CBF125 - The Wobble

[Zen Dog]

Denizens of the Workshop, your wisdom please. I bought a 2011 CBF125 off my mate's partner about a year ago, just for a bit of fun. She'd dropped it and lost confidence, and I wanted something I could absolutely thrash while staying vaguely within legal speeds.

It's been great (I mean it's awful, but I enjoy it and it's simple and reliable), but when I got it the handlebars weren't quite straight. It was only slight, and I assumed that the aforementioned drop was responsible and had slightly bent the bars, and didn't pay too much attention. A couple of months later it went over in the wind and properly bent the bars. So I bought replacement bars, but after fitting... the handlebars still weren't straight. So it was time to employ the time honoured technique of slightly loosening all the bolts in the front end, and holding the bike straight while repeatedly bonking it into a wall. This had the desired effect and the handlebars are now straight. I thought all was good until during the celebration/final testing phase, I took my hands off the bars, and they started behaving just like this - https://www.youtube.com/watch?v=O88djeVP4Bk

Clearly something isn't right. The bike feels fine to ride, just the same as before, although with perhaps a tiny feel of twitchiness as you pass over the fully upright position when switching lean from one side to the other. As long as you keep your hands on the bars, this weird exponential tankslapper doesn't occur, and it's easy enough to correct when it does. But I'm probably going to be selling this bike soon, and I don't want to sell it to some poor learner with some kind of inherent stability issue. So, so far I've checked -
Tyre pressures.
Rear wheel alignment. I measured this from the axle to the end of the swingarm, and if it's out, it's no more than a millimeter.
Head bearings. Feels fine on the ground, and smooth as silk with the front end in the air. I can't feel any notchiness at all.

Things I haven't done yet -
Get the wheels balanced, particularly the front.
Check the bolts again. I did all the bolts up to the specificed torque settings again when I did the wall bonking, but I should probably check them all (caliper/mudguard/axle/triple tree fork pinch bolts) again.
Check the overall straightness of the bike. I've never had a technique for doing this that I had any real faith in. Just getting the handlebars perfectly straight (and keeping them there) at a standstill is a hassle (though I know a few ways to do this), but accurately measuring "parallelness", particularly when front/rear tyres have different widths, with bits of wood and some string, just never seems satisfactory. I have just thought of a way involving a laser level on the front tyrewall though...

Any other suggestions would be very welcome.
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Current - '94 VFR750FR (Dead), '00 VFR800FI, 2011 CBF125 Previous - '10 Street Triple R, '92 MZ ETZ301, '05 TTR250, NSR125R, KMX125, "Honda" Win (chinese copy of an old Honda design with a C90 engine)
My bike trip around S.E. Asia 2010/2011

[stinkwheel]

That kind of thing usually originates in the back end somewhere.

Been through and checked all the bolts are tight? Tyres properly seated on the bead? Bushes in the rear suspension got any play in them? Wheel and sprocket carrier bearings in good order? (if you haven't had the wheel out, you haven't checked)
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[blurredman]

Don't take your hands off the bars? Some of my bikes do this fiercely.
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CBT: 12/06/10, Theory: 22/09/10, Module 1: 09/11/10, Module 2: 19/01/11
Past: 1991 Honda CG125BR-J, 1992 (1980) Honda XL125S, 1996 Kawasaki GPZ500S, 1979 MZ TS150.
Current: 1973 MZ ES250/2 - 18k, 1979 Suzuki TS185ER - 10k, 1981 Honda CX500B - 91k, 1987 MZ ETZ250 (295cc) - 39k, 1989 MZ ETZ251 - 50k.

[that_impulse_guy]

the banging against the wall technique I used on my 50cc loads of times. I'm older now and realise there is actually a step2 to this technique.

