Thorn Cycles Forum
Technical => Wheels, Tyres and Brakes => Topic started by: ianatstanage on June 17, 2019, 06:00:28 pm
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Hi,
What do folk feel about fitting 650B wheels to a Mercury?
I have a lovely Mercury, which I have had about 5 years now. Increasingly I spend more miles off-road, mainly tracks and smooth singletrack. I have tackled rough tracks without any major problems, but slowly.
I have fitted 40mm tyres (42 on the tyre, but 40 fitted to the 19mm rims) without mudguards that do a good job. However, I want something wider and the ability to fit mudguards.
I have put 650B wheels in from another bike and it looks like it would be possible to have a tyre of around 50mm wide, with still space for mudguards.
I am assuming that a 650B wheel and wide/taller tyre would almost be equal to a 700C and 35mm tyre combination. Therefore the handling is unlikely to be affected.
Am I correct with this assumption? It would be a very expensive mistake!
Cheers,
Ian
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Probably a question for Dave at thorn as few will have experience of this. Are you on disc or rim brakes. The latter might be an issue.
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Rim brake on the front, so new disc fork required. A fair bit of expense as would clearly need the rear wheel re-built as well as a new front wheel.
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I have 650B rims and Supreme 50 mm tyres on my old utility bike. The wheel circumference is 2094 mm.
I don't have a bike with 700C x 35 mm tyres, nearest is my old lightweight tourer with 700C x 28 mm, wheel circumference on that is 2108 mm.
So I reckon a 650B rim with 50 mm tyres will be about 40 mm smaller than 700C x 35 mm on the circumference, which corresponds to roughly 6 mm on the radius.
Don't know if this is enough to make any difference on the handling, it would correspond to going to 700C x 26 mm. I think 700C x 25 mm is possible on a Mercury, but would it be the same forks as for 700C x 35 mm?
With 650B x 50 mm you should be OK for tyre height, but a possible issue would be tyre width.
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Rim brake on the front, so new disc fork required. A fair bit of expense as would clearly need the rear wheel re-built as well as a new front wheel.
See also other threads and Rohloff handbook about the inadvisability of changing wheel size on a hub.
I'm not sure it's possible, the width and clearances will determine that. I'm even less sure that the gains are going to be very significant, or desirable. The Mercury is what it is, a fast tourer, it wouldn't be my starting point for an off road bike and by the time you've bought a new fork and brake, had the wheels re-built and put the hub at risk, you are IMO well on the way to getting another bike.
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I can only guess that you mean cracking of the flange around the spoke holes due to differant alignment of the spokes after rebuild? As to the suitabilty of the Mercury for the OPs intended purpose ...well the brochure extolls its virtues as a potential gravel bike and SJSC are now selling a convertible 700c/650b rim brake fork to fit so I would say give it a go.
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I can only guess that you mean cracking of the flange around the spoke holes due to differant alignment of the spokes after rebuild?
Yes, not only regarding a change of wheel size, but thy discourage any change from the initial spoke pattern. I see Thorn are offering a ten year warranty on flanges, that goes beyond what Rohloff offer, it could be expensive doing anything that invalidated it.
As to the suitabilty of the Mercury for the OPs intended purpose ...well the brochure extolls its virtues as a potential gravel bike and SJSC are now selling a convertible 700c/650b rim brake fork to fit so I would say give it a go.
Does it. I'll have to have a look, which brochure is that? My downloaded copy from when I bought mine describes the geometry as tight Audax style, that might be what some are looking for in a gravel bike, but it's a bit too lively off road for my liking.