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Author Topic: So when going up a few sizes in tires do you need to change the PSI.  (Read 996 times)

Offline ajay

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Re: So when going up a few sizes in tires do you need to change the PSI.
« Reply #40 on: April 13, 2011, 03:58:16 am »
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Offline capriracer

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Re: So when going up a few sizes in tires do you need to change the PSI.
« Reply #41 on: April 13, 2011, 06:51:52 am »
Yes, I was finished.  My conclusion was:

The calulated value results in a lower pressure, but matching the spring rate then becomes important, so i recommend doing that.

How does one calculate the pressure?  Why did it go down?

Starting with the placard pressure, you look up the load carrying capacity in a table.  Here's what one looks like:



Notice it is done by tire size. 

Then you find the new tire size in the table and then find the inflation pressure that gets the same or slightly greater pressure.  easy, once you have the table.

The pressure went down because the maximum load carrying capacity of the new tire was greater than the old tire.  Width was only part of the picture. 

And the comment about asking at a tire (or wheel) shop?  I'll bet the 99% of the shops out there do NOT know how to do this, so that's not very good advice.
« Last Edit: April 13, 2011, 07:09:28 am by capriracer »
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Offline No H2O

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Re: So when going up a few sizes in tires do you need to change the PSI.
« Reply #42 on: April 13, 2011, 10:30:51 am »
Quote from: capriracer on April 13, 2011, 06:51:52 am
And the comment about asking at a tire (or wheel) shop?  I'll bet the 99% of the shops out there do NOT know how to do this, so that's not very good advice.

99%...that pretty sad. So I guess you're not recommending anyone asking any tire shop for advice?

Now I know why I do my own work...cars, motorcycles, carpentry, A/C, furnaces, etc.  ;)
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Offline blur911

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Re: So when going up a few sizes in tires do you need to change the PSI.
« Reply #43 on: April 14, 2011, 10:51:09 am »
Quote from: No H2O on April 13, 2011, 10:30:51 am
Quote from: capriracer on April 13, 2011, 06:51:52 am
And the comment about asking at a tire (or wheel) shop?  I'll bet the 99% of the shops out there do NOT know how to do this, so that's not very good advice.

99%...that pretty sad. So I guess you're not recommending anyone asking any tire shop for advice?

Some tire shop guys know their stuff, but it's kind of rare.  I don't know how many times I've seen them mount tires backwards, inside-out, etc.
They usually just inflate everything to 32psi or so.

BTW, I remember Rick Bye once saying that on the track just about every radial seems to work best around 40psi hot, there are of course exceptions.
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Offline ktm525

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Re: So when going up a few sizes in tires do you need to change the PSI.
« Reply #44 on: April 14, 2011, 11:09:10 am »
I consider myself lucky if they can mount the tire correctly and achieve a halfway decent balance.
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