Those shields behind rotors

One of my greatest pet peeves is sports cars that come under braked. That is, brake systems unable to hold up under a few hot laps at a track day or even some spirited driving on the road. I find that brakes cause me more of a headache than any other system on a car and it just disappoints me when manufactures put crappy systems on so called sports cars.

The inside pad “melted down” forming this deposit on the rotor surface.

I recently did a brake job on my 350z and found that the rotors were totally destroyed. I pushed these brakes pretty much as hard as they would go on a track day about two years ago, and I did know that the rotors formed some cracks at that time, but I didn’t know that the surface of the rotors facing the inside of the car was as bad as it was (pictured). Amazingly, even though I’m sure the rotors have been bad for a long time, I haven’t noticed any real problems since I haven’t pushed the car hard for a while now and built heat into the system.

After discussing how bad my rotors were with a friend, I learned that back in June 2010, Car and Driver magazine tested a 370z which experienced enormous brake fade leading to a crash. My fiend suggested that my 350z might suffer similar problems.

I read the Car and Driver article and found that their 370z test car was equipped with the Sport Package which comes with Akebono sourced 4 piston mono-block calipers and 14″ rotors. This system is much better than the 2 piston slider arrangement and smaller rotors found on my 350z. Even though the 370z had this terrific brake package, they still overheat and faded.

The Car and Driver article concluded that there wasn’t enough air getting to the rotors so they were never able to cool. I agree that this was the case, but I don’t totally agree with them thinking that the lack of air was due to trying to make the car as aerodynamic as possible. What might be a huge factor is if there were restrictive splash shields on the intakes of the rotors preventing efficient cooling.

This is the rotor and shield off of my car (350z). You can see that the shield pretty much prevents any air from entering the rotor. The cut out on the left side of the shield is filled in by the caliper blocking air flow, and the window on the bottom goes against the upright and lower ball joint.

When I was doing my brake job, I noticed that these shields almost totally seal off the inside of the rotors from access to air. I don’t totally understand why these shields exist, especially on my 350z which has very large uprights that almost cover the back of the rotor anyway.

You can see from my rotor that they must have experienced extremely high temperatures to form this deposit seen on the surface of rotor. At the time of the track day, I was shocked at how much heat was pouring out of the wheels when I returned to the paddock; it seemed like it was more than any other car  I have raced in the past. I was also shocked that the cheap Prostop ceramic pads I got from Pep Boys were able to take all of this heat and never fade. I believe that the high temps encountered that day might not have been as bad if it weren’t for these shields. As far as the uneven rotor wear on the inside surface goes, I believe that its mostly caused from the calipers design. It seems like calipers with slider pins always bind up causing more wear on the piston side. Sliders are a terrible design, mono-block calipers are vastly superior.

A lot of people tend to think that these shields are to prevent dust from going someplace it shouldn’t. But there’s nothing behind these shields that dust would effect. I also don’t think these shields would be an effective barrier against dust since they don’t cover 100% of the swept surface and brake dust is just going to get blown everywhere anyway. I believe that this is a splash shield since wet brakes/rotors can cause increased stopping distance, or to prevent thermal shock from too rapid of cooling due to water spray.

It is a well known racers trick to remove these shields, and I have removed them with no ill effects from previous cars I have had, and have now removed them on my Z. Since I never experienced any problems, this further confounds me on why they exist. It seems strange that any sports car would have something like this that would so detrimentally reduce performance. But then again, very few people actually attend track days.

 

This is the front side of the rotor. This side doesn’t have the large deposits that the rear does, but the cracks are evident.

Here is the shield still on the car. Its located behind the hub assembly making removal somewhat difficult. Because of this, most people cut the shield off. I took the time and removed the hub so the shield wouldn’t be damaged. You can barely see the window on the bottom of the shield and how its directly against the upright providing no air flow.


This is the rear shield setup with the rotor removed. This covers 100% of the rotor and is worse than the front.

This is the back view of the rear rotor when it is installed (looking from the inside of the car out).  This area is usually covered by the caliper. The only area where air can enter is the small slit past the rust to the right of the swept area.

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4 Comments to "Those shields behind rotors"

  1. Jonathan's Gravatar Jonathan
    April 16, 2012 - 11:22 am | Permalink

    IMO, removing the shield helps, but is not a substitute for properly ducted brakes. I know a lot of guys who track BMW’s 3’s and M’s with stock brakes, and even with the stock ducting, have little issues. As for the uneven surface, it sounds like they are warped and there could be some pad transfer to the rotor. I also don’t think that the slider pins are designed for high heat applications, but you can buy some high temp grease at any auto parts shop, not sure if that helps.

  2. Chris's Gravatar Chris
    April 20, 2012 - 10:17 am | Permalink

    most often the shields are there to protect non-metal parts from being exposed to the heat that you saw pouring out of your wheels; specifically the rubber bushings of the various suspension components. Over time, the extreme heat will cause these pieces to break down prematurely.

  3. March 12, 2013 - 10:29 pm | Permalink

    I was updgrading my Tercel brakes today, to Corrolla brakes. Turns out that MX3 (1993-1995) Mazda discs will fit, using 93-97 Corrolla calipers, on a Tercel with no modifcations, they bolt right up. I can’t use my existing heat shields because the Tercel uses 238mm brakes, instead of the 257mm MX3 rotors. I got to wondering why the dust shields are there at all? I finally realized after reading this article and others that everyone is wrong. They aren’t splash shields, heat shields or dust covers. They are rock covers. They protect ONLY against rocks getting into the heat vents inside the hollow rotor. The rotor would fill with small pebbles and would have zero air flow if you were driving on a lot of dirt roads. That’s why they are there, end of story!

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