Date: Fri, 15 Dec 1995 08:16:53 -0500
From: read_at_engr05.comsys.rockwell.com (Pete Read)
Subject: Brake Dust Shields/Brake Temps
Larry Schwarcz asks:
>Anyone have any experience with those shields you put between the
>wheels and rotors to reduce brake dust on the wheels? Are there
>any negatives with them (overheating, warped rotors, reduced braking,
>>Warning!!...IMHO the negatives outweigh the benefit, which is
>>to keep the wheels clean...
>>...If the brake shields are put on, this stops the flow of air
>>(or reduces it... Reduced pad life, Brake fade due to overheating,
>>...premature wheel bearing failure, possibility of brake
Brake Dust Shields
You can't go wrong with REB's advice, his points are true. However
I'd like to balance this out with the good luck I've had, both with
my former 87 535i and present 88 M5. I'm using Kleen wheels
(from Bavarian) and they do, in fact, keep the wheels very clean.
There can't be much difference between dust shield brands. I think
the bottom line is if you drive easy dust shields are fine and
if you drive hard (many on this list) they don't work. I don't
drive very hard on the street, so they work for me, but they're
always removed for drivers schools.
As REB points out, dust shields reduce cooling air flow to the brakes.
This causes the heat related problems (brake fade, pad life, rotor
warping, wheel bearing problems). Most car brakes are sized (rotor
mass) so they can make a number of moderate stops without overheating.
Once the brakes get hot, cooling is very important.
My driving illustrates the point about heat. On the street, my brakes
with the dust shields are fine because they never get too hot. At the
track (dust shields removed), my brakes are also fine for the first
few laps until things heat up, then I need as much cooling as possible.
In fact, cooling is so critical for hard driving, I've installed cooling
ducts in place of the fog lamps and routed high temp ducting directly
to the inside of the rotors through the opening behind the calipers
(actually it was mostly done by NCC BMW Instructor Scott Low who's done
a number of M3 ducts - I helped). Next, I may remove the rotor splash
shields for even better cooling. The tradeoffs on removing splash
shields seem to be a little more wear (debris thrown on rotor) and
slightly delayed braking in the wet (more water on rotors).
Brake Temperature Rise
The "Brake Handbook" (Fred Puhn, HP Books) tells how to calculate
average brake rotor temperature rise for one stop (or slow to a
lower speed). A couple quick calculations, using my M5 as an example,
gives a good feel for the increased brake stress from performance
Average Rotor Temperature Rise for 88 M5
40mph to stop 43 F (moderate)
80mph to stop 172 F (hard)
120mph to 40mph 344 F (drivers school, e.g. Summit Point Turn 1)
Brake Temperature Rise Calculation
Here are the calculations if you're interesed.
>From the Puhn book, page 11.
Kinetic Energy of a moving car
K (ft-lbs) = Wc x S^2/29.9
Wc = Weight of car in lbs
S = Speed of car in Mph
Average Temperature Rise of Rotors (not calipers)
Avg Temp Rise (F) = Kc/77.8 x Wb
Kc = Kinectic Energy change (from speed change when braking)
Wb = Weight of all four rotors
1988 E28 M5s weigh around 3500 lbs and have 56 lb total rotor weight.
The 300x30mm vented front rotors weigh 19 lbs each and the 283x10mm
solid rear rotors are 9 lbs each.
For a 40 mph stop (40 mph to 0 mph)
Kc = 3500 x 40^2 / 29.9 = 187,291 ft-lb
Avg Temp Rise = 187,291/77.8 x 56 = 43 degrees F
For a 80 mph stop (80 mph to 0 mph)
Kc = 3500 x 80^2 / 29.9 = 749,164 ft-lb
Avg Temp Rise = 749,164/77.8 x 56 = 172 degrees F
For 120 mph to 40 mph brake (80 mph change)
Kc = (3500 x 120^2 /29.9) - (3500 x 40^2 /29.9) = 1,498,328 ft-lb
Avg Temp Rise = 1,498,328/77.8 x 56 = 344 degrees F
Hope this helps,
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