Topic: Weight and Wobble

[Pinhead]

I'm driving my '79 CM400 around until I finish rebuilding my CB650. I've found that the 400 will wobble above 65mph unless I have a decent amount of weight on the rear-end of the bike. If I have my laptop bag (about 10lbs) strapped to the back I can go about 80 mph with no problem. Riding two-up makes the bike substantially more stable, believe it or not, and can stably top-out.

This bike had a much worse wobble until I added a rear grab-bar which stiffened the rear end of the bike's frame.

It's got rearsets, clubman bars, and CB650 rear shocks which are about two inches longer than stock.

Besides adding weight to the rear (which is counter-productive to accel performance) what can I do to fix this issue?

[scottly]

It's got rearsets, clubman bars, and CB650 rear shocks which are about two inches longer than stock.

Besides adding weight to the rear (which is counter-productive to accel performance) what can I do to fix this issue?

My first guess is your rear shocks are too long, jacking up the back of the bike: more weight on the rear helps compress the shocks, restoring the rake of the front forks. You might try setting the spring pre-load to it's lightest setting when riding solo.

[Pinhead]

I was kind of thinking the same thing, but my 400 as-is has more rake and trail than my 650 does (and the 650 doesn't wobble). I'll try reducing the preload but IIRC it's already on the softest setting.

[scottly]

Don't forget to check all the standard stuff: loose steering head bearings, loose swing arm bushings, loose wheel bearings, wheel alignment.

[Pinhead]

Let's talk wheel alignment. Other than the "notches" in the swingarm when doing a chain adjustment, how else do I make sure my rear wheel is in-line with the frame?

[scottly]

Put a  couple of long straight-edges alongside both sides of the rear tire, and the front wheel should be centered between them, and parallel. Since it's a '79, does it have the nylon swing-arm bushings? (The WORST thing Honda ever did to the CBX!!!).

[Pinhead]

Not sure about the nylon swingarm bushings; I'll have to take a look. I do know, though, that I can't get the wheel to move left or right with the bike on the centerstand. Everything seems good and tight.

[scottly]

That's a good sign. I back to thinking it's the overly long shocks.

[Alan F.]

(I've got an 81 CM400C patiently waiting its turn to become a project, I have a picture of the rolling chassis on the desktop of my laptop so I can keep it in mind and figure out just what I'll be doing with it when the time comes.)

Anyway, I'm not 100% on the way the rear section of the frame bolts onto the backbone section.
I wonder if running a bead of weld on both sides of that connection would do much?

Seems to me that any flex at all in those areas would affect everything.

[Pinhead]

I don't know 100% about the top section flexing, but I do know that your lower red circle can use some stiffening; when I added aluminum plates for my rearsets the bike became much more stable (relatively).

[Alan F.]

I remember that from one of your earlier posts, I'll be doing something like that on mine for sure.