BAD VIBRATION ON STARTING LINE

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JENKINS528
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Joined: Thu Nov 19, 2015 3:04 pm

BAD VIBRATION ON STARTING LINE

#1 Post by JENKINS528 » Fri Nov 20, 2015 5:43 pm

Hey Guys..
Im new here on Drag Stuff,, I have been reading the Topics you guys put up here for a year or so,,, My dad and I have been fighting tire shake
for2 years, can not stop the car from doing it,, and with that being said, we go racen 8 to 10 times a year, some tracks does not do it much,, othere tracks, 60ft will jump all most a tenth,,, I went to VA to a mopar race, I went 5.82,,, 5 times in a row,, the nest time I race the car 5.75 too 5.85,, in the same lane,,,, just a few weeks ago, I went to Jacksonville nc, I went 5.77 6 times in a row, and 1.26 60 ft, times,,, and the next week, went KDS motorsports,, and 5.75 to 5.89 in 6 passes, 60ft, 1.25 too 1.35
so here is what I got,,
1968 DODGE CHARGER,, 452 BB mopar, 685HP 7200 RPM, 585TQ, at 5600 RPM,, car 2450LBS with me in the car, 51.5% weight on front of car,
4 Link,,, adj, front and rear shocks,, 116 wheel base, the car has never pick up the front end, no weight transfer to the rear, at this time 4 link 48 in out, 8 in off the ground
114% anti sqat, best time is 5.69,, at 119 MPH, ,, dana 60 4.56 gear, P/G with 1.80 gear, I think I have use every hole in the 4 link,, and
the tires still shake,, some a lot or alittle,, or tires will shake about 3 to 4 feet out, and stop,, and shake again, around the 60ft timer,
hoosier 33/15/15 D05,, reg, side wall,,, I have been told I need stiff side wall, and a firmer tire, like the Hoosier C07 ?? and maybe go to
a 4.10 gear,
I have run the 4 link, 55in too 59in out, 4 in from the ground,, the back of the car goes down 2 1/2in, ,,,
front springs 250lbs,, rear springs 110,,,
can some one please help,,, lol

thanks guys
MIke
12072593_1065011480177585_2491728280173300407_n.jpg

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John_Heard
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Re: BAD VIBRATION ON STARTING LINE

#2 Post by John_Heard » Mon Nov 23, 2015 10:56 pm

What have you tried besides 4 link changes? Tire Pressure Adjustment? How far have you went both ways? What was the result? Shock Adjustment? Same set of tires for last two years?

Stiffer sidewalls will likely help, but the stiffer they are the less forgiving they will be.

JENKINS528
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Re: BAD VIBRATION ON STARTING LINE

#3 Post by JENKINS528 » Tue Nov 24, 2015 10:34 am

The air psi,, from 5 1/2 psi to 9 psi,, it does some better at 9 psi,, but some times it will spin at the hit of the tire,, shocks,, redound have 30 clicks, and the compression has the same,, I have been lose and tight with them,, and inbeween,, does not really change,, tires are new this year, I had 14 /32 /15W,, D05,, 102 roll out,, I have 15/33/15 D05 104 roll out on the car now, car does the same thing, tire folds up in the front, if you do not run at least 9 psi,, The car does NOT transfer any weight to the rear of car, rebound on the front shocks are all the way lose, car just goes straight out,, I would like for the car to pick the front tires up 5 to 6in,, carry them 20ft, to 30ft out,,

JENKINS528
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Re: BAD VIBRATION ON STARTING LINE

#4 Post by JENKINS528 » Tue Nov 24, 2015 10:41 am

Im willing to take this car to some that knows how to set up a 4 link car, a pay them to do so,, and maybe teach me about at the same time,

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John_Heard
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Re: BAD VIBRATION ON STARTING LINE

#5 Post by John_Heard » Tue Nov 24, 2015 11:05 am

If you've changed the shocks from full loose to full tight and not observed any change, those shocks aren't working as they should. You should have noticed something different! You might want to look at getting some better or new shocks.

Perhaps you need to try more than 9 psi?

Do you have any good video of the car launching?

JENKINS528
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Re: BAD VIBRATION ON STARTING LINE

#6 Post by JENKINS528 » Wed Nov 25, 2015 8:29 am

I will say, when the shocks are tighter in the rear, vibratrion is not as bad, when you get 100 feet from the 1/8 mile marker, back of car starts moven around, alittle,car was rebuilt 3 years ago, every part on this car is new,, but the roof, a.b pillers, I have remove front and rear shocks, and rebound and compression does change, its like I can not find the cars happy place,, im going to a race next weekend, I will video the car,,
Im going to try a 4.10 gear in the car, and the 4 link will be 12in rear, 7in front, split, 57in out, and 7 in from the ground. 88.4% anti squat,
If I drop below 75% anti squat, and 60in to 65in out, 3 to 4in off the ground, the rear of the car will squat 2in,, with no front end lift, do I need to tighen the compression on the rear shocks, ??
little back ground on me, 30 years vw audi tech, I have drag raced for years with lader bars, help a lot of friends get there car working better,,
I have run late model stock, round track racen, won afew, I injoy working with shocks and springs,, THIS CAR CAR IS KICKING MY BUTT,,LOL
thanks for your help so far,, Im sure I will need more,,
Happy Turkey Day
Mike

