Chain Centerline with Grubee Box

[Duhawki]

I am on the waiting list for a 4 stroke kit from the Bird Dog guys. I'll be bolting the engine into an old Schwinn Cruiser that has an Atom rear drum brake. The backing plate side of the hub is 92.4 mm in diameter against the spoke flange. I would like to get started with the rear sprocket mounting; so, I ordered a 48 tooth 415 sprocket which I will machine to fit. Anyone know about spacing? How far from the centerline?
Regards,
Duhawki

 

[HoughMade]

The engine can be adjusted over the course of about and inch left to right. I just mounted the sprocket and adjusted the engine to match.

 

[Duhawki]

I assume you are using the frame mounted plate that came with the kit? Then how much clearance is there between the chain and the seat tube. If I know that, I've got something to work with. I'm not sure I will use the mounting plate.
Regards,
Duhawki

 

[ocscully]

It's not just the seat tube you need to clear. The widest object between the output of the engine or gearbox and the driven sprocket is usually the Tire. So the chainline you are looking for is half the width of what ever tire you are going to use plus half the width of your chain plus a clearance factor. The first measurment I would look at is the distance from the center of your rear hub and where you think you are going to mount your driven sprocket. This distance again should be at a minimum half the width of your tire plus half the width of your chain plus a clearance factor. Assuming it is at this distance or more from the center of the hub, then you will want to be sure you can get the motor and its gearbox output at this same distance. If the measurement from the center of the hub to the driven sprocket is less than half the width of the tire plus half the width of your chain plus the clearance factor then you need to add thickness to your driven sprocket mount to space it out to this minimum distance. (I hope this is making sense)

Perhaps Hough or Irish John can post a measurment from the center of their seat tubes to the center of the output sprocket of the gearbox just as a reference.

ocscully

 

[Duhawki]

Yep, clearance for the tire is the priority. I understand all that completely. I'm an engineer, and I'm in "design mode" right now. What I'm trying to do is find where the rear sprocket needs to be for clearance elsewhere. Thus, I need to know the location of the front sprocket. As a reference, if someone would please measure the chain clearance at the seat tube that would help.
Regards,
Duhawki

 

[HoughMade]

I wish I could give you measurements...I know engineers need measurements (my Dad is an engineer- missiles and medical devices for over 40 years).

However, my bike is completely dissasemmbled to be painted. I can only talk in generalities. My frame is a standard medium cruiser frame of the cantilever variety (it's under there somewhere), and I had absolutely no problem clearing the seat tube. There was probably a little over 25" clearance. Doesn't sound like much, but it's plenty.

What created some clearance issues for me was the tire (as mentioned above), but I am using 26 x 2.125. Even then, I was able to set it up to clear the tire by over 1/8" (ok, engineer... .125"). This required me to move the engine as far to the left as possible. All the way to the left in the slots, and then I tightened the right side plate to block bolts first so it pushes the plate another 3/16" to the left. Then I tightened the left side.

The alignment for the drive sprocket to the wheel sprocket is fine. I use a #41 chain that provides a little more space between the plates and I ran this setup for enough miles mocked up (probably 100) to know that the chain stays where it is supposed to.

I must use the chain tensioner, however, to gide the chain a bit so as to not rub on the wheelstay.

If I can find any pics that show the areas at issue, I will post them.