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,
>etc)?

REB answers:
>>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
>>fluid boiling...

Brake Dust Shields

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.

Brake Cooling

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

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

>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,

Pete Read
88 M5