Speed wobble from rear wheel

[TundraOG]

Hey guys.
I've had this issue for a few months now, it just sort-of "appeared", when exactly I can't recall, I only have the bike for about 4 and a half monts now.
When riding, especially on the highway, I feel the rear wheel wobbling about every few seconds. At first I thought it had something to do with my bad steering bearings so I've had them replaced (and the handlebars feel smoother), then I thought it had to be my worn tires so I swapped them for a set of Metzler Lasertechs and they do grip better but the wobble stayed.
I thought maybe I'm just paranoid and this is just how GS500s are, UNTIL I rode a friend's GS (she wanted me to check what's wrong with her engine). Her bike, despite being a year older than mine (01 vs 02) felt MUCH stiffer and more stable, I went into roundabouts at almost twice the speed I would on my bike and felt no hasitation while doing so.
I also let her take my bike out on a spin and she said it feels "shakey".
I took the bike to my mechanic. He rode her on the highway and said he understands what I mean, but everything he checked is perfectly set-up (chain tension link-by-link, sprockets, wheel alighnment, wheel bearings, checked if the tire is mounted correctly and the rim isn't bent, the rear shock is in good condition, etc).

My bike has a different problem as well - it has a small air leak in the intake manifolds between the carbs and engine block, wich is noticable by a sometimes slightly wobbly idle (100 rpms + or -, not constant) and power "pulses" in very slow speeds (up to 10-13 kp/h). I thought this was affecting my ride at higher speeds but when I pull the clutch at about 80 kp/h or more it feels pretty much the same, so it can't be that.

So far I've been riding with 30 PSI in the rear wheel to make the ride less bumpy, but I need a solution and I'd love some help...

[Bluesmudge]

I was thinking wheel bearing, but you said the mechanic ruled that out.

How is the front and rear suspension? I know my bike has felt wobbily when the rear shock was blown out or when the front forks had leaked most of their oil. Could be time for a fresh rear shock.

[TundraOG]

I was thinking wheel bearing, but you said the mechanic ruled that out.

How is the front and rear suspension? I know my bike has felt wobbily when the rear shock was blown out or when the front forks had leaked most of their oil. Could be time for a fresh rear shock.

Well, the bike has about 72K KM on her. The front fork fluid was checked a month ago and was in good condition, and yesterday he checked the rear shock and said it was in good condition too.
Will adjusting the rear suspension do anything?

[prmas]

It sounds like you might have loose swingarm bearings.
Loosen off the chain and see if there is any sideways freeplay in the swingarm.
If you do not loosen the chain first it will take up the slack in the bearings and you will not find it.

Macka

[mr72]

My bike has a different problem as well - it has a small air leak in the intake manifolds between the carbs and engine block

Once you get the wobble sorted out, this is an easy fix. You just need to replace the o-rings that are between the intake boots and the cylinder head. I bought a pair at the local Suzuki shop for about $5 total. Hardest part of the job is getting the airbox out. You don't have to remove the carbs to do this, just disconnect them from the intake boots and let them flop loose while still connected to the cables.

As to your other issue, I have no idea, but I was also going to suggest the swingarm bearing.

[TundraOG]

I checked the bearings myself and found no irregularities whatsoever, both the wheel and swingarm bearings seem to be rock-solid. I will have my mechanic inspect the swingarm bearings just in case.

And by the way, when I was riding yesterday for 200 KMs on the highway I noticed a large increase in stability and an overall smooth ride, even when cornering. The difference between this ride and my previous ones? Having my little sister on the back, as well as my rarely-used topbox.
My sister is fiarly skinny but her 50 killos were enough to make a BIG difference.

What to make of this? Should I stiffend the suspension?

[prmas]

I checked the bearings myself and found no irregularities whatsoever, both the wheel and swingarm bearings seem to be rock-solid. I will have my mechanic inspect the swingarm bearings just in case.

