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SafeSpeed
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PostPosted: Tue Jul 31, 2007 20:19 
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smeggy wrote:
To preserve the flow rate (of that from 70mph in good conditions) whilst maintaining an equally ‘safe distance’ based on half the available traction (which I guess is where the ‘2 to 4 second’ idea came from), the overall speed will have to halve.


I believe it comes from the increased risk of braking performance differential in wet conditions.

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ICEAFE
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PostPosted: Tue Jul 31, 2007 22:28 
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Yes - The coeffient of slipping friction is half that of rolling friction. That is why we have ABS and why we older drivers were taught to apply brakes intermittantly rather than just slamming them on.

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SafeSpeed
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PostPosted: Wed Aug 01, 2007 03:09 
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ICEAFE wrote:
Yes - The coeffient of slipping friction is half that of rolling friction. That is why we have ABS and why we older drivers were taught to apply brakes intermittantly rather than just slamming them on.


That's a risky over-simplification. The following approximate curves apply:

Image

Typical friction characteristics: (a) dry asphalt; (b) wet asphalt; (c) gravel; and (d) packed snow.

From: http://www.safespeed.org.uk/braking.html

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ICEAFE
PostPosted: Wed Aug 01, 2007 11:15 
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Fascinating! Yes - an over simplification; but easy to remember. However if I have correctly interpreted the definition of Slip (S) then we have 60% of the rolling frictional coefficient for wet asphalt, which not too far out. Perhaps you could confirm or explain. I am however surprised at the result for dry asphalt. Perhaps it is the result of improved tyre technology?

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