Drilling and welding idler sprocket to frame


New Member
Feb 5, 2016
Hello, I built a motorized bike a while back and welded some engine mounts for it. I did not take into consideration that if the engine is tilted the chain will hit the top of the engine case. No big deal though I bought a idler sprocket to use for a tensioner. My plan was to drill a small hole through the seat tube of the bike and have a rod of metal welded back into the hole to hold the idler sprocket. I was wondering how should I go about finding where to drill the holes. They have to be straight coming out the side of the seat tube or else the sprocket will be tilted. This is hard to do on a piece of tube with not much for a reference point. Here are some images of the bike http://m.imgur.com/a/XK1uI



Feb 10, 2013
Southeastern GA
Welcome Tom!

I think your best bet would be to utilize a jackshaft. Putting an idler gear like that is just asking for the chain to fly off unless everything is perfect. The jackshaft can even be a 1:1 ratio if you've already set it up for the ratio you need.


Well-Known Member
Nov 28, 2012
sf bay area
Is it possible to remove some material from the engine case that is in the way? If you can, do it. Better not to have an idler on the tension side of the chain drive (top), as it will suck up some power.. not a lot probably but why try when you only have an hp or two. Doing a jackshaft will add more drag and complexity and may introduce new problems like spacing side to side.


New Member
Feb 5, 2016
I considered that but I was worried when the clutch was pulled in it would snap off the cover. It may be possible to grind off what's hitting and reweld a tab and tap a new screw hole onto the top. Maybe a adjustable idler sprocket could help solve allignment issues, it would have to be strong though because of the force from the chain pushing up on it.


Staff member
Sep 15, 2008
Littleton, Colorado
First let me welcome you to the forum. There is a lot of bike building experience here to draw from.

You don't want to tension the top, or returning chain run. The tension needs to be applied to the lower chain run. Also unless properly fabricated your tensioner idea compromises the frame integrity by drilling into the seat tube as you plan.

My suggestion would be to discard that rear engine mount and to mount the engine as it was designed. Use the engine mount parts meant to attach the engine case to the seat tube then worry about the front mount. There is some very good information available here for ways to fabricate a front mount for curved or oversized down tubes.This will probably cure most of your alignment problems as long as you get the engine centered in the frame.

Concentrate on getting the engine drive sprocket and the rear wheel driven sprocket precisely in line and run your chain tension at 1/2 to 3/4" of slack. This might require a tensioner and there are many ways to go about making one or using an aftermarket design. I'm personally not a fan of any type of spring loaded tensioner for a motorized bicycle. There are much better ways of doing it.

Keep us informed on your progress.