GRRR
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| posted on 7/2/12 at 09:21 PM |
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Steve, no problems I didn't interpret your comment as harsh (blunt perhaps..!), of course I could be some armchair car designer cobbling a car
together out breadsticks and masking tape so your concern was justified. You're right, calculations need to be done but of course in the name of
cost compromise and trying to avoid custom bells and rotors, I was offering my initial thoughts about using OEM stuff. I'm looking at things the
wrong way round maybe, but not having luxury of R&D budgets, test facilities etc, 'borrowing' knowledge from elsewhere is a
necessity.
Calculations so far show under max braking the weight on the front axle is 410kg assuming a 40/60 f/r static weight distribution, indicating the discs
sizes could be quite similar. The Elise and Rev2 MR2 back this up, with the MR2 actually having larger diameter discs on the rear but a smaller piston
area and thinner disc.
Coyoteboy, interesting pic of the 911 brake size, fronts are enormous.
Hughpinder - cheers for the details about your setup. Road and Track magazine's test of an Exige got deceleration of a smidgeon over 1g on its
standard tyres. What are you running to get 1.9g!! Downforce?
[Edited on 7/2/12 by GRRR]
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coyoteboy
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| posted on 7/2/12 at 10:15 PM |
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quote:
With all due respect, if you're designing a car from scratch, you cannot randomly select braking performance based on opinions,
conjecture and what other solutions look like. At best it's poor engineering, at worst it's fatally dangerous. Not just to the driver and
passenger(s), but to other road users, pedestrians, etc...
Completely agree, but I read his post as "I'm looking for general opinions on sizes before I plough on and calculate the
"correct" size. Though I would say there's no 'correct' size, it is of course yet another a compromise, more of a
correct range.
I think I just have more trust in people's common sense and assume it won't be based on looks.
Considering the VAST array of sizes used on locost cars, plucked at random, used on vehicles with vastly differing masses, and relatively little
evidence of death and destruction from under/oversized brakes I'd be wary of suggesting it's a knife-edge decision with death on either
side. Not suggesting calculations can be avoided, or that you're wrong about what he should know, just that there was little need for the blunt
nature of the response, assuming the OP's not a kid.
quote:
he Elise and Rev2 MR2 back this up, with the MR2 actually having larger diameter discs on the rear but a smaller piston area and thinner disc.
Not sure about the Elise but the early MR2 runs drum handbrake inside the rear discs, this may skew your rear disc size a bit - not sure on the
pressure proportioning or piston sizes though.
Edit - actually I might be wrong on that, seems some of the early mr2's used cable-operated pistons too, but my point is sizing could be
packaging related too.
[Edited on 7/2/12 by coyoteboy]
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hughpinder
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| posted on 8/2/12 at 09:35 AM |
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Sorry, I didn't make it clear in my last post that those were calculated values. The track tyres are avon slicks, and avon quotes a coef of
friction in the dry of (2-((0.000185)*normal force in newtons)), approximately 1.6 - my calculation actually allows for the change in this value as
the weight changes during braking. Normal road tyres of the sticky variety have a coef of friction of about 1.0, but you would normally allow about
for 0.8 to get 'realistic' figures and this would give lock up of the front wheels at 0.98G. Toyo proxes GGG are somehat better. In the
wet the avons would be about 0.7, road tyres about 0.6, so you can reduce the figures again.
Regards
Hugh
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britishtrident
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| posted on 8/2/12 at 04:57 PM |
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The thing is a car with too much of the braking on the front is not enjoyable to drive especially in the wet, but a car that prematurely locks up the
rear can put you in the scenery in the blink of an eye.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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