What bore master cylinders do I need? I am making a bias bar system.

Adam

There's no quick answer without a lot more information about your car Adam. I've got a spreadsheet that might help at http://www.7builder.com/ in the downloads section.

It's usually something like .625" front .75" rear Adam. Smaller bore generates higher pressure.

I think I will go for the same front and back. I have got huge AP racing 4-pot front callipers off en escort rally car and tiny single-pot XR4x4 brakes on the rear, so as I am having a bias bar aswelll this would be better.

Adam

At a guess either 0.7" front and 0.7" rear or 0.7" front 6.25" rear should be about right

The maths say you want roughly 2/3 braking effort to the front

Workout the area of the caliper pistons on one side of the calipers only front and rear and find the ratio

as a rough guide
(piston area one side of a front caliper)/(front master cylinder area) = 2*(piston area one side of rear caliper/rear master cylinder area)

[Edited on 15/5/04 by britishtrident]

[Edited on 15/5/04 by britishtrident]

Just a thought, but you might want to keep in mind fluid requirements as well. The greater the piston area in the caliper, the greater the amount of fluid that must be pushed out of the master, and hence a longer stroke with a smaller bore or a shorter stroke with a bigger bore. If you want to do the math, the typical CoF I've been quoted for pad friction is about 0.40, road tires ~.75-1.0 and super sticky slicks have been known to go as high as 1.5. Could calculate the required pad pressure and then work backwards to mastercylinder size. Then only fine adjustments would be needed from the balance bar. Cheers!

What does CoF stands for.

Coefficient of friction I should imagine. I think we're getting carried away a bit here.

If brittish tridents theory is anything to go by then pete, you were right. .625 front, .7 back.

Here are my calculations:

Front:

Piston = 3.81cm

3.14x1.905x1.905 = 11.395

11.395x2 = 22.790

22.790/0.625 = 36.46

Back:

Piston = 4cm

3.14x2x2 = 12.56

12.56x2 = 25.12

25.12/0.7 = 35.89

That is the closest I could get going up in .025s.

Adam

[Edited on 15/5/04 by phelpsa]

Maybe it is over analysing Pete, but the brakes are a pretty important safety system, and I think having the shortest braking distance without rear bias instability is vital. More important than perfect bump steer or ackerman IMO, and absolutely critical for road or track.

Here's what I've found: the built in bias for most donor cars with 60/40 weights are unsuitable for a 7 style car, due to the 7's 50/50 weight distribution.

For 0.8g tires on a locost I get a 60/40 bias split, vs 66/33 on the donor. Using the 2/3 ratio a locost will be overbraking in the front and a longer braking distance will result (the rears won't be doing their share).

Adam, you need more than your caliper piston radius - you need your effective disc size and your wheel size, if they're different front to rear.

If you know the above and your pedal ratio, you can pick the right size of masters from the pedal foot pressure you want to lock the tires.

You need the right pressure ratio, if you're too far off your balance bar won't be able to move properly, esp if you ever have a loss of pressure on one side. I agree completely with crbrlfrost - start with piston pressure and work back.

Using my spreadsheet and some guesses about your car, I get that you want the front master larger than the rear, say 15/16" front and 7/8" rear for a panic braking foot pressure at the pedal of around 94lbs.

I've attached what I've done, check out sheet 4, "design". If you know some of your own values you can fiddle with it and see what it changes. Or not, if you think it's over analysing .

Mike, I hadn't looked at your spreadsheet until now, as brakes are sort of under my radar yet. Just checked out the one attached to your last post, and it's an amazing tool!

I've noticed that many builders have had difficulty getting their rear brakes to lock up, and this analysis explains that phenomenon very well.

A couple of questions:

1. Given a choice between a high-ratio pedal and a large master cylinder, or a lower-ratio pedal and a smaller m/c, which is preferable?

2. How do you determine brake line diameter?

Pete

Discs are same diameter front to back (250mm), fronts are vented, rears aren't. Remember that the weight is transferred to the front of the car when braking.

Adam

Looks like everybody is getting carried away again!!

0.75 front and rear or 0.625 front 0.75 rear, are the basic choices which you can easily get off the shelf and will work, any fine tuning can be done with the balance bar. I have used this combination on many vehicles with no problems at all. If you are using big 4-pots on the front then 0.625 will give a long pedal though so I would just stick to 0.75 front and rear.

Nat.

I'm not sure if it is possible to get carried away on brake design, unless you subscribe to the notion that bigger is always better, in which case, way overbraking (in terms of calipers and rotors) can cause a large performance drop as well.

