So my bike had a almost new Michelin Pilot Active in the rear and for some unfathomable reason a brand new Avon Road Rider in the front when I bought it last autumn. The bike has not been ridden more than 400 km since so the tires are still fresh.
Now a month ago I noticed this headshake. When i run at 80 km/h in a straight line, let go of the bars and let it coast to stop the shake starts at about 70 and gets progressively worse. It starts gently to oscillate at about 1 HZ and and the angle from the centerline gets bigger and bigger as the frequency increases until I have to grab it. As I grab it it stops, there is no real "force at this speed so it's not hard to stop it.
The bike is rock steady at speed and with the hands on the bar. I tried to run over bumps while tilted over in order to provoke it but nothing happens, only in the situation I described.
I took it to a Yamaha dealer and the mechanic said the steering head bearings were ok but he blamed the tire. I decided to fit a new Michelin Pilot Active to the front (another shop did it for me).
Now when I go up to 90 km/h and let go of the bars the oscillation starts at just below 80 km/h but much more gently and when speed drops to just above 60 km/h the bike sorts itself out without me having to lay my hands on the bars.
Here's the thing though. The shop that mounted the tire had inflated it to 2.9 kg. The bikes manual says 2.0 rear (2.25 if the rider is over 90 kg which I'm not, I'm only 72 kg) and 2.0 front. Michelin says 2.0 kg rear and 2.0 kg front no matter what.
Thinking that was excessive I decided to drop to 2.5 kg. just to see what would happen. Now the bike behaves as bad as with the Avon if not worse.
The first time I owned it I rode over 10 000 km with it and it never did this. Coupled with the fact that the Diversion/Seca is said to have a gentle nature due to it's chassis geometry I'm having a hard time to believe it's inherenet in the design as some claim.
