Chopper bikes a quite a sight to behold! Stretched frames, big tires, big bars- nothing looks more fierce than a motorized chopper. But building one is not as simple as putting an engine kit on a standard cruiser. Although there are many similarities, there are some considerations you will want to keep in mind when motorizing chopper bike. Rear Hub/Sprocket Mount: Most chopper bikes’ rear wheels are wide rear wheels to give it the classic look. However, this makes mounting a rear wheel difficult because the rear sprocket will sit inside of the wheel further than with a standard wheel, making it nearly impossible to align the chain. Rear chopper wheels, therefore, need special parts to mount a rear wheel.
Without special sprocket mounts, the engine sprocket will not align with the rear sprocket and the chain will be crooked. Moreover, the chain will rub against the rear tire, leading to damage to the tire and possibly the chain. There are a couple of methods on the market to set the sprocket out further than gaskets will. One solution is a mount that consists of a circular adapter (around ½” thick) which has ports drilled into it. This particular style of mount requires the installer to drill into the hub of the wheel and bolt the adapter to the hub. The sprocket is then mounted on the adapter and sets the sprocket far enough away from the wheel to clear the tire. Such mounts are great for OCC chopper bikes, as these only require about ½” of room from the wheel to clear the wheel. There are wider wheels on the market that require more than ½” of room from the wheel to avoid chain rubbing. In this case, something like the Manic Mechanic Sprocket and Adapter assembly will be perfect for this application. These mounts bolt around the hub and give around ¾” of clearance from the wheel.
Other solutions may include adding gaskets to the sprocket mounting hardware provided with the kit or simply using a smaller wheel. No matter what method is chosen, keep in mind that the chain should not run against the rear wire and at all times the chain needs to run on a straight path between the engine and rear wheel sprockets. Engine Mounting:
Mounting an engine kit to a chopper bike requires special engine mounts in order for the engine to sit in the center of the frame properly. One such mount would be a special chopper mount. The standard motor mounts on an engine allow for installation on the down tube and seat tube. However, most choppers don’t have the frame to do that. In this case the engine will need to be mounted toward the front of the bike. As such, this style of the mount will allow the engine to sit in a cradle toward the front of the frame. This will allow the engine to be mounted in the position needed for most chopper frames.
For frames with over-sized tubes, there are motor mounts- like the Manic Mechanic motor mounts- that provide a wider mounting base for the frame to go around. Available in sizes from 1”-2”, these mounts are perfect for those looking to mount an engine to a chopper with tubes larger than 1” or custom frames with oblong spacing. Extensions:
Choppers look so great because they have a stretched look to them. However, gas engine kits these days are made for standard 26” beach cruiser frames which are pretty compact. Chopper frames are usually stretched, so there are two components that need to be extended in order for the engine to function properly: the chain and the cables. The size of your drive chain is integral for the engine’s functionality. The 415 chains that come with most engine kits are sized for standard frames, so stretched frames require much longer chain. Doing so requires you to connect at least two chains, and to do so you remove the master links from both chains to lay the chains out. Connect the two with a single master link and measure how long you need the chain to be from the engine sprocket to the rear sprocket. The installer will need to take off some links in order for the chain to fit properly, but because every engine kit and chopper is different it is up to the installer in to determine how many links need to be removed. After the appropriate number of links are taken off a second master link will be needed to complete the chain.
The same thing goes for the throttle and clutch cables. If installed at the length they are, the rider risks clutch and carburetor damage. The installer will need to find cables long enough to give the slack needed for proper throttling and clutch engagement. Motorized choppers are a ton of fun but require some creative thinking. If you or someone you know is interested in motorizing their chopper bike, these tips will help determine what you need to take when embarking on such a build. With the right parts and preparation, your next chopper will be ready for the streets and bike shows!