Mike Leger's M5 low racer HPV recumbent bike chain chain management article

M5 Lowracer Idler/Chain Management modification
An article by Mike Leger  

Iíve been having problems with chain management on my M5 since purchasing it a year ago. After having the chain pop off  6 times in a recent race--always on a steep climb--I decided I'd had enough. Prior to the race, the chain would frequently pop off the idlers, or the return side of the chain would turn itself 90 degrees off axis and lodge in the inner idler instead of itís proper outer idler location. This would happen when I encountered bumps, or when shifting hard. In addition, there was an enormous amount of chain flop. I am unconventionally using a triple chain ring up front, and frequently, an 11-34 rear cassette. Iím sure this compromises the intention of the designerís chain management concepts. With that said, thereís some room for improvement over the original design. 

This picture is of the original M5 idlers with a few thousand miles wear. The bent aluminum piece below the idlers is intended as a chain keeper. It's a poor solution. I showed the original idlers to a local instrument maker named Tim who's interested in recumbents, having built 2 versions of leaning trikes. 
His solution to the problem was to create a u-shaped chain keep that maintains a very close tolerance between the idler walls and the keep. 

 

He started with 4" McMaster-Carr idlers, which when mounted required a little shaving of the elastomer stack at the front of the seat to to allow clearance for the new larger idler. The mounting for the elastomer stack required similar treatment from the barber.  The shaved elastomer can be dimly seen in this picture.
The new width of the idlers also required a longer idler rod.

In order to improve the tolerance of the McMaster idlers, Tim shaved 3/16" off the radius of both idlers. In addition he also opened up the V at the bottom of the idler groove to a U shape. 

 

This provided a nice tight little bed for the chain to occupy. The U-shaped chain keep was cut to fit within .025" of the idlers, a space which does not allow the chain room for flopping around.

The groove and tolerance can sort of be seen in this picture. One of the Tim's trikes is in the background. 

 
I've put about 75 miles on the new idlers. The smoothness of the new idlers was immediately remarkable. The chain management solution is excellent. The tolerance allows no space for the chain to dislodge itself, and minimizes friction compared to chain tube solutions. It's only problem is chain contact at the front lip of the U-shaped piece. I'm playing with the fore/aft angle of the piece to see if that makes a difference. It also acts as a 
chain flop dampener, and sort of amplifies the shift to and from the inner chain ring, due to the restriction of chain movement.
This picture shows the difference in size between the old and new pulleys. The new pulleys are not only larger, but are made of a harder plastic. Both of these features should increase the efficiency of the drivetrain.

These idlers are Fenner Drives "Powermax V Belt Idlers"
4" - VA4001
6" - VA6001
7.5" - VA7501
The 4" idler is available from McMaster Carr, and the 6" and 7.5" idler are available directly from Fenner for about $10 each. ($50 minimum order)

 

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