written by: Dennis Johannsen (Bubstr)
This is just a brief summary of how the drag race suspension got to where it is today. There is a 4 link chassis diagram on the last page that goes with this article and will be referred to from time to time.
We started off with a cross leaf suspension, with long trailing arms to locate the rear end. This had very little mechanical advantage, but it kept the rear end in place and handled the horsepower of a mild flat head even with hard tires. As drag racing evolved we went to a leaf spring. This was much better and with a few bars clamps and snubbers they really did a good job. The trouble with them was they where very hard to change the instant center relationship to the center of gravity, so if you were a bit low wheel stands and a bit high wheel hop. Then we went to a ladder bar. They started out very long and shortened up as years went by. Now we could adjust a preload and instant center height but almost impossible to set the instant center length. Then we went to a 4 link suspension. This not only allowed us to set preload, but instant center height and length plus it was a bit lighter than any of the other suspensions.
Some of the things that changed the type of suspensions, also changed the whole car. More horsepower and tires with better bite and faster top speeds also changed where we want the center of gravity and how much frontal area we want, even aero aids like wings and spoilers. The total traction package is a combination, just like you would have a combination of parts in your engine to make as much horsepower as you can in an RPM range you can use.
If you have a 4 link car, we will assume you have some choice of where your center of gravity is. Any thing that weighs something has an effect on it. There is no fixed spot where this should be, but total weight, wheel base and horsepower are the determining factors. The extremes would be a short WB car with a center of gravity too high or too far to the rear. You would have to control too much weight transfer. The other end of the scale would be a heavy long WB car with the center of gravity too far forward and too low. It would be hard to get weight transfer to the rear then. These are not impossible to control but the less energy we use to control forces, the more there is to go fast. Speaking of controlling forces, un-sprung weight is a drag racer’s enemy. Un-sprung weight is anything below the spring and half of spring, shock, and any control arms connecting the rear end to the chassis. This is more important to high horsepower cars that tend to tire shake. Less mass is easier to control.
Springs: What spring rate for your car? This is an almost impossible question, unless you know how the spring is mounted and what you need them to do. A front spring with a Chevy control arm could be 800lbs per inch and a Ford could use a 500lbs per inch and both end up with the same spring rate at the wheel. This has to do with the leverage the control arm has. The Chevy is mounted more inboard than the Ford. Rule of thumb is they should not coil bind (coils touching each other) but they should get at least an inch or two of travel, anything in between is spring tuning.
Front springs: The car tells you what it needs. Too much weight transfer to the rear says weaker front springs. Not enough says stiffen front springs. Think of them as a helper when you are trying to raise a ladder, if the helper is strong and throws his end up it makes it easier for you to get the ladder standing, weak helper, the opposite. The shocks just control the speed at which all this occurs.
Rear springs: They should be just soft enough to maintain tire to surface contact. Most strips are fairly smooth so a stiffer spring is a little better because it transfers weight to the tire contact patch faster on the start. Smoother/more traction equals stiffer spring, rough less traction softer spring. The shock here also controls how fast it happens.
Now we come to the scary part, all them adjusting holes on the 4 link. Each car has its own best setting depending on horsepower, weight, tire, type of transmission, torque converter or clutch, and center of gravity height. The best advice is to map out all the different settings in a note book or use one of the many software programs out there. If you choose to use paper and pencil, just draw a picture of the lines the 4link bars would intersect at if you extended them forward for each mounting hole. (arrow C points to the instant center for this car) Also, for each of these drawings make a few more for different ride heights. Remember the rear of the link will not change height but the front of link will. This will aid you later in case you want to change it at the track. These points are possible instant centers.
Then find the center of gravity. It's easy to find the front to back center of gravity, just balance car on 2 jacks with a stout piece of angle iron or box tubing. To find the vertical CG is harder. A good guess is about camshaft high for most, add for the hundred lbs of lead in the roof subtract for the chopped top. Now on all your pictures extend the center of gravity line forward to right above the front wheel center and if the front wheels are staggered, split the difference. From this point draw a straight line to center of rear tire contact patch, (where the rubber meets the road). This will be the percentage of Anti Squat (arrow B). The relationship of the instant center to the Anti Squat line determines the angle the mass of the car is being pushed at, too low mass wants to wheel stand and squat in the rear(compression), too high rear of car will raise, (separation). Rule of thumb, higher horsepower standard transmissions like less than 100% Antis Squat and less horsepower with torque converters like more. Really sticky track needs less Anti Squat, slippery track, more. Bad air or down on horsepower more.
You get the picture, the 4 link is infinitely tunable to keep from wasting horsepower doing wheel stands or raising the rear of the car. How far the instant center is forward controls the quickness of the anti squat. Farther forward is slower, to the rear is faster.
Now that we picked a starting point for the instant center, put your amount of weight in the drivers seat, level the ride height and adjust bars to square rear end with chassis, (not front tires, spindles or ball joints). Using top bars you set pinion angle. First time you set pinion angle it's a good idea to drop coil-overs and track the pinion angle of rear end during normal vertical movement to keep angles to a minimum as you may get 1 or 2 degrees of upward movement due to slop and strain on control arms under load. Next, put a couple flats, ( a flat is the flat part of the hex nuts that lock bar 2 on one end means 2 on the other) of preload in right upper bar, (shorten it). A little preload usually goes a long ways. This will actually increase the Anti Squat on the right side, to help control the compression there. Now it's time to run the car and see what it asks for. Drive it like you would in a real race. If you baby it here it won't act the same when you’re serious.
You want to look for several things:
1. Wheel stand.
2. Compression or separation in the rear quarter to tire.
3. Is it going straight?
A little tire off the ground isn't bad, if your front isn't lifting in the lights instead of rolling out and coming down smooth. The compression/separation and going straight go hand in hand and are correctable with more or less preload. Turning right =more preload, turning left =less preload. A WORD OR CAUTION HERE! If you’re running preload in your rear setup then you should be careful when brake racing on the top end! Sudden hard breaking at speed can upset a car with preload in the rear setup!
This final note by the Editor:
We’ve heard the term “Dave Morgan’s Sweet Spot” mentioned several times lately. This is the rectangular box (arrow A) in the chassis drawing. Dave Morgan wrote a book called The Door Slammers Chassis Guide some years back which is considered to be the “last word” for door car chassis info. This “Sweet Spot” that is referred to is dependant on all data being accurate, which most of the time it is not especially when dealing with the Center of Gravity or (CG). In light of this fact, the “Sweet Spot” is only a general starting point for initial chassis setup as it may or may not be where we think it is unless we have accurate data to support it.
Image taken from Performance Trends 4link Calculator computer program.
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