Rotor air gap - anyone checked various models?

[Duane Black]

Ill open with what I've checked now at leas . I'm curious how much manufacturers let this vary from one to another.

First up is a 2 piston slider. the 13.25" rotor. This particular model is a r-300 raybestos, but its air gap is the same as my trusty Wagners. It measured approximately .5 inche


I bought some powerstop for the 14" Brembo kit and noticed right off thr air gap was huge. It was approximately 5/8 (.625) inches. I also noticed this and the 15" gt500 centric i bought were a variable vane desigb where my sliding caliper rotors always straight.



I trief the s550 setup with the 15 inch gt500 centric rotors but it didn't fit. I never measured, but noticed it seemed the air gap was thinner. That shocked me. Id estimate it at 3/8 (.375) inches.


The theory is, if its a wider gap, it should cool faster, no?

Are similar measurements seen across all manufacturers? Anyone else care to share any measurements?

 

[Bill Pemberton]

Interesting thought Duane, and your logic seems quite sensible, but I wonder if it is a combination of not just width, but how distinctly styled vanes direct the air through the rotor? Looking forward to hearing from some folks with knowledge on this. Really thoughtful question and I know I for one do not know the definitive answer.

 

[JAJ]

There are a lot of factors in this - it's complicated. There are four main things to think about:

Part one - the lighter the rotor, the more the rotor temperature rises from the start of the braking zone to the end of the braking zone.

Part two - the easiest way to make a rotor lighter is to make a bigger air gap in the middle. This will help the rotor cool faster (as you said).

Part three - the design of the vanes inside the rotor. The more effective the vane design, the faster a rotor cools.

Part four - the amount of cooling air available to the rotor when you're moving. The more air at a given speed the faster the rotor cools.

So there you have it - the four things that determine the hottest temperature your rotors will hit at the end of a braking zone. A heavier rotor won't heat up as much in the braking zone but it might be hotter at the start of braking. A lighter rotor will cool faster but the bigger temperature rise might make it hotter at the end of the brake zone.

What you are trying to achieve is the lowest peak temperature at the hottest point on a lap, because high peak temperatures eat pads and boil fluids.

The way most people deal with this is to put as much cooling air as possible - the new Vorshlag plates or 4" ducts - and then use rotor paint to figure out if the temperatures are too high. If they're too high, then try a different rotor.

 

[Coz]

This is assuming that the gap measured at the edge is consistent across the rotor. Actually weighing or looking up the rotor weight may confirm whether one is lighter than the other.

Another thing to consider is the minimum/discard thickness. If a rotor with a wider gap has to be discarded sooner then you'll have to decide if the possibility of a cooler rotor is worth having to replace it more often.

Also check the nominal overall thickness. Some rotors, such as DBA are known to be thicker. My point is that a thicker rotor with a wider air gap might not be much different than a thinner rotor with a narrower gap.

For the reasons that JAJ outlined, you may be investing a lot of time into an effort that may not have any relation to real world performance.

You mentioned that the S550 Brembo Calipers did not fit on the GT500 (presumably 13-14) 15" rotors. Can you elaborate? I've never heard this to be a problem.

 

[Grant 302]

The two main factors for thermal performance in rotors are the inner surface area and mass of the rotor.

A wider gap does not mean more surface area. Vane count and designs will have more effect on total surface area. In the original GT350 fronts, partial height vanes are also used to create more surface.

Thermally, rotors are heat sinks. Given the same surface area, the rotor with more mass will see a lower peak in all conditions.

So unless the rotors in question have cutaway sections and weights of the rotors are compared, then comparing them will not yield the answer you’re looking for.

 

[Duane Black]

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You mentioned that the S550 Brembo Calipers did not fit on the GT500 (presumably 13-14) 15" rotors. Can you elaborate? I've never heard this to be a problem.

Clarification- didnt fit my wheels, i didnt feel like getting more invested so just got 4 pots for now

 

[Coz]

...or the Rotor air gap thread.