Hey all,

I'm running a 29er MTB frame with Salsa forks on 700 x 35 tyres and I get a BB height of 288mm

What is deemed a normal range for BB height on touring bikes? Oh I am using a 175mm cranks too.

I'm very interested in a switch to 650B wheels which are 38mm OD smaller on the bead, so 19mm lower on BB height if I switch wheels assuming the same tyre profile height. This would give a BB height of 269mm with 175mm cranks, does this seam to be too low?

I like to ride back roads and dirt / gravel tracks when touring.

I would be okay with that BB height, but I would not use such long crank arms.

Are you really going to keep the same tire profile? With 650b wheels you'll be able to run fatter tires, making up for part of the difference.

I think I've seen BB heights on touring bikes listed from the low 260s up to 270. Non-touring-specific bikes generally run a little higher. I think 269 is close to what my Long Haul Trucker was, based on measurements I'm seeing online. 170 cranks were no problem on flat surfaces. But once it got rocky, as in off-road, there was more care required to avoid pedal strikes. If you're looking to stay on roads, gravel or otherwise, you might be able to keep your 175s, but if you have any thoughts of about leaving the road, I'd probably leave the larger tires on to keep the BB up higher.

I think bikes intended for touring on pavement run lower bottom bracket heights than off road where ground clearance may be a bigger issue. Of all my bikes, I think my expedition bike (Thorn Nomad MkII) has the highest bottom bracket at 11.75 inches (roughly 298mm) with 50mm tires. When I run 57mm tires, it is even higher.

Lowest bottom bracket on my bikes is on a 1961 vintage Italian racing bike (Ideor) at about 10.625 inches (or about 270mm). On this bike I run a 170mm crank instead of a 175 to get the pedals a little higher. When I was a kid I would hit the pedals on the ground when I pedaled while cornering. But I am a bit older now, I do not pedal in tight corners, so I could probably run a 175mm crank. Every time I ride that bike and stop at a stoplight, I am always surprised how high the ground is when I put one foot on the ground. And it is easier to get my leg over the saddle when I get on it.

In other words, I think 270mm is ok for touring on pavement with a 175mm crank but I would not go lower, and forego the pedaling in corners. Since rails to trails routes are as good as pavement when it comes to clearance, that should be fine too.

A photo of the Italian bike with low bottom bracket, note that I have something propped under the pedal to hold the bike up for the photo. And yes, the bike has a few modern parts on it, it is not fully original.

OP, I think you'll have adequate clearance with bike configured as described even on fire/gravel roads.

As another has suggested, tires larger than 35mm would be nice in rough if you have tire clearance, which you should on a 29er MTB.

Even on low BB height bike (~260mm), I would experience pedal strike very infrequently while pedaling through a tight turn with bike leaning quite a bit, effectively shortening distance to roadway surface, with 175mm cranks.

What is the logic behind converting to 650B? Makes sense on a road bike so you can (potentially) fit a bigger profile tire....

damo010, The measurements for my Cannondale touring bikes: 10.5" BB height (32 mm tire spec.), 2.5" BB drop. I have 170 mm crank arms, BTW.

Brad

Thanks for the info :-)

I was looking at the new Marathon Supreme thats rated at 42mm but early reports say they come up small at about 39mm so it would be fractional bigger but I thought its so small thats its not worth bring up. I would have the option to tire up to 50mm too so thats an option too.

Reason for 650B is I like the wheels size and the midsize wheels see to put to bed the age old 26" vs 700 debate (to some extent) but I like the slightly smaller wheels size with slightly fatter tires with out going all the way down to 26". Also smaller wheels are stronger too :-)

I like low BB for touring and commuting. I had a Schwinn Circuit converted to 26" wheels that had a 254mm BB and it was a little low, but I never hit a pedal with 170mm crank arms. 260-265 is the range I like as I can put a foot down at stops without leaving the saddle, the bike feels as if I am riding "in" it more than on, and it just seems more fun. I have a Bridgestone RB-2 converted to 650b that is right at 260 and it is great. Fendered and racked it is my commuter usually.
Good luck,
Tom Palmer
Twin Lake MI, were we have had 20" of snow since Monday.

Would the smaller wheel size affect the way the bike handles?

Tire and tube replacement while touring would be more difficult for non-standard tire sizes. 650B tires and tubes would be hard to come by in many areas. For that reason alone, I would never consider going with 650B, but there are other reasons.

I commented above that when I was a kid I had pedal strikes when pedaling in sharp corners with my 10.625 inch bottom bracket height. But I forgot to say that was back before clipless pedals were invented. Pedals were wider and a little taller than modern clipless pedals are today. So, if you are going to run a modern clipless pedal, that would make the potential for pedal strikes less likely. And for touring, you are not going to go into sharp corners and pedal at high speed either, so that makes pedal strikes less likely too.

I think the strength considerations with 29er MTB wheelset while touring is a non-issue, especially if compared to a 650B road wheelset. Not trying to talk you out of anything or deter you, but don't do it for strength considerations.

Tires will be harder to find, but 26" and 700c tubes can work well enough if needed.

If it's not 272.439mm than you cannot tour on it!


Just kidding. However you might want shorter cranks as you might occasionally get some pedal strike. I don't think it will be too bad but certainly worth noting.

BB height is an interesting conversation. But!! Look at the frame geometry spec sheets. Larger bicycles generally have longer cranks. But the company generally does not adjust BB height. Touring bicycles tend to have lower BB height. But even with 180 cranks there should be no issue. I'll corner much faster on my road bicycle than a touring bicycle with any kind of load. I don't pedal through really tight corners, problem solved!!
I don't know anything about mountain bicycles on a trail with big roots and big rocks.

my Bike Friday pocket Llama has a rather high BB, so I Stop with 1 foot on the ground off the saddle.

its only a noticeable thing in town with a lot of stoplights.

because the trail is a ground measurement between the line thru the head tube and the line down from the hub axis,

they cross above the ground.. there will be a change to the trail , will it be noticeable? iDK ,

but the distance can be calculated .. it, as the base of a triangle, will be somewhat less .. as will the other 2 sides .

How does one measure crank length? I ask because I have an old 27" road bike I'm thinking of converting to a 650B gravel grinder and if I want to replace the crank arms, I'd like to know what I've got (and just in case it doesn't specify).

Measure C-C from the crank arm fixing bolt hole and the pedal spindle hole.

Brad

Thanks

Sometimes the number is printed on it or on a sticker on it or cast into the metal.

On touring bikes BB height is 10.5 to 12 inches, this is for road touring type frames as opposed to some kind of MTB which has different priorities. Modern bikes tend to have lower BB height.