Centrifugal clutch question

Discussion in 'Frame Mounted Engines' started by petite morty, Jan 31, 2008.

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  1. petite morty

    petite morty Guest

    OK, so I am in the process of modifying a motored bike to include a centrifugal clutch, which I got from Ebay. It's a comet clutch with a pulley, 5/8" shaft, pretty much standard looking clutch like you would find on a minibike or go cart. When I put the clutch on the motor (a Toro snowblower motor, maybe 35 or 50 cc's?), it bogs the motor down at idle so far it will barely run, and then kills it when I try to acelerate. When I take the belt off, and let the clutch freewheel, the engine runs fine. So my question is, is the clutch bad (it is old and rusty looking, and maybe weak springs are allowing it to engage at idle speeds), or will a clutch rated for too high a horsepower just drag a small engine down by virtue of the inherent friction involved even when the clutch is not engaged? Know what I mean? Does anybody have some experience with this? Thanks in advance for the help..you guys rock!:confused::confused::confused:

  2. Were gonna need to see some pics here. Maybe get a little stronger springs (or touch weld two touching spring wires to get it stiffer if you get what I'm saying) so more rpm would be needed before it engages?
    And what's your gearing like?
    Last edited by a moderator: Jan 31, 2008
  3. funker

    funker Guest

    I think that the clutch engage at very low rpms. 2 stroke engines have very low power at low rpm, thats why you engine may stop. I think you must put harder springs so the engine can rise up in rpm, and then engage the clutch to the wheel in a working point where the engine has more power. In racing bikes or karts with centrifugal clutch the engage point (at certain rpm) it set to the point in which the engine give the maximum power.
    Hope this helps!
  4. petite morty

    petite morty Guest

    Right now I am without a camera, but pretty soon I hope to post some pics. In the meantime, I will try to describe my setup. I originally had my engine set over the rear wheel, so the shaft (with a small pully on it) would lower onto the tire....I think friction drive is the term? Long story short, I wanted a mid frame mount and a centrifugal clutch so I could go slower, and I wanted the engine turned around so it exhaused out the left side, since I have a sidecar on the right. I did not want a high top end for this contraption, because I will be using it in crowds of people at Burningman, and need good control at low speeds. So I mounted the engine mid frame and put on a centrifugal clutch. The clutch is connected by belt to a jackshaft (3" pully to 1.75" pully). The jackshaft is mounted below and behind the seat so that it pushes against the rear tire and a acts as a friction drive. I can vary the position of the jackshaft against the tire so I can change the gearing by putting different diameter drive spindles on the jackshaft (and also change the pulley on the jackshaft). On the test run, I had the 5/8" shaft directly against the 26" wheel. The one time I got it moving without killing the engine, it went about 10-15 mph, more or less what I calculated based on a 7000 RPM top speed on the motor. So this gets me in the proper gearing range without adding some kind of a transmission. The only negative is that when I pedel the bike, it also turns the jackshaft and belt, which does add a noticible drag. It would not work well on the street, but is acceptable for the application I want it for.
    Now here is one more detail...because I wanted the engine exhausting to the left, I have to flip the drive belt between the clutch pulley and jackshaft pulley so it will drive the bike forward. It's shaped like an infinity sign. I know it sounds wierd, but it did actually work. Thats is why I got a pulley clutch, and not a chain clutch, so I could flip the belt. In the long run, I will add an idler wheel at the middle of the belt so it does not rub together.
    So anyway, back to my clutch question....the clutch I had is riveted together, so there is no way to get to the springs without drilling rivits, and maybe not getting it back together at all. So I am considering getting a new clutch....which is why I'm trying to figure out if it is a design flaw, and not the clutch, causing the problem.
    Adding to my dismay, I just looked up some general info about small 2 strokes, and found that they idle at roughly 1000 to 1800 RPM ( I had assumed it was much lower than that), while these go cart clutches engage about 1800 to 2000 RPM, So it may indeed be that the clutch is OK, but I need to find one that will allow me to change springs, or otherwise adjust the engagement RPM so its high enough to keep the engine from bogging down.
  5. gone_fishin

    gone_fishin Guest

    this seems to me to be a technical discussion...the "parts" area is for discussion of merits or drawbacks of components...i'm gonna move this into frame-mount.
  6. srdavo

    srdavo Active Member

    Hi morty,
    I'm trying to visualize your set up.

    a question ... is the 3" pulley on the clutch?? 1.75" on the jackshaft?

    if this is the case.... it sounds like you are over working your engine.
    To get better low end power, I believe you need a larger pulley on the jackshaft. at least equal size as the clutch pulley. (1:1 ratio) or maybe even larger. then you can adjust your overall speed with drive spindles.

    here's a pic of member Tom Bartlett's Zipcycle. http://www.motoredbikes.com/attachment.php?attachmentid=46&d=1185755361
    link to one of Tom's bikes http://www.motoredbikes.com/showthread.php?t=6045

    here's an amusing & factual look at friction drive gear ratios, in layman's terms.

