View Single Post
Old 10-31-2009, 11:15 PM
DCX35 DCX35 is offline
Motorized Bicycle Newbie
Join Date: Oct 2009
Location: Ann Arbor, Michigan
Posts: 3
Default A newby's chain idler pulley alternative

Hi all - I'm a newby to motor bicycling and to this forum. I have been reading your informative threads with great interest. Lot's of great tips there. I am running an 80cc Zoombicycle engine on a 27" Schwinn Cross-Fit 18 speed bicycle. It was a simple installation (slight exhaust pipe bend the only complication) and has been trouble free during the first two hundred break-in miles. I made what I hope to be an improvement over the original kit's chain idler as follows:

The standard chain idler pulley has a single ball bearing set in its center.

Here is an alternative chain idler pulley with two ball bearing sets, one at either side which I have been using with success:

** Purchase a standard 52mm skateboard wheel with the highest hardness available (durometer of 99a or 100a)

** Purchase a set of ABEC 9 (highest quality) skateboard wheel ball bearing sets

** A set of (4) Skateboard wheels with (8) matched ABEC 9 bearing sets are usually available on eBay or the internet for under $20 and will give you enough material for (4) chain idler pulleys.

Chuck up a skateboard wheel into your drill press’ chuck with a bolt through its center, secured by a nut and two fender washers, one at either side. Note: use tape wrapped around the bolt to adjust its diameter to perfectly fit into the center hole of the skateboard wheel to keep the skateboard wheel properly centered on the bolt.

Hold a coarse ½” wide file steadily against the circumference of the spinning skateboard wheel at the middle between the wheel’s sides until the file has machined a ½” wide chain groove (for use with #40, #41 or 415 chain) 0.200” deep, into the skateboard wheel. This will leave 3/16” thick chain guide fences at either side of the new chain groove. Add a slight taper to the sides of the chain groove by slightly tilting the file as the wheel spins so the configuration of the chain groove ends up similar in configuration to that of the original equipment chain idler’s groove. When finished, the chain groove will be 1.66” in diameter which is 0.300” larger than the original equipment idler pulley’s 1.36” chain groove diameter (a good thing - less chain flexing required as it passes over the idler).

IMPORTANT: When using standard 52mm skateboard wheels, there must be a 0.405” thickness steel spacer stack used between the two ball bearing sets. The spacers must have a 3/8” ID and no larger than 0.600” OD to fit inside the center hole of the skateboard wheel. 3/8” lock washers satisfy this diameter requirement. Flatten out five lock washers (placing them part way into a vice and bending them with pliers until totally flat). The 5 flattened lock washers should produce a spacer stack of 0.405” total thickness. If slightly over this dimension, grind several of the washers until the total stack thickness becomes 0.405”.

Press the ball bearing sets into sides of the grooved skateboard wheel with the stack of spacers between the bearing sets such that when the chain idler pulley’s axle bolt is tightened, the bearings will be prevented from pulling together which would cause side loading of the bearing sets and binding. The tightened chain idler pulley should spin freely when its axle bolt is fully tightened.

I have run this type of chain idler for several hundred miles with no problems or noticeable wear.
Reply With Quote