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Discussion Starter #1
During my recent road trip, I experienced something that I hadn't before, on my Sprint. When I approached semi trucks from behind, the bike would develop a headshake, and make the bike "wag." If I moved over to pass, this basically went away. I could repeat the effect consistently.

I am not sure if it is due to the changed geometry of the bike with the USD fork conversion, stiffer springs, or the fork or shock settings on the bike. I DO have the rear eccentric flipped (lower) since the fork conversion also lowered the front.

This only happened when I was in the turbulence of a semi truck.

Any ideas?

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Discussion Starter #2
Now I'm thinking the rear sag wasn't correct, since I set it without the weight of loaded saddlebags.

According to G.O.A.T. Dave Moss, this could be a cause.

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Discussion Starter #4
Haha! It's one possibility

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Discussion Starter #5
Still thinking about this. More particulars:

Tire pressures
Front: 36psi
Rear: 38psi

I'd have run 40 but it was near 0C, and I wanted some compliance

Rear preload was at 14 clicks in, rear shock 3/4 turn in. 1.0kg rear spring.

Bags had about 8# in each of them

I haven't experienced anything but super stable riding until I had weight on the rear

I'm more and more leaning towards settings on the rear. Thoughts?

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Possibly the turbulence from the truck and the rear weight of the bike loading that drafting the truck caused front end to lift up its stroke. I know when I’m changing lanes behind big rigs I get a little lift. Back in the day I would draft a truck up close in the still air. Ballsy but when it’s cold out it’s nice. I guess you can try jacking up the rear on next ride loaded and see if that helps. I just got back from Washington and between the stiff cross wind and the big rig turbulence, I would bounce about until I cleared the truck. Can be a butt pucker moment from time to time.
 

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Sorry for coming in late... what work have you done to the rear shock? That was the diabolical element in my ST's handling, especially when loaded.
 

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Discussion Starter #8
Dave, just the heavier spring from RaceTech.

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Try the standard spec 42 psi in the rear tyre and see if it helps. I've noticed really weird handling when the rear pressure drops off just a little, like when it's set for summer temps and then suddenly turns cold.

But 38 psi just seems too low anyway for any use other than a track day.
 

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MHS, it really sounds like the rear was 'low', and the steering geometry magnified the buffeting from the truck's wake.

If you can't get the rear high enough, you might lower the front tubes 5mm or so in the trees.
 

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MHS, it really sounds like the rear was 'low', and the steering geometry magnified the buffeting from the truck's wake.
That was certainly my experience when I first rode the 1050T loaded - I almost sold it, but bought an aftermarket rear shock instead. Very happy now.
 

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Scottie, get in touch with Tmod. He's in Idaho so logistics will be easier from your neck of the woods than from here in Cow Hampshire. Terry knows what he is doing with suspension and is especially talented with Triumphs. My Sprint was like two different motorcycles before and after his magic.

And believe it or not, the stock rear shock has very good potential with custom valving. He can probably do both ends of your bike for somewhere around the cost of an aftermarket rear shock alone...and it will be better.
 

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Discussion Starter #13
I've had Terry do work on my bikes before. It's looking a lot, by people's comments, that the rear is the issue.

I am going to go through each possible one at a time. Air pressure, preload, rear height.

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Scottie - I suggest that you don't ignore/underestimate aerodynamic effects, which may be significantly impacted by your suspension changes (e.g. lowering). When I first bought my 2003 Sprint, the previous owner had installed a Laminar Lip on the stock windshield. He was much taller than me and needed the extra wind protection. When I rode the bike, it handled normally until reaching approximately 68 mph. It would then "buck", causing the front end to "pogo" up and down as if there was a large weight imbalance on the front wheel. Removing the Laminar Lip eliminated the problem.

I've personally never experienced anything similar on other bikes, even when I was involved in land speed record attempts. I have come to believe the Sprint is particularly sensitive to aerodynamics. Hope you get the problem sorted.
 

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Here's what I would do:
1) find a place without cops and duplicate the riding conditions but without all of the weight in the luggage.
2) if it still does it, yank the luggage and duplicate the riding conditions
3) if it still does it, adjust the steering stem
4) if it still does it, take a look at the swing arm bearings
5) if it still does it, try raising the rear end some
6) if it still does it, try lowering the rear end some
7) if it still does it, try dropping the front end some
8) if it still does it, give up before you kill yourself and buy jer5ak's bike. You know you want to.
 
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