Brake calculator ~ single piston rear callipers?

Hi guys,

I’m sure this has come up before, I just have standard single piston rear callipers on the car, am hoping to run some calcs and tests using the hpa calculator, should I need to enter the brake piston size smaller to account for the fact that the sheet is going to multiply it by 2, but it’s not a simple case of entering my diameter in half is it.

Can someone point me in the direction of the right maths please as my brain isn’t working!

Thanks in advance !

No need to do anything like that! Just enter the diameter of the single piston and that's all you need to do.

In terms of piston sizing, your rear caliper is the equivalent to a fixed 2-piston caliper with the same size pistons.

In a fixed caliper the 2 pistons will move less than the single in the sliding caliper. The force on the pads and the total fluid displace is the same ignoring any compliance.

The sheet is designed to you just enter the piston sizes on one side of the caliper only - doesn't matter if it's sliding or fixed.

Hi Connor, Thanks for the quick response, Sorry for the delay my side!

Thats cool, that makes sense now thanks!

So plugging all the relevant figures into the sheet, the difference in master cylinder movement always seems to be pretty large, which seems to flag up the Bias Migration warning, I don't currently have a proportioning valve fitted. Would this help to solve this issue? Also, for these calcs, I'm leaving the Bias bar position set at 0.5%, I guess this is favourite as it allows a good amount of adjustment each way from the setup position.

Thanks again for your help.

Option 2 - Same issues but the other way around ..

That suggests massively overbraked rears/under braked fronts (more specifically, piston area), or, more likely, you've stuffed up the input values.

Could you please give a FULL run-down on the brake set-up you're wanting to use?

Certainly looks that way doesn't it !

I've attached a Shot showing my input values.

Brake setup is 4 Piston wilwood front Calipers with 280mm Discs, Single Piston (Standard) Rear Brake Calipers with standard Rear Discs (258mm), Linked up to an 'OBP' Bias Pedal Box, currently with 0.7 and 0.625 Cylinders, DS2500 and DS3000 Ferodo Pads.

Issue I have is the brake Pedal is very very solid (Not much Travel) and you have to push exceedingly hard to brake well... My Logger is currently showing a peak brake force of around 0.78g, Car weighs under 1000kg and is on semi slick AR1's. You have to push the pedal so hard that you cant modulate the pressure.

Also the Bias Bar is set almost the wrong way around to how the fitting instructions suggest you fit it - Suggesting something isnt right.

I Figured go back to basics and work out in theory what size cylinders I should be using and hopefully that will improve the situation...

Your help/ thoughts are appreciated!

General observations, NOT specifically "recommendations", but suggestions.

For the same line pressure, you have approximately 2.5 times as much brake torque, for the same line pressure, to the front as the rear, or approx' 71% front bias. This should be a good starting point, and easily changed with the bias bar and/or master cylinder sizes.

You have the pedal ratio as 4.5 to one - this is rather high, and would suggest a direct swap over from a power brake lever - altering the mechanical advantage closer to 7:1 will drastically reduce brake effort.

On that, 50kg is a VERY light value for the pedal force, even for a lightly built person - using a value closer to your weight would probably be be a better start point. Think of it as the force you need to stand on one foot, with the leg slightly bent at the knee - oh, on that, the seating position relative to the pedal can also make a big difference to the force that can comfortably be applied - I can't recall the ergonomic recommendation, but I think it was something like an 80 degree angle for the knee? I wouln't have the knee too straight, especially when applying "panic" braking force, as if/when you crash your leg is less likely to "give" and so broken bone(s) and other trauma may result from it.

You don't mention the specific brake pads being used - people will often select a "race" pad because it's a "race" car - but these do have problems and one of these is they usually give a VERY poor co-efficient until they're warmed up, and that means a LOT of brake effort, which is already compromised by your pedal ratio and effort limits. With a lighter car, with good sized brakes, you may never get them hot enough to reach their working temperature - especially if it's a lower power vehicle that isn't demanding on the brakes.

Agree with Gord, perhaps look at increasing the pedal ratio or just moving to a smaller diameter master first to get your pedal effort in Check.

I'm not sure i'd say 50kg is light though, I remember talking to Grant from Honed in Australia about their booster delete kits, they measured the stock boosted pedal effort with a load cell and it was 15kg on an EK civic and with their kit it's more like 30kg.

I think Daryl from Perusic said their max pedal efforts are around 80kg for pro drivers.

Standing on one leg is one thing, single leg squatting your weight over and over is another, and moving between a light throttle control to a heavy brake is something else to consider.

Looks like your driving an E30? 280mm front discs to fit under 15" wheels and small tires shouldn't be too hard to get heat into. Depending on your ducting setup. You could double check with some temperature paint to be sure though.

As I said, it depends on one's build - but I'm quite surprised at that 30kg figure, as I certainly expected it to be higher.

I should have asked Anthony if he'd tried driving in a low gear while applying the brake hard with his left foot, to see if that helped bring the pads up to operating temperature - depending on pad selection, of course, and there being no 'glazing'?

[edit] I missed the pad selection, depending on the use, they should work OK, if the 3000 is warmed up - https://wiki.seloc.org/a/File:Ferodo_pads.jpg Also forgot, some pads are very fussy about the bedding in process, if this wasn't done correctly?

Thanks for the advice guys, much appreciated, some interesting points there!

Yes the pads have worked well in a golf i used to have, although that did have a booster.

Ive double checked the instructions for the pedal box, this lists the ratio as 5:1, but i guess all depends where on the pad your foot is contacting, i think in reality its nearer to 4.5:1 and sadly not adjustable.

Im around 78kg body weight and average build..

Car is indeed an E30.

