death wobbles

[soyavenger]

anyone whos ridden a skateboard has experienced the death wobble, high speed, side to side motion, and trying to correct it only makes it worse. i experience a similar situation lane changing at high speeds on my bike. i just figured it was common for bikes until i took my bike to the metric shop for a mirror, borrowed the shops wrench to put it on in the parkinglot, and the mechanic looked at my forks and said, "those fork seals look worn out, does it wobble at high speed?" is the death wobble normal at high speed, or is there something wrong? 

[happycommuter]

Nope, never had it.  But you're correct that fighting it is the worst thing to do.
Supposedly there are many videos where a rider gets a lock-to-lock tank slapper and is thrown off, then the bike rolls along just fine without the rider.

[Bumblebee]

anyone whos ridden a skateboard has experienced the death wobble, high speed, side to side motion, and trying to correct it only makes it worse.

In aviation there's something similar called a dutch roll that'll do the same thing. What happens is that the natural instinct on when to put the control input in happens just after the point where inertia starts taking things back the other direction. The correct solution in that situation is when the instincts tell you to put the control input in, you put the opposite input in than the one you instinctively think is correct. By doing this, the dynamic forces are damped out, not amplified.

That said, this death wobble you're describing is not normal. Lane changes at high speed should feel solid...as in more solid than they do at slower speeds. Now if you're shoving the controls around unreasonably hard, that's forcing dynamics into a the equation that shouldn't be there, but that doesn't sound like the situation here.
Get the forks, triple T and head bearings checked out to find out what's not working properly.

[Hondo]

A tank slapper as it is called....

If you are getting into that while riding, you need to correct the problem asap -



Things you can do to correct the problem -

1 - adjust/set the steering head bearings
2- ensure proper tire inflation
3- ensure fork oil levels are the same
4- ensure that rear wheel alignment is correct/straight

[soyavenger]

yikes! hopefully the feces in his shorts cushioned the fall.

definitely nothing like that. its pretty minute. now that i think about it, it feels like when the roads are worn and rutted and you get into one and feel that shimmy, which may be the case here, the roads in this area arent the best. it wouldnt hurt to have it checked out, i always worry about the bike shop thinking they have a fish on the line. had a bad experience with a bad shop.

[Hondo]

You can do all of those checks yourself, just need the shop or Clymer manual.

[soyavenger]

cool, i do have the clymer manual. the alignment and psi are all correct, ill have to figure out how to check the bearing. i know the bike was poorly maintained by the last owner. im not sure of the levels, but could old fork oil cause this as well?

[IndyRider]

I love the tank-slapper video - nice touch! In response to the original post, I do not get a high speed wobble on my '84 700SC, but I did get a low speed one around 30 mph, if I let go of the bars as the bike is slowing. If I hang on while slowing (which of course you're supposed to do!), there is no wobble at all - everything feels solid.

This wobble disappeared when I put new tires on the bike, so perhaps the poster's wobble is related to abnormal tire wear?

[martian]

This wobble disappeared when I put new tires on the bike, so perhaps the poster's wobble is related to abnormal tire wear?

I have a similar wobble on decel with mine.  I've checked / replaced everything (including the steering stem bearings) but the tires so I also suspect the tires are the cause.  Mine are getting up there in years and mileage.  I plan to replace them next year.

[Custom82Hawk]

dunno about the fork seals being the issue.  my NH has no compression left in front forks and it doesn't wobble at any speed.  just makes a lot of clacking noises from the forks bottoming out :)  makes for a really interesting ride though, it's so bouncy from riding on just the springs, it feels like a boat.  next thing on my agenda after new intakes.  i figure running right comes before riding right, since the riding part isn't really that bad aside from what i mentioned above.

[soyavenger]

This wobble disappeared when I put new tires on the bike, so perhaps the poster's wobble is related to abnormal tire wear?

theres about 2k on the front rubber and less than 1k on the rear. theyre metzeler marathons, barely worn at all. i did notice that the old front was worn more on one side of center than the other, maybe the last rider took alot of right turns. the old rear was worn to the belts on one side (3oclock to 6oclock) and still had tread on the other side (9oclock to 12oclock) i didnt know if these were manufacturer defects or if there was some issue with the wheels, but it doesnt vibrate or anything with the new tires so they should at least be balanced properly.

[mwallette]

In aviation there's something similar called a dutch roll that'll do the same thing. What happens is that the natural instinct on when to put the control input in happens just after the point where inertia starts taking things back the other direction. The correct solution in that situation is when the instincts tell you to put the control input in, you put the opposite input in than the one you instinctively think is correct. By doing this, the dynamic forces are damped out, not amplified.

Very similar to Pilot Induced Oscillation (PIO) on landing -- pilot sees the nose come up a little and applies downward pitch just as the aerodynamic characteristics of the airplane cause nose-down pitch.  Then the pilot, seeing the nose drop farther and faster than expected, increases pitch -- again, just as the aerodynamic characteristics of the airplane were making the nose rise again.  Repeat until the airplane plants all three tires on terra firma or until the nose gear collapses, whichever comes first.

Problem is, those corrections happen almost at a sub-conscious level.  I didn't even realize I was pumping the control yoke on the airplane until my instructor pointed it out to me, so I can totally see how someone could end up with such a violent oscillation that the rider gets thrown, but the bike goes straight ahead for quite a ways afterwards.

[Hondo]

In aviation there's something similar called a dutch roll that'll do the same thing. What happens is that the natural instinct on when to put the control input in happens just after the point where inertia starts taking things back the other direction. The correct solution in that situation is when the instincts tell you to put the control input in, you put the opposite input in than the one you instinctively think is correct. By doing this, the dynamic forces are damped out, not amplified.

Very similar to Pilot Induced Oscillation (PIO) on landing -- pilot sees the nose come up a little and applies downward pitch just as the aerodynamic characteristics of the airplane cause nose-down pitch.  Then the pilot, seeing the nose drop farther and faster than expected, increases pitch -- again, just as the aerodynamic characteristics of the airplane were making the nose rise again.  Repeat until the airplane plants all three tires on terra firma or until the nose gear collapses, whichever comes first.

Problem is, those corrections happen almost at a sub-conscious level.  I didn't even realize I was pumping the control yoke on the airplane until my instructor pointed it out to me, so I can totally see how someone could end up with such a violent oscillation that the rider gets thrown, but the bike goes straight ahead for quite a ways afterwards.

Diliberate cross control pressures should stop that.

[mwallette]

Diliberate cross control pressures should stop that.

The problem is, your reaction time is exactly in sync with the aerodynamic response.  That's what causes the problem in the first place.  What you have to do is fight the urge to correct for small pitch changes (or steering changes on a bike) and maintain a steady pressure on the controls.  Otherwise, by the time you perceive the change and react to it, you will only amplify the oscillation.