iandollysprint wrote:Can you answer my question on what, with all your input, you would suggest as being the best set up?
Yes, though I'm not sure that question was entirely clear in its original context ("So whats your point."). And I'm sorry about the delay, it's not a trivial issue, and warrants some thought: which was my original point, I think, i.e. that you must think carefully before you substantially upgrade the effort from the rear brakes. And I wasn't intending to advise what anyone should do, just what to think about if you do upgrade the brakes, and why it matters. So it took a while to try to write about what I would do in such a way that it doesn't appear as a recommendation.
My take is this: I have no current intentions of upgrading or changing the brakes on my Dolomite Sprint. So, I suppose that means I suggest the best setup for me is standard. I'm certainly happy that I can use all the grip of the tires I have fitted, which are, admittedly, only Firestone F590 Fuel Saver tires, so perhaps a bit hard. I could fit tires with more grip than that, but don't see the point when it's just the daily drive and, more importantly, used for the school run: too much grip would only encourage me to excesses in driving; I know this about me.
But I have upgraded all the TR7's I've owned, except the factory TR7 Sprint. Usually that's just been to TR8 callipers, which is factory spec for the TR7 Sprint, and thus needs no thought. But where I've gone further – like with A TR7 16V the second, which had a bit of a cam, Dellorto 45s, some porting through, etc., to give (according to Dave Bogg) about 180 bhp, and (more importantly) wider wheels with grippier tires; and the TR7V8, which had SD1 heads over P6B pistons, etc., to give about 200 bhp (also on the Bogg standard) – I've use the big princess four pots and 4 speed TR7 rear slave cylinders to de-rate the rear brakes, as recommended by TriumphTune. Mind you, I think I worked out that the big princess brakes are something over 20 percent up on the piddling little HA Viva callipers fitted to the TR7 – though I've lost the envelope and, as a bachelor at the time, I had no children to speak of.
If I did want more effort from the brakes on the Dolomite, as far as I'm concerned, that would only be worthwhile if I upgraded the tires as well. And the straight-line braking model tells me exactly the maximum percentage change in brake effort when moving from one maximum CofF to another. So, if I've got a reasonable estimate of the grip figures for those I've got, and those I'm upgrading to, I can tell what brake upgrade is worthwhile, i.e. that would let me lock those grippier tires with the same brake pressure on the same surface. Even a reasonable estimate for the ratio of the grip factors, e.g. 0.9:1, or 0.8:0.9 (0.88:1) would let me get some good, ball-park figures for the range of upgrades that make sense.
For example (and only as an example), I can use some CofF data off the net (
http://hpwizard.com/tire-friction-coefficient.html), to work out the percentage change in brake efforts I would need if I were to replace "tourism" tires with a maximum possible CofF of about 0.9 with "hi-performance" at 1.0.
Using the same guess for the CofG as before, I would need about 14.5 percent more effort at the front, and about 3.4 less at the back. But the results vary for other guesses of the location of the CofG. Hence, if I were doing this job and were trying to do it in anything like a proper manner, and still couldn't get the actual location for the CofG (with just me in it), I'd have to use values at all four limits of the position I believe reasonable, and take the worst of those. Doing that at the four corners, I get about 15 percent as the most I might need at the front and 5 percent more at the back. It also gives the least that I might want as 14 percent more at the front and about -3.6 percent at the back.
Since
I would do this upgrade with more than half an eye to safety, I would probably look for a front brake upgrade that would give me no more than 14 or 15 percent, and look at the possibility of reducing the rears by 3 or 4 percent. But if that's too expensive for the relatively small reduction involved, I mightn't be too bothered about the small reduction in the safety margin. But I would want to test the braking, without the mod to the rear brakes, with tires worn to the limits on the front and new ones on the back, and somewhere dry (so not Lancashire then), as that should give me the greatest and worst imbalance in grip between the two ends.
It is a bit of a problem that the upgrade you get from changing to a different calliper, etc., isn't generally given as a percentage. However, all you need to work that out are the percentage change in the distance between the centre of the hub and the centroid (geometric centre) of the pad, and the percentage change in the areas of the pistons. Add those together, and you have a pretty good estimate for the upgrade as a percentage. I know that those specs have been got for an upgrade to the Vitesse front brakes.
I could do the same upgrade requirement calculations for slicks, e.g. with a grip of 1.1 or so. But I don't think my assumptions about the position of the CofG remain valid for a car that could use slicks effectively, I wouldn't have a clue where to set it, and
I aren't interested in doing it. But if someone did want to offer a range of values for how far back and how high off the road such a car's CofG might be, I'll willing put those numbers in. I also think slicks imply a manual balance adjustment, and that means upgrading the rear brakes isn't a problem any longer, presupposing that money is no issue.
So that's what I would do if I wanted to do anything. But it does depend on doing these calculations in the very simple situation of braking in a straight line at the limits of grip, front and rear. However, simple though it may be, that situation is a rather special case, at least from the perspective of the physics: it identifies the maximum brake force it can ever take to lock the wheels. That's because for any other case, where there must be additional side forces and sideways weight transfers, the wheels must lock at a lower brake force than predicted by the model of straight line braking.
I would also limit the upgrade to what is sensible for the change in type of tire. That's because if you upgrade the brakes sufficiently far beyond what the tires will support, you will have other problems. I believe
Dolly racer 33 was commenting on the more extreme effects of the brakes overpowering the grip from the tires in writing this: "If I run Toyo 888's or slicks I can brake very late and controlled into a corner. If I run a tyre with less grip, the brakes are a nightmare with no feedback, they lock up as soon as you apply any pressure to the pedal, I have to use a much softer pad to overcome this."
I also accept the point about some aspects of Triumph's production engineering being a bit shoddy, which I've always put down to penny-pinching by management (though I may be biased by personal experiences with production management; I'm not that impressed with research management to be honest). I assume the flaws in production are cited to suggest the safety margin is arbitrary and that attempting not to erode it is a waste of time. But if I thought that Triumph's design engineering, as opposed to production engineering, was that bad, I'd avoid all Triumphs like the plague and drive something where the engineering design was done by the Germans. But obviously, I don't believe that.
I also think that an understanding of the how the car responds to braking forces, how and why the limits change with grip, and what that means to where you can apply what force, is important if you are going to think about upgrading a safety critical aspect of the car, like brakes (and I do think it's important to think first). I therefore do think that explaining some aspects of that science in regard to a road car actually is constructive – and I think there are other responses that bear me out in that regard.
Graham
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