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Yamaha Continuously Variable Transmission Basics (CVT)

I wanted to revisit the basics of how the Yamaha CVT system operates because I am seeing so many questions from operators who are new to the system. For many of you, this will just be a repeat of your already known knowledge of the system. I hope this clears up how the CVT operates and minimizes the myths and/or incorrect information that I am seeing in the threads.

Starting with engine idling, the wet clutch is retracted or in a neutral position. As engine rpm is increased, the clutch pucks swing outward and begin to engage the wetclutch bell. At this point, the primary and secondary clutches begin to turn in their lowest ratio.

The CVT will continue to operate at this low gear position until the rpm and vehicle speed has reached the shift out phase. This is when the outward centrifugal force of the weights becomes stronger than the spring pressure on the secondary clutch. On the Rmax, I believe this is about 15mph in stock form. This means that all crawling and low speed driving is in the low ratio position, which has nothing to do with the size of weights installed or any shims or sheave modifications.

Now just to clarify, an added shim or lower cut sheave will produce a lower starting gear ratio. This means you will have more torque to the wheels in the 0 to 15mph range of the clutch operation, however due to the lower gearing, you may now be in a 0 to 12mph range before the weights make contact and begin the shifting cycle. There is no difference in noise, it is just lower gearing for taking off with big tires, crawling, climbing, etc. There is also no difference in rpm for your first shift, it will just come sooner in vehicle speed due to the lower gearing.

When the weights are spinning fast enough to overcome the secondary spring pressure, the weights will be forced against the camplate and slide the primary sheave inward. This forces the belt to ride in a higher position, increasing the gear ration to the secondary clutch. Because the belt remains the same length, the belt is lowered in the secondary as the belt raises in the primary. This gives you higher gearing automatically. It is during this process that the weight sizes effect the operation of the CVT. Heavier weights will apply more force at lower engine rpm. Lighter weights will require more engine rpm in order to obtain the same force required to push the primary sheave inward, over coming the rear spring.

After the CVT is shifted to its highest position, in order to accelerate the vehicle forward at a faster speed will require higher engine rpm. This is where most of the engine and transmission noise becomes a problem. The engine rpm can be lowered by increasing the weight size and/or machining the sheave to operate at a higher gear ratio.

Letting off the throttle reverses the process until the primary sheave slides back into a lower gear positions and the weights slide back to their starting point. There is a one-way bearing which engages to keep pressure on the clutch and limit its free spooling.

A shim will give you a lower starting gear, but it will subtract a small amount from your highest gear ratio. However, a machined sheave will give you both a lower starting gear and a higher top gear.




 

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Thank you for this. Coming from a 4 seat Talon as our first SXS, I am new to the CVT world.

I have the 22gram kit in my basket which includes the 1mm shim. What reduction in top speed should be expected in the RMX? Top speed is not very important to me, I see more need for the combination of a deeper starting ratio and lower operating RPM, but am curious.

The "funny" part is I don't have the machine yet, just the VIN this week. I've always been a tinkerer and will probably dive into the CVT not long after getting the machine.
 

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Thank you for this. Coming from a 4 seat Talon as our first SXS, I am new to the CVT world.

I have the 22gram kit in my basket which includes the 1mm shim. What reduction in top speed should be expected in the RMX? Top speed is not very important to me, I see more need for the combination of a deeper starting ratio and lower operating RPM, but am curious.

The "funny" part is I don't have the machine yet, just the VIN this week. I've always been a tinkerer and will probably dive into the CVT not long after getting the machine.
My Rmax 2 with a 1mm shim tops out at 66 MPH on the GPS speedo. I think it ran 71 when new, so, roughly 5MPH off the top end.

It’s a rare event if I ever get it up to 50 MPH anywhere I drive.
 

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Stock sheave ratio is 2.9:1
A 1mm shim produces 3.25:1 ratio.
It’s about a 15.5 percent increase.
Ive lost 8% of my gearing in tire size but I’ve gained back more than I lost.
It takes less throttle input to get the tires rolling when crawling in rocks than it did stock and less manipulation of the throttle to keep them moving from one big boulder to the next.
 

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"Heavier weights will apply more force at lower engine rpm." Does that mean more torque to the wheels at a lower rpm?
 

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Discussion Starter · #7 ·
I calculated a 4mph loss, actual will vary depending on tire size and terrain. It's not a significant loss in speed, but it is a pretty nice gain in torque.

I had stainless steel 0.5mm shims machined that are exactly the correct size. I have not put them up for sale yet though.

I have the 22gram kit in my basket which includes the 1mm shim. What reduction in top speed should be expected in the RMX?
 

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Discussion Starter · #8 ·
The shim will give you a boost to the wheels during the pre-shift of the clutch. The heavier weights will engage sooner at a lower engine rpm, so engine torque will be less due to the engine's powerband. The weights do not change anything until first shift out, which is about 15 mph.

To clarify. 0-15mph, 1mm shim gives 15% lower gearing so more power to the wheels. 15-40mph or full shift out, less torque due to less engine rpm while shifting, but quieter running. Full shift to top speed, same power as stock.

"Heavier weights will apply more force at lower engine rpm." Does that mean more torque to the wheels at a lower rpm?
 

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The shim will give you a boost to the wheels during the pre-shift of the clutch. The heavier weights will engage sooner at a lower engine rpm, so engine torque will be less due to the engine's powerband. The weights do not change anything until first shift out, which is about 15 mph.

To clarify. 0-15mph, 1mm shim gives 15% lower gearing so more power to the wheels. 15-40mph or full shift out, less torque due to less engine rpm while shifting, but quieter running. Full shift to top speed, same power as stock.
OK. Gotcha. Thanks.
 

