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Changing tire size on rims

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Changing tire size on rims

Old 12-06-17, 03:51 PM
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Changing tire size on rims

Hi guys - I know this question has been asked before, but the posts I just read kind of didn't answer the question straight out. I just bought two 700x35 tires for my bike. My rims have no info as to size as I can see (I'm assuming that info would be on the inside of the rim, exterior). Perf. Bike sent me one 35 and one 32 tire. My tubes aren't going to fit the 32 (they're larger), so the tire is kind of useless to me. Here's the question: I've been told that you don't have to always use the same size tire as what you originally found on the bike. So could I use the 700x32 if I bought a smaller tube? It's a loaded bike for touring/living on, so if I did that, I'd put the 35 on the rear and 32 on the front (I don't have front panniers). I was told by a mechanic that I could go from 35ish to 40 for a tire on my rims, but how does he know that? I don't have a measuring tape, so I can't readily measure the rim. It's probably the original one that came with the Specialized Crossroads 90's hybrid. Thanks.
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Old 12-06-17, 04:00 PM
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Anything "700C" uses the same diameter rim, so the tires will fit. "32" and "35" are the width, and there is pretty wide latitude regarding which width tires fit which rims.

There are limits regarding which width tires fit which width rims, but a rim that accepts a 35mm wide tire will probably accept 28mm to 45mm width tires without issue. Here's a good reference:

Tire Sizing Systems
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Old 12-06-17, 04:34 PM
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Yes, you could put a 32mm and a 35mm tire on the same bike. In fact, it would make a lot of sense to put a larger tire on the back wheel when you're putting a lot of weight on the back.

Also, are you sure you can't use the tube? 3mm isn't that big of a difference and tubes usually have a fairly big range of compatible tires.
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Old 12-06-17, 04:36 PM
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So it sounds like I'm going to have measure the rim to figure this out. Arrrr. Thanks.
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Old 12-06-17, 04:38 PM
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You will be fine. The 35 on the front will make it slightly more precise in steering - a good thing. I run a 35 and a 40 on my Crosstrail, and love it!
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Old 12-06-17, 04:38 PM
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Caloso- I just put the new correct size that arrived on the front and forgot to see what the tube said for its size. I did however order a tube at the same time and it is for 700x 35-38. So I'd only be able to use it for that. These older hybrids seem to have tire sizes that aren't so common.

And I should say that I actually prefer size 38! So going all the way down to 32 would be eww for me, but I don't like wasting either.

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Old 12-06-17, 04:45 PM
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Bead Seat Diameter standard for '700C' is 622mm.. ETRTO set the standards so rims fit tires though made separately,in many parts of the globe.

width of the frame clearance is another thing to measure.
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Old 12-06-17, 05:08 PM
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Originally Posted by travelinhobo View Post
Caloso- I just put the new correct size that arrived on the front and forgot to see what the tube said for its size. I did however order a tube at the same time and it is for 700x 35-38. So I'd only be able to use it for that. These older hybrids seem to have tire sizes that aren't so common.

And I should say that I actually prefer size 38! So going all the way down to 32 would be eww for me, but I don't like wasting either.
The tube size doesn't matter. While not optimal, you can stuff a 35-38mm tube into a 32 mm tire. You can also use a 32mm tube in a 38mm tire or a 23 mm tube in a 38mm tire. The tube expands to fill the space.
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Old 12-06-17, 05:24 PM
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Didn't know that, cycco. I would think if the tube is under sized, it wouldn't blow up well enough to keep the tire in place and rolling correctly.
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Old 12-06-17, 06:07 PM
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Originally Posted by travelinhobo View Post
Didn't know that, cycco. I would think if the tube is under sized, it wouldn't blow up well enough to keep the tire in place and rolling correctly.
You'd be surprised how big they'll blow up. Prove it to yourself by blowing up an unmounted tube.

In fact, I'd much rather use a "too small" tube than try to jam a "too big" tube into a tire. With a tube that's too big, you can accidentally fold it up inside the tire such that it pinches itself and doesn't properly inflate all the way round.
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Old 12-06-17, 06:25 PM
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a) if performance was in error the should fix it

if you are doing loaded touring, my guess is that you don't want any questions...get it right and get correct size, or under tubes....they are not that expensive
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Old 12-06-17, 07:33 PM
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Originally Posted by travelinhobo View Post
Caloso- I just put the new correct size that arrived on the front and forgot to see what the tube said for its size. I did however order a tube at the same time and it is for 700x 35-38. So I'd only be able to use it for that. These older hybrids seem to have tire sizes that aren't so common.

