Performance Sprocket Tuning
Tuning pro saws for peak performance involves many factors, and one of them is sprocket tuning. Sprocket tuning or "gearing," as it's sometimes called, affects the performance of professional chain saws, just as it does log trucks, race cars, and many other types of engine powered machinery.
Many pro saw users do not fully understand the role sprockets play. As a result, some don't get the most out of their saws. Sometimes this occurs because they don't understand the considerations necessary for making a good choice and other times those considerations are clouded by misinformation. The purpose of these next few paragraphs is to help users better understand their needs and to aid them in the selection of equipment that best suits them.
To understand sprocket tuning, the place to start with is with an understanding of pitch. Pitch, as measured on a saw chain, is the distance between the center of any 3 side rivets, divided by 2. Now, before any more of you write or call and ask why we simply don't say pitch is the distance between the centers of any 2 rivets, this is the reason: Some chain is designed with uneven pitch, so the distance between any two links is not always the same.
Fractional or Decimal?
Another thing that makes pitch hard to understand is the numeric format used to describe it. Today 3/8" and .404 are common pitches for pro saw chain. Strange as it may seem, one is expressed as a fraction and one as a decimal. It would be easier if we used the same format to describe both of them. However, in common practice they aren't described that way, so we are not going to suggest changing it. We will continue to refer to .404" as a decimal and 3/8" as a fraction, but we're the first to admit it's a clumsy thing to do.
Now if this sounds too much like a story problem in math class, please don't jump ahead. We promise it is just about to get fun. Even if pitch numbers are still confusing, the really important thing to know is that on pro saw chain, 3/8" is slightly smaller than .404". That's really it.
Since 3/8" chain is a little smaller than .404", when comparing sprockets with the same tooth count, the 3/8" sprocket is going to be smaller in diameter. This is important to remember because the diameter of the sprocket affects the "gearing" of the saw.
We like to use the word "gearing" when talking about sprocket tuning, because most of us have driven a vehicle with a stick shift. We know that first gear has good pulling power, but not much speed. We also know fourth has good speed but not much power.
Today's pro saws are direct drive. They don't have a transmission like the example above. There are no gears to shift. Gearing is set by the sprocket and it must be right from the beginning for the saw to perform its best. The ability to change the sprocket to one of a different pitch or tooth count is what allows the user to sprocket tune.
The pitch (diameter) and tooth count of the sprocket determines how much chain is moved on each engine revolution. In fact, sprocket size is usually stated: Pitch "by" tooth count. An example is the size: 3/8" x 8. This describes a sprocket that is 3/8" pitch with 8 teeth. That means each time the crank shaft rotates, it will move eight drive links of a 3/8" pitch saw chain.
Since we promised no more math, we'll do a little for you: This means a saw turning 13,500 RPM fitted with a 3/8" x 8 tooth sprocket is moving chain at 75 mph. Another way of saying it is the chain is moving 110 ft in one second. Pretty amazing, huh.
Now you know why today's high speed saws cut so well, but sometimes this is misleading. While it is true a fast moving chain leads to a fast cutting saw, the saw has to have enough power to cut. Simply moving the chain is not enough. It has to sever wood fiber and remove chips, too. This is where a lot of pro saw users get confused. Faster is not always better. The engine's speed and power have to be balanced with cutting characteristics.
How Sprocket Size Affects Speed & Power
Proper sprocket selection requires many considerations. These include: How much power does the saw motor really have? How long is the bar? How efficient are the cutter teeth? How hard is the wood? How sharp is the chain? How long will it stay that way? The secret is to find the place where all these factors come into balance.
Although pro saw users in the Pacific Northwest commonly use long bars to cut primarily large softwood trees, the setup is not the same for every user. Knowing some general guidelines can really help with sprocket selection.
The Best Sprockets For Saws With 65cc To 80cc Engines
For saws with engine displacements of four and five cubic inches, a 3/8" x 7 is a good all around choice. This category includes STIHL 044 and 046 and HUSQVARNA 371. These saws can effectively run long bars up to 32" with square ground chain when working in softwood trees. The effective bar length is reduced when cutting harder wood and when running saw chain with less efficient cutter teeth.
In situations where wood is soft, bar length short, and chain that is sharp and efficient, a 3/8" x 8 can work well, but it is rare. Usually when 3/8" x 8 tooth sprockets are run on saws in this size range, performance suffers.
