Peteff
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| posted on 18/5/04 at 09:15 AM |
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I've noticed that many builders have had difficulty getting their rear brakes to lock up,
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.
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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pbura
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| posted on 18/5/04 at 12:25 PM |
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I've noticed that many builders have had difficulty getting their rear brakes to lock up
quote:
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.
Defective memory at its finest (???)
I must have been thinking of a couple of accounts in which there was trouble getting the brakes (any) to lock for the test.
Should have noticed my head going into my ass when my ears started to bend 
Pete
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MikeP
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| posted on 18/5/04 at 12:30 PM |
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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.
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britishtrident
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| posted on 18/5/04 at 06:26 PM |
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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]
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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timf
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| posted on 19/5/04 at 07:18 AM |
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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]
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]
british trident forgot basic maths
area = pi * r * r
which is what adam used
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NS Dev
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| posted on 19/5/04 at 09:27 AM |
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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]
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MikeP
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| posted on 19/5/04 at 03:17 PM |
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 I figured we agreed, we were just approaching from different directions.
If only we hadn't lost Adam  with all of our drivel .
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phelpsa
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| posted on 19/5/04 at 05:31 PM |
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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
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MikeP
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| posted on 19/5/04 at 10:22 PM |
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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.
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NS Dev
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| posted on 19/5/04 at 10:47 PM |
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Yep, agreed!!
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