4 link adjustment

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  1. hotrod68

    hotrod68 Well-Known Member

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    took my 70 dart tube chassis four link car out to a test n tune for the 1st time yesterday. 1st pass just drove down and car drives nice and straight. 2nd pass left off the brake at 4000 and car comes up and wants to go left, let off easy. 3rd pass put more air in the left tire, left on the brake 4000 comes way up wants to go right, let off hit it hard at the 1/8 mark and went 10.48 @ 139mph. i know I can get it to go straight, any info on how to get the front to stay down? Would I be going in the right direction by lowering the front bars? It's my 1st big tire chassis car any help would be great.
     
  2. tinman2

    tinman2 73 Dart 340 Sport FABO Gold Member

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    That's pretty fast! You need to find someone who REALLY knows what they are doing to help you. I don't think a written description of a problem and and a written opinion is what you need. You could get seriously injured or worse and maybe take out someone else as well.
     
  3. yellow rose

    yellow rose Overnight Sensation FABO Gold Member

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    First off, congratulations on using a 4 link. Much better suspension than the rest of them.

    Second, why would you drop the front bar?

    I suggest you scale the car. Some guys can use graph paper, but I just do it on the shop floor, full size and then write down all the hole locations and Instant Centers (IC’s) in a notebook.

    You MUST know exactly what the bar is doing to the IC in every position or you’ll get lost right quick. Make notes of any changes you make.

    Shocks matter. Shock settings matter. Front shocks and front shock setting affect the rear, and vice versa.

    You must also know the true car weight, corner weights, center of balance and center of gravity.

    Center of balance is easy. It’s the sum of the end weights of the car, as a percentage, divided by the wheel base. Easy math because I hate math.

    100 inch wheel base and a 2000 pound car. The front weighs 1100 and the rear 900. You have 55% on the front and 45% on the rear. You can use either number, but I always work from the rear forward.

    45% of 100 inches is 45 inches (I love simple math) so you know that 45 inches forward of the centerline of the rear axle (or 55 inches rearward from the centerline of the front spindle gets you to the same place IF the chassis hasn’t been modified to change the wheelbase on each side to quicken vehicle reaction times...that’s why I always square the rear axle first and always go from rear to front) is the exact center of balance.

    Center of gravity is a bit harder to locate. Since I hate math, and I couldn’t do differential calculus with a gun to my head, for most anything we will do, we can use the centerline of the camshaft at the center of balance. Where those intersect is your center of gravity.

    You need to plot that out, accurately if you expect to be able to tune the 4 link. And a change in IC, whether it is in height or length or both almost always requires a shock adjustment. And probably on the rear and the front. Do one change at a time.


    Sooooooooo...where do you set the 4 link for a starting point? Back in the 1980’s (and probably before that) clear up to about 2001ish, chassis guys loved to make the bottom bar parallel to the ground. That way, you could raise and lower the top bar and move the IC forward or reward and not change the HEIGHT of the IC. Fat lot of good that was.

    The height of the IC is important, but so is the length of it. AND, equally important is the ANGLE of the bars. Especially the bottom bar. And parallel to the ground ain't the best postion for the bottom bar 99.9% of the time.

    Think about it. The bottom bar is what moves the car. Not the top bar. The bottom bar is in compression, the top bar extension. So as the bottom bar pushes, and the top bar pulls around the IC, you can get any manner of chassis rotation, or lack of it, even with the exact same IC and the bars in totally different locations.

    The best example of this I’ve ever seen is is thinking about a refrigerator, and trying to slide it on a floor. If you push in the center of it, it will slide in a straight up position. If you push on the top, about its center, it will want to tip over backwards. Pushing on the bottom will make it want to fall back onto you.

    Same thing in a car. You can make the car sit on the bumper or leave the line like a skateboard with 4 crazy wheels. Or anything in between.

    Back to the 1980’s. Back then, not only was getting the bottom bar parallel the de facto setting, but setting the IC at half the wheelbase and cam high was the other de facto setting.

    Both are wrong and the half wheelbase/cam high IC is about idiotic for almost anything, unless you have 100 lb/ft of torque and you need to hit the tire harder to make it happy. That almost never happens.

    Sooooo, I suggest the bottom bar run on an angle so it intersects the front ball joint not parallel to the ground. Wish I could draw it out. IOW’s, that bottom bar should, in most cases, be angled downward and then run through the front ball joint.

    That is going to be where the bottom bar levers the chassis. Remembering you want to hit the tire as hard as the tire will take without beating the tire to death.

    Then, depending on how much HP you make, what RPM you leave at and your center of gravity will determine where the top bar needs to be.

    A long, low IC (relatively speaking long and low) will make the car pitch rotate (wheelie) slower and higher than a shorter, higher IC. Think about pushing the refrigerator.

    You can also add some weight to the front of the car (or wherever it might need it) to alter the pitch rotation of the car, rather than making an IC or shock change. 50 pounds on the K member will change pitch rotation a lot, plus everything else.

    Slowing down the front shocks (more extension dampening) will reduce pitch rotation. You have to chose which adjustment makes the most sense for what you are trying to get done. Shock tuning before adding or moving weight.

    Damn this is long. I’ll use my last car as an example. When I put the 4 link in, the bottom bar was parallel to the ground and the IC was cam high and 53 inches out, which was 1/2 the 106” wheelbase. That was per the chassis shop that made the 4 link, and everyone else I could get on the phone.

    The car was a P.I.G. pig. It would porpoise. You could hear the tires chirping as it beat the tire into the track, and then the tire would bounce back and do it again. I couldn’t get the shocks tight enough to control the rear axle. The suspension was violent to say the least.

    So I went back to my note book and looked at all the numbers. The car needed to wheelie, and I was smashing the tire way too hard. So I needed to make the chassis hit the tire less, and to pitch rotate. With the original IC, it couldn’t pitch rotate.

    I forget how I figured out the bottom bar couldn’t be parallel, but I did know the IC had to be much lower (to reduce the hit) and further out to pitch rotate.

    I ended up being 108 inches out and 4 inches UP!!! That long, low IC made the chassis tuneable. And it was consistent. It would do the same thing every time. And, making small shock changes made a difference, where before shock changes didn’t do anything.

    To answer your question, you need to plot out all the IC’s, get the bottom bar running through the front ball joint and then look at all your IC locations.

    Then you can make an informed decision on where the IC should be to start. The higher the HP, the higher the RPM at launch, the longer and lower the IC will want to be.

    If the chassis beats up the tire, slow the rear shocks down. If that makes it spin, the IC needs to go forward, or down or both.

    If the tire spins because you aren’t hitting it hard enough, loosen the shocks, most likely on both ends. If you can’t make it hook, move the IC up, or back or both and then tune the shocks.

    If you can find the old Chris Alston Chassis book, buy that and read it. It’s very basic and simple, and that’s the beauty of it.

    Another great book is “Doorslammers” by Dave Morgan. It’s more complicated and in that book he teaches to make the bottom bar parallel. He was the editor for the IHRA equivalent of NHRA’s National Dragster. In late 2001 or early 2002 he wrote an article correcting that.

    Tuning a 4 link is about preparation, note taking and testing. It’s a PITA. The payoff you should be able to hook in a car wash when everyone else is struggling to hook on a National event prepped track.
     
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    • Rmoore

      Rmoore Well-Known Member

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      Tighten the front shocks down ! Limit front end travel !!
       
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      • hotrod68

        hotrod68 Well-Known Member

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        yellow rose, thank you!
         
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