I have spent some time now going over the geometry of the tourer/commuter bicycle that I’d like to have built and think I have come to understand some more of the balance of bicycle dimensions.
Bottom Bracket placement
Bottom bracket drop is the measurement of the distance between the centre of the bottom bracket and the plane or line between the font and rear axles. This measurement is not to be confused with bottom bracket height which is the height of the centre of the bottom bracket above the ground. Bottom bracket height changes significantly with tyre size and so is prone to variability depending on variables associated tyre choice. Bottom Bracket drop is most often used to describe bottom bracket placement as it relates more closely to the actually geometry of the frame. Having said that, the measurements are relative to each other and when added, should equal the height of the axle above the ground.
High bottom brackets with a 6 -7 cm drop are generally considered in situations where high clearance and pedaling around corners is required. Think velodromes, cross country racing and down hill racing and/or generally acting like a hooligan. Wheel size also plays a part in these very different disciplines, as this will raise or lower the bottom bracket relative to the ground. At the other end of the spectrum we have bottom brackets that go as low as 8 cms in drop or sometimes lower. These are road race and touring bicycles. Lowering the bottom bracket lowers the centre of gravity because in doing so you lower the height of the the top tube given that the optimal distance between pedal and hip is maintained. A low centre of gravity is great for fast descents and general touring stability. So bottom bracket height is a significant design feature functionally, but is also a factor in the aesthetics of your bicycle.
Lowering your bottom bracket also slightly increases the length of your chain stay and down tube, which are apparently the most stressed tubes in the bicycle frame. So bottom bracket height should also be considered with your choice of frame materials and need for frame stiffness. Longer spans means that a more elastic material like steel with provide a more pliant ride, whereas an un-lugged aluminum frame may not like the extra stress. higher bottom brackets, shorter chain stays and down tubes all add to stiffness, where as longer chain stays and down tubes, with the right materials may provide more comfort, and soak up more vibration.
All this got me thinking again about those sport picnic, randonneurs and french bicycles. If for example you quickened up the steering by reducing the trail, but didn’t lower the bottom bracket height, you might end up with a shimmy prone, and slightly twitchy ride. If however you wanted to keep the advantages of a low trail, and avoid some of the potential negative side affects, perhaps lowering the bottom bracket is part of the answer.
Crank length is a consideration for efficiency of pedal stroke and ground clearance. With regard to ground clearance, as you lower the bottom bracket, and effectively increase your likelihood of pedal strike on the ground, some may opt for shorter cranks. But the more important factor here is efficiency. Now I wouldn’t believe a word I said about the details of pedal stroke efficiency as I know nothing about it and there are some fairly serious bio-mechanics involved. But to summarise what I have gleaned, there are optimum limits for your leg and knees to operate in, and if you ride a lot it is worth having a suitable crank size to limit the possibility of knee injury and make the most of your effort. For more information, here is a calculator, and here is what Sheldon Brown has to say.
The established standards are 7cm bottom bracket drop and 170mm cranks. But what would you design? BikeCAD is here to help, select quick start to start quickly. I found this application pretty intuitive and within a short time had the dimensions of my bicycle in front of me. In my extremely limited experience of having custom built bicycles (zero – but talking with Mercian) I have found that it is a useful tool in designing and communicating what you want and what is possible. It’s also fun.