Why do longer bars require more power?

[brages]

I see the general advice is that longer bars require more power... that you're better off with a shorter bar if you mostly just cut small stuff.

Why is this?

If you are cutting through, say, 8" oak, will a 20" bar cut slower than a 16" bar? Why? Just the extra friction from the longer chain? I can't imagine chain friction on the unloaded part of the bar is really significant.

Or is the idea that longer bars need more power because a fully sunk longer bar is cutting more wood? Like my 210 can only really cut 16" of wood at a time, so if I had a 20" bar on it, it would be pointless because it would bog hopelessly in anything bigger than 16".

I guess my question is, so long as you are cutting smaller diameter wood, how much does excess bar length hurt you? And why?

 

[skid row]

:monkey: :monkey:

 

[bigjake]

You loose chain speed with a long bar its like changing gear ratio.

 

[Metals406]

I see the general advice is that longer bars require more power... that you're better off with a shorter bar if you mostly just cut small stuff.

Why is this?

If you are cutting through, say, 8" oak, will a 20" bar cut slower than a 16" bar? Why? Just the extra friction from the longer chain? I can't imagine chain friction on the unloaded part of the bar is really significant.

Or is the idea that longer bars need more power because a fully sunk longer bar is cutting more wood? Like my 210 can only really cut 16" of wood at a time, so if I had a 20" bar on it, it would be pointless because it would bog hopelessly in anything bigger than 16".

I guess my question is, so long as you are cutting smaller diameter wood, how much does excess bar length hurt you? And why?

A longer bar on a smaller cc saw is fine for smaller diameter wood... As long as you aren't exceeding the saws ability to oil said bar, and you aren't lugging the saw all day. I know a local crew that runs 28" bars on all their 361's, and have no problems. They do line clearance--so the longer bars are used to brush out. I've asked several times about their decision to run such a long bar... And they say they've never had an issue. They stated, "If we need to cut bigger wood 18"+ DBH... We pull out a bigger saw."

So, if you're "brushing" a longer bar may save some bending over (backache) to cut smaller diameter trees... But if you are cutting firewood, and it's small diameter, why not make a saw-buck, and get the stuff off the ground? Then you can use your smaller cc saw, with an appropriate bar length, and have no worries.

 

[brages]

I imagine a shorter bar would offer these benefits:

1. Quicker to rev up (less inertia)
2. Lighter weight (maybe better balance)
2b. Better handling - easier to toss around, not long and clumsy
3. Requires less oil (or, stays oiled better if you can't adjust your oiler)
4. Chains and bars are cheaper
5. Chains are quicker to sharpen

With these downsides:
1. Can't cut as big of stuff
2. Chains dull sooner (except for dulling caused by damage)
3. May require more stooping to use

My question - are shorter bars also faster in the cut, provided the short and long bar are both bigger than the diameter of the wood being cut? How much faster?

Does it make any sense to use a bar that's kinda too long for your saw, and just baby it through the odd big cut?

 

[stihl sawing]

I see the general advice is that longer bars require more power... that you're better off with a shorter bar if you mostly just cut small stuff.

Why is this?

If you are cutting through, say, 8" oak, will a 20" bar cut slower than a 16" bar? Why? Just the extra friction from the longer chain? I can't imagine chain friction on the unloaded part of the bar is really significant.

Or is the idea that longer bars need more power because a fully sunk longer bar is cutting more wood? Like my 210 can only really cut 16" of wood at a time, so if I had a 20" bar on it, it would be pointless because it would bog hopelessly in anything bigger than 16".

I guess my question is, so long as you are cutting smaller diameter wood, how much does excess bar length hurt you? And why?

You just answered your own question.

 

[RON58]

Was just thinking about this. If I had a 48" er I wouldn't even have to bend my back.

 

[bump_r]

You loose chain speed with a long bar its like changing gear ratio.

You may want to put a monkey smiley behind that statement lest someone think you were serious. PLEASE let that have been a joke...

Oh, and "lose" and ";"

 

[spike60]

Let's say that you are cutting a log that is 10" in diameter. With 3/8 chain, there will be no more than 8 cutters in the wood at any time, regardless of the length of the bar. 16" or 36"; it makes no difference.

The extra power is only really necessary when cutting much larger wood with the longer bar.

 

[Modifiedmark]

You loose chain speed with a long bar its like changing gear ratio.

You only lose chain speed when the motor dose not have enough power to pull the longer bar. The gear ratio only changes when you increase or decrease the size of the drive spocket.

 

[timberwolf]

Let's say that you are cutting a log that is 10" in diameter. With 3/8 chain, there will be no more than 8 cutters in the wood at any time, regardless of the length of the bar. 16" or 36"; it makes no difference.

