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Hi All,

I'm thinking of getting shot of my cyclocross bike and getting spare wheels with road (or CC) tyres for my mountain bike instead, but I have a few questions I am struggling to answer:

1) is this something that people do, or is it a stupid idea I should ditch immediately?

2) If I put 650b wheels on a bike that currently has 26" wheels on it, will the smaller profile of the road tyres (say 1.5 vs 2.25) compensate for the larger rim size?  ie will the overall diameter of a 650b wheel and 1.5 tyre differ significantly from a 26" wheel with a 2.25 tyre?  

3) are brake discs a standard size, or do I need to get the exact disc that is on my existing MTB wheels?

Question 2 is the one that I am struggling with the most.  Common sense says the 650b & 1.5" tyre would be the same size, as the difference in tyre thickness = the difference in the radius of the wheels, but common sense doesn't appear to generally apply to wheel and tyre sizing. 

Any insights gratefully received!

thanks

 

  

8 comments

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CXR94Di2 [2658 posts] 3 months ago
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650B is 27.5" so half that 13.75" compared to 13" for a 26" wheel.  Measure from centre point on wheels to se if there is enough clearance to frame to take larger rim.  If tyres are very close to frame with 26" currently it might be close, conversely good clearance should work well

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Drinfinity [236 posts] 3 months ago
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Brake discs are standard sizes. If you have a 160mm at the moment, any other 160 mm will do. They are fitted with 6 bolts or a  Centrelock  to the hub. All that matters here is the disc you choose has the matching fitting for the hub on your new wheel.

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jackiefatenbread [2 posts] 2 months ago
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CXR94Di2 wrote:

650B is 27.5" so half that 13.75" compared to 13" for a 26" wheel.  Measure from centre point on wheels to se if there is enough clearance to frame to take larger rim.  If tyres are very close to frame with 26" currently it might be close, conversely good clearance should work well

Thanks.  I THINK there's a decent amount of clearance, but need to remeasure with a vernier to get the exact tolerance.  Probably more clearance on the front fork than the rear.

I know the wheels will be exactly 1.5" different in size.  However will the difference in the tyre sizes (between a 1.5 width and a 2.25 width) be as exact?  Is the height of a 1.5 width tyre exactly 1.5", and the same for a 2.25?

 

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CXR94Di2 [2658 posts] 2 months ago
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Have a little look here, for a bit of reference. https://www.tredz.co.uk/component-buyers-guides/mountain-bike-tyres

Pop into Halfords or somecycle shop and measure a few tyres for profile height to get a better indication

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ktache [1851 posts] 2 months ago
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Good find CXR94Di2.  I think you probably need to find a mate who is running the size tyres on the rims you would like to use and pop them in just to see, or a very nice LBS.  I would find it a difficult to justify and possibly expensive experiment.  Or start with second hand rims.

You can still get narrow tyres for 26 inch rims.

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CXR94Di2 [2658 posts] 2 months ago
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Another option is you can now get 26+ tyres which bring outside diameter to the size 650b(27.5")

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Argos74 [517 posts] 2 months ago
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Quote:

... spare wheels with road (or CC) tyres for my mountain bike instead, but I have a few questions I am struggling to answer:

1) is this something that people do, or is it a stupid idea I should ditch immediately?

2) If I put 650b wheels on a bike that currently has 26" wheels on it, will the smaller profile of the road tyres (say 1.5 vs 2.25) compensate for the larger rim size?  ie will the overall diameter of a 650b wheel and 1.5 tyre differ significantly from a 26" wheel with a 2.25 tyre? the size 650b(27.5")

Not saying it's a dumb idea, but there are simpler solutions. 26" tyres for road / light trail use.  Maxxis Detonators, Continental Doublefighters, Schwalbe Smart Sam. Plenty of other options. Still plenty quick, if you're worried about speed.

Quote:

3) are brake discs a standard size, or do I need to get the exact disc that is on my existing MTB wheels?

As long it's the same type (centre lock / 6 bolt) and size, you should be fine. If you're not sure what you got, post pics or take to your LBS.

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jackiefatenbread [2 posts] 2 months ago
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Cheers for the comments.  

I'm beginning to think that getting a spare set of 26" wheels and buying road tyres for them is the sensible option.  An inch or so off the diameter of a wheel isn't going to have a noticable effect on speed at my level of cycling, and there's only 3/8" clearance on the forks and frame.

I'm just slightly swayed by the fact there is a set of lightly used 27.5" wheels with rear cassete and 160mm discs for sale locally for £40!