Tank Slap

zsr

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#1
01 CR250. Under hard acceleration my son is experiencing viscious tank slapping. We are running the 20" front, which he says makes no diference to the bikes handling AT ALL by the way (more money wasted - looks cool though), though the bike was the same with the 21". The front has been resprung / revalved by the U.K.s top bloke and the stock shock set up as per his recommendation.

It is probably a question of fine tuning I'm sure, but we seem to have been round the block several times and cannot cure it. He can rip the local practice track apart on his CR125, but the 250 just scares the ertha kitt out of him every time he gets on the gas.

Any suggestions much appreciated.

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98 CR125 01 CR250 99YZF R1

Thanks to: Motorex / D.I.D. / Vesrah / YES Helmets
 

MX265

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#2
How exactly are you reffering to "Viscious tank Slapping"? This usually occurs to a guy when he clears a very large jump and his feet slip off of his pegs when he lands. This often leads to excrutiating pain for hours if not days. Also tends to elevate the vocal tones for awhile. I sure hope this is not the tank slapping you are reffering to.
 

zsr

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#3
Aha, obviously different terminology this side of the water, I'll remember that next time I'm in California!

The bars turn violently from lock to lock under hard acceleration. The 'instinctive' thing to do is back off though I suspect the right thing is to ride through it. As it's my sons neck not mine, I'd rather cure it that make him ride around it.

I've heard it referred to as headshake too - but here thats when you take a piss!!

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98 CR125 01 CR250 99YZF R1

Thanks to: Motorex / D.I.D. / Vesrah / YES Helmets
 

MikeT

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#4
One thing to do is to slide the fork tubes down as far as possible in the triple clamps. This will help remedy the headshake. I would also check the rear sag. If there is not enough race sag, say 95-100mm, then the rear will sit high and pull the nose in. That will also help add to the shake problem. Start with thr tubes though.
 

zsr

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#5
Thanks Mike T

The forks are already right thru the clamps (so front rides high) , sag without rider is set at 25mm, with rider its about 97mm.

:(

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98 CR125 01 CR250 99YZF R1

Thanks to: Motorex / D.I.D. / Vesrah / YES Helmets
 

MX265

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#6
The steering stem has been greased extremely well and is tight as well corerct? It was tightened with the top triple clamp bolts loosened first, correct? There is Zero steering head play? If so, is there any kind of play in the front wheel bearings?

Just a few simple things that are often overlooked. If I see anything on a 2001 CR250 reffering to this issue, I will post it here.
 

KawieKX125

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#7
The tire could be dryrotted or have the wrong pressure, wrong spring, wrong oil level, it can be ALOT of things. Also, no difference with the 20, i've got one on my 99KX125 and it made a night and day difference. I can't get it to wash out or slide. What conditions are you using it in, it sucks in mud.

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Aaron's web site
 

zsr

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#8
The bike has only been ridden 5 times from new, headstock was regreased before the first ride, but bolt wasn't tightened with triple clamps loose. Didn't know that and will recheck.

Tire is brand new M59 in mud just soft enough to leave a print without sticking (perfect?), pressure set at 14 psi. My son liked the bikes ability to turn even with a 21", we switch to the 20" hoping the front would be a little more compliant. Still running his 125 with a 21" which flies, no problems.

A national class rider was at the track today, rode our bike and concluded it was more likely handlebar / (therefore) body position. (????????????). I was trying to avoid the "which bars are best" issue, as I'm sure thats such a subjective thing.

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98 CR125 01 CR250 99YZF R1

Thanks to: Motorex / D.I.D. / Vesrah / YES Helmets
 

marcusgunby

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#9
What is your sons weight?What mods were done to the front forks?Im thinking the std rear spring may be too stiff.
 

zsr

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#10
He's 16 years old, 175lb. The rear spring is stock, sag set to 97mm (25mm without rider) which seems right (if working by numbers). On big jumps where his 125 bottoms badly, he feels the 250 is spot on. He says the front seems to 'skip around' and when hard on the gas he gets the 'slap' I have described.

The forks were given the next softer springs from stock and revalved. I can't detail the revalve but it seems to be a standard job here on the 01. The bike is used for MX only, no trail stuff.


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98 CR125 01 CR250 99YZF R1

Thanks to: Motorex / D.I.D. / Vesrah / YES Helmets

[This message has been edited by zsr (edited 04-02-2001).]
 
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#11
I just fixed the same headshake/front end hunting in the lower stroke on my 250sx today.I had to increase the fork compression.I set the damping a little harder than I felt it really needed.When we got to the track today,it was really whooped out.Big rollers and short choppers all over.It never felt out of control and stopped"hunting its own line". Hope this is some help.

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KTM250sx(mine)
BSA B50MX 500cc(mine)CR80expert(son) MX Madman

[This message has been edited by MX Madman (edited 04-03-2001).]