Something's bent

Commando rear wheels can often be built/rebuilt with the wrong offset so it could be worth checking the rim/tyre is correctly aligned with the frame centre line but of course that depends on the swingarm etc. being straight to check.
 
concours said:
Hope is an emotion.


Have a skilled eye look at it.
Easy to see where problems are when a person knows what to look for.

Report back please.
Click to expand...
What would be the points to look at / for without dismantling? How much can I check without taking the thing apart? I don't mind taking it apart - I will at some point but having just bought it, it would be nice to have a fair idea of what's going on to make the back wheel & swinging arm sit at several degrees out. Is there some way of assessing the state of the Isolastics with the bike on the bench? Or is it just a matter of seeing how much the isolated section moves by hand in relation to the frame?
 
Tigernut said:
What would be the points to look at / for without dismantling? How much can I check without taking the thing apart? I don't mind taking it apart - I will at some point but having just bought it, it would be nice to have a fair idea of what's going on to make the back wheel & swinging arm sit at several degrees out. Is there some way of assessing the state of the Isolastics with the bike on the bench? Or is it just a matter of seeing how much the isolated section moves by hand in relation to the frame?
Click to expand...
Well, if I was there, I could answer that question.

With all respect, there is much to see.

If you know what to look at/for.

If you want, to open up the mind & preconceived notions, there is a lot to see.

I don't expect a layperson to have the skillset. But, a talented fabricator can see much.

Only if you want to.

Best regards🍻
 
Take your rear wheel off ,including rear sprocket/chain (the chain can stay on the sprocket ) , take off the two bottom rearshocks bolts , such your swingarm is now free of moving up and down , bike on main stand (of course !!) , then you can measure the distance between shock holes and shock top holes (or bolts as in that case they could stay on the frame ) , both sides should be equal , not a very accurate but if there is a big (what is big ???) difference .....
 
If you can get a piece of plate glass cut to fit on top of the swingarm without touching other other weld spatter and such, it should sit flat. Much like how some people check to see if the fork tubes are aligned.
 
I would also pull rear wheel but first set the bike up on the centre stand on a flat and level concrete or ashphalt area. Check the area for flat and level with a 2' or 4' hand level.

With the rear wheel removed place a short hand level on the cross tube at the front of the swing arm. I have a 6" long plastic level that geta used a lot for such things. These are cheap to buy at any hardware store.
If it the cross tube isn't level then shim under the appropriate centre stand foot until the bubble is in the centre.
Now measure the the distance from the swing arm bottom/ back to floor.
If both sides are the same +- 1/8", your swing arm is not twisted or bent.

Glen
 
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I had a new front chain guard mounting welded onto my 750 swing arm (The old one was worn through by the chain) and it noticeably bent the swingarm. The chap who did the work is an excellent welder and fabricator and he simply welded a bead on the opposite side of the mounting and it promptly went straight again.
 
With all the responses discussing what could possibly be causing the rear wheel to tilt, I took a close look at mine again out of concern, as it does list a couple of degrees to starboard. Most obvious is the wheel is parallel with the shocks, and the entire bike leans over, not just the wheel. I am satisfied that one or both of the cradle bolt holes for the center stand are oversized from wear. Check yours again Tigernut, maybe it’s as simple as that.
 
Can ypu use a front wheel chock to support bike on both wheels, vertically? This takes CS out of the issue. Then check wheel truness. You can also assess iso and swing arm bushing sloppiness by pushing rear wheel left or right, and top or bottom. Watch for movement at swing arm to z plates and for iso, movement at primary cover and z plates.​
 
franko said:
If you can get a piece of plate glass cut to fit on top of the swingarm without touching other other weld spatter and such, it should sit flat. Much like how some people check to see if the fork tubes are aligned.
Click to expand...
This seems a very good suggestion, thanks. Anything that I can rely on to be properly (if not 100% truly) flat, that will span the swinging arms and extend as far fore & aft as possible, will tell me whether it is twisted and if so, by about how much. I'll do this. I'm hoping it turns out to be more or less untwisted, and that the whole issue is due to isolastic rubbers being buggered. I was expecting to be replacing them anyway.
 
