Tyres or wheels being non concentric with axle.

Tyre walls having hard and soft spots.

Tyres or wheels being out of balance.

Regards
pat   pprimmer@acay.com.au
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Many of the problems indicated by Pat can be the result of poor inflation.  My 4Runner's tires got VERY lumpy after running underinflated for a while

TTFN

Sen:

Depends what you mean by 'bumpy'. What sort of vehicle is it, and might it have been 'modified' by some previous owner who thinks that rock hard suspension makes you go round corners quicker?

Harsh, jarring ride (even on a good surface) can be caused by seized dampers, or over-firm adjustment of dampers or hard springs. Low profile tyres increase harshness, too.

Could be any of the tyre problems Pat mentions but the bump will be very regular (every 2 metres or so, depending on the rolling circumference of the tyres).

Increasing damper forces will help if the 'bumps' are long, slow, high amplitude inputs from a wavy surface. Otherwise, in my opinion, it will make things worse.

Cheers - John

Thanx all..
Well, the vehicle is a minibus (7.5Ton GVW) brand new vehicle.
By bumpy ride i mean the vehicle oscillates up & down once it crosses a bump..
Should i increase the shock absorber damping.. if so is there any rule to relate suspension stiffness & damping?/ (Its a rigid axle suspension-Shackle ended)

Cheers
sen

That is not, typically, described as a bumpy ride.

Anyway, yes, you should look at the shock absorber tuning, but you might also find a mismatch between the front and rear axle spring rates as well.

Check out Olney's book on ride and handling for a description of the 'flat ridw' criterion and related subjects.

Cheers

Greg Locock