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Steering oscillation - cause and solution?

saizai

saizai

New member
Joined
Jan 29, 2006
Location
San Francisco
Moto(s)
Red '98 Ninja EX250
Name
Sai
Yesterday I was going from Oakland to Berkeley via 580, medium traffic. Probably going about 40-50.

I was leaned into a mild right hand turn of the highway - not all that hard - when suddenly my bike's steering started oscillating back and forth very strongly. I reacted immediately by easing off the throttle, off of front brake if any (I'm not sure if I was using it or not, but I was covering at least as I always do), and trying to make a compromise between the direction it was pushing me - towards the outside of the curve, which would put me into the car to my left - and staying within the lane.

This worked and it subsided within a few seconds, but was still rather scary... and I don't know what caused it.

I was going normal speeds (lower, if anything), the road at that point didn't seem at all unusual, etc.

I don't like having this happen and not know why, 'cause it means I can't avoid doing whatever it was that caused it in the first place.

So, two questions:
1. What probably caused it? Or put another way, what are the typical cause(s) of this?

2. How can I best stop it ASAP if it happens again? What if I have less margin room than I did? (I started in the right wheel track, and ended up near the left border... farther off my desired line, especially with a car there, than I'd like.)
 
called headshake, the best thing is to NOT back off the gas but keep some on. It is caused by many things, bumps uphill, improper suspension adjustment, bad headstock bearings, etc etc.

They are pegs, not pedals.
 
Causes:

Uneven road surface
Poor tire inflation

When I hit an un-even interchange at 280 and 85 south, I was sent into a headshake at 70+ mph. I sat up from my mini tuck, let off the throttle and it subsided on it's own. Fools here told me to wheelie out of it.....ppffffttt:x I can't wheelie:shhh http://www.bayarearidersforum.com/forums/showthread.php?s=&threadid=115166
 
In my experience, the rider usually has something to do with starting an ocillation. While the other factors mentioned in this thread are definitely related, something has to deflect the front wheel off axis and that's usually the rider.

Often, the rider has a small amount of pressure on one bar or another and when the bike hits a bump, the steering head cocks slightly to the side. When the tire gets weighted again, the wheel tries to snap back to center and goes a little too far -- this is the beginning of the headshake. Rolling off the gas and loosening up on the bars usually settles the front down. If you've ever seen a rider get flicked off a headshaking bike, often the bike straightens up and keeps going without him. If you can simulate that without getting tossed off, that's usually the solution.
 
tzrider - like I said, I was leaned into the curve... so of course I had pressure on the inside bar. :p

I'm not sure whether I hit a bump or not; I was a bit distracted by the "oh shit" control. :laughing

The video linked above shows a guy on a big bike getting up to speed, taking hands off the bars, and purposely knocking one bar to start it shaking. After a couple seconds, it calmed down.

However, he was going in a straight line. A straight line in my situation would have put me inside a coupe, which isn't where I prefer to be while riding. :p

So, summary so far:
* if it's in a straight line, be steady or ease off a bit on gas, loosen up on bars, and let it ride out
* if it's in a curve... ?
 
saizai said:
tzrider - like I said, I was leaned into the curve... so of course I had pressure on the inside bar. :p
Click to expand...

Not sure if you know this, but once leaned over, you shouldn't need bar pressure to hold your line. The bike should track the arc on its own as long as you maintain a smooth roll-on with the throttle. The bike may not hold a line if the front tire is worn, but generally it will.

Many riders do maintain bar pressure through the turn and this is a definite contributor to headshake over bumps.
 
Hm. I'll try testing that next time I'm out to see what happens.

FWIW, my tires are getting a bit worn. They're on the list of stuff to get replaced in the near future.
 
tzrider said:
Not sure if you know this, but once leaned over, you shouldn't need bar pressure to hold your line.
Click to expand...

It has been my experience that it's rare for a (stock) bike to hold a line without constant pressure on one of the handlebars. I spend the time and sometimes the $ to enable my bikes to be neutral steering. It sure is nice when you find the combination of suspension/chassis/tires that enable neutral steering.
 
Enchanter said:
It has been my experience that it's rare for a (stock) bike to hold a line without constant pressure on one of the handlebars.
Click to expand...

Most of us who aren't ~140 lbs need to at least adjust the sag before the bike will act right. With the sag set correctly, tires in good condition (and inflated properly) and good T/C most bikes I've ridden are pretty close to neutral.

Sometimes body position can make the difference too. Leaning the upper body into the turn can do it, but some sportbikes seem to work better with the rider hanging off a little. Most often though, I still find that throttle control makes the most difference.

