Dirt bike question.

[irismary]

I have a 6 months old Rieju Tango 250 dirt bike/ enduro/ scrambler. It 'shakes' at 95 plus km/hour on tarmac. It's not a dangerous shake, it can be stopped by a firm hold of the bars, and it doesn't get progressively worse, but it is tiring, and spoils ones enjoyment.
I've had several bikes in the past, but they were road bikes.
I complained to the boss of the shop that sold it to me, thinking it was perhaps a manufacturing fault; he said it was because it had nubbly off road tyres, and that all bikes with them shook like that. The bike is very light ( one of its attractions), with a short wheelbase, and upright front forks; he says these all help the handling on dirt, but make the shake more likely.
Is he trying to snow me?
I can imagine that it would be far simpler for him to convince me that it is normal, than to enter into an investigation, with a possible fight with the makers on my behalf.
Apart from this problem, the bike is great, it goes like unwanted detritus off a shovel, and if it has lowered me to the ground now and then, it has probably been my fault.

[El Cid]

If it was a car it would be wheel balance. Is that something that affects bikes? If so it could be the reason for the problem.

Sid

[Nimrod]

Wot Sid said

[Colinm]

He could be right. A dirt bike is designed for off road and the settings for off road just will not work properly on road.
Presuming that wheel balance is ok, have you checked the steering bearings?
Is it new or used bike?
If its used then the previous owner probably set the bike up for his own use and preferences. If its new then you will have to set the bike up. What works off road will not work properly on road. If you do both, then its trade off time.

[scotty]

Check your tyres. Are they running true ? Have the wheels been balanced ?

[irismary]

Thanks for your thoughts on this.
Yes, we have checked the simple things, like tyre pressures, wheel tracking and balancing, but I have had unbalanced wheels, and they produced a vibration, rather than an oscillation of the handlebars. Also they usually do it at a specific speed, and you can drive through it.
The only thing that I didn't mention is that it is better if I sit as far back as possible, reducing the weight on the front wheel. This makes it different to a high powered bikes 'tankslapper', which I think is usually said to be caused by the front wheel lifting.

[irismary]

I've just been back to the UK for a week, and I popped into a motocross dealer, and asked them the same question. They said that most bikes fitted with nubbly tyres 'shook' on tarmac, though it is worth trying to lessen it by refitting the front tyre, to make sure that is correctly fitted on the rim.