I have always been unhappy about the VxT
brakes and have been through a variety of pads and disks. The RS14 pad seems a good compromise for those doing road and track work. However, I still found that the brake system suffered severely from overheating when driven hard.
So much so that I felt I was close to boiling new DOT 5.1 fluid. Certainly I have been blueing disks which I believe doesn't happen below 720C (its something to do with a change in iron / carbon organisation to form a different allotrope with different physical characteristics). All of this is etiher nasty or dangerous.
I did a bit of modelling of the energy dissipated when braking the VxT from speed. I selected a slowing from 100-50mph at 1G deceleration which is pretty severe but not unreasonable. This process dumps approximately 2*10^6 Joules which is equivalent to drawing 8,000 amps from your main supply for 1 second - err, running 670 of the biggest heaters you can run off a 13Amp socket. AsS most of this is dumped into the brake disk the temperature goes up very rapidly indeed. Have a look at the top line in this graph which shows an uncooled 3kg front disk hitting 476C in the 4.6 seconds to knock off 100mph (I've allowed for 2:1 disspation front / rear).
I tried to make an estimate of the cooling effect of having the disk in a real airstream but there is no straightforward analytical approach as modelling turbulence in the wheel / wheel arch etc is a mystery understood only by God and some physicists doubt even that. Still, suggesting the disk surface is moving through a column of air the length of the braking period and maybe 1.2 cms wide with a transfer efficiency of 50% seemed to yield reasonable looking numbers. The middle line shows the temperature of the disk with this estimate of cooling effect.
I expect the cooling rate is not a linear function of the volume of air passing the disk face / second. If anyone knows more about this or has real observations to plug into the model I would be glad to hear of them.
The bottom line shows the effect of improving disk cooling by shunting a circular column of air (75mm dia) moving at slipstream speed to the centre of the disk. This shows very significant improvement in disk temperature as it stays well below any threshold for change in frictional charcateristic and well below fluid boiling temperatures. Thats why the foglight ducting has worked so well. Chopping slots is the next thing for me to have done.
Interesting as this primitive analysis was I wasn't entirely convinced it told the whole story. One can suppress temperature rise by using a bigger (weight) disk but one still has to get the same amount of energy out of the system and thats easier at higher temperatures. One can increase the disk diameter (the braking effect goes up as a square of radius) which improves modulation. It still doesn't feel like the entire answer.
The other half of the heat dissipation issue is the pad: we have evidence that the disk surface was reaching 720C+ (and not knowing how high) even if the expected whole disk temperature is maybe only half that. We are trying to stuff an unconscionable amount of energy into a very tiny surface area.
I have noticed that with Mintex 1144s I ate 6-7 mm of pad in one track day - not much more than 90 minutes driving. Thats got to argue that the pad material has crossed a thermochemical threshold and is breaking down in some way. The SBS pads also exhibited a phase change and had poor friction when hot which did not improve with load. The RS14s also exhibit a dramatic change in characteristic although they respond again when pressed harder. Published figures for RS14 suggest the friction characteristic is fairly flat up to about 650C. I think this all argues the pad surface is getting too hot - its a localised skin phenomenon.
OK, so by increasing pad area we can reduce the energy density. Its probably why the change to 4 pot calipers is regarded as succesful. Its not so much to do with brake balance / torque its a lot more to do with keeping the pad in the correct energy handling range in watts / sq cm.
Unfortunately I don't have the time to make a series of experiments to validate this contention. So I am going to kill it all in one go: the biggest diameter disks you can get in a 16" wheel, increased mass of rotor, better internal ventilation from greater disk cross-section, bigger pads all round, increased slave piston area, hoovering great brake ducts feeding the front disks.
I had planned to modify the front end with bigger disks, caliper spacers and some bigger Brembo calipers. In the end it turned out to be faster to persuade Geary at Elise Parts to modify his standard offering to take bigger disks (caliper spacers). Whilst discussing this with him I found I greatly liked the idea of his GT rear uprights which alter the roll-couple and allow a lower ride height. It didn't hurt that they also supported the toe-link properly. By bodging the pads (milling 1mm off the face) we can squeeze the 28mm disks into the radial mounting handbrake caliper (HiTec Ultralight). Hopefully it will all be on the car and at the track next weekend.
I plan to instrument this with an IR temperature probe into a data logger. Has anyone got a source for a cheap one? The one sold to F1 teams seems just a tad on the pricy side. Anyway, this make an attack on the brake problem and to some degree on the handling issues. And, with the new toe-link I can run slicks. Now that is going to be fun.
I'm looking forward to hearing how you get on with them. I've had my eye on the Eliseparts uprights and calipers for a while now. Will this give you much of a weight saving?
The corners are lighter than the originals by at least a kilo each - I should get exact weights on Monday. The rears lower the car about 15mm straight out of the box.
However the toe-link inboard mounting is a mess. 
The link fouls the plate / flange unless its spaced off so far that the box section (which puts the mount into double sheer) can't be fitted. We thought about it and we don't like this bit much (particularly the rivetted mounting). So we are going to fabricate a new mount which will also introduce some of the strengthening and load spreading Lotus introduced as a kit and now fit on the Exige. I'll post some piccies when we get it together.
There are other minor niggles, the handbrake cable supplied needs to be made 15mm longer, the rear brake pipes need a RtAngle union rather than straight and so on. But nothing too major. It all looks very nice and will be strong, stable and adjustable when finished. Its going to be fun 
Stunning 
A full write up is necessary! 
