Brakes again - Page 2

JCL · #11 08-25-2011, 07:57 PM

Quote:

Originally Posted by jdd

Re: Point 1; I understand what you are saying however; every ABS equipped car I’ve come across has had a higher proportion of rear bias than an equivalent car without ABS. A number of years of circuit racing various cars have provided this experience.
I believe the reasoning behind this is because it is undesirable to have the rear wheels lockup before the front so quite a bit of safety margin is built in so that this can’t happen on non-ABS cars.
With ABS equipped cars, this safety margin is removed and more rear bias applied as it doesn’t matter if the rear wheels reach maximum braking point slightly before the front under heavy braking as the ABS will take care of it.
I know some of the older cars I’ve worked on over the years can do 3 or even 4 sets of front pads to one set of rear pads.
Many newer cars wear front and rear pads at a similar rate and some even wear the rear pads faster than the front. I’m aware some of this is due to pad surface area and in some cases material difference between front and rear, but some of would also be due to the additional rear bias.

Many vehicles (including BMW) with ABS are not using a proportioning valve to achieve brake bias, but rather using electronic control through the ABS controller. This saves them installing the proportioning valve hardware, since the ABS controller can already do variable control based on wheel speed. So, instead of a fixed 70/30 or whatever it used to be, or even a variable proportioning based on line pressure, it can be continuously adjusted. BMW call it electronic brake force distribution. There is still an additional built in mechanical bias based on the smaller rear disks, and lower clamping pressure from the smaller rear pistons.

While the above will achieve maximum stopping effectiveness, in theory. It may also wear the pads faster than a fixed brake bias might have done. That is where I agree with your point of faster rear brake pad wear compared to older designs. Even with these changes, though, actual brake bias in practice will follow the effects of forward weight transfer, and it is easy to get sufficient transfer to have twice as much weight on the front wheels as the rear wheels. That suggests that faster wearing pads on the rear than the front on the same vehicle won't be due just to bias, but something else.

The effects of front brake bias can be seen on the E53 with the brake dust accumulation on the wheels. The fronts are much worse than the rears, simply due to the forward brake bias.

I think the traction control system, which uses the brake pads, and likely intervenes on the rear first (due to the front/rear torque split) is a potential culprit, but I can't prove it, it is just an opinion.