Compromises for having 700c and 650b compatibility

[cyan]

What are the big/small compromises for making a frame that accommodates both 700c and 650b wheels? Handling? Ride feel? Weight? Cost?

BB height might be an issue but I've also heard that with larger 650b tires the difference is minimal (so is the risk of pedal strike)

The context here is a metal all-road bike that does not venture to single tracks, rocky descents, or similarly gnarly terrains.

EDIT: to clarify, my question is intended for the effects on the frame, i.e. how a frame that accommodates both 700c and 650b compares with one that just takes 700c, for the desired application (see above). Of course, riding on 700c vs. 650b will be different, but that's not the focus here.

[slowpoke]

What are the big/small compromises for making a frame that accommodates both 700c and 650b wheels? Handling? Ride feel? Weight? Cost?

If you're switching wheels sizes to increase/decrease rubber and air, then ride feel will change because that's the point, right?

There's a great interactive chart here that gives you 26" / 650b / 700c equivalent diameters: https://bikecounterculture.com/2017/08/17/bike-wheel-diameter-calculator/ E.g. 650b x 48 ≈ 700c x 28

Outer diameters being equal, handling may still change because wider tires will yield increased pneumatic trail (supposedly more stability). However feeling that depends on tire pressure and the rider.

You may also want to check on older threads..

Firefly has offered a frame like this: https://forums.thepaceline.net/showthread.php?p=2397835#post2397835

And more recently "rain dogs" has happily done the swap: https://forums.thepaceline.net/showthread.php?t=252952

[cyan]

If you're switching wheels sizes to increase/decrease rubber and air, then ride feel will change because that's the point, right?

There's a great interactive chart here that gives you 26" / 650b / 700c equivalent diameters: https://bikecounterculture.com/2017/08/17/bike-wheel-diameter-calculator/ E.g. 650b x 48 ≈ 700c x 28

Outer diameters being equal, handling may still change because wider tires will yield increased pneumatic trail (supposedly more stability). However feeling that depends on tire pressure and the rider.

You may also want to check on older threads..

Firefly has offered a frame like this: https://forums.thepaceline.net/showthread.php?p=2397835#post2397835

And more recently "rain dogs" has happily done the swap: https://forums.thepaceline.net/showthread.php?t=252952

Thanks for the info/links. Sorry I didn't make myself clear, but I'm more interested in differences resulted on the frame rather than how 700c and 650b ride differently.

Updated my OP to clarify this point.

[slowpoke]

Ah, apologies for misreading that.

Maybe a chainstay crimp needs to be larger to accommodate wider tires for the 650b variant, but I don't see why a designer would really need to compromise on anything else.

[Jaybee]

For truly wide 650b (like 2.2" and above) you have to start making decisions about chainstay length, bottom bracket standard, possibly limit to 1x, maybe introduce a funky chainstay shape or yoke. I'm not sure why you'd go that big unless you were putting knobbies on(which, BTW, is awesome). For the more common 650b x47mm, most gravel/cross bikes can fit them despite being designed around 700c x 40 or whatever. If low bottom brackets are important to you, that might be a case where 650b compatibility isn't optimal.

[robt57]

I'm more interested in differences resulted on the frame rather than how 700c and 650b ride differently.

I'll add my experience with regards to BB drop. First All Road bike I did has a heavy 8CM BB drop [82mm]. I started with 175 cranks and went to 170s fast. BB drop, crank length, and Q factor all play a part.

I wound up doing another bike with 66mm drop with 650b in mind. So IMO 65-7mm drop if you are going to go smaller than 48x650s, especially with a wide Q chain set, else you may strike a lot if you tend to pedal thru as I do.

I think 65mm BB drop may be the sweet spot with 42-48x650b.

Even with the 170 cranks I strike the 82mm drop bike, even with 28x700. on pave diving into turns. It is like having a over lap to the front wheel road bike, you forget until you kick the tire. Same, you forget until you get home and have to file/sand/polish the end of the crank arm. ;)

I do not like +40x700s on the All road, make it feel 29er trucky. So it seem to do best with 35-38x700 and 47-8x650b.


There is some trail changes if you go back and forth from a skinny 28x700 to a fat 650b. but the steering feel does not bother me either way after 300' rolling/acclimating.

[slowpoke]

Even with the 170 cranks I strike the 82mm drop bike, even with 28x700. on pave diving into turns.

I'm curious what the resulting bottom bracket height is on that setup. Probably something around 26cm or less if you're striking so much.

Bringing back to OP's question, this issue seems more rooted in too ambitious of a BB drop rather than a frame trying to accommodate different wheel sizes.

[ColonelJLloyd]

Lots of good posts here already.

On my bike I'll run 650b with 51-55mm tires and also run 700x35. 170mm cranks with 156q and Shimano A600 pedals. It has 75mm BB drop and 430mm chain stays. For me, it all works out really well. There are definitely compromises when you start changing wheels/tires, but I've been happy what it has afforded me. Pedal strike has only ever been an issue on rocky, rooty single track and I don't think 5mm BB drop in either direction would've made a difference. On the road, I'm pretty cognizant of potential pedal strike from time riding fixed on converted road bikes.