Mounting Engines: What to Watch Out for Before Your Next Build

Motorized bike builds are awesome: it’s all the fun of a moped or motorcycle without the weight, a special license, and insurance. But if you’ve built a few motorized bikes- or know anyone who has- chances are not every build was a piece of cake.

If a frame is too big special mounts need to be made, if the chain rubs against the tire a special sprocket adapter is needed… The bad part of an engine not fitting a bike is that when it just won’t fit you’ll either need to do plenty of fabrication or swap the bike entirely. So before you go shopping for a new bike and engine (that may not work together), here are a few things to consider:

2-Stroke Mounting Precautions

The 2-stroke engine is simple to upgrade and fairly simple to fix. But installing a 2-stroke engine… now that can be tricky. There are plenty of things to consider when installing a 2-stroke, but when it comes down to it there are three big challenges 2-stroke riders are going to face:

The first big hurdle is making sure the engine motor mounts will fit on to a bike frame. That means you need a frame with 1.5” (38mm) down tubes, seat tubes, and top tubes to make sure the engine and tank will fit on the bike. If your frame is bigger than 1.5” you can use vibration motor mounts and shape them to fit the motor properly on to the bike.

Another big issue for 2-stroke riders is the chain and sprocket clamp assembly. If you’ve got tires that are 3” or bigger your chain is going to rub against it, wearing down your tire. In addition, the rubber sprocket clamp assembly you get with engines is often too weak to keep that sprocket aligned at all times. If you chain’s misaligned and rubbing against your tires that’s a mounting issue that can lead to some serious danger. In cases like this it’s a good idea to use an upgraded sprocket clamp assembly (like the one from BBR Tuning) to give your rear sprocket more security, and it would hurt to upgrade your chain idler and align it with your rear sprocket for better, smoother riding.

The height of the engine may also give you issues when mounting. Most bike frames will accommodate the 11” of clearance you need for that engine. Lowrider bikes, though, fit only the engine and have no room for a carburetor. That’s unless they use an offset intake manifold to shoot the carburetor to the side of the engine.

4-Stroke Mounting Precautions

There are two main types of 4-strokes bike engines: chain driven and friction driven motors.

  • Chain driven motors sit in the center of the frame and are run with a chain from the transmission to your bike’s rear wheel
  • Friction driven engines use a friction bar that sits on top of your tire to drive your bike. Simple enough, right? Well… here are a couple of things to keep in mind when building 4-strokes:

Chain-driven motors require the same frame sizing as 2-strokes, but with a twist. Because the chain-driven 4-stroke is wide you’re going to need a bike you can extend the pedals on. If the bike is a good fit for the engine, also keep in mind that there are only a handful of performance upgrades for 4-strokes, primarily mufflers. However, the upgrades available (like the 2-stroke adaptable manifold) will definitely make your motor run smoother. So while these motors may not be race bikes, they can definitely make fantastic commuters.

Friction drive motors are more universal, and we’ve seen them on just about any style and size of bike you can think of. What you’ll want to look out for is weight displacement and tire size. Remember that when a friction drive is on your rear wheel you are too, so if you have a bike that may be lighter up front be careful when starting not to accidentally wheelie yourself (trust us, we’ve seen it happen on a friction-driven BMX). Also, most friction drive mounts will fit up to 3” wide tires, so anything that size or larger may not fit.

Chopper Bike Mounting Precautions

No one can deny how awesome a motorized chopper bike is, but try building one and you’ll learn that there’s a lot more to it than tapping the handle bars and bolting on motor mounts. While they’re great looking builds they are difficult, so here are three things to keep in mind when motorizing a chopper bicycle:

Mounting the engine and gas tank has got to be one of the biggest challenges facing chopper builds.

  • Most 2-strokes have a special cradle mount that allows the motor to mount toward the front of the bike instead of under the seat where this is no way to mount the motor
  • 4-stroke chopper builders also have to rig special mounts to fit the engine in the frame which can take lots of fabrication to execute the right way

With a special mount comes the second hurdle: extending the drive chain and throttle/clutch cables. Chopper bikes often have stretched frames, which most motorized bike manufacturers don’t account for. Usually most choppers require 2x 415 chains to be put together in order to make up the distance. And if you’re a chopper rider with ape hangers and a long frame, you’re going to need to custom make cables long enough to reach from the grips to your motor.

Lastly, you’ll want to consider a new sprocket adapter. Chopper bike tires are at least 3” wide, so the standard rag joints usually won’t do. A sprocket adapter that sits the rear sprocket out from the wheel will make sure the chain doesn’t rub and burst your back tire.

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