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Re: General Chat for advice
Posted: Thu Dec 31, 2015 11:02 am
by GrahamFountain
Carledo wrote:the Sierra caliper is a sliding one which allows the force of one piston to be transmitted to 2 pads. My brand of theory says this should make no difference
Yes, you're right and I was wrong - see above.
Graham
Re: General Chat for advice
Posted: Thu Dec 31, 2015 2:27 pm
by Carledo
GrahamFountain wrote:I've been doing a bit more research on the Net and I've found the discrepancy between the calculated change in effort with the 1 pot Sierra calliper and what the TJ kit gives. And it turns out, embarrassingly, that when I said I was wrong I was in fact mistaken (LOL). So please ignore the previous apology and accept this free of charge replacement: Sorry.
I thought you just use the area of a piston multiplied by their number – it seemed logical at the time. But I'd missed that the cylinder end face in the slider acts in the same way as the back of another piston - the hydraulic pressure, as a scalar, pushes in both directions at once. So there's the full force pushing the piston onto one pad and the same from the other end of the slider pushing onto the other pad (less any extra stickiness in the slider mechanism). So its not the force on one piston split between two pads - it would only be that if the disc were floating between two pads, one pushed by the piston and one fixed to the suspension. So you calculate the area the fluid acts on as twice that of the piston's, just as with a two pot calliper. (See Escort RS Turbo Series 2 piston area calculation at
https://www.rallydesign.co.uk/pdf/desig ... brakes.pdf)
Anyway, that means that the earlier calculated values of something like 44 to 51 percent overall increase in front brake effort from the TJ set (depending on where the pressure centre is between the pad centroid and piston centre), should be right.
I go back to calling that a "massive gain", in relation to what you can take advantage of with 10 or 12 percent grippier road tires. It's therefore rather too much for what I would want, were I to upgrade my road car.
The TR8's 54 mm piston callipers on the 220 mm solid disc should give about 22 percent more effort, which seems a much more reasonable figure. From what I remember, it's a fair bit bigger pad too - same as the Stag's I think. So there should be some improvement in fade as well – not near as much as a vented disc would give, but some. But I've no easy way of seeing if they'd fit and the only set I have belong to the TR7 Sprint.
Graham
Some serious thought last night brought me to the same conclusion, because the sliding caliper acts as a second "piston", the pressure goes both ways so even though there is only 1 piston, it counts twice and we are back to the (roughly) 50% improvement! I just worked it out in my head! (insert "smug" smiley here!)
It bothered me that nearly every production car these days uses the single piston sliding caliper (excepting only exotics with multi piston fixed calipers) and I couldn't see manufacturers using a LESS efficient design, no matter how much cheaper it would be to produce! Even stuff like Shoguns have a sliding caliper with 2 pistons on the same side (effectively a 4 pot)
Regarding your theory that the TJs increase would be too much for a road car, I can only suggest you actually DRIVE a TJ fitted car and see for yourself! It really is the dogs doo-dahs!
Steve
Re: General Chat for advice
Posted: Thu Dec 31, 2015 2:35 pm
by tony g
Ive been reading this with interest along the way, but had nothing to add so I didnt
Graham, I fitted a TJ conversion on my sprint but with KA calipers (these fit where the sierra ones would, so they could be swapped to sierra if needed.) The KA has a smaller piston than the sierra so less increase in overall efficiency I guess, but road manners are very much improved. Ive never had a near lock up nor have I had to brake so hard I was worried about it locking. Overall mine feels like a very nice set up with good feel and progressive braking and certainly an improvement over the wooden feel of the standard set up. Youre welcome to try mine if youre in the Warwick Cov area any time
Tony
Re: General Chat for advice
Posted: Thu Dec 31, 2015 8:14 pm
by GrahamFountain
Steve and Tony,
The thing is, I'm simply not unhappy with the brakes I have. I'm even running a smaller 1500 servo at the moment, because the Sprint's went for reconditioning, and I haven't put it back yet. I mightn't, at the moment, be able to lock the wheels in the dry (because I'm still waiting for a dry day to find out), but the idea of upgrading by 40 or 50 percent from what I have, plus going back to the right servo, seems more than a bit too much.
So stared to wonder if I was spoiled, or is it ruined, by the TR8 and big Princess sets on the TR7 Sprints; or maybe just old. So I worked out their brake efforts, relative to the Doly's. And they come out, on the 248 mm TR7/8 rotors, about the same as these Sierra callipers – 43 and 53 percent up on the Sprint's, respectively. Even the much derided TR7 brakes are nearly 30 percent better – so you can see the TR8 and Princess callipers aren't huge upgrades to the 7, just relative to the Doly Sprint. And as there's little difference in kerb weights between TR7 and Doly (1100 vs 1015 kg), none of that makes any sense at all.
The only thing I'm not sure about in the comparison is the servo boost ratio, which is 2.3:1 on the 7. If the Sprint's is over about 3:1, then it might start to come together.
Graham
Okay,.....
Posted: Sat Jan 02, 2016 1:08 pm
by sprint95m
From page one....
GrahamFountain wrote:I still find it odd that none of the disk upgrade kits give the increase in effort, and no one seems to mind.
Sorry, is this really what you meant or is it just poorly worded?
I think the cliche is
the proof is in the pudding......
if you change from a solid to a vented disc you get more braking from the same effort.
Every car manufacturer cannot be wrong?
Ian.
Re: Okay,.....
Posted: Sat Jan 02, 2016 1:16 pm
by GrahamFountain
sprint95m wrote:
Furthermore, it should be worthwhile to read Chris Witor's articles where he (at considerable expense) actually
tested different set ups (on the same car).
Don't seem to be able to find one covering the Sprint. Have you got a URL for this tech article?
Graham
Re: Okay,.....
Posted: Sat Jan 02, 2016 3:07 pm
by GrahamFountain
sprint95m wrote:From page one....
GrahamFountain wrote:I still find it odd that none of the disk upgrade kits give the increase in effort, and no one seems to mind.
Sorry, is this really what you meant or is it just poorly worded?
Yes, I think that's what I mean and I don't see any solecisms in it. Essentially, I don't see how you can choose between possible upgrades unless you know, at least approximately, what difference they will make to the ratio between foot pressure and deceleration. Admittedly, that also means you have to know, at least roughly, what it is you want from an upgrade. And maybe I'm unique in having some idea what that is. But that would surprise the hell out of me.
sprint95m wrote:if you change from a solid to a vented disc you get more braking from the same effort.
I simply don't agree that vented disc give more "braking from the same effort". All that vented discs do is allow for more braking effort, e.g. in terms of the number of times you stop, before the system fades: All else being equal, the first time you use the brakes and for some number of times after, they will give exactly the same deceleration for the same foot pressure. The solid disc system will, however, overheat for a much smaller number of times they are used heavily, and fade well before the vented system. But if you don't drive like you're on the race track, that should never happen with either system.
And I think you'd have to be decelerating the car from a speed well over what's legal or possible on the road, for the solid disc brakes to fade before you actually stop. I admit I don't know what that speed is, but I've never had the Sprints' brakes fade. I've only had it happen to me twice with the 7s. Both times because I was following a car/driver that simply outclassed me: Dave Wynn in a Porsche 92somethingorother, and Dave Brown in his White Lightening 6. And they were on twisting country roads, where I was using the brakes hard at every turn. So I got quite a few braking events out of them before they went.
So if you want more deceleration you have to increase the gain of the system, which vented discs simply cannot affect. You can do that by increasing the piston area or moving the pressure centre further out or using a bigger gain servo or a smaller diameter master cylinder. But pad size and disc cooling don't do it.
Graham