LOL, put a 32 inch bar on an 026 and see how well it limbs 4 inch branches. If it makes no difference it should be just as fast with 32 inch bar.

It takes HP to move the chain, easy 1/2 a horse just for the accelerations to make the chain go round at full speed. Then add friction, the longer the bar the more friction there is and to compound it the chain must be kept tighter on longer bars to keep the chain snug that adds more friction on top of that.

 

[romeo]

My brother couldn't explain it to the guy who had him put the 54" bar and CARBIDE chain on a wild thingy either.:hmm3grin2orange:

 

[scotclayshooter]

LOL, put a 32 inch bar on an 026 and see how well it limbs 4 inch branches. If it makes no difference it should be just as fast with 32 inch bar.

It takes HP to move the chain, easy 1/2 a horse just for the accelerations to make the chain go round at full speed. Then add friction, the longer the bar the more friction there is and to compound it the chain must be kept tighter on longer bars to keep the chain snug that adds more friction on top of that.

+1

 

[SawTroll]

LOL, put a 32 inch bar on an 026 and see how well it limbs 4 inch branches. If it makes no difference it should be just as fast with 32 inch bar.

It takes HP to move the chain, easy 1/2 a horse just for the accelerations to make the chain go round at full speed. Then add friction, the longer the bar the more friction there is and to compound it the chain must be kept tighter on longer bars to keep the chain snug that adds more friction on top of that.

:agree2:




- sorry Spike!

 

[TreePointer]

You only lose chain speed when the motor dose not have enough power to pull the longer bar. The gear ratio only changes when you increase or decrease the size of the drive spocket.

By definition, a simple ratio compares two things. If we consider the drive sprocket as one gear and the chain loop is another gear, then changing either one would change the ratio. Right?

 

[timberwolf]

Chain is not a gear, it moves in a straight line where it contacts the wood. if you look at number of revolutions the whole chain loop makes yes, a longer chain makes fewer full revolutions every min. How ever the liniar speed is the same. Well actually, it is slower due to increased friction. Prove it to yourself, tune a saw with a 16 inch bar and new chain measure the RPM make some cuts and time them, then with a 32 inch do the same (edit without adjusting the carb). The smaller the saw the bigger the difference will be, an 088 is going to be less effected as the friction/acceleration losses are a smaller part of total power. 1/2 a horse to an 088 is much less difference at the wood than with an 026.

 

[Modifiedmark]

By definition, a simple ratio compares two things. If we consider the drive sprocket as one gear and the chain loop is another gear, then changing either one would change the ratio. Right?

Wrong, the chain loop is not a gear, it is drivin by the gear. The drive sprocket is the only thing that determines the speed of the chain relitive to crankshaft rpm's


Sorry Timberwolf typed faster and is correct.

 

[bump_r]

By definition, a simple ratio compares two things. If we consider the drive sprocket as one gear and the chain loop is another gear, then changing either one would change the ratio. Right?

That would be akin to chaning the length of the road being travelled in your car to change ratios. I guess that explains why a drag car can make better times on a 1/8 mile track versus 1/4. Now we're getting somewhere.

 

[bigjake]

You only lose chain speed when the motor dose not have enough power to pull the longer bar. The gear ratio only changes when you increase or decrease the size of the drive spocket.

let think about this your saying if you have a 20 tooth chain or a 200 tooth
chain the speed is the same .the moore teeth the slower the speed.
i will put any amount of money you guys wish to wager.Mark a tooth an
a 16'' bar and count the number of times you turn the sprocket until it gets back to the point were you starte now do this with a 32'' bar. now we know
the saw operates at the same rpm with the 2 bars:monkey: :monkey:

 

[B_Turner]

LOL, put a 32 inch bar on an 026 and see how well it limbs 4 inch branches. If it makes no difference it should be just as fast with 32 inch bar.

It takes HP to move the chain, easy 1/2 a horse just for the accelerations to make the chain go round at full speed. Then add friction, the longer the bar the more friction there is and to compound it the chain must be kept tighter on longer bars to keep the chain snug that adds more friction on top of that.

+3 Plus there are some length thresholds based on saw size (power). My 5100 would not as happy cutting anything with a 42 inch bar as with an 18 incher, but my 880 can't tell 42 from 50 very much unless the 50 is buried. (Looking back on TW's first post, I'm repeating what he said.)

A seat of the pants test is to measure WOT with a short and then long bar on the same saw. Long bars, lower WOT rpm.

And don't forget depending on the angle of the cut vs the grain (chip shape and size) and the type of wood, there can be additional friction happening from cutting certain wood (e.g. very pitchy) that is increase with a longer bar. And a dull chain usually compounds the issue.

Another inefficiency of longer chains is the dolphins have more freedom to swim out of line.