Purpose of the plate behind the rotor
#1
11-28-2004, 06:13 PM
Purpose of the plate behind the rotor
This is a response to the question of what's the purpose of the back plate behind the rotors. Thought it might be ineresting to some.
We have been asking this question for 20, maybe 30 years now. When Porsche made factory race cars, one of the first things they often did was remove these plates. Then later, when they needed to improve the brakes further, they designed new back plates and put those on. Yes the plates are intended to prevent garbage from getting on the rotors, but also they are designed to direct heat into the eye of the rotor. Rotors are not much cooled by air blowing on the face of the rotors from the side, they are cooled by air being blown into the center of the rotor, then thrown outward through all the cooling channels inside the rotor, between the two rotor faces. The hot air comes out the edge of the rotor, and then hopefully is carried away to the atmosphere.
Because of this, it is hard to tell for any given car if the plate is helping or hurting. So you can measure it, with brake temperature paint for example. You would need to be able to produce consistent test runs. If you can do that, and prove it with consistent readings, then remove the plates and run some tests again. Then put the plates back on and check it again to see if it is really changing. We don't want to measure changes in weather, changes in track conditions, or the tires going away. So it is a lot of boring work to do this. But this is the only way to know for sure.
Now, as for what you can do for your car, I hate to say it but there are no easy answers. You are trying to stop a 3300 pound car at high speeds on the track. Brakes convert momentum to heat. They will get hot, and the surface will crack. The cracks by themselves are OK, the cracks and rotor thicknesses have to be continually checked and measured and compared to the specs in the factory workshop manual to know when they are gone. When they are, the good news is, the rotors are relatively cheap, and easy to replace.
If you really want to make an improvement, the first improvement is to save some weight in the car. Bypass the mufflers, put in light weight race seats, etc., the first hundred pounds will come off the car fairly easily. After that though it gets harder and harder.
The next step is to install larger front brakes, especially up front. This increases the capacity of the brakes to handle more heat. For your car it is easy but not cheap, you just bolt on the bigger yellow calipers, and their matching rotors which are available at a reasonable price from Porsche Motorsport. The calipers are available from dealers and the usual parts suppliers.
The next step after that is to go to a racing brake rotor, many of these are not drilled but instead slotted. These will last a lot longer, but because of the lightweight hats and other engineering & manufacturing details, they cost a lot more.
Joel Reiser - PCA WebSite - 11/27/2004
We have been asking this question for 20, maybe 30 years now. When Porsche made factory race cars, one of the first things they often did was remove these plates. Then later, when they needed to improve the brakes further, they designed new back plates and put those on. Yes the plates are intended to prevent garbage from getting on the rotors, but also they are designed to direct heat into the eye of the rotor. Rotors are not much cooled by air blowing on the face of the rotors from the side, they are cooled by air being blown into the center of the rotor, then thrown outward through all the cooling channels inside the rotor, between the two rotor faces. The hot air comes out the edge of the rotor, and then hopefully is carried away to the atmosphere.
Because of this, it is hard to tell for any given car if the plate is helping or hurting. So you can measure it, with brake temperature paint for example. You would need to be able to produce consistent test runs. If you can do that, and prove it with consistent readings, then remove the plates and run some tests again. Then put the plates back on and check it again to see if it is really changing. We don't want to measure changes in weather, changes in track conditions, or the tires going away. So it is a lot of boring work to do this. But this is the only way to know for sure.
Now, as for what you can do for your car, I hate to say it but there are no easy answers. You are trying to stop a 3300 pound car at high speeds on the track. Brakes convert momentum to heat. They will get hot, and the surface will crack. The cracks by themselves are OK, the cracks and rotor thicknesses have to be continually checked and measured and compared to the specs in the factory workshop manual to know when they are gone. When they are, the good news is, the rotors are relatively cheap, and easy to replace.
If you really want to make an improvement, the first improvement is to save some weight in the car. Bypass the mufflers, put in light weight race seats, etc., the first hundred pounds will come off the car fairly easily. After that though it gets harder and harder.
The next step is to install larger front brakes, especially up front. This increases the capacity of the brakes to handle more heat. For your car it is easy but not cheap, you just bolt on the bigger yellow calipers, and their matching rotors which are available at a reasonable price from Porsche Motorsport. The calipers are available from dealers and the usual parts suppliers.
The next step after that is to go to a racing brake rotor, many of these are not drilled but instead slotted. These will last a lot longer, but because of the lightweight hats and other engineering & manufacturing details, they cost a lot more.
Joel Reiser - PCA WebSite - 11/27/2004
#2
11-28-2004, 07:20 PM
We used to call this part the "Snow/Rain/heat Shield" depending on application.
On a non-drilled or slotted rotor if the rotor gets wet braking can become very exciting for the first second or so of application untill the offending water is wiped away.
Depending of the car the shield is also used as a heat shield to limit heat to bushings, ball-joints, wiring, or sensor.
Anyhoo. Just some practical knowledge stored in the old noodle. Have a good one.
Jeff Moss
On a non-drilled or slotted rotor if the rotor gets wet braking can become very exciting for the first second or so of application untill the offending water is wiped away.
Depending of the car the shield is also used as a heat shield to limit heat to bushings, ball-joints, wiring, or sensor.
Anyhoo. Just some practical knowledge stored in the old noodle. Have a good one.
Jeff Moss
#4
11-29-2004, 07:25 PM
I thought those plates were some cruel trick....They are notorious for trapping small rocks and scraping against the rotors. The sound is really great, you think your car is about to blow up.
#5
11-29-2004, 08:23 PM
Originally posted by ypshan
What I don't get is that the exterior side of the rotor is completely exposed. Wouldn't you get water on that side?
What's the purpose of blocking only one side?
What I don't get is that the exterior side of the rotor is completely exposed. Wouldn't you get water on that side?
What's the purpose of blocking only one side?
Not really sure. When we first played with removing them we did so on a car with undrilled/unslotted rotors. It was scary for the first second of application in the wet. Not really sure why the outside face did not seem to matter, but it did not seem to.
When we went to a slotted rotor the problem in the wet went to next to nil. If it was snowing I am not sure, was not tested by me.
Jeff Moss
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