26 inch vs 650b for tiny frame

[Stevemikesteve]

Building up a Soma Double Cross disc for my wife. Size 42, the smallest size. Soma recommends 650b for the smaller double cross frames and claims 700c would also work. A friend is arguing that 26 inch wheels are a no-brainer and will help the fit for such a small rider. Dudes at the bike shop are saying to go with what the manufacturer recommends for the frame, 650b.

Far as I can tell clearance won't be helped by using 26 inch wheels. It's a 2016 frame. Soma altered the geo in 2017 to clear wider rubber. So with fenders it should work with 650b x 42 or roughly 26 x 1.8. I tested with rat traps and no fenders, they don't clear.

Also curious if 26 inch wheels would have a negative effect on handling. Thanks in advance!

[nublar]

Yes going with a smaller diameter rim will reduce the BB height and "gear ratio" unless you use a wider tyre to compensate. Reduced BB height may increase stability and reduce toe overlap but result in increased pedal strike.

[HTupolev]

Smaller wheels can sometimes help the geometry of smaller bikes by designing the smaller bikes around the smaller wheels. For example, the stack can be made lower because the fork crown can sit lower so the top of the head tube can be positioned lower. If the frame is already built, I don't see how going to smaller wheels would improve "fit" unless the rider is having issues with toe strike on the front wheel, or with standover.

26er vs 650b isn't a big difference, though, so you could probably get away with whatever. Keep in mind that the bottom bracket will be over 2cm closer to the ground for a given tire width, if pedal strike is a concern.

As far as handling goes? Assuming that the only thing that changes is the rim BSD, a smaller wheel will reduce rotational inertia and trail. This will tend to lighten the steering, and make the bike less inclined to lean out of tight turns. Someone who dislikes the difference would probably say that it makes the bike feel twitchier.

[zmalwo]

toe overlap is going to be a huge problem for 700c wheels on a 42cm frame. I'm not even sure that 650c wheels can eliminate that problem but def better than overlapping from 1' clock position all the way to 5' position with a 700c wheel.

[R3awak3n]

I would do 650b just for the tire choice... a lot of good 650b tires out there right now, from 38mm which would be great for road, all the way to 48 for gravel

[stien]

I would say 26 has more tire choices overall depending on clearance. Compass makes a slick 26 iirc.

-Alex, who wants a Polyvalent set up with 26s

[R3awak3n]

Originally Posted by stien

I would say 26 has more tire choices overall depending on clearance. Compass makes a slick 26 iirc.

-Alex, who wants a Polyvalent set up with 26s

there is more tire choices with 26 but not as good imo. Compass does make 4 26in tires but that is kinda it for good 26in road tires (that I am aware of). With 650b you have compass, gravel king (which is a great tire, more affordable), schwalbe, WTB, ect...

[colker]

IME bigger wheels ride smoother. ymmv.

[Doug Fattic]

Putting 650B tires on a small frame makes no sense at all. The outside diameter of a 650b and 700c are essentially the same. That means a 650b is not a small diameter tire. The result of putting such a big tire on a small frame is that there is going to be massive toe overlap. While some overlap is not an issue when riding at speed it is guaranteed to put the rider on the ground if they are doing a slow Uie turning around on the road.

The 2nd way to avoid massive toe overlap is to design the frame with a very steep seat angle. The problem with this approach is that this is a lawyer’s solution and not bike fit solution – especially if it is a women rider that often likes their handlebars to be somewhere close to the height of their saddle. 95% of normal riders would not like their saddle to be that far forward if they were being fit on an adjustable sizing bicycle.

This subject often comes up in my framebuilding classes and some short students want to put on big tires for whatever reason. However after finding their best bicycle position on a sizing bicycle and then creating the frame design around their selected seat/handlebar/pedal relationship, they finally realize big wheels and small frames is not a good combination.

[Stevemikesteve]

Thanks. Appreciate the input.

To clarify, 700c hasn't been a consideration. Also, a valid point that I'd already been noticing a bit online is the broader selection of quality tires available in 650b (Compass Naches seems to be the only decent 26 inch tubeless slick on the market that will work with fenders for this frame).

Doug Fattic- You make a good point. Are you implying that 650b and 700c are equally lacking for a build of a tiny frame like this?

Also probably helpful to mention this is for a relatively casual, recreational rider.

[R3awak3n]

Originally Posted by Stevemikesteve

Thanks. Appreciate the input.

To clarify, 700c hasn't been a consideration. Also, a valid point that I'd already been noticing a bit online is the broader selection of quality tires available in 650b (Compass Naches seems to be the only decent 26 inch tubeless slick on the market that will work with fenders for this frame).

Doug Fattic- You make a good point. Are you implying that 650b and 700c are equally lacking for a build of a tiny frame like this?

Also probably helpful to mention this is for a relatively casual, recreational rider.

What he is mentioning is that a 38mm 650b tire equals say a 23mm 700c tire so the rider will have similar toe overlap.

[HTupolev]

Originally Posted by R3awak3n

What he is mentioning is that a 38mm 650b tire equals say a 23mm 700c tire so the rider will have similar toe overlap.

If you're debating two options for the same purpose, though, you're probably not comparing 700x23 to 650x38.

[R3awak3n]

Originally Posted by HTupolev

If you're debating two options for the same purpose, though, you're probably not comparing 700x23 to 650x38.

I am not debating anything, just pointing out that 650x38 will give you same toe overlap as 700x23

[everbeek]

Try 650c (not b) wheels.
-Mike

[HTupolev]

Originally Posted by R3awak3n

I am not debating anything, just pointing out that 650x38 will give you same toe overlap as 700x23

Right, I know you weren't. I'm saying that Doug Fattic's objection to using 650b to improve toe clearance is making confusing assumptions.

If someone is considering a 700x23 as one of their options, they're likely riding on smooth surfaces, and likely want to go narrow. So if they're also considering a 650b tire, they're likely going to look at something like the 25mm 650b Schwalbe Pro One, not a 38mm allroad tire.

Similarly, if someone is considering a 650b 38mm tire as one of their options, they probably value tire fatness. So, if they're concerned that switching to 700c could harm clearances or something, they're probably envisioning a fat 700c allroad tire, not a 23mm tire.

In this case, the frameset is designed for CX/gravel, and the OP is talking about using tires that are nearly 2" in width. Skinny 700c road tires are likely not part of the discussion.