[shinyribs]

The bike is a '80 CB750 F.

Stock rear suspension raised about 1.5''

Forks are off a '91 CBR1000 F.

Triples are '92 CBR600 F-2

Wheels are '87 CBR600 F-1 Hurricane (17'')

In the twisties,in town or just cruising anywhere under 60 mph it's fine(mostly,more on that later). In fact,I think it handles fantastic,very fast turning bike,but it doesn't seem 100% stable.It's a tad twitchy.

On the highway at 65 mph + it feels a little nervous. The bike doesn't wander,but the least bit of cross breeze or a wind gust from a passing semi and the bike ''wobbles'' .

The wobble I'm feeling is not a headshake or any sort of turning of the bars. It's like you are literally tilting the bike side-to-side. It's not severe,but very noticeable.

Yesterday I had been on the highway for a fairly long amount of time and decided ( for some odd reason) to stretch out my legs by hanging them on the passenger pegs. As soon as I did this the ""wobble'' started. Instantly. If I put my feet back on the drivers pegs it stops instantly. The longer I keep my feet on the rear pegs the wobble gets stronger and stronger. After about 10 seconds it gets to the point where it feels pretty severe. Scary,in fact.

Also,occasionally, while cornering hard to the right ( only to the right & at any speed) the bike has the same wobbling feeling. Again,it's not severe at this time,but it's definitely not stable. The bike just falls in to a turn great,lay right over,but it seems like it wants to try and stand back up while turning. Hard to explain really. It's lays right over like it wants to turn,but it's hard to make a smooth,100% stable turn. Just feels unsteady. Again,this is ONLY while turning to the right and leaning pretty hard. Normal riding it does just fine.And when I say "leaning hard" I am running on the middle lean marker on Diablo Rosso II's.

Things I've checked.

-Steering stem. Seems tight. I have not disassembled anything. Just put the bike on the center stand and jacked the front tire off the ground via the frame tubes. Turns smooth with no catchy spots. No apparent slack.

-Fork straightness. Eyeballing them they look good(not sure how to get hard measurements on this). Everything tightened like it should be.

-Swingarm. Pivot seems tight with no apparent slack. Again, I haven't done any disassembly. Just pried around on it with the bike on the center stand.

-Wheel bearings. I felt them all when putting on new tires -300 miles ago. All seemed well. Tight with no play. Nice and smooth,no catchy spots.

-Rims. Checked them for any wobbling/out of round when putting the tires on. They turn true.

What should I be looking at now? First thing when I get in tomorrow I'm going to pull strings to verify that the rims are centered and inline.I'm not 100% sure that everything is spaced correctly since I didn't build this bike(shame on me...) but the rest of the build quality on this bike is very nice and clean. After that I'm gonna need some help. Thanks for any input and advice!

 

[RatFighter]

I would use a pretty much static point on the bike that is totally symmetrical (axle centers are a good place to use, as well as the pivot bolt ends) and see if your rear wheel is perfectly aligned. Use a ratchet strap to pull slight tension on either side for the measurements too, (measure both sides, left and right, pulling one way, and both sides the other way) if it is out of alignment you will find it that way, even if it is only a few millimeters it will show up with proper measurements.





Or yet... you gotta pull that swingarm and replace the bushings and bearings. I bet all my dollars that in the end of this the mystery wobble is related to worn pivot bushings/bearings allowing the swingarm to get a shimmy going and the oscillation just gets worse and worse. the more load bearing the rear becomes the worse it is.


Sounds like you are finding out why there is a term for bikes like this:


"Handles like it has a hinge in the middle."




It appears BikeBandit has the bearings, but the thrust bushings are a question mark to me, I would think getting some new ones spun up on a lathe to fit your application (if they are really that bad at all, I bet it is just bearings) you could overhaul the whole thing. The center sleeve in the swingarm is harder than anything I have ever seen, they don't wear out, even if it has some slight pitting I wouldn't worry about it for even a second. It'll go longer than the earth itself with the treatment they got from the factory.

 

[94yzf75fighter]

I was going to say check the rear wheel alignment and also when you put new rubber on the rims did you balance the tires?

 

[Azharok]

Yeah, I was thinking simple as wheel balancing.

Cauz my bike starts wobbling at a 100+ and it all factory and trusted shop work.

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[shinyribs]

Dang, Rat. "Handles like its hinged in the middle ". You hit the nail on the head there! That's actually a great way to describe it.

And yes, we did balance the tires. They took very little weight. We were actually trimming weights to make them small enough. Thanks for the idea still.

I'll look at the swing arm again. Probably be best to remove the shocks and all the weight off of it to do it correctly.

Rat, I got to thinking after reading your reply and thought about something I've felt a few times on this bike. Feels like slack, or something "catching up " when I take off. Only did it a few times and I thought it was just me not being used to the clutch. Its pretty grabby. I betcha its slop in the pivot area, like you mentioned. I always finish a stop on the rear brake only. That probably pulled the swing arm backwards and then it banged forward when under power again. Seems to make sense at 4 am at least .

