Why not a 100mm bb?
 

Why are 100mm bbs not used on bikes for big guys? Generally speaking the pelvis/hip width for a guy 6'4" 250lbs is larger than a 5'10" 130lb guy. Seems like the wider bb will position the pedal stroke in a more ergonomic position than a 73mm bb. I know that this difference for the big guy can be lessened by using cranks with different q-factor but why not use the 100mm as a starting place to pick the right q-factor cranks? The 100mm will also increase heel clearance with the chain stays if the rider has large feet(like me, size 50). Negatives are less pedal clearance to the ground in turns and perhaps a bit more stress on the st/dt bb area because off the wider lever(spindle). I am posing this question for a rider that typically will tour/commute/adventure but not do any racing or "radical" riding. What are your thoughts?

thanks, Brian

I'm not sure there is any objective way to do a measurement for desired q factor. Obviously, you can get cranks and bb shells for 100mm because of fatbikes. I have found that it doesn't bother me that much riding a fatbike, although it does seem to make saddle selection more important.

Pedal extenders will allow you to increase the Q-factor with your current frame and components.

While the concept of proportionally sized components is not new, and is currently done to a small extent, the lack of all aspects of bikes doing so is challenged by basic economics. The cost to get to market, a range of component sizes in otherwise similar grade/levels, has to be supported by the consumer spending the $, and doing so for long enough to establish this as the "way to do it".


An excelent example is the Terry bicycle. A solution for many smaller riders but never became large enough to sustain bikes as a stand alone. Early on many main line brands did their copies of the Boston/Terry design (some even without shortening their top tubes!) but soon learned that the market wasn't there to support all the added effort to bring their terryocki bikes into your LBS. (And some of this is also shared by the LBS staff and their often lack of enthusiasm). Soon the big brands turned to a dual 650c design as their sole offer to short riders, and again soon enough they pretty much all don't any longer.


So where the market will support it proportional sizing is already done (shoes, cranks, stems...). New aspects of sizing may or might not popular depending on people like the OP to step up and fund the trial. Andy.

I should have made it clear that I was referring to new custom builds, not retro fitting an existing bike or production runs.

thanks for the input, Brian

actually, I think John's suggestion may be best, unless you are going to go ahead and use a fatbike drivetrain and 170 or 190mm rear hub. Because otherwise the chainline is going to be a problem. I guess there is no reason not to use a fatbike drivetrain on a road bike, but I'm not sure it's worth the hassle. I know one thing for sure, before I built a bike like that I would make sure the resulting q factor works better for the rider.

you can get different spindle lengths for Speedplay pedals.

This is the main reasoning right here. If you want to avoid the pedal spindle issue, then take a fatbike and swap out the rims for smaller ones to make a road bike/hybrid/regular mtn bike, so that all the driveline components work properly. It's basically what the builder will end up doing anyway, except you may be able to avoid the cost of a custom build with non-standard components, and you know that you can buy replacement parts on the regular that will fit. The rider can also try the bike through rental before going ahead with a purchase/build.

Interesting thought, and very doable. I think the Q-factor is on the large side for people of average size and down already. I use 200mm IRD cranks, which are also a bit wider than an S brand crank, which in turn are pretty wide compared to bikes of 30 years ago.

Switching back and forth to my MTB, I do like the wide cranks, especially out of the saddle. But when switching back, narrow feels more spinnable, and more aerodynamic. Other than that, I haven’t experimented enough to know that a certain width would be better. Compared to length, it seems less important.

I would not mind reducing dish in my rear wheel by going a little wider. Maybe I will next time.

It would seem like it would depend on what you are planning to do with that extra space.

So, say you're building a steel bike with standard tubing sizes, and welding on a 68mm bb shell, or a 100mm bb shell, then the forces on the seattube and downtube would be the same, assuming you would get your cranks and pedals in the same place.

Using narrow crank spacing, plus some sort of pedal extenders or long pedal spindles has an advantage of reducing crank rub (which might be a problem for those wearing size 15 shoes).

MTB cranks may already have somewhat wider Q-Factors than Road cranksets, but it would also be easier to build one frame & BB, but multiple lengths and Q-Factors of cranksets.

Perhaps have the Q-Factor proportional to the crank length. So, 170mm cranks, narrow Q-Factor. 180mm cranks, a little wider Q-Factor, and 200mm cranks, even a bit wider of a Q-Factor.

Ok, chain line, didn't consider that at all, don't want to go with a fat bike type drivetrain so that answers that. And..... I didn't know there was such a thing as pedal extenders, I am ordering some now, very interested in how it will change the ergonomics for me.

thanks for the input guys, really appreciate your knowledge and experience, Brian

Fat Bikes have extra wide BBs.. to clear the fat tires ...