Bar wiggle at high speed

[socalrob]

Last weekend I was on a fast trip to Fresno California. On the way back, buzzing down highway 99 on my RR, traffic was light & I was "playing" around with a couple of compact car jockeys. We were generally going between 80 to 95 mph, but at one point I was boxed in with a truck, one of the cage drivers was a bit close, I got clear, there was about a mile a clear straight road ahead with no evidence of LOE's, so I opened her up to about 120 indicated. Cars up ahead, I applied the brakes a bit late & fairly hard (was having a bit of fun), and during the start of the deceleration, I noticed a mild head shake of the bars. The shake was limited to an inch or 2 of bar end travel, just enough to wake me up.

My question, as I don't have alot of experience throwing a bike around at speeds over 100, was the head shake caused by my quick deceleration, some bumps on the road, or what? I did have the system cases on, but pretty lightly packed. Never really noticed any head shake on the trusty RR before, sort of surprised me & woke me up to the fact maybe I should be showing a bit more respect for the speed.

[GPM]

Pretty tough to diagnose. One thought would be to ask how tight a grip did you have on the bars when you grabbed the brakes? Did it subside as you relaxed? A death grip could induce a bit of a shake.

Perhaps Dean will chime since from what I hear, his RR spent most of her life at those speeds.

[BobFV1]

Rob, Rob, Rob -

Seriously - I do NOT want to give the CHP the opportunity to scrape you off the highway so KNOCK IT OFF!!! The freeway is no place to "experiment" with speeds over 100MPH on a naked RR with system cases! Seriously - you could kill yourself.

Now - if you want to play around and go fast - do it at a track day. I just lost a friend at the end of November who decided to experimment with putting his knee down out on a public road. Too much throttle, too much lean angle, low side - his body went under the guard rail but the rest of him didn't.

Okay - first, you were at high speed, in turbulent air (from the traffic you were coming up on) under heavy braking. Your bike shifted all the weight to the front under the sudden heavy braking, making your rear tire very light on the pavement. Application of your rear brake under these circumstances caused your rear tire to lose contact with the pavement and the bike to become unstable, resulting in your handlebar waggle. Your bike is not a high performance bike and does not have a steering damper, so the handlebars just started whipping back and forth. This phenomenon has KILLED a lot of riders! The first time I took my R1 on the track and put it under heavy braking I accidently got on the rear brake and almost became road kill.

Ride safe and please ride conservatively on the open highway....

[geechie]

I think it's interesting that my 'old fashioned' 1100R does have a steering damper. I've never had a significant head-shake at speed. I get a neat little one though, on this one particular left turn from a stop where I occasionally whack the throttle as I'm standing the bike back up. Feels like it lifts the front wheel about an inch or so, and shakes briefly as it touches down again. Got to let the inner squid out a little bit sometimes.

George

[supermotoC]

What BobFV1 said.

Fresno? Going there made ME shake my head.

The quick way to "kill" the tankslapper-in-the-making is to get off the brakes just long enough to settle the bike down, even if it means just backing off them 50%. Also, like on a bicycle, a white-knuckle deathgrip can much more easily induce bar wag, even if you're just decelerating under engine compression. A firm-but-relaxed grip is key, especially while braking, when wheels want to oppose the action you are telling them to induce.

If you need a steering damper, then you must be riding on a track.

[Deans BMW]

Rob, don't know what to say. I would first want to look at the bike, how many miles, tire air pressure, tire condition, road condition, is every thing tight, bearings, etc and on and on. I know that both Dave and Allan will also have good inputs. If I had thje opportunity to sit on your bike, I could tell you rather quickly. I would suggest to have your dealer do a quick look see. We used to do that all the time at no charge with our good customers.

[socalrob]

Bob,

Yeah, I hear what your saying. OTH, Highway 99, in many stretches, has a newish asphalt overlay, long flat straight aways that go on for dozens of miles at a time, and sporadic traffic. I was basically just riding from one traffic grouping to the next, a brief spurt of speed (although the traffic does run in the 80-90 range) to distance myself from a cager driven a bit closer behind than I like, a cold day that the RR was running pretty much perfect. While I did brake hard enough to upset the bike's geometry, I don't think it was anywhere near hard enough to completely unweight the rear, but I'm glad you brought that up, as I think it helps to know what to watch for. Your comments on my speed are also why I know I can't own a K1200R, I would get myself into serious trouble.

