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Tire Sizing (Or How to Buy A Tire)

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Skyguy9999 Offline
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Posts: 122
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Post: #1
 
Are there any good hints on how to properly size a tire for a rim? I figured out pretty quickly that you need to match the inner tube to the tire in terms of size (26"x1.75 for tire and tube both). But for the rim, I ran across something curious. One of my bikes had 26"x1.95 tires and tubes on a 26"x1.50 rim.
Now my question is this: What made it so that particular combination works and the 1.75 tires and tubes I tried to replace one of them with (and failed) not work? I noticed the 1.95 tires were pretty flexible and more balloon-like than the 1.75 tires (which are kevlar-lined street thread from what I can tell - man them were fast when I got them on my other bike with 1.75 rims), so that is probably what made them work on the 1.50.
Maybe more to the point: How do I know what should work on a rim and what shouldn't? I looked at two or three sites (including the Sheldon Brown one) and still didn't come away with a good explanation for the above.

Why is it that they make adult bikes that'll generally work for 5'9" or above, yet when you pedal these same bikes they only work for someone who is 5'4" or so?
Feb 15, 2009 07:16 PM
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Skyguy9999 Offline
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Posts: 122
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Post: #2
 
Okay, I just went shopping (physically), and on the packages for the tires it says the rims they work on. So that question is resolved, to a point. I don't see that information online, though, for any tires that are in online stores (I checked before I made the first post).
So, the big question now would be whether there are smaller diameter tires than 1.95 made for 1.50 rims (the entire stock I saw stopped at 1.75 for smallest, except for road bike tires, which would have been too small)?

Why is it that they make adult bikes that'll generally work for 5'9" or above, yet when you pedal these same bikes they only work for someone who is 5'4" or so?
Feb 16, 2009 09:04 AM
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cyclerUK Offline
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Post: #3
 
You are talking about diameters when I think you mean width? I have 26" rims (diameter) on which I have fitted a range of tires from 1.5" up to 2.1". (I have never bothered about the width.)
According to Sheldon Brown's site :-
http://www.sheldonbrown.com/tire-sizing.html
You can measure the rim width and then there is a chart giving the recommended tire widths.
You can get as small as 1" tires for 26" rims but you will also need to fit the matching size inner tube as you have said.
If you fit a wide tire to a narrow rim then you run the risk of "pinch" punctures when going over rough stony ground. If you just run on smooth surfaces then you will find it much easier with narrow "slick" tires inflated hard.
Hope this helps?

Ride hard or ride home alone!
Feb 16, 2009 10:05 AM
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binzer Offline
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Post: #4
 
Would there be a problem fitting a tube that was originally in a 26x1.95 tire in a 26x1.5 tire?
Feb 16, 2009 02:42 PM
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cyclerUK Offline
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Post: #5
 
The biggest problem fitting an oversize tube is making sure it doesn't get trapped when re-fitting the tire. Once the tire is on there is no problem with inflation.
There may be trapped folds in the tube which I suppose could cause the tube to rub and puncture? Usually it states on the tube the range of sizes that it covers.

Ride hard or ride home alone!
Feb 16, 2009 04:01 PM
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binzer Offline
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Post: #6
 
The reason I asked is because I just did this. I was careful to make sure that the tube wasn't pinched between the rim and tire before inflating it. I also inflated each tube to about 3/4 of it's maximum pressure and then deflated it before reinflating them fully.
Feb 16, 2009 05:06 PM
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Skyguy9999 Offline
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Posts: 122
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Post: #7
 
Quote:You are talking about diameters when I think you mean width? I have 26" rims (diameter) on which I have fitted a range of tires from 1.5" up to 2.1". (I have never bothered about the width.)
Yes I meant tire width. And that was the "Sheldon Brown one" that I referred to. Basically, I was just wondering why the 1.95 width tire works and the 1.75 tire didn't work on a 1.5 rim (I guess because the 1.95 was really flexible material), and how I would know that a particular tire would work. When I was trying, I was thinking "surely the 1.75 would work if the 1.95 did".
So I guess the best main rule is to match up tire size to rim size as well (to make sure to get the right thing, plus the other factors you described), or see if the info is there where the manufacturer will guarantee that their tire will work on a particular rim?

Why is it that they make adult bikes that'll generally work for 5'9" or above, yet when you pedal these same bikes they only work for someone who is 5'4" or so?
Feb 16, 2009 07:28 PM
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cyclerUK Offline
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Post: #8
 
I don't understand why the 1.75 won't fit.
What "size" rim have you got?
My MTB rims are about 3/4" wide ( 559 x 17 [26"]) 17mm inside the lip of the rim.
I have 1.5 and 2.1 tires on them. (I have 2 sets of wheels.)
Will your physically not go on?
There was the old 26" rims that was on bikes of yesteryore which I think where/are a different 26" ?????

Ride hard or ride home alone!
Feb 17, 2009 01:06 PM
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Skyguy9999 Offline
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Post: #9
 
I don't understand why the 1.75 won't fit.
Neither do I. That's the original question I put into this thread.
What "size" rim have you got?
As was stated in the thread, 26"x1.50". This is what is etched into the rim itself.
Will your physically not go on?
Yes, the 26x1.75 tire would not go on this rim. One side of it went on with a LOT of work, and the other side would go under the rim but would pop back out. This makes me think that the particular 1.75 tire would not fit the rim.
But as the start of this post indicates, if the 1.95 fits, why not the 1.75? And how would I know a particular tire would fit (obviously someone knew this particular 1.95 tire would fit on this rim before they were purchased, or they wouldn't be there) a particular rim?

Why is it that they make adult bikes that'll generally work for 5'9" or above, yet when you pedal these same bikes they only work for someone who is 5'4" or so?
Feb 17, 2009 04:37 PM
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DaveM Offline
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Post: #10
 
There is a lot of variation in how tight tires fit on the rim. Usually mtn tires are fairly loose because they're not made for high pressure. But occasionally you get a tire that is just very tight. I suspect that this is the problem with your 1.75 tire. It technically is the right size, but just fits much tighter than you're used to. On a 1.5 rim, you should be normally be able to fit anything from a 1.25 tire up to 2.2. Also note that tire widths are notoriously inaccurate. One companies 1.75 might be wider than another's 2.125.
I doubt that it is the issue here, but do note that sizes with decimals (1.5, 1.75) are completely different than sizes with fractions (1 3/8, 1 3/4). Any tire with a fraction measurement will never fit on a modern 26" mtn bike wheel. These are old incompatible sizes with a different rim diameter even though 26 x 1 3/4 is mathematically the same as 26 x 1.75.
Feb 19, 2009 10:24 AM
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