CVT Continuously Variable Transmission
The most common type of CVT in use for ATV’s is the V-belt system employed by Yamaha and several other manufacturers.
Gear ratio. Although a CVT does not contain true gears (the High and Low ranges on a Grizzly are gears but they work outside of the CVT), the ratio of the number of turns the engine shaft makes compared to the number of turns the drive shaft makes is the effective gear ratio of the CVT. For example; if the engine turns 10 RPM for each 1 RPM at the drive shaft, you have a 10-to-1 (10:1) gear ratio = a low ratio/low gear. If you have 1 RPM at the engine for every 1 RPM of the drive shaft, you have a 1:1 ratio = a higher ratio/higher gear. Any ratio above 1:1 would be considered a high ratio/high gear.
How the CVT works:. The basic principal behind the V-belt type of CVT is that you have two pulleys that sit right next to each other with a V-belt running between them. These pulleys� can effectively change size so that the transmission’s gear ratio can be changed. Each pulley is made up of two sheaves, which are the halves of the pulley that the belt is squeezed between (imagine two large straight-sided salad bowls placed base-to-base to form a V-shaped space between them, getting larger as you move out from the center). The primary sheaves, which connect to the engine, can move closer or farther apart by virtue of a centrifugal weight system and the secondary sheaves, which connect to the drive shaft, have a large spring that hold the two sheaves together. The weights, which look like ~3/4” long pieces of ~1 ½” pipe, live in the outer or “sliding”� primary sheave and ride on ramps so that, as the weights are forced by the spinning of the engine shaft toward the outside of the sheave, they force the sliding sheave closer to the rear/stationary sheave making that pulley� bigger and bigger. Being spring-loaded, the secondary sheaves simply take up the slack in the belt by squeezing as close together as they can, given whatever belt is left to them by the primary sheaves. This is how the Grizzly CVT works and it automatically compensates for load and engine RPM to give torque when needed and speed when desired (within the limits of its design).
Although the CVT is sometimes referred to as a clutch, it actually is not (at least in Grizzlies). The Grizzlies do actually have a separate wet clutch that lives inside the engine/transmission case in the oil bath directly behind the CVT. This clutch disengages at very low RPMs (i.e. idle) so that no power is transmitted to the CVT. At RPMs above idle, this clutch essentially “locks” so that all power is transmitted to the CVT. This allows the Grizzlies and other CVT systems that employ a wet clutch to maintain contact between the drive belt and sheaves at all times rather than relying on a loose belt to slip at idle (wearing out the belt and sheaves and causing unnecessary heat). The wet clutch system prolongs the life of the belt and the sheaves and just works better than the clutch-less “slipping belt” systems.
The following are the clutch mods I have encountered to date, although different techniques are used similar results are achieved which is: lower “bottom” end torque while maintaining top end speed with focus on a strong performance throughout top to bottom ranges.
During my research I have found many flavors and methods but all come back to these three…
• JBS Performance
• Hunterworks
• Coop45
Below are the basic “clutch mods” of which I am aware of with “my opinions” on each.
1) Weight Changing
a) Roller Weights
b) OD – Over Drive Weights
2) Shimming
3) Machined Sheave
a) Face
b) Channel
4) Cam Plate Cutting
5) Roller Lubrication
a) Grease
b) Greaseless
6) Secondary Spring Change
7) Wet Clutch Springs
8) Wet Clutch Slugs – information to follow
1) Weight Changing:
Heavier weights:
Referring to the weights in the sliding primary sheave, heavy CVT weights get “flung out” with more force than lighter weights would at the same RPM thus causing the sheaves to press together (raising the gear ratio) faster and requiring lower RPMs to go the same speed…but with less torque. You do not gain any top-end speed (you have not changed what the highest attainable gear ratio is) but you do loose low end because the heavier weights are forcing you into higher gear ratios faster. You end up at top gear ratio at a lower RPM than you would with lighter weights.
