Thread: tire diameter
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Old 09-24-2008, 12:49 PM   #50 (permalink)
CapriRacer
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Quote:
Originally Posted by tasdrouille View Post
Capri,

Keeping everything else the same, but just changing tread width, would you agree that the contact patch will keep the same area, but change shape becoming shorter and off course wider?

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No. My understanding of footprint length is that at a given load/pressure the length is essentially independent of the width - everything else being equal.

Quote:
Originally Posted by tasdrouille View Post
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Most of the rolling resistance in a tire comes from the flexing of the sidewalls (~80%).

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My understanding of RR is that most RR comes from the deflection of the tread and that the flexing of the sidewalls is coincidental. Put another way, if I take 2 identical tires, but one is pretty much worn out and the other tire is new - and then use the same load / pressure, not only would the footprint be the same size, but the worn tire will have substantially lower RR.

Quote:
Originally Posted by tasdrouille View Post

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When you shorten the contact patch by going to a wider tire, you will flex less sidewall but more tread. The impact of the reduction of sidewall flexing will outweigh the penalty taken on the tread flexing.

In real life though, everything else rarely is the same.

Choosing a rim/tire combination is always a matter of compromise. It would be easy to make a recommendations if you were always driving down the highway at 85 mph (skinny and tall), or cruising along country roads at 40 mph without ever stopping (midsize and tall), or driving 100% city with stop and go all the time (short and wide). But that's generally never the case.

Also, I would like to comment on pressure and rolling resistance. Higher pressure reduces hysteresis, which is the main variable determining rolling resistance. There's hysteresis between pressure and RR.

My understanding is that the thing that creates the most hysteresis (heat generation) is the thing that has the most mass - the tread rubber.
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