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  1. #1
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    Is 70psi enough?

    We are a heavy (360 lbs) tandem team. After the latest in a series of tire blowoffs, I bought a Bontager Select K (700x28) from the repair van that was assisting the ride we were on. I pumped it up to 100 psi and we finished the ride without further problems. Later I noticed that the recommended pressure on the tire is 70 psi. 70 seems so low that I'm afraid we'll get pinch flats. Will it hold us at 70? At 100, I'm afraid we'll have another blowoff. Any experience with this tire, and if so at what pressure have you been running it?

    Thanks,

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    Senior Member RdRunner's Avatar
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    I know you will get answers from people with a lot more experience and knowledge than me, but I have to say that 70 seems too low to me. I would agree with you that pinch flats would become a very real possibility. But, I don't know anything about the Bontager Select K, so I reserve the right to be wrong.
    My stoker ain't no slacker

  3. #3
    Velolutionary IowaParamedic's Avatar
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    I bought that tire too. It seemed low to me, but I put 70 psi in. We had a pinch flat about 10 miles in. We pumped back up to 70 psi, but it seemed very low. We made it home.

    We were running 700x26 tires. I am not sure if I am going to continue using those tires, but they are working if I pump them to 80-85 psi.

    I thought blowoffs would happen more if the tire was over inflated or the rims were heated by the brakes.
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  4. #4
    hors category TandemGeek's Avatar
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    No experience with Bontrager tires, but I did sleep at a Holiday Inn Express last weekend so here goes...

    Short Answer: 20 - 30 psi above the rated tire pressure "shouldn't" be a major concern on a 700c tire -- even the Bontrager.

    -------------Long-winded, superflous commentary -----------

    In my opinion, tandem teams should run tires anywhere from 5 to as much as 30 psi above sidewall ratings (or what you would normally use for the same size tire on your 1/2 bike) to compensate for having 70-100% more weight than the average 1/2 bike cyclist that most tires are designed for. As for max. pressure ratings, they tend to be very conservative, something on the order of 1/2 of what it would actually take to blow that particular tire off a rim -- assuming the rim is able to handle that much pressure. In fact, old rims with worn sidewall/brake surfaces and rims that are too narrow for wide tires probably pose a bigger risk for a wheel failure due to an over-inflated tire than the tire's rim bead coming unseated on a rim of the correct width.

    We weigh about 280lbs (127kg) as a team and run either 25mm tires rated for 135 psi that I inflate to 140psi or 23mm tires rated for 145psi that I inflate to 145 - 150psi. If we were to run 28mm tires I'd have to get wider rims and, even then, I'd run the tires at no less than 115psi, up and to or above their recommended rating as necessary to get the right "feel" and "shape" of the tire(s) with our weight on the bike.

    Assuming you have a tire that's the right width for your rim, the general rule of thumb on finding the right tire pressure for road and touring bikes is that there should be a slight bulge in the tire when everyone's on-board the bike. If there is no bulge when rider weight is put on the bike, the tire(s) is/are over-inflated and you'll end up with a harsh ride and are more likely to get a puncture from road debris than you would on a properly inflated tire. If the tire bulges equal to or greater than twice it's normal width, the tire(s) is/are under-inflated which will make the bike feel "mushy" and, as you noted, be very susceptible to pinch flats.

    Yes, I run our tires a bit over inflated but I like how the tandem feels through the corners on "hard" tires and try to be good about not riding through debris on the road. Of course, we're spoiled rotten by having exceptionally good roads here in Georgia which is probably why I get away with so few punctures (knock on wood).
    Last edited by TandemGeek; 01-30-05 at 08:50 PM.

  5. #5
    hors category TandemGeek's Avatar
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    Originally posted by IowaParamedic
    I thought blowoffs would happen more if the tire was over inflated or the rims were heated by the brakes.
    That is a common belief, but the real culprit seems to have more to do with the affects of heat on the tire or tubes, not expansion of the air in the inner tube.

    Without getting too verbose, with hook bead rims and tires (aka. clinchers) the inner tube is always pushing outward against the tire and rim which is what keeps the tire on the rim. As a rim is heated by the bike's rim brakes on a long descent, the heat "sinks" into the tire and tube which are also being heated by road friction. As the tire continues to heat bead strip adhesives can begin to lose their grip and the tire casing softens, either one of which can eventually allow the heated inner tube to "peek out" from under the tire bead or perhaps even through a widening cut in the tire casing and then kablamo... Yes, heat may cause a small amount of expansion in the inner tube, but not enough to independently cause a tire to come unseated from a rim.

