Different sized tyres front and back - check my geometry sums!

neeb
neeb Posts: 4,467
edited October 2018 in Road general
I was thinking about running a 23mm tubular front tyre with a 25mm rear. It occurred to me that as the 25mm tyre is 2 or 3 mm higher this might affect the bike's geometry slightly.

Let's say the bike has a wheelbase of 977 mm. I work out (using basic right-angled triangle geometry) that if the rear tyre is 3 mm higher than the front this should increase the head tube angle by 0.18 degrees, so roughly taking it from 73 degrees to 73.2 degrees. It seems to be that the effect of this on bike handling should be practically undetectable.

Does that seem right?

Comments

  • cooldad
    cooldad Posts: 32,599
    It does, but why a fat tyre on the back?
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  • pblakeney
    pblakeney Posts: 25,774
    Put a 25mm tyre on the front.
    Problem solved.
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  • cougie
    cougie Posts: 22,512
    Not going to be noticeable. There's more weight on the rear tyre anyway.
  • Fzs1000n
    Fzs1000n Posts: 22
    according to a chart i found the circumference of 23mm is 2096 and 25mm 2105 so approx 3mm diff in diameter, which divided by 2 as there's a top and bottom of the tire so would only be about 1.5mm diff in bike height
  • sungod
    sungod Posts: 16,551
    i've run different front/rear tubs for years, it works fine and makes no difference to geometry

    currently run 22/23 front rear on my obermayers (veloflex sprinter/criterium)

    the narrower front matches the rim better which improves aerodynamics, go any wider and the front will 'catch' in high wind as yaw angle changes, it's an amazing difference for such a small change

    the 23 rear lasts longer and allows slightly lower pressure - i've tried 25mm on the rear but in hard cornering they are a bit too squirmy on the narrow rim
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  • cycleclinic
    cycleclinic Posts: 6,865
    The tyre is higher until you sit on it.

    Also narrower front is more aero. Wider at the back adds comfort. I would not over complicate the decision.
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  • craigus89
    craigus89 Posts: 887
    If ever something has been overthought....
  • imposter2.0
    imposter2.0 Posts: 12,028
    Craigus89 wrote:
    If ever something has been overthought....

    Next week: 'Does thicker bar tape make you less aero?'
  • cougie
    cougie Posts: 22,512
    Can you get bar tape that's thin on top and bottom and thicker on the back edge for Kamm tail aerodynamics ?
  • stueys
    stueys Posts: 1,332
    I run a 23 on the front and a 25 on the back. I've yet to notice any difference in the bike handling, though the front wheel seems a smidge more susceptible to cross winds. No idea why, maybe because the rim now overlaps the tyre where as on 25's the tyre just overhangs the rim...

    Either way, it's fine.

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