Keep or trade in

[JohnR]

As to a 650b Mercury, I have a long bare foot stand over measurement (89cm) so have 175mm cranks. I could be wrong but I’m pretty sure there’s a warning in the Thorn brochure somewhere against 175mm cranks with 650b wheels (Tyre size might be the determining factor here).

There's plenty of clearance on my Mercury (size 55L with 50mm 650b tyres + mudguards and 170mm cranks) and I can't see that 175mm cranks would create a problem. That's unlike a bike which I've just dismantled which had 175mm cranks on a 49cm frame and, from time to time, my toes would clash with the front mudguard.

[mickeg]

I am not researching the Mercury, but in regard to 650b wheels on it and longer cranks, could the issue be a low bottom bracket height?

If you had the same size tire on a 650b wheel as on a 700c wheel, I think the axle on the 650b wheel would be 19mm closer to the ground.

[Moronic]

Didn't see the warning and Thorn was happy to deliver my 650b Mercury with 175mm cranks and 50mm tyres.

I've not noticed an issue with them. What was the problem supposed to be?

[JohnR]

I am not researching the Mercury, but in regard to 650b wheels on it and longer cranks, could the issue be a low bottom bracket height?

If you had the same size tire on a 650b wheel as on a 700c wheel, I think the axle on the 650b wheel would be 19mm closer to the ground.

That could be the reason should someone opt to put narrow tyres on 650b rims.

[PH]

Didn't see the warning and Thorn was happy to deliver my 650b Mercury with 175mm cranks and 50mm tyres.
I've not noticed an issue with them. What was the problem supposed to be?

There isn't going to be an issue with that, as mickeg says the radius difference between 700c and 650B is 19mm.  So 650B with a 50mm tyre is potentially has the same BB height as 700c with a 30mm tyre, I say potentially as there's also a difference in tyre deformation if you're running both at appropriate pressure, but that's quibbling. Thorn may have baulked if you'd asked for 650B with a 30mm tyre, but who'd want that? Maybe someone with a small frame and big feet, but then they wouldn't want 175mm cranks...
There's a fair amount of leeway - MTB'ers might be concerned with pedal strike, and crit racers with cornering, but for most of us +/- 10mm isn't going to make any difference, I have that much variation between pedals.
I did at one time consider building a 700c Nomad MKIII , but the high BB in that wheel size put me off, it might have been OK, but I didn't get on with the one bike I've had like that, though that may not have been the cause.

[mickeg]

I have not measured bottom bracket heights on any of my bikes, but I can say that my vintage Italian bike has an unusually low bottom bracket that I frequently ground a pedal when cornering, but I can't remember ever grinding a pedal on any other bike on pavement.

I find that every single one of my 700c bikes has toe overlap when I use fenders, but it is so common that I am quite used to it.  If I have to make a very sharp slow turn, I am used to remembering to stop pedaling during the turn and to position my crank arms up and down.  It has mostly been a hassle when touring with a heavily loaded bike, a heavy bike accelerates much slower and a slower bike means more steering corrections, thus I have often hit a fender with a shoe when starting out.  But otherwise have not had a problem with toe overlap, I call it a nuisance, nothing more.  My 26 inch bikes do not have toe overlap.  My only 650b bike is a three speed from the 1960s, it does not have toe overlap, that bike came with fenders on it.

My Nomad Mk II has a high bottom bracket, but I have not measured it.  I just feel that I am higher off the ground when on that bike.

I do not have a dedicated mountain bike, I was pretending that my Nomad Mk II was a mountain bike on a trip to ride mountain bike trails.  I fitted it with a 100mm suspension fork for that.  There were a few places like in the first photo below that there was enough erosion that the trail was quite a bit deeper than the ground on each side, occasionally I dragged a pedal on the side, which was most inconvenient.  I have no idea if the other mountain bikes that were more common on that trail had unusually high bottom brackets or not?  But usually the eroded trail was just wide enough for a crankset with pedals and not much more clearance than that.

Second photo, my Nomad on that trip, we were car camping in campgrounds and riding our bikes from the campsites.  I also fitted a cheap (mostly ineffective) suspension seatpost on that trip.