    Personally, I'd be more concerned about old tires, tires that don't seat tightly against the rim, and/or under inflated tires being more likely to fail vs. a good tire that is perhaps a bit over-inflated.

    Just my .02.

    Mark

  6. #6
    Velolutionary IowaParamedic's Avatar
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    I found something interesting on Road Bike Rider http://www.roadbikerider.com

    Tandem Tire Inflation

    DEAR UNCLE AL: I always enjoy reading your comments and have learned quite a bit, but Iíve been most impressed with the dramatic increase in tire life Iíve experienced since taking your advice on not inflating to maximum pressure.

    So I decided to try the same thing on our tandem. I went against the conventional twofer wisdom that says to overinflate, and instead run about 10 psi below the recommended max pressure. The result? As with our singles, we are seeing longer tire life and a more comfortable, responsive ride. Iím also not seeing any of the sidewall cracks that used to plague us.

    Wear, blowouts and related tire issues seem to be a hot topic on tandem websites, and I now wonder if at least part of the problem is that tires are often being run at 10-15 psi above recommended max.

    Are we just lucky since we quit our old-school inflation habits, or do you think your inflation recommendations are also valid for tandems despite their additional weight? -- Ray M.

    UNCLE AL FIRES BACK: Sure, Ray, lower pressure applies to tandems as well. You're proving it every time you take yours out.

    I know a couple of RBR subscribers who weigh as much as some tandem pairs. I recommended that they run bigger tires, such as 700x28C or 32C, and run them at lower than max pressure. They report the same good results youíve had.

    If you're running 23s or 25s on your tandem, you need to run them pretty hard so you donít pinch flat with the greater weight. You are definitely on the right track, and now experimentation will prove to you what works best.

    Blowouts on tandems are a nightmare. A main cause is running too much pressure, then overheating the rims from braking or hitting something sharp. Running lower pressure will certainly reduce the chance for such problems.
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    Visit Bicyclists of Iowa City -- Ride AHCAST on Sept 18 & 19

  7. #7
    SDS
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    With a 340 lb team, I won't use a tire smaller than 700 X 32 at 120 psi for general use. Rarely I will use a 700 X 28 for events where I know the road surface will be good. In North Texas, on the narrow farm roads we use, narrower tires don't seem to be sufficiently resistant to pinch flats. My stoker is opposed to flats, so I use wide tires because the volume of high-pressure air seems to better resist pinch flats.

    I had trouble on one occasion because of a bad gauge on a floor pump that was reading high. I got a pinch flat crossing some railroad tracks. I was later able to figure out how much the pump gauge was reading high by comparing it with a good gauge. I had had 100 psi in the tires.

    Which leaves the matter of tires and the margin of safety that is acceptable. If you are uncomfortable with a tire, you should can it. If you captain and do the maintenance, your stokers are completely dependent on you to keep them safe. One bad accident that could have been prevented could cause them to lose interest in getting on a tandem with you.

    I can't find a tire that I can wholeheartedly recommend. In recent years I have only used:

    1. Avocet Duro 20Ks (?) with Kevlar belt. These are wired-on slicks that seem to be fairly sticky. I eventually decided that they were unsatisfactory because of the loose-fitting wired-on tire bead and abrasion failure of the sidewalls.

    2. Panaracer Paselas, wired-on. Don't know if they had Kevlar belt or not. One had a sidewall failure when I got that pinch flat with the low tire pressure crossing the railroad tracks mentioned above. The loose, wired-on tire came completely off the rim before we stopped upright, both beads off on one side of the rim. The other delaminated the tread off the carcass. I considered this to be poor job performance and fired them.

    Both of the above tires were used in 700 X32, though a few times I also used Avocets in 700 X 28.

    3. Panaracer T-Serv for Messenger. Folder, Kevlar belt, size 700 X32. I am hoping that the black rubber sidewalls will have satisfactory abrasion resistance, and that the tighter-fitting Kevlar folding bead will provide better post-flat performance. They seem to provide slightly less traction than the Avocets. So far they seem to be entirely satisfactory after one year of use.

    It could be that all of the above tires pop out of the same molds in a tire factory in SEA. Certainly the Duros and the Paselas seem to have a close relationship. I know that. For me there seem to be sufficient differences so that they are not interchangeable.

