Does anyone (other than BMW's engineers) have the specs for the the coefficient of friction and the working temperature range of the OEM MINI brake pads? Most aftermarket pads clearly list these numbers; it would be nice to have the specs of the stockers for comparison.

 

I'd disagree to a point and say that sadly many of the aftermarket pad makers do not publish this data. I know of a few; Wilwood, Ferrodo, Pagid, and maybe a couple of more (add to the list if you have a link) but most only offer a glimpse of what they are.

Comments like "high temperature applications" or "ultra sticky'' come to mind.

EBC makes an attempt to show some data but it's not to specific in the charts.

Hat's off to those who put their numbers on the table. What are the others affraid of?

If you wonder what most street pads are, they come in at about .35-.40Mu
The aftermarket pads tend to push this upwards of .45-.50.
Higher end race pads take it from there; .52-.65.

Remember too that adding 'mega pads' to the stock rotor, while improving bite, will also elevate rotor temps, wear and can alter the vehicles bias also.

 

OK, I'm ignernt.

What's vehicle bias? Does it have anything to do with the behavior of the DSC or front to back braking balance?

John

 

vehicle bias IS front/back balance.

 

Thank you! Now I'm a little bit less ignernt!

I can see how changing just the front pads (because they wear out quicker) would cause bias problems. If all the pads are replaced with something grippier, I wouldn't expect any imbalance. Thoughts?

John

 

I'll expand in simple terms. If you want more specific info there's plenty of it out there to be had.

Bias refers to the % of front to rear brake 'power'. On paper it's simply 'static bias' meaning one knows the % based on a constant pressure appied to the system.

The MINI runs about 68/32% by my calcs. That means that 68% of the work is performed up front. This makes sense as a 50/50 would perhaps place to much emphasis on the rear brakes thus allowing rear wheel lock up.

Much of the changes come from three places; rotor diameter, piston area, and pad Cf as you asked. Think of it as; LEVERAGE, CLAMPING POWER, and SAND PAPER grades. The same basics apply to sanding a piece of wood...push harder on light paper or work less with coarse paper and shorten your arm it again changes. Luckily the stock system has some internal valving to prevent foolishness on the part of input pressure. Too much input results in a lower output- that's pretty much a proportioning valve.


The kicker to consider is 'dynamic bias' which goes one step further in considering the the weight distribution on said car under certain conditions. (you with me? you look pale...lol)

When you apply hard braking to the car a larger % of the mass is shifted to the front of the car. Thus the fronts can do more work as they are intended. Depending upon; springs, ride height, pad temp, shocks/dampers, tires, load (you only or you and three friends) tires, tire pressure, road condtions, and probably some things I'm forgetting. The useable % of bias front to rear will change. With a longer MINI (a Wagon?!) the rear can do more work as the wheel base effects this as well. That's why what kit may work on a Mustang or Camaro won't be right for a MINI.

One common thing found in some kits is the intended shift too more rear bias for improved overall braking. While in theory (and to an extent practice) this make perfect sense it may or may not take into account the dynamic needs of the car under ALL conditions. That is simply not possible. If it were completely correct then one would have no reason to put adjustable proportioning valves in a car to tune to conditions, it would be self correcting and we know this is not so. So the key is to find the 'sweet spot' and keep it.

How much rear brake can one use however?? That's the $M question. The answer is; only as much as the dynamics of the car allow for the wheel to stay in contact with the road at a given weight to be efficient. We all know how a short wheel base car can three wheel through a corner. Now, if I gave you masive 13" rotors back there what are you going to do with them? They will reach the same point of bias and skid as stock ones do. BUT, if you plan ahead you may be able to do this with a different amount of pressure. Remember; as the rotor got larger the demand for clamping decreased to the same overall rotor torque.


Properly thought out kits or any brakes will keep in mind the F/R relationship. Putting huge 14s on the front with massive 13s on the rear may also keep this % real. They are simply bigger in relation to one another.

Pads often look to have higher Cf, this to provide more bite. And they do. The problem comes when a 'mega pad' is used to stop a small rotor. The temps go up and the fade comes back. 'Power' is up, but life may be down. For this reason the larger rotor allows for lower and better managed temps thus the need for 'mega pads' is reduced.

Anyhow, this is an ongoing issue for everyone and the key to success is to not go overboard one way or the other. Especially too much rear...

 

Wow Todd!

Thanks for taking the time to write such a clear description! My ignorance is now greatly reduced. (I hope)

I'm not planning on changing out rotors etc just yet. I'm only going to replace my OEM pads with some basic green stuff pads sometime this weekend.

John

 

I second the thank-you, Todd! I was looking for hard numbers that would allow me to gauge how much more aggressive than stock a given replacement pad set might be. That would allow me to do two things: I could avoid going too extreme with the first set of replacements. Then, if I wasn't satisfied with the first try, I'd have a point of reference for how much higher to go with the cf to get the car where I want it to be.

 

Keep in mind that pads have different peaks based on their temperature ranges also. That means some pads 'come in' at higher temps where others begin to fade.

See my pad page or the Wilwood catalog for some ideas.

A good pad, such as the Poly Q is nice to have as it 'ramps up' when temps increase to match that of the added temps usually associated with harder use. On the other hand the Poly D pad is a great auto cross pad given its low temp bite.

Most of these pads however are used in my kits and other aftermarket calipers only and are not available for street calipers. But then if you know how they work you might be able to better shop you own needs from what you have to select from.