Saturday, July 2, 2011

Part 14 - Rear Wheel Centering

Before the rear wheel was aligned, the rear swing arm was removed to install the belt. The drive belt is a Panther 1-1/2"  139 tooth purchased from Demons Cycle eBay store for $139 or $1 per tooth.

Panther 1-1/2" 139 Tooth Drive Belt

Swingarm removed to install the drive belt (click to enlarge)

The length of the belt was calculated using an online calculator from You need to know the number of teeth on the front and rear pulleys, as well as the center to center distance between pulleys. Make the measurement with the rear wheel roughly in the middle of it's forward/back adjustment position to allow for future adjustment and changes between the ideal belt length and available stock belt length. The transmission pulley has 32 teeth, the rear drive pulley has 65 and the distance between centers is approximately 630mm.

The calculator determined the required belt length to be 138.5 teeth. Rounded to 139 which is fortunately a typical stock length. You may need to play around with your forward/back wheel placement to get a measurement that is close to an easily available stock belt length.
Once the rear swingarm and the drive belt was in place, a laser level was used to align and center the rear wheel with respect to the pro-street frame backbone.

Rear wheel and belt installed

To find the center of the rear wheel, a strip of blue painters tape was placed crosswise over the tire and a mark was made where the tape crossed the tire case edge on each side. The tape was lifted and placed on a flat surface (kitchen counter...) and the distance between the two endpoints was measured and the midpoint between the endpoints was marked. A second piece of tape was run next to the first on the kitchen counter and the marks transferred to it. Then both pieces of tape were returned to the rear wheel roughly 5 inches apart. 2 strips of tape were run lengthwise down the tire one on each side of the center marks to highlight the center of the tire.
Marking the tire center line

Don't use the mold parting line on the tire as the center reference for the tire. For some reason, tire molds aren't balanced around the center of the tire, so the mold parting line (small ridge of excess rubber you see on a new tire - called "flash") is off center by at least 1/4"

Rough alignment of the rear wheel using a laser (click to enlarge)

The laser was sighted down the backbone and the rear wheel was moved into position by sliding it left and right on the axle until the laser indicated the approximate center of the tire. The rear wheel axle adjusters where turned to straighten the rear wheel so that the center line of the tire marked by the tape was in alignment with the laser and thus the backbone. The 7/16 wrench used to manipulate the axle adjusters had to be ground down to fit in the opening. Here's a picture of the customized rear axle adjustment wrench.

Rear wheel axle adjusters Adjustment Wrench

With the rear wheel centered, the lengths for the spacers were measured using telescoping gauges, a digital caliper and a bit of guestimation for where the telescoping gauge wouldn't fit properly. This resulted in the following lengths:
Left Spacer:    2.145"
Right Spacer:  2.130"
Right Spacer between Pulley Brake Bracket and Swingarm:  0.325"

A 1-1/2" OD, 1" ID by 1 foot long stock 6061 aluminum pipe was ordered from for roughly $16. The ID of the tube was on the small side of the tolerance (0.990) so had to be bored out to fit over the 1" rear axle. The spacers were roughly cut to size using a hacksaw and then machined to final dimensions on a  lathe. Machined 2 handlebar spacers to fill in the gap between the handcontrols and handgrips while I had access to the lathe. 

Machining a rear wheel spacer
After the spacers where turned, they had to be sanded smooth and eventually polished. This was a time consuming process but considering the financials of 5 spacers for $16, worthwhile. The final polished spacers ended up as shown in the image below. The remainder of the stock tube is on the left, followed by the 2 handlebar spacers and 2 rear wheel spacers turned on the lathe, a purchased spacer for comparison and finally the small spacer between the pulley brake bracket and the swingarm also turned on the lathe):

Rear wheel and handlebar spacers

Left spacer installed

Right spacers installed (click to enlarge)

The tension on the belt is determined such that a 10lb upward force on the lower belt at approximately the midpoint between the pulleys results in a deflection of about 3/8". A caliper can be used to check the distance from the rear of the axle mounting to the axle to make sure the wheel is correctly aligned with the frame. The measurement should be the the same for left and right right.

Checking wheel alignment - repeat on right side

The axle bolts need to be torqued to 50 ft.lbs for final assembly. Here's a site with useful information regarding belt tension adjustment and belt care in general

A belt tension gauge for applying the required 10 lbs upward force is available on ebay for around $30.

Belt Tension Gauge

Visit eBay store ridersdiscount and search for MOTION PRO BELT TENSION GAUGE 10LB

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