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I had stainless steel 0.5mm shims machined that are exactly the correct size. I have not put them up for sale yet though.
Sounds good, I will keep an eye out for that. I will either stick with stock 29's (my gut says this is where I'll be) or possibly go to 30's. 1mm could be more than needed, but we do mostly slower riding and run Moab, Sand Hollow a couple times a year where the deeper starting ratio would be most useful. TBD once I see how the machine behaves stock.
 

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The shim will give you a boost to the wheels during the pre-shift of the clutch. The heavier weights will engage sooner at a lower engine rpm, so engine torque will be less due to the engine's powerband. The weights do not change anything until first shift out, which is about 15 mph.

To clarify. 0-15mph, 1mm shim gives 15% lower gearing so more power to the wheels. 15-40mph or full shift out, less torque due to less engine rpm while shifting, but quieter running. Full shift to top speed, same power as stock.
Ready to buy a 1mm shim if you have them available. Is your eBay store UTV engineering?
 

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Ready to buy a 1mm shim if you have them available. Is your eBay store UTV engineering?
In case he isn’t back on the forum for a bit, yes that’s correct, UTV Engineering
 

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I've been reading up on the CVT to better understand it but have some questions. I have the 21 rmax 4 and want to start running 32s on a square setup. Currently I'm still on 29s but will be installing the pv3 tuner with the wells racing runes and whiskey bent 2.5" tip in the coming days. I've read where alot of ppl aren't doing any CVT tuning running the 32s, how does this affect the performance of the CVT and or machine? I feel like it needs to be adjusted for the new tire size. Ideally, I was leaning towards the machined sheave to get the best of both worlds of low torque and a lil more top end, is this correct? Also would I need to change the weights or not? I'm looking to get the right setup on the first go around if all possible. Thanks
 

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You will notice an immediate difference with the 32's. The machine has plenty of power to turn them in most trail riding situations but your probably not going to do doughnuts in sport mode like you could before. It will crawl like a beast and a 4+3 offset wheel will make an even bigger difference in stability. The small loss of low end torque and acceleration can be overcame with the mods you mentioned. Read Budro2 's threads. I think he covers it very well
 

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I've been reading up on the CVT to better understand it but have some questions. I have the 21 rmax 4 and want to start running 32s on a square setup. Currently I'm still on 29s but will be installing the pv3 tuner with the wells racing runes and whiskey bent 2.5" tip in the coming days. I've read where alot of ppl aren't doing any CVT tuning running the 32s, how does this affect the performance of the CVT and or machine? I feel like it needs to be adjusted for the new tire size. Ideally, I was leaning towards the machined sheave to get the best of both worlds of low torque and a lil more top end, is this correct? Also would I need to change the weights or not? I'm looking to get the right setup on the first go around if all possible. Thanks
Yes, a machined sheave is always a great option for the best of both worlds on the top and bottom especially when going with larger tires. Most machined sheaves give an increase of top speed but also give a better starting ratio, or a “gear reduction” to get the bigger heavier tires rolling.
To my knowledge the only machined sheave available is a JBS and last I knew it doesn’t offer much benefit for the low end. They say if you want more bottom end to use a shim.
Its rumored that WELLER will come out with a sheave that accomplishes both.

In the past it was standard to use lighter weights with bigger tires to get engine RPMs higher and in a better torque curve to handle the loss of performance from the larger tires.
The Rmax comes with pretty light weights and high RPMs already. I’m not aware of a lighter weight for it than the 18s that come stock in the 2 seat. If big hill climbs, deep snow, sand etc are your riding style you could take out your stock 20s and put in the 18s to help it out but I haven’t heard of anyone going that direction.
The most noticeable difference you feel is in the mid range. Most everyone is going to the heavier weights like 22s to lower RPMs and make the noise level of riding more enjoyable.
I ran 32s in stock fashion and was not terribly disappointed. The larger engine handles it better than what people experienced with the 850.
After installing 22 gr weights i did find it a little disappointing when using the 32” tires.
My honest opinion as well as others is that the tune pretty well put it back to feeling similar to its stock performance, possibly a bit better even. If you need a lot of high end performance for sand hill climbs etc then don’t go heavier weights but if your a crawler or general trail rider and don’t have your foot to the floor a lot your will appreciate the better cruising RPMs and lower noise level by using 22s. The 24s are not advisable with the larger tires.
If you crawl or do deep mud/snow, just simply adding a 1mm shim will give you back the starting ratio you lost with the bigger tires. It will do it better than the JBS sheave and a shim is only a few dollars. In fact, if you buy the 22 gr weights through UTV ENGINEERING, they now include the shim with purchase. You will lose around 5 mph off your top speed. I don’t know to many people that spend much time needing to drive one of these faster than 65 MPH.

That’s pretty much the options available at this time to us and your riding style and terrain should dictate what your decisions are, on how set it up.

For me, 32s with Weller tune and 22 gr weights along with a 1mm shim is a great combo 👍
I’m a crawler and slow mover not a racer. Stock performance was always more than acceptable for my riding.
If I did lots of big hill climbs I’d probably go back to a 20 or 18 gram weight. 20 is probably a good compromise and you already have that. Then again if I did hill climbs I’d probably only run a 30” tire as well.
 

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Good summary @Budro2 I will be installing the 22gr weights with carbon sliders, .5MM shim and Tinkseal in the near future. I have the Weller tunes and don't intend on going larger than 30s. Our riding style is slower and more rock crawling/trail riding so adding to the starting ratio with the .5mm shim (~8% increase) is a good match.
 
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