Re: Brakes vs Engine

[albert jenab <jenab@xxxxxxx>]

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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