CVT trouble - back wheel locks up

[Peter the blacksmith]

Well, I managed to make a spacer and move the front disc 6mm out. Now the discs are in line, and hopefully this will work. Now, the only trouble seem to be to find a belt...I thought Comet 30 was a very common cvt ? But here about - no belts

[gilburton]

I should think you might be able to get a universal belt but here is a link in the UK.

http://www.geminikarts.co.uk/

[Peter the blacksmith]

Thanks, gilburton !

[Peter the blacksmith]

Okay guys, ther neverending story goes on
Now I have spare belts (two of them ) , the primary and the secondary drive discs are in line - but I still have the bloody same problem - when turning down the gas, even moderately, the back wheel starts to brake and the bike is truly difficult to handle.
I understand what is happening ; the engine rpm controls the primary drive and the secondary drive just spins on neatly and follow, ok ? But when the decrease of rpm is fast, is is like driving with a normal bike in 4th gear and then all of a sudden kicking in the first gear...
Even when I drive slowly 40 kmh and shut off the gas, (moderately) the backwheel brakes I hate the thought of what will happen when an elk steps out on the road 100 meters ahead of me when I'm cruising at 90 kmh - there is no way to stop this bike and still have full brakeing control in such a long distance at that speed. . A bit scary, in fact.
So - the question is how to tune the primary drive pulley so it does not brake that fast when rpms are reduced ???

I have a Suzuki QuadMaster 500 cc, with cvt-drive, of course, but no problems at all when turning off the gas, the engine brakes slowly and under full control.
Well it has a petrol engine, of course, but that can hardly have any much significance, since the system is similar, basically.

Or - would it be an easier solution to get a gearbox with clutch, like this one http://www.geminikarts.co.uk/files/Come ... nsions.pdf ??
Then one would be able to pull the clutch when braking and thus have more control...?

Thanks for any inputs !

[gilburton]

Sounds like something is not working correctly possibly the weights sticking??
Can you safely get the back wheel off the ground and run the bike and observe the pulleys to see if you can figure out if it is the front or rear pulley that is the problem??
The other option is to strip both pulleys down to see if you can find any wear??
It was noted that your chain appears to be hitting the pulley so if it still locks up with the wheel off the ground try taking the chain off.
It could be the flex on the chain jamming it against the pulley??
Make sure it is safe and try and get someone to support the bike.

[Peter the blacksmith]

There are no weights in the primary pulley.
And the secondary must be ok, since it is just following the primary, and if it would get stuck in "high gear", there would be no braking effect when the rpms are decreased. Or am I wrong ?

It is not the chain grinding at the secondary pulley, it is the builder that took a grinder and cut som off, to make more rrom for the chain

[coachgeo]

There are no weights in the primary pulley.....

hmmm.. ok something is fubared here. No weights? We need pics. there must be weights or cams or something to transfer cyntrifical force to one sheave of the primary pulley to make it move outward so the belt can ride lower and visa versa higher as the RPM drops.

also to help you we need pictures and data on the secondary ramps/helix

I get the feeling this CVT is wayyyyyy out of calibration or not atiquate for the hp/torque rating of the engine.

[Peter the blacksmith]

Ok, I'll take it down to parts tomorrow and take some pics

[gilburton]

The front pulley must have some sort of weights.
From your original posts the bike uses a Comet type transmission??
I'm not entirely familiar with these but don't they have a centrifugal type system with weights on the front pulley?
You'll need to check that this is not sticking and the pulley is free to move in and out.

[gilburton]

http://www.youtube.com/watch?v=C9yPHtfJO9w

[albertaphil]

Okay, so lets talk this out. Peter is getting too much braking force at his rear wheel when he lets off on the throttle.
This braking force then is either from the engine through the CVT setup or it is from the rear brake, chain, or ? and only happens when he is off the throttle such that the wheel is trying to push the engine.

So is there any possible way the when the back wheel is pushing the engine that the rear brake gets activated? Wheel hub rotating and allowing the brake lever to be moved at the hub? When the chain is tight on the underside of the swingarm rather than the top of it that it interferes with the brake? I'm just throwing these unlikely possibilities out there.

So then it is just the engine itself that can be causing the problem. And it sounds like the engine is operating properly, and must be making about the right power if Peter's top speed is 90kmh. So then the only reason the engine would be providing more than normal braking force is if the the transmission setup is "shifting to low gear" any time reverse torque is going through it. Isn't it so that these CVTs are designed to ratio down or "gear down" when reverse torque goes through them to provide engine braking?

Are we simply dealing with a CVT that is undersized for the engine? Perhaps that had something to do with the original belt shredding so quickly beyond just the pulleys not being aligned. If the CVT is undersized, could any reverse torque be causing the secondary pulley to immediately go all the way to slowest ratio (largest circumference) thus multiplying the engine braking force?

