So I've had a chance to ride my newly acquired 08' SD 990 a bit now to familiarize myself with it. Awesome motor, pulls like a freight train, sharp handling and the sound......

I am having an issue though. The bike has some med/high speed instability. Bordering on going into a full on tank-slapper. I noticed it initially in some med speed (60-70 mph) sweepers the bike seemed to wallow a little and wasn't exactly confidence inspiring. Felt like something was upsetting the entire chassis and causing the rear end to walk around. Yesterday I took it up on the highway, I don't often ride slab as I live in an area that I can get almost anywhere I need without it, and honestly I hate riding on the highway----boring! I got behind a line of cars that were bunched up in the left lane and the right was clear. I was in 6th at about 75-80 and without downshifting I just rolled-on to motor past them. This was on a dead straight 1-2 mile section of slab. As I was accelerating past them I detected a slight wobble, continued to accelerate until the wobble became significantly worse to the point I was concerned it was going to go past the point of no return. I loosened my grip on the bars and relaxed my arms as I rolled off the throttle gingerly, things settled down and I continued about 1/4 mile to my exit at about 80-85.

Is this a characteristic of the duke? Short wheelbase, steep rake, light weight adding to this?

The bike is setup with Pirelli Supercorsas that have a trackday on them, the center of the tyre is fine, sides have significant wear but not down to the bars. Scotts steering damper that IMO is set pretty stiff. The suspension was just rebuilt by a reputable local suspension builder and I checked all the damping settings that are set to the "normal" range according to the manual. The sag was set for my friend (the P/O) who only weighs 10lbs less than me and it's got a set of aftermarket bars (forgot the make, Fatbars-something?) on it that appear slightly higher rise than stock. Other than that chassis-wise the bike is stock.

Anyone experience something similar that can point me in the right direction?

Take the damper off for now. It's only going to mask the real issue.

How far are your forks legs sticking out of the top triple? It's pretty common to push the forks down until they're just about flush with the top triple, mine sit 5mm above it.

The other thing is suspension. I would dial all clickers back to zero, count each click and see where your settings were. Check the preload on the forks and the rear shock, then get a friend to set your sag.

The only time I've have headshake issues is under hard acceleration where the front tyre is skipping on the ground or when my front tyre was cupped.

The 05-06 bikes were pretty prone to headshake regardless of your settings, they relaxed the rake in the 07+ models.

Forks caps are level with the top clamp. And yeah I was going to take the damper off. I was thinking that the damper is transmitting anything from the bars directly to the chassis and getting into a cycle. Kind of like what happens when you try to fight a tank-slapper. Honestly I've never had a bike with a damper set that stiff but I've also never had a bike with an adjustable damper either, just the stock unadjustable units. It's VERY stiff, as in even with the wide bars it takes a concerted effort to turn the bars from lock to lock at a standstill.

Per the manual I turned all the suspension damper clickers in all the way, and then backed them out to the "normal" setting in the range chart. My SMS 630 did the same thing at about 100mph, and I could alleviate it by sliding my butt back in the seat to get the weight off the front end, being a thumper sumo I hardly ever went on the highway with it.

The suspension on the SD does feel stiff. The suspension builder is a mutual friend of me and the P/O, I never checked the sag I just called him and asked if he'd set the sag for the P/O which he confirmed he did......I only weigh @10lbs more than him so I just assumed it was right. He mentioned that at the last trackday lots of guys were commenting that the rear tyre was moving around a lot on him and mentioned the swingarm was a noodle? Not sure if that's the case (does it have a rep for moving around/bendy swingarm?) and even mentioned having the S/A braced (not into doing that)

I'm going to put her up on the lift this weekend, take off the damper and thoroughly check a few things (rear tyre alignment, tyre pressure, static sag, etc) then take her for a spin.....is raising the forks in the tubes 5mm the stock setup? Reccomended?

5mm is just my personal setting. It felt like a good compromise for a quicker turn in on my lazy feeling sport touring tyres and corner stability while leaned over.

It sounds like your on the right track. What I mentioned were really the only bike specific quirks that would cause head shake.

Something definitely has to be off though. I've taken my bike up to 140mph indicated and it was pretty stable.

This thread might help:

Thanks man that was a good thread to remind me of things I'd forgotten about. I adhere to the theory of only making one change and then going for a ride to ascertain what effects the change made. If you make multiple changes there's no way to know which change did what.

Since I've already felt that the suspension was on the stiff side I checked the sag in the rear. It was set at 20mm for me on the bike which is a little on the tight side. I backed off the preload adjuster and got it to 35mm. Going to take a ride on Sunday (tomorrow is daughters big softball thing---all day event, band, contests, BEER).

