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  • Tyre width… The nerdiest post I will ever make
  • Northwind
    Full Member

    Disclaimer: If you don’t like bikegeekery, this post is not for you. Even if you do, it probably is still not for you, it is pretty tragic.

    So. Tyre width. Everyone knows that sometimes tyres aren’t the size they say they should be, and most people know that this is because tyre manufacturers are pisstakers. But sometimes, out come the excuses, and one of them is “Different sized rims will make tyres different sizes”. And obviously they will, but I’d never really paid any attention to how much, other than having a gut feeling it’s less than people think.

    So, I got a new Conti Baron, and found that it’s very close- 2.3 claimed, 2.28 actual, measured at the widest point. This was on a DT540, pretty typical of the sort of rim that these tyres will be fitted to (slightly wider internally than a Mavic 721).

    But how much difference would a smaller rim actually make? On my commuter is a 1990s Weinmann/Bontrager BCX3. It’s just 17.1mm wide internally, compared to the DT’s 22mm, so realistically you’re not going to stick a 2.3 tyre on it normally. It’s also 1mm deeper in section which will pinch the tyre further.

    And at the same 40psi, it comes up (drumroll) at 56.31mm wide, or 2.21 inches. So, despite the exaggerated difference in rim size, the difference in width is just 1.6mm.

    That’s practically science, that.

    paulosoxo
    Free Member

    It’s the profile that changes. Innit.

    druidh
    Free Member

    Do you want to redo the test with one of these??


    Does my bum look big in this? by druidh_dubh, on Flickr

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    aracer
    Free Member

    Where you’re going wrong there is having an insignificantly small difference in rim widths – I suspect you’d notice rather more of a difference if comparing with the 42mm rim I have my 2.4 RR mounted on!

    twohats
    Free Member

    Challenging looking terrain there druidh, overbiked I’d say…

    Northwind
    Full Member

    aracer – Member

    Where you’re going wrong there is having an insignificantly small difference in rim widths

    Nah, it’s not insignificant… The difference is 4.9mm, over 20% smaller. That’s a big change in terms of width, very few rims will fall outside that variation (it’s enough to go from rims that don’t recommend a 2.3, via rims that are just spot on, up to rims that don’t recommend anything smaller than a 2.3)

    I would like to do it with a bigger “normal” rim and really exaggerated change, though… I had a doublewide in mind, if I can find one, but they only have a 33mm internal width… what’s your 42mm out of curiosity?

    I think fitting a 2.3 tyre to a 100mm fatbike rim could have interesting results :mrgreen:

    iDave
    Free Member

    This is why I usually avoid the bike forum….

    aracer
    Free Member

    Nah, it’s not insignificant… The difference is 4.9mm, over 20% smaller. That’s a massive change in terms of width

    Not compared to the width of the tyre it’s not – less than 10%

    what’s your 42mm out of curiosity?

    I don’t know – here’s all the info I have http://www.unicycle.uk.com/unicycle-spares/rims/26-nimbus-dominator2-rim.html fairly standard wall thickness though I think, so the internal difference should be close to the external difference.

    This is why I usually avoid the bike forum….

    Yeah – all this pointless arguing, I see your point.

    Northwind
    Full Member

    aracer – Member

    Not compared to the width of the tyre it’s not – less than 10%

    Not really relevant, though, is it? The range of variation covers practically every rim you’ll find on a trailbike. I’d be interested to know what happens with a much wider rim but just out of curiosity.

    Thanks for the rim details btw… Think I need to avoid spending money on this, that’d be proper mental.

    aracer
    Free Member

    Thanks for the rim details btw… Think I need to avoid spending money on this, that’d be proper mental.

    At some point I’ll try mounting the same tyre onto that and a more normal MTB rim and get some numbers. A bit of a faff to take the tyre on and off though (it should be easy to guess what it’s on – far more hassle than any bolt through), so not going to happen until I have the wheel off for some other reason (maybe for the tubeless conversion, given I’ve already had more thorn punctures on that in a couple of months than I normally get on an MTB in a couple of years).

    I did measure up the previous tyre which was on, but unfortunately didn’t keep the figures as I was only interested in the difference between that and the big RR – unless anybody has a 2.4RR mounted on a more normal rim I can compare notes with?

    Superficial
    Free Member

    Have a read of this: http://www.pinkbike.com/news/Tech-Tuesday–Wider-Rims-Are-Better-and-Why-Tubeless-Tires-Burp-.html

    There’s certainly some questionable science in the article but it’s quite a good description of how things change with wider rims.

    P.S. I thought tyres were technically measured on their height and it’s a misconception that the number printed on the side refers to the width of the tyre?

    Northwind
    Full Member

    Superficial – Member

    Have a read of this: http://www.pinkbike.com/news/Tech-Tuesday–Wider-Rims-Are-Better-and-Why-Tubeless-Tires-Burp-.html

    Aye, it’s a good read that, interesting even though it’s full of kind of weird conclusions (ie, “This tyre burped on our non-ust rim, we compared it with a wider UST rim and it didn’t burp”, must be because of the width”)

    I need to measure the thickness of the rim but I don’t think the height thing is right- certainly with this tyre, the width is almost bang on what’s on the side whereas the height is way off.

    Northwind
    Full Member

    <too late for an edit- should have said is probably way off>

    Onzadog
    Free Member

    This is why many moons ago, mbuk talked about a STAR rating for each tyre tested. (standard tyre and rim). All tyres to be measured on the same reference rim.

    buzz-lightyear
    Free Member

    thought tyres were technically measured on their height

    Disparity occurs because some manufacturers don’t include the height of the knobs in the profile measurement, they just measure the profile of the carcass. For example, you may have noticed that newer Maxxis models have a taller profile than the older models for the same quoted measurement.

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