To try this the other way...if the entire CVT is undersized for the torque of the engine or if the secondary pulley has too light of a spring, it would tend to "gear up" too quickly on acceleration, right?

Peter, when you give the bike full throttle, does it start to move and then seem to bog down until you are nearly up to speed. That is, does it seem to be "shifting to top gear" too soon? If so, a heavier spring on the secondary pulley might fix that problem.

Now what I don't know for sure is whether too light of a spring on the secondary pulley, or a generally undersized CVT setup would actually cause the transmission to "gear down" too quickly under reverse torque.

[gilburton]

When you strip it down it would be helpful to all the Comet "experts" out there if you could give any ref numbers stamped on the components and any colours painted on various items as these can illustrate different specs of springs etc.
All this assumes it is a genuine Comet and not a Chinese clone??

[Peter the blacksmith]

Hi guys ! sh*t, I did not get the time to tear the cvt apart today - too busy earning my living I'll try again tomorrow.
@gilburton - when giving the bike full throttle, it moves with a slight hesitation at first, then pulls strongly until it reaches top speed (90 km/h). The acceleration feels ok. A bit slow, but strong. I'm happy with that. No bogging down before reaching top speed.
So, with my little knowledge of cvts, I assume if accelerating is ok, the secondary pulley is ok. Since it is just transferring the power from the primary to the back wheel. BUT when shutting the throttle, even moderately, it seems the primary pulley moves down to lower gear too fast. No weights on the pulley, maybe adding some small weights (magnets ?) could do wonders here ?
From 90 km/h I need about 350 - 400 meters to stop, if I don't want the backwheel to start brakeing. As you can imagine, it is a bit annoying...
Anyway, I'm really grateful for your help lads, thanks, I'll post some pics tomorrow

[gilburton]

Ok I've been doing a bit of research as I don't know these comet systems.
You do have a weight.
When you open up the drive(front) pulley you will find a circular spring device,yes? (spring and flyweight assembly in diagram)
This is what opens up and rides up the face of the outer pulley. If you look one side of it is shaped to fit the chamfer or curve of the pulley/outer cover.
Look at the diagrams and make sure the spring ends are facing in the correct direction. I'm not sure if it can be fitted the wrong way round??
This has to be clean and in fact is supposed to have some light lubrication to allow it to slide.Nothing else needs light lubrication just the inner face of the outer cover and the outer face of the moving pulley.
l'd open it up and fully clean it with brake cleaner or similar and make sure the splined hub is free to move as in the video I posted.
I think what MAY be happening is that it is opening up but not returning smoothly and then suddenly letting go??
Once you do this and assemble it then I suggest you test it by disconnecting the chain and watch the pulleys to see what happens. That way you shouldn't cause any damage.
If it appears to be functioning ok then refit the chain and raise the rear off the ground and see if it still works.
That way you will not be putting any stress/weight on the pulleys or belt and if the problem is at the rear it should now show up.
Once again if that tests ok go for a gentle ride.
These clutches are incredibly simple with nothing much to go wrong

[coachgeo]

Contact the seller of the bike and ask him about the CVT.. as in was it installed new off the shelf? did he order it to specifications of some kind? Was it used? If new where did he buy it?

I ask cause if it was used...... maybe the helix ramps on the secondary/rear clutch or it's spring is not correct for the application on a diesel bike.

[Peter the blacksmith]

The cvt is brand new, so is the engine. Bought from shops in Holland.
BUT there is no spring in the front pulley. I remember I was a bit surprised when I took the thing apart last time. Maybe it is still in Holland...
Hopefully, this is where to problem is.
No photos today either, it is our 26th wedding anniversary (almost forgot that) so I am about to make some nice food and open a bottle of wine...
Tomorrow it will be ok to spend the evening in the garage, but not today

[gilburton]

The weights must be there otherwise the pulley wouldn't open so I presume the assembly isn't returning when the revs drop and,as I said before, suddenly letting go when the revs get low enough causing the problem you described as suddenly changing down to 1st??
I don't think they are too expensive to buy as a unit but the only thing I am concerned about is whether it is a genuine Comet or a clone from China??
If you are still in contact with the original owner,as you say, the part may still be there or he can verify the make??
Get some pics of the various units on the internet and compare your flyweight assembly with those although most seem to have 2 spring ends or guides coming out of the unit and 2 coil springs wrapped around it ?
This seems to be the source of your problems

[skoleskibe]

The reason for that comet model to use ASYMETRICAL belts is that it'll make it possible to have both the stationary driven and the driving sheaves fixed parts at the same side, ie for a more compact construction. You can't have that with a solution based on a symmetrical belt.
I do agree this CVT is probably too flimsy for the diesel engine. My comet 40 with the same engine is just about shot, after 3-4 years of use. The inner walls on the driving sheave is worn down to next to nothing, the bushes has been changed as the wear on the inner part of the driving part was more than 0,5 mm, giving lots of slack, jerky shifting and belt wear.

