ChasR
RR Helper
motivation
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From my RST thread (see my signature) I recently went from Wilwood calipers to Sierra Cosworth 4x4 calipers, but staying with a 283mm 2WD Cosworth disc. I never would have thought that changing calipers would have such a dramatic effect on the braking power on the car, despite the unsprung weight increasing. The post below will talk of my experience and potential reasons. Some people were saying that the braking performance would get worse I would say that they are at least twice as good as the Wilwoods. Well, I have somehow managed to vaguely get the car ready for the MotoFest! It is now on some TRW Cosworth 4x4 brake pads (of the slightly larger width - they sweep near enough the entire disc. Despite being in the process of bedding in I was in for quite a shock! If I am honest the gentle brake response require a little more of a shove than I was used to in this car but not too bad for the old Fords I have driven over the years. The biggest difference is in stamping your foot on the brakes. WOW! is one way to put it! Despite still bedding in they do really hurl the car to a stop, and Christ they don't half slow you down! You are gripping on for dear life when you slam the anchors on at a speed higher than say 60! Lord knows what they will be like once they have bedded in! I am not surprised at all as to why the TVR Chimaera guys like this 'budget' setup! Upon doing the sums via a Total Piston Area method, it seems to become clear as to why this is the case! Total Piston Area (TPA) = 3.142 x r x r x n where r is the radius of the pistons n is the number (sliding calipers are assumed to have two pistons for this equation). If you look at my Wilwoods the piston diameter is 34mm, giving each piston a radius of 17mm meaning: TPA (Wilwood) = 3.142 x 17 x 17 x 4 = 3632mm (squared) With the Cossie calipers it is: TPA (Cosworth) = 3.142 x 30 x 30 x 2 = 5656mm (squared) The pads on the Cosworth calipers are probably 60% larger so this may well have affected things given that both setups are using the same discs. As a side effect the high speed vibrations are near enough gone in addition to the handling seeming to have improved despite the unsprung weight increasing with the heavier calipers. The turn in is better for one. Maybe the front axle width of the car being more in line with the rear has helped things somewhat. Good news all round for now it seems. I also changed the oil for Millers 5w40 CFS. Whilst the hot running oil pressure is now spot on the oil does not half smell! Jesus the car stinks of castor oil inside! People were say that slider caliper pistons cannot be judged against the superiority of 4 pots due to the compromise of weight and the sliding parts. My question to you is just how much effect do the following have in a brake setup regarding the grand scheme of things? -Disc area/size -Pad size -Caliper piston size -Weight -Other? I was under the impression that the disc area had the most effect with the piston size and pad size have less effect in descending order. However, a friend of mine diasgreed with the above (working in the industry) saying that the piston size would simply aid locating the pad. What are your thoughts and experiences on the matter? I may well go out and buy a book on the matter! |
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adam73bgt
Club Retro Rides Member
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Club RR Member Number: 58
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Being a brakes engineer (albeit a fairly inexperienced one) I shall have a think about this and see if I can come up with a short answer...
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Hi, I think it's a combination of. The ratio of master cylinder bore to calliper piston bore. Disc diameter rather than swept area, because the further from the centre the more mechanical advantage. The pad surface area and material. I don't think weight will come into it, that's more to do with ride and handling.
Colin
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mylittletony
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Colin pretty much has it nailed.
Braking is applied as torque = force x distance. Distance comes from brake diameter, force comes from how hard the disc is gripped.
That's a combination of pad area, friction coefficient and piston force. Piston force comes from pedal effort and the relationship between master cylinder and piston cylinder bores.
I.e. large, sticky pads, pushed really hard at a large diameter will be great.
