A question came up recently about how each of the clutch mods work and the advantages & disadvantages of each. It sounded like a perfect opportunity to create a thread we could all share in building to be the definitive "clutch mods"¯ thread. Please share your opinions, point out where I got it wrong, or add new insight (or even other mods I missed).
I tried to keep this as non-technical as possible so we don't get caught up in techno-babble that has to be defined in order to be understood but figured it might be nice to define a few terms so we are all on the same page and to explain some basic concepts:
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.
. 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 1/2"¯ 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.
EDIT: See this thread
for an excellent write-up and review by Jack (the ModFather) of a greaseless setup using a machined sheave and a stronger secondary spring. Good info as well as great pics for visual aids while reading this post.
See this thread
for a spectacular 2-part video by Butch450 and The Modfather showing step-by-step installation of a machined sheave and greaseless weights and sliders on a Grizzly 450!
Below are the basic "clutch mods"¯ of which I am aware with my thoughts on each.
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.
- Going any heavier than the weights required to place you at top gear ratio at a reasonable point below red-line has no advantage at all.
- You loose bottom end torque without any gain in top speed.
Light weights get 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.
- Gives you greater low end torque. Inexpensive.
- likely to reduce your top-end speed.
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.
- gives you more bottom end pull for climbing rocks and plowing through high-torque situations. Inexpensive.
- you loose top end speed to gain the low-end torque.
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.
Sheave machining is the only clutch mod I'm aware of that can increase/lower your low end while not reducing (or even increasing) your high-end top speed.
- Customizable RPM/torque/speed curve throughout the entire range.
- NOT inexpensive. Could turn out very badly if not done properly or it does not match your riding style/desires.
Cam plate mods:
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. An example of cam plate modification is the Airdam mod as discussed in this thread http://grizzlyriders.com/forums/gene...h-setup-5.html
Greaseless vs. greased:
Just about every manufacturer other than Yamaha that uses a CVT uses a greaseless system. I don't know if that means Yami knows something they don't or vise-versa.
A greased system uses (special) grease on the weights and sliders of the sliding primary sheave.
- 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.
- The grease is a major pain in the
to deal with. Cleaning and maintaining the CVT clutch is a time-consuming hassle.
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.
- No messy grease to deal with except on the shafts. Easier cleaning and maintenance.
- Cost, other than that, none, if it works properly.
Stronger secondary springs:
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, modders will often replace this spring with a stronger one.
- Reduces slippage in the CVT that can often be caused by other modifications.
- Increases stress on the CVT shaft bearings, possibly causing them to wear more rapidly.
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.
- Sounds like a major PITA to do for no real gain. I almost expect to be challenged on this one.