I was having a chuckle at people whining that the Demo is still running an old school 73 mm BB / 135 mm rear end, and it got me thinking about the advantages and disadvantages of wide BBs. My thoughts are below, and it turns out that narrow BBs may actually have some benefits, at least theoretically.
NOTE: For those that dont know what Q-factor is, its the term used for the spacing width between your feet. DH bikes have a wide Q-factor due to 83 mm BB shells and wider cranks, roadies generally run a narrow Q-factor.
Pedaling: Theory has held that narrow Q factor is better for pedaling, but this is primarily for seated pedaling, and therefore not relevant to DH. What is relevant is standing pedaling, and a wider Q-factor will require more body movement (or bike movement) to get your weight over your driving leg. See-sawing the bike back and forth to get more power out of your pedal stroke is a waste of energy. -1 for wide BB.
Stability: A wider stance most likely will feel more directionally stable when not pedaling. nuff said. +1 for wide BB.
Cornering: This one is not so straightforward. Lets consider flat corners, since this is more critical in racing. Motorcycle road racers keep their weight low and inside on flat corners, and the same applies for mountain bikes. A wider Q-factor places the force driving through your outside leg further from the bikes centerline. This is not ideal, and the moment generated by the force from your outside leg has to be countered with more inside force at the handlebars.
For the best cornering stability, requiring minimum correcting force at the handlebars, youd want the force from your outside leg driving right down the centerline of the bike. This is obviously not possible, but a narrower Q-factor does place the force closer to the centerline.
Furthermore, like the motorcycle road racer, the best position for your weight from a dynamics point of view is low and inside, and the narrower Q-factor would be preferred. -1 for wide BB.
So maybe Specialized know what theyre doing with their narrow BB. (Rear wheel strength/stiffness is another issue. As is chain clearance. There are obviously trade-offs.)
I would love to ride two identical bikes, one with a 68 mm BB and narrow cranks (XTR?) and one with an 83 mm BB and wide cranks to see if there was any noticeable difference or if Im just blowing smoke Probably the latter, but bike design is getting pretty sophisticated these days, so you cant blame me for thinking about it
NOTE: For those that dont know what Q-factor is, its the term used for the spacing width between your feet. DH bikes have a wide Q-factor due to 83 mm BB shells and wider cranks, roadies generally run a narrow Q-factor.
Pedaling: Theory has held that narrow Q factor is better for pedaling, but this is primarily for seated pedaling, and therefore not relevant to DH. What is relevant is standing pedaling, and a wider Q-factor will require more body movement (or bike movement) to get your weight over your driving leg. See-sawing the bike back and forth to get more power out of your pedal stroke is a waste of energy. -1 for wide BB.
Stability: A wider stance most likely will feel more directionally stable when not pedaling. nuff said. +1 for wide BB.
Cornering: This one is not so straightforward. Lets consider flat corners, since this is more critical in racing. Motorcycle road racers keep their weight low and inside on flat corners, and the same applies for mountain bikes. A wider Q-factor places the force driving through your outside leg further from the bikes centerline. This is not ideal, and the moment generated by the force from your outside leg has to be countered with more inside force at the handlebars.
For the best cornering stability, requiring minimum correcting force at the handlebars, youd want the force from your outside leg driving right down the centerline of the bike. This is obviously not possible, but a narrower Q-factor does place the force closer to the centerline.
Furthermore, like the motorcycle road racer, the best position for your weight from a dynamics point of view is low and inside, and the narrower Q-factor would be preferred. -1 for wide BB.
So maybe Specialized know what theyre doing with their narrow BB. (Rear wheel strength/stiffness is another issue. As is chain clearance. There are obviously trade-offs.)
I would love to ride two identical bikes, one with a 68 mm BB and narrow cranks (XTR?) and one with an 83 mm BB and wide cranks to see if there was any noticeable difference or if Im just blowing smoke Probably the latter, but bike design is getting pretty sophisticated these days, so you cant blame me for thinking about it