And by the way, when I was riding yesterday for 200 KMs on the highway I noticed a large increase in stability and an overall smooth ride, even when cornering. The difference between this ride and my previous ones? Having my little sister on the back, as well as my rarely-used topbox.
My sister is fiarly skinny but her 50 killos were enough to make a BIG difference.

What to make of this? Should I stiffend the suspension?

I always have my rear preload at maximum as I prefer the ways it feels like that.
I have also put heavier oil in the struts (15w).
It is not that I am heavy (85kg) or carry a heavy load (rarely use the top box), I just prefer the "tighter" feel.

On my Honda, which is a much heavier bike, I have the rear preload at minimum when riding solo or even with a light to moderate load in the panniers. When fully laden with camping gear/food/water etc I still only have it at about 60-75% as otherwise it is too harsh in the rear. As for the front, I have done the opposite and added small spacers to the top of the springs to add a little preload.

Perhaps you just need a "stiffer rear end", pardon the pun ! 

Macka

[Watcher]

You said you only feel it every few seconds?  If it was a hop in the wheel or a seized link in the drive chain it would be near constant at highway speeds.  And I think a bearing would be repeatable by input, so rolling on/off throttle or specifically leaning left or something would create the sensation.
But you ruled pretty much everything mechanical out...


Next time you are riding highway see if you're passing anything on the road surface itself that may be the cause.
If the rear end is set up very stiff you could be feeling irregularities in the road itself.
Portions of the highway I used to commute would feel "hoppy" and would give me a bump every other second or so.

Adding the weight on the bike effectively reduced the preload, same with lowering the tire pressure (which I would NOT advise doing).  I'd try running the suspension down a notch and see how it feels.

Does the manual suggest a preload based on weight?  Anyone?
I know my Buell manual has a whole chart and several methods for setting up the adjustable suspension...

[Bluesmudge]

I believe correct preload is 1.5". When you sit on the bike, it should sag 1.5" from a reference point.

[mr72]

Given you said that it didn't happen when riding 2-up I think it's clear that you are topping out the shock over certain road surfaces and this is causing your "wobbly" feeling. Too much "preload" is likely the cause.

BTW, it may be better to think of the preload as a "ride height" adjustment. If you adjust it too high, then you can top out the suspension. Too low and you bottom. It's really just tuning it so the suspension is working with the load you are carrying so that it doesn't top out or bottom out. FWIW it doesn't make the suspension stiffer at all, that would require a different spring. The shock probably damps poorly at the ends of its travel so that may be contributing also if you are topping it out. You want to keep the shock in the middle of its travel.

Anyway, maybe try turning it down some and see if the issue goes away. Wouldn't that be nice?

[TundraOG]

Given you said that it didn't happen when riding 2-up I think it's clear that you are topping out the shock over certain road surfaces and this is causing your "wobbly" feeling. Too much "preload" is likely the cause.

BTW, it may be better to think of the preload as a "ride height" adjustment. If you adjust it too high, then you can top out the suspension. Too low and you bottom. It's really just tuning it so the suspension is working with the load you are carrying so that it doesn't top out or bottom out. FWIW it doesn't make the suspension stiffer at all, that would require a different spring. The shock probably damps poorly at the ends of its travel so that may be contributing also if you are topping it out. You want to keep the shock in the middle of its travel.

Anyway, maybe try turning it down some and see if the issue goes away. Wouldn't that be nice?

Thanks for the advice!
When you say "turning it down" you mean putting it on a lower (1-3) or higher (5-7) setting? mine is currently setup as a standard 4. At 1.88 m high and 86 kg weight (about 6"3 and 190 pounds), what's the recommended preload?

[Watcher]

That actually sounds about right.  I'm a little heavier at somewhere between 200-220 and I think I ran my GSs on the higher side.
I never set mine up scientifically, just went by feel.

But being that its adjustable you should feel free to play with it.  If it's on 4 bump it down to 3 and see if the sensation goes away.