Re: [ST] Tire Pressure on a Worn Rear

[Craig Tate <craigtt@xxxxxxxxxxxxx>]

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Steve,
Well, at least we agree on one thing - wrong thinking (unfortunately, we 
don't agree on whom to assign it).  No offense taken or intended; I've 
benefited from many of your past posts.

- - I've snipped our posts to reduce size.

At 07:34 PM 3/2/2004 -0600, you wrote:
>-Snip-
>>Craig
>
>     But your conclusion is in error concerning an
>     already worn tire.  Under inflated tires wear on the shoulders
>     precisely because of the flexing of the sidewalls, as you mentioned.
>     NOT because of any abrasion.  The heat generated by under inflation
>     causes the cords/belts in the sidewall to breakdown.  AND this heat
>     IS transferred to the main body of the tire (in this case already at it's
>     limit, and therefore, faster wear).

Although it certainly doesn't appear so at first glance, I'm not sure our 
thinking is diametrically opposed.  Perhaps it's a matter of degree, 
context, and assumptions.  I used the term "under inflated" in a general 
sense.  By that I meant "less than fully inflated."  Rather than running a 
"center" worn a tire at maximum pressure, temporarily reducing the pressure 
by several PSI will place a larger "contact patch" on the road.  Given the 
same riding style, speed, and weight, the same force is spread over a 
larger area, resulting in less wear to any specific point on the tire 
(i.e., the worn center).  This is basic physics and very different from 
running a tire grossly under-inflated - which would cause excessive heat 
and abnormal wear or failure, as you suggest.  Without trying it on my 
bike, I'm not sure how low I'd put the rear tire, but off hand, I think you 
could safely reduce it from 42 to 36 PSI - perhaps lower.  I assumed our 
Hero had the sense to to not ride with a grossly under inflated tire.

I also assumed that up to the point of this discussion, the tire had been 
properly inflated and that the wear pattern was not abnormal and the 
projected failure was not premature.  Our Hero gave no indication that he 
had experienced any sidewall breakdown or other abnormal/premature tire 
wear - he just had too many miles on the tire.  I think you and Blake were 
focusing on a different scenario.  I'm not suggesting he run the tire 
grossly under inflated, but the last thing he should do is run it at max 
pressure because doing so reduces the contact patch to the area of his tire 
that is the thinnest.

I suppose then we could argue about whether reducing the tire pressure a 
few PSI causes premature sidewall breakdown.  I personally don't think so, 
but I don't have any test reports to back it up - I'm not in that line of 
business.  However, based upon previous posts to this list, many riders 
suggest reducing tire pressure by several PSI for better traction on the 
road and track.  I recall several list members reporting that racers always 
run tire pressures below the manual specification.  And, to put this in 
context, our Hero wasn't planning on running a tire to the point of 
sidewall breakdown - he just wanted to buy a little time until he could get 
the tire changed.

>     And finally, are you suggesting that
>     our hero run his last remaining miles on the SHOULDERS of the tires?
>     Hell, even Rossi can't do that.  We ride on the tread, and further
>     weakening the shoulders will not extend the life of the tread portion.

No, read my post again - I suggested he replace the tire.

I don't mean to be rude or inflammatory, but if you disagree with what I've 
written here, we may, in fact, hold diametrically opposed positions that 
neither is likely to concede.  I'm sure we agree on other points though.
Craig   

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