As for .404" pitch chain, there isn't any sprocket size that yields good performance. This chain is too heavy and inflexible to work well on saws in this size range.
The Best Sprockets For Saws With 85cc To 100cc Engines
For saws with engine displacement between five and six cubic inches, the options are greater. This category includes the STIHL 064 and 066 and the HUSQVARNA 288 and 394. Saws in this category perform well with both 3/8" and .404" chain. Cutting is usually faster and smoother with 3/8", but .404" offers more durability, a wider kerf, and easier filing.
As for sprocket size, for bar lengths up to 32", a 3/8" x 8 or a .404" x 7 works well. This gearing provides good chain speed, yet still offers plenty of stump power.
When running a 36" bar and .404" pitch, the .404" x 7 remains the best size. When running a 36" bar and 3/8" pitch, a 3/8" x 7 offers more power, while the 3/8" x 8 favors chain speed.
When running a 42" bar with 3/8" chain, the 3/8" x 7 is the clear choice. This is really a pretty good combination because a 42" bar pushes the limits on saws in this category. The lower gearing that a 3/8" x 7 offers over a .404" x 7 really helps. 3/8" chain is also more flexible and lighter than .404", and this helps, too.
OREGON DURAPRO chain, or the "X" chain as some call it, works well with saws in this category. It's heavy duty cutter tooth cuts a wider kerf than standard saw chain. This is a benefit for saws in this category because this chain is tough and the wider kerf helps keep the longer bars from binding in the cut.
When running a 42" bar with .404" chain, the .404 x 7 is the best option. Even in soft wood, this pushes or exceeds the limits of the smaller saws in this category. Most pro saw users find this combination suitable for use only an occasional basis.
Bars longer than 42" do not perform well on saws in this category with any sprocket combination.
The Best Sprockets For Saws With Engines Over 100cc
For saws with engine displacements over 6 cubic inches, the .404" x 7 is usually the best choice. Saws in this category have the power to pull the large chassis of .404 chain and are not hampered by its mass and reduced flexibility. It's wide cutting kerf, high chip clearing ability, and tough design are characteristics that make .404" chain the best choice.
3/8" chain can be used on these large saws, but performance usually suffers. If 3/8" chain is used, a 3/8" x 8 is the best choice.
Large Sprockets Affect Clutch Hub Wear
Large diameter sprockets tend to wear clutch hubs faster than smaller sprockets. The larger the sprocket, the more leverage it places on the splines. As a result, saws using .404" x 8 tooth sprockets will wear splines more quickly than any of the other sprockets. Conversely, the 3/8" x 7 is the easiest on them.
Although hub wear may not be a big factor in determining the best sprocket, it is a good idea to know the larger sprockets tend to wear them faster and more frequent inspections are a good practice when running them.
Racing sprockets such as 3/8" x 9 or larger seldom work well on production saws. They are designed to produce extra chain speed for motors that make more power than regular pro saw models.
They work only on large displacement saws with "hopped" engines. Using one on a standard pro saw will usually gear it so high there is not enough power to cut well. It will also lead to frequent chain derailing because their large diameter holds the chain out of the heel of the bar for a long distance.
Over-Speeding Saw Chain
The use of large diameter sprockets may lead to other problems such as over speeding the chain. Although we see this problem on mechanized harvesters more than hand held chain saws, all saw chain is designed to run at certain speeds. Running it beyond this speed may improve cutting speed for a short time, but it will lead to premature chain failure.
The dynamic forces generated when operating saw chain beyond its recommended speed will cause the chain to self destruct. Such breakage is dangerous and wasteful. Operating the chain within its design parameters will yield both good cutting performance and long serviceability.
As you can see, sprocket tuning and pitch selection involve a number of factors. Like many things there is not one "perfect" combination. You must understand your saw and consider all the factors that affect each component. In the end, the best set up is a compromise that allows the components to work together to achieve top performance.
We also want to thank you for your feedback and the many compliments on the "shop tips" we provide. You're ongoing encouragement has already got us working on more sections like this for the future. Drop us a note and let us know what you are interested in. Let us know what topics are most important to you. It is true, sometimes a little information goes a long way towards unlocking better performance from your pro saw. Helping you get the most out of your tool is why we're here... That's why we're the Pro's Choice.
Got questions about sprocket tuning? Call or stop in.