1973x75 said:
I have the same issue, 1973 750,while on the center stand. I took the swing arm off and it was straight. New isolastics didn’t make a difference. I followed it while my son rode it and everything looked good, fortunately, so I’m assuming it’s how the center stand is connected to the cradle. The bolt holes are slightly wallowed. I’m not too concerned, just living with it for now.
Click to expand...
When I rebuilt my '72 750 combat the swingarm was warped about 3/8-1/2 in. I am the second owner an I am pretty sure it was never down hard. I assumed after examination that the chain torque twisted the swingarm up on the primary side, twisting it. I have seen this twist on a lot of 750 swingarms so that is plausible to me. Don't know if this occurs on the 850 as I have never owned one but next time I build a bike and repaint the swingarm I will be inclined to add some reinforcement to stiffen it from twisting.
 
ilikevelos said:
When I rebuilt my '72 750 combat the swingarm was warped about 3/8-1/2 in. I am the second owner an I am pretty sure it was never down hard. I assumed after examination that the chain torque twisted the swingarm up on the primary side, twisting it. I have seen this twist on a lot of 750 swingarms so that is plausible to me. Don't know if this occurs on the 850 as I have never owned one but next time I build a bike and repaint the swingarm I will be inclined to add some reinforcement to stiffen it from twisting.
Click to expand...
If there is a bend to the swinging arm on mine, causing the tilt of the wheel, it's the other way round - the left side is down and the right side is up. But I'm beginning to think it's the Isolastics, not the swinging arm...
 
Tigernut said:
But I'm beginning to think it's the Isolastics, not the swinging arm...
Click to expand...



When checking the cradle note that it (therefore the engine, gearbox and front end of the swingarm) are offset approximately 1/8" - 3/16" to the left of the frame centre line. The head steady is also built with the same offset so the swinging arm isn't symmetrical as the axle pads at the rear must line up with the frame or the shocks would lean over.
 
L.A.B. said:
When checking the cradle note that it (therefore the engine, gearbox and front end of the swingarm) are offset approximately 1/8" - 3/16" to the left of the frame centre line. The head steady is also built with the same offset so the swinging arm isn't symmetrical as the axle pads at the rear must line up with the frame or the shocks would lean over.
Click to expand...
Perhaps it's the head steady that's the cause, are the rubbers goosed etc. It's surprising how far the rear wheel can tilt especially if the rear ISO clearances are too wide.
 
I would always take the swingarm out.
It is not so hard to straighten them, but make sure the bushings stay in line.
Whatever you do ( weld, bend..), always keep the bushings and the spindle in place.
There is more to it than swingarm and isos, but once lined out correctly, a Commando is a joy to ride.
I have hollow wheel spindles and front iso bolt, which makes it so much easier to check:


Something's bent
 
I've jacked the bike up under the Z plates and put axle stands uner the footrests. The rear wheel is now 1" or so off the ground and is nice and straight - all the tilt has vanished, so I don't think it was anything being bent (although I'll still check the swinging arm once I get the whole thing apart).

At least, now I've seen this (and also now that I know the slight 'hump' to the frame top tube is normal) I can feel that it's worth doing the work. If the frame had been badly bent I'd have felt like giving up!
 
Never say give up! It's blood in the water!
The sharks will start circling.
You will receive private messages of help that offer to take this hopeless project off your hands!
 
Ludwig wins the prize!! Another way is to use two 8-foot fluorescent light bulbs. If your fenders are removed, you can stand the bulbs straight up gently touching the rims or tires. Two friends can gently hold them there or, if you don't have two friends then use duct tape. Now step back and eyeball the bulbs to see if they are in the same plane. (Parallel with each other.) This method can also be used to adjust the tension in the head steady if you have an adjustable one.

Just remember that the bulbs being parallel (in a vertical plane) does not indicate wheel off set. You can, however, run one bulb under the bike and see if your tires or rims are parallel when you hold the steering straight. When the front rim is parallel to the rear rim the distance from the straight edge to the front rim or tire should be half the width difference between your front and rear tire/rim.
 

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