I've been playing around with bicycle design lately, and I'm thinking about all the adjustable angles and lengths that need to blend into something called a bike frame.
But right now, I'm thinking about BB drop - or the vertical distance between the center of the BB spindle and the center of the rear axle. Below are thoughts on what I've learned and a few follow up questions. Please do respond, but I don't expect to everything.
I've read
this article that suggests a shallow BB increases the ability of the rider to lift up the front wheel/bunny hop, and suggests a deeper BB drop creates a 'sitting in' the bike feel. There's also
this great discussion that suggests a shallow BB drop will increase the rigidity of the frame (through shorter tubes), while a deeper BB drop will increase comfort (through compliance). This article also tries to debunk the claim that a higher BB will raise the rider's center of balance and therefore cornering stability, pointing to lean and ground conditions instead. However, I do believe the good author could have addressed how a high positioned rider compared to a low positioned rider may feel more 'on top' and potentially more playful while riding. There's
this one that discusses a growing interest in deeper BB drops on cross bikes as clearance isn't much of a problem during cross racing, toe clips aren't a problem (mostly), and jumping off/on may be easier while racing over barriers. Finally, there's
this post on the triathlon forum that suggests deeper BB drop might be more aero. However, I would suggest aerodynamics is more of a function of fit and rider flexibility than BB drop. Do other's have ideas on how changes to BB drop will impact ride quality?
Secondly, I'm wondering how frame designers set the BB drop to a bike given a customer's preference? What indicators do you use to define the BB drop? I would think the following are important:
- Use case - is the customer going to corner tightly (crit racing, xc mtbing) - points to shallow BB drop
- Crank arm length - are long or short crank arm being installed - long arms point to shallow BB drop, and short deeper (at least is available)
- Customer preference - does the customer want to sit 'in' the bicycle or does the customer want to bounce around on top. - latter points to shallow, the former, deep.
Another angle to this question might be, are BB drops informed by customer preference or designer preference? Thinking about a designers 'style' here.
Also, what's the deal with BB drop + BB height = ~ radius of 26" wheel? -> see
paragraph 3 here. Anything?
I understand this might be a little long. Please comment with thoughts on any part. I don't expect anyone to respond to everything here.
Thanks in advance!