4link experts
#3082614
10/03/22 12:44 AM
10/03/22 12:44 AM
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Joined: Apr 2022
Posts: 158 Kansas
Thelma133
OP
member
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OP
member
Joined: Apr 2022
Posts: 158
Kansas
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I recently back halved my 67 cuda. The car goes straight now, and 60 is 1.25. Tire size is 32x14x15. Son is driving and he said car has tire shake about 110-115mph, so he lifted. No shake according to him at all from front end. I have done the obvious, pulled shaft out, shop said u joints were fine and it was perfectly balanced. Had him replace them anyway. Prior to second pass we tightened springs on rear big guns about half an inch. It was obvious after 1st pass the tires were hitting top of wheel tubs as there were lines on both tires in same place, so chassis installer had us change ride height using springs. After he returned from second pass and said it still vibrated, I loaded up and went home. Tire pressure was 7.5lbs, no changes made there. Son and checked pinion angle and it was down one degree. Installer changed instant center by moving both upper and lower bars up. This change was after multiple calculations by him on bickels program. Change was due to what he called a violent launch. It doesn’t do that now. The one thing I did notice was there are NO weights anywhere on the rims. Aren’t rims supposed to be balanced even with slicks???
I’m not really sure what else to look for, for safety sake I have ordered new (31x1x15) Hoosiers, but they are back ordered everywhere, as many have already stated.
Interested in any opinions,
Thanks, Bill
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Re: 4link experts
[Re: Thelma133]
#3082644
10/03/22 09:11 AM
10/03/22 09:11 AM
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Joined: Sep 2014
Posts: 4,457 Washington
madscientist
master
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master
Joined: Sep 2014
Posts: 4,457
Washington
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You’d have to post up all the numbers for all the holes and the CG to see why he moved the IC and then said it would be less violent.
As a general rule, the longer and lower the IC the softer the tire is hit and the longer it’s planted. And the higher and shorter the IC, the harder the tire is hit and it’s planted for less time.
The angle of the bars relative to each other also affects how the tire gets hit. And of course, your shock settings will change with any change you make to the bars.
IMO, under 1k HP needs the bottom bar running down hill through the front ball joint (or close to it) and the IC should be 88% or so wheel base out and 6 inches up to start.
And, you have to consider stall speed, two step RPM, tire pressure and track prep. It all works together (already conspires against you as it may be) to get the car moving.
If you are fighting wheelie issues, my first move is shock tuning. If that doesn’t clean it up then I would rather move 50 pounds up to the front or I’d rather add 50 pounds up front before is used the IC to push the front end down.
All these cars should be doing a wheelie. They don’t need to scrape the bumper but they need to pitch rotate.
Last edited by madscientist; 10/03/22 09:15 AM.
Just because you think it won't make it true. Horsepower is KING. To dispute this is stupid. C. Alston
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Re: 4link experts
[Re: Thelma133]
#3082857
10/03/22 02:15 PM
10/03/22 02:15 PM
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Joined: Jan 2003
Posts: 19,363 Las Vegas
Al_Alguire
I Live Here
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I Live Here
Joined: Jan 2003
Posts: 19,363
Las Vegas
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You vibration is not very likely to be coming from the 4 link setting IMO, given your stated pinion angle at least. Nothing else back there can cause that. If it is rubbing the tubs thats likely tire growth thing. The 4 link is not causing it to rub unless there is something VERY VERY wrong with the set up or shocks settings. I for the vibration I would be looking for a new driveshaft myself.
"I am not ashamed to confess I am ignorant of what I do not know."
"It's never wrong to do the right thing"
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Re: 4link experts
[Re: Al_Alguire]
#3082906
10/03/22 03:45 PM
10/03/22 03:45 PM
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Joined: Mar 2003
Posts: 2,498 PA
moparacer
top fuel
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top fuel
Joined: Mar 2003
Posts: 2,498
PA
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Pull the slicks off and have them balanced. Did it shake the tires on the burnout? That's a sure sign trouble is coming with slick balance.
IMO 7.5 lbs. air is too low too. Especially with a 1.25 60 ft.
And yes if the IC is low enough on the 4 link settings the car might squat after it launches.
Lots to figure out here.
Last edited by moparacer; 10/03/22 03:48 PM.
67 Barracuda street/bracket car 11.27-119 68 Dart 502 BB 8.70s-152 414 cid SB Dragster 7.65-174
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Re: 4link experts
[Re: moparacer]
#3083197
10/04/22 11:31 AM
10/04/22 11:31 AM
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Joined: Jan 2003
Posts: 16,925 NC
440Jim
I Live Here
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I Live Here
Joined: Jan 2003
Posts: 16,925
NC
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Pull the slicks off and have them balanced. Did it shake the tires on the burnout? That's a sure sign trouble is coming with slick balance.
