My bike, 2018 Bobber with 11k miles has developed a rear/driveline harmonic vibration that occurs from 50-60 mph and can be felt in the seat. The vibration comes on and off like a heart beat from 50-60 mph and in 4th gear.

The odd thing is that the problem developed just as I put a new tire on the rear. Tying to rule out a balance issue, I have had the tire balanced 3 times. The last time the tire was broken down and rotated on the wheel. I still don’t like the fact that this tire is taking far more weight to balance than any previous tire. Part of me still wants to believe that the coincidence of the vibration and the timing of getting a new tire points to the fact that I may have a bad tire.

The odd part about the vibration is that it is only in 4th and only in the 50-60 mph range. It is quite noticeable. It is present both on and off of the throttle but seems to disappear when the clutch is engaged. All bolts are tight, swingarm has no play, chain is in spec, and wheel is perfectly aligned. I am wondering if it is possible that a bad tire is being felt at this speed/rpm because the harmonic vibrations coincide with what the engine is doing in this gear/speed. I am tempted to purchase another new tire to rule this out. Worse case scenario there is that I have a another tire for the next time that I need one.

I am afraid that if the wheel/tire is not the issue, I may have a problem in the tranny with 4th gear. That would not be good.

Has anyone heard of any tranny/4th gear problems with this generation of motor? I wish one of you guys was close to me. I would try and talk you out of your rear wheel for 15 minutes so I could rule out where this is coming from.

You say the vibration disappears when you pull the clutch in? That would seem to rule out the wheel or drive.

Do you have a paddock stand? if you have, get a piece of chalk, use the end of the rear fender to rest the chalk against, then rotate the rear tire without letting the chalk move up and down, you should be able to tell, with in a few rotations, if the tire is round and running true.

That is a great idea! Never thought to do that.

I replaced the tire to rule it out. The previous tire took a lot of weight to balance and the new tire balanced with minimal weight. While it actually feels better, the 4th gear harmonic vibration remains. The dealer also acknowledged the presence of the vibration. The bike will go in for service in a couple weeks to try and find the issue. In the mean time I am riding it to see if it gets worse.

Bike is in the shop. They can’t seem to determine the cause the vibration yet. We will see.

Mine did same (80-100kmh and only in 4th gear), service loosed chain and all gone

Update:

Bike is still at the dealer and I guess they are scratching their heads at this point. They cannot find the source of the vibration. I am told that they are talking with triumph about the issue.

They swapped the chain to rule it out. No difference.

If you feel since 80kmh - 110kmh in 4th gear(and no other speeds) 1-2 second interval vibration from handlebars/foot begs and has quiet wooz wooz wooz sound. This is too tight chain, it must more loose than i tightened(on any other bike i never had tightening issue).
It's easy to rule out, make your chain pretty loose and try out.