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Old 10-09-06, 10:15 PM   #1
TallRider
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does it matter that road tires get mounted true?

So we all know about trueing wheels, and making sure that rims are true and round. But what about tires? Specifically, narrow road tires.

Quite frequently, in mounting a tire on the rim, the tire fits rather snugly and it takes some pushing to get it all over the rim, and then the tire is unevently tensioned around the rim. So (before inflating the tire) when you spin the wheel, you can see the tire is out of round even if the rim is perfectly in round.
Often, even after inflating the tire, the tire is higher off the rim at some points than it is at others.
Sometimes the tire will spread out and be distributed more evenly on the rim after deflating the reinflating the tire, but not always. Sometimes you can tug on the tire to get it more evenly spread around the rim, but not always.
And worst: sometimes you can feel the unevenness or "hop" in the tire while riding. So the effects are similar to a hop in the rim that's bent out-of-round, although this matters less when the tire is inflated at lower pressure.

Does anyone have comments on this problem, especially methods for getting a tire evenly mounted on the rim? It's something I've not thought carefully about before, and wanted to raise it here in the forum.
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Old 10-09-06, 10:22 PM   #2
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Your tires should always run true. It is critical for safety to have the bead up in the lip of the rim. If the tire is too low in one spot, it can more easily blow off the rim.

Best practice is to always inflate tires that have no air in them by laying it flat on the floor. Then the air pressure can act equally on all sides.

Nice tires tend to seat themselves pretty well. Are these dried out discount "blow-outs"? No pun intended.
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Old 10-09-06, 10:28 PM   #3
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Usually you can pump'em up partway and then work the tire with your thumbs where the bead is too far below the rim. Some stupidly tight...let's call them small...tire beads won't respond to that treatment. Deflate...lubricate bead/rim interface with talcum powder, soapy water, whatever, and then inflate fully. Even then they may not seat perfectly until you ride on them for a while. On the bright side, you can feel very confident that this is a tire that is not going to blow off the rim!
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Old 10-09-06, 10:34 PM   #4
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I agree with you completely. We true wheels to within 1mm and all is fine. But we don't account for variation is tire manfacture and mounting. Tires are a composite of a fabric casing and wear surface (tread). I doubt the manufacture and installation of a tire is as exact as the trueness of a rim/spoke/hub. And once you throw in the variability of $3 tubes, meeting a 1mm overall tolerance cannot be guaranteed.

A true rim has a big effect on braking performance (assuming rim brakes).

An analogy would be the fallacy of improving my performance with light-weight components, titanium fasteners, drilling out and gram counting, but completely ignoring the fact that I am 15 lbs overweight.
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Old 10-09-06, 10:35 PM   #5
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Originally Posted by BikeWise1
Your tires should always run true. It is critical for safety to have the bead up in the lip of the rim. If the tire is too low in one spot, it can more easily blow off the rim.
Best practice is to always inflate tires that have no air in them by laying it flat on the floor. Then the air pressure can act equally on all sides.
Nice tires tend to seat themselves pretty well. Are these dried out discount "blow-outs"? No pun intended.
Current ones are nice Serfas 27x1.25 tires on single-wall 27" Araya rims on my fixed-gear bike. Tires seem slightly too tight fit over the rim, which means there's not enough slack for them to "even out" once they're over the rim. But as Starclimber notes, a too-tight tire is also a tire that's not likely to blow off the rim.
And yeah, I do inflate just-mounted tires with the wheel sitting on its side so the ground isn't pressing one part of the tire closer to the rim during inflation. Should have mentioned that in my OP.
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Old 10-09-06, 10:48 PM   #6
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Weird phenomenon you state. I've noted badly mounted tires on my 27" rims, but they've always been real cheapos. My nicer tires (i.e. nashbar housebrands, Continentals, Panaracers) seemed to seat fine. I've never used Serfas, though. Maybe they're just manufactured with a slightly small diameter (maybe a mm or two smaller than others)? Or is it possible your wheels are a little out of round? Wouldn't this create a bit of added tension on one part of a wheel and tire?
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Old 10-09-06, 10:49 PM   #7
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Quote:
Originally Posted by timcupery
Current ones are nice Serfas 27x1.25 tires on single-wall 27" Araya rims on my fixed-gear bike. Tires seem slightly too tight fit over the rim, which means there's not enough slack for them to "even out" once they're over the rim. But as Starclimber notes, a too-tight tire is also a tire that's not likely to blow off the rim.
And yeah, I do inflate just-mounted tires with the wheel sitting on its side so the ground isn't pressing one part of the tire closer to the rim during inflation. Should have mentioned that in my OP.
As a mechanic, I'd never let one go out the door that way. Pump it up to 150 or so and see if it seats. Chances are if it seats that way it'll stay seated after you let it back down to riding pressure. Earplugs are a plus.

