Overheating rear brakes

[Rob Walker]

I am having a problem with overheating rear brakes. The calipers are the stock Ford sierra claw type and my pads are Mintex 1144. Following a few laps of either Donnington Gp or Oulton the fluid Comma dot 5.1 in the calipers has boiled and I have excessive peddle travel and the discs have overheated/turned dull and the pads have glazed.

 

Has anyone come up with a solution or must I bite the bullet and buy an uprated rear brake set up. Several people have suggested going up to 1177 pads or Pagids, AP 600 fluid and fit a rear brake bias valve to reduce the rear braking effort.

 

I have to say that the latter suggestion sounds dubious . I can understand wanting to reduce the rear braking effort on the up rated rears but on the single piston claw calipers sounds undesirable.

 

 

 

Edited by - Rob walker on 31 Mar 2002 10:37:49

[F355GTS]

Rob

 

Can't help thinking that more aggressive pads in the rear will make it worse!

 

At the end of the day fluid boiling is caused by heat, you could try AP600 or Castrol SRF fluid but that won't stop the pads overheating or the discs going dull, it might stop the pedal going soft but this is a mixture of the fluid and glazed pads

 

Have you got 4 pot fronts? if not this would help enourmously as it sounds like the rears are doing too much of the work

 

If you upgrade the rears you have to be careful not to upset the balance front to back, you probably want something like 60% front, 40% rear or even 70/30, the brake bias option would help here

 

Mark

[Carburet]

Yep, go for more powerful front brakes, 4 pots or more aggressive pads.

[felix.klauser]

How fresh is the fluid? If you're doing track days flushing it every 6 months is probably a good idea to keep its water content low.

 

DeDion Caterhams with standard brakes tend to be over-braked at the rear; I would suggest the standard pads. Race pads might well have too high a coefficient of friction when they got hot, overbraking the rear further and putting extra, unnecessary heat into the system.

 

I run a 200+ bhp Zetec engined 7 on slicks with standard brakes front and rear. The front pads are Hawke Brake carbon metallic and the rears standard Caterham fitment. This set-up works very well except that the rear pads get eaten up after 4-5 track days because of the heat. A bias valve would help as would some sort of cooling duct. However, the set-up works well enough so I've yet to make either mod.

 

My recommendations: make sure the fluid is fresh, use standard rear pads and try to brake less.

[paul jacobs]

Hi Rob,

 

I've got the opposite problem on my 21 now. Since upgrading the front discs to 280mm vented and drilled with the AP 4 pot calipers, installing a competition master cylinder and putting in Pagid pads all round, but still retaining the standard XR4 rear calipers, I'm now finding that the fronts are locking under heavey braking, so I'm looking at putting in a valve to limit the front pressure! This is probably a better problem I fancy.

 

I expect that you've got the 4 pots with the 26omm discs on the front, the competion m/cylinder makes a big difference, really shortens the pedal stroke and hardens it up, should help combat the softness when the fluid heats up.

 

The problem with having different pads front to back is that as they heat up they will give different characteristics over a period of time, so you won't know exactly what the car is going to do next.

 

As Felix says, it may be best to brake less - hmm......

[Rob Walker]

Yep I have the uprated AP fronts and the standard M/S. I think the advice regarding fitting higher temperature pads to the rear is to stop them glazing up and thus stop the overheating getting worse and leas`t the rears will be working and not putting all the bias towards the fronts.

 

Edited by - Rob walker on 31 Mar 2002 20:06:53

[Carburet]

I'm gonna have another stab at this now you've said you have uprated front brakes. I don't think correctly fitted rears should overheat but if they are the pads will go crumbly. Excessive pedal travel is due to the need for a big master cylinder (even Arnie seems to finally agree with this) and probably new brake fluid will help too. Maybe the big m/cylinder has a different front to back ratio. There is no way with properly functioning 4 pots on the front that they should not lock first. I would think twice about an AP set up at the back, it presents all sorts of hand brake problems which I'm not sure anyone has overcome. A final thought, is it possible that the rear pads were not installed properly and the pistons not wound back into the calipers,this would cause the rears to rub I think.

[jonhill]

Rob,

 

I have this problem all the time. I've just fitted some ducting, and will know after Cadwell if it's made any difference. Fitting a pad which will endure a higher temperature doesn't really address the real problem i.e. heat build up behind the shrouded wings and wheels, and won't stop the fluid from boiling.

 

I used the (interior) heater piping that a friend got off a Ford at a scrappy, and found a couple of pieces already bent appropriately. I've put some pictures on the world's crapest web site: http://freespace.virgin.net/jon.hill7/

 

Cheers,

 

Jon

[EFA]

I've got some 2nd hand AP 4 pots plus discs and 2 deg ears that the buyer has not paid me for yet. Interested at over £250? (Includes a set of little worn "red stuff" pads.)

