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dumb tire/tube question

Mark Regier

Mark Regier

Husqvarna
AA Class
So on my recent ride of the White Rim trail in Canyonlands, I flatted my rear tire. I only had a 21 inch front tube as a spare, so I put it in the 18 inch rear tire. The rear tire only has a few hundred more miles before it gets a new Shinko, so I'm wondering... (and being lazy)... am I crazy to just leave it in there for the next few trips down the highway?

I know what OSHA would say, and I know what the conventional wisdom would suggest with moto tires: change it. But I'm asking for more than speculative opinions on this one. Someone with some experience in the area, please share.

Thanks in advance
 
for slab speeds no way in hell would i leave it in there. too much heat generated, too much risk of a high-speed off when it blows, too much death involved.

for trail rides I'd pack another tube, and be prepared to change it when/if it lets go.
 
Nordland said:
90/90x21 inch tire on a 3x18 inch rim? good joke :cheers:

Please show me a foto of it :D
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I dont have any photos but we do it all the time. front tubes are lighter to carry , and work well (in a pinch) for both front and rears.
 
Sceep said:
for slab speeds no way in hell would i leave it in there. too much heat generated, too much risk of a high-speed off when it blows, too much death involved.
Click to expand...


I'm curious. If a 21" tube in a 21" tyre at 80MPH is safe, how much less safe would the 21" tyre be in the 18" tyre?
I'm seeing the rubber stretched further inside the tyre, which will make it thinner, which might make it more susceptible to a puncture, but how does the heat play a part. The rear wheel will run hotter than the front, just because it has less cooling because it's in the wake of the front wheel, but under normal circumstances is a rear more susceptible to a spontaneous heat related failure than a front ? Wouldn't the extra 3 inches of rubber create a greater surface area for dissipating heat? I'm not trying to be argumentative. I'm interested in understanding the physics of the forces at play in this case.

I only carry a 21" spare when I go riding (I've never used it), and I rarely go over 100kph/60MPH. I find on the Terra that if I don't look at the dials and just ride where it feels and sounds right, and without too much wind annoyance, about 95kph on the Terra is the sweet spot for me. I know when you guys in the USA talk about slab speeds you're talking about 70-85MPH, and having myself ridden down a section of I-70 through Utah where the limit is 80MPH and most traffic is doing 90MPH, I was travelling slowly because my wife's bike could only do about 70MPH, and 20MPH below the traffic speed on those roads is scary (especially when you're from a country where you're accustomed to vehicles overtaking on the right hand side - I shit bricks when trucks doing 90 passed on my left).
 
nev.. said:
I'm curious. If a 21" tube in a 21" tyre at 80MPH is safe, how much less safe would the 21" tyre be in the 18" tyre?
I'm seeing the rubber stretched further inside the tyre, which will make it thinner, which might make it more susceptible to a puncture, but how does the heat play a part. The rear wheel will run hotter than the front, just because it has less cooling because it's in the wake of the front wheel, but under normal circumstances is a rear more susceptible to a spontaneous heat related failure than a front ? Wouldn't the extra 3 inches of rubber create a greater surface area for dissipating heat? I'm not trying to be argumentative. I'm interested in understanding the physics of the forces at play in this case.
Click to expand...


you thought way more into it than i did, and you could be right on.

yes the rubber would stretch further cross-sectionally in the wider rear tire, but it cant shrink the overall circumference of the ID and OD. it will have wrinkles and folds that will rub on each other creating heat and wear.

at least, thats how i envision whats going on in there.

someone make us a transparanet tire representative prototype so we can shove tubes in it and see whats going on!
 
I guess I would think in the other direction in that the rear tire only has a few hundred miles of life left, go ahead and replace it now with a new tire, correct tube and have piece of mind when going down the road. Not worth it to me to squeeze a couple hundred more miles out of a tire.
 
Wouldnt an 18 tube in a 21 tyre be more effective as rubber streaches it wont have any wrinkles on the part touching the rim.

Off on a tangent here has anyone sucsessfully used that tyre weld stuff on an innertube?
You know the rubberised black stuff that also inflates your tyre.
 
juicypips said:
Wouldnt an 18 tube in a 21 tyre be more effective as rubber streaches it wont have any wrinkles on the part touching the rim.
Click to expand...

It would be interesting to see through a transparent rim and tyre to see exactly what the situation inside was like. I can see how an un or under inflated tyre might wrinkle and generate friction heat where the tube wasn't a single layer fit within the tyre/tube. I wonder, if the tyre was inflated to normal road running pressure of 30-35PSI, whether there would be any friction at all given the internal air pressure. Any heat issue would be generated on the tyre side of the tube, not the rim side, and the tyre side, although a couple of inches longer than the internal diameter of the tyre, would have much more lateral room to expand once it was inflated.
 
I have ripped off a valve stem at 45 mph and done some asphalt surfing. It's not something I want to repeat. I'd change that tube back out ASAP.
 
Thanks all for your thoughts. FWIW, I've been commuting on the highway at 80 mph and put on 270 miles since the 21" tube went into the 18" rear. I'm pretty conscientious about tire heat, and so I often put a bare hand on both front and rear after a trip just to check. I haven't felt any difference in heat, but the hand is a crude instrument for measuring temperature (unless you're a mother doing a fore-head check, apparently). While I recognize the possible dangers, I think they're remote. I'll keep on keeping on, and after a few more trips I'll load another Shinko and post pics of the tube/inside of tire-- just for grins.
 
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