Author Topic: Quick swing arm centering question (Read 1987 times)

wicks · « **on:** March 27, 2020, 04:10:31 PM »

I'm installing my swing arm after putting in a new u-joint and boot (and fresh spline slime and fresh greased bearings, new seals, trans output seal, fresh clutch rod seals, etc), and I'm doing the technique I've read about here (thanks!), but an option is occurring to me. The swing arm/drive shaft should be centered on the trans output, not on the frame. Those two, in an ideal world, are the same. But...

So when I have the swing arm at rest and then gradually tighten the pins evenly on both sides, increasing to a light, even torque, they are almost at identical depth. So I think it's very close to perfect, but it occurred to me that an enhanced way to do this might be to install the rear wheel and then have a friend (with a mask on) spin the rear wheel constantly as the pins are evenly torqued on both sides. This way the axle is in rotation and so it more likely to be exactly on it's center, making the swingarm get into an ideal position for running with the joint and axle straight.

The swing arm should lifted a bit so the tire is off the ground but the swing arm is at running angle (no springs installed) while doing above.

Does it make sense? What am I missing...

Muzz · « **Reply #1 on:** March 27, 2020, 04:41:23 PM »

The bike?

mtiberio · « **Reply #2 on:** March 27, 2020, 04:42:18 PM »

You are over thinking this. Guzzis are rare in the motorcycle driveshaft world in that they have a "dual" u-joint or a homo-kinetic u-joint or to put it yet another way, they have a constant velocity u-joint. Back in the day (it might be different now) there was only one yamaha that had this besides guzzi. What this means is that the u-joint will tolerate some side to side offset. Now you do want to minimize this and measuring the large set screws is only a first order solution. There may be better ways to do this, I don't know one. When I raced, I had a 4" rim and I used to run 140 tires on early narrow swingarms. I had to offset the rear wheel .4" or 10mm to the left for the tire to clear my notched swingarm. They only way I got any sembelence of handling back was to run the swingarm offset to the right as much as the set screws allowed. I never had a u-joint failure on my race bike. I also used to run tall rear shocks and would have to grind on the u-joint body to clear the u-joint chamber at full shock extension.

LesP · « **Reply #3 on:** March 27, 2020, 04:54:43 PM »

Machine checker.
Set swing arm level and adjust pins until the coupling slides smoothly on output spline indicating on centre/center.

Check swing arm bearing pin height with Mitutoyo depth micrometer.
Scratch head when two heights turn out to be with thousands of each other.
Conclusion, not all you read on the internet is BS.

wicks · « **Reply #4 on:** March 27, 2020, 05:34:01 PM »

ooooh that's a cool tool!

It's my 77 LM1

Antietam Classic Cycle · « **Reply #5 on:** March 27, 2020, 05:40:37 PM »

Quote from: mtiberio on March 27, 2020, 04:42:18 PM

You are over thinking this.

delrod · « **Reply #6 on:** March 27, 2020, 06:58:38 PM »

Just for the sake of conversation. In my experience in another power transmission application. Many diesel firepump applications use a short conventional u joint driveshaft. Most of the setup charts avoid or caution against dead true alignment. Not always but most setup check lists in this application. I'm not an engineer but just a dumb sprinkler fitter. None of my setups have experienced driveline failures. My vote goes for close is good enough YMMV

wicks · « **Reply #7 on:** March 27, 2020, 09:49:08 PM »

HAH thanks all. I tried both methods and the position was the same. So she's all bolted back together now and ready to ride.

SmithSwede · « **Reply #8 on:** March 27, 2020, 09:55:14 PM »

Quote from: delrod on March 27, 2020, 06:58:38 PM

Just for the sake of conversation. In my experience in another power transmission application. Many diesel firepump applications use a short conventional u joint driveshaft. Most of the setup charts avoid or caution against dead true alignment. Not always but most setup check lists in this application. I'm not an engineer but just a dumb sprinkler fitter. None of my setups have experienced driveline failures. My vote goes for close is good enough YMMV

Hmmm. That is very interesting. Anyone have an explanation for why such charts would warn against a dead true alignment?

