Harbor Freight 2.5 Chopper(In The Works)

ChainDriver · Jul 16, 2011

Harbor Freight 2.5 Chopper(Need clutch/sprocket help)

I picked up a harbor freight greyhound from dirt cheap today for $42 and am searching craigslist for a occ/schwinn chopper bike. I hope to frame mount the engine with a 10t cent. clutch and sprocket on the rear wheel. I would like to eliminate the need for a jackshaft but that would mean that I would need a giant sprocket on the rear wheel, something like 70t which would give me a
7:1 ratio and 30.6mph at 3600rpm and 42.5mph at 5000 rpm (which i believe is close to the limit of this engine without the govenor) Is this realistic or am I missing something? Also I was looking to use #41 chain because of its strength, but the only 40 pitch 70t sprocket i can find is $62 before shipping!! would it be possible to use a #35 sprocket and chain or would that be too weak? I dont know how i'm going to mount the sprocket to the rear wheel, but I am considering a manic mechanic hub adapter, could jim also make me a sprocket to go with that? I also have a 2 stroke kit that doesnt run that I will be taking parts off of for this build. I will try to keep you guys updated on my progress once i get a bike. Until then I will be buying all the parts i need online.

ChainDriver · Jul 17, 2011

After seeing this:

I thought maybe a similar setup would be nice using pulleys a jackshaft and a 16" kids bike rim bolted to the 20" rear wheel to act as a pulley. Using this calculator: Burris Racing Jackshaft Calculator I determined that a 2" pulley on the engine going to a 6" on the jackshaft then to the 3" max torque pulley clutch and to the kids bike rim (i assume 10" diameter, maybe someone has one they can measure?) should give me a 7:1 final ratio and the clutch would be on the jackshaft instead of the engine. If anyone sees any problems with this setup please let me know. Does anyone know where i can get these pulleys with a 5/8 id and keyway? Also where would i buy parts for a jackshaft?

ChainDriver · Jul 17, 2011

After actually doing the math on that pulley reduction I would only be getting a 7:1 overall ratio which gives me 30 mph with no load at 3600 rpm. Is this right/realistic? on the other hf 79cc builds i have seen they were using 12:1 ratio, this just seems way too low.. showing 17 mph at 3600 rpm I would like to have a max speed around 40/45 and the budget isnt very big so i guess ill end up having to use a jackshaft anyways. What sprockets would you suggest on the hf 79cc engine, jackshaft, and 20" wheel?

culvercityclassic · Jul 17, 2011

I say use these items and you will have no problems and it’s a proven system:

affordablegokarts: 79cc Parts

Also add a MM sprocket adapter with a 56 tooth sprocket.

This all adds up and may be more than your budget but it will be reliable and fast.

BTW: where did you find the motor?

ChainDriver · Jul 17, 2011

I have a welder and was considering building my own jackshaft to save money, the engine i got from hudsons dirt cheap, they often have harbor freight returns at 60% off. I like the
MM sprocket adapter and will probably use one of those tho, my biggest question right now is what ratio should i use for this engine on a 40lb bike with a 135lb rider?

cannonball2 · Jul 17, 2011

12:1 is a good all around ratio, if hills arent an issue then maybe 10:1, lower than that and its going to be hard on the clutch, with a sucky bottom end. The HF engine will top 5000rpm pretty easily, so you will have better top end than what 3600 shows. To get into the 40/45mph range you are really gonna have to have a high ratio and twist the engine pretty hard. A 10:1 @ 5500 gets you 42mph or so.

ChainDriver · Jul 17, 2011

I got the bike today and immediately set about cutting out the seat post and removing the pedals. Here is a pic of the engine sitting in the bike. Its a great fit without the gas tank and muffler, almost like its made for it. usflg

I dont know how to post pics so heres the link to my photobucket:
DSC01952.jpg picture by sk8terdudeguy - Photobucket

culvercityclassic · Jul 17, 2011

Looks good. You are going to have to off-set it to the right to get the drive to work, you may also have some crank sprocket issues. I am sure you will figure it out. This looks like a fun project. Keep us posted on the progress.

