideal (with 40 at front and 60 in the back), but I am quite sure that the diameter will be too large (in general for boosted brake MCs) and probably the available stroke is the wrong way around.

With 0.625" being the smallest MC around (well available), you'd need about a 0.8125" for the front. If my calcs are correct this will balance out your brakes and seriously reduce pedal effort (less than half of what it is now).

Is the rear brake radius correct? I'd assume a 126.5mm disc radius - 18.5mm piston radius = 108mm brake radius, maybe a bit less if the brake pad overlaps the piston.

I would think that the calipers aren't too far off and going to separate MCs would be well worth the effort. Probably quite a bit cheaper than changing calipers.

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Swapping the ports on the master cylinder will do nothing for the brake balance as the bore is the same.

All it will do is mean the rear piston could move a lot more if able to as it has the longer stroke

1.1g is quite high deceleration. 0.8g is more commonly used.

[Edited on 9/9/13 by jeremy]

Yeah, with 0.8g decel (which I would think is on the low end, 1.1g is surely on the high(est) end of the range) a load distribution of 50% / 50% is found.

With the current calipers that turns into the rear pressure being twice as high as the front.

If you'd assume the front calipers are also used at the rear, you'd logically end up with equal pressure front and rear (assuming the 0.8g, with 1.1g you'd need 56% front and 44% rear)

However, pedal force still remains high (albeit lower than the current setup) and I would suggest going to a smaller bore tandem MC in that case.

So yeah, matter of price / work etc I'd guess. Both routes could lead to a much improved brake performance.