Recently I have been tweaking the brakes on my middy and I have a bit of a problem - basically I am under-braked on the rear. The front brakes lock up waaay too easily compared to the rears so I need to increase braking efficiency to the rears (remember this is a middy)

First of all, here's my original setup - Front: 285mm vented discs with Dynalite 4 pots.
Rear: 273mm solid discs with Powerlite 4 pots w/handbrake.
Bias bar connecting to 2 x 0.625" m/c

My first thought was to look at the bias bar - all is set up correctly with full bias to the rear circuit. The pushrods are equal in length when all slack is taken up.

Next I changed the Smart Pads (which I found rather wooden at slow speeds) to Mintex 1144 - big improvement in feel and braking efficiency at 'road' speeds (yet to try on the track)

So now I'm a bit stumped and need some ideas that don't involve spending hundreds of ££ I did try and speak to Rally Design (I originally bought the kit from them) but Lee just went into 'pendantic arse' mode and was extemely unhelpful, spouting lots of rubbish about pedal ratios and hydraulic problems (which I had already been through in the design stage)

As I see it, one option is to use a bigger disc to make the most out of the existing caliper, moving it further away from the axle. Does anyone know what discs will fit my Sierra rear hubs?

I could try and play with the pushrod lengths, making more bias to the rear but I am reluctant to go against RD's setup instructions.

Any help will be appreciated as always

[Edited on 22-7-09 by RazMan]

From my limited experience put a bigger master to the rears. Either a 0.7 or 0.75

quote:

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Originally posted by nitram38
From my limited experience put a bigger master to the rears. Either a 0.7 or 0.75

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that should be the other way around nitram, more brakes to the rear needs a bigger cylinder on the front brakes.

it should make a big difference to balance.

initial thought wouldnt think there would be a problem with the size of the rear disks being to small or pad friction area size but hate to say it not enought hydraulic force going to the rear calipers, (or to much to the front) is the ballance bar working properly and not jamed, have you wound it fully the other way to see?

I been here before and got a right earful for my trouble.
Both my rear engine cars had larger rear master cylinders, both passed sva.

Are you sure you are winding the balance bar the correct way ?

For more rear bias the bias bar should be closer to the cylinder conected to the rear brakes .

Also the bias bar should be set up so the trunnions at either end have just enough free side clearance to allow the bar to pivot.

Any Bias bar should work properly within about 4 turn of the central position --- one turn towards the rear is normally enough to make the difference between dry and wet settings (you need a greater % of braking from the rear for track use in the wet).

If the that doen't work think about putting a 0.7" dia master cylinder on the front circuit. However this will increase pedal pressure.




[Edited on 22/7/09 by britishtrident]

I think it may be worth having another look at how the bias bar is set up. I found the attached very useful when doing mine. The push rods are set at different lengths.

and another doc

Easy way to find out.
Buy a bigger master and fit it.
Try the brakes.
If I'm wrong then just switch the circuits.
Either way, the cost is the same in parts.

Yep, I set up my bias bar exactly how the Wilwood link states so I am confident that things are ok there - the rods are level when moderate pressure is applied.

I used to have a larger (0.7" ) m/c on the front circuit but that required eye-popping amounts of pedal pressure to stop even at slow speeds so I went back to the 0.625". I realise this is not ideal and could have a lot to do with my imbalance, but I am hoping to find another solution / compromise. Ideally I would like to fit an even smaller m/c on the rear circuit but as far as I know there is no such animal.


[Edited on 22-7-09 by RazMan]

quote:

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Originally posted by wicket
I think it may be worth having another look at how the bias bar is set up. I found the attached very useful when doing mine. The push rods are set at different lengths.

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quote:

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Originally posted by RazMan
Yep, I set up my bias bar exactly how the Wilwood link states so I am confident that things are ok there - the rods are level when moderate pressure is applied.

I used to have a larger (0.7" ) m/c on the front circuit but that required eye-popping amounts of pedal pressure to stop even at slow speeds.

[Edited on 22-7-09 by RazMan]

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I just noticed the words middy -- with a middy you should have the front calipers with only a slightly larger total piston area than the rears.

