Need help with wobble problem

[Derby Monkey]

I need your help...

My son's car finished 1st overall in his pack race yesterday. His car was easily faster than the other cars there... however, there is a problem that I must work out before our district derby.

Yesterday, the car raced on a 32 ft wood track. The starts were very clean and the car jumps to an early lead. However, during the last 10 feet of the run it develops a severe rear end wobble. The back of the car noticeably wobbles quickly from side to side like a fish tail. Several other folks noticed it also and made a comment.

My test track is a 20' Best Track... so, I never noticed any wobble. Yesterday, on the 32 ft track, the wobble started after the 20 ft mark.

I am convinced that the wobble slowed the car down... just not enough for him to loose any heats though. However, I must rectify this problem before we go to district or I'm afraid it may cost us a victory there.

Any thoughts???

Thanks,
Steve

[khouse]

I'm pretty new at this but it would seem your car fishtails at slower speeds? Why not start your car half way down your test track? Is your COM toward the front? What does your car weigh? How are the wheels and axles prepared? I would also think that maybe your front wheels have more friction compared to the rear would make it fishtail too? I believe a lot of problems are fixed with a rail rider. I know it helped me.

[Derby Monkey]

Thanks for the response!!!

The CG/COM is pretty aggressive at somewhere around 11/16" and 3/4" in front of the rear axle.

The alignment allows the car to turn in toward the rail 1" over 6'.

Rear axles have 2.5 cant and installed for neg camber.

The dominant front axle has 1.5 cant and installed for pos camber.

I just ordered an extension kit for my 20' test track. So now I can test at 39'.

More thoughts???

Thanks,

[khouse]

Yes. The more aggressive your COG the more you need to steer into the rail. I would suggest at least 5 inches in 6 ft. I have to run some of my cars at 4 inches in 4 feet. But I'm sure 1 inch in 6 feet is too little. Does you rear wheels gravitate to the head of you axle in forward AND reverse? I used to think that just forward was good. But it's not! If it doesn't work BOTH ways your rear wheels are scrubbing the tread all the way down the track.

[derbydominator]

You definitely need more steer. The minimum is 1 1/2-2" over 4 feet.