Centrifugal clutch question


petite morty

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:
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?
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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!
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.
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.
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)

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!!
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!
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.
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.
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.

1.75" (jackshaft pulley) to 3" (clutch) = 0.58:1
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.

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!

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??
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......