russbost
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| posted on 17/7/13 at 07:07 AM |
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quote:
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 
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 ..............????????????
I no longer run Furore Products or Furore Cars Ltd, but would still highly recommend them for Acewell dashes, projector headlights, dominator
headlights, indicators, mirrors etc, best prices in the UK! Take a look at http://www.furoreproducts.co.uk/ or find more parts on Ebay, user names
furoreltd & furoreproducts, discounts available for LCB users.
Don't forget Stainless Steel Braided brake hoses, made to your exact requirements in any of around 16 colours.
http://shop.ebay.co.uk/furoreproducts/m.html?_dmd=1&_ipg=50&_sop=12&_rdc=1
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NOTE:This user is registered as a LocostBuilders trader and may offer commercial services to other users
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russbost
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| posted on 17/7/13 at 07:07 AM |
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quote:
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
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 ..............????????????
I no longer run Furore Products or Furore Cars Ltd, but would still highly recommend them for Acewell dashes, projector headlights, dominator
headlights, indicators, mirrors etc, best prices in the UK! Take a look at http://www.furoreproducts.co.uk/ or find more parts on Ebay, user names
furoreltd & furoreproducts, discounts available for LCB users.
Don't forget Stainless Steel Braided brake hoses, made to your exact requirements in any of around 16 colours.
http://shop.ebay.co.uk/furoreproducts/m.html?_dmd=1&_ipg=50&_sop=12&_rdc=1
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NOTE:This user is registered as a LocostBuilders trader and may offer commercial services to other users
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russbost
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| posted on 17/7/13 at 07:08 AM |
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Sorry, no idea why that came up twice!
I no longer run Furore Products or Furore Cars Ltd, but would still highly recommend them for Acewell dashes, projector headlights, dominator
headlights, indicators, mirrors etc, best prices in the UK! Take a look at http://www.furoreproducts.co.uk/ or find more parts on Ebay, user names
furoreltd & furoreproducts, discounts available for LCB users.
Don't forget Stainless Steel Braided brake hoses, made to your exact requirements in any of around 16 colours.
http://shop.ebay.co.uk/furoreproducts/m.html?_dmd=1&_ipg=50&_sop=12&_rdc=1
|
NOTE:This user is registered as a LocostBuilders trader and may offer commercial services to other users
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onenastyviper
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| posted on 17/7/13 at 08:39 AM |
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quote:
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 ..............????????????
You are correct - in a manner of speaking.
The big difference between an ABS system and a proportioning valve is that the ABS system performs self-checking/testing and is (hopefully) designed
to fail-safe whilst the proportioning valve does not.
Edit: ...and, just to be clear, I do not believe you are a Fool.
[Edited on 17/7/13 by onenastyviper]
"If I knew what I was doing then it wouldn't be called research would it?...duh!"
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jeremy
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| posted on 17/7/13 at 10:06 AM |
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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.
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Mr C
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| posted on 17/7/13 at 10:37 AM |
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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.
Girl walks into a bar and asks for a double entendre, so the barman gave her one
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MarcV
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| posted on 17/7/13 at 11:35 AM |
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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.
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)
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onenastyviper
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| posted on 17/7/13 at 12:04 PM |
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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]
"If I knew what I was doing then it wouldn't be called research would it?...duh!"
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coyoteboy
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| posted on 17/7/13 at 12:37 PM |
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quote:
The good thing about brakes (besides them stopping you) is that they are unlike rockets. Actually they are quite straightforward and can be
calculated.
Rockets are also quite straight forward and can be calculated
However I'd personally replace the system with one that has been properly designed and use biasing to make small changes, rather than start
plumbing in additional hardware to start trying to put a plaster over a gaping sore, but that's my opinion. But I do have a set of 9
spreadsheets that I'm using to calculate the correct brake setup for my car rather than just buying something that fits so I'm probably
biased myself after the effort I've put in (and still not finished).
