XXLHardrock

Suppose the location of the saddle relative to the pedals and handlebars is good and working well. That places the BB, or the centre of the pedal rotation in a particular location.

Now keep the contact points at the same locations relative to each other but change the BB location because of a longer or shorter crank.



Does the BB’s location alter fit and or comfort and efficiency?

Iride01 

It's all a compromise. That why we have different styles of bikes, each with it's own somewhat unique BB position.

Using a road bike frame as an example-- the normal BB position on a properly sized road bike with a seat tube angle of 72 to 74 degrees.

Road bikes are endurance bikes, lending themselves for fast long distance riding.

Those that don't do fast long distance riding on their road bike tend to like seat posts with more setback putting the BB further forward and more matching the position they'd be on a properly sized cruiser style bike with a seat tube angle of 65 to 69 degrees. Cruisers being the bike for leisurely low effort slow riding in an upright position.... IMO.

Those that go the other way and do shorter very fast rides are probably going to want a BB further back than on a road bike. As evidenced by the fact that Time Trial bikes typically have seat tube angles of 77 degrees or more. That lets the rider in the very aero position put more power into the cranks without having their thighs shoved up in their gut as would such a position on a road bike or worse a cruiser.

So where do you want to compromise? Comfort? Speed? Power efficiency? Can't have it all.

AnthonyG

Yes it changes everything, yet, the truth is that the BB is the true zero, zero point in regards to bike fit. You don't move the BB, you move everything else.

Carbonfiberboy 

Do not change the relationship of the BB and the saddle. Crank length doesn't matter to that relationship. I just went through that with my wife when she went from 165 to 151mm cranks.

mr_pedro

With longer cranks you need to bring the saddle and handlebar down to be able to reach the pedal in bottom position in the same way.
That will give less wind resistance since you are lower on the bike. It also makes the knee more bend when pedal is in top position, which can be more or less efficient/comfortable for you depending on your size and flexibility.

XXLHardrock

OK. Are you talking about not changing fore/ aft or height? With a 14mm crank change, it seems at least one of those would have to change.

AnthonyG

Yes, crank length changes things.
If you use Knee Over Pedal Spindle (KOPS)as a guide (guide only), then the saddle needs to go back with the shorter cranks, as well as up to maintain KOPS and this is a GOOD thing. As the saddle goes back the weight on your hands/shoulders reduces and you will likely find that you're still comfortable with the handlebars staying where they currently are.

Carbonfiberboy 

I just meant fore-and-aft relationship. Of course the height has to change. KOPS is an approximation which works sometimes, but only with standard crank lengths. Many riders prefer being somewhat further forward or aft even with standard cranks. I go by balance, but in this case if balance is good on one bike, make the other the same, CG of rider in same relationship to BB.