cvt performance theory boss length and belt length idea/?

[Sir Scoots-A-Lot]

I recently bought a new variator for my 50cc 4t. it has 5gram roller weights, I am currently using a 729 17.7 30 belt and Im fairly happy with speed at about 25mph on steep long uphills and 37-40 mph on long flat stretches with favorable or no wind. my variator kit came with what looks to be a fairly standard length drive boss that is within 1 mm of my stock boss.

I noticed that my old variator kit seemed to always get to 40mph on flat ground with no windwhereas the new kit usually only gets to 37-38 mph unless a good tailwind is present then it might hit 40mph. so I looked at the drive faces on the stock and new variators, which seem to be the same circumference, and the stock setup showed belt wear on the drive face at least few mm closer to the outside-- meaning my stock setup was achieving a bit of a higher gear, maybe because of slightly different roller weights or weight of the variator itself or maybe my new drive belt that claims to be the same size is slightly different.

My idea to achieve a higher gear on the new setup is to shave a couple mm off the drive boss, then test it, then maybe shave off more or not until im happy depending on the results.

My second idea is to install a 743 20 30 belt to get an extra half inch in belt circumference, a wider belt and hopefully achieve a higher gear. I got this idea when installing the new variator and belt I made sure the clutch was all the way open so the drive belt only had to go around the smallest part of the clutch, in the front I tried to see how far to the outside of the variator I could strecth the drive belt by hand and it seemed a bit short of getting all the way to the outside of the drive face although maybe the engine can strecth the belt better than my hands.  then I tightened on my variator and noticed the variator is at least a couple mm short of grabbing the belt with  the engine off and the belt around the smallest (inside) part of the variator.

so what do you all think? is it a good idea to get the half inch longer and 2.4mm wider drive belt and shorten my drive boss in increments? (my goal is to achieve a slightly higher gear and therefore a higher top speed -- assuming the engine rpm will stay the same) or will the slightly longer belt run the risk of jumping off? on that last note I did notice some wear on the outside of my drive belt where the belt must be hitting the cvt cover right where the two halves of the cover come together and im not exactly sure why that is happening if my belt isnt even hotting the very outside of the variator drive face?--my only guess to that outside belt wear is maybe my contra spring is so light that the clutch switches to high gear ( smaller clutch circumference for belt to go around) before the light variator weights open the variator so that the variator is still in low gear while the clutch is in high gear meaning the front and back pulleys are both short at the same time causing belt slack temporarily.

one last crazy idea to increase gear ratio is to get wider rollers for my 139qmb. stock rollers size is 16mm wide (diameter) x 13mm height (height from top to bottom of cylinder when standing on end)... so I found some other size rollers like 20mmx 15 mm but in higher weights than I want. my crazy idea is to machine it down to 20x13 so it will still fit in my variator but I am hoping it will push open the variator more with the same amount of engine effort and therefor possibly achieve a hogher gear. do you all think this is a good idea? will it work? or crazy idea that will break stuff??!!


any input is greatly appreciated because I know the cvt confuses me a little altough I would like to think I grasp the concepts individually, I know I get my thinking messed up at times at the very least when taking into consideration all the moving parts at different rpms and also changing the parts to different ones and how each little change has a big effect on the overall scoot performance

[90GTVert]

Don't do a couple mm at a time on the drive boss. A couple of mm can be a lot of change. I'd do 1mm at a time. You can take more off and then shim. Remember that there are 2 sides to this. You may help belt travel on the speed end, but on the low gear side for takeoff it will be squeezing the belt more and you may slow it down on takeoff.

A 2mm wider belt is also a big deal. I've never really enjoyed the results of extra wide belts. The standard width that the CVT was designed for has worked best for me in most cases. Longer belt may help on both ends. Too long can have slack at idle and slow takeoff or could potentially come off of the pulleys.

20mm from 16mm would be another really big change. That also will hold your variator open at rest/idle and hurt takeoff. 16x13 sliders are likely the best choice. If you wanna try larger, see if you can find something in 17mm that is appropriate.

It sounds like you have the basic idea, but you've gotta learn that a little goes a long way in the CVT sometimes.

[oldgeek]

CVT tuning can be confusing to say the least. There are gains to be found, usually through lots of trial and error. As 90GTVert posted very small changes can have big effects on the operation of the CVT. There is usually a trade off too. For example, sometimes sacrificing low end for more top end. It all depends on your preferences.

Sept 9, 2018 8:05:50 GMT -5 Sir Scoots-A-Lot said:

..... then I tightened on my variator and noticed the variator is at least a couple mm short of grabbing the belt with  the engine off and the belt around the smallest (inside) part of the variator.