I'm going to bring some tools in my backpack in case I need to make some minor roadside adjustments & take off the steering damper. I have a feeling the preload was cranked up too much. If it's still there I'm going to yank the damper mid ride.

I'll post up my findings Sunday evening from work.

Oh and I was mistaken.....my forks are 5mm above the top clamp too (2nd line)

change your tyres for fresh rubber

Definately on the right track setting static sag.
Start with minimal damping and increase till it gains some control. over-damped is bad for me, loosing feel and feedback. just watch how the tt bikes operate. Really live with just enough damping to control the wheels movement but not enough to stop it recovering for the next shock which will be only fractions of a second away.
Keep a log and go two clicks at a time to get close and then fine tune. Rebound first. depends on your riding style but for road work too much compression can reduces the effectiveness of the much needed suspension. You need just enough to stop squat and pogoing when opening the throttle exiting bends but not enough to restrict the range of movement, for smooth track work dial in more.
Obviously this is all my preference but I had the same problem when I first got my bike. I found the pre load was too high and conversely, which I cant really work out, I found increased stability from dropping the yokes 5mm, doesn't really make sense that bit but suspension only works well when both ends are in sync with each other and now I have achieved that and the correct sag my 05 is stable and fast steering, solid under power cranked over and still absorbs a fair bit of bumpy road, up to a point.
As with all things that are not adjustable on the fly it is a matter of compromise, keep at it

Ohh one more thing, you can't hold onto this bike with your hands, you will transmit wind buffeting to the sensitive steering and induce wobbles. Try to brace your torso and grip with your knees and just push the bars to turn with a light touch. The bike will go straight until the rider upsets it.

My 2008 SD 990 has never exhibited any sort of instability of the type or degree you are describing. I regularly ride this bike at speeds of over 100 and up to its top speed of about 130 MPH (gearing restricted).

Minor chassis wiggle over railroad tracks or minor front end wiggle when the wheel is light/airborne under hard acceleration on a rough surface is all I've ever experienced. Pavement grooving can cause some tracking changes, but never chassis instability. It has always been an incredibly predictable and stable bike.

I weigh approximately 220 in gear. My bike has no steering damper and is set up on the stiff side of the stock suspension's adjustments. It has 27,000 plus miles on it and the suspension has never been mucked with.

I also suspect your tyres' wear patterns or balance issues as prime culprits. Other strong possibilities include fork or chassis misalignment(s); bent rim(s) (but you should feel that at low speeds too); bad wheel bearings; loose or damaged steering head bearings; and other worn or damaged suspension bushing or bearings. Check all your suspension and wheel mounting hardware for proper torques too!

I logged on to the forum just looking for accessory seat advice for a new 1290 SD R, but saw your post and found it so scary I had to respond!!

Ok, took a ride today with the P/O. On the way to meet up with him I was on a meandering 2 lane with plenty of sweepers that on my R1 id have no problem hitting at triple digits. I tried different corner entrance methods to try and isolate when it would happen. On a corner that I had a very good line of sight I decided to set my entry speed (@70-80) early and go through at neutral throttle (not accelerating/not decelerating) and this is where I had the biggest issue. I was light on the bars (if you were able to grab my wrist whilst I was riding you'd be able to lift my hand off the bars with little effort, I always hold the bike with my lowers extremities) and just gave my input for the turn.

Completely unstable. The bike pogo'd clear to the other side of the road, slight pucker moment for sure.

So next turn I set my entry speed lower that what's necessary for me to comfortably negotiate the turn. As soon as I made my steering input I began to seamlessly get on the gas and accelerate through the turn.

Dead stable. Like telepathy the bike held a laser accurate line as intended

Then I tried to trail-brake into a slower, tighter corner. Slight chassis instability and it ran slightly wide than intended. Wasn't seriously bad but wasn't confidence inspiring either.

We had a chat about the bike. His cousin bought it new put @3k miles on it. Never been down. He bought it from him and put another 3k miles on it. The suspension was just serviced/rebuilt by a mutual friend who has a very good reputation and has services many of the club racers in the area. I asked about the head bearings and if they were ever replaced-services to which he replied that when his cousin owned it the steering head bearings became loose and actually clunk-clunked when on the brakes. (I think we have a suspect).

Based on my evaluations with different corner entry methods and my conversation with the P/O I'm going to check the head bearings. Anyone know of a supplier of timkin rollers for the SD? If I feel even the slightest of resistance or notchiness they're getting replaced. (I'll probably just replace them anyway).