You can allow for it by fitting a bigger master cylinder on the front but you will still another problem the the rear brakes will warm up at a different rate from the fronts ie you will have cold front brakes but hot rears -- more so if they are vented discs and the rears aren't.

quote:

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Originally posted by nitram38
My point exactly, put the 0.7 on the rears and try it..............cost zero

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quote:

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Originally posted by nitram38

My point exactly, put the 0.7 on the rears and try it..............cost zero

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Humour me.....try it.
Sometimes trying things work.
You have the cylinder already. All it will cost you is some time.
As I said, it worked on both my rear engined cars.

As to your example (britishtrident) of pressure, that is all well and good accept when you run out of master cylinder volume before the pistons on your calipers are fully out.
You may find that the .625" is bottoming out, which would require a double pump to shift the correct volume of fluid.
Something you don't want to do is pump the pedal twice!
Fitting a bigger master will require more effort but maybe it won't be exhausted of fluid by the time the pads are against the disc.

[Edited on 22/7/2009 by nitram38]

Its not a problem with the volume of fluid in the master cylinders compared to your rear calipers is it? e.g can't displace enough fluid to fill the pistons of the rear calipers properly to push them against the disks? If you have the bias bars adjusted to give maximum pressure to the rears as you say, does that limit the travel of the piston in that master cylinder more than in the front circuit?
When you replaced the brake pads for ones with higher friction material - you did just do the rears (as these are the ones you want to increase the power of) didn't you, otherwise you increase both circuits equally, so the imbalance remains.
If you change to bigger rear disks I would have thought the major problem would be moving the caliper mounts to match!

Don't know if any of that is any use to you
Good luck
Hugh

quote:

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Originally posted by hughpinder
Its not a problem with the volume of fluid in the master cylinders compared to your rear calipers is it? e.g can't displace enough fluid to fill the pistons of the rear calipers properly to push them against the disks? If you have the bias bars adjusted to give maximum pressure to the rears as you say, does that limit the travel of the piston in that master cylinder more than in the front circuit?
When you replaced the brake pads for ones with higher friction material - you did just do the rears (as these are the ones you want to increase the power of) didn't you, otherwise you increase both circuits equally, so the imbalance remains.
If you change to bigger rear disks I would have thought the major problem would be moving the caliper mounts to match!

Don't know if any of that is any use to you
Good luck
Hugh

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All good stuff so far guys - thanks.

The volume of the m/c shouldn't really be an issue because both front and rear setups are similar in size (fronts are marginally larger pistons) so they both move roughly the same amount of fluid and have similar pushrod travel once all the slack is taken up.

I am presently waiting for the rear pads to arrive so at the moment I only have Mintex 1144s in the fronts and Smart pads in the rears which compounds the problem a little more, although if I am to believe Rally Design's advice today that 'won't make any difference to braking efficiency' despite being told the exact opposite last week I ordered the pads on their own recommendation! It seems advice changes according to your salesman.

Changing the subject slightly, I had a close look at the rear calipers and it appears that the pads are not in 100% contact with the disc. There is a crescent shaped 'overlap' on the inside of the disc, so a small percentage of the pad area is not in contact. Quite a shocking bit of mismatch from Rally Design imo. If the swept area of the disc was increased by 10mm to the inside radius, that would improve things a little.

I will try to illustrate .... dammit I can't upload a pic


[Edited on 22-7-09 by RazMan]

You can check whether it is volume of fluid that is causing the problems by using the handbrake.

Light application of the handbrake takes any "slack" out of the circuit. This statement is incompatible with the "eye-popping force requirement" unless the pedal stroke is tiny.


I think you have already answered your question:
"I used to have a larger (0.7" ) m/c on the front circuit but that required eye-popping amounts of pedal pressure to stop even at slow speeds so I went back to the 0.625".

You shouldn't need stupid force, unless there is something wrong. Loads of people run 0.7 and 0.625 masters and Sierra rear calipers are a burden many of us live with. Either something is wrong with some of your components (i.e. glazing, binding etc.), or the pedal ratio needs altering, or the pedal is too short.

Good luck,

Matt

The handbrake on the Powerlites is very crude in design and it simply pulls the pads onto the disc with a cable - pretty crap for anything other than stopping the car rolling on a gentle hill. As a result there is next to no slack in the caliper - I was referring to the slack in the bias bar and m/c stroke before fluid gets pushed.

