650b wheel/tire radius for calculating BB height

[John H.]

I have a Seven that I have used with both 650b and 700c wheels.
This frame has 75mm bb drop.
I do feel like the bb is noticeably lower with the 650b wheels- easier to strike pedals. But this only comes into play on really rocky trails.

I recently got another bike with 73mm bb drop. Feels about the same as the Seven in terms of bb height.

[NHAero]

John, what tire size do you run on the 650b, and what crank length?

Originally Posted by John H.

I have a Seven that I have used with both 650b and 700c wheels.
This frame has 75mm bb drop.
I do feel like the bb is noticeably lower with the 650b wheels- easier to strike pedals. But this only comes into play on really rocky trails.

I recently got another bike with 73mm bb drop. Feels about the same as the Seven in terms of bb height.

[ColonelJLloyd]

As a data point I briefly owned a custom frame built around 54-559 tires (I ran 42-584). The CAD drawing showed it with the 26" wheels as a 270mm BB height/65mm drop.

[NHAero]

A tall friend in Seattle measured his Davidson for me, with 650bx38 tires - pretty sure Gravel Kings. He's done a bunch of mixed surface trips on this bike.

He got 333mm axle-to-ground, so 666mm wheel diameter.
With a spec'd and measured 70mm drop he has 263mm BB-to-ground. He runs
180mm cranks so has 83mm pedal axle to ground.

What I take away from this is that running 170mm cranks with 38mm tires I'd have the equivalent pedal axle to ground with 80mm drop, and so with the smallest tires I expect to run, 32mm, I could likely increase BB drop a bit over the 70mm we are currently carrying in the design.

Thanks all for the input, it's really helpful.

[Doug Fattic]

Just as another data point, I made myself a coupling travel bike once with smaller wheels. I set the BB height on that one at 250mm. I did it partly as an experiment and partly because the angles made my lugs and BB shell work better. I've never had any trouble with pedal strike. Sometimes I would use it on fast training rides because the smaller wheels allowed me to sit tighter on the wheel in front of me. I needed every advantage with my modest abilities so I wouldn't get dropped. One time I was still with the leaders at the end and as we accelerated to maintain position before the final turn and crosswalk finish line, I pedaled right through the corner without thinking. Surprisingly my pedals didn't hit. But afterwards I thought, I shouldn't have done that.