Death wobble question

[NEWHD74FAN]

An oscillation that builds on input energy rather than being damped by losses is what causes the wobble. The losses are related to contact patch friction loss, steering preload (bearing friction losses i.e. fallaway setting), cable/wiring dress between bars and frame, the riders' arms and grip (affecting transfer)...and that is just part of front end steering "dampening".

As you probably know, all mechanical things like bikes have a "natural vibration frequency" like bridges and such...continually putting energy into that frequency will make it oscillate unless friction or dampening losses are imposed...to "detune" this, otherwise bad results will occur.

Short answer: if you have a wobble...do the fundementals to find root cause 1.) what was the last accessory or service done to the bike? 2.) did a check conditions that may have induced the wobble...lsolo/2-Up/loading, speed, accel or decel ? 3.) check tire/wheel pressure/condition, fallaway/lock to lock movement, lubrication and remove unnecessary load high up?

If still there SIMPLIFY conditions to the basic factory equipped bike. Steering dampers, Tru Trak and such are add-ons used to "detune" the oscillation but do not REMOVE the cause...so are a "Quick" (read expensive) way to mask the symptom. HD would not intentionally sell a bike with a "death wobble"...and they have invested millions in R&D to prevent such...Product Liability is scary to ANY manufacturer.

 

[R_W_B]

This was posted on another forum, thought I would paste it here to share the info.

I have 2 friends with FXDX's that both have this "death wobble". Both bikes have been 'fine tooth combed" front to back. One has low bars, small fairing and steering stabilizer, the other tall bars, 1/4 fairing, fork brace. Both develop at about 80-85mph and if you are really brave you can power thru em at about 100mph. I've ridden next to both bikes while it's happening and the front tire seems to bounce much like a severe imbalance..but swapping front wheels with known good units off of bikes with no wobble does nothing to eliminate it. My DX doesn't do it at all. I've had the bike up to..well lets just say I ran out of speedo before I ran out of room and it's smooth as glass. All three bikes though have different suspension setups. The 1st has 11in Works shocks, next has 12in lowrider shocks and mine has stock fxdx shocks. We've all swapped shocks and still no change. It's definately scary for them while blasting along and until we can pinpoint it we all have to ride the speed limit which kinda (EDIT) lol.

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[porkchop]

Also how fast were you going?
Where you on a curve?
What do you think kept you from going down?

Some say to let go of the bars, others say to hit the throttle. I think most just need new underware like you.
I've read that in cases where the rider is thrown from the bike early on, the bike will straighten back up on its own and continue without the rider weight and shift going on.

I was on a straight away and let off the throttle coming up on a stop light, It was about 40MPH when it started the wobble and I just hung on with dear life till it quit. Been great ever since the tire change.

 

[karlsbike]

It seems that both the Dyna and Tourer frames are prone to wobbles at 'normal' speeds. The touring frame (up to 2008) has: one low/front central rubber/biscuit mount with a lateral stabilizing link, one lateral upper stabilizing link between the heads. At the aft end, the engine is solid mounted to the tranny, which again is semi-solid connected to the rear swingarm. This whole set-up is supported in the frame thru' rubber donuts at the ends of the swingarm axle, that fits into two frame brackets (that also double as rear footboard support). It is a soft connection, and the flex can easily be felt in turns (on mine, anyway) as a rear steer/hinge in the middle kind of feeling.
How is the dyna set-up?

 

[Hoople]

How is the dyna set-up?

Dyna is set up with the rear fork attached to the transmission with a pretty strong pivot rod. Transmission is bolted to the engine, so the rear wheel/rear fork/transmission/engine can almost be looked at as one assembly. The rear fork pivot bushing don't seem to be an issue. There is 1 rubber isolator from the tail-end of the tranny to the frame. One front engine isolator at the voltage regulator area. One stabilizer link under the tank from top of cylinders to the frame.

Lots of places to develop a problem in stock form especially if the isolators are old and "gummy bear".

 

[karlsbike]

Dyna is set up with the rear fork attached to the transmission with a pretty strong pivot rod. Transmission is bolted to the engine, so the rear wheel/rear fork/transmission/engine can almost be looked at as one assembly. The rear fork pivot bushing don't seem to be an issue. There is 1 rubber isolator from the tail-end of the tranny to the frame. One front engine isolator at the voltage regulator area. One stabilizer link under the tank from top of cylinders to the frame.

Lots of places to develop a problem in stock form especially if the isolators are old and "gummy bear".

Intersting - thanks Hoople:
The main set-up with the engine/tranny/swingarm and front & upper connections sounds very similar on the two frames.
If I understand it correctly, there is only one isolator connecting the rear end of this ass'y to the frame? How is it mounted - is it sitting on top of the frame cross-brace?

 

[Hoople]

there is only one isolator connecting the rear end of this ass'y to the frame? How is it mounted - is it sitting on top of the frame cross-brace?

Karl, Here are some pictures of what I have on the 2009 Dyna as far as drive train mounts. Over time if the mounts get oily and gummy, there is not much keeping it together.

 

[karlsbike]

Very interesting - thanks.
The set-up looks more thought through than on mine, but it also seems more prone to wear. The FLH mounts, in particular the donuts (item 33 in the picture) are just lums of steel reinforced rubber and seem very 'accidential' by desing. A big room for improvement, in other words, and this is why I do not think H-D spent much money on researching these...
Ronnie's Harley-Davidson's 1995 Harley-Davidson® Motorcycle FLHR ROAD KING® (FD)

 

[Brutal FXDX]

My FXDX does the same as RWB posted. Ive checked everything from tire pressure to swing arm bearings. So thats why i said in another post im going to try the sputhe stabilizer for fxd. Think about the dyns platform, you only have 2 rubber isolator connecting the powertrain to the frame (front & rear) And a stablizer link at the upper mount. For those of you riding dynas, next time you have your fuel tank off, remove the stablizer link and move the engine around side to side with your hands and watch the rear wheel and swingarm. Its moves around like crazy. This is what happens to my bike at high speeds in a curve or hiting a dip on the road. It feels like the whole powertrain is moving. I know most people dont ride their dyna like i do, so they dont experience the wobble like i have. Its been so bad at 100mph i thought about bailing. But didnt and rode her out. Very scary folks.

JD

 

[sgmfisher]

I have run into this at times over the years. First time was with a '47 Knuckle Head back in '56 when I was young. The old bikes, Harley, Brits, German, you have it, had a steering damper knob built in at the top of the forks. When you were in town at low speed, you losend the ratched knob up. When you headed back out on the highway, the knob had to be tightened back up or katie bar the door some where between 45 and 60 mph. The second is to hit a hard corner with some road bumps and the frame will flex. The flex may be in the swing arm, the soft tail, or somewhere along the back tube around the neck. No real fix for this situation except to find it and weld some reinforcement to the frame in the latter. I think that too much weight transfered to the neck bearings and/or neck bearings being too lose may have something to do with it. The forks themselves have been known to flex and cause the problem and a fork brace will be a big improvement if it is, or even in combination with a steering damper. The best frames I ever rode were the Triumph, BSA, and Norton bikes. They always seemed to be rock steady to me. That's why I rode them for years, that and they were realy quick bikes. My '06 WG seems pretty solid but I don't ride nearly as hard as I used to. It is solid up to 100mph though and I have a small NC Street Shield X on it to get the bugs. It doesn't seem to catch enough wind or traffic blast to have any effect. In the wind.

Pops