What would cause this on a motorcycle?

[notbike]

I remember a while back when I was riding different bikes all the time I had a weird problem with some of them and I never really figured out what was causing it. It's not that I care but I'm just curious cause it popped into my head again all of a sudden.

On my bike I could take my hands completely off and keep going in a straight line without having to balance at all, but on some of the other bikes one of two things happened. Either the handlebars would start shaking violently and I'd have to catch them, or the bike itself would immediately start veering over to the left or right as if it were going to fall down.

Not saying I ride with my hands off the bike often but I have let go of the handlebars now and then to see what the bike does out of curiosity.

I already know that bikes should stay upright, my ninja stays upright when I let go, so what was wrong with those other bikes?

Firstly why would the handlebars shake, and secondly why did some of them immediately start veering to the left or right as if they were getting heavy?

[Pigeon]

Unbalanced wheel(s).
Miss-aligned wheels.
Incorrect tyre pressures.
Squared off / uneven worn tyre.
Loose / worn head bearings.
Bent forks.

Could be a huge number of things.

[SQL]

Tyre cupping, wheel alignment, uneven shocks.

[Nash GT]

Witchcraft
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Suzuki GSF600N

[barrkel]

Sometimes bikes are just a tad unstable. I've owned three SH300, two of them from new, and they've all had headshake at 20 when slowing from 30 with no hands and stock topbox attached. Nothing that threatens loss of control. Some bikes' geometry just prioritizes steering over stability.

Possibly related, at other speeds, where it doesn't go into headshake, the steering turns to the left from the weight of the front brake caliper and the scooter countersteers to the right. This may happen on other bikes with one-sided front calipers.
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Bikes: S1000R, SH350; Exes: Vity 125, PS125, YBR125, ER6f, VFR800, Brutale 920, CB600F, SH300x4
Best road ever ridden: www.youtube.com/watch?v=s2MhNxUEYtQ

[wristjob]

its the nut between the handlebars and the seat

[Fisty]

What Pigeon said.

Or.....

https://geektechpop.files.wordpress.com/2015/01/ancient-aliens.jpg
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Quietly and consistently taking the piss.
TL1000R | Hayabusa | ZXR400 | TL1000S | Bandit 400 V
Fatter and faster than Fret

[Wonko The Sane]

[Andy S]

My vfr handlebars used to wobble if I let go of them only if the top box was on.
Disruption of airflow i pressume.

[fatjames]

[wots]

Fireblade wandered sharply to the left when I bought it, new rear tyre fitted (it was nearly shot anyway). It was back to neutral. A mate who has a near perfect 'blade same year, rode it and thought it was twisted somehow.

I actually think it was more to do with wheel alignment, than the tyre, in my case. As the guy I bought it off, told me he'd done the rear brake
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Currently: Yamaha DT 125 LC2,Repsol Fireblade, and Bumblebee MSX 125

[Northern Monkey]

[wots]

[Northern Monkey]

A pillion is normally in front or above the rear wheel. A top box is behind the rear wheel
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Fisty: after polishing the tank with the glistenng beads of sweat from my full hot scrotum, I filled the headrace bearings with 10cc of my manmilk

[Pete.]

Top-box can definitely upset the handling, especially the square ones.

Get up behind a Transit or Renault Master at 60mph and you'll find out why.
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a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[Pol Anorl]

divvy shook like mad with topbox on it, fazer has no wobble although does go off to the left when my hands come off the bars..

[scorpia4]

My cbr125's handlebars don't shake at all no matter what speed I'm going. It'll easily go round soft corners no handed

[anthony_r6]

Missing Engine Mounting Bolt!
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Ted : "Maybe he's agoraphobic."
Dougal : "Jack scared of fighting? I don't think so, Ted."

[Pol Anorl]

[Andy S]