phelpsa
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| posted on 15/5/04 at 06:48 AM |
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Master cylinders
What bore master cylinders do I need? I am making a bias bar system.
Adam
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MikeP
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| posted on 15/5/04 at 01:50 PM |
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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.
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Peteff
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| posted on 15/5/04 at 02:03 PM |
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As a guideline.
It's usually something like .625" front .75" rear Adam. Smaller bore generates higher pressure.
yours, Pete
I went into the RSPCA office the other day. It was so small you could hardly swing a cat in there.
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phelpsa
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| posted on 15/5/04 at 06:12 PM |
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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
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britishtrident
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| posted on 15/5/04 at 06:47 PM |
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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]
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crbrlfrost
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| posted on 15/5/04 at 07:07 PM |
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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!
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phelpsa
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| posted on 15/5/04 at 07:12 PM |
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What does CoF stands for.
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Peteff
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| posted on 15/5/04 at 07:25 PM |
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Coefficient of friction I should imagine. I think we're getting carried away a bit here.
yours, Pete
I went into the RSPCA office the other day. It was so small you could hardly swing a cat in there.
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phelpsa
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| posted on 15/5/04 at 08:00 PM |
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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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MikeP
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| posted on 16/5/04 at 12:44 AM |
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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   .
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pbura
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| posted on 16/5/04 at 02:06 AM |
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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
Pete
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phelpsa
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| posted on 16/5/04 at 08:05 AM |
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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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NS Dev
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| posted on 16/5/04 at 08:08 AM |
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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.
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crbrlfrost
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| posted on 16/5/04 at 05:04 PM |
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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!
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crbrlfrost
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| posted on 16/5/04 at 08:33 PM |
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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.
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NS Dev
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| posted on 16/5/04 at 08:49 PM |
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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
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MikeP
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| posted on 16/5/04 at 11:04 PM |
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quote:
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
Hi Adam - have a look at page 2 on the sheet, it shows how the weight shift works and lets you compare it to your donor. A locost, with it's
50/50 weight and lower CoG doesn't shift as much weight and so doesn't need as much braking force on the front as most sedans. Vents
don't change the braking force available, they help with heat dissapation.
Watch out for the net wisdom out there. The better sites will talk about how they put bigger brakes on the front, admit their braking distance
*increased*, and show how they fixed it. The worst one I found recommended setting the bias bar by testing on a dirt road because the wheels locked
easier! A very bad idea - they're in for a very scary time on their first panic stop on a dry road.
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MikeP
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| posted on 16/5/04 at 11:15 PM |
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quote:
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?
Thanks Pete, from a fellow obsessor  .
I don't recall Puhn prefering one over the other, I think it comes down to what kind of room you have and how straight you can keep the throws.
The larger masters might be preferable with more fluid, better heat resistance, and Puhn does say that you can go to smaller masters if the pedal
force is too high. Most pedal assembly ratios are 6:1 or 7:1.
Puhn recommends 3/16" all around. 1/4" moves fluid easier but 3/16" is easier to work. I'm used to 3/16" on the rear,
1/4" on the front.
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pbura
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| posted on 17/5/04 at 12:38 AM |
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GTS Tuning has some 5:1 pedal sets, very reasonable. Maybe even worth ordering from across the Big Water.
Pete
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MikeP
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| posted on 17/5/04 at 12:34 PM |
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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.
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phelpsa
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| posted on 17/5/04 at 01:33 PM |
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Would .7" front and rear do?
Adam
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pbura
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| posted on 17/5/04 at 02:04 PM |
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quote:
Originally posted by MikeP
I couldn't find the pedals on GTS
They're on the chassis page, I think; can't get on the site right now. Brake, clutch, and throttle, all three for UKP40 or 45, depending
on style. I'd prefer using the donor pedals myself to keep costs down and because I prefer top mounts, but these are a great value, and a
possible backup plan. Summit Racing also has Howe pedals at a good price.
I was looking at some old posts about adjustable proportioning valves vs. bias bars in reference to a problem derf's having in another thread.
It seems that they do the same job, but the valve is much less expensive. Maybe our UK friends don't use them because they might poke their
eyes out on the knob (SVA)?
Pete
Pete
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NS Dev
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| posted on 17/5/04 at 04:06 PM |
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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]
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MikeP
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| posted on 17/5/04 at 11:02 PM |
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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.
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NS Dev
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| posted on 18/5/04 at 07:05 AM |
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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!
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