Unstability with Superbike bars!

I changed the bars on my CB900F to a set of superbike bars which are
more forward and lower and narrower. The hands now pretty much line up
with the triple tree clamp looking down from the new riding position,
compared to the much more pulled back original bar. Also the new bars
have no weights inside. The steering is much different now. The bike
wants to "fall" or pull into into corners, is this called oversteer?
I cant say its not stable, but I have to keep my hands on the bars to
feel in control. I used to be able to take my hands off the bars for a
few seconds without sensing an urgency to grab on again. What is
wrong? Is the missing bar weight the problem or the fact my hands are
more forward?

 

No, it is not oversteer. When the rear end is trying to pass the front
end, you have oversteer. You see oversteer on dirt tracks usually, but
the very best motorcycle roadracers are oversteering their racers too.
The best guys have dirt track experience, and this is called "backing
it into the corner". Then the front wheel of a motorbike has to point
to the outside of the turn to prevent the rear wheel from passing the
front wheel. That's oversteer. It looks squirrely, it feels
squirrely...

What you are feeling is a steering neutrality issue. The handlebars
should require no more than about one pound of force to hold the bike
down in a turn, once you've countersteered it to a cornering attitude.

If you then have to continually push on the outside bar to keep the
motorcycle from falling into the turn, that's more nerve wracking than
having to continually push on the inside bar, holding the motorbike in
the turn.

Your upper body weight is a little further forward and this contributes
to front tire grip. The front tire is just doing its job a little
better. You will probably get used to it in a short time.

The feeling I get from a motorbike with standard highway bars is that
it's pivoting around the rear tire contact patch when I turn it. The
feeling I get with shorter bars is that the pivot point moves further
forward. With clip-on handlebars, it feels like the motorcycle turns
around a point directly under my chest.

This feeling actually arises from a steering phenomenon called "out
tracking". The first thing a normal street bike does when you
countersteer it is the front tire out tracks. It tries to follow a much
wider radius turn than the rear tire follows. Then it settles down and
goes along with the rider's wishes, it tracks closer to same radius as
the rear tire. The higher the handlebars and the more rake and trail in
the forks, the more out tracking you get.

When the front tire tries to track inside the radius the rear tire, you
get the feeling that the machine is "falling into the corner". You
usually see this on motorcycles where the owner has lowered the front
end by pulling the forks up through the triple clamps.

I did that to a GSXR-750 once, in order to raise the riding position a
bit. I moved the handlebars from underneath the upper triple clamp,
pulled the fork tubes up an inch, and clamped the clip-ons to that inch
of fork tube. Problem was, it upset the steering neutrality. It was OK
in big sweeping corners, once the motorbike was set into a big sweeper,
it would continue around the turn without any further effort.

But the lack of neutrality upset the handling is fast esses where I had
to go left-right-left-right rapidly. Once the motorcycle was headed off
to one direction or the other, it wanted to keep going that way.

So I put the handlebars back where they belonged and moved the fork
tubes back to the correct position. Riders had been telling me that
they could feel the difference in steering with the fork tubes 5mm
higher in the clamps than stock. My fork tubes had been 25mm higher
than stock.

If you just can't get used to the feeling of the superbike bars, you
might try going the other way. Push the fork tubes down 5mm or 10mm in
the upper triple clamp and experiment until you get the steering feel
you want.

Another way to go is to a pointier front tire. But the downside of a
pointy front tire is that it might wiggle too much while you're riding
straight up...

 

Thanks I didnt know what to call it.

It takes more effort to countersteer it into the corner, then it goes
easily as you have said, and then it takes more effort to get out of
the turn.

Maybe this is the root cause, body forward position altering geometry!
Maybe I'll experiment by sliding back on the seat a little and see if
the effect goes away.

I havent done this, but I had thought about lowering the front a
little "for the aggressive look" and I knew this would change the
geometry, but your explantion helps me to see this would probably not
be a good thing to do

Thanks for your advice

 

In addition to Krusty's excellent dissertation, you may have issues with
worn steering head bearings, which may be amplified by the greater front
end wieght bias (compared to the old bars & less weight on the front tire).

 

Some of that is just cause you are in a different position and your
perspective has changed. Some is the increase in front end weight (you
are up over the tank now) affecting the handling. Some may be because
the ride height was changed when you put the tubes back in...

Hell. I've stuffed an 18mm spacer above the rear shock to get the tail
in the air. (Left the front end alone) Bike is more stable at speed and
I'm getting used to the fact it isn't a Duc.

But, it's light and easy to toss...

We usually move 'em about 3mm at a time at the track

--
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'77 CB750K '78 CB750K
'00 ZG1000 '96 Ducati 900SS
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