Rear wheel drifting right during jumps

[Jasle]

Well Jr. crashed hard and I was right behind himduring the jump. He came up at a double about 50' he changed lines coming from left to right but seemed to straighten out before actually hitting the face. Immediately as he left the face the rear wheel started to drift to the right and when he landed he was about 45º to the track and high sided. Now I've seen him bring the wheel around like that before several times and I sorta thought he was trying to whip it on purpose. During this particular jump I thought the rear wheel drifting was due to the change of lines and maybe some momentum being carried to the right.
Someone suggested that the gyroscopic effect of the wheel could cause this if the rider was to stay on the throttle during the jump? Could this be a cause?

The rider is a 9 year old 72lbs rider on a 65

 

[Chili]

When my son was on 80's his KX did that. We looked at sprocket alignment, wheel truing, suspension setup and never did figure out why it kicked to the right everytime he jumped.

 

[oldguy]

That is what caused Spider to crash and bust his collarbone a couple years back. At the time Jer Wilkey made an adjustment to his rear shock and removed the problem. My problem now is that I cannto remember which adjustment he made. I'll look thru Eric's book and see if Jer mentioned it in his susp section.

 

[High Lord Gomer]

Not enough rebound damping can cause the rear of the bike to kick more, but that would happen both left and right depending on if you were leaned slightly either way on takeoff.

If he is consistently whipping to the same (right) side and doing so on multiple jumps (some always throw you one way or the other if they are out of a turn), my best guess would be that he is putting more weight on the left footpeg.

The most common causes of getting sideways are turning on the jump face, hitting a rut (even worse is having the front tire in one rut and the rear in another going up the jump face...the bike then swaps in the air), and loosing traction on a slippery jump face. Less likely would be having the rear wheel misaligned or a flat tire.

 

[Jasle]

bummer so I'm not homing in on a route cause. I'll have the suspension guru here watch him ride and see what he thinks. thanks guys.

 

[High Lord Gomer]

Was this a one time incident or does this happen to him often.

Going back and re-reading it, it sounds like he was still turning/leaning left a bit and that would cause the whip to the right.

One good thing to help keep the bike from kicking/drifting on takeoff is to squeeze the bike with your/his legs all the way up the jump face and into the air.

 

[Radiator Ron]

Dont know what 65 you have but if its kx/rm they are terrible. I had to respring and revalve my boys bike, problem solved. My experience on bikes going sideways even on smooth faces. 1 springs too stiff bike not setting into face, 2 face slippery, 3 application of throttle, either accelerate and chop or use at least 1/2. Changing springs should solve your problem of kicking sideways, if its rm/kx i'd revalve it. I used racetech goldvalve for shock, emulators in the front forks, springs for 75lbs. My son actually increased @1/2 of a lap distance on 1.5 mile track during moto (@10th to 3rd), and stayed there consistantly. ktms look seem to have good valving i'd just respring and try it.

 

[Jasle]

we're riding the ktm 65. I thought this might be a one time thing but i have seen him drift right many times. this is the second wreck where he landed sideways and highsided. He's usually good with pinching the tank but I have not paid attention to while he's jumping. He can whip the rear on command and does so sometimes and he can birng it back. I asked it he was trying to whip and he said no. We'll have to see when he comes back. I'm going to video some jumps from the side and behind to see if we can spot anything. It might take some time cause I know he's going to be skittish for a while. Its one of those things I'll just have to let him do when he's ready to do the jumps again.
It was the last moto of the day and the track was packed hard. He might have gotten some wheelspin. He did the jump most of the day in 3rd and was coming up about 1 foot short. He tried it the last 2 times in 4th and was still accelerating when he left the face. New tire 14psi. Stock spring with about 3" of sag.
As far as the suspensions we've ridden the KX and immediateley we needed suspension. We also have a Cobra DC65. Its got the motor but we're still sorting the front suspension. He rides the KTM because he feels most comfortable on it. We barely had to change anything suspension wise. Just fork oil weight.

Thanks guys.