Tuesday, October 09, 2007
Is it time to re-think the derailleur?
One area of the bicycle that seems to be overlooked when it comes to equipment performance is friction; especially in chain drives and derailleur gears.
From an engineering standpoint the derailleur gear is an inefficient system. However, since its invention in France in the 1920s, no one has come up with anything better.
The system of a chain drive running out of alignment on multiple sprockets is not used on any other application except a bicycle as far as I know.
When the chain is not pulling in a straight line, there is extra fiction on the side plates and the bearing pins of the chain. There is also friction on the sides of the teeth on the chainwheel and rear sprocket.
When a chain is in alignment there is only slight friction on the bearing pins as the chain goes around the top portion of the rear sprocket and chainwheel. There is little or no friction on the side plates of the chain.
One thing a person notices the first time they ride a single speed fixed gear bike, is the smoothness of the transmission and the lack of friction. This is because the chain is in alignment, and there are no pulleys the chain has to run around.
The pulleys on the rear derailleur are the other source of friction; there is the friction of the pulleys themselves, and the chain has to go around a constant "S" curve. Turning the links of the chain, first in one direction, the other.
One derailleur popular in the 1930s and 1940s was the Osgear. (Left.) It had chain tensioning arm with a single pulley just under and slightly behind the chainwheel.
This meant the chain ran in the same direction and was not made to go around an "S" curve; there was also one pulley instead of two. At that time freewheels only came in 3 and 4 speed.
The Osgear had its shortcomings; it would not work with a double chainwheel because the tension arm was fixed. However, had it pivoted on a simple ball joint and had sideways movement, it would have aligned itself as the chain switched from one ring to the next.
The other drawback was, the fork that shifted the chain on the rear sprockets was over simplistic and shifting was not that good. Had it been designed like a modern front derailleur it probably would have worked much better.
A modern front derailleur is very efficient in that it will shift the chain over a ten teeth span or more, and once it has shifted the chain it is no longer in contact with the shifter and so causes no friction.
The Osgear had fell out of favor by the 1950s when the French made Simplex and Huret derailleurs appeared; they shifted better, and worked with 5 speed freewheels and double chainwheels. The Simplex and Huret rear derailleur had the chain wrapped around two pulleys in the "S" fashion; the way all modern rear derailleurs are designed today. I do feel the Osgear was a very efficient design that was never fully developed.
Campagnolo’s Cambio Corsa derailleur (Below.) patented in the 1930s but developed in the 1940 was a masterpiece of engineering for its time, but extremely difficult to use. A long lever released the quick-release, the wheel moved forward on a rack built into the frame’s rear dropout thereby loosening the chain. Another lever shifted gear while back-pedaling. At the same time, the wheel moved back tightening the chain, and the quick-release was re-tightened. There were no pulleys to tension the chain, so no friction.
Was the Cambio Corsa developed to its full potential? Has anyone ever experimented with sprockets that slide sideways on the rear hub so the chain is always in alignment? It would not be necessary for the hub to be wider, or the rear wheel dished more; the hub could be large enough for the sprockets to slide inside. Another idea, fixed rear sprockets, and a chainwheel that freewheels, gears could be shifted while coasting.
If the chain is to remain out of alignment, how about a chain with spherical rollers at each joint so it will run out of alignment without the friction of the side plates. I know all these ideas will cost more, but with the price of the top of the line bike what it is today, what is a couple of hundred dollars more for a drive train with less friction that will allow a rider to go faster.
The derailleur gear has remained basically the same for over fifty years; all improvements have been in shifting and the number of gears. Friction is overlooked because you can’t see it; and if everyone is using the same design equipment it is not an issue.
In some of my recent posts I have waxed nostalgic and longed for simpler times. I am not against change if it benefits the bicycle and the cyclist. Many changes I see benefit the manufacturer, and then sold to the consumer after the fact.
I am just throwing out a few off the wall ideas that may or may not be practical, but would it hurt one of the manufacturers to put a little money into some research and development to find out just how much of the rider’s energy is wasted overcoming friction?
Posted by Dave Moulton at 10/09/2007 04:52:00 AM