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Old 07-29-14, 04:17 PM   #1
flan48
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Simple Question Regarding Tube Size

Hello all,
I have 700-32 tires on my Trek 7.4FX. The other day I noticed a very slow leak in the rear tire, so I took it off, checked all over and found nothing, and ultimately replaced the tube.

When removing the "problem" tube I saw that it is a 700/35-40. This had been put in by my LBS a couple or 3 months ago. In general, is this so-called incorrect tube problematical? Being wider, could the 80 pounds of pressure I put in have caused a minor valve leak?

Thanks and best regards
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Old 07-29-14, 04:20 PM   #2
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No. Look elsewhere for the cause.
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Old 07-29-14, 04:38 PM   #3
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Agreed. The slightly wider tube might make is a bit harder to install in the tire without having it get pinched under the tire bead but it shouldn't cause problems once properly installed. Not uncommon for something (e.g. tiny glass shard or thorn) to cause a puncture and then work it's way back out so there's no clearly visible evidence left in the tire.
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Old 07-29-14, 05:07 PM   #4
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Tube nominal sizing has little to do with actual sizing. Sure, the nominal size defines the ballpark, but that's about as close as it gets.

What determines the smallest tire a tube will fit well is the actual unstretched diameter which has to be smaller than the inside diameter of the tire to make mounting easy. One easy way to know if a tube will fit well when buying is to compare the flat width to the tire diameter. The unstretched tube will have a diameter equal to about 2/3rds the flat width, so you want that to be less than 1-1/2 times the tire width. Err slightly low to allow for the tire wall thickness.

It's very safe to go smaller since the tube can easily stretch to twice it's size, but going higher makes it hard to stuff it in when mounting.
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Old 07-29-14, 08:33 PM   #5
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Originally Posted by prathmann View Post
Agreed. The slightly wider tube might make is a bit harder to install in the tire without having it get pinched under the tire bead but it shouldn't cause problems once properly installed. Not uncommon for something (e.g. tiny glass shard or thorn) to cause a puncture and then work it's way back out so there's no clearly visible evidence left in the tire.
Yes, this makes sense - thanks very much, and thanks to all for your responses.
Best regards
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Old 07-29-14, 09:13 PM   #6
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Originally Posted by flan48 View Post
Hello all,
I have 700-32 tires on my Trek 7.4FX. The other day I noticed a very slow leak in the rear tire, so I took it off, checked all over and found nothing, and ultimately replaced the tube.
Sticking it in a sink full of water usually reveals the hole. Then a simple patch job sets it right.
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Old 07-30-14, 08:08 AM   #7
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Sticking it in a sink full of water usually reveals the hole. Then a simple patch job sets it right.
Good morning,
Yes, I haven't yet tried that.
Thanks!
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Old 07-31-14, 09:59 AM   #8
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Update: This morning I pumped up the tube and immersed it in a tub of water, valve stem down first. Right away I could see slow and very small air bubbles coming about 3/8 of an inch from the valve's base. I dried the tube thoroughly, patched the tube, re-inflated it somewhat and re-immersed it. The patched spot is fine, but now I see some air escaping from just below the valve head!
I'll try pinching the head with pliers, but at this juncture I'll simply buy a new tube!

Best regards
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