Thanks for the quick input guys. I'll check out that swinger tomorrow.









Damn...looks like I gotta park my "new " bike.

 

[shinyribs]

I still havent looked at the swinger on this bike. Instead I took it camping,loaded with luggage on a 800 mile trip on the Parkway. :nuts:

With the weight of the luggage on the back it actually did much,much,much better in the wobble department. Before,with my feet on the rear pegs,I'm thinking it's causing me to lean forward more and put weight on the front vs the rear. These two things have me thinking that maybe the geometry is off on the front end. Like a rake and trail issue.

I need to find some specs on the original CB trees vs the CBR trees that are on here now. I'm thinking the current CBR trees are running much less offset in the trees than the stockers. Less triple offset+stock rake=less trail,right? And a low trail number=twitchy steering AFAIK.

I still plan to inspect the swinger. Especially now that I've put some miles on the bike and can see that the chain run is off a tad. It's not bad,but the chain is running pretty tight against the inner edge of the rear sprocket.

 

[rstar45]

Less triple offset+stock rake=less trail,right? And a low trail number=twitchy steering AFAIK.

I still plan to inspect the swinger.

I'm thinking a bit of both. With the rear raised + triple offset... Probably twitchy, but in both left and right equally.

So axle aligned with swinger pivot giving the one sided difference?

Or could the frame be tweaked? With the rear wheel held plumb, the front and rear wheels aligned, is the front also plumb?

Edit: Could it be dog tracking? (a wheel off center )


Anxious to hear what you find.
(My kid ran my xs650 into a rock wall this spring. I'm going to be checking all this stuff soon.)





.

 

[shinyribs]

Wow. Is your kid alright?


I didn't build this bike so I'm just gonna tear it down and go through it all. Its a Hurricane wheel on the back with a spacer to align the sprockets. Either that spacer is too thick or the rim just isn't centered. Hope to find out soon.

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[rstar45]

Wow. Is your kid alright?

Yes he is fine. Thanks for asking. Severe black eye, couple stitches in the chin, concussion and bruises.

Helmets save lives...

He doesn't remember a thing for at least 2 hours. Lucky kid.





.

 

[jmartin1110]

I think it might be a combo of bad bearings and the different offset of the CBR trees. It could also have something to do with the 1.5" of extra lift. I have heard of those little lift blocks making these bike handle funny but I wouldn't think it would cause the wobble, just the twitchy steering. When gearing down while coming to a stop, mine will twitch to the left but I know for a fact that I have a little play in my swingarm pivot that I have to shim up.

Keep us posted on what you find

 

[shinyribs]

The rear is lifted to level the bike out. Supposedly ....it should be sitting "stock " now. But I don't trust it and wont know for myself until I measure it. Maybe ill find time this evening.

Thanks for all the ideas. I'm connection check into all of them. Keep em coming guys. Much appreciated!

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[shinyribs]

Wow. I've been slow on this!

I just got done checking out the swing arm. I peeled the rim and shocks and all outta the way and gave it a good looking at. It's nice and tight, so no trouble there.

After the paint dries I'll pop the rear back together and check out the front end.

 

[Beef supreme]

If the bike is lifted that could definitely cause your issues, lifting/lowering a bike affects the flick-ability

 

[shinyribs]

I totally agree. But this bike is a little different. As in the angle of the neck in relationship to plumb is still stock. It has been raised equally on both ends. That's not to say that swing arm geometry isn't fubar'd since its all jacked up now.


But! I think I found the gremlin. The rear tire is tracking 11.5 mm to the right of the front tire.

Right. No typo. Not 1.5 mm ...11.5 mm! DAMN!

Time to fire up the lathe....

 

[jmartin1110]

What was the guy looking at when he centered that rear wheel?!?! 11mm is very obvious to the eye...I would say that could be a major cause of your problem.

 

[gsf1200R]

lol

 

[shinyribs]

LOL for sure. :nuts: Its all good now. The front rim checked out centered on the forks so I adjusted it all at the rear.

PO had this sprocket spacer which is now unneeded. Reduced rotating mass :thumbsup:

I tell ya what, if you didn't build it you'd better give it a good looking at. Strings don't lie.

 

[shinyribs]

What was the guy looking at when he centered that rear wheel?!?! 11mm is very obvious to the eye...I would say that could be a major cause of your problem.

Dude, I'm ashamed that I didn't spot it. After I saw the measurements I eyeballed the rear and was like..."....oh my...." :doh:

As good as it handled before I cant wait to see what's up now.

 

[jed.only]

:doh::doh::doh:

 

[jmartin1110]

Dude, I'm ashamed that I didn't spot it. After I saw the measurements I eyeballed the rear and was like..."....oh my...." :doh:

As good as it handled before I cant wait to see what's up now.

Its not your fault man...the guy that made the swap should have caught it.