 

Fat Arn

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[Rob Walker]

Carburet,

 

IMO fitting the big master cylinder only treats the symptom and not the cause. I want to prevent the rear fluid from boiling not merely cover up the problem with the high capacity big master cylinder. I do not like the feel of the big master cylinder the peddle is like a rock and needs a lot of effort to brake hard. The other factor is that if the rear calpipers are over heating to the degree that fresh dot5.1 fluid is boiling then I must now worry about the state of the piston seals. Sorry just covering up the problem won`t do for me, I want my rear brakes to work and work efficiently giving a consistant front/rear balance and a working handbrake.

 

Jon,

 

See you at Cadwell is that the L7 day or slipstream or both ? We can discuss the problem further.

 

 

[Rob Walker]

Carburet,

 

IMO fitting the big master cylinder only treats the symptom and not the cause. I want to prevent the rear fluid from boiling not merely cover up the problem with the high capacity big master cylinder. I do not like the feel of the big master cylinder the peddle is like a rock and needs a lot of effort to brake hard. The other factor is that if the rear calpipers are over heating to the degree that fresh dot5.1 fluid is boiling then I must now worry about the state of the piston seals. Sorry just covering up the problem won`t do for me, I want my rear brakes to work and work efficiently giving a consistant front/rear balance and a working handbrake.

 

Jon,

 

See you at Cadwell is that the L7 day or slipstream or both ? We can discuss the problem further.

 

 

[jonhill]

Rob,

 

I'm at the L7C day at Cadwell, 7th May. I'm also about to go to Castle Combe to watch the Roadsports today, so might see you there?

 

Cheers,

 

Jon

[Paul Ranson]

I think you have to change to the 'big' rear brake setup.

 

Arnie probably has chapter and verse on piston areas, but the AP calipers will conduct less heat into the fluid. AP600 boils at a higher temp but it's not hugely higher and of course the heat is still there and the temperatures haven't changed.

 

Or brake less.... I don't know Oulton but I'm surprised you'd have a brake temp problem at Donington, even on the GP circuit.

 

Paul

[Asklepios]

Rob,

If most do not get this problem,it points to a flaw in your system.Heat damaged discs would have me considering the brakes dragging.The seals can go off and restrict pad retraction by a few mm and overheating,esp. in the cauldron of a track,ensues.If indeed your disc is overheating to this degree a brake fluid change would be only a partial fix.Remember that the plumbing of the brake lines can permit poor bleeding and you may have a pocket of old, water vapour rich fluid.This doesn`t explain damaged pads and discs.

[Rob Walker]

Paul,

 

I think you may be right, the design of the claw caliper is flawed for high performance use in that the piston has a large flat face that is in full contact with the pads where as the race calipers have holow face presenting a very small contact area to the pad and thus very low heat transfer. This problem has only presented itself since I installed my 230 bhp K and removed the screen/ excess weight my entry speeds into the corners has significantly increased. I find Donnington hard on brakes with coppice, melbourne hairpin, goddards and redgate all being fairly tight corners following high speed straights.

 

Asklepios,

 

I can assure you that I am not the only person with this problem and my brakes are correctly assembled and not dragging.

[Rob Walker]

Paul,

 

I think you may be right, the design of the claw caliper is flawed for high performance use in that the piston has a large flat face that is in full contact with the pads where as the race calipers have holow face presenting a very small contact area to the pad and thus very low heat transfer. This problem has only presented itself since I installed my 230 bhp K and removed the screen/ excess weight my entry speeds into the corners has significantly increased. I find Donnington hard on brakes with coppice, melbourne hairpin, goddards and redgate all being fairly tight corners following high speed straights.

 

Asklepios,

 

I can assure you that I am not the only person with this problem and my brakes are correctly assembled and not dragging.

[rgrigsby]

Rob,

 

I've experienced similar problems although not just at Donington. I run standard front and rear brakes with a bias adjuster and the uprated master cylinder. On 032r's with 5.1 fluid and red stuff pads I was just about ok although the pads were still getting to hot and giving off lots of dust.

 

I've since moved to ACB10's and the 5:1 fluid boiled on the first track day so I've upgraded to AP600 which is much better, giving a rock solid peddle all the time.

 

Still got pad problems though. The red stuff are good in that they cope and aren't too expensive they just don't last very long, combined with the huge amounts of dust on the wheels suggests I'm still overheating them.

 

I know a few people who've fitted ducting on the front brakes which has worked well, Mick Smith fitted some on the rear brakes but I don't know the end results. I'm guessing if you can get some air onto them it will help quite a bit.

 

I think a combination of ducting and upgraded brakes is the only way to go if you are getting brake problems on track. The red stuff pad also seems to be a fairly good compromise road/track pad especially as it's not to expensive.

 

I think it really depends how you drive on track, I'm very hard on brakes on track where as I know other people with similar set-ups that don't have any problems at all.

 

Cheers

 

 

 

 

Rob G

www.SpeedySeven.com

[EFA]

Even the big rear brake setup does not cure 100% if you are going to be heavy on the brakes lap after lap at speeds on the straights in excess of 120mph. Normally the pedal goes soft after around 20 mins on a circuit like Brands GP which is fairly heavy on brakes at about 5 points.

