This was also posted on Maintenace Forum:

Now that many of us are beginning to look to next track season, I guess I need to settle some issues on the car from last season. . . . Wondering if the brake gurus had some suggestions.

I had Pagid blue pads with Brembo slotted rotors on front and rear last year from late spring on (three DE schools and street driving). At all three events, I noticed the rear wheels were dusting up black something fierce vs the fronts. Thought it might just be from the fact that the fronts get some *marginal* airflow from the stock front bumper air ducts vs the rears, so more dust is expelled from the wheel well . . . . (yeah, I know, not very strong theory, just wishfull thinking).

By the end fo the season, I had gone through significantly more rear pads than fronts! These are all the same compound pads, shouldn't it be the reverse? I have SS lines, otherwise the brake system is stock.

So why am I consuming more rear pad than front at the track? I noticed that during some of the hotest track days, my empty trunk was blazing hot to the touch (is this from the rear brake extra thermal letoff or my rear diff??)

I will be going to a dedicated track pad/street pad setup this season, but would still like to understand whats going on.

Has anyone had similar problems or have heard of someone else who dealt with this??

Thanks for the advice,

ben

That sounds unusual. Not sure what to tell you. We generally wear through fronts first on my dad's car, even with a Brembo F40 setup on the front with no ducting and the stock caliper on the back.

Maybe your rear calipers are sticking and need to be rebuilt? Have you ever rebuilt them?

Yes, I have gotten this question from others before when I have asked at the track about this issue . . .

Left rear was rebuilt (actually replaced with a rebuilt caliper under warranty by BMW NA) for hanging last June- still had dusting of that wheel after that at DEs.

The right rear was starting to drag intermittently this winter. I pulled the outer boot off and cleaned gunk off of the piston two weeks ago and reassembled as a temperizing measure until ambient temps reach a comfortable level for me to do a full rebuild of that caliper- car no longer under warranty. The right rear outer boot is starting to become crispy toast and crumble like the left one did when it was hanging last summer. I presume this is because of the added heat . . . . Either way the right rear seems to be working fine now in the mean time.


Hey, speaking of, how often are you track junkies having to rebuild your stock calipers? Unless this is all because of the added heat issue, regular caliper rebuilds could start to become a pain in my ass with this car!


Any other thoughts?

Normally when the rears heat up more than the fronts:

1. The rear caliper piston is dragging

2. The rear pads and rotors are significantly inferior

3. There's poor heat convection at the rear, basically really poor air flow

4. The front brakes are already wasted, so the rears are doing the majority of the work.

5. The heat IS NOT coming from the brakes. And is more than likely the rear wheel-bearings.

Outside of giving you the posibilities it's hard to tell you exactly which applies to you.

IMO: It's either 1 or 5 since you have stock brakes with matching pads. Odd dusting characteristics points to #1.

But, without seeing and driving I can only speculate.

Caviallo, Thanks for the imput. A few questions about your comments:

With option #1: Assuming that the rear brake hydrolic system works independantly R and L in the E36 M3, I guess I don't understand why after getting the left rear caliper rebuilt, I would still get severe dusting of both rears on the track . . . .

With Option #2/4: Using Pagid blues, a decent street/track pad, I would assume that unless the fronts a very low (which they weren't at the beginning of the season), the stress to the rears would not be significant enough to cause such serious consumption of the rears. Maybe I should have gone with a more front biased setup with pads, i.e. a lesser pad in the rear-- but this solution doesn't make much sense to me. I know people that track with Pagid Orange up front and blues in the back.
I assume that by going with a slotted brembos in the rear did not cause this problem-- I guess most people I know that track E36 M3s have left the rear rotors solid.

With option #5: unfortunately, don't know much about rear wheel bearings, in terms of why they would be getting hot and how to prevent it. Can you say a bit more on this topic. What about rear heat being generated by the rear differential? I have seen many finned aftermarket diffs for the E36 (e.g. Alpina or RE variety) for sale. Is heat what they are attempting to prevent with these diffs?

Thanks again for the help

You make a point about the rear brakes. Actually the system is diagonal, one front and the opposite rear are on the same hydraulic circuit.

I agree that option 4 is only viable if the front pads are very low friction, even worse than stock, or the front pads/rotors are very worn.

The slotted rotors would not cause this problem. Slotted rotors do create more dust, but you have slotted rotors front and rear so both ends will dust more.

