Technical > Roo Man's Room

shut downn bounce

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wideopen231:
 so as not to highjack Dave thread on wheelie bars I figured maybe better section for question.

Mainly asking Roo Man and Bruce.What are yout thoughts as to main contributing factors that cause bounce in shut down.Obvious are of coarse lifting quick and braking hard w/o chutes.Anything in chassis setup that can help or hurt is main thing I would like to hear thoughts  on.

rooman:
If the car has a flexible frame and arches quite a bit or has a torsion bar etc, snapping the throttle shut and getting on the brakes at the same time can cause the frame to whip down to the limit of its travel and then bounce upward setting up an oscillation that quite often magnifies itself. Unless the track is exceptionally short it is better to let the car come down off the tire a little before getting on the brakes.
  On my last run in Horan's NT/F (which had a push brake) I must have clipped the brake handle when I hit the chutes and it slowed the wheel speed while the tires were still 220+ mph tall, putting some daylight under them. In that case the bouncing was lateral but it was still a rough ride for a while.

Roo

dreracecar:
If you get on the brakes too hard it winds up the tire, when the spring effect overcome the winding, it launches the car in the air, comes down and starts over again

digster:
Roo and dre. Have you seen the bounce problem worsen when chute tow line was not mounted at the correct height? I'd imagine pulling the hind end of the car up at deployment wouldn't help.

dreracecar:
I also have seen it pull the front end up. The problem is that its not consitent, most of the time not , but then it does it. Some tracks have a horrible shutdown surface which only adds more to the problem
  Because what we do is not NHRA S/C where all the cars are the same, To really find out where the tow line should be mounted for any particular car or chute design(YES that does affect it) a 3/8" cable should be mounted to the top of the cage and lower frame rail, the tow line should be on a pulley that rides the cable. at the end of the run with the chute deployed a video would show the optimum mounting posistion by drawing a straight line down the tow line to the back of the chassis.

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