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adam73bgt
Club Retro Rides Member
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Club RR Member Number: 58
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I agree with the above points in that the bigger diameter disc you have, the more brake torque and therefore braking power you can generate. Sliding fist type calipers may help with this as (for the same size wheel) you can fit a larger disc with a sliding fist due to them having a lower profile over the top of the disc compared to the opposed piston. The amount of pedal effort will be affected by the total piston size of the calipers compared to the piston size of the master cylinder. Opposed piston calipers can give a stiffer pedal due to the increased stiffness compared to a sliding fist caliper. The bonus of opposed piston calipers is that you can have more, smaller pistons which allows you to fit longer brake pads without getting as many issues with taper wear on the pad. If you have a big single piston fist type caliper with a long pad, you will probably find that the leading edge of the caliper will wear more due to the movement of the disc pulling it in more than the trailing edge. (That wasn't worded great.. apologies) On opposed piston calipers, you can find that the leading pistons are slightly smaller than the others in order to balance out the tendency to wear the leading edge of the pad more I'm not sure that sliding fist calipers should be immediately disregarded in favour of opposed pistons. They do have more flexibility in them which does cause issues with uneven pad wear, but they have big advantages in terms of packaging, cost and NVH which is why most OEM's fit them as standard. Obviously your priorities are with performance  but if you look at a Jaguar F Type, they have sliding fist calipers (carbon ceramic brakes aside..) so clearly sliding calipers can work in performance applications That was a bit rambling and probably a bit vague as I've only been a brakes engineer for just over a year so I'm not 100% on the details still. But I think ultimately, if you feel an improvement with the Sierra calipers over the Wilwoods then go with them, no amount of theory will convince you more than a car that drives well and has brakes that give you confidence  I have a book somewhere which does go into the theory a bit more of calculations and stuff that we go through to choose an appropriate brake size for vehicles, I'll see if I can dig it out for you |
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mylittletony
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I've only been a brakes engineer for just over a year I thought you were the face of Halfords?  |
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adam73bgt
Club Retro Rides Member
Posts: 4,864
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I've only been a brakes engineer for just over a year I thought you were the face of Halfords? Sadly I've been replaced! My face is no good for recruiting people to work at Halfords any more it would seem (not that it ever was...) |
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Dez
Club Retro Rides Member
And I won't sit down. And I won't shut up. And most of all I will not grow up.
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theres a lot to take in there, but from what youre saying, the only thing you have changed is the actual calipers and pads?
then, ignoring all other factors such as disc size (which makes by FAR the biggest difference)
the wilwoods were being run with the wrong master cylinder.
the hydraulic ratios of the master/slave setup is massively critical to both the realtionship between pedal effort and braking effort, and pedal travel.
for the master/slave ratios to the the same between the two setups, the master for the wilwoods would have needed to be a lot smaller, about 1/3 less piston area. you simply cannot expect calipers with piston areas of 3600mm.squ and 5600mm.squ. to run off the same master cylinder.
running it with a too large dia. master cyl would massively increase the pedal pressure needed to operate them (force= pressure/area and all that).
the issue wasnt that the wilwoods were 'worse', they were just been run with incompatable parts.
theyd have been just as good if theyd be run with the correct m/c.
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ChasR
RR Helper
motivation
Posts: 10,191
Club RR Member Number: 170
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Precisely Dez. I only changed the calipers. Oddly enough, Rally Design recommend a larger pistoned caliper for my car which is similar to what the S2 RST/Cosworth piston size is; I am assuming for the reasons you stated.
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slater
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You could write a book on this.
There's a bit of personal preference in the equation too ofcourse you cant just go on altering the cylinder ratio to get more and more leverage as each time your pedal will get longer and longer. Some people prefer a shorter harder pedal and some people a longer pedal with more feel. Ultimately it's your leg that's pressing the pads onto the disc and if you push harder it's better.
A big disc is good ofcourse both for radius and for avaliable swept area but this is limited by your wheel so you get to a point when you have it optimised quite quickly
Alot of caliper design is about getting the maximum piston area under your wheel but also about spreading the force your generating over the pad evenly to eliminate pad flex. Eliminating flex in the caliper is a big issue too.
Then there's keeping it all cool. Bigger swept area will run cooler for a given stopping power but aerodynamically might not be quite so good.
Pad material is a big influence. You need it to work well at the right temperature. Run pads too hot or too cool and you have big reduction of brake efficiency!
Weight is all about handling. Reducing unsprung weight is good, as ofcourse is overall weight.
Then there's pedal box. Reducing flex. Saving weight. Getting a good pedal leverage.
Then bias. Fit big brakes up front and it shifts. Car might stop better still with an adjustment rearwards.
