Overheating rear brakes

[paul jacobs]

Peter, Could you explain why you say that a proportioning valve should not be used in the front circuit?

[wg_mulholland]

So what is everyone changing too if arnie and peter are selling the ap big brake rears? Drums? Anchors? Parachutes?

[Miraz]

There is a race-spec SLR vented rear brake setup that seems to be flavour of the month at the moment.

 

Miraz

[David Hopkins]

Fitting a prop valve in the front is undesirable for the following reasons ,

 

As you brake there is weight transfer to the front axle which on a 7 is not as pronounced as on say a frt wheel car but is present nonetheless . Therefore as you get weight transfer to the front axle you can effectively get more braking from the fronts before wheel lock . With the rears the situation is reversed , as you get higher decels the available braking is reduced as the rear axle effectively gets lighter , the reason for fitting a valve is to manage the rear wheel lock point . The car must not have rear wheel lock before 0.8G decel and rears should always lock up after the fronts otherwise the car will be unstable . Therefore unless you have the front brake size wrong ( too big )you shouldnt need a prop valve in the front circuit .

 

A question from me now . Why doesnt anyone seem to use the ventilated version of the exisitng rear caliper . Is there a foul between the caliper and the wheel since the disc grows in the outboard direction ie moves closer to the wheel . All other installation details are the same as current , so as far as I can see you simply change the disc and bolt on the vented version of the same caliper, handbrake cables , function etc is the same .

.

 

 

[Hibster.]

David

 

There's nothing like bringing an old thread out of retirement

 

Do your comments/questions (retorical?) as to why don't people don't use the ventilated version of the Sierra caliper apply to the standard braking setup where the discs (on all 4 corners) are Spitfire/Herald items and I guess are only available as solid ?

 

I've just changed to Greenstuff all round with new discs on the back, I've only done 60 miles of gentle braking during run-in but I'm finding that the rear discs are much hotter than the fronts (in fact the fronts pads have been on longer so this might be a red herring and down to extra bed in time).

 

Should I just give the setup more time to stabilise ?

 

Paul

[CHRIS CLARK]

I have seen two examples of ducting to help cool the rear brakes. Mick Smith uses a sort of funnel effect & ducting on either side mounted under the car to 'guide' cool air to the brakes. Lol Pilfold has some forward facing vents mounted on his solid boot cover (full blown race car) that duct air down to the brakes.

Rob. You are obviously not alone with this problem.

Good Luck

[millsn]

a CERTAIN sTEVE fOSTER IS ALSO TRYING ducting at the moment. Perhaps he'll chome in soon with his experience of ducting?

 

Nigel Mills - 2.0 Zetec carbs

[Graham Perry]

Who was it who had a screen washer motor wired into the brake light circuit , so that when the brake lights came on it pumped water over the rear discs ?

[stevefoster]

I had small ducts and was getting a calliper temp of around 220 deg C.

I blocked one off to test and got problems so even the little duct were doing something.

Calliper temp was upto 230'ish.

I have now built bigger ducts and pipes and after a 30 min race at Lydden on Sat using AP telltale strips and the temp measurer gun to confirm, I get a calliper temp of 180 deg max and pad temp of ~200 at max.

 

My brakes have never boiled the fluid on the track when used in anger. I only put some ducting on after fitting big rears that are not as open a spoke arrangement as I would like and when I went Pagid race compound. Lydden is fairly hard on the brakes I guess but you are not going in to hairpins at 120mph. Actually I don't know how fast I go into the Devils Elbow at but I don't think it is 120. More like 100 and something on the start finish straight and braking into Pilgrims is not so hard.

I had no problems at Brands last year with 7" rears, no cooling, fresh fluid and amb temp of 30deg C!

 

Lol BTW has standard Sierra Callipers and his car is road legal.

 

 

My racing pics, 7 DIY, race prep. Updated often here

Photo's of the year here

Hants (North) and Berkshire Area club site here

 

 

Edited by - stevefoster on 14 Apr 2003 08:55:18

[J.R.]

Steve.

 

You could be breaking much harder if you wanted to

 

Both my relatively light Blackbird engined cars cook the rear pads and I have found greenstuff to be the worst.

 

I ran standard pads at croix last week (very hard circuit on brakes) and in the 1/2 hour endurance arce at lydden (which I won) 😬 😬 and they have not degraded at all and show much less wear.

 

Next step is to try the cheapest £5.50 patterm pads that I have.

 

When Lol tried the break ducting initially it melted ☹️ on the mark 2 set up the rear pads did not get up to temperature & he had to go back to standard ones so ducting obviously is of benefit on the heavier Vauxhall car

[SvenDriver]

Rob,

If the rears are boiling and the fronts are not it might be a f-r bias issue. I have had similar fun but less extreme. Several of the instructors at the Heyford day suggested the same thing to me to fix it.

 

The 'correct' solution is to fit a brake bias value (if you only have one master cylinder). This will let you adjust the bias until it is the fronts that are doing the most work at the speeds at which you drive.

 

The cost of a valve is 120+Vat from Caterham or 60ish from Hoopers of Bristol or about 65 from DT. They are fitted inline on one brake line so you would need to cut the rear brake line (generally) to fit it.

 

Given that this is a cheap performance item compared with everything else on your list I would fit this as the number 1 one thing to do and see if you can get the fronts to boil instead. I suspect that if you get to this position with the rears working hard and the fronts boiling then it will be time for the more expensive toys.

