Main cylinder sizes

I have driven my car on track for a couple of years. It brakes good, but the biasbar is "suboptimal" to say at least. This was not a problem for me until this course. Now i Know I have a problem. The formular stated that I should change MC on both rear and front. But after som tweaking with tires and stuff it worked out fine just to switch front and back.

The pic here shows front pressure in blue and rear pressure in red. The thick black is G force. It maxes in this log at 1.52G Wich I am pretty satisfied with. You can see thet the front press is dropping from araound 60 to 40 bar. This is the ABS working. I obviously lock the fronbreaks first. When I have maximum G retardation the pressure is higher on rear than on front. So it seems like the formular is right. I have 15.9 (0.625) on the front and 19.05 (0.75) on the rear. Optimal on semislicks is 17.8 (0.7) on both and on the slicks I get 19.05 front and 15.9 rear. Just a simple switch. In my ears it sound sketchy but The math usually doesnt lie. Today I have a some whiat long stroke in the pedal, and that would be much better with this setup. I will ofcaourse try, but it is at least 4 month left before the season starts here in Sweden again.

What do you think?

If you're happy with the balance of the car under braking, when the brakes are up to temperature, and the balance bar is around the middle, souns like it's all sweet.

Don't get hung up on the actual line pressures, because and change in master cylinder bore sizes and/or balance beam change will change the pressures. Use them as a tuning tool to help size the caliper bores, by all means, though, and if you have the option of displaying the comparitive pressures it allows a quick and easy way to adjust the 'bar for different tyres, pad materials, weather, etc. if you've made note of what worked before.

Well thats just the thing. I was pretty pleased. But now I know whats not to be pleased about. I have too much sway in the bias bar and I would like to have a stiffer pedal. When breaking on track I reach aprox 60 bars. In the garage when testing I can reach 75bars. So Maybe it could be right to switch... Its not much work really. Just switch the pipes and bleed.

Again, don't get hung up with the line pressure - if 60 is what it needs to hold it on the verge of locking/ABS kicking in, you don't need 75 because you'll never reach is - unless you have bad brake fadeor pads that don't work well when cold.

You're going to have to make the call that suits you, though, where the trade off between pedal travel, pedal force, and master cylinder bores lies for you.

I'm not sure what you mean by "sway", or by a "stiffer" pedal - is it having a bias one way or the other, and a pedal that isn't solid but one you can push down against the pedal force?

No, Iam not "hunting" pressure. I just observed in my log that I have maximum redardation when I have aprox 10 bars more pressure on the rear than on the front circuit. I have a soft pedal that I dont like. I also have far from a optimal biasbar. I got the numbers from the formular, but at first thought it was wierd with a smaller MC on the rear, than on the front. But maybe that is whats right for my system.

Hi Henrik,

It's probably best to have a smaller MC on the rear if you can, since the rear callipers will need less fluid displacement, so in the end the bias bar tilt will be minimised.

From what you've explained I definitely think it would be worth experimenting with swapping the master cylinders around. As you described it should make your pedal shorter and firmer and also allow you to have the bias bar more centralised so you have more for adjustment.

No harm in trying.

Yes. I have decided to swap. This is what I got from the formular.

Smaller M/C will mean more pressure for the same applied force, depending which way the balance beam is off-set from centre that could be a good or bad thing.

It will also mean more travel is required for the same fluid displacement and, again, this may be a good or bad thing - probably the latter because you seem to be more concerned with pedal load? however, if you're running small rear caliper bores you may need the pressure to generate the clamping force?

That calculation software has some assumptions, they may have been calculated and entered from elsewhere - but how did you calculate the load transfer, did you check the static loadings, are you using pads with approximately the same CoF - and what is/are them and their temperature characteristics, because that can have a big affect on required line pressures?

As Connor said, no harm in trying the swap of the M/Cs, but have a good think first.

Regarding that "formulae page, I have some other issues, such as -

There isn't a pedal to master cylinder leverage ratio value - was that also produced elsewhere, because it would be required for the nominal pedal force, and the travel HAS to be an assumption because there are several factors not mentioned that will have a significant affect on this - such as the fluid used, the flexible hoses used and lengths, the caliper deflection, and the compressibility of the pads - oh, and of course, any 'knock back' of the pads that may occur from slight run-out or stub axle/strut flexing under load.

TBH, while it has it's place, there are many assumptions that could send you in a completely wrong direction unless they were addressed elsewhere - it's just another tool, not "the" answer. To use an old computer acronym - GIGO.

Yes. I will use my complete breakingsystem as it is. Just swap the MCs. If it doesnt work, I will just swap back.

The thing is, that all my logs point at the same thing, so I look forward to testing. It could be exact right way to go. Not much work and 2 bottles of break fluid. Totally worth the test. Always awsome to test theory IRL. Its one of my favourite thing about racecars.

Cheers for the help Gord,

For reference the calculator featured in our brake course which Henrik is using does include considerations for pedal ratio and effort, brake compound (front & rear individually) and load transfer in the input section.

Some assumptions are required to make this work possible without the use of tire data, but these result in relatively small changes in practice.

There's also considerations for pad travel around "knock-back" and compliance built into the calculations, of course at a level that make's this accessible to an enthusiast-level without an unlimited budget.

So I see, having some more info' on the full 'calculator' package.

It's a long time since I did any writing, and even then it was much more basic, from what I recall - is it a 'smart' program where you can find any unknown if the rest of the data values are entered?

At this stage it will allow for experimentation with different braking components, be it callipers, pads, discs, master cylinders, pedal setups, and tires as well to see how the resulting pressures and bias compare to a theoretical ideal setup for your vehicle based on load transfer.

Of course there's a lot of other considerations with brakes to get a power, balanced and controllable setup, all of which we also discuss in the course.

Awsome course btw Connor.

thanks Henrik, really appreciate it.