In terms of pedal stiffness, it is well known that racers tend to prefer a "wooden" pedal, or a very stiff pedal with little travel. This is due to the fact that humans can sense pressure far better than distance, and thus a larger pressure differential between initial bite and lockup helps to modulate with greater consistency. However, on a street car those kinds of pedal pressures could get quite fatiguing. On the other hand you have many large trucks that are very overboosted and made to be easy to actuate. One I drove travelled about 2in prior to biting, and with very little feedback (very disconcerting on panic stops). Gotta go, will finish up later. Cheers!

Ok, just thought I'd finish my thoughts from the last post. On the cars I've worked on, I've aimed for .25in or so on the take-up/biting period, as it allows for a little tactile feel prior to getting hard on the brakes. I like my travel pretty short, 1-1.25in, but thats me. May be good to go measure a street cars you like the brakes on and see what the throw is, as that can be changed by altering the leverage to the master cylinder, rather than the cylinder itself. Might also be worth thinking about the orientation of the brake petal to the throttle at both medium to heavy braking to help facilitate heal-toeing. I know this be starting to get excessive, but greatness is in the details.

cbrlfrost, I completely agree on the pedal travel and height relative to throttle, both of these will make a lot of difference to how the car feels to drive (and how easy it feels to you to drive it) but as you also said, both of these things are fairly easy to fix with pedal ratio easier to fine tune for little money than cylinders, and pedal height adjustment is built in when using most master cylinders as most have plenty of "spare" thread on the pushrods.

Cheers

Nat

quote:

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Originally posted by phelpsa
Discs are same diameter front to back (250mm), fronts are vented, rears aren't. Remember that the weight is transferred to the front of the car when braking.

Adam

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quote:

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Originally posted by pbura
A couple of questions:

1. Given a choice between a high-ratio pedal and a large master cylinder, or a lower-ratio pedal and a smaller m/c, which is preferable?

2. How do you determine brake line diameter?

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GTS Tuning has some 5:1 pedal sets, very reasonable. Maybe even worth ordering from across the Big Water.

Hi Pete - I couldn't find the pedals on GTS, but I did see a balance assembly. That's what I'm using, but it's cheaper over here - wilwood and tilton both have them for under $100CDN. Wilwood have complete drawings on their site. Very handy for me, as the pedal vendor assemblies don't fit - I can pick my own ratio and use my donor pedal.

I can't see matching the brake pedal height to the throttle height as a big issue, I need a custom throttle pedal which I intend to build to the proper height once I have the brake setup.

I'm still not sure I understand how one could recommend a standard bore size without asking about wheel and tire selection, intended use, pedal ratio, disc radius and caliper size. I'd be concerned about too much or little foot foce (the driver can't lock the wheels without excessive effort) and mismatched front/rear balance. Isn't it best to use the bias bar for fine tuning and track conditions, rather than compensating for a mismatched design?

I realize most masters are only available in 1/8" increments, which does limit the design somewhat. But I'm pretty sure with is big front brakes Adam is better off with a larger front cylinder than the rear, or at least a pair of the same size.

Would .7" front and rear do?

Adam

quote:

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Originally posted by MikeP
I couldn't find the pedals on GTS

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"I realize most masters are only available in 1/8" increments, which does limit the design somewhat. But I'm pretty sure with is big front brakes Adam is better off with a larger front cylinder than the rear, or at least a pair of the same size."

quoted from MikeP.

Possibly but probably not, the bigger front brakes take even more weight off the rears during heavy braking which then need even less effort, so will probably need a bigger cylinder than expected in order to avoid having the balance bar wound right onto the front cylinder.

.7" front and rear sound ok.

[Edited on 17/5/04 by NS Dev]

We must have different physics teachers NS . I'm pretty sure the weight transfer onto the front tires is a function of the decelleration rate and would still happen even if the brakes were on the rear wheels only. The decelleration rate is limited by the available traction of the tires. To get more weight transfered to the front, you need to use stickier tires, not bigger brakes.

On your question Adam - I agree with NS, a pair of 3/4" will probably work, from another pass I did using your disc sizes. Your pedal ratio, front and rear pad size (so I could figure out effective brake radius) and tire type would help get closer. Doubtless most any size will stop your car, and the balance bar can cure most ills - if you're careful not to run out of travel or bind up in any condition (including a complete loss of pressure on one side). Worst case is you can switch one or both cyinders later if you have a problem.

Hi again Pete - the bias valve works slightly differently than the bar - try playing with different values at the bottom of sheet 4 and you'll see what I mean. Puhn complains that it uses up pedal travel without adding brake force, but he likes the way it compensates better for conditions and allows for more even front/rear pad wear on a road car.

Yes Mike, but you still have to prevent the rears locking up before the fronts to pass the SVA test, and also to not crash!

