Inflated tyre diameter for 700c wheels

Discussion in 'Cycling Equipment' started by j.r.hawkins, Jan 25, 2007.

  1. j.r.hawkins

    j.r.hawkins New Member

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    Hi All,

    I'm looking to switch from a slick-tyred MTB to a drop-bar road bike for my commute, and I need some help with some numbers.

    Can someone tell me what the typical rolling diameter is for a 700c wheel with say a 21 or 23mm tyre on it?

    Also, for the same amount of effort, how much faster do you reckon I'd go up hills dropping from a 14.5kg slick-tyred MTB to say a 9.5kg roadie? (I'm doing 13-15km/hr on those sections at present.)

    The point to these geeky questions is this: I've worked back some numbers for cadence on some of the steeper hills on my route and I'm trying to work out what gearing I'll need on the roadie to ensure I don't overstress my surgically altered knees. I'm hoping to avoid the need for a triple chainring on the front.

    Thanks for your help! Much appreciated. :cool:
     
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  2. 531Aussie

    531Aussie Well-Known Member

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    I assume you're adjusting your speedo (a 'computer' is what I'm sitting in front of right now:))? Cateye says it's 210cm, but, at 90kg, I seem to get a more accurate on-the-bike roll-out at 208cm. The best way is to do an on-the-bike roll-out on a marked track.

    On hills, any drop in weight as a percentage of the bike and rider combination is pretty much commensurate with a similar % speed increase. For eg, if you're 80kg, your weight combination is obviously 94.5kg, so dropping 5kg off this is 5.3%, therefore you should expect (roughly) a 5.3% speed increase. But, it's not that simple, because the road tyres will roll faster, your new bike sill most likely have new smoooooth hubs, and the motivation of a cool new bike might give you another 1kph! :) Also, at slightly higher speeds, even on hills, any aero advantage with a road postion will give you a bit more speed.

    A 5.3% speed increase from 15kph to 15.8kph might not sound like much, but that would cut a 10km time from 40mins to 37:58mins, Two mins is along time to be waiting for your mate at the top of the hill

    This is a cool calculator which figures out all sorts of grouse stuff :p
    http://www.kreuzotter.de/english/espeed.htm

    By the way, you should be able to easily get a new bike under 9.5kg! :)
     
  3. alienator

    alienator Well-Known Member

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    First, gearing is gearing, so whatever gear ratios are working for you now will work for you on a road bike. Whether it's easy to get those gear ratios with road components is a different question. Typically road gearing is taller than mtn. bike gearing. You already know what combos are giving the cadence you like. FWIW, a 700x23mm road tire has a circumference (average) around 2097mm. A 26"x1.5" mountain bike tire has a circumference(average) of 1985mm, so if you divide your cadence on your mountain bike by 1.056 (2097/1985) that for the same gear ratio, your cadence on the roadbike will be 95% of that on your mountain bike (ex: 90rpm on the mtn. bike vs. 85.5rpm on the road bike) at a given speed.

    The weight difference between the bikes will have very little--probably negligible--difference on your performance.

    The different body position on a road bike may improve your performance on the same rides.

    Rolling resistance may be different for the different sized slick tires. In general, larger diameter tires have lower rolling resistance than tires with smaller diameters. Likewise, wider tires have lower rolling resistance than narrower tires. Higher pressure tires generally have lower rolling resistance than lower pressure tires. How all of these trends combine in a comparison between road bike tires and mountain bike tires, I don't know.
     
  4. j.r.hawkins

    j.r.hawkins New Member

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    Absolutely.

    But I'm consistently advised that road bikes are "so much faster", so I'm assuming that a lot of that difference is accounted for by the lower weight, lower rolling resistance, and less upright position, and is also partly why the gearing on road bikes is so much taller.

    So, if speed uphill goes from 15 clicks to 20 then I can probably do without the triple and live with a 39/53 chainring, but if it's less I'll need to go the triple or try to source a bike with compact gearing.

    Thanks for your help!

    Postscript: with WTB Slickasauruses at 80PSI, the rollout with me on the bike at 78kg is 2005mm.
     
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