11-23-2007, 12:44 PM
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Member
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Join Date: Sep 2007
Location: Boston
Posts: 212
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Quote:
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Originally Posted by STALKER
The ABS system does measure the overall diameter. Its a far superior system to the one that BMW uses on its cars now. I own vehicles with both, and he M system has been complete reliable.
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Yes, I too owned both vehicles the E46 M3 and the E70 X5 and agree with you that the M system (the rotational speed system) has been flawless. I never once had an issue with the way it worked.
BUT the ABS system does NOT directly measure diameter, it is back calculated from Velocity = Omega x Radius (V=wr), Radius being one half of the wheel diameter.
What Penguin is saying is correct; I do not agree with him about reliability of the system, (based on my experience with it), but he is none the less correct. If all 4 wheels went down in pressure equally, there would be no wheel-to-wheel difference, and thus no alarm.
The ABS system is looking at a difference in Radius in the V=wr equation – since the car speed is constant, if there is a difference in w (rpm) there must be a difference in radius.
That is how the system is backing out a pressure difference – from the change in tire diameter, it is not directly measuring tire diameter.
Cheers,
KEF
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