Wednesday, February 21, 2007

Bottom Bracket Height

Let me explain the difference between bottom bracket drop, and bottom bracket height. Bottom bracket drop is the measurement from the bicycle’s wheel center, to the center of the bottom bracket. Once a frame is built this measurement is fixed and never varies, therefore it is the most accurate.

However, bottom bracket height is easier to visualize and so is widely used. It is the measurement from the center of the bottom bracket to any level surface that the bicycle is sitting. This measurement can change because fatter tires will raise the bottom bracket height.

On the spec sheet for my Fuso frames, [PDF file.] I listed both bottom bracket height 10 5/8 inches, and 2 ¾ drop. If you add the two measurements together it is 13 3/8 inches, the radius of an average size wheel. (26 ¾ inch dia.)

The argument usually put forward for a low bottom bracket is that it lowers the center of gravity and therefore improves stability. I do not subscribe to this theory because center of gravity is not really an issue on a bicycle, and raising or lowering it has little effect on stability.

On a three or four-wheel vehicle a low center of gravity is important because when cornering at speed the centrifugal forces generated can cause the vehicle to tip over. However, a two wheeled vehicle leans into a corner, and the centrifugal forces actually push the bike down onto the road, which assists traction.

You seldom hear of a bicycle or motorcycle tipping over or falling outwards on a corner; if the rider goes down it is because they leaned too far and the bike slid out from under them. Alternatively, they fell because of road conditions like water, ice or loose gravel, but once again the bike slides out from under the rider, and it is loss of traction not center of gravity that is the issue.

If C of G were an issue, a bicycle would be a lot more difficult to ride; the bicycle can weigh less than twenty pounds and the rider a hundred pounds and above. The center of mass is somewhere in the center of the rider’s body some four feet or more above the ground; proof of this is the racing tricycle. These fascinating machines, rarely seen in the US, are very unstable on corners and it takes a great deal of skill to corner at speed and not tip over.

Picture from the [UK Tricycle Association website.]

This is why I maintain raising or lowering the bottom bracket on a bicycle has little effect on its stability, the center of mass is still very high.

The advantages of a high bottom bracket are obvious on an MTB or a cyclo-cross bike going over rough terrain. Pedal clearance on a road bike when cornering is another, but with clipless pedals this is less of an issue that it was in the 1980s.

The disadvantage of a high bottom bracket is that it makes it difficult to reach the road with your foot when you come to a stop.

Raising the bottom bracket even a little, shortens the chainstays and the down tube on the frame; conversely, lowering it will lengthen them. This is because the wheel center remains constant and so do the rear dropouts. The front fork remains the same, so does the bottom head-lug of the frame.

If these points of the frame remain constant, raising or lowering the bottom bracket shortens or lengthens the lower tubes in the frame, it also raises or lowers the top tube and therefore lengthens or shortens the head tube.

If I raised the bottom bracket on a criterium frame, it was not just to achieve more ground clearance; it was to make a more rigid and responsive frame. The head tube became longer, but as this is the least stressed tube in a frame, it had little affect. On the other hand, the down tube and chainstays are the highest stressed tubes in a frame and shortening these is a definite advantage.

If I lowered the bottom bracket on a touring frame, it was to lengthen the tubes to make a more comfortable ride. It had nothing to do with stability.

With any design aspect it is best not to go to extremes, the 10 5/8 inch (27 cm.) bottom bracket height or 2 ¾ (7 cm.) drop was where I built most of my frames, and is still a good average.


Anonymous said...

love your blog. i'm constantly learning more and more about cycling, how frames are built and how and why they respond the way they do. i feel like it is also making me a better rider. thanks.

I was wondering what the BB height was on the track frames you made and how or if the bank of the velodrome comes into play there?

Dave Moulton said...

I usually built track bikes with around 11 inch (28cm.) bottom bracket height. If a rider is moving slowly, they are sometimes vertical on a banked track so it is possible for the right pedal to hit the track surface.

Frames built specifically for very steep tracks can run as high as 11 ¼ inches. (28.6cm.)

Korps said...

First off, I want to thank you for your past and present education and hopefully many more in the future of your blog postings here. My very first road bike was a John Howard, built by you. I still own the bike, and now in the process of making it into a awesome fixed gear. Jsut wanted to say thanks!!!

Paul said...

It's not actually fixed, though (drop, once a frame is built). I primarily ride mountain bikes, and as we who assemble our own bikes know, there is huge variance in available axle-to-crown distances of mountain forks. Of course, this also plays havoc with steerer angles, which is why people with mountain bikes from 10 or 15 years ago are mostly being forced to buy new frames to accommodate today's longer-travel suspension forks.