    I like what you are doing.....get us some pics!!
  7. petite morty

    petite morty Guest

    Wow, thanks for the zipcycle pics, Davo. That is indeed the basic setup I have, engine to jackshaft, and jackshaft to tire. Here I am, thinking I came up with something different, and it's already been done. I will try to borrow a camera to post some pics anyway.

    So about gearing ratios..........originally I had the engine with about a 1.5" spindle directy on the tire (28.6" diameter). That was 28.6 divided by 1.5 = 19:1 total reduction. It went about 25 mph top speed.

    Now I have two reductions, 1.75" (jackshaft pulley) to 3" (clutch) = 0.58:1
    and 28.6" (wheel) to .625" (jackshaft, with no spindle) = 45.8:1
    Multiplying the two together... 45.8 x .58 = 26:1 total reduction. So I went from 19:1 to 26:1. Now my top speed is about 10- 15 mph. So really my overall reduction is greater than before (although I will allow that the clutch, belt, and bearings are sucking up lots of extra energy through friction). Because of that, I am doubting changing the gearing will help. If anything, I want it to be able to go slightly faster when I am done.

    Picture this...I have the centrifugal clutch on the engine shaft, with the engine idling, and no v belt attached. The clutch drum/pulley is spinning freely. I take a v belt and carefully loop it over the clutch pulley by hand, and gently pull it tight until the clutch drum stops spinning. It almost kills the engine (or does kill it). So I think either:
    A. the clutch springs are weak, allowing it to engage at engine idle speeds, or
    B. the clutch is OK, and it just normally starts to engage at the idle RPM
    of the little motor
    I just don't know which is true. I did try fiddling with the idle screw, etc. But nothing I tried got the engine to reliably start and run with the belt attached.
    I'm in to it so far now, I am tempted to just buy a new clutch and try it. This engineering on the fly stuff is not for the weak at heart!
  8. kerf

    kerf Guest

    I don't know if this matters but a 1.5" roller on a 26" tire is NO ratio reduction at all. If you were to run the 1.5 against a 20" wheel, your speed would be the same as running it on a 26" wheel. In a friction drive, it's like you're direct driving a 1.5" wheel directly against the pavement. Any gear reduction must take place prior to the drive roller.
  9. srdavo

    srdavo Active Member

    ok.....your original set up... the 1.5" spindle, directly to the tire.....seems pretty close to where you want to be. a smaller spindle will give you more low end torque.

    now ...your 1st gear reduction.... from the clutch, with a 3" pulley, to the jackshaft, with the 1.75" pulley is not a reduction. 0.58:1
    it is an increase in speed to the jackshaft. nearly 1:2
    not good for low end power.
    when your clutch engages, it's really gotta work hard to get you rolling.

    bear with me here.... is there anyway you can change your clutch to jackshaft ratio to at least 1:1 ?

    this would be a lot cheaper to try, before you buy a new clutch.

    I will not purposely steer you wrong.
    there are many real mechanics here , who are not afraid to point out any bogus info!! ;):lol:

    here's a link to some of our homebuilt bikes http://www.motoredbikes.com/showthread.php?t=4770

    one more thought..... could your infinity belt be putting undue pressure on your clutch??
  10. petite morty

    petite morty Guest

    Oh, yeah. I see where my calculations were wrong now. With the frictions drive, the circumference and RPM of the drive spindle, not the rear tire, dictates the speed of the bike. So it's not like a two stage reduction. Thanks for straightening me out.

    And yes, when the vehicle, and v belt, is actually moving, there is certainly some friction caused by the infinity shaped belt rubbing against itself. I think that throwing in an idler pully where the belt crosses over itself would probably mostly mitigate that problem.

