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Wednesday
Feb212007

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.

Monday
Feb192007

75 Bucks for Scrap Metal on Ebay


After I wrote about this Masi frame on eBay February 10th; it was withdrawn then re-listed item number 230091061463. I don't believe the seller saw my blog because he still listed it as a 1984 when I pointed out that it was in fact built in 1981.

My only interest was that it was originally built by me, and it gave me the opportunity to write about the Masi numbering system. The frame looks like it has been run over at some point.

The seat tube is cut out; it was probably damaged like the top tube. But why were the rear chainstays cut off just in front of the rear dropouts? This appears to be one of the few undamaged parts on the frame. The front fork was also probably undamaged, but it is missing.

This item sold for $45 plus $29.99 for shipping; a penny shy of 75 bucks for a piece of scrap metal. It never ceases to amaze me what people will pay for stuff on eBay.

Saturday
Feb172007

Old tech, new tech


[Click on picture to view a larger image, use back button to return.]

I came across this old ad from the early 1900s; it states “Removes one great drawback of cycling, viz. Perineal Pressure.”

Just goes to show, old tech becomes new again if you wait long enough. When it comes to the bicycle there is not too much that hasn’t been tried at least once before.






Left: The San Marco Caymano Arrow-Head Gel saddle, one of many similar designs on the market today.

Wednesday
Feb142007

Custom Touring Bike

Built in December 1982 this custom touring bike is quite rare; only 20 of these were built. This one has mounts for front and rear panniers, and mudguard eyelets.

It was built for my long time friend and photographer David R. Ball, who still owns it; it is his regular ride. David gave me a total freedom to design this one including this one of a kind paint job.


Before delivery the frame was to be a show piece for the Interbike Trade Show.

The two-tone dark and light green metallic finish called for some very intricate masking. The white striping that separates the two colors was done with automotive striping tape.

This meant I could easily make the perfect straight lines, however it did take 8 to 10 clear coats over the striping tape, with sanding in between to completely “bury” the tape for a smooth to the touch finish.

The amount of man-hours involved in doing this particular paint scheme made it impractical, and I never did another like it. However, inspired by this frame, a simplified version came over a year later on the production Fuso frames. (See picture below)

In 1983, Bicycling Magazine did a road test on my touring model. This was back in the day when Bicycling had some decent articles. You can read it here as a PDF file.


Monday
Feb122007

Interview

Last Friday (Feb 9.2007) Industry Outsider did me the honor of an interview.

When someone else asks the questions, it brings out material that I normally wouldn’t bring up myself, or stuff I wouldn’t even think of.

Thanks IO for the opportunity.