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Thread: Clincher tyre tubes and pressures

  1. #21
    Join Date
    Sep 2005
    Location
    Sarasota, Florida
    Posts
    3,710

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    I never thought of motocross tubes, Steve. As I stated years ago, I've had a lot of trouble with modern tubes, and mostly on my bicycle. I now run a quality tube from a local bicycle dealer and have had fewer blow-outs. I'll look into the Malcolm Smith tubes.
    Eric Smith
    AMCA #886

  2. #22
    Join Date
    Aug 2013
    Posts
    623

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    Eric, thank you for your reply. i cannot remember where i got the motocross 4 mm tube idea, it was from somewhere, i am thinking this thread or the link on page 1 of this thread "OcchioLungo wordpress." Know that there are no 20" MX tubes available, so i fit a 19 inch tube, rather than a 21" tube, so that there is no bunching of the tube one could expect with a 21 inch tube. i will suggest the heavy 4mm thick tube will give a bit more sidewall inflexibility.....? thoughts anyone?

    before i write further, i feel it necessary to state i am discussing solo riding the Coker 3.85x20 diamond tread clincher tire on a new Al McRoberts rim (and i weigh 180 pounds). (the Rider's Handbook refers to this tire as a 3.85x27 and the tire is also known as a 3.85x20) clearly blowouts are a worrisome concern for us folks who are willing to risk a "like original" accident due to a clincher blowout.......

    and - i am not trying to over-think this topic, i am simply trying to understand what physical forces are in play on the clincher tire in it's under and over-inflated states. clearly my questions/musings may be the same to some as trying to figure out how many angels fit on a pin-head..... i just want to make sure i have done my due diligence by checking tire psi every time before i go out on a ride and what folks feel appropriate psi for solo riding should be. Considering the number of conditions that can contribute to blowout, i cannot help but wonder if clincher's are not getting an unnecessarily bad rap despite their inferior design.

    the reason i am giving this thread a little CPR is because my local buddy is getting ever-closer to the day he'll have his 29JD on the road and he and i have had some back and forth convo's about clincher tire psi, i.e., follow Rider's Handbook settings, follow what other people are saying their experience is, over-inflate vs. under-inflate, so forth, so on. he has been told by a rider of recognition that 20 psi is all that is needed in a 3.85x28 (20 inch rim) and of course the Rider's Handbook says inflate front = 15 psi and rear 20 psi. (i have been running 40 psi in my tubes without incident thus far.) another reason for keeping this thread on life-support is there may be new experiences people can share since we last discussed the topic in 2012-2013.....

    thoroughly reading the OcchioLungo article, it seems the consensus of the information in the article and folks with hundreds if not thousands of miles is a range between 40-45 psi.

    again, what i have noticed absent in these discussions is:

    1. the condition of the rim in which blowouts occurred ?
    2. what psi guys were running that had these blowouts ?
    3. what type of tubes were fitted in these cases of blow outs ?
    4. what is the condition of the bead in which blowouts occurred?

    Clearly, any one of these factors are critical and could cause a blowout, and any combination of these factors one could assume would even further increase the risk for blowout.

    can ANYONE think of a good reason why the factory would publish 15/20 psi for the 3.85x20 tire ?? and why we are saying inflate to 40-45 psi ??

    it seems reading this thread, there are at least four conditions that can contribute to a clincher blowout....
    1. the tire rolls of the rim due to under-inflation....correct ??
    2. the bead gets "cut" and simply pops off rim from under-inflation....correct ?? and..... WHAT DOES CUT MEAN ?? the bead actually is severed from the body of the tire...?? Would this "cut" be caused by a sharp bead edge of an old original rim...? or by any rim due to under-inflation....???
    3. over-inflation, the bead literally blows off the bead....??
    4. the valve stem severs due to under-inflation because the tire actually "walks" on the rim and as a result the stem shears at the point where the stem comes through the hoe in the rim...???

    it seems obvious, in the case of under-inflation, the tire could literally roll off the bead, especially in corners. and i guess if the tire is under-inflated, as the tire is walking, especially if the bead is sharp, then the bead will get "cut" ?? in the cases where the tire blew off the bead of a parked motorcycle, what was the psi? What was the rim condition? Was the tire in hot sun and psi increased?
    Last edited by Steve Swan; 07-01-2018 at 12:23 PM.

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