Heavier rollers will get you into your mid range curve sooner. Lets say that Heavier rollers will shift out of 1st gear sooner & gets you into 2nd sooner. (You get wheel speed faster to clean your tires unless your bogging then the rollers are to heavy)
Advantages – Going any heavier than the weights required placing you at top gear ratio at a reasonable point below redline has no advantage at all.
Disadvantages – You loose bottom end torque without any gain in top speed.
Lighter weights:
The lighter weights are flung out with less force than heavier weights would at the same RPM. This causes the sheaves to press together (raising the gear ratio) more slowly thus taking higher RPMs to go the same speed…but with more torque. You gain more bottom-end torque because it requires higher RPMs to raise the gear ratio. Assuming you do not increase your engine’s top-end RPMs, you may loose some top-end speed or it may take longer for your quad to get up to top speed (lots of runway). As a general rule, the weights in smaller displacement Grizzlies are lighter than those in the higher displacement ones. This is why you will read of such things as replacing half of the weights in a 660 with weights from a 450.
Lighter rollers will stay in 1st gear longer & in which reduces bogging BUT (Its kind of like having a drag car that does excessive wheel stands–if the weight transfer from the wheel stand is not needed, then your just losing ET by forcing the car into the air instead of traveling forward.) So if your not bogging than you don’t need the lighter rollers. Also the lighter rollers will make you engine turn more RPMs while path riding because the lighter rollers wont let the belt shift into 2nd gear or 3rd per say.
Advantages – Gives you greater low-end torque. Inexpensive.
Disadvantages – likely to reduce your top-end speed.
2) Shimming:
The shim mod increases the maximum distance between the primary sheaves and effectively lowers the entire gear ratio to give you a lower low end (more grunt) at the cost of taking the same amount off the top-end speed. It was Originally called the Coop45 Mod and will give you basically a gear reduction due to making the belt ride lower in the primary sheave & will make the belt ride higher on the secondary sheave. The shim will give you more torque multiplication as shimming lowers your max speed a avg. 2mm shim 5mph give or take 1mm shim maybe 2-3 mph
Advantages – gives you more bottom end pull for climbing rocks and plowing through high-torque situations. Inexpensive.
Disadvantages – you loose top end speed to gain the low-end torque.
These are the Yamaha part numbers for the shims used with this mod, others have purchased from hardware stores etc. to achieve similar results. JBS Performance is the only one I am aware of beside Yamaha to manufacture their own.
JBS Performance has shims available in .5 mm, .75 mm & 1.1 mm thickness
Yamaha Part # 90201-225A4-00 washer plate, Thickness = 1.0mm
Yamaha Part # 90201-222FO-00 washer plate, Thickness = .5mm
3) Machined sheaves:
Sheaves can be machined for one of two basic goals or a combination of both:
a) More torque at the low end and/or a longer low-end range (greater amount of RPM range before quad speed is increased much). This is done by either making the “lower” part of the gear ratio curve flatter and, thus, increasing the slope of the remainder of the curve (making the speed to RPM ratio clime faster once out of the low range) or by actually lowering the low end (like the shim mod)…or both.
b) Higher top-end speed by making the primary sheaves come to minimum separation at a lower RPM. This also causes the gear ratio slope to be steeper since it is, within the same RPM range, ending at a higher point.
c) A combination of both. This leads to a longer and/or lower low end and a higher top end and a gear ratio slope that is very much steeper than the original.
Advantages – Customizable RPM/torque/speed curve throughout the entire range.
Disadvantages – NOT inexpensive. Could turn out very badly if not done properly or it does not match your riding style/desires.
4) Cam Plate Cutting:
The cam plate (also known as the fix plate) sits on the outside of the weights and is the other half of the equation for how much the sliding sheave has moved when the weights are at a given position from the center of the sheave. Technically, all of the adjustments made by a sheave machining can be duplicated by modifying the contour of the cam plate but, in reality, all cam plate modifications of which I am aware only involve adjusting (increasing) the angle of the of the cam plate so that the rate of increase from lowest gear ratio to highest is increased and the top end ratio is attained at a lower engine RPM.
