Why does bike do this?

[DougRoost]

I usually ride my KTM 300exc but last weekend rode my friend's '01 CR250 and noticed something different going over a jump. My bike jumps straight and true. But when I jumped the same jump on his CR250 the rear of the bike would hang out a bit to the right as I went in the air. When I brought this up, he said it did the same thing on him and always has.

I did notice watching the Unadilla outdoor national last weekend that all the fast guys' bikes did the same thing coming off those big jumps. They just gassed it before they hit and it seemed to straighten out.

Anybody know why this happens? I was guessing it's due to the torque of the rear wheel, but the "right hand rule" we learned in physics would say that would push the rear left, not right. It also doen't explain why that doesn't happen on my KTM.

 

[High Lord Gomer]

Bikes shouldn't push one way or the other in the air. Most likely causes are:

• Weighting one peg more than the other.
• Still turning when hitting jump.
• Uneven jump face.
• Misaligned rear wheel.

It's hard to use what the really fast guys do as a comparison. Most of them are whipping the bike in the air to help scrub off height so they can get back on the ground (and accelerate) quicker. Some jumps it is preferable to whip left, some right. Depends on the approach and what follows the landing.

 

[Shig]

Misaligned rear wheel.[/list]

Dang it Gomer...let someone else be the smarty pants for a change ;)

 

[DougRoost]

Thanks Gomer. Since this doesn't happen to me on my KTM, and since my friend just had his swingarm bearings repacked at a local shop, we'll doublecheck the wheel alignment. I know sometimes the little guide marks in the back of the swingarm aren't exact, so maybe that's what happened.

See you again at DirtWeek this year!

 

[High Lord Gomer]

Dang it Gomer...let someone else be the smarty pants for a change ;)

Hey, I just grab the easy ones...I leave the tough ones for you fast guys!

 

[markthomps]

This is just a WAG*, but it could be the difference between the KTM's linkage-less suspension system and the link-type suspension on the CR and all the other japanese brands.


(*wild-ass-guess)