Doug68
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| posted on 26/4/08 at 10:22 AM |
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Brake Pedal Movement.
I've written up some issues I've run into with my project on my Blog.
this leads me to ask for some group wisdom on the following regarding brake pedals:
1. Would be the theoretical amount of fore-aft travel possible for the foot pad if allowed to freely travel i.e. you were bleeding the system or it
had no fluid in it.
2. What stops the travel of the brake pedal in this condition? Does is A: Run into a part of the car or B: The master cylinder runs out of
travel?
3. If answering A to Q2 how much of the possible travel of the master cylinder is it actually used by the full range of motion of the pedal?
4. How much can the pedal actually be moved in the operating condition? i.e. pushed as hard as you can when there's fluid (and no air  ) in
the system.
I'm not interested in leverage ratio's, I can work that stuff out for myself, what I'm looking for is typical and safe values for
pedal and MC movement.
Thanks.
Doug. 1TG
Sports Car Builders WA
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RazMan
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| posted on 26/4/08 at 10:45 AM |
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2. My pedal comes to a halt when it hits the firewall (which is nearly the limit of the m/c anyway.
4. My pedal moves about 2" under normal braking but about 3" in a 'brown trouser' situation.
Don't dismiss lever ratios too quickly - 5:1-6:1 is usually a good starting point.
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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blakep82
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| posted on 26/4/08 at 10:50 AM |
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erm, i'm not sure what the proper answers would be, but i think the leverage ratio would have a big impact on the answers to your questions (if
that makes sense)
eg, 1) witha really bad set up, in theory you could have 10" of pedal trave, but only 1/2" on the master cylinder. or at the other
extreme, 1/2" on the pedal and the master cylinders gone through all its travel. dependant on leverage ratio...
ideally i'd say the pedal should travel about 4" with no fluid though
2) i think the pedal should go to a stop on the car before hitting the end master cylinder
3) very nearly all of it. like 98% ?
4) don't know 2" maybe
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Doug68
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| posted on 26/4/08 at 10:58 AM |
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quote:
Originally posted by RazMan
2. My pedal comes to a halt when it hits the firewall (which is nearly the limit of the m/c anyway.
4. My pedal moves about 2" under normal braking but about 3" in a 'brown trouser' situation.
Don't dismiss lever ratios too quickly - 5:1-6:1 is usually a good starting point.
So at the "Brown trouser point" how much travel it left before you'd get to the fire wall?
BTW I'm not dismissing leverage, I think I've got the theoretical side of things under control. It the 'real world' values
I'm missing.
Thanks,
Doug. 1TG
Sports Car Builders WA
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RazMan
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| posted on 26/4/08 at 02:59 PM |
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quote:
Originally posted by Doug68
So at the "Brown trouser point" how much travel it left before you'd get to the fire wall?
I would say that 1" sounds about right - but that is with two feet on the pedal and a very scared expression on my face. 
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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Doug68
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| posted on 27/4/08 at 08:01 AM |
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Razman
Thanks for that.
On the face of it that'd make the pedal to mc ratio roughly 4 to 1 which seems a bit high?
Anyway now I've got some thinking to do.
Doug. 1TG
Sports Car Builders WA
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RazMan
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| posted on 27/4/08 at 10:36 AM |
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Doug, you must have some fairly large pistons in your calipers if 4:1 sounds rather high to you - what's your setup?
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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Doug68
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| posted on 27/4/08 at 11:31 PM |
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Razman,
Sorry I was referring to your set up from the numbers you give there's 4" of pedal movement to ~1" of mc movement (I guessed).
Here's the brakes I'm using, from the math done to date I'm using a 1"
front and 0.75" rear mc's
The leverage ratio for the pedal box I have as delivered is 6.25:1 I think this'll need to go up to 5.6:1 as I need to shorten the pedals to get
them in the right spot for my foot (see Blog link for reasons why).
[Edited on 27/4/08 by Doug68]
Doug. 1TG
Sports Car Builders WA
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RazMan
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| posted on 28/4/08 at 06:47 AM |
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Its a little difficult to give exact measurements after final assembly and to be honest I have forgotten the rather involved process I went through to
arrive at my present (and working) setup.
My original pedal was around 4:1 iirc and it was as scary as hell - no feel and considerable effort was needed to stop, even gently.
I remade my pedal to around 5.5:1 and everything was immediately transformed - I could stop on demand!
From your specs I would say that a similar ratio would be a good starting point.
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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RazMan
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| posted on 28/4/08 at 07:35 AM |
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I have just found a handy brake calculator which might help you figure things out. It was originally posted by Fred WB iirc. As you are using 6 pot
front calipers you might need to adjust things to suit the extra fluid being pushed into them.
[Edited on 28-4-08 by RazMan]
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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Doug68
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| posted on 28/4/08 at 08:19 AM |
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Yeah I've been using a slightly different version of that workbook.
Doug. 1TG
Sports Car Builders WA
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