Good Point well made RE the pads as i did have some issues with brakes seizing on due to adjustment / freeplay when i first took the car out, i have since removed pads and cleaned off the glazing, but they do always look a little glazed so maybe not bedding in properly even now...

Although i have ducts fitted on the front these are currently blanked off (Was waiting until i cook them then i know that the ducts are definately working and the brakes are getting up to temp.

@conor your recommendation would be to maybe look at smaller dia master cyls in order to bring the pedal effort down? issue is its hard to reduce the size of both and not flag up all sorts of warnings on the sheet!

thanks again

Sounds like you might have a pad issue, but it's hard to know for sure.

Also seems like your hydraulic leverage might be an issue.

With the calculator remember they are just warnings for potential issues, not complete deal breakers. Our Honda city setup is just over the limit for the difference in master cylinder travel, but in practice it doesn't seem to be causing any issues.

From what you've said you have a very short and hard pedal, moving to a smaller master setup should help this, maybe making the front MC smaller relative to the rear is a safe way to go even if the difference in master travel isn't ideal.

Just as an example the Perusis pedal box uses a very short pedal ratio as well (3.5 ish I think) to keep the packaging compact. As a result, they use quite small diameter Girling master cylinders, which also have a longer max travel.

As Connor implies, the travel limitation(s) of the master cylinder are warnings - the actual travel will depend on how much fluid is used to counter the 'flex' of the various brake components, such as hoses, calipers, pads, and brake pedal assembly mounts, and taking up any 'knock-back' that might move the pads from the pistons.

Minimising those will reduce the master cylinder travel, which will mean that a greater pedal leverage can be used for the same pedal travel - and the greater the leverage at the pedal that cn be used for the same pedal travel, the lighter the pedal force and/or greater the line pressure, all else being equal.

It's a trade-off between pedal force and travel - the former for the brake force, the latter depending on 'clearance' and flexing being taken up. One important factor, if you do decide to modify the pedal assembly, is to make sure that even when one master cylinder bottoms out fully, such as would happen with a failure, there is still travel available at the other to ensure the brakes will work on it or, better yet, both master cylinders will bottom out before the pedal hits anything.

Sorry for the delay in replying guys, i appreciate your help and advice!

I think looking at the rarity of a 0.5" Master cylinder, only a couple of places seem to sell them, this would suggest that it's not required/ common to go that small. there are plenty of cars running a similar setup to this without issues, so I think maybe i have killed the discs &/or pads in that initial running of the car when they overheated and thats whats causing this issue.

I'll head out to another test day with new discs and pads all around and see how we get on!

thanks again

What pedal box, or conversion, are you using for the dual master setup? Some are definitely better than others, but it shouldn't be too difficult to modify the pedal, or (have) another made with some tweaking to the pivot and push-rod locations - even a 10% improvement will definitely be felt.

From a purely hydraulic pressure viewpoint, simply reducing the master cylinder bore will solve it, BUT that also makes a corresponding increase in travel for the same fluid displacement, and that's usually something one would wish to avoid.

Hi guys,

So a little later than i had hoped, but i eventually managed to get a test session booked in the car, fitted new Front and rear discs and pads, this time 'CL' front pads and DS2500 Rear pads, careful bedding in.

While the brake performance did feel better, it's still not what i would hope for the weight of car and brake setup. I'm yet to look at my data for max braking G, but id be surprised if it was much higher than before.

The pedal box I'm using is an OBP Floor mount box, master cylinders are their standard supplied ones.

I'm wondering if maybe i dont have the front/ rear bias set correctly, of if maybe i just need to be more manly on the brake pedal !

Any advice/ ideas on the best way to set the bias up for a relatively amateur driver? currently its just set so that with the car jacked up and a small amount of pedal force, the front bites before the rear, nothing more technical than that.

Thanks again for all your help!

Peak Brake force Seen this week still only 0.77g!

Hi Ant,

As we discussed in the course, you need to setup your pedal box/bias bar properly before getting on track so the bar is level at full pressure.

From there I would start testing on track with some straight line stops, starting gently and working up to full power.

We also discuss this in detail in the course. Start with the bias all the way forward, so the front brakes lock first, slowly dial it back until the rears start to lock first and then move it forward again. If you can't achieve this range your master cylinders or sizing of other components is probably to far out of the ideal spec.

Connor, at this point he's unable to get any significant braking force from the system when applying as much force as he can. If it was just a balance problem, he should be able to lock up one axle, or the other, and doesn't seem able to do that.

That said, Anthony, with the car stationaly, have someone look at the balance bar when you're applying as much force to the pedal as you can, just to check it's position. It is possible Connor's right and the balance isn't helping.

Oh, what sort of pedal feel is it when hard braking/pushing on the pedal? It should have a little bit of free movement as the 'play' is taken up and the replentishment ports in the m/cylinders are covered, then have a firm/hard pedal with little, or negligible, movement. If it's got a long travel, or feels spongy, it is possible, in extreme cases, to run out of pedal travel before full line pressure is reached.

Most modern power assisted brakes require quite low pedal forces - is it possible you've become aclimatised to that, and just not applying as much force to the pedal as you think you are? A simple approximation check would be to have someone position a common bathroom scale between the pedal and your foot, if you have access to fit it there, and have them read off the force you're applying?

Ergonomics - the seating position relative to the controls, including the pedals, can also play a big role in the force you can apply - some vehicles place everything so you can get a clear push in-line with the pedal, but some need more of a "push down", which can make it hard to apply a lot of force.

Hi guys,

It's occured to me that i didnt complete the course fully, i got to the bit about designing the system and got distracted putting numbers into spreadsheets, I'll revisit the course and the setup of the pedal box and report back!

I'll also do both the trick with the bathroom scales and have someone look at the bias bar (Not at the same time obviously!

thank you again for your help, ill report back !