5) Roller Lubrication:
Just about every manufacturer other than Yamaha that uses a CVT uses a greaseless system. I don’t know if that means Yamaha knows something they don’t or vise-versa.
Grease
A greased system uses (special) grease on the weights and sliders of the sliding primary sheave.
Advantages – It is hard to be sure about this so, I’m pretty much speculating, It could be that a greased system may be less prone to failure in highly dusty environments since the grease could allow the dust particles to move out of the way more easily than they might in a dry situation. Of course, enough dirt will eventually contaminate the grease to the point where it becomes a gritty paste and the clutch fails because of that. It is also possible that a greased system may work better in environments where it is more likely to have water enter the CVT housing (read, deep water crossings) because the grease will keep the water from causing corrosion of the aluminum sheave in the areas where the weights and sliders ride.
Disadvantages – The grease is a major pain in the to deal with. Cleaning and maintaining the CVT clutch is a time-consuming hassle.
Greaseless:
A greaseless or “dry”� system uses weights with a special low-friction covering and sliders made of low-friction material that allows them to move freely within the sheave without using any grease. Note that even greaseless setups use grease between the CVT shafts and the sheave bodies but you only have to deal with that if you completely disassemble the CVT.
Advantages – No messy grease to deal with except on the shafts. Easier cleaning and maintenance.
Disadvantages – Cost, other than that, none, if it works properly.
Stronger secondary springs:
6) Secondary Spring Change
Since the intent of the spring on the secondary sheaves is to take up the slack in the drive belt left by the primary sheaves, it stands to reason that this spring needs to be pretty beefy in order to squeeze the belt tight enough to where it does not slip excessively under power. Whenever you perform a modification that gives you more low-end, you run the risk of exceeding the ability of the stock secondary spring to keep the belt from slipping too much. For this reason, mod’ers will often replace this spring with a stronger one.
Advantages – Reduces slippage in the CVT that can often be caused by other modifications.
Disadvantages – Increases stress on the CVT shaft bearings, possibly causing them to wear more rapidly.
7) Wet Clutch Springs:
There are springs in the wet clutch that hold tension on its weights and set the RPM at which this clutch engages. I have heard that some people have changed these to slightly stronger springs so as to increase the speed at which this clutch engages. This would sort-of have the effect of giving you more low-end but really, it would just make the engagement happen at, what I would consider, an incorrectly high RPM. Stiffer internal clutch springs allows a higher rpm engagement before the bike moves in which its side effect is shorted internal clutch life & more contamination of the oil. (So internal springs depends on riders preference)
Basic Notes
PS after adding shims
(Bolt the cage on & run the motor before putting the clutch housing cover on so the belt will set in place & not rub the housing cover at start up.)
Things that alter belt travel:
Primary & secondary overall
1) Roller/weight diameter – larger pushes belt out farther on primary which may shorten inner belt travel on primary so the inner roller travel path may need to be lengthened/machined
2) Machining the roller/weight path Mod. (Outer travel area) – extends the area the rollers travel to allow the faceplate to be pushed farther in which pushes the belt out farther in which increases top speed
3) Shimming – allows the belt to ride lower in the primary in which increases gear multiplication/low end pull also reduces the angle on the face plate, in which reduces the push on the belt to the outer rim of the primary. Reduced top end speed
4) Machining farther down into the primary Mod. (Depth Cut) allows the belt ride lower into the primary but with no change to the faceplate angle in which increases the low-end gear multiplication while machining to get the depth cut into the sheave, the angle is changed DUE to cutting for the depth
5) Faceplate angle Mod – pushes the belt farther out on the primary which is done to increase wheel speed and also done to counteract the angle loss when using the shim Mod (Smaller cc atvs may only notice its effect in a low range gear due to parasitic loss in the drive train or any other power robbing sources)
6) Machining farther down into the secondary Mod. (Depth Cut)
(While machining to get the depth cut into the sheave, the angle is changed DUE to cutting for the depth)
(This Mod pushes the belt farther down into the secondary in which increases speed BUT this area is run through before the face plate tops out the top of the primary)
(Maybe this mod will help counteract the shim mod issue of reducing the faceplate angle & regaining the extra push the faceplate needs to push the belt out on the primary)
Things that alter belt shifting speed.
