NitroDann wrote:Total mass (assuming all materials between different calipers disks etc have the same volumetric thermal capacity) and surface area touching fresh air along with relevant efficiency of pumping ventilation design are the most important things really. So long as the brakes feel good and can lock the tyres when at the right temperature, only keeping them at that temp is important.
Dann
Dann, what you have said above is not wrong, by any means, but I think it is out of context for a weekend track/hill climb car, even a dedicated track car. I am also yet to see a road registered mx5 with rear brake ducting!
The average person isn't going to be able to influence anything to do with the specific heat capacity of materiel except the brake pads, which will come down to cost and what they are used for.
If you are going to go to all the trouble of looking at heat capacities, wouldn't you need the friction coefficients (which are temperature dependant)? Now if you have all this complicated data, we can plot the equations as a function of speed.... Now as fun as this would be to do, it’s not going to change what is available on the market, might give you a better idea on the brake pad need for different tracks, dependant on entry speed, but now you have to think about wear... Let’s assume you calculate all the data and you know the most appropriate gear to be running and you finally head out to the track.
Do you really think this is going to help? I’m pretty sure you would agree when I say the person driving the car has the most influence on how ‘quick’ a car is. I know for a fact I’m not a world class driver, so it is probably going to make little difference to me except to my wallet. More experience on track would be yield faster times.
The reason I started the thread is I was curious to see what the geometrical differences were between each series of mx5, because some basic analysis would show whether it is worth spending the extra money on rear NA8 brakes. This could mean that your brakes aren’t balanced perfectly but there is only a 10% difference in area assuming they are rectangles, with a 4 mm difference in height. I think the actual difference would be less than this when you look at the pads – go to bendix site. The front upgrade is an easy decision.
Now this analysis would be a little subjective, because I am cheap.
If some basic assumptions are made, we should be able to come up with a comparative approximation for the performance of each set of brakes. A brake performance coefficient you could call it. Now, friction from the pad produces a moment which counteracts the motion of the wheel. The friction is pressure (caliper) and temperature (pad) dependent (fluid will also have some influence). Assume you have the same pads and fluid, this removes the main temperature dependence. Now dann i assume you know most of this, but for the benefit of anyone else following the discussion i’m going to step it out.
Now, we know that Pressure, P=Force, F/Area, A
Therefore F = PA
The moment, M caused by the brakes can be approximated at a distance, d, which equals brake disc Diameter – 0.5*Pad width (centreline of pad).
M = F*d, substitute in Pressure.
M = PAd
Now are known information here is the Area and approximated distance. P is unknown, but we can calculate A and d to give us our brake performance Coefficients it is P is proportional to M.
So, the variables
NA6: Rear Disc Diameter: 231 mm Pad Area = 3978 mm^2 Pad Height = 39 mm
NA8: Rear Disc Diameter: 250.5 mm Pad Area = 4386 mm^2 Pad Height = 43 mm
Now: NA6 Disc and Pad gives;
M(NA6R) = P * (231-(0.5*39)) x 3978
M(NA6R) = 0.841P Nm
M(NA8R disc, NA6 capliper) = 0.919P Nm
M(NA8R) = 1.00P Nm
So, comparing the coefficients, there is a 9% increase in moment switching from NA6 Discs to NA8 discs with NA6 capliers/pads and a 19% increase when going from NA6 disc and calipesr to NA8 disc and calipers. This means there is a 10% difference between the NA8/NA6 combo and the NA8 set. Now this is basic and only a guide! But, basic math and sound reasoning can be much more helpful than the specific heat capacities of each component.
I could have come and asked “should I upgrade my rear brakes from na6 to na8?”, but wear is the fun in that?! Would you prefer to spend the extra money for calipers, or use it for ‘better’ pads?