If it still wouldn't seat, I'd be looking for a better rim/tire match.
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Old 10-09-06, 10:49 PM   #8
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A tire mounted correctly should have no noticeable hop in it. If it does, the lazy me usually pumps it up a bit more and it "seats" itself fine. Deflating and carefully trying to mount it again also doesn't seem to do the trick.

Higher pressure must be used.

This particular issue baffled me once while remounting the tire after a flat fix. The hop was very noticeable when riding.
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Old 10-09-06, 10:53 PM   #9
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Originally Posted by operator
A tire mounted correctly should have no noticeable hop in it. If it does, the lazy me usually pumps it up a bit more and it "seats" itself fine. Deflating and carefully trying to mount it again also doesn't seem to do the trick.

Higher pressure must be used.

This particular issue baffled me once while remounting the tire after a flat fix. The hop was very noticeable when riding.
Yeah, it's uncommon but when you get one of those, hugely annoying. BOUNCE BOUNCE BOUNCE...
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Old 10-10-06, 05:33 AM   #10
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Originally Posted by BikeWise1
As a mechanic, I'd never let one go out the door that way. Pump it up to 150 or so and see if it seats. Chances are if it seats that way it'll stay seated after you let it back down to riding pressure. Earplugs are a plus.
If it still wouldn't seat, I'd be looking for a better rim/tire match.
As I said, I'm not worried about this popping off. It's not seating properly because it's too tight - part where the bead is seated too low on the rim is not counterbalanced by part where the bead is seated to high and in danger of popping off.
I've pumped it up to 100 (the recommended max pressure) and it doesn't change how it's seated.
My guess is it may be a QC issue with this tire manufactured just a little too small.
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Old 10-10-06, 07:03 AM   #11
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It's not seating properly because it's too tight - part where the bead is seated too low on the rim is not counterbalanced by part where the bead is seated to high and in danger of popping off.
If that is really the case, i'd put on a different tire. Personally I can't stand anything that would create such a bouncy effect as a non true tire. Dare I say it might even be dangerous on high speed descents?
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Old 10-10-06, 07:07 AM   #12
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Originally Posted by operator
If that is really the case, i'd put on a different tire. Personally I can't stand anything that would create such a bouncy effect as a non true tire. Dare I say it might even be dangerous on high speed descents?
Yeah, I'm going back to a different set of (mismatched) tires that both seat properly and easily. I may try the problem tires again once I get some talcum powder, see if I can get them to seat properly.

The problem seems to initiate because the tires fit tightly around the rim. I really need to push to get them over, especially the last part of the tire that I force over the rim's edge. Once I've done this, the tire is seated too low on the rim on the opposite side of the rim from where I forced the last bit of the tire over.
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Old 10-10-06, 10:17 AM   #13
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Originally Posted by operator
A tire mounted correctly should have no noticeable hop in it.
+1
There is no good reason for an out-of-round tire but it does happen. From my experiences tires with a noticeable hump are a sign of a manufacturing defect, damage to the casing, improper mounting, or often a sign that a small part of the tube is caught between the rim and the tire. With high pressure road tires if any part of the tube is between the tire and any part of the rim not only will you notice a hump, but you are about to experience SARD - Simultaneous Automatic Rapid Deflation.

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Old 10-10-06, 11:09 AM   #14
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Originally Posted by timcupery
Quite frequently, in mounting a tire on the rim, the tire fits rather snugly and it takes some pushing to get it all over the rim, and then the tire is unevenly tensioned around the rim. So (before inflating the tire) when you spin the wheel, you can see the tire is out of round even if the rim is perfectly in round.
I reject your premise. I see no basis for your assumption that a tire is "unevenly tensioned". An uninflated tire has to tension on it. It may sit unevenly on the rim because the bead is not yet seated.

