4link experts

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

Try checking the tires for run out on the car, both tires.first. If they have more than 1/2 inch run out that may be the problem
I have always bubble balance my slicks due to owning a good bubble balancer, years ago they took more weights than they have in the last three new sets
I have tried moving the tires around on the rims when they were going to take more than 6 ounces to balance them, the tire stem can cause some of the balancing being off a little bit off
You could have the wheels balance bare and then redo them with the tires on to see how bad those tires are out of balance
I had one set of 12.25x31.25 x15 W Firestone years ago that were unusable do to being way out of balance, Firestone did replace them with better set

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.

Thanks Cab and Mad. Car leaves well now, briefly on the bars, but not like it was before 4 link adjustment. Is it possible the 4 link change would now force the tire higher in the wheel tub?? I will try to put video up so you can see it. I’m grateful for your input. Will also put up position of 4 link.

Thanks,

Bill

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.

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.

I have ran mickey thompson tires for years, not sure of what brand you have now. 31 10.5 W 15 inch on a 10 inch rim 14 to 15 lbs is repeatably the best tire pressure. years ago I ran goodyears 10 to 12 lbs seemed to be the best, lower than 10 lbs and the car would move around on the big end.

Thanks Al. I sent my driveshaft to Tulsa driveshaft (that was my first suspicion) and they installed new joints (but said old ones were fine) and they said it was perfectly balanced. I can see where the tires had rubbed during first pass. They had lines in the same spot on both tires. My son was driving, he said nothing about shake (I.e. burnout) till about 110/mph. Prior to second pass, we adjusted outer spring on rear shocks one half inch clockwise. I’m assuming this changed ride height. I know nothing about 4 link which is why I’m asking you gents. It was the 4 link installer that was telling me what to do. Tires did not rub (32x114x15) prior or adjusting 4 link setup. He now says I should put on smaller tires so they won’t hit the tubs??????? Don’t mind spending the money, but will that fix the problem. Thanks for your help

Hoosier tires. Hard to find anything now. Thanks for input

Yes lots to figure out, thanks mopar racer.

Raise the car with the shock bracket not the spring.

Balance the rears.

Do they have tubes?

No tubes,, will balance. TrYin’s to find new tires. Thanks to all of you!

My bet is tire balance. Sometimes I have to balance mine more than once throughout the season.

I did have an M/T radial slick go bad once.

Not a vibration issue however, just made the car turn right at the launch.

However I think trying another set of tires is a logical next step.

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).

Thanks Jim, demon and cuda. Appreciate your input. I have spent a boatload of money on this car trying to make it right.

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.

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?

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.

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...

If it went the wrong way, slowed down with the bar change, then you eventually want to go back to the original and work from there.

Yes, the chassis can be made to squat down onto the tire with the 4 link. Tightening the bump on the shock or moving the front of the bottom bar up will reduce or eliminate that. Excessive squat may cause the tire to rub in a tub if it is too close early. Typically, once the car has moved off the starting line a bit, (before 60') it will come back up off the suspension, depending on shocks, etc. Again, down track, on a car that's not high horsepower, radial and/or something else peculiar, it should come back to near ride height in the rear.

Thanks Al. Ordered new slicks today. Wow so many opinions on tire balance, but my 4 link installer told me the same thing you did, he doesn’t balance.

Thanks cab.

Much for me to learn here. My fear was the 4 link installation was installed wrong. How do tell anstaller these suggestions DIPLOMATICALLY??? You gents are all very smart, this is all magic to me. I ordered new tires and went down one inch in width and height. I hope this helps. I have watched some of tim mccamis videos, they helped somewhat.

Thanks cm

Track is great bend, it’s a nice track. I will call drive shaft guy tomorrow and ask all questions raised. This is rocket science to me, but trying to eliminate one by one.

Test it on jackstands on all four corners at the same height above ground and see if it shakes or vibrates with the rear tires and driveline revved up in each gear like you race it
Let us know what you find out

I always stayed after tire balance. Having access to a good machine made it easy, and free except for some time. I have buddies who have an old bubble machine and swear by it.

I understand some don't, and don't have issues. BUT, if I were after a vibration, I would have them checked. I've seen guys catch wheels bent by an idiot installer.

Still like to see some picture of the bars and brackets in the car. I see the 4 link chart but woudl like to see the stuff n the car before you throw anyone under the bus so to speak...I can post them if you like you can text me the photos if thats easier....

Here are some photos of 4link and shock

Here is one of the passes we made. 5.62 in eight. Son lifted due to shaking. Anyone tell me how to attach this video I made? I could do it.