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Old 02-24-2009, 04:25 PM   #1
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Default RX7: Advantages of RB's two-piece rotor design

This is a message originally posted on 2/24/09 on

Originally Posted by primerGrey View Post
I wouldn't have guessed the RB rotor would work as well as it seems to be for those that have it. Just from the pictures, I would be worried about:

- The RB rotor design seems to be less than ideal from a cooling perspective. Because of the gaps in the rotor webbing, there is no way to force air from the low pressure eye through the vanes in the rotor. All other rotors I've seen ensure that air forced into the eye (by the simple ducting every production car has, or by a complete duct solution that pulls air from the nose of the car) must exit through the rotor vanes. With the RB design, air forced into the eye can just shoot out the other side - there is no way to duct it at all. The RB design thus relies only on the pump effect of the curved vanes to move air from the eye to the perimeter. This seems a lot less efficient than a closed hub design.
The heat transfer efficiency is determined by the flow rate of cooling media (air) and temperature difference between the inlet (t1) and outlet (t2).

Therefore better rotor cooling can be achieved by:
  1. Increasing air flow, at the same temperature difference (t2-t1); or
  2. Widening the air temperature difference between the inlet temp (as low as possible), and the outlet temp (as high as possible), at the same air flow rate.
However, there is something else to consider in heat removal when it comes to a rotor application compared to a normal cooling system, and that is temperature uniformity. A ventilated rotor consists of two friction plates with cooling vane (ribs) in between the plates. Ideal rotor cooling, in addition to efficiency, must maintain as uniform a temperature as possible throughout the entire friction surface - from inboard to outboard surfaces and from inner to outer radial edges. From our extensive brake experience we have found this temperature uniformity factor sometimes can be more important than just the cooling itself especially under extreme heavy braking, as most ill functions such as vibration, pulsation, rotor cracking or uneven pad wear can result from non-uniform disc temperatures during heat surge cycles.

RB center mount rotors are patented and uniquely different in design and function from traditional surface mount rotors and have the following advantages:
  1. It pumps more cooling air (from both inboard/outboard sides) unlike surface mount rotors which admit air only from the IB side, which is often obstructed by a dust shield. Even another brake company realizes the importance of outboard cooling air and offers a directional hat (Aerohat) which admits some air from the outboard side. But these inlets can quickly clog up with brake dust and debris only after a few heavy stops and are much smaller than the spaces in our center mounted rotors.

  2. It increases cooling efficiency by widening the temperature difference between t2 and t1 through our patented cooling vane design - Notice our outer edge has more number of cooling vanes than inner edge plus more overall cooling surface area.

  3. Our rotor rings are unidirectional – same for the left or the right side. You need only one spare ring, saving your replacement costs.
We have many customers (EVO, STi and Corvette Z06) who had previously installed cooling ducts but still had brake problems, not realizing that if the cooling ducts are not piped properly, they can worsen the heat balance and do more harm than good. After replacing their rotors with RB two-piece rotors and removing the cooling ducts, the brake performance improvement could only be believed by the people who made the change.

We are not objecting to using cooling ducts; however we don’t like the idea of inducing the “forced” air at an angle into the rotor's inboard surface while the outboard surface remains unducted. Our objective is to keep the rotor from overheating while maintaining a uniform temperature throughout the entire braking surface. Inducing extra air to be blown toward the rotor does not necessarily mean more effective cooling.

Since brakes are subject to heat surges (rather than being under a constant heat load), we still believe a design with internal circulation can generate a more uniform and efficient cooling system. However, we will just leave this to other car enthusiasts' evaluation and testimonial.

- Looking at Howard's comparo shots. the rotor faces don't appear to be much thicker than the stock rotor - it is wider overall, but most of that is empty vane. Thus there is not as much iron as there could be to absorb heat. It looks like what other vendors would call a "lightweight rotor" with relatively thin faces. On the up side, that means less rotating mass, too.
RB designs brakes based on application and need. The suggestion of thicker plates is valid if we are making rotors for a heavy truck trailer application driving at street speed. In that case, you need a large heat sink (rotor mass) due to a slow heat dissipation rate. But here, we are dealing with race cars where every driver is trying to shave pounds and is driving twice as fast as a truck's street speed. These applications don't require heavy rotors but do require quick heat dissipation. A good example is Corvette Z06 front rotors. Stock one-piece rotors weigh 26 lbs each with 9mm plate thickness. In contrast, our two-piece rotors weigh just 17 lbs each and have a 7.5mm plate thickness. For street driving, the heavier rotors are fine, but for track racing, not only are our two piece rotors adequate to handle the heat, but they also drastically improve braking response.
- The rotor tabs that connect the rotor to the hat have bends in them that seem a great place for a crack to form (I have actually seen this on RB rotors on other cars at the track). In a more standard design, there are no bends between the hat and rotor.
No one makes perfect products, including RB, but we 100% back up what we sell. Our center mount design requires less overall height as compared to surface mounted designs and actually reduces the amount of cantilever stress on the mounting tabs when the brakes are applied.

We are sure there are quite a few RX7 owners who have installed RB brakes and their experiences and opinions are what really count.
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