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Mr C
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| posted on 17/7/13 at 05:08 PM |
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quote:
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]
Its two cars which both have an identical brake setup. the only fundamental differences other than lights colour etc, is mine runs 220bhp and
Jeremy's is around 250bhp. I'm considerably heavier than Jeremy and carry a passenger most of the time we both add 200kg to the car.
I find my pedal hard with little travel, requiring considerable force to be effective.
Girl walks into a bar and asks for a double entendre, so the barman gave her one
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rdodger
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| posted on 17/7/13 at 05:29 PM |
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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!
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russbost
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| posted on 17/7/13 at 05:43 PM |
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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 no longer run Furore Products or Furore Cars Ltd, but would still highly recommend them for Acewell dashes, projector headlights, dominator
headlights, indicators, mirrors etc, best prices in the UK! Take a look at http://www.furoreproducts.co.uk/ or find more parts on Ebay, user names
furoreltd & furoreproducts, discounts available for LCB users.
Don't forget Stainless Steel Braided brake hoses, made to your exact requirements in any of around 16 colours.
http://shop.ebay.co.uk/furoreproducts/m.html?_dmd=1&_ipg=50&_sop=12&_rdc=1
|
NOTE:This user is registered as a LocostBuilders trader and may offer commercial services to other users
|
MarcV
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| posted on 17/7/13 at 08:28 PM |
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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.
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jeremy
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| posted on 18/7/13 at 07:00 AM |
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Thanks Marc
I will try and get those measurements over the weekend - would you be able to plug them into the spreadsheet?
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MarcV
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| posted on 18/7/13 at 07:27 AM |
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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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onenastyviper
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| posted on 18/7/13 at 09:10 AM |
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quote:
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!
No problem
"If I knew what I was doing then it wouldn't be called research would it?...duh!"
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rodgling
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| posted on 18/7/13 at 10:56 AM |
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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?
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jeremy
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| posted on 5/9/13 at 09:01 AM |
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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
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adithorp
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| posted on 5/9/13 at 11:13 AM |
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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.
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
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jeremy
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| posted on 5/9/13 at 11:32 AM |
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quote:
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.
I've read up on this quite a lot and for a 4 pot fixed calliper you use all 4 areas (this is backed up by the specs on the AP website for the
calliper).
For a floating calliper, you times the piston area by 2.
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rodgling
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| posted on 5/9/13 at 12:24 PM |
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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.
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jeremy
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| posted on 5/9/13 at 01:09 PM |
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quote:
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.
The spread sheet that I have (based on the "Brake Handbook" by Fred Puhn) from
LocostUSA, and a load of websites say the effective piston area is the sum of
all the pistons in a fixed calliper and double the sum of the pistons (often only one) in a floating calliper.
Here's the AP product sheet (you have to click on the Specification tab) that agrees with that:
Product Sheet
And this website:
Brake Calculations states Newton’s Third Law: every force has an equal and opposite
reaction
and a reaction force from a sliding calliper is the same as an opposed piston one
Meaning, a sliding calliper effectively exerts double the force of an opposed piston one.
Overall, it doesn't change the ratio front: rear but it does effect the braking force you have to exert (And if I only use half the calliper, I
have to exert over 500 lbft to stop the car with a maximum of 0.8g deceleration.
With all 4 piston areas plus double the rear piston, the force goes down to 250lbft. Given that you should only have to exert about 80lbft, something
is clearly out!
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jeremy
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| posted on 5/9/13 at 01:18 PM |
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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.
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ashg
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| posted on 5/9/13 at 06:12 PM |
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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.
Anything With Tits or Wheels Will cost you MONEY!!
Haynes Roadster (Finished)
Exocet (Finished & Sold)
New Project (Started)
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jeremy
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| posted on 5/9/13 at 06:15 PM |
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Read that article a long time ago
I'm using an updated version of that very spreadsheet!
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