Redesign of kit car brakes
Discussion
I'm not impressed with my current set-up - not able to lock anything except on the loose! I've tried both road and DS2500 pads. I can't change:- pedal ratio, and don't want to change pedal effort too much, could slightly reduce travel; can't change disc diameter or dual servo. Can change:- dual M/Cs, calipers. It's fitted with balance bar and hydraulic handbrake (which works fine)
I think a change from single piston calipers to 4 or 6 pots is the way to go, but haven't got my head round the calculations of caliper piston areas. Since it's rear engined, the rear calipers could do more than the current ones from a front engine/FWD car. Does pad area matter?
Can somebody give me some clues?
Thanks!
I think a change from single piston calipers to 4 or 6 pots is the way to go, but haven't got my head round the calculations of caliper piston areas. Since it's rear engined, the rear calipers could do more than the current ones from a front engine/FWD car. Does pad area matter?
Can somebody give me some clues?
Thanks!
jimmystratos said:
Does pad area matter?
Think you mean piston area, but it is far from the only consideration. It's like levers, the longer the leaver the greater amount of weight you can lift. I think Haynes do a book with a title along the lines of "designing kit car brakes" though I may have dreamt that
Though I just can't see halving or doubling pad area making much of a difference all other parameters being fixed.Pad area has negligible effect. If you double the pad area you halve the contact pressure, and the two factors cancel out.
Regarding piston areas... Imagine you've got two calipers, one is a single-pot sliding OE caliper and the other is a 2-pot caliper with opposing pistons. In both calipers the pistons are the same diameter. For a given braking effort (everything else being equal), both calipers require the same operating pressure and fluid displacement. I'm finding it hard (at this time of dayv without any coffee inside me) to explain why in words, so you'll just have to trust me!
Essentially you need to ensure that your replacement calipers have a greater total piston area ON ONE SIDE than the sliding caliper. In other words, if you have a four pot caliper the sum of the area of two of those pistons must exceed the area of the single piston in a sliding caliper. However this will result in increased pedal travel.
4-pots have a further advantage in terms of increasing the effective diameter of the disc (two smaller pistons nearer the edge of the disc) and remove the disadvantage of sticking sliders that OE-type calieprs usually suffer from, but I don't think you'll see much advantage without increasing your piston area.
Other options are reducing the size of your master cylinder. If you've got dual MCs you could consider replacing that with an OE-type tandem MC. Either way you increase pedal travel.
Another slightly whacky option is to use a remote (hydraulically activated) servo as used on proper Mini Coopers (and on Escort Mk2 works rally cars). But again you'll increase pedal travel.
Alternatively, reducing the weight of the car is another way of increasing braking efficiency!
Regarding piston areas... Imagine you've got two calipers, one is a single-pot sliding OE caliper and the other is a 2-pot caliper with opposing pistons. In both calipers the pistons are the same diameter. For a given braking effort (everything else being equal), both calipers require the same operating pressure and fluid displacement. I'm finding it hard (at this time of dayv without any coffee inside me) to explain why in words, so you'll just have to trust me!
Essentially you need to ensure that your replacement calipers have a greater total piston area ON ONE SIDE than the sliding caliper. In other words, if you have a four pot caliper the sum of the area of two of those pistons must exceed the area of the single piston in a sliding caliper. However this will result in increased pedal travel.
4-pots have a further advantage in terms of increasing the effective diameter of the disc (two smaller pistons nearer the edge of the disc) and remove the disadvantage of sticking sliders that OE-type calieprs usually suffer from, but I don't think you'll see much advantage without increasing your piston area.
Other options are reducing the size of your master cylinder. If you've got dual MCs you could consider replacing that with an OE-type tandem MC. Either way you increase pedal travel.
Another slightly whacky option is to use a remote (hydraulically activated) servo as used on proper Mini Coopers (and on Escort Mk2 works rally cars). But again you'll increase pedal travel.
Alternatively, reducing the weight of the car is another way of increasing braking efficiency!
A servo generates more hydraulic pressure which means you have more hose stretch and more caliper flex. Also, the pads are elastic to a degree so the more you squeeze them the more they squish. You don't need much extra fluid displacement to create a noticeable increase in pedal travel.
Of course, you may be able to fit a larger MC (or MCs) if you use a servo, which would counter the increased pedal movement.
Of course, you may be able to fit a larger MC (or MCs) if you use a servo, which would counter the increased pedal movement.
I put a remote servo (off an old London taxi as I recall!) on my Relaint (don't laugh!) once, many years ago. It was a 4-legged Reliant and I put the discs off a Herald Vitesse on it (same front suspension) instead of the standard Herald drums that it originally had. The servo DID increase pedal travel considerably. Partly, I think, for the reasons stated above but also partly because of the servo itself and the components within it. (in other words, with the vacuum pipe disconnected, there was still more pedal travel for the same amount of effort than before I fitted it.
If your problem is not being able to LOCK the wheels, would it not be better to go to softer pads, or does that just give you more trouble with fade at high speed?
If your problem is not being able to LOCK the wheels, would it not be better to go to softer pads, or does that just give you more trouble with fade at high speed?
Reducing the size of your master cylinders will increase pressure on the caliper and is the simplist thing to try..... but it sounds like you need 4pots all round IMO.
I had huge calipers on my Ultima, but they were sliding ones (ie pistons on one side) and the pedal feel was awful and the rears were just inadequate.... on a mid engined car with close to 50/50 weight split and much larger tyres on the rear you really want the same size caliper on the rear as the front.
I had huge calipers on my Ultima, but they were sliding ones (ie pistons on one side) and the pedal feel was awful and the rears were just inadequate.... on a mid engined car with close to 50/50 weight split and much larger tyres on the rear you really want the same size caliper on the rear as the front.
Gassing Station | Suspension, Brakes & Tyres | Top of Page | What's New | My Stuff