Like you, I do what it takes to get the bike to hold a line without bar pressure, as that pressure can absolutely lead to slappers.
 
i agree with what a few have said. letting off throttle increases your front end load, and will consequently increase any action taking place.

a very good reason why you should be loose in the arms (bar pressure), and not death gripping it.

going right back to a roll on or even a maintenance throttle would help tremendously.

my .02
 
Out of curiosity - what exactly does it take?

AFAIK my suspension is not adjustable, so if there's anything it probably means replacing parts...
 
replacing parts? are your forks bent? thats too extreme, assuming everything is hunky dory.

i'm not sure if the suspension on the 250 is adjustable or not: but if it is, read this article. in fact, you should read it anyways. it very well could be a suspension issue. but at 50mph, i wouldn't think so. those issues usually only show up at higher speeds. i am, however, no physicist.

but like others have said, low tire pressure might cause it, but it would have to be seriously low. but if its something bad with parts or other mechanical issue, you should talk to the person you bought the bike from... or let us in on the crash you had.

either way, the turns on 580 aren't so tight that rider input would have caused something like that. especially at those speeds.
 
saizai said:
Out of curiosity - what exactly does it take?

AFAIK my suspension is not adjustable, so if there's anything it probably means replacing parts...
Click to expand...

How long do you plan on keeping the bike? If you are going to keep it- it's worth getting the suspension setup right. Even changing the weight of the fork oil could help. But take it to a pro and have them set it up...
 
I'd like to keep it indefinitely. Mainly it'll depend on how long I'm still in the area - if I move, it's probably not worth taking the bike along (vs. selling and re-buying).

This definitely goes outside the range of stuff I'm willing to do myself though. :laughing

I'll see how it runs after I swap out the tires. If it still has issues, I'll look into the suspension changes.
 
Saizai,

Let's not get you too worried about the condition of your bike. It could be suspension, but you've told me three things that are very likely connected:

1) You had scary headshake.
2) You were keeping pressure on the inside bar to hold your line.
3) Your tires are worn.

Let's work backwards from the tires. One charateristic of a worn front tire is that it gets a flat surface bevelled onto the sides at the lean angle you use most often. That angle probably isn't as deep as you may corner on a back road, as you ride in other situations that don't call for or allow deep lean angles. When you *do* try to lean the bike a little past your "typical" lean angle, the bike rolls onto the outside edge of that flat spot. Once there, the bike tends to stand up so the flat spot is in full contact with the ground again. This can be a *very* pronounced effect -- not subtle at all. You would probably have to maintain some pressure on the inside bar to keep the bike leaned beyond the flat spot.

With pressure on the inside bar, you may have hit a bump or some other feature in the road. It wouldn't take much. This momentarily reduces the tire's contact with the road and as a result suddenly reduces the resistance the bars are giving you. Since you have steady pressure on the bars, the bars cock to the side a little bit. The tire regains contact and snaps back onto the arc and a little beyond. Any pressure you happen to put on the bars amplifies the oscillation.

A variation on this is the worn tire is inherently less stable if the bike is leaned far enough to ride onto the outside corner of the flat spot. A bump you didn't notice could be enough to destabilize the front end.

My suggestion: Go look closely at the wear profile on your front tire. Has it been worn into a shallow V? As you look at or feel the tire, can you detect that outside corner I've been describing? It may feel like a slight radius change if you run your finger across the tire starting from the center towards the outer edge. If you do detect this, you'll know what to look for as your next set of tires wears. They all do this and inspecting the tire now will give you a basis to decide when it's time to replace the next set.

Next, get new tires. You've said you need them anyway.

Then, go riding. Go around some corners and see if you can get the bike to hold a line with no bar pressure. Correct control operation in the corner to accomplish this is:

- Press forward on the inside bar to initiate the lean into the corner.
- Sit still on the bike and go with it; don't lean the opposite way.
- The moment the bike reaches the lean angle you want, stop pressing on the inside bar and the crack the throttle open.
- Roll on smoothly through the turn. You just want a very light feeling of acceleration.
- Notice if the bike is holding its line without bar pressure. If it tries to stand up, lean your upper body slightly farther into the turn (without putting pressure on the bars) and see if that helps the bike hold the line.

Let us know what you find out.
 
Last edited:
tzrider- Thanks for the very helpful reply. That definitely makes sense.

It was pointed out to me by another rider - David I think? - that my front tire has 'scalloping'. I felt that when it was pointed out - a sort of high/low irregularity along the length of the tire. I didn't notice any irregularity from side to side, though... but then, I wasn't really looking for it either. Will check next time I can.

I'm not sure though that I'd distinguish it from the radius difference that I know is supposed to be there in a lot of tires - they're not just elliptical in profile. Are you saying just that a worn profile is going to have sharper, or irregular, edges between those different areas?

Will try what you suggested, anyhow. Definitely will replace tires in immediate future.

Thanks again for your well-articulated response. :thumbup
 
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