Anyone running 650b wheels on a road bike for gravel?

I want a new bike which is primarily a winter trainer but has the option to go off road once a month or so. My plan was to get a gravel bike with one set of road wheels and one set with wider gravel tyres. Now I’m wondering about getting a road frame with 30ish mm clearance and then a set of 650b wheels with gravel tyres that I could swap in. (The reason - much wider range of frames and better choice at my fairly low budget).

I’m pretty sure it would work, but anyone got any experience of doing this and any downsides to it?

I dont imagine you would fit very big 650b tyres in there. The benefit to 650b on gravel is you can fit much bigger tyres in a standard size frame. If the frame only clears 30mm (id call that a standard modern road bike TBH, even the aero ones clear 28-30 now) you might get lucky enough to squeeze 38 or something in there in 650b. But you will be running a smaller rolling diameter, and you will have trouble finding tyres. The road plus gravel standard tends to be around 47mm, although I believe there are a couple tyres available in 42mm

Better off with a gravel frame and adequate clearance IMO

I was referencing the below which doesn’t agree with what that you said there at all…

650B tires, such as the popular 47mm Byway or 48mm Switchback Hill, will have a diameter of about 678-679mm, approximately the same as a 700C x 28mm wheel/tire combo. Here are five useful equivalents:

700C x 23mm = 668mm = 650B x 42mm
700C x 28mm = 678mm = 650B x 47mm
700C x 32mm = 686mm = 650B x 50mm
700C x 35mm = 692mm = 27.5 x 2.1”
700C x 38mm = 698mm = 27.5 x 2.25”

That’s saying I can run a 50mm tyre on a frame with 32mm clearance

The rolling diameter may work but the tyre will struggle to clear the chain and seat stays due to its width. I was referring to the width of the tyre, not the height.

My CX frame clears a 700x40mm tyre with a little room, however my gravel wheels which are road plus (650b X 47mm) only just fit in the chainstays. My frame has way more clearance for a wide tyre than a road free would have, even one that takes 32s

You may want to look carefully at your proposed road frame, and where the clearance limits are.

If the limits are the clearances at the fork crown or seat tube, then switching to 650B will give you greater clearance.

If the limits are at the sides of the tire, then going to 650B won’t help you much. Note that 650B will move the tire/rim interface back a bit, which might give you an extra mm or two lateral clearance as the chain stays widen out, but the difference won’t be huge.

Note also that switching to a wider internal rim width for gravel will slightly increase the installed tire width and volume for the same nominal tire width.

It may also be worth checking around as to what width different frames can handle. The BMC Roadmachine and Canyon Endurace for example will comfortably handle 32+mm tires, even though they are intended very much as road bikes.

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Ok makes sense thank you!

So what would you guess is the biggest that you can fit inside a 30mm clearance frame on a 650b wheel? I know it will depend on the frame but as a rough guide.

Would a 35mm with small knobs fit?

The standard for reporting tire clearance is 4mm either side.

IF the manufacturer has been generous and allowed more than this, and
IF the rims are relatively narrow and not causing the tire to go wider than it’s nomincal width
IF you can can move the tire back to the wider part of the chainstays through using 650B wheels, and
IF your gravel riding will mainly be on hardpack (ie. not mud which can accumulate and abrade the frame/wheels,

then you might be able to squeeze 35mm on there. But that is a lot of “ifs” piled on top of each other, and things may not work out so well in reality.

Again, I would recommend looking at bikes in manufacturers’ “Endurance” category, as they can often fit 700x32 by spec, which makes it a lot more likely you can get 35’s on there.

This might give you some insight:

I think that table is just equating outer diameters, not saying the 650b tires will clear your frame.

If you are planning to ride gravel, best to get a frame with the proper clearance. There is an increasing number of gravel bikes these days, meaning you should be able to pick one up for a similar price to a road bike/frame.

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When you say “go off road once a month or so,” what kind of riding/terrain does that entail, exactly?
There’s not much question that one “could” fit 650b wheels into a bike originally delivered with 700c wheels on it, but there may be some grey area around which combinations will actually perform well for the intended purposes. You might actually find that some bikes shipped with 700c wheels perform better on the road with 650b wheels in them and better off road with 700c wheels in them, depending on the terrain and the cornering you do.

Just giving this a bump, as thinking of doing the same with a 2016 model alloy Giant Defy. I can just abut run slick 32mm’s on it at 700cc, and that’s basically because they are running narrow. 28mm is the stated limit. 32 CX tyres don’t work.

I’ve measure at every pinch point 325mm from the centre of the axle, which I’ve read is approximate widest part of 650b rim and tyre combo, and coming up at 47mm (chain stays the narrowest point). I’ve found 1.5inch gravel tyres (38mm) so even allowing for running a little wide…

I feel like I’m missing something?

It’s probably going to be occasional use, and to test my enjoyment of the local fire roads and trailes before committing to n+1. A new frame or bike isn’t an option at the moment. A gravel specific bike is a possible option, but I want to test the water meaningfully first.

Macy, how did this work out? Thinking of the same.

It ended with purchase of a Topstone Tiagra! :grinning:

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