As for pedal ratio, it certainly was an issue in the early days but it still passed SVA - they just put it down to brand new pads & discs needing bedding in. I remade the pedal to improve this situation but the pedal is fairly short (not excessively though) due to the design limitation of my footwell.

TBH I have never had confidence in the Wilwood pads. They were originally intended as a cheap 'get me started' pad so I guess the Mintex can only improve matters - it certainly did in the front brakes with a much better feel. I guess I will wait and see what the rear pads do before I start doing anything too drastic. I have got a spare 0.7" m/c which I can bung in there as a trial along with the new pads.

Meanwhile I am going to have a look at some alternative discs to solve the pad overlap problem. I am sure I saw a 'disc database' somewhere, giving all dimensional stuff of various oem discs as a kind of 'mix & match' guide. Anyone?

[Edited on 22-7-09 by RazMan]

quote:

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Originally posted by hughpinder
Its not a problem with the volume of fluid in the master cylinders compared to your rear calipers is it? e.g can't displace enough fluid to fill the pistons of the rear calipers properly to push them against the disks? If you have the bias bars adjusted to give maximum pressure to the rears as you say, does that limit the travel of the piston in that master cylinder more than in the front circuit?
When you replaced the brake pads for ones with higher friction material - you did just do the rears (as these are the ones you want to increase the power of) didn't you, otherwise you increase both circuits equally, so the imbalance remains.
If you change to bigger rear disks I would have thought the major problem would be moving the caliper mounts to match!

Don't know if any of that is any use to you
Good luck
Hugh

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Trident is right - you definately want a smaller MC for the rear to give a bigger braking effort for the same input force.

I've never seen insufficient fluid volume the cause of the problem, I am using the smallest 0.625 MC's with willwood 4 pots all round and the cylinder moves barely 1/3 of a stroke. If it moves more your pads are backing off or you have excessive runout on the discs which is a seperate issue... I suggest dropping a size on the rear if you can.

quote:

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Originally posted by cloudy
I suggest dropping a size on the rear if you can.

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Why cant you change the pedal ratio?

I dont think more pad area acting on the disc will improve rear braking effort. More pad area with the same force acting on it will reduce the pad pressure.

It would probaby be best the balance the brakes with line pressure, eg 0.7 front to change balance and change the pedal ratio to increase pressure to rear. This will leave you with some fine adjustment using your bias bar.

Paul

Thanks Paul, that's just about what I have decided to do now. I have already remade the brake pedal to improve the ratio, and this reduced the pedal pressure and gave better feel, so increasing the size of the front m/c is my next step after trying the new pads. As you rightly say, I should then use the bias bar to fine tune the balance ..... in theory

[Edited on 23-7-09 by RazMan]

Compound levers --- some clever forum bod sugested it yonks ago but it didn't strike me as practical at the time.
Since then I had a close look at the pedal set up on the Rover 75 it uses a bell crank -- the servo and master cylinder centre line runs accross the car at 90 degres to the usual direction.

Razman...lets go back to basics.I assume your front Dynalites have 1.375 pistons and your rear Powerlites have 1.0 pistons.The rears are kept to small piston sizes and a 4-pot with 1.0 pistons has the same swept area as say single pot VW calipers or Sierra rears...it is so easy to overbrake the rear of a conventional seven with front engine and rwd.However your mid engine would need some more braking to the rear....this would normally be achieved by bigger pistons on the rear but you have selected the Powerlites with integral handbrake and they cannot accept bigger pistons.Assuming 1.375 pistons on the front then 0.625 master cylinder is a well proven combination and should stay,if you go up to 0.7 on the front you will have a hard pedal on the front with no feel.
On the rear you need to achieve a higher velocity ratio ,AP racing have a 0.5 cylinder,it is expensive but would achieve more power to the rear.
At present the front master cylinder needs to move substantially more fluid so the front pushrod length should be longer than the rear,at static.
You are using all round small piston sizes in all your calipers so you need to achieve more power either mechanically or hydraulically....if you up the pedal ratio to 6:1 and change the rear master cyl to 0.5 that will achieve more power/less effort as economically as possible.
Everybody blames pads when their brakes are rock hard with no feel but pads are not usually the problem it is mostly due to incorrect hydraulics...your brakes squeal because the lack of power is not cutting the pad against the disc,lack of power causes glazing which agravates squealing. The Mintex is in my opinion a better all round pad over the Smart pad but will only make minor improvements it is getting the hydraulics and mechanical advantage correct that will reward the best brakes.
Hope that helps.