And I should say that I actually prefer size 38! So going all the way down to 32 would be eww for me, but I don't like wasting either.
There's almost zero chance that you can't use a tube intended for 35-38mm tires in a 32mm tire. It'll be a little heavier than it could otherwise be, but for your purposes that's probably no big deal.

I'm currently using some 25-28mm tubes in a pair of 32mm tires. It's been fine.
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Old 12-06-17, 07:56 PM
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Originally Posted by travelinhobo View Post
So it sounds like I'm going to have measure the rim to figure this out. Arrrr. Thanks.
No... it's pretty difficult to measure a rim and come up with an answer that make sense. Bicycle rims are designated by the diameter at the bead seat- that is, the inside diameter of the tire where it contacts the rim. A flexible tape measure (and a little math) might do the trick, but it's a waste of time.

Here's an info page for your bike:
https://knowyourbike.com/specialized/crossroads/1995
It says your bike came with Specialized Nimbus III tires (622mm x 38mm) and Araya PX-35 rims. The 622mm is the bead seat diameter, which means you have "700C" tires and rims.

(Don't get hung up on the 700C designation- it's just a name, like "T-Rex" or "Secretariat". If you want to know the whole history, well, drink some coffee beforehand because it'll put you to sleep otherwise.)

As I said above, those rims will accept 700C tires in a variety of widths.

And back to your original question: tubes are stretchy. Tires are not. Stuff the tube in the tire and be done with it.
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Old 12-07-17, 09:12 AM
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Originally Posted by travelinhobo View Post
Didn't know that, cycco. I would think if the tube is under sized, it wouldn't blow up well enough to keep the tire in place and rolling correctly.
As long as the tube is made of something elastic...i.e. rubber...it's not a problem. As something of an indication of how much tubes can expand, we had someone at my local co-op that blew up a 700c tube out of the tire so that it was about about 6' across.

caloso is correct that a larger sized tube in a smaller tire can be problematic. They tend to get wrinkles in them but I've never found that to be much of a problem. Wrinkles in the tube can make the tire a little lumpy but it won't cause any harm.
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Old 12-07-17, 09:32 AM
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Originally Posted by travelinhobo View Post
Didn't know that, cycco. I would think if the tube is under sized, it wouldn't blow up well enough to keep the tire in place and rolling correctly.
Tubes are like party balloons. They don't contain pressure when you add more air, they're highly stretchy so they get bigger, until they either explode or press up against something that *will* contain pressure without stretching much (like a tire).

And that rubber can stretch quite a lot before it explodes.
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Old 12-07-17, 09:45 AM
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Yes, as others have mentioned there's lots of leeway in changing the width of tires. I have gone anywhere from 21mm up to 38mm on the same rims on my touring bike. And tube sizes really don't matter as long as you're careful during the installation. Tubes nominally marked as 18-21mm have worked fine in the 38mm tires. Fitting wider tubes in narrow tires you need to be careful not to pinch any of the excess width under the tire bead.

For that matter I've even used tubes of the 'wrong' diameter without any issues; i.e. 700c tubes in 26" tires to help out people stranded with a flat on their MTBs. Even used a 700c tube once in a 12" trailer tire to rescue someone who had towed his child up Mt. Diablo and had a flat at the junction. You just have to fold over the excess length when you install it.
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Old 12-07-17, 03:57 PM
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Originally Posted by caloso View Post
You'd be surprised how big they'll blow up. Prove it to yourself by blowing up an unmounted tube.

In fact, I'd much rather use a "too small" tube than try to jam a "too big" tube into a tire. With a tube that's too big, you can accidentally fold it up inside the tire such that it pinches itself and doesn't properly inflate all the way round.
I deliberately use slightly undersized tubes for those reasons. Makes installing a breeze and makes it much easier to prevent pinching it between the rim and tire bead. Plus I save a few grams so I'm way faster.

Of course, I do suspect that I have decreased puncture resistance. I'll live with that.
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Old 12-08-17, 09:01 AM
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Your limiting factor is more likely to be frame/fork clearance than rim dimensions. As noted above, a 700C (622mm bead seat diameter) will mount on any 700C (622mm) rim. A particularly wide tire on a particularly narrow rim will increase hoop stress on the rim, limiting the upper end of inflation pressure. But lower pressures are why we use wider tires in the first place, yes?
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Old 12-09-17, 01:53 PM
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Jeff - thanks for the page. This bike, which I absolutely hate, was bought as a replacement for my best friend when it was stolen in the spring. Didn't buy this bike because I wanted to - it's too large for me, to begin with. So I never looked up the info and didn't think to.

I've put the 700x35 tire on the rear and the 32 tire on the front. Both tubes are good from 28-38 or so, so I'm okay with that. But the tube which I just bought is for 35-38, so we'll see what happens when I need it. THanks for all the help and comments!
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