 

 

I've just upgraded to the venterd rear setup and will report on the performancer of this combined with "red stuff" and AP600 in the not too distant future.

 

Fat Arn

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[David Hopkins]

A couple of points .

 

- How have you determined that the rears are boiling . If you have vapourisation in either the front or rear line the pedal travel will be extended similar to if you had a hydraulic failure in one circuit .

 

- The presure in the frt and rear circuit will be exactly the same since the diameter of the m cyl pistons in each circuit are the same . The ratio someone talked about is actually the available fluid displacement for each circuit . This should not be causing you problems though even with the bigger frt brakes .

 

- There is a vented version of the same caliper as fitted to Escort Cosworth / Scorpio 24v . This should overcome the problem if in fact it is boiling . The vented disc grows in the outboard direction from memory so you need to check if there is any problems with a foul to the road wheel .

 

- Really sounds as though you need to get the fronts doing more work .

[Carburet]

Yeah that's what I'd like to know, how do you know the rears are boiling?

[Rob Walker]

How do I know that the rears are boiling, because I bled them in the pits as soon as the peddle travel increased and only the rears bled out LOTS & LOTS of gas bubbles I went back on track five laps later same problem. The rear discs had also changed colour and all my braking effort had transfered to the fronts as the rears had glazed up. The rear brakes have not worked efficiently since the last track day despite deglazing the discs and refacing the pads Its my experience that once the pads get severely overheated they never work again properly. I am in little doub`t that fitting some 1155, 1177 or Pagid Blues would ensure that the pads do not glaze up and continue to bite into the disc giving consistant braking, the question is will this generate more heat than the softer pads that have glazed up and are clearly not biting into the discs. If so the fluid is still going to boil so therefore the only option would be to fit sime cool air ducting or spend lots of wonga on the uprated rears and dubious hand brake setup.

[EFA]

In the past I have used a brake pyro and can confirm the rear temperatures were higher by a significant margin than the front.

 

 

 

Fat Arn

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[David Hopkins]

Rob ,

 

OK if you are sure its the rears that are boiling then ,

 

- You first need to check that the frt / rr balance is correct . When the brakes are in good condition ie I mean with unfaded rears does the car suffer from premature rr wheel lock ie they lock before the fronts ?

 

- What friction are you running ? You can alter the balance first by altering the pad material and use a rr pad with lower co efficient of friction . Std material is marked D8106 or MXD 8106 ( ears of pad are light blue ). There is another ferodo material with shightly lower coeff this may help . If you use a lower coeff friction you get less braking output for a given pedal effort . This will transfer more work ( energy to the fronts ), hence lowering the rear temps .

 

- If this is not sufficient then a balance valve is the next step . This works by reducing the gradient at which the pressure is rising in the rear circuit depending onthe ratio and cut in point of the valve . So for instance up to say 30 bar line pressure you will get equal presssure frt / rr then above that when the valve cuts in if the ratio is say 2:1 for every bar increase you generate at the mcyl you will get 1 bar inc at the frts but 0.5 bar at the rears . The std rear caliper has a large piston as rear calipers go so you cant say a valve isnt required .

 

- Thirdly the rr set up I mentioned is exactly the same as you currently have ie handbrake cable fitting installation etc . Basically when Ford produced the hi performance Cosworth variants including 2.9 24v Scorpio they went to an identical caliper as std Scorpio but with a vented disc .

 

If you want to give me a call my number is 07785 287712 and I will go into all of this in more detail . I work for Girling as was so this is my field of expertise .

 

David

[Peter Carmichael]

Rob,

 

You have two viable approaches:

 

1. Big brakes of one sort or another. I have a set to sell as well (reconditioned with new seals) , complete with naff handbrake calipers.

 

2. Sort out some ducted air to the calipers - this solution can be applied with the big brakes as well, as you need to get rid of the heat rather than just make the system heat-tolerant. A discrete opening on the front of the rear wing could be used to channel air to the caliper area.

 

IME, you can get severe overheating of the rear with a standard 1.6 Superlight on sticky tyres. I have had a set of Mintex 1144 pads crumble because of the heat.

 

You should *NOT* run a proportioning valve in the front brake circuit, although that didn't stop the Superlight racers until 2001 season.

 

For track use, you should aim to have a slightly overbiased rear braking set up and a proportioning valve to make minimum adjustments to get the front just locking up first. Either that or a setup that always locks the front first, even in the stickiest dry conditions - this will be less than optimum in the wet.

 

A 2 pot caliper is infinitely superior to the claw type and gives improved feel in its own right. An uprated master cylinder is not necessary with 4 pot fronts and 2 pot rears. Having said that, if you fit a higher friction pad material, the race master cylinder can give you the same braking with the same pedal weight, but less travel.

[EFA]

I know what else you've bought now......

 

Fat Arn

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