Custom Drive Train Alignment Procedure

 OK. Lots of you have been talking with us about the proper way to line up the drive train on your custom scoot. Here’s how we do it.

 Let’s start with the basics.

 Setup

If you are starting with a bare frame, load the rear axle into the frame through your rear wheel, and block up the front of the frame at the height you would like to see at the end of your build. If you have the front end and wheel, load them up as well. Just make sure that the frame is stable enough to load in the transmission, and engine with primary drive.

Engine and Transmission

Mock up the engine position (we usually use oversized gauge pins to locate the holes in the engine mounts), then drop the transmission onto the mounting plate, and start the anchor nuts (or bolts) into the plate, but don’t tighten them at this time. If the transmission mount plate has anchor bolts, get them started as well, but do not tighten them up. Install the primary drive inner case (or backing plate) firmly to the transmission. Using the primary case (or backing plate) as a guide, locate and align the transmission to the engine at the front of the primary case, and install the four bolts to hold it in place for now (snug, not tight). You should find that as the engine, primary and transmission are bolted together there is a little movement side to side in the bolt locations. This is where you look down from the top of the motorcycle, and visually align the primary case seam (or the drive backing plate) with the backbone of the motorcycle. For a more precise check, you can use a plumb line off the backbone, and measure over to the case or backing plate edge to verify parallel. You may also notice that the transmission is not seated flat on the mounting plate, or mount locations. This is normal, and can be shimmed out later. For drive train alignment, the transmission will be held in the proper place by the primary.  Your engine and transmission are now aligned with the frame.

Rear Wheel

Using a bright colored string tied to a weight (try a large nut), attach the unweighted end of the string to the steering neck, so that the string can be draped over the rear tire centerline with the weight holding the string in place on the tire. Using this line as a guide, step directly behind the motorcycle, and optically center the rear wheel on the neck, and backbone of the frame (distance helps here). If you like at this point, you can machine a couple of “dummy” spacers for either side of the wheel to keep it centered as we move to the next step. Or, you can simply leave the string in place as a ready reference.

Take a look again from the top of the motorcycle with the string in place. You can even use a plumb Bob and measuring tape to verify that the primary plate is parallel to the centerline of the motorcycle. When it is, snug a few bolts in the following order:

            Rear Engine mount bolts FIRST.

            Check to see that the front mounts are flat and well supported on the frame mount points. If they are not, Shim them with shim stock or shim washers until they are. You may have to disconnect the primary plate from the engine case to be sure the primary and transmission are not influencing the engine position.

Front engine mount bolts next.

Re-attach the Primary plate (or inner primary case) to the engine firmly. As you tighten the bolts, observe the transmission. It may move up, or tilt away from the mounts.

It is important to shim and be certain that the entire drive train is secure and NON Binding as you finalize its position in the frame. As you will see in the steps to come, the rear drive alignment is dependent on the wheel centering and engine/trans alignment as a baseline. It’s like the foundation in a house. If not done properly, everything else you do from here will have to be adjusted as you go, and will not operate as smoothly in the end.

The transmission mounting plate should be attached directly to the transmission without shims. The plate should then be shimmed to the frame and affixed so that the transmission can be removed from the plate, and re-installed as necessary for service without shimming again. If your trans mounting plate is part of the frame, then the transmission must be shimmed to the plate for proper mounting alignment.

Rear Drive Alignment

Once you have the engine and transmission installed, remove the primary plate to expose the transmission drive sprocket.

Check to make sure that the rear axle is square in the frame (front to back adjustment is the same on both sides usually is adequate). Sometimes this method does not assure that the rear wheel is perpendicular to the centerline of the frame. To further verify that the wheel is square, you can place two long straight edges on either side of the rear wheel, and use them to project forward to the front wheel, and measure the distance on either side from the straight edge to the wheel, making small adjustments to the rear axle adjuster blocks until both sides are the same.

Now you have the engine and transmission aligned, the rear wheel centered and straight, and the rear drive can be aligned. Take a look from the rear of the motorcycle forward across the rear wheel hub to the transmission sprocket. Does the rear hub mounting flange extend out far enough for the chain width to clear the tire? If not, use spacer washers to move the drive sprocket away from the hub far enough to allow the chain to clear the tire by a safe margin (.150” Minimum).

Use a straight edge held to the outside face of the drive sprocket to project forward to the outside face of the transmission sprocket. If it lines up perfectly, YOU’RE DONE! (Relax, it never does……)

If the straight edge does not touch the outside edge of the transmission sprocket, determine the amount of adjustment needed to make the two faces line up. If it is less than .200”, you can adjust the alignment by eliminating some of the washer space under the rear sprocket (Yes, this moves the chain closer to the tire), and then make up the balance of the distance by moving the rear wheel off of the centerline to make the two match. Moving off the centerline is not ideal, but sometimes it has to be done to clear frame rails (or even to balance the bike).

If the straightedge goes past the outside edge of the transmission sprocket,  add more washers under the rear sprocket to move it outward and line things up.

If the adjustment needed is greater than .200”, or you want to make it align perfectly on center, you will need to select another transmission sprocket with a greater (or smaller) offset dimension to get things to line up. In this case, you can remove the transmission sprocket, and measure from the inside edge of your straight edge to the outside face of the bearing spacer that is revealed when you remove the trans sprocket. Then give us a call, and we can help you find the closest stock offset available, or build you a custom sprocket to get your project to line up correctly. Sometimes it’s a matter of extending the rear sprocket further, and selecting a greater offset at the transmission to get things sorted out at the rear wheel. Sometimes the frame or swing arm can get in the way.

In the end, you must position the rear sprocket and transmission sprocket within .050” for proper chain operation, the closer to absolute, the better.

Any way you build it, there’s a way. Most frame manufacturers have already sorted out a drive solution for the frame they provide you, and some come with the necessary primary spacer kit and/or extended transmission shaft to get it done. We’re here to help.

The GUYZ

Thanks for taking time to read through this. We’d appreciate your input and comments. If you have something you’d like to add, drop us a line by CLICKING HERE