However, tires are not perfectly balanced or true. But, unless the tire is defective, it's negligible.
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Old 10-10-06, 11:34 AM   #15
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I reject your premise. I see no basis for your assumption that a tire is "unevenly tensioned". An uninflated tire has to tension on it. It may sit unevenly on the rim because the bead is not yet seated.
However, tires are not perfectly balanced or true. But, unless the tire is defective, it's negligible.
You're actually wrong here, at least in reference to my case (which I've observed a number of times over the years). The uninflated tire, when mounted on the rim, has a spot that sits lower in the rim than the rest of the tire, and this spot depends on what's the last part of the tire I push over the rim.
My sense of it is that the tire is tight enough against the rim, with enough friction (no talcum powder yet) that limits it from spreading out evenly around the rim.

I took these (basically new) tires off a wheelset that came with a bike I bought on eBay. I mainly just wanted matching tires on my newly-built-up fixed-gear bike. They mounted correctly enough on the wheels I took them off of. I'm going to try reinstalling them on other 27" rims and see if they show the same mounting behavior.
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Old 10-10-06, 09:12 PM   #16
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Update: by sprinkling talcum powder on the inside of the tire and also along the rim strip (and subsequently a bit gets on the rim's bead), I was able to get the tire mounted cleanly, a constant distancce from the rim and spins with no hops prior to and after inflation.
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Old 10-10-06, 11:09 PM   #17
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Originally Posted by timcupery
You're actually wrong here, at least in reference to my case (which I've observed a number of times over the years). The uninflated tire, when mounted on the rim, has a spot that sits lower in the rim than the rest of the tire, and this spot depends on what's the last part of the tire I push over the rim.
My sense of it is that the tire is tight enough against the rim, with enough friction (no talcum powder yet) that limits it from spreading out evenly around the rim.

I took these (basically new) tires off a wheelset that came with a bike I bought on eBay. I mainly just wanted matching tires on my newly-built-up fixed-gear bike. They mounted correctly enough on the wheels I took them off of. I'm going to try reinstalling them on other 27" rims and see if they show the same mounting behavior.
If the problem is that the tire doesn't seat onto the rim when inflated, then you simply need to add more air pressure until it pops into place. I see that you found the use of talcum useful. A little soapy water also helps. The problem isn't one of uneven tension, it's just that the tire bead is a little bit small and needs some encouragement. Continental tires often have this problem, in my experience.

Sheldon Brown addressed this here a few months ago. As he said, if the tire doesn't pop into place, you haven't added enough air.
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Old 10-10-06, 11:47 PM   #18
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Originally Posted by supcom
I reject your premise. I see no basis for your assumption that a tire is "unevenly tensioned". An uninflated tire has to tension on it. It may sit unevenly on the rim because the bead is not yet seated.
Quote:
Originally Posted by supcom
If the problem is that the tire doesn't seat onto the rim when inflated, then you simply need to add more air pressure until it pops into place. I see that you found the use of talcum useful. A little soapy water also helps. The problem isn't one of uneven tension, it's just that the tire bead is a little bit small and needs some encouragement. Continental tires often have this problem, in my experience.
Sheldon Brown addressed this here a few months ago. As he said, if the tire doesn't pop into place, you haven't added enough air.
I do think that the problem was one of uneven tension (and talcum powder allows the tire to slide a bit more around the rim to equalize the tension of the bead; soapy water would help too). If it were just that the bead were small (which is a necessary component of this problem) then there'd be no reason why there would be a low spot where the tire is seated lower on the rim; the tightness and too-closeness-to-the-rim would be distributed equally through the whole tire.

But you're right about the upshot of the problem (whether "unequal tension" or not) is that the bead is not properly seated. And on the front tire - which had given me lots of trouble even with talcum powder, after I'd gotten the back tire seated properly on the rim... well, the front tire popped into place when I got the pressure up to 100. (100 is the official max for the tire; I usually run 27 x 1.25 tires at 80.)
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