99 CR80 wants to tank slap

[VintgeRiderVintageGuy]

My son has raced for a while on a KTM 65 but just moved up to a 99 CR80. First off, the bike has huge hit (no powervalve) but that's another story.

When he was riding it across an open field in 3rd or 4th gear, it wanted to tank slap (i.e. wobble the bars rapidly). Naturally he is concerned and so am I.

Does anyone know what could cause this and what I could do to correct it?

Thanks

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Me - 94 RM250, NOT good
The boy - 99 CR80, getting better

 

[c3hammer]

It's pretty easy to fix, unless you have something broken.

Reduce the preload on the rear shock spring to lower the rear ride height in the back. Lower the fork tubes in the tripple clamps. Or both of the above.

These will reduce the steering / head tube angle, making the bike much more stable. More like a chopper but much more subtle.

Solve the hit issue by adding two or three teeth to the rear sprocket. This will get him running up on the pipe all the time and away from the pop of it coming on the pipe.

I went through all these same things with my boy when he switched from a PW80 to a CR80.

Good luck,
Pete http://www.verts.com/slickrockfeb01/ One year later!

 

[VintgeRiderVintageGuy]

Yeah, another guy suggested setting the race sag, which involves setting the spring preload. It's supposed to be 3.3 inches, it was 2.1. That means it was set too stiff for my flyweight son. Not surprising, the kid we got it from moved up to a 125 and my son came off a KTM 65. Now it sits a little lower, which he likes anyway. We also bleed the front brake and put Nstyle graphics on it last night. He's ready to rock, we just need the weather here in Central Illinois to cooperate, and that might be a while!

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Me - 94 RM250, NOT good
The boy - 99 CR80, getting better

 

[VintgeRiderVintageGuy]

I checked out your web site. Where are those pictures taken?

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Me - 94 RM250, NOT good
The boy - 99 CR80, getting better

 

[c3hammer]

Vintage:

That's on the Slickrock Trail near Moab, Utah. The first picture of endless slickrock is looking north toward the Colorado river and Arches National Park.

1.1 inches is absolutely huge! No wonder your boy was having trouble. 5 or 10 mm will make a substantial difference in handling. Good catch on finding that.

The NStyle graphics could have been a bad choice! Graphics make kids think they're bullet proof. Well, adaults too, I suppose. Next thing you know your boy will be hucking 50' table tops and such. Hopefully the first big crash won't trash the graphics too bad

Been there seen that :0

Pete

 

[Jeremy Wilkey]

VRVG,
I agree with pete but also try to reduce fork compression.
Regards,
Jer

 

[zsr]

I'll just add that the stck springs are probably WAY too stiff if he's just come from a 65. My son has always been real big for his age but we ran soft springs to cure the slapping problem. We had the advantage of a suspension guy spend the day at the track with us (it didn't cost much more that sending the stuff away for someone to set up 'blind'). Some people also I think run an extra base gasket which reduces the compression a little, but you should check with a local tuner that this won't upset the ports or cause the ring to snag.

Going to the track with that suspension guy was the best money I ever spent on the bike.

Good luck to your son.

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07