1) Centrifugal force. (Roller weights)
Heavier rollers shift faster.
Lighter rollers shift slower.
2) Secondary spring pressure.
More, equals slower up shifting & faster down shifting.
(Less is prone to a slipping belt)
3) Sheave angles. (NOT depth angles)
(But controlled by spring pressure)
4) Face plate angle length.
a) Steeper equals faster
b) Less equals slower
5) Roller path length.
a) Shorter equals faster
b) Longer equals slower
6) Sheave surface texture
a) Ruff in which equals more friction.
Belt ware may increase due to better back shifting & tracking of the belt & the heat created due to friction.
b) Smooth in which equals less friction.
Belt ware may be reduced due to less back shifting & tracking & less heat generation.
(Spring pressure maybe more important here but of course spring pressure increases friction also)
Things that reduce horsepower in the clutch system
1) Friction.
Things that induce horsepower in the clutch system
1) Lighter parts
(Weight reducing Mod (of the Sheaves)
(This increases engine acceleration due to less parasitic loss)
(But at this extent are we really noticing it???)
The belt starts at the base of the primary in which is the low-end power area.
Things that allows the belt to lower more into the primary
1) shimming.
2) Machining the primary depth.
PARTS
Primary Rollers
The 350s has 6 rollers total
The 400 & larger ATV’s has 8 rollers total
6 roller housing
6ea x 12 = 72 total
3ea x 12 + 3ea x 14 = 78 total
3ea x 12 + 3ea x 16 = 84 total
6ea x 14 = 84 total
3ea x 12 + 3ea x 18 = 90 total
3ea x 14 + 3ea x 16 = 90 total
3ea x 12 + 3ea x 20 = 96 total
3ea x 14 + 3ea x 18 = 96 total
6ea x 16 = 96 total (stock 350)
3ea x 14 + 3ea x 20 = 102 total
3ea x 16 + 3ea x 18 = 102 total
3ea x 16 + 3ea x 20 = 108 total
6ea x 18 = 108 total
3ea x 18 + 3ea x 20 = 114 total
8 roller housing
8ea x 12 =96 total
4ea x 12 + 4ea x 14 = 104 total
4ea x 12 + 4ea x 16 = 112 total
8ea x 14 = 112 total (stock 400 & 450)
4ea x 12 + 4ea x 18 = 120 total
4ea x 14 + 4ea x 16 = 120 total
4ea x 14 + 4ea x 18 = 128 total
4ea x 12 + 4ea x 20 = 128 total
8ea x 16 = 128 total (stock 550 & 600)
4ea x 14 + 4ea x 20 = 136 total
4ea x 16 + 4ea x 18 = 136 total
8ea x 18 = 144 total (stock 660)
4ea 16 + 4ea x 20 = 144 total
4ea 18 + 4ea x 20 = 152 total
8ea x 20 = 160 total (stock 700)
Stock 07 350, 09 550 & 01 600 has 14-gram rollers, with cover =16 grams.
Yamaha roller part# 4WV-17632-00-00
Stock 07 400 & 450 has 12-gram rollers, with cover =14 grams.
Yamaha roller part# 5GH-17632-00-00
Stock 07 660 has 16-gram rollers, with cover=18 grams.
Yamaha roller part# 5KM-17632-00-00
Stock 07 700 has 18-gram rollers, with cover=20 grams.
Yamaha roller part# 3B4-17632-00-00
