Help me understand the CVT

[scottso]

My son was trail riding with our Kasea 50 this weekend. He got hung up with both rear and front wheels - essentially rocks were chocking the tires pretty efficiently. He could'nt get it to overcome the blockage. Understanding that a 50cc bike has it's power limitations - when that limitation is reached -where does the slippage occur? Even with a brand new belt - will the belt always be the slippage point - or will the clutch allow the slippage? I'm trying to decide if a belt inspection/replacement is in order.
I can't make this comparison to my Polaris 500 cause that will just spin the wheels in the same situation.

[Samson]

Clutch works by centrifigul force (basically a brake shoe). If the engine can't build up enough rpms, it won't engage. Doubt the belt is slipping. Not enough HP.

You can install lighter roller weights in the front variator for additional take off power but it may cost you some top end. Add a pipe and you'll get the top end back back.

[wilkin250r]

:confused: :huh :confused:

The whole purpose of a CVT is to keep the engine at a constant RPM. If you're going uphill or loading the motor, the CVT increases drive ratio (better acceleration) to bring RPM back up. With a light rider, little load, or going slightly downhill, the engine wants to rev faster, so the CVT decreases the drive ratio for more speed, and brings the RPM back down.

As such, a pipe will not increase the top end. You might get an increase in top-end motor output, but unless the CVT is designed to allow the motor to operate at that RPM, you won't be able to USE that power.

[wilkin250r]

On the original question, I highly doubt the belt was slipping. The problem you had was more a symptom of engine design than anything else.

2-strokes do not have much power at low RPMs. At the same time, as Samson pointed out, the clutch is centrifugal. This means it begins to "grab" at a certain engine speed.

Unfortunately, these two aspects combined together to create a problem for you. The clutch begins to grab before the engine makes good power (but the clutch doesn't grab completely). So the clutch is starting to grab and try to spin the tires, but the tires don't spin. This puts an extra load on the engine, which causes RPM to drop. As RPMs drop, the clutch grabs less, which puts less load on the engine, and allows the engine to rev again.

All this happens very quickly. Within one second, it reaches an equilibrium. The engine isn't strong enough at low RPMs to overcome the load, and the clutch doesn't allow the engine to rev higher.

If your question is simply concerned that you're burning belts, rest assured that is not the case. It is the CLUTCH that is slipping, and it won't harm the clutch unless you keep it up for an extended period of time. Five or ten seconds won't harm the clutch (it WOULD harm the belt), just don't let your son keep on it for several minutes.

If you want a solution, it would be to either get stiffer springs in the clutch, or lighter clutch pads to allow the clutch to engage at a higher RPM where the motor builds more power. Unfortunately, this also increases difficulty to ride and manage the power.

[scottso]

Yes wilkin250r the belt was my concern - I did inspect it last night and it appears to be in perfect shape - so you appear to be right on. It's a small motor - I don't expect massive power - I just wanted to be sure the slippage wasn't going to be burning up components.

[Samson]

Originally posted by wilkin250r
:confused: :huh :confused:

As such, a pipe will not increase the top end. You might get an increase in top-end motor output, but unless the CVT is designed to allow the motor to operate at that RPM, you won't be able to USE that power.

Don't sas me boy!

You'll turn more rpms with a good pipe. Good pipe is tuned to a higher than stock rpm. Thus the rollers must be changed at a minimum with a decent pipe or you have a lag at take off. Also stiffer clutch springs can help raise the stall speed and a stiffer rear torsion spring will help prevent belt slippage as you add hp.

Lighter rollers, will in affect, force the cvt to hold a lower gear longer. And you aren't going to overrev these little 2-strokes with slightly lighter rollers.

As for the original question, I thought I already said it was the clutch? :macho :p

[scottso]

Don't get mad Samson. Sorry I didn't credit you.:ermm:

I just love causing fights;)

[Samson]

I'm not mad, wilkon's one of my favorites on here. :)

[scottso]

Hey man - I'm on board. I got it. :) Thanks for the info guys

[wilkin250r]

Originally posted by Samson
As for the original question, I thought I already said it was the clutch? :macho :p

Don't get cocky with me, I wouldn't want to embarrass you. :blah: :macho

You're right, you said it was the clutch. But you didn't give an explanation with it.

Also, while I'm on my high-and-mighty chair, I'll go ahead and point out your error on the rollers. The way you put it, you can get lighter rollers for more take-off power.

Lighter rollers would NOT help this situation. Regardless of how light or heavy the rollers are, they only have an effect when the wheels start turning. If you're hung on a rock and the wheels don't turn, the weight of the rollers is useless. Until the wheels start turning, it's ALL in the clutch.