Lately I have been paying very close attention to what you mentioned in your post. I have been having good success using a boss length that just barely starts to grab the belt. Any slop at that point and you are wasting available variator travel. Too much grab at that point and you lose some takeoff/launch.


Using a sharpie marker to mark the face of your variator will help you determine actual belt travel. Another thing to watch for while tuning, is to always be certain your variator is properly fitted and tightened. A loose variator nut can ruin the splines on the crank.

[Sir Scoots-A-Lot]

Sept 9, 2018 9:56:30 GMT -5 90GTVert said:

Don't do a couple mm at a time on the drive boss. A couple of mm can be a lot of change. I'd do 1mm at a time. You can take more off and then shim. Remember that there are 2 sides to this. You may help belt travel on the speed end, but on the low gear side for takeoff it will be squeezing the belt more and you may slow it down on takeoff.

I appreciate the advice and I will try to go with your suggestion of 1mm at a time since I can always take off a little more if I want to and I like that idea better than taking too much and using shims.

I also have an extra drive boss so one I will leave stock and the other I will try to shorten to see what I can get out of my top speed. i will also use a marker to check belt travel on the variator and clutch as I have seen you suggest in videos before

[Sir Scoots-A-Lot]

Sept 9, 2018 10:58:26 GMT -5 oldgeek said:

Sept 9, 2018 8:05:50 GMT -5 Sir Scoots-A-Lot said:

..... then I tightened on my variator and noticed the variator is at least a couple mm short of grabbing the belt with  the engine off and the belt around the smallest (inside) part of the variator.

Lately I have been paying very close attention to what you mentioned in your post. I have been having good success using a boss length that just barely starts to grab the belt. Any slop at that point and you are wasting available variator travel. Too much grab at that point and you lose some takeoff/launch.


I appreciate the idea. I think thats exactly what I am aiming for is to have the belt to either just start to be grabbed with the engine off or maybe leave the tiniest bit of a gap. right now it seems like there is at least a few mm, maybe 5mm or more of space between the variator faces and belt with the engine off. I am wondering if I do have the belt already grabbed at lowest gear when I start the scooter, would that make it harder for the scooter to start? I feel like that shouldnt be a big deal by itself but sometimes, like right now on my scooter, the valves go out of adjustment and that is another factor to add to hard starting.

I am thankful to have two good answers on here so quickly to my thoughts. This is a great forum where I have pretty much learned everything I know. I hope one day, maybe next year, I will get the chance to buy a 2T or a 150cc 4T so I will have something fast enough to keep up with traffic at least around town. I was looking at some builds like Brent's Triton and it has me dreaming of going faster. I would be a lot happier with my mostly stock 50cc 4T around town if it wasn't for all the cagers speeding and tailgating so I have to let them pass often for safety

[snaker]

Sept 9, 2018 15:06:03 GMT -5 Sir Scoots-A-Lot said:

Sept 9, 2018 10:58:26 GMT -5 oldgeek said:

Lately I have been paying very close attention to what you mentioned in your post. I have been having good success using a boss length that just barely starts to grab the belt. Any slop at that point and you are wasting available variator travel. Too much grab at that point and you lose some takeoff/launch.


I appreciate the idea. I think thats exactly what I am aiming for is to have the belt to either just start to be grabbed with the engine off or maybe leave the tiniest bit of a gap. right now it seems like there is at least a few mm, maybe 5mm or more of space between the variator faces and belt with the engine off. I am wondering if I do have the belt already grabbed at lowest gear when I start the scooter, would that make it harder for the scooter to start? I feel like that shouldnt be a big deal by itself but sometimes, like right now on my scooter, the valves go out of adjustment and that is another factor to add to hard starting.

I am thankful to have two good answers on here so quickly to my thoughts. This is a great forum where I have pretty much learned everything I know. I hope one day, maybe next year, I will get the chance to buy a 2T or a 150cc 4T so I will have something fast enough to keep up with traffic at least around town. I was looking at some builds like Brent's Triton and it has me dreaming of going faster. I would be a lot happier with my mostly stock 50cc 4T around town if it wasn't for all the cagers speeding and tailgating so I have to let them pass often for safety

I've been watching the Vikings game and I can't wrap my head around most of what your up here but this post caught my attention. I don't understand intentionally making the drive pulley sheave opening wider than the belt. Because your drive pulley has no sort of return spring, the moveable sheave would just be loose to the extent of the gap (5mm is a lot). The flyweights would also be loose.These systems have the belt spinning any time the engine is running due to the clutch being farther downstream on the driven pulley.