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John_Heard
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Re: BAD VIBRATION ON STARTING LINE

#7 Post by John_Heard » Wed Nov 25, 2015 9:34 am

75% anti-squat sounds low, but I'm a leaf spring guy, we have a ton of anti-squat in them, but they are small tire hard sidewall radial cars so hard to equate? lol... Good video would help a lot to see what the car is actually doing, if you can find someone with one of the new iphones they take some pretty decent high speed video that can be slowed down very nicely..

Remember not to confuse smashing the tire sidewall with rear end squatting, those big tires move a lot.

Back of car moving around on the big end is probably those big squishy soft sidewall slicks, you would love radials (if you could make them work).

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clutchdude
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Re: BAD VIBRATION ON STARTING LINE

#8 Post by clutchdude » Tue Dec 01, 2015 2:51 pm

My car will shake if i have the compression to tight on the rear shocks.

CHampshire
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Re: BAD VIBRATION ON STARTING LINE

#9 Post by CHampshire » Thu Dec 10, 2015 11:11 am

Some questions on your setup:

What does the car weigh front to back?
Where is cam c/l?
Have you tried moving the 4 link bars closer to the rear end to achieve the same IC? The closer the bars are to the rear end housing, the less likely it is to shake the tires.
Is this car equipped with a wheelie bar?
Do you have double adjustable shocks in the rear?
Is this an a-arm or strut suspension in the front?
How much shock travel do you have in the front until the tire comes off the ground? How much shaft is showing when the car is ride height?
Are there any travel limiters on the front of the car? Chain or cable...etc?

If the IC is below the neutral line the rear end will separate from the body and drive the tires into the track. Therefore this is controlled by shock extension not shock compression. You can control how hard the tire is applied by changing IC height.

You can throw different IC at it all day long, but the front springs need to help pick the car up on acceleration to transfer weight. The rear shocks need to control the movement of the rear end. Everything needs to work together.

I would recommend Jerry Bickel's chassis book that he sells on his website. Very good book to learn off of, and for the 70 bucks it costs it is priceless. Also, if you have not done so yet, measure out your 4 link and use one of the various programs available to plot out IC's. Makes life way easier to make a change to the bars and know what you are going from and two. Also, it would be a good time to hang some strings and measure things to make sure nothing is bound up and out of square.

JENKINS528
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Joined: Thu Nov 19, 2015 3:04 pm

Re: BAD VIBRATION ON STARTING LINE

#10 Post by JENKINS528 » Tue May 30, 2017 10:21 am

Hey guys,,
I have not been up been up here in a while,,
my dad got sick,, lung cancer, he passed away in oct, 2016,, we have raced together over 30 years,, what a great time I had with him,, and he is greatly..
want to say thank you for all,,, for your help, I have made some changes to the car, its some better,, But still doing the same thing,
so this is what I have right now for set on the car,
car has 112in wheel base
I have put a 4,10 gear in the car,
inst center 3.6in
inter cet, for the 4 link 54.8in
anti squat 43.6%
rear 4 link bar spred 11.7in
front 4 link bar spred 7.5in
car weight 2460 with driver, and I have move the fuel tank to the rear, weight is 50% front and rear
strut front end, with 1in showing shaft of the shaft, 3in stroke struts
rear shock are double adj, 30 clicks rebound, 22 clicks on compression
compression is 14 clicks from full soft
rebound is 20 clicks from full soft, avco rear coil overs,,
front springs are 250
rear springs are 110
I have gone up to 10 psi on the tires,,,
the last time I run the car 2 weeks ago, I had 128.5, and 128.6 60ft, went 5.85, both times,, no car vibration, track temp 112
1st round 132.5 60 ft 5.91 othere guy went red
2nd round 133.9 60 ft 5.93 othere guy went red
3rd round 136.1 60 ft 5.96 I went red,, lol,,
would luv some help,,
thanks every one,,
Mike

Z28SBC427
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Joined: Fri Aug 12, 2016 6:25 pm

Re: BAD VIBRATION ON STARTING LINE

#11 Post by Z28SBC427 » Sat Jul 08, 2017 5:59 pm

Have you checked the pinion angle?

ironmike
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Joined: Wed Jul 26, 2017 3:54 am

Re: BAD VIBRATION ON STARTING LINE

#12 Post by ironmike » Wed Aug 02, 2017 2:52 am

Sorry to hear about your father. Hope the issue got sorted out.

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