Handlebar twitch when braking - any ideas?

I have sold my SO LS650. However, I had another one in bits that I had
intended to use for spares. As I no longer needed it for spares I decided
to recommission it as a sort of semi-rat runabout - just for the fun of
doing it.

It is now back together and seems to run OK although it needs a bit more
fettling. However, I have noticed that when I use the front brake the
handlebars have a noticeable initial "twitch" to the right. It has a single
front disk brake on the left with new pads. Fork seals have been renewed
and equal amounts of oil put in each fork leg. Forks seem to damp ok.
Forks are non-adjustable so it isn't a case of unbalanced preload adjusters.
If I keep on braking it comes to a halt without further obvious movement
rightwards. If I take my hands off the handlebars when riding it continues
in a straight line so wheels don't seem to be seriously out of line.
Overall it seems to handle OK (for a SO LS650). Any suggestions about
possible reasons for this "twitch".

 

under inflated tyre

warped disc

involuntary clenching of seat weighting peg

 

Tyre was inflated just before test and it is holding pressure.

Warped disk usually results in "pulsing". Also - if the problem was with
the disk I would have expected a twitch to the left - which is the side the
disk is on.

I can repoduce the twitch when not riding - push it forward and grab front
brake - bars twitch right.

 

Something's loose.

Check the handlebar clamps, and the triple clamp bolts. In fact,
check everything.

 

Only does it under braking? Inadequate fork bracing.

 

Handlebar Clamps - checked
Fork yokes/clamps - checked
Wheel spindle/nut -checked

But it still does it!

 

Yes - I thought that too - but why twist the opposite way to what one would
expect from a left-sided single disk set up? Plus the other one that I had
was identical up front and didn't show any such tendencies.

 

Twisted frame?

Does it run straight if you get up to 30 or 50 (not 40) and take your
hands off?

 

Hmm - I will double-check front bearings. If the axle/spacer configuration
was wrong then the disk probably wouldn't line up with the caliper but I
will double-check.

Worn fork bushes - again, I will double-check but there is no obvious play.

Head bearings were replaced during recommissioning.

 

Hmm..I will try it at a range of speeds next time I test. However, AFAIK it
was in daily use before being retired with an engine problem so it wasn't a
crash write-off.

 

Especially if it's only one front disc & it pulls to that tune.

 

Yebbut...disk on left - twitches to the right.

 

Ah, but you steer left to go right; counter steering and all.

Unless you're saying that the right bar comes back (and hence you go
left)

 

Using the patented Mavis Beacon "Hunt&Peck" Technique, TMack

Have you adjusted the built-in butt-plug to the correct ride height?
Only, I can imagine that causing a slight involuntary steering input
under braking if it's set a bit long.

--
Wicked Uncle Nigel - "He's hopeless, but he's honest"

I have already made the greatest contribution to the fight against climate
change that I can make: I have decided not to breed. Now quit bugging me and
go and talk to the Catholics.

 

You've got a fast, heavy spinny thing that effectively kicks the left
fork leg forwards when you hit the brakes, which is why it twitches to
the right. Probably.

 

Gyroscopic precession.


--
Beav

VN 750
Zed 1000
OMF# 19

 

Y'see, I don't like your use of the word 'twitch'. And old bike with
a single disk can cause a small turning moment on the steering, but
this should be continuous while the brake is applied, not a initial
'twitch', which to me implies some movement or slack is being taken
up.

 

Yebbut it will even do it at less than 5mph, when countersteering doesn't
work. It will even do it when damned near stationary. Anyway, its a
noticeable twitch rather than a steering movement. If I keep the brake on it
seems to stop without veering right.

 

Hmmm - the front brake lever is on the right.
Maybe you are feeling a twitch through the lever due to a sticky caliper
not moving fully until enough force is applied.
This release of pressure would cause your hand to twitch the steering.

Just another theory ....

 

Play in the left fork, between the slider and the stanchion. You might be
hard pushed to see it, but you'd probably feel it if you put your hand on it
at the dust seal. This should be doable even just standing beside the bike
and pushing.