[Rob Walker]

Gosh who dug up this old thread its more than a year old ? However still valid. Since posting the thread last year I have done a lot of research into the rear brake set up. I stuck with the Girling Claw Calipers until the last Llandow day March 2003 and that was the final straw for me. I had taken advise from a very well known race team and a brake specialist who both advised me to go Pagid RS14 pads all round, uprated master cylinder, brake proportioning valve in the rear circuit, flexible rear brake lines and finally strip and clean the entire system and refill with Castrol SRF. In addition I noted that my discs were fouling the inside of my calipers when hot so these were reduced to gain clearance. The result excellent fade free braking effort with good feel from both cold and hot on and off the track but sadly after a few reasonably quick laps the spongy peddle returned and the brakes needed bleeding yet again.

 

I have formed the view that I am wasting my time with these calipers as they were never intented for this type of extreme use and as a result will never be up to the job. The piston has a large flat face that is in direct contact with the back of the hot pad, its made of chrome plated cast steel and readily transmits too much heat ito the caliper. Behind the piston inside the caliper is the handbrake mechanism this traps a small amount of fluid between the hot inside face of the piston and the locating plate for the handbrake mechanism. This fluid gets very hot and cannot convect this heat easily throughout the rest of the fluid in the caliper body resulting in localised boiling and the gas produced gives the spongy peddle and is very difficult to bleed it out without inverting the caliper.

 

IMO fitting the Pagid pads into the Girling Claw Calipers was bad advise and has in fact made things worse, sure they give excellent bite but the extra heat that they create causes problems.

 

I am currently fitting uprated rear brakes and will report on my brakes after Cadwell on the 6th May.

 

Rob

 

SvenDriver giving me advise whatever next, you cheeky Monkey!

[catflap]

I have been having this trouble myself, i had no clue what it was. Kept having to re-bleed the rears, as the pedal would go off, and spongy.

 

Anyone got pictures of ducting to the brakes, especially from the rear boot type area, i thought of some carbon type tubing that followed the staves of the role bar an hade slightly wider openings at the top, with flexible hose to rout the air to the brakes.

 

I am also waiting for hi spec to finish their rear brakes, a direct replacement for the sierra ones i currently have.

 

[SvenDriver]

The brake-light switch has given me an idea - a lateral one.

 

PC fans run off typically 12v but dont have much throughput.

 

Model aircraft ducted fan units are VERY powerful. They are available in different sizes . Some might be able to be driven off 12v. I have several 3v units that soak up 7A each!!

 

It might be possible to mount one of these in a tube and drive it off a brake light or by a lever microswitch mounted in parallel to the brake light solenoid on the brake master cylinder and with its own 12v supply. The reason for using a tube would be because the fans are all plastic and 200+c of brakes would melt them.

[felix.klauser]

Not sure that's enough running time and you really want the fan running after braking's completed. 7A at 3V = 21W; at that comsumption a fan could run continuously and draw no more current than a brake lamp.

[Rob Walker]

There is no doubt that some ducting must help with the problem however its never going to be possible to cool the rear of the pads and thrust face of the piston. I am told that Performance braking market a Mica shim that can be fitted between the brake pad and the piston. Mica is an extremely poor conductor of heat and as such should prevent all lot of the heat build up getting into the fluid. I think the shims are about 1mm thick . Could be worth a try along with some ducting.

[SvenDriver]

Mica!!!! Mica is used as an insulator with Transistor heat sinks because it is a GOOD conductor of heat.

[stevefoster]

A better castlelation in the piston would help. Anybody see a reason not to cut some in?

 

 

My racing pics, 7 DIY, race prep. Updated often here

Photo's of the year here

Hants (North) and Berkshire Area club site here

 

[julians]

Ressurecting a very old thread here, but you never reported on whether the uprated rears sorted the problem.

 

I have the same problem ( small brakes up front and normal rears using castrol srf fluid), in fact they get so hot they transfer heat to the wheel making it too hot to hold for 20mins after stopping.

 

I'm just about to change the fronts to the ap 4 pots in the hope it may take some of the strain .

[julians]

BTTT , any answers?

[alien]

AP600 fluid solved my problems.

You need big fronts for sure.

 

Just re-read this thread and had a chuckle to myself at JR saying I could brake harder. 😳

I've got a nice close up pick up him out braking himself from the May 2003 Caterham race at Lydden and hitting the tyre wall pretty hard at the devils elbow. Gravel flying everywhere.

"A man must know his limits" as Dirty Harry once said.

That said, JR was miles down the road by that time, & probably in the lead but we all moved up a place after that.

My racing was all about having fun on the lowest possible budget.... well that's what I told the wife at any rate.

I reckon big brakes on the front and the standard sierra rear can go a long way but have limitations.

I can only repeat that I've found Ferodo race pads and pagid RS14 rears the best combo for road and track. Never felt the need for a brake bias valve with that combo.

I ran big under car ducts and pipes. These have subsequently got scraped to bits on the road. ☹️

 

 

 

Edited by - SteveA7 on 24 Jul 2004 19:32:39

[Rob Walker]

I am pleased to report that since fitting the uprated rear brakes I have had no further problems whatsoever. I have just returned from Le Sept in particular Dijon, air temp, track temp were high and the brakes were put to extreme use without an trace of the dreaded spongy peddle.

[julians]

Cheers, I'd better save up for the uprated rears then.

 

Hows the handbrake?

[Rob Walker]

The kit comes with two airheart spot handbrake calipers. These are pretty crude but do provide a handbrake facilty, my car passed its SVA in March 2004 with the spot calipers so there should be no issue with the MOT in future. Whilst at my local Rover Dealer last month I noticed that the new Rover 75 V8 was fitted with AP two pot rear calipers which looked identical to my Caterham supplied calipers the only big difference being that these now had an INTEGRAL HAND BRAKE facilty that was cable operated. I would certainly check this option out before perchasing the Caterham kit.

[Graham Perry]

As its on the V8, they probably won't be cheap but it looks like an option for the future