TR7/TR8 brakes

[A TR7 16V]

Does anybody have the measurment for the distance between the piston centre and hub centre on the TR7 and TR7 front brake callipers?

Graham

[Steve AKA vitessesteve]

Not sure I understand what measurement you are after.  Happy to measure on my TR7 tomorrow.

[Beans]

Me neither 🤔

[A TR7 16V]

Sorry. It's the distance out from the centre line of the stub axle to the centre line running through the two calliper piston centres. It's essentially the leverage the calliper has on the brake disc. It's not necessarily the exact same distance as hub to pressure centre, but it can't be far off. And if the two callipers are basically similar, the percentage change in leverage should be the same as the change in that distance – near enough for such a comparison. So if you're comparing another calliper as an upgrade, like from TR7 to TR8 callipers, you have to multiply the change in piston area by the change in that distance.

If I had to guess I'd put the centre of the 51 mm TR7 piston at about 98 mm from the hub centre, and the 54 mm TR8 piston at slightly less – 97 or 96 mm –, giving an area change of 13 percent but a loss of about 1.6 percent from the pressure centre moving inwards on the same diameter disc. That gives, approximately, an overall gain of about 11 percent.

Again guessing at the leverage for the big princess callipers, I get that as an overall increase in front brake effort of just under 20 percent – about 12 percent from area gain and 6.5 ish from the centre of the smaller 4 pot pistons being further out from the hub.

I think 20 percent is a pretty good increase, considering you can only really take advantage of it by increasing the grip from the tires, and H rated road tires on a standard wheel should only vary in their coefficients of friction by about 10 or 12 percent, e.g. from about 0.8 to about 0.9 - depending on what source you take; some give it as 0.9 to 1.0.

Graham