Driven Pulley Problem

michael whiteman

Active Member
Joined
Mar 2, 2021
Messages
139
I have a 5:1 belt drive trans on my 4 stroke engine. The 6.25" cast aluminum driven pulley that has 100 teeth also has .050 lateral run out, or side to side wobble. Who knows why, maybe a bad casting or the 1/2" shaft hole was drilled at an angle? At about 2/3 throttle this will produce a vibration that remains to max RPM. The worst case scenario fix is to buy another trans and hope for a better pulley this time. There seems to be NO replacement parts for any of these transmissions. Does anyone have a junk trans that would sell me a replacement?

I was thinking maybe chuck it in a lath and machine the sides square. This would narrow the tooth width but I don't think it will be a big deal. After this I would attempt to balance the pulley by drilling lightening holes in the heavy area. I thought about heating the pulley with a torch while it was in the lath and trying to pull it back into shape but working with aluminum is pretty scary. It all goes to s*** in a hurry if you're not paying attention. Any other ideas more than welcomed. Thanks in advance. michael
 

Nirve

Member
Joined
Jun 12, 2020
Messages
17
Hey Michael, I doubt it is the big pulley that causes these vibrations. I had the same issue with the same gearbox when I started my build.
First, what caused my pulley to wobble that much was the spacer behind it. If that's not 100% flat, you screw down the pulley at an angle. It took me many tries of the orientation of both the pulley and that spacer. Once dialed in, I marked it down with a sharpie to recreate the orientation every time.
Here are some pictures of that spacer:
IMG_6901.jpgIMG_6946.jpgIMG_6947.jpg

By now I upgraded both my transmissions to the best gearboxes on the market, from Grubee, and the big pulleys still wobble a bit.

Anyhow, your actual problems with the vibrations, are you staying below 6800RPM? That was key for me.
To proof that the vibration is not caused by the big pulley you can either take the belt off or the whole trans and rev the engine. I bet you won't see a change.
 

michael whiteman

Active Member
Joined
Mar 2, 2021
Messages
139
Thanks for your quick reply and pics Nirve. Did you check to see the thickness of your spacer was the same at multiple spots around the circumference ? My pulley comes off with a lot of effort, so I really don't believe there is any slop on that fit which could be affected by a poorly machined spacer. To create the .050" runout I'm experiencing the shaft would have to be pretty sloppy in the pulley. I realize those spacers are just cut with a saw, but I will double check this next time I'm in there. I did remove the belt and ran the engine to max RPM without feeling any vibrations, so I know it's coming from the trans. I do not have a tach so I'm guessing the vibrations start around 4800 RPM if 6800 is the top end.
 

Nirve

Member
Joined
Jun 12, 2020
Messages
17
Thanks for your quick reply and pics Nirve. Did you check to see the thickness of your spacer was the same at multiple spots around the circumference ? My pulley comes off with a lot of effort, so I really don't believe there is any slop on that fit which could be affected by a poorly machined spacer. To create the .050" runout I'm experiencing the shaft would have to be pretty sloppy in the pulley. I realize those spacers are just cut with a saw, but I will double check this next time I'm in there. I did remove the belt and ran the engine to max RPM without feeling any vibrations, so I know it's coming from the trans. I do not have a tach so I'm guessing the vibrations start around 4800 RPM if 6800 is the top end.
Of course!
I did check that back in the day and it was not the same thickness all around.
Maybe I don’t get it right but .050“ is really not that much? I had at least that if not more. I just worked to get my wobble a little better so the big pulley stops dinging on the clutch bell.

Never run your engine to max rpm. It will damage the engine (I did that). 6800 is just redline. It is hard to say from here, unless you have a rpm limiter you should not guess your rpm.

Assuming you don’t have an RPM limiter and you go full throttle thinking 6800 is max, those engines go up over 10k rpm if you let them. Then your 4800 is actually somewhere in the 7-8k‘s, producing tons of vibration.
Just throwing this out to cover every possibility.

But if, as you say, you don’t have the same issues with the gearbox off.. are both circumstances exactly the same? Stable bike, no load, rear wheel jacked up, etc?

To make sure 0.050“ are 1.27mm, correct?

I just don’t see this little much side to side wobble cause any crazy vibration.. wobbly mounting does not equal imbalance in the actual piece
 

JerryAssburger

Active Member
Joined
Apr 20, 2017
Messages
174
Sorry, Michael for your issues. I think we're both victims of Engine/Belt Drive getting dropped during shipment. Mine arrived with the belt cover pushed up against the pulley, and then the pulley has a total wobble of about an 1/8" at the rim. And you're right- no replacement pulley- at least the ones I looked at ended up being more expensive than just hunting down another belt drive.
Also, as far as I can tell, our "Budget 53cc" kits are really the 49s, with the 6800RPM limiter. I'm okay with that, I don't appreciate the false labeling, though.
 

michael whiteman

Active Member
Joined
Mar 2, 2021
Messages
139
Jerry, mine showed no signs of shipping damage. I would have returned it. How did you correct your wobble? Did it cause a vibration somewhere mid range? It would be impossible to sort out all the mis advertising that is everywhere without the help all our members share about how things really are. I can't tell you how many hours of reading and notes I made last spring on these subjects.
 

michael whiteman

Active Member
Joined
Mar 2, 2021
Messages
139
Nirve, I agree with everything you wrote 110% and yes it really is .050". My large pulley was kissing the clutch bell also, like I imagine is the case with so many more. I removed the black metal guide ring to cure this. It is just pressed on. The belt still runs perfectly centered without it. I haven't looked yet at the industrial drive belt market for a similar pulley. Probably expensive if available.

With my tire size and gear ratios the calculator says my bike will go 25mph @ 6800rpm. The vibration starts before 20mph. I never run my engine anywhere near WOT (wide open throttle). It amazes me still that this valve train design will rev this high.

I just can not think of anything else that would cause a vibration. Definitely not the clutch bell or the belt, so what's left? The large pulley is the only spinning part with enough mass to create this issue, right? Thanks for your thoughts
 

Sidewinder Jerry

Well-Known Member
Joined
Mar 14, 2011
Messages
2,337
Nirve, I agree with everything you wrote 110% and yes it really is .050". My large pulley was kissing the clutch bell also, like I imagine is the case with so many more. I removed the black metal guide ring to cure this. It is just pressed on. The belt still runs perfectly centered without it. I haven't looked yet at the industrial drive belt market for a similar pulley. Probably expensive if available.

With my tire size and gear ratios the calculator says my bike will go 25mph @ 6800rpm. The vibration starts before 20mph. I never run my engine anywhere near WOT (wide open throttle). It amazes me still that this valve train design will rev this high.

I just can not think of anything else that would cause a vibration. Definitely not the clutch bell or the belt, so what's left? The large pulley is the only spinning part with enough mass to create this issue, right? Thanks for your thoughts
Assuming you're using 26" tires you'd have to have a 21:1 total reduction to be doing 25 at 6800 rpm.

If it is 26" wheels; I'm assuming the reduction belt drive is 5:1 and you're using a 10:44 drive.
 
Top