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  1. #1
    Senior Member GeorgeBMac's Avatar
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    Wheels and tires question

    Sorry if this has been covered before (and I'm sure it has) but,

    I changed tires on my 2012 Trek DS 8.5 for the first time yesterday and measured the inside rim width. The inside width of the rim is 16mm and the tires the bike came with are 700x38 -- which seems like a mis-match to me.

    When I bought the bike, I tried to get information on the Bontrager rims it came with but this model was not listed anywhere on their website -- which is why I had to resort to measuring with a tape measure. (Admittedly my measurement might be off -- it wouldn't be the first time).

    But, according to everything I have seen or read, the width of the tire should be no more than 2x the inside width of the rim. I assumed that the tires the bike came with would be somewhere in the middle of what these rims should be able to safely handle. But, according to my measurement they are significantly wider.
    ... So, I am wondering: Did Trek screw up?

    For me it is a moot question: with my new tires I dropped down to 700x35 -- which is just a little above the limit for the rims as I understand it and I am thinking I can push it that far. But Trek could have put ANY size rim and ANY size tire on there -- so it puzzles me that they would use a rim that was too small for the tire...

    But part of the Trek advertising for the bike bragged that the bike had lots of wheel clearance and even implied it could run a 29're tire.

    On the other hand, they also said it was "fender ready" -- but I had to replace the front derailer to get clearance for a rear fender...

    ... Am I missing something here? Or is Trek just not putting enough thought into the components they are combining?
    --------------------------------------
    bikes: 1992 Cannondale R500, 2012 Trek DS 8.5, 2008 LeMond Poprad

  2. #2
    Senior Member Mobile 155's Avatar
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    I don't think they put a 18mm rim on your bike. Tell us what is says on the rim? Is it a bontrager rim? The spec sheet shows it has AT-650 rims and they are wider than 18mm. http://www.trekbikes.com/us/en/bikes..._series/8_5_ds

    They use the same rim on tires up to 2.2 inches.
    Life is like riding a bicycle - in order to keep your balance, you must keep moving. ~Albert Einstein.

  3. #3
    Senior Member GeorgeBMac's Avatar
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    Quote Originally Posted by Mobile 155 View Post
    I don't think they put a 18mm rim on your bike. Tell us what is says on the rim? Is it a bontrager rim? The spec sheet shows it has AT-650 rims and they are wider than 18mm. http://www.trekbikes.com/us/en/bikes..._series/8_5_ds

    They use the same rim on tires up to 2.2 inches.
    Thanks - but that's the 2013 catalog. On my 2012 the specs only said: "Bontrager Nebulla Disk, 32 hole rims". And that is how the rims are labeled: with a big "Nebulla" printed on them.

    But, when I went down to double check, I noticed they also had some (very) small print next to the "Nebulla". It was:
    "made in China 622x15'

    So I guess my 16 mm measurement was overly generous.

    Now, I'm even wondering if the 700x35's I put on are too big for those rims -- much less the original 700x38s (which I planned to put back on in the summertime)?
    --------------------------------------
    bikes: 1992 Cannondale R500, 2012 Trek DS 8.5, 2008 LeMond Poprad

  4. #4
    Senior Member Bill Kapaun's Avatar
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    I don't see any problem.
    Do you ACTUALLY SEE one?

  5. #5
    tougher than a boiled owl droy45's Avatar
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    I think you will be fine. The (2x the inside width of the rim) rule is a guide but it doesn't have to follow that precisely. As long as your too far off it should work. You can try it and see how it rides.
    For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life. John 3:16

  6. #6
    Senior Member GeorgeBMac's Avatar
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    Quote Originally Posted by Bill Kapaun View Post
    I don't see any problem.
    Do you ACTUALLY SEE one?
    No, the tires seem to perform OK (to me).

    I was going by the Sheldon Brown site that said:

    "A general guideline is that the tire width should be between 1.45/2.0 x the inner rim width. "

    and:
    "If you use a very wide tire on a narrow rim, you risk sidewall or rim failure. This combination causes very sloppy handling at low speeds. Unfortunately, current mountain-bike fashion pushes the edge of this. In the interest of weight saving, most current mountain bikes have excessively narrow rims. Such narrow rims work very poorly with wide tires, unless the tires are overinflated...but that defeats the purpose of wide tires, and puts undue stress on the rim sidewalls. "
    --------------------------------------
    bikes: 1992 Cannondale R500, 2012 Trek DS 8.5, 2008 LeMond Poprad

  7. #7
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    I don't think it's a problem, either. I've used tires from 21 to 41 mm on the same wheels on my Atlantis (i forget the rim width, and I'm not going out in the snow to check). No issues at all.

  8. #8
    Senior Member BlazingPedals's Avatar
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    Sheldon Brown's tire article here:
    http://www.sheldonbrown.com/tire-sizing.html
    has a chart which is admittedly conservative, showing 'good' combinations of tire width vs rim width. If I can extrapolate, your 18mm inside width would be good for at least a 40mm tire.

