GP 4000 or 5000 tyre size on 17mm rim

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raythebike2
Posts: 2
Joined: Mon Jun 17, 2019 10:42 pm

by raythebike2

A very good youth racing cyclist I coach has 42mm Giant SLR1 carbon wheels, fitted with tubeless Giant 25mm tyres which come out as 26.3 diameter and a rolling circumference of 2.13m. With a BC gear restriction of 6.45m, and using 52/17, this overshoots by approximately 50mm due to the tyre size. I need a tyre with a circumference of 2.11m to match 6.45m. Can anyone with this wheelset recommend a good racing tyre that matches this size?

BTW, It does not have to be a tubeless tyre, but i would be interested to know of any problems mounting standard clincher tyres.

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alcatraz
Posts: 2321
Joined: Mon Aug 29, 2016 11:19 am

by alcatraz

Why not change the gearing to match? A tire is more important for racing.

Is it a fixed gear race?

They use a 17C rim (same as yours) on bicyclerollingresistance.com. You can look up all the width and height measurements there.

Furthermore, a new tire stretches by ~0.5mm within the first couple of rides/days.

raythebike2
Posts: 2
Joined: Mon Jun 17, 2019 10:42 pm

by raythebike2

Thanks @alcatraz . Its the gearing that matters here as the gears are restricted on young riders, it is imperative to be right on the mark, as being under geared is a great disadvantage. I looked at the site, but it details the width and height, but I am unclear as what measurement to use to calculate the circumference of the tyre on a wheel. i guess i'd have to try a couple of tyres and do an actual measurement.

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pdlpsher1
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Joined: Thu Jul 19, 2012 6:09 pm
Location: CO

by pdlpsher1

What’s the rationale behind junior gearing? Their leg muscles not fully developed for a tall gearing?

I recently went from a 28 to a 32mm on my tandem’s rear wheel. The 32mm was 4mm taller and I could detect the change in the gearing from the larger circumference. I welcome the change as I frequently spin out on the 50x11.


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alcatraz
Posts: 2321
Joined: Mon Aug 29, 2016 11:19 am

by alcatraz

Sounds to me like cadence and breathing drills can give a rider the edge they need. If you (as a kid) can hold 120 rpm cadence you'd have 33% taller gearing than someone pedaling at 90 rpm.

On brr.com I'd look up the tire you measured, was it a GP4000 25c and note it's dimensions. Be it 27mm width and 24mm height (arbitrary numbers)

If you want to decrease the circumference by a differential of 50/6450=0.8% then the radius needs to decrease by the same value. I measured a wheel here and got 675mm diameter. 0.8% of its radius is 2.7mm ~ 3mm. If this tire is 24mm tall maybe look for 21mm height?

I'd also drop the pressure during the measurement. :D

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