For option 5, the wheel-bearings can very hot if the lube starts to go, this heat will effect the brakes. Normally you will start to get lots of noise from the hub at this point though.
The finned diff covers, and it's just the cover, is to better dissipate heat. More surface area better heat conduction, that's why radiators have all those fins.

As far as the heat in the trunk can you tell which area is the hottest? Is it the spare wheel tub or the fender area?

It is odd to have accelerated rear pad wear compared to the front on a BMW, but BMW's actually have much more even wear front-rear compared to front heavy cars.

Are the rear tires significantly wider than stock?

Have you changed the weight distribution of the car, shifting the weight to the rear?

Have you lowered the car? Or stiffened the suspension, decreasing dive under braking?

If you increase the friction force on the rear tires this will use the rear brakes more, therefore more wear and heat. Brake systems are designed based on a cars weight, weight distribution and suspension settings. Change those enough and you can change how the brakes perform.

The only other thing I can think of right now, do left foot brake (you didn't mention if the car is auto or manual) or something similiar? If you apply the brake and accelerated together often this will accelerate rear wear on a rear-wheel drive car or "ride" the brakes.

Caviallo, thank you again for your thoughtful and informative comments--

Originally posted by caviallo
The finned diff covers, and it's just the cover, is to better dissipate heat. More surface area better heat conduction, that's why radiators have all those fins.
Sure, I am familar with the concept and it makes sense to me to increase surface area for improved convection of heat. Most of the aftermarket BMW diff covers also increase diff oil capacity by 1.5qt, further increasing the size of your thermal resevoir for better heat sink. I guess my original question had more to deal with if rear differential heat production was enough of a problem at driving schools (vs all out racing) to cause significant heat rise in the rear of a street car, and thus potentially affecting rear brake heat levels . . . .

As far as the heat in the trunk can you tell which area is the hottest? Is it the spare wheel tub or the fender area?
If I remember correctly it appeared as though the tub was the warmest area. I remember thinking how uncomfortable it was to touch the sheet metal in the tub putting the spare back in packing up to go home-- By the way the trunk is gutted at the track, but only the bottom carpet is removed.

It is odd to have accelerated rear pad wear compared to the front on a BMW, but BMW's actually have much more even wear front-rear compared to front heavy cars.
This is partly why this problem is so complexing for me.

Are the rear tires significantly wider than stock?
Have you changed the weight distribution of the car, shifting the weight to the rear?
This past season I was running the stock 225 width in front with 245 in rear. This season switching to a 235 all around.
No additional weight shift occurs at the track other than to gut the trunk of tool kit, spare tire, carpet and a few misc junk we all carry in our trunks for no good reason, thus shifting weight away from the rear.

Have you lowered the car? Or stiffened the suspension, decreasing dive under braking?
Car has been lowered mildly with H&R sport springs with Bilstein struts. This is mild spring rate increase vs other cars (eg my wifes audi S4) that have gone to a full coil over system with teeth cattering street performance. But either way, you really don't feel much dive at all at the track with this caliber of vehicle, compared to, lets say, a 1981 Ford Granada.

If you increase the friction force on the rear tires this will use the rear brakes more, therefore more wear and heat. Brake systems are designed based on a cars weight, weight distribution and suspension settings. Change those enough and you can change how the brakes perform.
This is an excellent point, unfortunately I don't think that any of my modifications are very different than "average" mods on E36 M3s, yet I can't seem to find anyone else with this problem (insert scratching head smilie figure here)

The only other thing I can think of right now, do left foot brake (you didn't mention if the car is auto or manual) or something similiar?
The car is a 5 spd. I am not gutsy enough to try left foot braking on the track yet. Nothing tricky with the feet, I am busy using too much energy with the hands and eyes to stay on the track! :)

Thanks for the feedback

Perplexing :95

I guess it is possible that the heat from the diff is indirectly causing the brake issue.

This is just a possibility and I have seen this when wheel-bearing get very hot.

The heat causes the rotor to expand, causes the caliper piston to expand and causes the seals to soften. It also causes the brake fluid to expand as it starts to go to the gas phase.


One or any combiantion will basically allow the brake pad to move against the rotor. It's refered to as heat induced brake drag. Brake drag will definetly increase pad wear.

Oddly being on the rear and having the pad already moved up against the rotor you may not even notice it through the pedal. Normally the heat would cause a softer pedal feel but on the rears it's possible to not even notice.

It's got to get pretty hot though. And even then the heat from the diff or bearing is not enough, but coupled with the heat already priduced by the brakes, it is possible.