[Samson]

Yes, but he wouldn't have gotton hung and stopped in the first place with the lighter rollers. :devil:

[LT80]

Ummmmmm,,,,,, CVT 101 :D :D
Lighter rollers would indeed help here. The motor would have a higher stall thus the clutch would hit a lil harder and spin the wheels.
It's simple, remove one of the rollers and see what the effect is.;)
The rollers ARE part of the clutch.:rolleyes:
LOL :D

[wilkin250r]

LT80, you have more experience racing these little buggers then I do, so I'm hesitant to second-guess you, but I don't get what you are saying.

The centrifigul clutch has it's own shoes, and the rollers are attached to the belt pulley. They are physically separate objects.

The rollers also work by centrifugal force, and have NO effect at all until the wheels start turning. I don't care if your rollers are 10 grams or 250 grams, your drive ratio is exactly the same when both pulleys are still. As such, I don't see how lighter rollers would help this situation, or help the stall speed.

[LT80]

I agree, 2 separate things, but they must work with each other.

The rear clutch with its centrifical shoes:: weight of the shoe or spring strength is what we adjust to get it to work with the front clutch.

The front with the variator:: We adjust the roller weight for a number of things but basically to work with the motor and the rear clutch.
The correct roller weight will allow the motor to get to xxx rpm's before the centrifical forse throws them hard enough to increase belt speed.If the roller weight is too heavy then the motor will not achieve proper rpms and will bog (like the topic starters machine did). When the belt moves fast enough the rear shoes throw out and start the actuall machine movement This is the stall speed part of it.
Now when the motor turns on (high rpm) it throws the rollers even harder thus the movement and ratio changes of the CVT, this is where the torque spring takes effect.

You stated that the ratios are the same when "still" (which of course you are correct).BUT,, The variator and rear clutchs are never still. They spin at idle.

I realize you understand the CVT. I prolly mentioned things that you obviouslty allready knew, please take no offence, I just thought I'd go through it all.:)
I'm terrible at explaining! Hope that helped. :)

[wilkin250r]

Correct me if I'm wrong.

The RPM that engine normally operates is determined by the rollers. However, the clutch engages long before before this RPM is actually reached.

If your engine makes peak power at 8000rpm, you want to adjust your roller weight so the CVT allows your engine to operate at that peak, at 8000rpm. However, your clutch does not engage at 8000rpm, it engages somewhere around 4000rpm. Once your engine reaches 8000rpm, THEN your rollers begin to change the drive ratio.

If you want your clutch to ENGAGE at a higher RPM, say 6000, the solution isn't in the rollers, it's in the weight of the clutch shoes.

[LT80]

Well, not completely wrong IMO. :)
The rear shoes indeed have alot to do with the stall, I just think that the rollers play a small part in it also.

What I don't agree with is::: "The RPM that engine normally operates is determined by the rollers".
I feel it's the carb/pipe/porting that sets the rpms. We adjust the roller weight/clutchs/springs to work with the motor.

I do understand and agree with how you explained the clutch working ie: my motors start the engagement at 6500. 10 feet out of the hole they are at 11,000 and now the clutchs work.

Thanks for disgusing this, we all can learn. :)

[Samson]

I would argue the rollers are part of the transmission and not necessarily the clutching system. But I think we're all beating around the same thing....pretty much.

:)

[LT80]

LOL! I think your correct. :)
Zuki calls it transmission 1 and 2 plus a gearbox. Then we have clutchs, variators,pullys, etc. All the same parts just different peoples definitions. :D

[wilkin250r]

Originally posted by LT80
What I don't agree with is::: "The RPM that engine normally operates is determined by the rollers".
I feel it's the carb/pipe/porting that sets the rpms. We adjust the roller weight/clutchs/springs to work with the motor.

Yes and no. The carb/pipe/porting determines where the engine makes peak horsepower. However, it is the function of the CVT to change the drive ratio to keep the motor at a certain RPM.

If your engine makes peak HP at 9000rpm, but your rollers are too heavy, they may keep the engine at 8000rpm. If the engine rpms rise above 8000rpm, the outward centrifugal force of the rollers change the drive ratio, which will drag the RPMs back down to 8000rpm. Even though the engine makes peak power at 9000, the rollers will never let it get there.

It's slightly backwards thinking from a design standpoint, because you certainly don't design your engine to work with your rollers, but rather the other way around. However, when it comes to actual operation, it's your rollers that determine where your engine operates, regardless of where peak HP is.

This of course is up until the rollers are all the way out. Then, when drive ratio is maxxed, the rpms will continue to climb as high as possible.