My good friend Allan uses a Comet Magnum 44 which seem to be a all in all a much better more durable construction, spreading the load between the fixed and the movable sheave on a significantly larger surface than the Comet 40

Regards

Skoleskibe

[Peter the blacksmith]

It appears there is at least one part missing from the cvt-drive, and that is a bronze bushing that is supposed to be in the primary drive. Now the belt is touching the driving axle, and I can imagine that is the problem. I'll see if I can find one bronze bushing, aply it and see what's happening. Thanks for all the support !

[coachgeo]

Did it ever get established that this is for sure a comet unit or Comet Clone? If it is do not think all of the comet units had the bush; if your speaking of the one that sits on the axil allowing for tiny bit less wear on the belt when at idol. Though I think they can be added. Course maybe that bush does alot more than what Im recalling.

Still looking forward to pics of the whole bike and esp. the clutches.

[Peter the blacksmith]

Here, some photos, hope they are not too large :-/
Yes, I found a product sheet of the cvt in the engine manual. It said it was a Comet 30.
From there I also noticed the bronze bushing was missing. I think the belt sticks to the axle when I let go of the throttle, and that is causing the too fast gearing down that locks the backwheel. Could that be right ?

[Peter the blacksmith]

Here is the whole bike.

[gilburton]

Looks like a lot of damage to the sliding faces??

[coachgeo]

thanks for the pics. Was very helpful. NICE BIKE.

I had forgotten how the comet primary/drive unit works. I can see the weights and spring.

Bushing may have something to do with it... but I still get the feeling the whole thing is not set up right. On the PDF found below.... the very last paragraph, on the last page, on the right margin of the page is a discussion about spring tension in the sencondary/driven clutch. Take a gander at that and see what you come up with

http://www.bmikarts.com/PDF/Comet_30series.pdf

[Peter the blacksmith]

Yes I was surprised at the wear of the sliding faces. But - they may have been there before I got the bike...back then the pulleys were not in line, and I know the builder/previous owner had to buy a new belt, due to that, suppose he lost interest then...well anyway that could be some old marks.
I'll read the link, thanks coachgeo !

[gilburton]

I think you are too quick in dismissing the marks/damage.
In order for it to work properly it should be smooth and damage like this will only cause problems.
If you blow the pic up there is some serious damage.
I still think your weights are sticking and either this has caused the damage or something is causing your weights to stick.
It could be the rear chain has caught on the rear pulley and caused the cvt to instantly come to a halt and the damage was caused then??
I know you said the former owner shaved some off to give clearance for the larger chain so he was obviously having problems??
I still think you should try running it without the rear chain to see how the cvt performs and observe if it is opening/closing smoothly.
After that refit the chain and with the rear wheel off the ground see if it all operates smoothly.
Don't get sidelined by the lack of bush in the pulley as this is a side issue that won't be affecting the front pulley.
I think you should be looking at getting the damaged parts replaced?

[BertTrack]

Are the rear pulleys able to turn separately from each other so that the torque feed back mechanism works? the slanted/angled pieces.

My feedback system pushes the cvt in "overdrive" when i let go of the throttle.

If yours doesn't it may not be designed for it or it's not working. Could you take a picture looking from above onto the secondary cvt. focussing on the helix/angled parts?

[coachgeo]

.... Could you take a picture looking from above onto the secondary cvt. focussing on the helix/angled parts?

better yet, put her up on blacks (or centerstand) and video seperatly from above looking down at both the pulleys.... then just one pulley.. then just the other pulley while going from idol to say 2500? rpm and then complete throttle release back to idol.

[Peter the blacksmith]

As if the cvt-trouble was not enough, the fuel injection pump broke down some days ago. So now I can do no test before the sparepart arrives.
Think I'll re-name the bike and call her mrs Murphy...

[gilburton]

This is the picture you posted some time ago about the pulleys not being in line.
That rear chain still worries me as it's very close the the pulley and despite the former owner telling you about reducing the pulley edge it still looks to me that on the overrun or braking that the top run of the chain will become slack and is possibly touching the pulley and causing it to seize.
This would then cause the front pulley to stop and,as the pulley is still open with the weights fully out,it causes the pulley sides to turn against the weights.
If you blow the pics up and look at the state of the rear pulley inner it doesn't suggest that the edges have been ground off with any precision.
Is there any sign on the chain of rubbing or bits of alloy??
See if you can slide the chain sideways on the sprocket as he may have left the original size sprocket on there which is too thin for a heavier chain??