Brake calcs

[Nutzy]

Forgive me for posting in this section, but to blow a little smoke up some butts, the smartest dudes are in here!

So, I'm putting together my new braking system, and I was about to buy some new Wilwood calipers and then had pause for thought.

The system is this:

Stock e30 master
Front - E46 325mm rotors
Rear - Z3M 315mm rotors

The Z3M calipers are 43mm (1.57") with a surface area of 1.95" sq.
Front was going to be 4 pot 1.75" with a surface area of 4.8" sq (calc is halved due to opposing pistons).

Now, the tricky bit here is that the rear may have the effective swept volume of a 3.9" sq piston, due to its sliding caliper nature, but what does this mean for actual clamping force in real terms? A lot of the web-based calculators out there suggest that their calcs are based on DOUBLING the piston area (and hence clamping) for sliding calipers, yet this probably isn't true.

Isaac says:

Piston forces.JPG

So old mate Newton says different and so did my head, hence the finger, paused, hovering over the 'Buy it now' button!

So this would imply that I'll have a piston ratio that is 2.4 times bigger in the front.

Am I going to totally roger my bias here? Is a 1.38" 4 pot the better choice? This would give 3" sq on the front as opposed to 4.8" sq

Is it just going to be that hard to get right through sizing alone, that I'm better off getting an adjustable bias valve?

Thanks for any input.

Pete

[Nutzy]

No pearls of wisdom? :-/

Well, the only way I can work this out is:

Newton's Third Law...forces have equal and opposite forces.

The 1.94" sq Z3M rear caliper is of the sliding variety, however, you apply 100psi of fluid pressure to its piston and the piston will exert a force of 194lbs.

If the caliper had two 1.94" sq opposing pistons and the same 100psi pressure was applied to each, then the clamping force would still be 194lbs. It won't be doubled, but the pistons will only have to move half as far as the same sized piston in a sliding caliper to meet the disc as compared to a sliding single piston type.

I see lots of excel worksheets and web calculators out there that state that the sliding caliper clamp force is DOUBLED for its given piston size. I also see calcs say 4 pistons calipers are worked out only for one side, which would be correct. Then I have seen calcs say that the clamps force is worked out on ALL pistons. Such scattered info!

The sliding caliper has no magical mechanism by which it can get mechanical advantage, its plane is linear. So how can it be doubled?

So, assuming equal applied pressure of 100psi:

A stock 43mm e30 front will produce 224lbs clamp
A stock 33mm rear will produce 130lbs clamp

A 4 pot 1.38" piston Wilwood will produce 300lbs clamp
A 4 pot 1.75" piston Wilwood will produce 480lbs cllamp

A stock 43mm Z3M rear will produce 194lbs clamp
A stock 60mm Z3M front will produce 437lbs clamp

Anyway, in keeping with the Z3M rear piston size, the closest matching caliper from the Wilwood range would be the 1.75" piston unit. I'll have to go large MC bore, I'd guess.

EDIT: I goofed some of those calcs! Edited now.

[digger]

sliding caliper has piston on one side only but the force is still applied on the opp side equal and opposite.

[Nutzy]

sliding caliper has piston on one side only but the force is still applied on the opp side equal and opposite.

Ye, I get that. Basically the backside of the piston (the caliper housing) is the equal and opposite force. This then pulls on the opposing brake pad. However, it does not follow that the clamp force is doubled.

[StRaNgEdAyS]

Clamp force is distributed

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[JimsR]

Why do they call you Nutz..."because I'm crazy"

[Nutzy]

Why do they call you Nutz..."because I'm crazy"

I might be.....but I hope not!

[someguy2800]

I can't reason out why anybody would think the piston area should be doubled for sliding caliper. Thats seams quite moronic to me. I wasn't going to post since this is not my area of expertise but 1.38 4 pot seams a better choice to me given the rear brake size and master cylinder bore size.

[StRaNgEdAyS]

Check this out to see what you come up with
http://www.tceperformanceproducts.com/bias-calculator/

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[Nutzy]

I can't reason out why anybody would think the piston area should be doubled for sliding caliper. Thats seams quite moronic to me. I wasn't going to post since this is not my area of expertise but 1.38 4 pot seams a better choice to me given the rear brake size and master cylinder bore size.

A fair comment, Perry, but with the much large Z3M caliper in the back, due to the Z3M rear end swap, keeping front caliper piston sizing inline with the rear is probably a good idea. I'll need to install a 25mm E32 cylinder, the 22mm E30 cylinder is going to be too small and have too much pedal travel.

Check this out to see what you come up with
http://www.tceperformanceproducts.com/bias-calculator/

That's a good link, thanks for that. I do note that they are quoting the rotor clamp force as being double for opposed piston and sliding calipers though, but as long as they are being equal about it, it doesn't matter much.

Punching in some numbers....