  9. #9
    Senior Member GeorgeBMac's Avatar
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    Thanks guys... I guess that's the trouble getting your info from books and such rather than from experience. Plus I've been a bit overly fussy about my bikes in order to compensate for the lack of solid foundation of knowledge and experience.
    --------------------------------------
    bikes: 1992 Cannondale R500, 2012 Trek DS 8.5, 2008 LeMond Poprad

  10. #10
    Senior Member osco53's Avatar
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    George,,I used to work in a bike shop when I was a teenager, always had bikes, now recumbents.

    Being detail oriented is a good thing, can save your bacon and saves money too.

    I always double check things in forums like these and will continue to do so. We, ALL of us, We never stop learning....
    Scott Aspect 940 29er, Tour Easy LE, Sun EZ-3 sx, Walmart Thruster :P

  11. #11
    Certified Bike Brat Burton's Avatar
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    I'm not sure that absolutely everything on the Sheldon Brown site is still applicable to everything on the market today.

    If you use a very wide tire on a narrow rim, you risk sidewall or rim failure. This combination causes very sloppy handling at low speeds. Unfortunately, current mountain-bike fashion pushes the edge of this. In the interest of weight saving, most current mountain bikes have excessively narrow rims. Such narrow rims work very poorly with wide tires, unless the tires are overinflated...but that defeats the purpose of wide tires, and puts undue stress on the rim sidewalls.
    The implication is that high internal tire pressures can peel the rim sidewalls outwards. In theory that might be possible. In practice, wide tires have their own PSI ratings that are considerably lower than the 120 PSI rating found on some road bike tires. Personally I' ve run 700 x 50c Marathon Supremes on Mavic Cosmic Elites and Mavic A719's for three years now with absolutely no rim issues. Tire pressure has varied from 30 PSI to 65PSI depending on where I was driving. The Shimano Deore XT rims have had Maxxis Hookworms on them (26 x 2.5") at 50PSI for several years without incident. None of those can be considered wide rims.

    And - to further muddy the waters - we had a young lady come into the shop last summer with a rim on an expensive road bike so badly worn that the lip was actually separated for a length of two inches. Surprisingly - the tire wasn't bulging it outwards. When I pointed it out to her she mentioned that she was aware of it and had been driving that way for a couple months. I do think it was a serious risk - mostly if the brake pads ripped it loose.

    Which brings us to another question - how much pressure do the brake pads exert on the OUTSIDE of the rim? I don't have any numbers my self for rim brakes, but the guys at Hayes have some interesting numbers for clamping forces for their hydraulic brake pads - up to 1,200 lbs and hydraulic pressures up to 2,500PSI.

    Yeah - I know. Those aren't rim brakes, the diameter is smaller etc. The point is - it only takes so much force to lock up a wheel, and even if a rim brake only needed 1/10 the PSI as a hydraulic brake - thats still about as much force from the outside as a high pressure tire would put out from the inside. If someone wants to crunch some numbers - feel free. I'm happy being 'close enough' and 'approximately right' when thats all it takes.
    Last edited by Burton; 01-20-13 at 01:47 PM.

  12. #12
    Senior Member GeorgeBMac's Avatar
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    Quote Originally Posted by BlazingPedals View Post
    Sheldon Brown's tire article here:
    http://www.sheldonbrown.com/tire-sizing.html
    has a chart which is admittedly conservative, showing 'good' combinations of tire width vs rim width. If I can extrapolate, your 18mm inside width would be good for at least a 40mm tire.
    Yes, I saw that chart -- which is one of the reasons that I asked the question.
    ... But the rims are only 15mm. I measured 16 but the fine print on the rim says 700x15. So, according to the chart, my tires are too big for these rims.

    But, from reading all the posts here, it sounds like that chart is only used in an ideal world.
    ... i.e., if you have 15 mm rims and 35 mm tires, just put a little more air in them!
    --------------------------------------
    bikes: 1992 Cannondale R500, 2012 Trek DS 8.5, 2008 LeMond Poprad

  13. #13
    Have bike, will travel Barrettscv's Avatar
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    Quote Originally Posted by GeorgeBMac View Post
    Yes, I saw that chart -- which is one of the reasons that I asked the question.
    ... But the rims are only 15mm. I measured 16 but the fine print on the rim says 700x15. So, according to the chart, my tires are too big for these rims.

    But, from reading all the posts here, it sounds like that chart is only used in an ideal world.
    ... i.e., if you have 15 mm rims and 35 mm tires, just put a little more air in them!
    The trend has been away from narrow rims.

    15mm is about as narrow as they come. Will these rims accept the 700x38 tire? Yes.

    It it an ideal fit? No.

    I use a 24mm wide outside-dimension rim for tires in the 700x38 to 700x50 size.

    I use a 23mm wide outside-dimension rim for tires in the 700x24 to 700x38 size.
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