Honda GX35 Frame Mounted Cruiser

I've been lurking for awhile now and I thought it time to start a build thread. This is my first build. This will be pretty much a real-time thread, as I am just beginning the build process. Since this is my first post I thought it appropriate to outline what I intend to do, what decisions I've made and what decisions I haven't. My goal is a reliable, quiet bike capable of cruising 20mph smoothly. That is what is legal in Illinois where I intend to ride it(I work there). I prefer 4-stroke engines to 2-stroke engines, I understand the advantages of 2-strokes, but I prefer the civility of 4-strokes. I chose the Honda GX35 and I got one from SmallEngineWarehouse. I also got the Honda clutch drum because I don't intend on using a gearbox. It's expensive at $75 but it's made out of billet steel, and is very strong. When I did my research here, I wasn't impressed with the reliability of any of the available gearbox's. I will get my primary gear reduction with a chain-driven jackshaft.
What I haven't 100% decided yet is the final drive configuration. I don't know if I am going to do a friction drive or a belt direct drive. I have no doubt I could make either work well. Friction drive would be simpler. Wayne Z is doing great work with his Predator friction drive, his results are impressive to say the least. But at the moment I am leaning toward a direct drive set up. I would attach a bicycle rim to my rim similar to what harry76 did with his brilliant Villiers project. This path will require way more work, the frame would have to be modified for the belt to clear and building that rear wheel will be a project in itself, but I think the results could be pretty neat. There is something about the belt and that big hoop on the rear wheel going around and round that is cool to me.
Anyway, I have some work done already, but I don't want this post to be too long, I could ramble on forever! Oh yeah, I probably should mention the bike I got for this build is a mid 70's Murray Westport Cruiser I scored off Craigslist.

I've begun the mount for the motor. First I removed the fuel tank because I want to run a remote tank anyway and I need to get under the motor. The only mounts for this motor are on the face of the clutch housing. Four bolts that are 1/4-20 thread? I want to frame mount the motor and I don't want the motor hanging off the side, I want it centered, so here's what I'm doing. I welded two pieces of 1/4" plate together at 90 degrees and drilled the front for the clutch housing bolts and the crank.


The bottom of the motor has two bosses.


They are drilled but not tapped. I will tap them and run bolts through the bottom of the bracket. With this done I will have a nice surface to attach the motor bracket to the bicycle frame. I haven't quite figured exactly how I will do this yet.
What I am thinking at this point is cutting some tubing from a donor bicycle frame and cut it to run between the downtube to the seat tube and weld it. Then the bracket I made could easily be attached to this tube.

Good point happyvalley, sometimes I automatically think the hard way is the best way. Not sure what's wrong with me.

Thanks HV.

I have a question for those familiar with the GX35. At approximately what speed is a happy cruise RPM for it? I want to take an educated guess at what my total gear reduction should be. I can achieve anything I need with the jackshaft setup.
I've pretty much decided on the direct drive route for the final drive. I picked up an old Huffy ten speed with 24" rims. I plan on attaching one of the rims to my rear wheel. If I drive it with a 2" pulley, that's about 12:1 reduction right there. I can easily do 2:1 on my primary, which would give me 24:1 total. Which is about 22mph at the 7k redline. Obviously I don't want the motor near redline at my intended cruise speed of 20mph, so I will need to reduce the amount of reduction on the primary. But I need to know where this motors' sweet spot is in order to figure out how much.

If I don't get any advice, I will shoot for 5500rpms @ 20mph. I thought I read somewhere that the clutch fully engages at about 4000rpm. 5500 is halfway between that and redline. What do you guys think?

Thanks again HV! I'm glad that I was wrong on the clutch engagement RPM, the wider the usable powerband, the better. I'll shoot for the 5500@20mph and fine tune it from there. One of the reasons I wanted a chain primary was the ease of changing gear ratios. I'm using sprockets from the local farm center and 40 chain. I invested in an American-made chainbreaker and plenty of extra chain. I realize this stuff is kind of heavy, but I'm OK with that. I'm expecting to pedal this bike from a stop and then adding power. 35cc and that little clutch can only do so much. Illinois is flat as a board and my top speed is reasonably low, so I'm hoping the extra mass of the chain drive will serve as a sort of flywheel effect and help to smooth the ride. I'm not concerned with it sapping acceleration. Of course, I've never done any of this before, so these are all just thoughts in my head, I could be way off.

What kind of tach did you use, happyvalley?

I got a high quality TNT Revolver BMX flipflop freewheel hub off of Ebay that I will replace my cheap coaster brake with.


I'm a freewheel guy, comes from my BMX days in the early '80's. We built RACERS not freestyle bikes!

I'm less stressed now because I finally found the proper way to measure ERD (Effective Rim Diameter) in order to calculate spoke length. Just have to actually do it now to get the spokes ordered. If anybody is interested, go here Spoke Length Calculator and click the question mark next to ERD.

Anybody know what size drill bit I'll need for 12g spokes?

Thanks Cam, I hadn't seen that thread yet, a very interesting read. A three speed hub for a jackshaft is a great idea! I don't think I need it for this build because I'm only looking for 20mph. But I am very curious, how fast was that bike capable of going?