Headshake/mild tank slapper issue - Triumph Forum: Triumph Rat Motorcycle Forums
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post #1 of 15 (permalink) Old 10-13-2019, 08:06 PM Thread Starter
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Headshake/mild tank slapper issue

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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I probably need an intervention because of the bikes that keep 'following' me home. Current garage:
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post #2 of 15 (permalink) Old 10-13-2019, 08:23 PM Thread Starter
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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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I probably need an intervention because of the bikes that keep 'following' me home. Current garage:
2001 Daytona 955i, 2 yes two, 2003 Sprint 955i STs (spouse rides one), 2016 Speed Triple, 2001 Kawasaki ZRX 1200R
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post #3 of 15 (permalink) Old 10-13-2019, 10:17 PM
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Glad we could be of help!
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post #4 of 15 (permalink) Old 10-13-2019, 11:08 PM Thread Starter
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Haha! It's one possibility

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-- Scott

I probably need an intervention because of the bikes that keep 'following' me home. Current garage:
2001 Daytona 955i, 2 yes two, 2003 Sprint 955i STs (spouse rides one), 2016 Speed Triple, 2001 Kawasaki ZRX 1200R
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post #5 of 15 (permalink) Old 10-14-2019, 12:41 AM Thread Starter
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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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-- Scott

I probably need an intervention because of the bikes that keep 'following' me home. Current garage:
2001 Daytona 955i, 2 yes two, 2003 Sprint 955i STs (spouse rides one), 2016 Speed Triple, 2001 Kawasaki ZRX 1200R
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post #6 of 15 (permalink) Old 10-14-2019, 01:04 AM
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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 Iím 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 itís cold out itís 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.

GO CHIEFS!!! Iím with Mahomes!
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post #7 of 15 (permalink) Old 10-14-2019, 06:37 AM
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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.

"I just want to ride" Joe Satriani, Flying in a Blue Dream, 1989
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post #8 of 15 (permalink) Old 10-14-2019, 12:30 PM Thread Starter
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Dave, just the heavier spring from RaceTech.

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-- Scott

I probably need an intervention because of the bikes that keep 'following' me home. Current garage:
2001 Daytona 955i, 2 yes two, 2003 Sprint 955i STs (spouse rides one), 2016 Speed Triple, 2001 Kawasaki ZRX 1200R
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post #9 of 15 (permalink) Old 10-14-2019, 12:52 PM
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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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post #10 of 15 (permalink) Old 10-15-2019, 10:47 AM
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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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