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Restless
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1,672 Posts
Discussion Starter #1
Can anyone explain the theory behind this setup? As I was going back together with my 3.37 gearing and after torquing the comp nut to spec and the clutch hub nut I went to rock the assembly back and forth in gear with the rear tire. What I'm wondering is why does the sliding cam slip back and forth against the sprocket? Why isn't the sprocket just splined onto the shaft extension then secured with the comp nut? What purpose does the sliding cam serve against the sprocket? And why, under torque can you see it slip and move?

I did a search on this and couldn't find any info.

Thanks for any insight.
 

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XLIII
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10,078 Posts
I'm just an idiot like you, but I think it's to "cushion" the load when you give it the gas, so the shock is spread out instead of hitting the driveline all at once.
My "idear" is, of course, worth exactly what you paid for it... :D
 

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>>> The Curmudgeon <<<
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352 Posts
MegaGlide said:
... I think it's to "cushion" the load when you give it the gas, so the shock is spread out instead of hitting the driveline all at once.
...
Close enough for a cigar.

When a cylinder fires, there is a sudden twisting "thrust" on the crank. On engines with several cylinders, these thrusts are close together, and tend to be less distinct. Those engines with fewer cylinders tend to have much more distinct thrusts, with longer intervals between.

With each firing impulse, the crank rotates forward, and with each interval between firings, the crank "coasts". This rotate-then-coast cycle is felt as vibration, and in addition, contributes to stress on the drivetrain. You could say then, that multi engines have a higher frequency of vibration, and twins and singles have a lower frequency. Lower frequencies are more noticeable, and probably more stressful.

With the compensator, a portion of the energy in each forward rotation is stored in the compensator spring, and with each following coasting interval, the majority of the energy stored in the spring is returned to the crank. (The remainder would be converted to heat from friction).This tends to smooth out the cycle.

Many motorcycles use a rubber cushion in the rear hub to do much the same thing. This is probably effective on a multi, but less so on a v-twin or single.

The entire compensator can be removed and replaced with a solid pulley, at the expense of somewhat more vibration and stress on the drivetrain.
 

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Restless
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1,672 Posts
Discussion Starter #4
It's just weird watching the two parts move against each other. Now I understand where that nasty knocking noise is coming from some people hear when their compensator nut works loose. I guess it's the sliding cam and the compensating sprocket clacking. After awhile you'd think the points on the sliding cam would wear. But I guess the primary oil prevents alot of that.

I just find it interesting. I've been an ambulance mechanic for 14 years. And have really only moderately wrenched on bikes for the last few years. So when I run across something like the compensating sprocket setup I've just got to ask a couple "why" questions. It's definitely something you don't see on a Powerstroke.;)

Thanks.
 

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Highly Seasoned Rider!
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4,884 Posts
Compensating or "cushioning" sprockets have been around on motorcycles since the 1920's, parrticularly on British bikes. It prevents chain snatch at low speed and eases strain on the lower end. It's a really good idea.



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Premium Member
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Here is a good comparison for you to think about:

Put your television on the floor and move it from one end of the room to the other by pushing at it with the bristles of a broom. Then try moving it back again but this time by hitting it on one side with a ball peen hammer.
Make sure you try the broom method first or you will be sweeping smashed parts back toward a dust pan.
 

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Registered
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6,413 Posts
Timbo,

I've got a friend who has a 75 FLH (shovel). He's cheap so when the spring inside the cup busted on his comp sprocket setup, he just opted to install the setup without the compensator stuff. Straight sprocket on the splines. Not a fun bike to ride for me as lots of vibration. Course he rides like an ol woman so it doesn't bother him.
 

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Highly Seasoned Rider!
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Ed Y said:
Timbo,

I've got a friend who has a 75 FLH (shovel). He's cheap so when the spring inside the cup busted on his comp sprocket setup, he just opted to install the setup without the compensator stuff. Straight sprocket on the splines. Not a fun bike to ride for me as lots of vibration. Course he rides like an ol woman so it doesn't bother him.
The early bikes were like that, no compensator sprocket. They would often come loose, even back in the panhead era. I had to have my panhead bike towed in once when the engine sprocket came loose and let the key fall out and get lost. HD added the cushioning sprocket to smooth things out and cut vibration.
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