    Bit again, my primary problem is just getting the motor to start and idle. Lower gearing will help the bike get moving easier, but simply slipping the belt over the clutch pulley will kill the engine as often as not. The bikes got to be able to sit in one place and idle. Adjusting the gear ratios will be no problem after that......
  11. petite morty

    petite morty Guest


    Well, having run out of any other options, I purchased a new centrifugal clutch...and it worked! The bike starts up and idles with no problem. And I was able to get a refund on the bad clutch as well. I put a more appropriately sized driven pulley on the jackshaft (thanks srdavo), so I now have a 3.2" clutch pulley to a 4.5" driven pulley on the jackshaft, and a 1.5" drive spindle. Top speed = about 15-18 mph, which is maddeningly slow on the street, but will be perfect for picking my way through crowds at Burningman. I still need to put a idler pulley where the v belt rubs where it crosses over itself, and build a cover over the clutch/belt/pulley, but basically it's together and working. I borrowed a camera, so here are a few pics.:grin:

    Attached Files:

  12. (ahh. It's welded.)
    Dude. If you flip the friction drive to the right and turn your engine to the right,would that allieviate you having to twist that belt like that?

    Wait. What teh...does that engine spin counter clockwise?

    We need to look at your rope starter. I have a feeling that's on backwards or wound wrong so when you're pulling the rope the engine's actually running backwards...yes..this is possible.
    Last edited by a moderator: Feb 11, 2008
  13. petite morty

    petite morty Guest

    I really dont want to flip the engine the other way because I have a sidecar on the right, and don't want the engine exhausting in the sidecar riders face.

    BUT...the engine does turn counter clockwise, looking at the end of the drive shaft. Thats the way it rotated when I pulled it off of the snowblower I scaveneged it from. I took a look at the pull starter case, and it does look like it could probably be rewound to turn in the other direction. Would the engine run in a different direction just by pull starting in a different direction?
    I seem to remember some references here on the forum that you would have to change the orientation of the magneto to change the rotation of the engine.....

    If I can get the engine to run "backwards" without major engine/magneto mods..that would be great for simplifying the design...
  14. petite morty

    petite morty Guest

    Well, I guess I answered my own question. I pulled off the pull starter cover and just wrapped a pull cord around the flywheel pulley, and tried starting the engine in the other direction. It won't start.

    Of course another course of action would have been to flip the engine and build an exhaust pipe and muffler so that the exhaust would not blow in the sidecar riders face, but..... I dont have any tubing bending equipment, blah blah blah, and it just seemed like a bigger job to do it that way. Besides, even though it seems kind of wierd, the infinity belt actually seems to work OK. Even without an idler pulley where the belt crosses over, the belt does not seem to rub much on itself. When the bikes moving, the belts flaps around. It's more like the belt is just slapping against itself, rather than continually rubbing....

    But still, if I found a reletively easy way the reverse the engine rotation, I would go for that....any input out there about reversing engine rotation?

    Oh yes, and one other thing I forgot. I will need to bend the crank to allow for the wide engine as well if I am going to peddle it much. Or I might just be lazy and use the pedals for footrests and never peddle it.
  15. kerf

    kerf Guest

    Could turn the seat around and run in reverse, steering outboard style. Keep plenty of bandages in the the ol tool bag, could be a rough ride.:grin:

    Your best option is to turn the engine and handle the exhaust issue.
  16. Maybe you can snag up them wide cranks that come with those 4 stroke kits.

    It's actually pretty cool seeing that belt like that. Heck,if it works,it works!

    So what's this about a sidecar?

    We need some pics!
  17. srdavo

    srdavo Active Member

    I like your design. specially the way your engine mounts to the frame.nice work!!
    I wouldn't change a thing.
    It should be easy (for you) to build a belt guide of some sort, for your infinity belt. :D

    I agree with Large about the wider pedal cranks. they should be easy to find. do you have one piece cranks? I know of a vendor who used to carry wide one piece cranks.

    you could squeeze out a few more MPH's If you went 1:1 on your pulleys....but as is, I'll bet you have great take off power. you'll need that hauling passengers.
    once again....nice work!
  18. petite morty

    petite morty Guest

    Thanks for the input...I will start learning to drive backwards...hehe.

    It does have a LOT of torque for taking off, which really helps for carrying a 170 lb sidecar passanger on dirt. Which is one reason I may not worry about the crank, because its really hard to peddle a passanger under those conditions. Even using low gears. Still.....if you know where to find a wide one piece crank, Davo, please let me know.

    I will take some pics of the sidecar when I get it out of storage soon, Large P.
    Last edited by a moderator: Feb 11, 2008