I know a lot of folks have opinions about this guy, but I'm just putting this video here to save me typing alot of words.
https://www.youtube.com/watch?v=pNEmOqY02EU
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Gone: Yamaha DT50lc, Suzuki DR500, Suzuki A100, Kawasaki z250ltd, RD350YPVS, Suzuki DR Big, Kawasaki AR125, Kawasaki KMX200, Suzuki GS1000S, Katana 1100, GS550M, Suzuki RGV250
Now: Suzuki GSX400X, Suzuki RF900R, NS400R

[rpsmith79]

[Zen Dog]

[blurredman]

[Zen Dog]

[blurredman]

Can I have it?
____________________
CBT: 12/06/10, Theory: 22/09/10, Module 1: 09/11/10, Module 2: 19/01/11
Past: 1991 Honda CG125BR-J, 1992 (1980) Honda XL125S, 1996 Kawasaki GPZ500S, 1979 MZ TS150.
Current: 1973 MZ ES250/2 - 18k, 1979 Suzuki TS185ER - 10k, 1981 Honda CX500B - 91k, 1987 MZ ETZ250 (295cc) - 39k, 1989 MZ ETZ251 - 50k.

[Zen Dog]

I had a look at the rear end generally yesterday. For a start, I gave it all a damn good wiggle, and I can't detect any play in anything, horizontal or vertical. Rear suspension feels smooth with minimal damping (but I suspect thats normal). So I had the rear wheel out.
https://i.imgur.com/cNMHCxjl.jpeg

Bearings on the wheel are smooth, as are the ones for the sprocket carrier. Cush drive rubbers appear normal. Drum brake wearing evenly and in good nick.
https://i.imgur.com/FzM4RvFl.jpeg

Swingarm is definitely rusty in multiple places, but nothing structural. The only thing I found that didn't look exactly as I'd expect is the right-side swingarm mount. There's more of a gap between the bits than I'd expect. But even then, nothing is loose. I regreased the rear wheel assembly and put it all back together and torqued the axle up as specified. Took the bike out afterwards and it's just the same, still wobbling.
https://i.imgur.com/HzocxuAl.jpeg

I've ordered a new rear tyre so I've get that changed and balanced on the weekend. Maybe I should get them to balance the front at the same time, just to exclude that as a possibility.

[stinkwheel]

Have you checked the subframe bolts (if it has a bolt on subframe). I got a horrible wobble on my VFR once as i was leaving a campsite and it turned out I'd lost one bolt and another was loose. Bike was all over the place. Hadn't noticed at all on the way there but I'd been hooning so my arse wasn't really on the seat much.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Zen Dog]

[Robby]

You've done the obvious stuff, which gets everything aligned if everything was straight to begin with. Now you need to check what is bent.

First, easiest step is to see if the wheels line up. I hate using string for this, I use a couple of long bits of 2x4 held together by some very long bolts or threaded bar. The lengths of wood are clamped around the rear wheel, then I can get a decent measure on how out of true the front wheel is.

If the front wheel is out of true, I then check the forks. I don't have a shortcut here, I just strip them down and check the stanchions for straightness by rolling them on something flat. A sheet of glass is ideal, but a paving slab is pretty close. If it appears to roll and not behave like a banana, I'm happy. I put a straight edge against the fork lowers, but I've never had trouble with them, it's always been bent stanchions for me.

For the swingarm it's often difficult to find a suitably straight edge to check, so I just measure and compare what I can from side to side. I do remove the front swingarm bolt and rear example, to roll them on my workbench and look for bending like on a stanchion.

Then wheels. The axle is mounted in my vice, with the vice gripping the nut and the axle pointed straight up. The wheel sits over the axle, so I can spin it. I mount up my dial guage pressing against the rim, and spin the wheel to look for runout. The correct allowed amount of runout is recorded in the Haynes. For a general rough idea, I don't worry about 1mm and I do worry about 4mm. I check the wheels with or without the tyres mounted.

If all of that is straight, then it could be the frame, but I can't measure a frame. Best I've managed is removing everything and seeing if it goes twang when I remove a major stressed member, like the engine. Or seeing if it's difficult to replace the engine, when it relies on 3 long bolts going through the frame in 3 different places. Those holes don't line up on a bent frame.

So I would take it all apart, measure what I can, then put it all back together. I will generally find the problem when I measure, or when I try to put it all back together.

The most bastard part of the whole job is routing the clutch and throttle cables. I can never get that right. I also tend to replace the handlebars as a matter of course, just in case they were the problem all along.

Lots and lots of grease on reassembly. The factory never use enough, and I want this to be better than new.

On a CBF, all of the above is only a day's work. Good to order fork seals and oil before the stripdown, they are the only bit I know I can't re-use.