HF 79cc Utility Bike

A

atombikes

New Member
A long time ago I bought one of the Harbor Freight 79cc 4 stroke engines one sale, got a great deal. Due to work around the house, the engine has sat in the box for months. Now, my time is freeing up, and I am planning my build. After long thought, I think I know what I am going to build.

Here is my plan. I am going to build a custom "longtail" utility bike, similar to this one:
bikefinished.jpg

by using pretty much the same technique shown on this person's website:
Long-Tail Cargo Bike

Also, I am going to modify the seatstays and the area directly behind the seatpost to add an engine mount for my 2.5hp Harbor Freight engine. The goal would be to make enough space to leave the HF engine as it comes out of the box (leave gas tank/air filter/etc on). I think to keep things simple, the engine will drive the rear wheel on the left side, and the bicycle crankset will drive the rear wheel on the derailleur side. So the drive will be like a minibike on the left side, using a centrifugal clutch.

I solicit help from you guys because you have experience. I plan on a 26" wheels. I will use the engine mainly to keep my speed up between 20 and 30mph.

Questions-
  1. What is the best/cheapest source for a 5/8" clutch for this HF 79cc engine? Northern Tool has this one, is it appropriate? Hilliard Extreme-Duty Centrifugal Clutch — 5/8in. Bore, 12 Tooth, 35 Chain Size | Brakes, Clutches, + Hubs | Northern Tool + Equipment
  2. What is the easiest clutch to adjust for idle rpm? The Hilliards have different spring kits to adjust the clutch; maybe all these types of clutches are similar?
  3. What is the approx. idle speed for the Harbor Freight 79cc engine?
  4. Will a size 35 chain be appropriate?
  5. Should I plan on using a jackshaft?

Sorry for such a long list of questions,
Thanks, Ed
 
In spite of having just finished a greyhound build I'm afraid I can't give you much advise for what you want to do. As for the cheapest clutch, I don't know, but I think you're going to need something to change the gearing, as you say, perhaps a jackshaft ... maybe some kind of CVT. Unless you're good at fabricating you'd best follow what someone else has done and even then it may be difficult. I know my own limitations, so I got a ready made transmission to transfer the power from the engine to the rear sprocket. Be forewarned that just because you got a great deal on a motor and it is a nice motor, no doubt about it, that will be one of the cheapest parts of your build. Be prepared to spend more getting the power to the rear wheel than you spent for the engine. Nobody wants to spend more than they have to on a build, but there are some things you just can't get around. Hopefully someone more knowledgeable will jump in here. I would suggest reading up on other greyhound builds so you start to get an idea of what's up and which direction to take. Good luck to you.
SB
 
Thanks for your response, Silverbear. I have been following your latest build, it looks great. Seems (to me) like one reason for these different transmissions that I see discussed on this board would be to change the engine speed to match what is needed at the rear wheel. And that is a balancing act between required torque and speed. And since most people are trying to motor existing bike frames, it is best to combine everything onto on plate and keep things compact, bolts right to the engine and doesn't require custom fabrication. But these solutions typically cost several hundred dollars. I think I can keep my cost down a little by fabricating the jackshaft mount into the frame or engine mount. But if I do that, I will also have to figure out how to tension that chain....

I guess mainly what I need to know right now is, for those that use centrifugal clutches, is there a preference of one over another? Like Max Torq over Hilliard? I can run thru the gear calculators later to determine how large each sprocket must be to get in the desired rpm range.

Thanks
 
Glad you like my Greyhound build, thanks. I'd say if you can fabricate something yourself, go for it. I love to see low budget, quality builds. It's a lot simpler if you have deep pockets and pay for the best of everything and then just bolt it together. I shouldn't say "just" as there is a lot of skill necessary anyway. But you know what I mean about having to figure things out as you go. Lots of satisfaction in that. Somebody smarter than me will have to advise you, though. I know my limitations. Elmo and Neattimes (not sure how that's spelled) are both pretty sharp with this stuff. If they don't see this thread I'd PM them asking for help. I can cheer lead though...
SB
 
Seems to me that Rusty built a jackshaft into the frame, but I think that was with a 2 stroke. Camlifter is another one good with jackshafts and such.
SB
 
That long tail would be awsome. If you could find an 8 tooth centi clutch you might be able to get away without a jackshaft. I don't think you would be happy with it though.

Here is an equation I came up with a while back. It will show you how much reduction you need. All you have to do is plug and chug. You will need to know is your RPMs and your ratio.

A= Motor rpms
B= total ratio from motor to final sprocket
C= rpms at the wheel
D= feet per Min
E= Feet per Hour
F= Mph

A/B=C

Cx6.7=D

Dx60=E

E/5280=F


Top rpms would be about 5500. If you were to use that clutch thats 12t and the biggest driven sprocket is probably 72t so your reduction would be 6/1(no jackshaft). Thats a top speed of about 70 mph. Thats just not realistic.

20/1 would put you around 20 mph 15/1 is 28 mph.
 
you have gears , hills w/and weight hauling,you want a shift kit,the jack shaft gears can be fenagled to adjust the standard motor to wheel ratio in high gear and the lower ones are for hills etc
 
He wants his speed between 20-30 mph all I think he needs is a jack shaft.

