Chain Speed vs. Torque

[Jonny Quest]

I've seen some posts where folks talk about changing the "gear ratio" of the saw/chain by changing the sprocket. Example: go from the standard 7 tooth to an 8 tooth. It seems that most talk about going bigger - longer gear ratio.

In "normal" (non race) applications, why would this make sense? In my mind, I would want a little lower gear ratio so as not to bog the saw down. When I pull a load up a hill in my truck, I come out of overdrive. I realize that its 2 stroke vs. 4 stroke, but isn't the concept similiar? Get the rmps of the saw up into its "power band" and sweet spot and then use a lower gear ratio for better torque.

What magic is there in a higher chain speed with less torque that would produce better results than lower chain speed and higher torque?

Is there any higer risk of throwing a chain with a smaller vs. larger sprocket?

If you were to gear down with a smaller sprocket, could you pull a full comp chain where you were using a skip chain?

JQ

 

[Mad Professor]

Big bar, high load, small sprocket

Small bar, light load, big sprocket

 

[2dogs]

Big bar, high load, small sprocket

Small bar, light load, big sprocket

Yep. In my case I run an 8 tooth on my 260 with a 16" bar with great results. My other saws run stock 7 tooth sprockets except sometimes with the 361 and an 18" bar. I am not a racer nor a full time faller so speed is not a big concern for me. Most of my saws run a long bar and need torque vs speed.

 

[timberwolf]

Something to considder too is chain pitch 3/8 7 pin is roughly = to .325 8 pin in terms of the gear diamiter. .325 has more cutters, but they are smaller so that part equals out quite a bit.

There is no one answer, it depends on saw, bar length, chain type and size, wood type, size of log, elevation, how good your sharpening is....

 

[cjcocn]

I bought my Jonsered 2186 used and it came with a 9 tooth sprocket and a 32" bar.

Bought a new 3/8" 8 tooth sprocket, clutch needle bearing, and a 24" bar.

It cuts really well and I have had no issues with it bogging down.

Was I wrong to consider a 24" bar a "small bar" and use an 8 tooth sprocket? My figuring was that the bar was small considering that it is an 85cc saw?

Thanks

 

[Jonny Quest]

OK - restate the question...

When is chain speed more important than torque?

When is torque more important than chain speed?

Is this a function of clearing chips during cutting? Do the chains cut more efficiently at higher speed? Is there a problem to slow the chain down to apply more cutting force or pressure? Is the increased chain drag the main reason for a lower gear ratio and more torque for longer bars?

JQ

 

[madrone]

When I'm bucking softwood with the 660 with a sharp, full comp, square ground chain, it rarely bogs down. I run a 8 T. More speed means faster cutting.
When I bury the bar in oak i'd be better to run a 7 T and the smallest bar I can use. More torque, less wear on the saw.
It come down to this: if you have the power go faster. Rarrrr

 

[redprospector]

When I was cutting timber for the mill I started running an 8 tooth sprocket on a 288xp. I knew I was sacraficing some torque, but I was spending a lot more time cutting limbs than at the stump or bucking. It all depends on your situation.
Yes, there is a greater chance of throwing a chain with the 8 tooth than the 7. The tail of most bar's are sized for a 7 tooth so with an 8 the chain travels a little farther before it gets into the rails.

Andy

 

[stihl only]

When is chain speed more important than torque?

When is torque more important than chain speed?

Is this a function of clearing chips during cutting? Do the chains cut more efficiently at higher speed? Is there a problem to slow the chain down to apply more cutting force or pressure? Is the increased chain drag the main reason for a lower gear ratio and more torque for longer bars?

JQ

One is not more important than the other, but they work together. If you have good torque and a shorter bar, you can run a bigger sprocket and cut faster with the higher chain speed. If you are down on power or need to run the max size bar for the powerhead, you can run a smaller sprocket to keep the engine from bogging.

It's all about keeping the RPMs in the range where they make max HP.

As far as chain speed VS high bar pressure, more pressure is going to cause more bar wear, more AV wear, more bearing wear, etc.

A sharp chain is #1 and unless the saw is way too small for the job, the operator can keep the RPMs where they need to be. If you are dogging the saw in and can't slow the thing down, a bigger sprocket should make for more production.

Most guys are never going to really fine tune all of this, and they will still lead a long happy life!!

 

[mountainlake]

If your dogging in and can't slow your saw down check the raker hieght and sharpen the chain. I'd think with a dull chain with the rakers a little too high chain speed would win. Steve

 

[cjcocn]

When I'm bucking softwood with the 660 with a sharp, full comp, square ground chain, it rarely bogs down. I run a 8 T. More speed means faster cutting.
When I bury the bar in oak i'd be better to run a 7 T and the smallest bar I can use. More torque, less wear on the saw.
It come down to this: if you have the power go faster. Rarrrr

Good point that I forgot to mention. I am cutting in softwood (with a new chain).

Birch is the hardest wood that I will be cutting around here. I put the 2186 into some decent-sized birch on its trail run and it ate that up as well.

I will keep an eye on it, but so far it seems to be working out for me.

Thanks for all the comments!

 

[Banshee]

If you was wanting to check you chain speed here is a formula.
I kind of thought this formula up myself so it may not be accurate.
(chain pitch), times 2, times (sprocket size) times (rpm's) divided by 12 to convert to feet, divided by 5280 to convert to miles per minute, times 60 to convert to mph = chain speed in mph.

example for a 066 with a 8 pin and 3/8 pitch chain.

(chain pitch x 2.) 3/8 (.375) x 2 = .75
(.75 x sprocket size) .75 x 8 = 6
(6 x rpm's) 6 x 12500 = 75000
(75000 (divided) 12 to convert to feet) 75000/12 = 6250
(6250 (divided) 5280 to convert to miles) 6250/5280=1.18
(1.18 x 60 to convert to mph) 1.18 x 60 = 70.8

So the chain speed for a this an 066 with 3/8 pitch and 8 pin sprocket is 70.8

 

[computeruser]

I think it's more of a seat-of-the-pants thing, where torque and speed work together to get you to a productivity a sweet spot. Technique, bar length, chain type/condition, and user preference play a big role in finding this sweet spot.

Saws require a certain level of load to be placed on them in order to actually get the engine to smooth out and work at max efficiency with a particular bar/chain combination. This can be accomplished by one or a combination of three methods: (a) higher chain speed (overdrive), (b) lower rakers, or (c) more cutters in the wood at once. The common attribute of all of these tweaks is that the saw is being asked to gnaw more wood per unit of time; whether you come up short of or whether you exceed the saw's max gnawing per unit of time, the result is the same: efficiency will be less than it might have been under optimal conditions. For the guy who is blocking up firewood in his yard, it might not matter. But for the volume cutter, the desirability of being in the peak production zone should be obvious.

By way of example, this setup was still 4-stroking when buried, unless you leaned on it a bit. 111cc, 42" bar, brand new loop of 3/8" round chisel skip chain, 8t sprocket. Productivity with the 8t is up markedly over the unstoppably torquey 7t setup it had before, but still wasn't optimal; the rakers could definitely come down from factory height (.025"??).