Mike ,
A test you can easily do to check the shockers (but not an accurate test , only an indication).
Bounce the car by pressing down in the bumper bars at each corner of the car .
If the body goes down a few inches and comes up quickly and bounces uncontrollably the shock absorbers probably need replacing .
The rear shocks being Koni should be adjustable . I you press down on the LH & RH side of the rear bumper bar you should not get much more movement than about an inch . Anything between 1/2 an inch to none , points to the Koni are set too hard.
I have Koni on one of my 400 and they are set quite soft .
The front shockers need to be set quite soft as the leaf spring is quite hard.
The rears should be set a little harder as the torsion bars are quite soft .
My GUESS is that the rear Koni could be adjusted way too hard.
I don't know the Joyroad tyres that are fitted but you should check and adjust the shock absorbers before you consider buying tyres.
I don't think your tyres are the cause of hard riding especially at 28psi
I inflate my 175/80 Austone Taxi tyres to about 35psi and the 400s ride smoothly.
Check with a tyre retailer for a recommendation of tyre size for the 406 with 4.5 wide rim.
As I mentioned in my earlier post the 175 is the ideal size and 185 will work OK but they are a little to wide .
Geoff
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