For a while I blamed this on a springy suspension, but I've noticed that even on perfectly flat pavement that I'm seeing an amplitude modulated vibration when cruising around 50kph (~30mph). The vibration is ~4 Hz and modulates at about a 2 second interval. At low and high speeds, the vibration goes away.
I'm guessing it's a due to the planetary gearbox, because I can't imagine how a simple wheel balance issue would be amplitude modulated. Has anyone else seen this problem?
The vibrations which I experienced were due to PG Box problems.
They were/are synchronous with wheel rotation frequency.
Your vibrations seem different because the wheel would spin about 14 times between peak intensities.
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There is always a way if there is no other way!
I definitely think this is due to wheel balance now. Given the average wheel circumference, I should get resonance at 23 & 46 kph. I believe the modulation at 2 second intervals is due to the slightly different wheel sizes. It's kind of like the beating effect you get playing two notes that are slightly out of tune.
If balancing the tires fixes the problem, great. However, I firmly believe it won't change the gearbox vibration. I spent a lot of time (and money) fixing this and the result isn't worth the price of the fix. It has to do with machining tolerances within the gearbox. Some Vectix scooters are better then others. In trying to reduce the noise from the gearbox, I found machining errors in the planet carrier, Improper backlash in the geartrain and out of round ring gears. Making new parts or re-machining the existing parts made the gearbox run smoothly (no vibration) but did not lower the noise level enough to make it worth the time and effort. There are several older threads where all this has been covered. BTW the gearbox vibration is at a much higher frequency then tire vibration.
So just to follow up on this, I replaced my original Pirelli tires with some Michelin Gold Standards this week and the original amplitude modulated vibration I was seeing when cruising around 50kph is almost completely gone. The front end wobble is also greatly reduced. Overall, the ride seems *much* smoother.
While I was only able to balance the front wheel (since the rear has no hub when removed), I'm suspecting that this was more than just a balance issue. Vectrix issued a Technical Service Bulletin (NBTB0807-Rev 01) that reported, "It has been found that the front tires on some scooters may have deformed slightly during shipping. This deformation can result in varying degrees of front end wobble." The recommended fix is to replace and balance the front tire.
Sure enough, when I examined my front tire closely, I found some ballooning in the tread. I didn't get a chance to look at the rear one that closely, but I'm betting that was the cause. While I'm disappointed Pirelli and Vectrix didn't have better quality control, I'm more relieved that it didn't result in a blow-out.
Bottom line: If you're experiencing this vibration or pronounced front end wobble, replace your tires ASAP.