    Use enough volume and pressure to reliably support your load.

    If we were just a little bit lighter or if the roads were better I might use Continental Gatorskins, but the last time I checked they were not available in 700 X 32.

    100-mile club ride on tandem starts today at 0645. See you there...

  8. #8
    hors category TandemGeek's Avatar
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    Uncle Al seems to contradict himself in his own response (e.g., under inflate is good, but if you're running 23 or 25mm tires run them hard), but his general advice and Scott's regarding "go bigger" is spot on for most tandem teams. Sub 300lb teams can get away with narrow tires; those over 300lbs usually can't. The bigger and heavier the load, the wider the tire. Again, I would defer to the "general rule of thumb" for tire inflation except on narrow racing tires (19-25mm):

    Proper Inflation: There should be a slight bulge in the tire when everyone's on-board the bike.

    Over Inflation: If there is no bulge when rider weight is put on the bike, the tire(s) is/are over-inflated and you'll end up with a harsh ride and are more likely to get a puncture from road debris than you would on a properly inflated tire.

    Under Inflation: If the tire bulges equal to or greater than twice it's normal width, the tire(s) is/are under-inflated which will make the bike feel "mushy" and will be susceptible to pinch flats.

    If you can achieve the slight bulge with a 700x28 or larger diameter tire at 10 psi below the recommended max pressure for the tire, then that's the right tire pressure. If you can't, put in more air until you get the right shape. As Uncle Al notes in his closing sentence, experiment until you figure out what works best for you -- and that may have been the tire you had to replace at given pressure. So long as you are happy with the performance you get from your tire selection and the amount of air you put in to it, what other folks do or don't do may not matter a hill of beans so long as you know why you made your choices on tires and pressure.

    As you can see, Scott likes tires with larger volume for his needs whereas we're more content with nothing bigger than 25mm. I pay a price for my rock-hard racing tires in terms of comfort (it can be tough when we encounter "shake and bake" roads), reduced tire life (2k - 2.5k at best if I run them on the front for the first 1k - 1.5k miles before condeming them to the rear), and of course cost since the go-fast tires tend to cost more and wear out their softer tread compounds must faster than touring tires. But, at least for now, that's what 'works for me'. Who knows, we may one day end up on larger volume tires... well, at least in back. Our carbon forks will only accommodate a 25mm tire.

    As for tire blow-offs, see my comments above. I put the overinflated tire blow-off into the same category as concerns over putting inflated bicycle tires into an aircraft's cargo hold... cycling lore and mythology. The only thing we do know about tire blow-offs is that they occur when a tire isn't properly seated or, for what ever reason, comes unseated after being subjected to severe overheating. There are other posts on this subject in the archives.
    Last edited by TandemGeek; 01-30-05 at 08:51 PM.

  9. #9
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    Thanks for all the info. I think I'm going to mix the suggestions here. I'll start with the 80-85 psi range that Iowa recommends then adjust from there until I get the slight bulge that Mark recommends. Since most of our rides for the rest of this year will be of the around the neighborhood variety, it should be a good time to experiment.

    The blowoff issue has haunted us for some time. We've never riden in areas with enough hills to cause overheating from the brakes, but all of the occurances have been on days with long mileages in very hot (90+) weather, or on the day after such a ride. Its happened with both new and old tires. I've started to wonder if there is a problem with the rim (its always the back tire), but I can't see anything wrong with it.
    The moral arc of the universe is long, but it bends towards justice. M.L.King

  10. #10
    hors category TandemGeek's Avatar
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    What kind of tires have you been using and what rim?

  11. #11
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    Its a Bontager Clyde rim (original equipment on a 96-97? Trek T200). Have had blowoffs with a Specialized Armadilo (700x25) with 200 miles on it, a Conti something or other el-cheapo (700x28) with 800 miles, and a Gatorskin (700x25) with 500 miles.
    The moral arc of the universe is long, but it bends towards justice. M.L.King

  12. #12
    hors category TandemGeek's Avatar
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    Given that you are having trouble with different brands of tires and based on what you've described, I would have to say the rim is certainly suspect. A new rim laced up to your current hub would be a relatively good investment (should be less than $100 for rim, spokes and labor) if only to eliminate the rim as a variable.

    Mark

  13. #13
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    Thanks for the advise Mark. You're right, if I can get it done for $100 its worth it. I hate flats!
    The moral arc of the universe is long, but it bends towards justice. M.L.King

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