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 Post subject: Re: Mid Track Vibration.
 Post Posted: Tue Apr 20, 2010 11:10 am 
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Posts: 400
not sure Shrinker havn't been able to run it yet, I'll be running it mid May. Basically the car bounced hard in the shutdown under too hard braking (my bad) and it just snowballed bouncing harder and higher side to side. I took my hands off the wheel waiting for it to roll on the next hit but it didn't. Firecrew said I bashed my head off the cage pretty bad and they thought I was going to go end over. In doing so, the car bottomed out, buckled a wheel, screwed a tire, bent a halfshaft, took out the belly pan, hit the trans pan on the ground which bust the mid plate mounts and flattened the bottom rails. It's all been jig repaired and replaced so hopefully the bouncing is due to the ociliation in the tyre induced by the excessive eccentric hub rotation.

I was running 8.5's at 150mph in a hardtail 125in wheelbase with very little wing, and with hinesight should have been pulling the chute. I've since changed my chute release to a simple throw lever as previously it was a T-Bar pull on the cage and no use to me in an emergency. They are a bitch to drag along in the shutdown and to pack, so for the time being i've throttle it back to run 9.5's, want to start enjoying racing again instead of chasing problems ! Once I run the car and feel the bouncing has gone, I'll build back up to running it flat out again. Cheers for the input guys.

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 Post subject: Re: Mid Track Vibration.
 Post Posted: Tue Apr 20, 2010 11:30 am 
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gearhead1011 wrote:
Wow Ken that's a lot of air. I run 5 in mine. M/T told me that the lighter the car the less air it will require. For my car they they said I should be around 4-6 lbs.


heres the MT recommendations on pressure http://www.mickeythompsontires.com/tech.php?bulletin=s3 From the testing I've done on my altered, 5psi and below induced some sway mid track within the sidewalls, above 6.5psi it got a little too bouncy in the shutdown and on poor tracks prone to spin on the high gear shift.

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 Post subject: Re: Mid Track Vibration.
 Post Posted: Tue Apr 20, 2010 12:01 pm 
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What I found on the bounce thingie was that if you snap off the throttle quickly then get on the brakes quickly that induced the bounce on my hard tail diggers. I learned real quick to "ease" off the throttle then wait until the chassis completely unloaded before getting on the brakes. Bob Boulton (car builder) told me that I should always use the chute first but that just isn't an option for a bracket car, especially in later rounds.

Yeah Chuck, 10.5 is a lot but that's what it wants? Tried lower pressure but short times went down and did the same with more than that. These tires are 14" wide on 15" wide wheels so that may be the reason it likes it.

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 Post subject: Re: Mid Track Vibration.
 Post Posted: Sun Apr 25, 2010 11:02 am 
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Ken0069 wrote:
What I found on the bounce thingie was that if you snap off the throttle quickly then get on the brakes quickly that induced the bounce on my hard tail diggers. I learned real quick to "ease" off the throttle then wait until the chassis completely unloaded before getting on the brakes.


I think that's sound advice for any type Drag car.

Don

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 Post subject: Re: Mid Track Vibration.
 Post Posted: Mon May 17, 2010 7:31 am 
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well it seems the problem is now fixed. I ran the car this weekend and did 9 runs over 2 days. The new halfshaft, wheels and balanced tires have sorted it out and it now pulls hard and straight all the way thought. Shutdown is a lot more stable. The motor is limited to half pedal travel and it's running between 9.27 and 9.35 @ an average of 140mph. This seems to work best for me as it calms the 60ft a little and has dropped the MPH so I don't need to use the chute. Plus most other cars who I run in the Wild Bunch are low to mid nines so the racing is reasonably tight. On the flipside when I run Pro ET Brackets with an index of 11.99 to 9.00, I'm the chasing car so have an advantage there too (plus the first or worst foul works in my favour ie, I can see my opponent red light)

One issue I did have was with front wheel shimmy at low speeds, coming out the burnout, and at about 30mph in the shutdown and on the return road back. I stop it by dropping into low and gunning it and it goes, braking seems to make it worse. We checked the toe and it's still running 1/8in toe in. Current theory is that the steering box has some slop in it and it's being amplified by the front wheels and any slop in the drag link rods ends etc. But all in all happy the cars running without bouncing and I've got lots more confidence back in the car.

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 Post subject: Re: Mid Track Vibration.
 Post Posted: Mon May 17, 2010 9:05 am 
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If that has front suspension you need to check the bumpsteer.

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 Post subject: Re: Mid Track Vibration.
 Post Posted: Mon May 17, 2010 10:04 pm 
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A VW steering damper is light weight, small and not bad looking, it may be just the thing to cure the shakes. Worked for me.

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 Post subject: Re: Mid Track Vibration.
 Post Posted: Tue May 18, 2010 8:16 am 
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no suspension, solid tripod front with castor and camber built in. Only thing I can think of is a little slop in the kingpin boss on the chassis and some play in the steering box.

II'd like to see a set up with a damper as I'm not sure how I'd mount one.

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