Lots to think about
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Jun 29, 2015 19:05:30 GMT
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It depends how far you want to take this. Most of the factors affecting how much torque can be applied to the discs has been covered already.
Once you have decent brakes, the limiting factor is actually your tyres, or rather the available grip. That is a combination of suspension control and tyre compound.
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Jun 30, 2015 12:16:31 GMT
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I fitted new (brand new!  ) calipers to my existing disks and the performance upgrade was amazing Stuff that might have made the difference : pads in the new calipers ran at the edge of the disk, pads on orginal calipers were 5mm in from the edge (so better mechanical leverage) old calipers were overdue a rebuild as pistons were sticky so not very efficient new pads had better / different compound? No more info so I'm guessing new calipers had different size pads & disks to originals? I'm guessing again  |
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ChasR
RR Helper
motivation
Posts: 10,191
Club RR Member Number: 170
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Nov 17, 2016 10:48:50 GMT
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I fitted new (brand new! ) calipers to my existing disks and the performance upgrade was amazing Stuff that might have made the difference : pads in the new calipers ran at the edge of the disk, pads on orginal calipers were 5mm in from the edge (so better mechanical leverage) old calipers were overdue a rebuild as pistons were sticky so not very efficient new pads had better / different compound? No more info so I'm guessing new calipers had different size pads & disks to originals? I'm guessing again A thread from the dead but it popped up on a search . adam73bgt 's and Dez 's in addition to your findings were interesting food for thought. Regarding your questions they are as follows: Pads-The new pads did not run quite to the edge of the disc, whereas the Wilwoods did. -There was quite a difference in pad size between the two however: Cosworth Pads:  Wilwood:  The old calipers were in better condition than what I put on in all honesty. The car was dry stored prior to my ownership and I don't think they had more than 4,000 miles on them retrorides.proboards.com/thread/180062/wilwood-brake-calipers-various-warwickPads-Wilwoods had Hawk pads in them. I did think maybe I didn't heat them up but even when really pushing the car they didn't want to work at all -New ones just had stock TRW pads in them Discs-Same discs were used for both . |
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So if I have a new master cylinder and my Wilwoods would/could smoke the sticky front tyres if I wanted to, in the dry, yet they had enough "feel" to make town driving fine, I kind of assume I had pretty much maximum effiency in my set-up?
So if my new calipers have at least the same TPA, if not a little more, and using similiar pads, then I should end up with identical stopping power?
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slater
Club Retro Rides Member
Posts: 6,390
Club RR Member Number: 78
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Pretty much. If you can lock the wheels you have enough 'stopping power' A lot of the development in brakes is more about making them work consistently over a long period of hard use (like say on a track). On a road car there's little too it as really you only use the brakes hard once in a blue moon.
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You may well be getting all you can out of your fronts, but are your rears pulling their weight?
What master cylinder do you have, and what diameter wilwoods and rear brakes (amd what diameter discs)? If you let me know I'll do the maths and find out what your F/R split is. Might be some more you can pick up by resizing either the rear calipers or discs.
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It depends how far you want to take this. Most of the factors affecting how much torque can be applied to the discs has been covered already. Once you have decent brakes, the limiting factor is actually your tyres, or rather the available grip. That is a combination of suspension control and tyre compound. Very true, most owners of older vehicles, (i.e. no ABS), will have had occasional experiences with brakes locking up but the car continuing serenely on whilst all 4 wheels are immobilised. |
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It depends how far you want to take this. Most of the factors affecting how much torque can be applied to the discs has been covered already. Once you have decent brakes, the limiting factor is actually your tyres, or rather the available grip. That is a combination of suspension control and tyre compound. Very true, most owners of older vehicles, (i.e. no ABS), will have had occasional experiences with brakes locking up but the car continuing serenely on whilst all 4 wheels are immobilised. That's why us old gits learnt to cadence brake :-) |
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Well, I have somehow managed to vaguely get the car ready for the MotoFest! It is now on some TRW Cosworth 4x4 brake pads (of the slightly larger width - they sweep near enough the entire disc.
but if you look at a Jaguar F Type, they have sliding fist calipers (carbon ceramic brakes aside..) so clearly sliding calipers can work in performance applications
) calipers to my existing disks and the performance upgrade was amazing 