And again since it's the rears you may not even notice the decline in brake power.

Just a theory.

Is it possible that the stock staggered tire sizes could have an effect?

ie, BMW didn't plan on staggered tire sizes in developing the braking, then they were added for looks and to increase understeer.

Originally posted by Pinecone
BMW didn't plan on staggered tire sizes in developing the braking, then they were added for looks and to increase understeer.

I don't know about the asthetic issue with BMW NA, but they surely believe that Americans can't handle oversteer and are deathly afraid of our billions of lawyers salivating for our business . . . .

If your diff were creating so much heat that you were experiecing braking problems, you'd have spewing diff fluid and you'd smell it every time you came off the track. By now, the fluid would probably have broken down and you'd have a ruined diff. Besides, unless you've rebuilt the diff and increased the lockup to around 75%, there should be absolutely no reason for it to create that much heat.

The heat you are feeling in your spare tire well is most likely from your exhaust. You may want to check and make sure that the exhaust heat shields are still in place.

Are you sure your parking brake isn't dragging? Also, check your caliper brackets, and guide pins to make sure that the pads and calipers are sliding correctly. Seems to me that something is dragging - it sure seems like there's too much heat for it to be transmitted from the diff/exhaust/wheel bearings. If there's that much heat from those sources, I'd guess that the ABS wiring, brake hoses, speedometer sensor/wiring, and everything else around that area would be cooked to a crisp by now...

Jim

BMWRacerITS: Thanks for the suggestion, once a bit of the snow melts away evough for me to get under the car (grunt) will check the exhaust shields. The exhaust is still stock, but I am the second owner, so who knows might might be lurking in the depths . . . .

Jim O.: Yes, you are correct, I would be seeing more major crispiness than just the outer piston boots if something was majorly dragging. My independant mechanic checked out my parking brake, etc the time my first rear piston was sticking last June (see earlier post on this thread), so I doubt they are an issue [old track habits anyway, never use parking brake anymore].

Thanks for the suggestions guys, I appreciate the communal head scratching.

benaj -

You might want to take your car over to Brett Anderson at Koala Motorsport - he is in Novelty (near Solon), and is probably one of the top 10 BMW mechanics in the country. He reponds to lots of the tech questions in the Roundel. If you need contact info, drop me a PM.

FYI: Heat induced brake drag is always there. As the brakes heat up and the rotor expands the pads will drag on the rotor. This is just the laws of physics.

For the rear you would not require that much heat to cause this, just a bit of heat from another source would create more drag than common. Because the rear rotors are small and there isn't much airflow.

As far as ABS wires, seals, etc. They are rated to take over 1000 F, since a front brake rotor could reach that and more.

The problem he seems to be having is excessive brake wear on the rear, not brake failure, noise or vibration. Just a bit more brake drag than expected would cause this.

For instance dust shields alone will do this.

I agree with jblack, take it to a good tech and have him inspect and measure out the rear brakes to make sure all is in spec.

Just FYI:

Most brake systems have constant brake drag. An interference design where the pad is always slightly touching the rotor.

This is done to minimize pedal travel.

This drag usually has a minor parastic power loss and decreases fuel mileage by about .01 MPG when the brakes are dead cold.

Everyone of course tries to minimize this interference and on some cars they don't have it. Sacrificing pedal feel for fuel economy.

Of course as the rotor expands from heat the drag increases.

Originally posted by jblack
benaj -

You might want to take your car over to Brett Anderson at Koala Motorsport - he is in Novelty (near Solon), and is probably one of the top 10 BMW mechanics in the country. He reponds to lots of the tech questions in the Roundel. If you need contact info, drop me a PM.

Yes, actually the plan was to spend some time trying to figure it out on my own. Then I was heading back to my indenpendant shop. They are a race shop, focusing more on Porsche than BMW, but have done a great job on track prep for the car thus far. Last season we had agreed to address this problem this spring (my mechanic wasn't too pleased that he had recommended the pagid blues to me and then I went through the rears like butter on hot rotors . . . .)

And finally if we were all still stumped, I was going to contact Brett. He is actually a stellar guy in person, if you have a chance to ever talk with him. He also does consultation remotely (e-mail, tele) for a fee.

FYI, I am 90% sure that e36 m3s do NOT use a diagonal brake proportioning system. There are two separate channels in the front, and one for the rear. This may explain something. Can you easily spin the rear wheels on jackstands? I can usually hear my pads dragging just slightly, but it still turns freely. Good luck.

benaj you have mail