Muzz · « **Reply #9 on:** March 27, 2020, 10:53:44 PM »

Quote from: SmithSwede on March 27, 2020, 09:55:14 PM

Hmmm. That is very interesting. Anyone have an explanation for why such charts would warn against a dead true alignment?

Possibility of harmonics of some description?

LesP · « **Reply #10 on:** March 27, 2020, 11:35:26 PM »

If not mistaken they are talking about drive shafts (same as Automotive) with universals not a compact unit as per a Moto Guzzi.

wirespokes · « **Reply #11 on:** March 27, 2020, 11:53:13 PM »

The dead alignment has to do with the U-Joint being directly in line with its driven shaft. The U-Joint needs to 'work' - input and output should be at angles to each other.

nc43bsa · « **Reply #12 on:** March 28, 2020, 12:03:33 AM »

Quote from: mtiberio on March 27, 2020, 04:42:18 PM

You are over thinking this.

No!!

No one ever does that on this beeb.

LesP · « **Reply #13 on:** March 28, 2020, 12:22:40 AM »

Quote from: wirespokes on March 27, 2020, 11:53:13 PM

The dead alignment has to do with the U-Joint being directly in line with its driven shaft. The U-Joint needs to 'work' - input and output should be at angles to each other.

There will be folk running to the shed to wind one pin in another 2 turns, the other 2 turns out.

wirespokes · « **Reply #14 on:** March 28, 2020, 10:26:46 AM »

With airheads (lots of experience there) I've always used an allen wrench to judge spacing between frame and swingarm. Get about the same wiggle one side to the other = good to go. Never even thought about stresses side to side on the shaft. Never a problem I can recall - never a U-Joint issue.

But now, next time, I'll have attention on it and take a look. Like others have said, it's not an issue so I'm definitely not concerned.

wicks · « **Reply #15 on:** March 28, 2020, 12:48:02 PM »

Quote from: LesP on March 28, 2020, 12:22:40 AM

There will be folk running to the shed to wind one pin in another 2 turns, the other 2 turns out.

This *could* be a method to temporarily "fix" a worn u-joint or loose carrier bearing! Hmmmmmmm haha

Denis · « **Reply #16 on:** March 29, 2020, 10:04:37 AM »

Quote from: nc43bsa on March 28, 2020, 12:03:33 AM

No!!

No one ever does that on this beeb.

Haha!

Denis · « **Reply #17 on:** March 29, 2020, 10:47:43 AM »

When I align mine I thread in the pins until they bottom out and eyeball them to get them as even as possible.
Then I slowly back them out until the swing arm moves freely up and down. Then I adjust the pins left to right in small increments until they are even.

It's not difficult.

pehayes · « **Reply #18 on:** March 29, 2020, 04:21:53 PM »

Many years ago in my early days with Guzzi I restored a crashed 73 Eldorado. Someone advised me to lace up a 5.10 X 16 rim to the rear hub. Bigger tire = more weight carry capacity. Doing so, the fatter tire hits the driveshaft tube. So, you have to adjust the spoke lacing so the hub is substantially "out of dish" in order to make the tire fit the swingarm. Requires cutting and modifying the drum rear brake anchor rod. Also requires the swingarm pivot pins to be substantially mis-matched (by about a cm! differential) in order to get the center of the rear tire to track the center of the front tire. I drove it 100,000 miles that way before I converted back to a standard rear wheel/tire and equally centered swingarm pins. The odd adjustments never seemed to bother the u-joint. BTW, handled a LOT better with the stock, narrower rear wheel/tire.

Patrick Hayes
Fremont CA

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Author Topic: Quick swing arm centering question (Read 1987 times)

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