Gravity Clutch

deacon

minor bike philosopher
Jan 15, 2008
8,117
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north carolina
I know we are all familiar with the gravity clutch, but I'm not sure anyone ever explained all the variations to it. So join me in adding new things you figure out or invent for the gravity clutch design.

As Comfortable shoes said in a recent post, the gravity clutch is nothing more than a system to lift the engine off the wheel. Everyone can have a gravity clutch design and trust me I have stolen bits of mine from everyone I ever saw I think. So add yours.

First of all you have to mount the engine so that it pivots somehow in all the designs I have seen. I suppose you could have it lift straight up, but that would seem to me to be a lot more trouble than it would be worth.

The engine would either pivot toward the bike or away from it. I have actually seen designs for both. I have never build one but if the engine pivoted away from the bike it would be much easier to build a clutch.

A gravity clutch has to overcome the weight of the engine and your tension spring as well. You can lift your engine with cable or rods. You can rig devices to lock the engine up. I really do recommend that you add that feature. It makes it easier to start, warm up, and work on the engine. I have one engine that really has to be warmed up well to ride. Not to mention with DIY bikes there is always the possibility of a long pedal home.

The main thing I have to get right when I build a friction clutch is the angle of pull for the cable or the lever fulcrum. I use both types at the moment.

Cable pull and rod pull are the same thing. The best and easiest clutch (I have not done this) might be a very heavy cable. Like an engine lift cable from the hardware store, The reason I say this is that my brake cables needed constant adjustment.

this is a simple rod clutch setup. You can always substitute a heavy cable for the rod.

One good thing, with a gravity clutch you do not need a return spring. Gravity and the tension spring also bring the engine back in place.
 
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SpiderSpartan

New Member
Jul 29, 2008
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Cincinnati
Thank you for starting this thread. I'm getting ready to start my second motorbike that's going to be a friction drive and I'm going to have to start designing a gravity clutch for it. I'll be keeping an eye on this post for ideas and I'll be sure to post what I come up with too.
 

deacon

minor bike philosopher
Jan 15, 2008
8,117
5
0
north carolina
I hope everyone adds to the body of information. Glad you are going to try friction drive I love its simplicity. I expect you will as well.
 

LordMaximo

New Member
Aug 31, 2008
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High Plains Desert, Roy UT
Friction drive is good for slow moving objects and short distances, like moving a heavy load in your bake yard. But to go friction drive all the time is a quick way to burn your small cc engines up. This is why motor cycle developers went to a rear wheel drive some 80 plus years ago.
If you want to do a sort of gravity feed drive unit, it is more advantageous to use a belt with a lever to engage the wheel with a spring load to absorb the rough spots. Instead of a direct drive.
The use of a 3 wheeled pulley system gives you control of desired WOT ratio speed to the road. Plus it will give the engine a longer life expectancy over direct friction drive. This would be the same as used during the industrial revolution of machines back during the turn of the century. Many textile mills used the over head belt and pulley systems to feed their equipment. They had several belts across the rafters, turning several spindles, each loaded with many shivs and waiting to be engaged by a canter lever belt actuator for use of operation. They would sprinkle water onto the 6" wide belts when first engaged, this would help keep the belts cooled and safe from catching fire during take off/ start up of their intended power transfer.
You would be doing the same with this type of drive unit. But not requiring the cooling fluids. Good luck, I will try to draw up a plan or find some pictures to help bring this to light.:)

Maximo
 

deacon

minor bike philosopher
Jan 15, 2008
8,117
5
0
north carolina
To be honest I find the friction drive every bit as dependable and much easier to deal with than the chain driven bike I have with the china kit. I actually find it much more fun to mess with. I hardly ever start the other bike now. The truth is I force myself to ride it once a week.

I'm from cotton mill country I am familiar with the old belt and pulley systems.

I go for friction because it is the simplest drive system there is bar none. I am a simple kind of guy.

It is also cheap to use no gears to buy no parts to have machined. It's just a simple cheap and easy going kind of drive system. But then it isn't for everyone.
 
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comfortableshoes

New Member
Jul 22, 2008
606
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Beverly, MA USA
We used belt driven stuff all over the farm- winnow machines (Blow the leaves out of the berries) and a harvesting unit that used a series of belt driven wheel with a 3.5 HP engine among various other things. I remember them being a total pain in the rear.

I wouldn't want to put a belt drive on my bike, that's a personal thing, I'm sure some people like it, it's not for me. I like friction drive. Its simple and easy. Weedwackers are $25 on Clist, if I burn it up I can buy another for $25 and xfer over all my systems to the new engine. It takes me about 15 minutes to ready a ryobi for friction drive. I can build the frame in another 2 hours and have it all set up on the bike in about 3 more.
 

deacon

minor bike philosopher
Jan 15, 2008
8,117
5
0
north carolina
Actually if the bike burns up an engine, mine usually shake apart because I don't get them tight to begin with, you just unbolt it and bolt on another twenty five buck engine and off you go. In my case the gas tank is generic so you don't even lose your gas.
 

comfortableshoes

New Member
Jul 22, 2008
606
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0
Beverly, MA USA
I uploaded a diagram of y pulley system. I used screen door rollers as my pulleys, I drilled them out of their cheap housing and mounted them to some 1/8th alum bar stock, mounted on some all thread and bolted into place.

Pulley one directs the cable into the cable stop and pulley 2 give the system the leverage it needs to lift the engine. I only need about 1/2 inch of throw to get teh engine up and off the tire but the system lifts it a good inch.

I picked up a cheapie brake lever that had about 2 inches of throw and it works really well. The pull is hard but not too bad. To hold the break lever in place when I need to for long periods of time I have a velcro cable strap. I had 2 cheap ones that came from a USB cable from work, which were ultra cheap and I broke them already. I found one in the gas station parking lot the other day and it's much nicer.

To adjust hte whole thing I have a nut and bolt with some washers that the cable loops around and then I tighten that to hold it all in place.

I've found that the pull has gotten easier as I've used it more. the lift system is the most expensive part of the build but I like the pulley system because it keeps the whole thing low profile and you can barely tell that it's there. After about 10 or 20 miles all the stretch has gone out of the cables and it's doing very well.

To hold the system down on the tire I use a turn buckle and door spring.
 

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