Hot brakes

[Tim Harber]

Did HSCC at Brands Grand Prix track at the weekend and at least 3 of the Minis (including me) lost the brake pedal I.E. Pedal has to be pumped to get pressure , so not fade but boiling fluid I guess. Air temperature wasn't that huge

Question is : I've never had this problem before . I'm using standard S discs with Carbon metallic pads and DOT5 fluid. Is this a common problem? Anyone has similar experience?

[Vegard]

Just as if I had written this myself. Last race in Sweden, the EXACT same thing happened to me. In race one, when it was 3 laps left. In race two (after having bled the brakes) It occured when 13laps left. It makes you loose focus if not anything else.

I've been thinking internally leak master cylinder (Dual circuit) also. You've got a single circuit car?

[Ronnie]

If there is water ( humidity) present in the fluid, the water and not the fluid can boil.

[Pete]

Yes I've heard about the water trace in the fluid problem before but maybe the high ambient temperature combined with heat transfer through that particular pad was just too much cos Brands ain't that taxing on the brakes is it ? Do some use none metallic spacers ?

[Tim Harber]

Single circuit brakes

I worked out that I used the brakes 7 times in a 2.65 mile lap so they were getting some grief. The fluid has been in there a year which does'nt seem that long.

Pete . What do you mean by non metallic spacers - on the back?

[Ronnie]

Tim have a look at the last picture posted by WNX ( you will have to log in to see the picture). http://forum.minicooper.org/showthread.php?p=115243

[Vegard]

My Carbon metallic pads are half worn. How are yours? Worn pads don't help as much as an insulator either.

[Tim Harber]

Mine are also half worn too. The only person that I spoke to who did'nt lose their brakes was Roger Godfrey , but we established that he doe'snt use the brakes as much ; he has the John Rhodes approach to slowing the car down IE Chuck it into scrub off speed using the tyres. He also used a Ford Anglia in a "Lewis Hamilton" overtake at one point

[Pete]

Tim have a look at the last picture posted by WNX ( you will have to log in to see the picture). http://forum.minicooper.org/showthread.php?p=115243

Well that's one approach though it'd have be proud of the piston, I'm sure I've seen a spacer on the back the pads before to similar effect.

[Vegard]

I'm ordering new pads before the next race meeting.

After talking to Whizzo about it he also said that play in the wheel bearing can make the disc knock the pistons back in the caliper. This obviously will increase the travel needed on the pedal. Although this makes perfect sense, my bearings don't have a lot of play.

[Pete]

He also used a Ford Anglia in a "Lewis Hamilton" overtake at one point

That car's certainly been through the wars, don't think there's a straight panel on it, as you say light on the brakes !

[pad4]

just thinking aloud here - working near some very old buildings i managed to get some asbestos blocks and wedged them in my pistons (as the works cars did), now its only a road/rally car but i am quite heavy on the brakes up in the lanes...

cant say if it actually works or not but worth a try ?

PAD

[5portsrock]

If the is fluids boiling ?? ( check temps of disc and caliper using heat paints and temp strips ) try using a proper racing fluid. AP racing 600 or 660 .has a far higher boiling point than dot 5 fluid. Also more frequent changing cuts down on the hydroscopic absorbsoin of the fluid .Another thing ive done is make some thin titanium plates to go between the pads and pistons to insulate against the heat transfer. Hope these ideas help

[Tim Harber]

Sounds like I will be changing fluid at least and pads probably. Thanks for advice

[Yorkie 'S']

Hi Tim,

We had a similar problem at Oulton earlier this year, we changed the fluid for Donnington to a Racing type (DOT 4) and hey presto the Pedal returned ! I guess the fluid probably gets to a point where it has absorbed so much water that all of a sudden it heats up over its current boiling point and the performance falls off the edge of a cliff ! I think in future we will probably chnage twice a year, since canvassing various opinions it would appear some people have a fat enough wallet to even change for every race !!!

[Ronnie]

Hi Tim,

We had a similar problem at Oulton earlier this year, we changed the fluid for Donnington to a Racing type (DOT 4) and hey presto the Pedal returned ! I guess the fluid probably gets to a point where it has absorbed so much water that all of a sudden it heats up over its current boiling point and the performance falls off the edge of a cliff ! I think in future we will probably chnage twice a year, since canvassing various opinions it would appear some people have a fat enough wallet to even change for every race !!!

Meters can be obtained to determine the water content/boiling point, it just depends how much you want to pay.

http://www.liquid-levers.com/bfsm.htm or if that is to much try this http://www.amazon.co.uk/dp/B003934V50/r ... B003934V50

[sandman]

Be sure to change brake fluid atleast once a year.... More often if you do alot of races.

I've never had any brake problems myself, but then again that was my intention when I decided to cash out for Castrol SRF brake fluid.

[Tim Harber]

One of my helpers looks after a leading car in one of the historics; he uses SRF fluid and carbon metallics. The car has 6" Mambas so it has a bit more air going round them , but at recentl rally (extra hard on brakes) even with some ducting to the brakes they were lasting only 8 minutes before they would go. Other competitors only managed 5 minutes. When he comes to bleed the brakes, steam comes out!

[WNX700H]

Tim,

Leave it with me since brakes is my area , let me see what I can come up with but some thought prompters......

1. What rear linings are you using if they fade early on then more energy will be being dissipated in the fronts, I realise the rears do less work but the more you get out of the rears the less energy the fronts have to deal with .

2. Are the friction characteristics of the fronts and rears matched ie that if worst comes to worst they fade at the same time.

3. Need to think about brake balance so you are getting the rears to do as much as is possible whilst still retaining vehicle stability

4. What friction are you using on the fronts ? There may be a better alternative that transfers less heat into the caliper...... key to solving this issue is getting the fluid temps down..... where the two caliper bodies meet there are cross drilllings to link the two halves of the caliper hydraulically , these bridge the disc and heat transfer from the disc goes into that area .... that is why many racing calipers have external bridge pipes.

5. If there is room try something in the bridge area of the caliper which can act as an insulator .

I will give it more thought and let you know when I have consulted some more expertise.

David

[WNX700H]

Also , the amount of energy you are having to manage is a function of the the vehicle mass and its speed . The mass will remain pretty constant but the effect of extra speed is not linear ie the energy dissipated at 100mph is not double that which would occur at 50 mph. The effect of velocity is a squared function so exponentially increases as speed increases .