RE: Brake upgrade advice

["Bill Hussey" <list_squid@hotmail.com>]

>From: "George Turner, Jr" <getone@mediaone.net>
>Date: Tue, 14 Nov 2000 07:04:54 -0800
>
>FWIW the last time I was out on the track with 17's, no one with smaller
>wheels beat me, even the stock m3's.

This more than likely says a lot more about your driving ability than it 
says about the difference in handling/speed between 17", 16", and 15" 
wheels.  Most of the BMWCCA track instructors run with 15" rims because it's 
much easier to find Hoosier/BFGoodrich DOT approved slicks in 15" sizes than 
it is to find them in 17" which is the stock rim size for both the E30 M3 
and the E36 M3.  The main advantages to a larger wheel are the following:

1) Increased clearance for larger rotor/caliper combinations

2) Increased airflow through the wheel itself

3) Less rotation of the wheel/tire for a given distance, this keeps the tire 
marginally cooler since there is a longer duration of time before the same 
spot on the tire is forced to bear load again which "squishes" the tire 
causing friction and heat.

However, there are some disadvantages as well:

1) Increased unsprung weight.  General rule of thumb is that 1 pound of 
unsprung weight is roughly equal to 3 pounds of sprung weight in it's 
performance degradation.

2) Increase rotational forces (gyroscopic?) which makes it physically harder 
to turn.  Compensation is made in the form of increased driver effort which 
increases fatigue, or greater power/hydraulic assist in the steering system 
which reduces feedback.

3) Reduced fender clearance .. on a race car with spring rates roughly 20 
times stiffer than that of it's road car equal, this is not as much of an 
issue, but on a road car with "sport" springs, rubbing can easily become a 
factor, especially if the car has been lowered.


  The primary factor is wheel size selection is most often brake clearance, 
that simply can't be ignored.  After that however, it comes down to using 
the lightest (i.e. smallest) wheel possible that will still clear the 
brakes, because the unsprung weight can really hurt you.  The increased 
weight means that there needs to be more friction (heat) to slow the car 
under braking, more weight to accelerate, and more load for the tires when 
cornering (and braking).

  Back to the original question that was asked.  I believe you mentioned 
that you already had cross-drilled rotors?  you might want to look into 
having them slotted as well, but I fear that the loss of surface area will 
negate any advantages in cooling.  A good set of quality pads will go a long 
way, but there are two areas you should really pursue.

  Investigate what would be involved with allowing your brake system to use 
DOT 5 brake fluid.  DOT 5 has a considerably higher boiling point than DOT 
4, but it's highly corrosive to some metals and cannot be used in most OEM 
applications.  You may also want to consider a rigging up a simple brake 
cooling system.  All that's necessary is a few feet of dryer exhaust tubing 
("dryer" as in washer/dryer where your clothes come out all wrinkly =), a 
hole punch (the little hand held kind that you used to use in grammar 
school), and some zip ties.  Point one end of the dryer tubing out into the 
air flow somewhere, either behind the front grill/fascia or under the front 
bumper (remove the plastic scrape guard thingy under the car) and lead the 
other end back to a point right in front of the rotor but out of the way of 
suspension movement.  Punch holes in so you have a place to tie it through, 
and lock it down to the car nice and tight .... voila.

Bill
'98 GLX
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