quote:

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Originally posted by ghostrain
Razman...lets go back to basics.I assume your front Dynalites have 1.375 pistons and your rear Powerlites have 1.0 pistons.The rears are kept to small piston sizes and a 4-pot with 1.0 pistons has the same swept area as say single pot VW calipers or Sierra rears...it is so easy to overbrake the rear of a conventional seven with front engine and rwd.However your mid engine would need some more braking to the rear....this would normally be achieved by bigger pistons on the rear but you have selected the Powerlites with integral handbrake and they cannot accept bigger pistons.Assuming 1.375 pistons on the front then 0.625 master cylinder is a well proven combination and should stay,if you go up to 0.7 on the front you will have a hard pedal on the front with no feel.
On the rear you need to achieve a higher velocity ratio ,AP racing have a 0.5 cylinder,it is expensive but would achieve more power to the rear.
At present the front master cylinder needs to move substantially more fluid so the front pushrod length should be longer than the rear,at static.
You are using all round small piston sizes in all your calipers so you need to achieve more power either mechanically or hydraulically....if you up the pedal ratio to 6:1 and change the rear master cyl to 0.5 that will achieve more power/less effort as economically as possible.
Everybody blames pads when their brakes are rock hard with no feel but pads are not usually the problem it is mostly due to incorrect hydraulics...your brakes squeal because the lack of power is not cutting the pad against the disc,lack of power causes glazing which agravates squealing. The Mintex is in my opinion a better all round pad over the Smart pad but will only make minor improvements it is getting the hydraulics and mechanical advantage correct that will reward the best brakes.
Hope that helps.

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1. Find a 0.5" m/c for the rears (if one exists) *edit* just re-read your post and will seek out an AP jobbie
   
2. Increase the size of the rear disc (£400 for custom bells and rotors )
   
3. Increase the size of the front m/c and increase pedal ratio (tricky due to my footwell constraints)
   

Just had a look on Demon Tweeks website and found an AP CP2623 Compact Master Cylinder with a 14mm bore (the smallest they do) At £76.65 - £99.26(exc VAT) they might be expensive, but a whole lot cheaper than new discs. I have requested info I might even replace both front and rear while I'm at it.

Hmmm I wonder if a 14mm/15mm combo would work better. Does anyone know how much extra braking force will be given with a 14mm m/c compared with a 0.625" (15.87mm) one?


[Edited on 30-7-09 by RazMan]

Taking the ratio of the diameters ^ 2 you get 196 to 252 (14mm and 5/8" respectively) so you'll get about 20% more fluid pressure with the 14mm bore for the same pedal force.

Aha! so it looks like a plan then - a 14mm m/c will be ordered [grits teeth]
I just found a lovely little brake performance calculator created by a guy called Mike Polan. It basically confirms that my combination of brakes requires the same sizes that this thread has just deduced It should give me a 60/40 front/rear ratio which is about right for a middy.

Update:

After studying the AP drawings it appears that their m/c has a shorter stroke than the Girling so I have decided to buy a pair of APs to avoid any mismatch on the bias bar - 14mm & 0.625"

That should sort my brakes out once and for all

Final Update:

Well my AP master cylinders arrived from Demon Tweeks and they seem nicely engineered, although I still wince when I look at the invoice - ah well the price of perfection I suppose

Fitting them was fairly straight forward, in fact they are quite a bit shorter than the Girling units so there was a little more room to play with. I had to shorten the pushrods by about 10mm or so to get the right pedal position but everything bolted in and it was soon time for a test drive.

As expected the pedal feels a bit 'longer' but the front / rear bias is now much better, although I still need the bias bar to be set fully to the rear at the moment - I will have to take the car to my local garage's brake test rollers to be certain. The Mintex 1144 pads have certainly improved the feel and I can predict and control lockups much easier now.

Brakes are now sorted - now where did I put those lumpy cams for the V6?