ChainDriver · Jul 18, 2011

Ok so after many many hours sitting at the computer thinking of different ways to get that magic 10:1 ratio for the least money possible I have thought of possibly the cheapest one yet, and I would like some input on if it will work, My idea is to weld a 20" rim to the outside of my 20" rim to act as a 16" pulley and then weld a 1.5" pulley to my clutch for a final ratio of 10.6:1. I would of course need a belt tensioner, but that shouldn't be too hard. I think the most difficult part would be welding the pulley on the clutch perfectly centered. What do you guys think?

wayne z · Jul 18, 2011

You can buy clutches with a pulley instead of sprockets. I don't think you can weld on a clutch without causing problems.

ChainDriver · Jul 18, 2011

Well the smallest pulley clutch i could find for 5/8 shaft was a 3" pulley that would mean i would need a 30" rear wheel! Maybe if i used a bushing clutch and welded only very small beads and then let it cool it would work.
I have found a way to make a jackshaft for $95 but that is still more than twice what I paid for the engine! Are there any low priced torque converters near this price range? Everyone seems to think torque converters are better than cent clutches what is the advantage? Also if im going to put forth $95 just for one gear, maybe i could shell out a little more for a 2 or 3 gear setup? Do they make 2 gear torque converters?

EDIT: I saw the htd clutch:HTD Clutch
Will this work with belts? if so it may be just what im looking for.

EDIT 2: could I use a max torque driver instead of a cent clutch?
Max Torque Driver 20 Series (400805), www.bmikarts.com : BMI Karts & Supplies : Go Karts Parts : Racing Go Kart Parts : GoKarts : Versailles, Ohio : USA : Indoor Track : Tips
It would have to have an engagement size of around 1.5" or less

cannonball2 · Jul 18, 2011

Heres a jackshaft I made for my shifter bike. Some scrap metal plate two $8 flange bearings and a $7 5/8 jackshaft. Doesnt have to be expensive.

ChainDriver · Jul 18, 2011

If i use the pedal crank for the jackshaft all ill have to do is buy one sprocket and a clutch, i just need to find a square hole sprocket or a way to mount a 10t sprocket to my pedal crank... hmmm

cannonball2 · Jul 19, 2011

I have built a bike like that in fact still have it, You will have to replace the crank bearings with high speed sealed bearings if you want it to last long. I ran a belt on the primary side, #35 chain on the other. The reduction is somewhere in the 10:1 range. This bike was built nearly 30years ago when there was no such thing a s a sprocket adapter or rag joint, atleast not in this neck of the woods. I centered and welded a Gokart sprocket to the coaster brake sprocket for the final drive. This set up has survived thousands of "kid" miles. Just be sure your local laws allow pedal-less bikes to be legal.

ChainDriver · Jul 20, 2011

Ok so i ordered the parts today and i'm going to have a machine shop turn the pedal crank shaft down on a lathe so i can slide a 1/2" hub 10t sprocket on it and the ratio works out so i can still use my original 20t freewheel sprocket on the rear. So once i get that done Ill just have to mount the motor and hook up a gas tank, throttle and brakes and ill be good to go! Ill be sure to keep you guys updated on my progress. .shft.

WayneC · Jul 21, 2011

CB2,
On that blue bike, is that mount bolted through the frame? I thought that was tabu. How's it working out / holding up?
WC

cannonball2 · Jul 21, 2011

Yeah, its through, but there is a thick walled seat tube driven down it to the crank area and extends up past the the jackshaft. So far so good. I believe the 4stroke with the auto clutch is harder(wear&tear) on a shifter bike than a 2stroke with a manual clutch in some ways. Shifting while not hard, requires just the right touch when bringing back throttle to avoid a pretty good drive train snatch. If it is gonna tear up shoulda done it during my learning curve! Hope this was of educational value to Chaindriver!

ChainDriver · Jul 23, 2011

I was actually thinking of making this a shifter bike, will any sprocket set (5 speed) just thread on to the hub after i take the freewheel off? And do i just drill a hole in the frame and hook up a derailer? I am also going to lower the rear wheel 6" so the chain will clear the frame. Here's a picture of my plan, sorry for my poor editing skills, but i think you'll get the basic idea.

cannonball2 · Jul 23, 2011

You are using the pedal crank as a jackshaft?

ChainDriver · Jul 23, 2011

Yes, that is my plan. I will probably install a grease fiting in the crank housing and keep it full of grease. Then if my crank bearings fail, I will replace them with high speed sealed bearings. I wouldve liked to keep the pedals, but doing it this way was just so easy I couldnt say no!

Harbor Freight 2.5 Chopper(In The Works)

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