Stock e30 = 60% front bias

E30 with 280mm FR rotor + 1.38" 4 pot BBK = 71% front bias

E30 with (325mm FR rotor + 1.75" 4 pot BBK) + (315 R rotor + 1.69" sliding caliper) = 68% front bias

Bear in mind there are other variables that aren't correct, or known, such as friction coefficient, pad radius height etc, but the link provided above with interesting and most likely quite close to the actual outcome. Note also that bias valve is not taken into account.

[someguy2800]

A fair comment, Perry, but with the much large Z3M caliper in the back, due to the Z3M rear end swap, keeping front caliper piston sizing inline with the rear is probably a good idea. I'll need to install a 25mm E32 cylinder, the 22mm E30 cylinder is going to be too small and have too much pedal travel.



That's a good link, thanks for that. I do note that they are quoting the rotor clamp force as being double for opposed piston and sliding calipers though, but as long as they are being equal about it, it doesn't matter much.

Punching in some numbers....

Stock e30 = 60% front bias

E30 with 280mm FR rotor + 1.38" 4 pot BBK = 71% front bias

E30 with (325mm FR rotor + 1.75" 4 pot BBK) + (315 R rotor + 1.69" sliding caliper) = 68% front bias

Bear in mind there are other variables that aren't correct, or known, such as friction coefficient, pad radius height etc, but the link provided above with interesting and most likely quite close to the actual outcome. Note also that bias valve is not taken into account.

Your using the 43mm rear calipers regardless right? I put the piston area of the Z3 rears at 14.5 Sq cm, the 1.38" 4 pot fronts at 19.3 cm and the 1.75 4 pot at 31.0 cm. The 1.75 is pretty close to the stock Z3 front of 28.3 so mabey that would be a better choice if you have enough master volume for it.

i don't know shit about brakes.

[Nutzy]

Your using the 43mm rear calipers regardless right? I put the piston area of the Z3 rears at 14.5 Sq cm, the 1.38" 4 pot fronts at 19.3 cm and the 1.75 4 pot at 31.0 cm. The 1.75 is pretty close to the stock Z3 front of 28.3 so mabey that would be a better choice if you have enough master volume for it.

i don't know shit about brakes.

Neither do I......

Stock Z3M rear - 43mm (1.69") which equals an area of 2.243"
Stock Z3M front - 60mm (2.36") which equals an area of 4.38"

Wilwood 120-15131 - (4 pot 1.75") which equals an area of 4.8"

The 120-15131 looked like the best match I could find for the 325x25mm rotor, and also to match the relative balance of the OEM piston sizing.

My 22mm OEM master won't be big enough....IMO

bias.JPG

[chikinhed]

Keep in mind that the stock bias is for stock suspension, stock size tires and stock pad compounds.

[Def]

I think you're getting caught up in semantics, but the old addage of sliding calipers having "double the clamp force as an opposed piston fixed caliper design" is very much true. This is from the standpoint that 1 sq in. of sliding caliper piston area does equal the same result as 2 sq in. of opposed piston caliper clamp load.

Generally when doing bias calcs with a mix of sliding and opposed piston calipers, it's easiest to just double the piston area on the sliding caliper to get the proper bias. Otherwise you're going to estimate way more clamp load on the axle with a fixed caliper.

[Nutzy]

Semantics....yes, but I'd say that that is only true IF you choose to use the total sq area of the opposed piston caliper as in input to working out clamp force. The link up above does so, but then they doubled the sliding caliper, too, so the result is the same in the end. That's fine for working out bias on a fairly basic 'back of the envelope' sort of way, but it'd never fly in the field of hydraulics.

I'd prefer to actually understand the exact calculation method, but that is just me. In any case, I'm pretty happy with the outcome of the deliberations, and some sexy red Wilwood 4 pots have now arrived.

I think you're getting caught up in semantics, but the old addage of sliding calipers having "double the clamp force as an opposed piston fixed caliper design" is very much true. This is from the standpoint that 1 sq in. of sliding caliper piston area does equal the same result as 2 sq in. of opposed piston caliper clamp load.

Generally when doing bias calcs with a mix of sliding and opposed piston calipers, it's easiest to just double the piston area on the sliding caliper to get the proper bias. Otherwise you're going to estimate way more clamp load on the axle with a fixed caliper.

[Def]

Semantics....yes, but I'd say that that is only true IF you choose to use the total sq area of the opposed piston caliper as in input to working out clamp force. The link up above does so, but then they doubled the sliding caliper, too, so the result is the same in the end. That's fine for working out bias on a fairly basic 'back of the envelope' sort of way, but it'd never fly in the field of hydraulics.

I'd prefer to actually understand the exact calculation method, but that is just me. In any case, I'm pretty happy with the outcome of the deliberations, and some sexy red Wilwood 4 pots have now arrived.

I don't think it has anything to do with the field of hydraulics, it's a result of the free body diagram of a fixed caliper vs. sliding caliper, and the resultant clamping forces on each brake pad.