Re: Brakes vs Engine

[Michiel van Wessem <jmvw@xxxxxxxxx>]

Hello Albert,

I'm glad you liked that post. I guess I'll just stay away from attacking
innocent audiophiles. I'm not sure, but I think brakes might only make
wheels lock at lower speeds. Don't know why. But if that's true, then
100mph to 20mph times would be more useful in determining the engine power
required.

Maybe with soft race or even drag race tires you could avoid the sliding too.

Turning a wheel from a stand still with the brakes on may required some
more torque then when it's already moving, similar to how tires have more
friction when they're rolling then when they're sliding. Brake pads aren't
flexible like tires, though, so I don't know if there's as much difference.
I don't know much about friction.

Maybe we need someone with a timing device and very grippy tires.
Interesting :)

Michiel

At 10:15 AM 12/17/98 -0500, you wrote:
>Michiel,
>Cool post!  I ran the numbers for braking from 80 mph for the M3 sedan (204
>ft, from road test summaries in the back of Road & Track).  This works out
>to over 1 g of deceleration and nearly 265 kW to stop.  But then I got to
>thinking that one-shot braking performance is limited more by tires than
>anything else.  I don't think there is a car out there that can't lock up
>its wheels under braking (with ABS off).  
>
>What you would really need is the minimum stopping distance from a certain
>speed independent of tire characteristics, which means replacing the tires
>with spikes or teeth or something that won't allow slippage between the
>road surface and the contact patch.  I think maybe other mechanical
>components would fail long before this happened.  So I am betting the HP
>required to overcome the brakes is well above what you calculated.  Imagine
>putting your car up on jack stands, removing the wheels, having a buddy
>apply the brakes, and then determining the torque necessary to break the
>rotors loose from the pads.  You'd probably be popping bushings and bending
>suspension links by that point.
>
>In short, no way in hell a modern car's engine can overcome its brakes.
>-Al
>
>At 11:28 PM 12/16/98 -0500, Michiel van Wessem <jmvw@domain.elided> wrote:
>>A little while ago in the discussion about cars accelerating out of
>>control, I said that brakes have more force then the engine. I thought it
>>might be interesting to see how many hp you'd need to overpower your
>>brakes. Plus I get a chance to refresh high school physics.
>>
>>In something I found on-line, called 49 Code of Federal Regulations
>>(10-1-1997 edition) Chapter V, paragraph 571.105 (I'm not sure what all
>>this means), I see that passenger cars in the US are required to be able to
>>stop in 673 feet from 100 mph in a first test. There probably are a number
>>of conditions under which this criteria must be met, but this two numbers
>>seem fine for my purposes.
>>
><snip>
>
>

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