IMO 7.5 lbs. air is too low too. Especially with a 1.25 60 ft. Tire size is 32x14x15. ... Tire pressure was 7.5lbs, no changes made there. Don't know what to think on the vibration. But at some point you could look at increasing tire pressure. Depending on weight, I am thinking 9-10 psi could work. I have run 9.5 psi at 2750 lbs with Hoosier 33x15x15 and 1.19 sixty foot (PG trans).
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Re: 4link experts
[Re: Thelma133]
#3083224
10/04/22 12:57 PM
10/04/22 12:57 PM
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Joined: Jan 2003
Posts: 19,363 Las Vegas
Al_Alguire
I Live Here
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I Live Here
Joined: Jan 2003
Posts: 19,363
Las Vegas
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AS said you NEVER should use the spring perches to increase ride height. That is what the shock mount is for. If the slicks are old start there, possibly they have some issue.
FWIW I have not bothered to balanced a slick in 15 years. Had a set mounted and balanced at the trailer at a division race and out of curiosity we checked them after that race, they were not even close to balanced 11 passes later. Have not bothered to balance a slick ever again, but when the truck does my tires they always do it. I have been just under 200mph and never had an issue with slick balance causing a vibration. More often than not these ays we mount our own tires on with the beadlocks, one less thing to do at the track.
I have seen more than one "good balanced" driveshaft be an issue with vibration or broken tailshafts for sure.
"I am not ashamed to confess I am ignorant of what I do not know."
"It's never wrong to do the right thing"
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Re: 4link experts
[Re: Thelma133]
#3083228
10/04/22 01:02 PM
10/04/22 01:02 PM
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Joined: Jan 2004
Posts: 8,249 fredericksburg,va
cudaman1969
master
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master
Joined: Jan 2004
Posts: 8,249
fredericksburg,va
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Does your driveshaft maker-balancer turn the shaft at a high speed? Balancing for highway speed won’t work gotta spin that shaft to finish line speed @least 5000 rpm to balance. Also raise the car with shock brackets not the spring, you’ll run out of shock travel which could be making it bounce at speed? Raising the car also changed the link settings. Does the rear raise or squat when launching?
Last edited by cudaman1969; 10/04/22 01:04 PM.
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Re: 4link experts
[Re: Al_Alguire]
#3083273
10/04/22 03:38 PM
10/04/22 03:38 PM
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Joined: Dec 2005
Posts: 11,551 Fulton County, PA
CMcAllister
Mr. Helpful
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Mr. Helpful
Joined: Dec 2005
Posts: 11,551
Fulton County, PA
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You vibration is not very likely to be coming from the 4 link setting IMO, given your stated pinion angle at least. Nothing else back there can cause that. If it is rubbing the tubs thats likely tire growth thing. The 4 link is not causing it to rub unless there is something VERY VERY wrong with the set up or shocks settings. I for the vibration I would be looking for a new driveshaft myself. All of this. First thing to determine is what kind of vibration. It's not a rough track with a tight shock issue? Tight shock in bump on a rough track can be sketchy. What tire on what width wheel? Soft sidewall? Narrow wheel for the tire? Radial? Some grow more than others, depending on tire construction and the wheel it's on. A driveshaft is a high frequency buzz. When it's bad, it makes things blur. Think a die grinder with a bent burr. A tire is a much lower frequency. Think a tire out of balance or a brake pulsation at interstate speeds. The shaft may be balanced, but does it have run out? Many places are unable to spin a shaft at the speeds we need them to be right. I have had a slight out of true shaft balanced multiple times. It was OK for a few runs and the buzz came back. Ended up in the scrap. Tires have to be balanced. And I made it a habit to rebalance them about half way through their expected life, because they do go out. 4 link on your car is along for the ride at that speed unless you are making Top Sportsman type steam. Moving the bars is useless when looking at a vibration. Cranking up the shocks is not the right way to adjust ride height. Adjustable mounts are. IF raising the car and/or eliminating the tires hitting fixes the problem. then I would want the sheet metal fixed so you can run the right tires on it at the desired ride height.. Vibration frequency. High speed is something in front of the ring gear. Lower speed, tire, axle. Driveshaft, balance and run-out. Tire balance, excessive weight needed means the tire is faulty. Tire hitting the body somewhere. I'm skeptical. I've seen tires hitting bad enough that they would smoke in the lights without vibrating. Easy stuff first.
Last edited by CMcAllister; 10/04/22 03:41 PM.
If the results don't match the theory, change the theory.
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Re: 4link experts
[Re: CMcAllister]
#3083342
10/04/22 06:47 PM
10/04/22 06:47 PM
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Joined: Jan 2003
Posts: 19,363 Las Vegas
Al_Alguire
I Live Here
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I Live Here
Joined: Jan 2003
Posts: 19,363
Las Vegas
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We have almost rubbed through the C pillar on the TS car this is a car that the rockers are 3.5" off the ground at static ride height. Happens on PS cars alot, we have repaired a few that tried to come through the body in the c pillar. It can happen in a 4 link but you need to be making some power and have a high growth large tire, has nothing to do with the 4 link...
"I am not ashamed to confess I am ignorant of what I do not know."
"It's never wrong to do the right thing"
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