Something else you might want to look at befor you mount your engine is how much space it is going to take up. If you look at my rack mount hf it is centered up on the axle yet I am practically sitting on the gas tank. You might be better off putting the motor in the frame if at all possible.
 
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i have a idea to make a electric bike with a razor scooter that is only 100 dollars.. can a friction one do the job ? or how about making it with a big sprocket and a centrifical clutch??? that has teeth or a belt.
and how do i get to find posts taht i have made and people respond to???
 
we will say the power allows the elec. scoooter wheel to move you along at 15 mph,if the tire is used (in reverse,face the scooter backwards) it will move the bigger bike wheel along at the scooter speed ,you can go to a smaller wheel on said scooter and get more hill climbing power as a friction drive set-up,,, but the gear bolted to the bicycle wheel and tire would have to be the same size as that bicycle wheel and tire for the scooter motor to have the power to move the bike,,or the motor would have to run through 2 - 3 jackshafts to get the ratio the original scooter wheel/tire/and sproket, and motor shaired,by the time it reaches the bike wheel sproket to have the ability to movethe bike,if you notice ,the sproket on the scooter wh. is almost as big as the wheel ,the scoot is just so fast and that will limit the bike to that speed,its all the power/speed it has to give
 
Felton said:
He wants his speed between 20-30 mph all I think he needs is a jack shaft.

Something else you might want to look at befor you mount your engine is how much space it is going to take up. If you look at my rack mount hf it is centered up on the axle yet I am practically sitting on the gas tank. You might be better off putting the motor in the frame if at all possible.
Click to expand...

I maybe should have been more descriptive or posted a drawing of my plans. I'm working on the drawing, not quite done with it yet. But the plan IS to put the engine in the frame, almost like a tandem build with the engine behind the front seatpost. Just last night I was looking at someone else' post that did something very similar (maybe Elmo?).

Thanks for the speed calculator, I assume the 15/1 ratio you reference means for every wheel revolution the engine does 15? If so, assuming I have a 10 tooth gear on my clutch, I would then need 150 tooth gear on the rear wheel (assuming no jackshaft)???

When you say that 70mph is not realistic, is that because as speed increases torque decreases? The engine would just stall due to the gearing?
 
atombikes said:
I.

Thanks for the speed calculator, I assume the 15/1 ratio you reference means for every wheel revolution the engine does 15? If so, assuming I have a 10 tooth gear on my clutch, I would then need 150 tooth gear on the rear wheel (assuming no jackshaft)???

When you say that 70mph is not realistic, is that because as speed increases torque decreases? The engine would just stall due to the gearing?
Click to expand...

Yes and Yes
 
I miss understood where you were putting you motor, I think thats a great idea.

For the first part of your reduction I would suggest something between 2/1 and 3/1. With a 2/1 you could run a 10t sprocket off the jack shaft to a 72t sprocket on the tire. That would be a total of 14.4/1 or a top speed of 29 mph. If you used 3/1 you could use the same 10t on the jack shaft but run it to a much smaller 48t sprocket on the tire for the exact same reduction of 14.4/1.

You might be able to use that clutch the problem would be trying to find a keyed sprocket that would be between 24t and 36t and runs a 35 chain or (40/41)

I will see if I can find one in either 35 chain or 40/41. (Norther tool has the same clutch that runs a 40/41 chain)
 
I ran a ten tooth clutch to a 30 tooth then on the same shaft a 12 t to a 48 on the back wheel for a 12 to 1 reduction. if you are going to pull heavy loads go with a 11t to 50 on the wheel, that will give you 13.? to 1 and should take care of the load and still have decent speed.
 
the 30t sprocket was hard to find. I used 41 chain. 35 would work well but still hard to find 30t sprockets.
 
 
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Thanks, Felton. So would it be easier for me to use the jackshaft to get to the right side and then drive that thru a freewheeling crankset, finally back to the rear wheel (ala sickbikes shiftkit)??? I can see how I might have issues getting large sprockets for a 5/8" jackshaft.
Click to expand...

Lets get this back on your page.

There a couple large sprocket above that I posted they might work. You could go to the peddal side if you want,(I know nothing about going to peddal side but I could still help with calculations) or you could make some sort of a set up like I have using a sprocket instead of pulleys. My "tranny" is notheing more than a jackshaft set up.

You might be able to do a double jack shaft set up like tyreslider did. I have been wanting to do that myself. You could go with smaller sprockets then because you would be adding a reduction.
 
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or just throw a regular v-belt pulley on the engine and use a belt-tensioner from a lawnmower/snowblower/car engine hooked up to a cable so you can engage it with a bike-handbrake setup. chains are pains.
 
Build-a-beast workshop said:
or just throw a regular v-belt pulley on the engine and use a belt-tensioner from a lawnmower/snowblower/car engine hooked up to a cable so you can engage it with a bike-handbrake setup. chains are pains.
Click to expand...

How well does this setup work? Does it cause the belt to wear quickly? Sounds like it would be quieter than a centrifugal clutch if set up properly. Any pics or links you know of to such a setup?
 
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