Ivan
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| posted on 5/3/16 at 09:18 AM |
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Mike Polan Brake Calculator - Question
I have extensively changed the front brakes on my Cobra with bigger discs, Willwood calipers with bigger pistons than the original Jag ones but left
the rest of the braking system the same.
I now suspect that the front is way over-braked compared to the rear (Car is squirmy under heavy braking just like it was the last time the back
brakes stopped working) and want to switch to dual master cylinders as it will save a couple of kilos in weight getting rid of a heavy booster and
enable me adjust brake bias.
I am using Mike Polan's spreadsheet to run some calculations but it is designed for identical width front and rear tyres and the Cobra runs
anything from 275 to 315 rear tyres (275 in my case) and 245 front. The spreadsheet allows one to adjust front grip to compensate for tyre width
differences but I am not sure what percentage difference to use - do I take a factor of say 245/275 = 89% or maybe half the difference sat 94.5%?
Some thoughts from the knowledgeable would help.
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ettore bugatti
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| posted on 5/3/16 at 01:31 PM |
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Do you have a link to this calculator?
A way round is to do two calculations.
One with the 245mm all round and one with 275mm all round and see what the difference is.
But I can't imagine that the 275mm is going to bring much. At braking you got weight transfer to the front and even a 245mm wide rear tyre will
have more grip to brake than the fronts.
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Ivan
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| posted on 5/3/16 at 05:33 PM |
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No link I'm afraid. But if you U2U me your email address I could send you a copy.
It's all about brake balance - you do want as much braking at the rear as you can get with lockup at the front first - in the case of the Cobra
some 40+ % of the braking should take place at the rear at high G stops.
A comparative check with different Tyre width changes, front to rear brake bias at a 0.8G stop changes from:
57% F and 43% R for same size tyres
to
54% F and 46% rear with wider rear tyres than front
So it has a not insignificant effect.
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Bluemoon
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| posted on 6/3/16 at 12:08 PM |
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Linky:
http://wascb.org/?p=407
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ettore bugatti
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| posted on 7/3/16 at 08:50 PM |
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How do you adjust the tyre grip level in that calculator linked by Bluemoon?
I can't see tyre/road friction coefficient.
You might to dive into tyre modelling (Pacejka curves) to get the answer you after.
The only reference I found in race car design by Derek Seward about tyre grip and width is that the maximum cornering force varies with width to the
power of 0.15, but that's lateral not longitudinal.
But for the sake if we do use that relationship for longitudinal grip (275/245)^0.15=1.017, so 1.7%
Now try CoG Height 2" higher or lower. How accurate is your CoGH number anyway?
Some food for thought about tyre width and grip:
http://tentenths.com/forum/showthread.php?t=117345
http://www.f1technical.net/forum/viewtopic.php?t=12762
http://www.eng-tips.com/viewthread.cfm?qid=102250
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Ivan
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| posted on 8/3/16 at 04:06 AM |
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quote:
Originally posted by ettore bugatti
How do you adjust the tyre grip level in that calculator linked by Bluemoon?
I can't see tyre/road friction coefficient.
You might to dive into tyre modelling (Pacejka curves) to get the answer you after.
The only reference I found in race car design by Derek Seward about tyre grip and width is that the maximum cornering force varies with width to the
power of 0.15, but that's lateral not longitudinal.
But for the sake if we do use that relationship for longitudinal grip (275/245)^0.15=1.017, so 1.7%
Now try CoG Height 2" higher or lower. How accurate is your CoGH number anyway?
Some food for thought about tyre width and grip:
http://tentenths.com/forum/showthread.php?t=117345
http://www.f1technical.net/forum/viewtopic.php?t=12762
http://www.eng-tips.com/viewthread.cfm?qid=102250
In line 8 of the "Analyse" worksheet he allows for a "Rear Grip factor" - but a subsequent check shows that the factor is not
used in any of the calculations. So my original question is moot and I have decided not to make any adjustment to rear balance due to tyre grip as
that will be a safer method.
In fact at the moment I sit with the problem of being way over braked in the front and will either have to significantly increase braking at the rear
and upgrading cooling to the rear and downgrade front braking.
Will start another thread about this when I give it more thought and research.
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