Am I wrong in saying that during braking, the weight on the rear tyres decreases, and increases on the front? If this is indeed the case (which it is!) then the higher the rate of decelleration, the more the weight transfer to the front of the car, and the more the brake ratio (front to rear) needs to be biased to the front, because the maximum braking effort available at the front tyres has increased due to the increased friction available, and at the rear it has decreased. (think of a racing motorcycle braking hard for a corner, and watch the rear wheel come off the ground!!, don't think the rear brakes do a lot there!!)

This was my understanding anyway and it seems to have worked on my cars, but I stand to be re-educated if anybody knows better!

I just reread the thread and noticed this from Pete Bura. I always thought the idea was to stop the rears locking, at least to a degree, that's why we reduce the effort to the rears with various devices. I used the standard Cortina front and Capri rear with the Sierra M/C and no servo. It works and doesn't need any balance adjustment. According to the parts shop the 3 cars all used the same rear cylinders.

quote:

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Originally posted by Peteff
I just reread the thread and noticed this from Pete Bura. I always thought the idea was to stop the rears locking, at least to a degree, that's why we reduce the effort to the rears with various devices.

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NS, I think we do agree, but perhaps differ on the details slightly. Starting off with the answer to Pete's question - you definitely want the fronts to lock *just* before the rear, or you'll have a very nervous car under braking. But all 4 tires can generate stopping force, and you want all 4 to contribute as much as they can for maximum decelleration.

I think of it like this - imagine all 4 wheels have the perfect braking force, and are just at the point of locking up. Now put bigger brake pistons on the front but keep everything else, including foot pressure, the same. There's extra hydraulic advantage from the same line pressure on the fronts now, right? The front wheels will lock up from the extra clamping force, or the driver will back off and the rears won't be stopping at their best**.

Same thing happens with donor parts on a locost - too much front braking force, and worse the rear bias valve is more trouble because the rears don't contribute enough anyway (the bias valve can only bring rear pressure down).

For Adam's problem - I'm sure he's long given up on us - I'm muttering because I can't tell from the info he's given us whether we've got the right F/R ratio for him (though I agree 3/4 & 3/4 is probably close), and whether his pedal will be too soft or hard - whether he should go smaller or larger overall. I like what crbrlfrost said - start with pressure at the brakes, work back to get the right master sizes and save the bar for tuning.

While I tend to shrug off ackerman and can live with a little bump steer, I personally sweat about braking distance - 5m might make all the difference to me some day in my amateur welded chassis...

**Not to mention that with less stopping force there's less weight transfer, so the fronts carry less load and will lock up even sooner . This is the common "I put bigger brakes on the front and my brakes got worse!" problem. The pedal effort will go down, so it feels better if they don't actually measure their stopping distance.

quote:

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Originally posted by phelpsa
If brittish tridents theory is anything to go by then pete, you were right. .625 front, .7 back.

Here are my calculations:

Front:

Piston = 3.81cm

3.14x1.905x1.905 = 11.395

11.395x2 = 22.790

22.790/0.625 = 36.46

Back:

Piston = 4cm

3.14x2x2 = 12.56

12.56x2 = 25.12

25.12/0.7 = 35.89

That is the closest I could get going up in .025s.

Adam

[Edited on 15/5/04 by phelpsa]

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quote:

---

Originally posted by britishtrident

quote:

---

Originally posted by phelpsa
If brittish tridents theory is anything to go by then pete, you were right. .625 front, .7 back.

Here are my calculations:

Front:

Piston = 3.81cm

3.14x1.905x1.905 = 11.395

11.395x2 = 22.790

22.790/0.625 = 36.46

Back:

Piston = 4cm

3.14x2x2 = 12.56

12.56x2 = 25.12

25.12/0.7 = 35.89

That is the closest I could get going up in .025s.

Adam

[Edited on 15/5/04 by phelpsa]

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Got to use areas of master cylinder pistons not diameter.

bias pedal boxes have always tended to use 0..626" and/or 0.7" because the bias bar limmits the leverage ratio


[Edited on 18/5/04 by britishtrident]

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MikeP, see where you ae coming from now, make the fronts too good and they lock prematurely taking load back off the front, hadn't really thought of it from that angle!!

As you say though, and I obviously agree, the brakes need to be just how you want them, but as we have both said, given a balance bar and the cylinder sizes we have mentioned, that should be possible. Then you can have confidence each time they are used that you will always have that 5 metres to spare!!!

Cheers

Nat

[Edited on 19/5/04 by NS Dev]

I figured we agreed, we were just approaching from different directions.

If only we hadn't lost Adam with all of our drivel .

I am getting quite confused. What do you recommend or shall I just get the standard ones Stuart Taylor sell with the bias bar?

Adam

hehehe, you're still here Adam!

I think the consensus was that 3/4 & 3/4 should be pretty close with what you've told us about your car.

If you want to be 100% sure, try page 4 of that spreadsheet tool I posted for you - u2u me if you want help figuring out what to put in some of the fields.

Yep, agreed!!