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Old 01-13-2011, 05:15 AM   #21 (permalink)
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Originally Posted by stovie View Post
Sorry i was trying to get to this conclusion but i was in a hurry. i was also just wondering if the bulg of the tread could have a pretty big factor in RR for example my dad has 31 inch tires with 1/4 inch bulg will i have 29's with 3/4 inch bulg both pairs are new?? Thanks agian
Let's start with how tires are tested for RR. All RR test procedures specify a load and inflation pressure for each test point. The standard test methods specify the load as a percent of the rated load and specify the inflation pressure based on the rated inflation pressure. Both of these are found in the load tables as published by the tire standardizing organizations.

Please note: The rated inflation pressure is NOT necessarily the sidewall inflation pressure - and this is particularly true for passenger car tires.

Some RR test procedures specify a single test condition, and some have multiple test conditions. The result is sometimes expressed as a single point and sometimes expressed as multiple points.

However, enough testing has been to be able to say that tires behave in similar ways regardless of the test procedure and the various test conditions - that is, the test procedures will give fairly good rank order when comparing different tires.

Needless to say, you can not compare different tires if the test procedures are different - without knowing more.

So if you want to know the difference in RR between your Dad's truck with 31"er's with a 1/4" bulge and your truck with 29"er's and a 3/4" bulge - well, that's not enough information.

Obviously a tire with a 1/4" bulge is more lightly loaded than the same tire with a 3/4" bulge - and by "more lightly loaded" I mean a percentage of the load at the inflation pressure being used. Notice that I am referencing both the load and the inflation pressure.

But your question involves different sized tires- and likely a different make/ model. If you've followed along, you would have seen that make/model has a HUGE effect on RR. There's also an effect caused by the difference in size.

So there are difference in tire size, load, inflation pressure, and make/model. Too much information to make an estimate.

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