The reason I brought the whole episode up was that I had never "noticed" any unstability in the RR before, not that the unstability I did notice was too significant, it was more just enough to let me know that it could exist. It settled itself quickly, more of a shake than a building tankslapper kind of thing. Just a curious occurance. Has anyone had a real tankslapper on a telelever BMW?

[Dallara]

Rob,

To be quite honest, without actually seeing the stretch of pavement and taking a really close look at your bike it would be literally impossible for anyone to give you a definitive answer as to what might have caused your wiggle. The possible causes are nearly infinite in this frame of reference, and many bits of conjecture could have you going down the wrong path.

Suffice to say, the first thing you should do is check anything and everything you can think of on the machine... Tire pressures (yes, improper inflation can cause wobbles), various pinch bolts in the suspension, the tightness of the front axle and the rear wheel bolts, the swingarm pivot, the telelever pivot and anything associated with the telelever system, tire condition (is there any evidence of a tread delamination or other tire anomalies?), right down to the tightness of the handlebar pinch bolts... You can start a minor wiggle under the right circumstances if you have even just lost a wheel weight.

Most likely everything is fine, and what you ran into was simply one of those rare occurences where the planets, pavement, power pulses, placement of mass, plethora of various geometries, and powerful gyroscopic precession of the front wheel all combined at just the right reasonant frequency to excite an oscillation.

Everything has a reasonant frequency... The bike's frame, the telelever, the front fork tubes, the wheels, the swingarm, etc. Most of the time the various reasonant frequencies are enough apart (or damped) that they do not "get together" and excite each other into self-energizing oscillations. Different types and constructions of aluminum excite into reasonance at fairly low frequencies, where steel reasonants at higher frequencies, and they all have different, varying amplitudes as well. Most of the time when a few of these get in synch the tire's self-aligning torque is sufficient to damp these away...

But every now and then enough different components get to *singing* and all end up *harmonizing* in just the right *key* and BOOM, you have a wiggle, wobble, or wind-up. Many times just the right kind of ripply pavement (it may seem smooth to you, but the tire and wheel may see it differently, and sometimes it is just the type or coarseness of the pavement that can help set something off...) with the right kind of chassis loading - front end unloaded just enough or not enough, etc. - that can excite some things to reasonant together.

Now in most of today's modern bikes, with pretty substantial frame and suspension structures, even when a reasonant-based oscillation tries to start up the frequency is so low that the tire and geometry can more than handle it and damp it away before you ever feel it. High frequency oscillations are the ones that are hardest to control, and even though they manifest themselves in what feels like a lazy, gentle oscillation that doesn't mean that they are *ringing* at a much higher frequency at the source (like the steel tubes at the top of the sliders) and translating to a larger amplitude, lower frequency shake where they effect the steering. When this happens any number of things, or inputs, can amke 'em worse, or make 'em better... It just depends on the situation, road and traffic conditions, rider skill and experience (to sense what is causing it), and velocity (and our acceleration or deceleration rate) to formulate a recovery solution.

Chances are it will never, ever happen again... But it doesn't hurt to check over the machine to make sure nothing is amiss to help the issue along.

If you don't find anything wrong, and are pretty brave... You can go out on a deserted section of road somewhere and see if you can duplicate the conditions and start the oscillation up again. If you can try and note whether it self-amplifies and gets worse or if it self-damps and sort of cures itself. If it self-amplifies, try adjusting your grip on the bars a bit, or slightly changing your riding position and posture and try again. It very well may be you that is exciting the problem, and a minor correction in your "style" can rid you of the problem once and for all.

Sorry this has droned on so long, but chassis oscillations and resonant frequencies of materials and systems is a complex beast. Understanding it is your best weapon, and good, logical thought and analysis are really the best way to cure it once and for all.

Hope this helps, but I doubt it does...

Cheers!

Allan (Dallara)