The best advice we can give is to not over-think it, as long as you are familiar with the basic concepts of anti-squat, anti-dive, and roll center, and the axles move nice and smooth when you cylce the suspension, you will be in great shape. The first step in either building a 4-link suspension or troubleshooting an existing suspension is to download one of these Triaged calculators created by Dan Barcroft and plug in the dimensions and weights it asks for.
While they may look complicated, they are actually very easy to learn by just entering numbers and watching the outputs and graphics change. Anti-squat in a linked suspension system determines how the rear end of a vehicle moves under acceleration or upon the rear axle contacting an obstacle at speed. The anti-squat value is determined by the vertical angle of the rear links as they relate to the front axle position and the center of gravity of the vehicle.
These characteristics are desired for drag racing and heavy acceleration applications because the forces that push the rear end up also push the rear tires down for more traction. At speed, however, when the rear tires impact an object, that immediate increase in traction will cause the power applied to the rear axle to raise the chassis up at the same time as the suspension is trying to compress and absorb the impact.
These characteristics are desired for desert racing to absorb rough terrain at speed because the impact forces are transferred directly to the rear suspension. Under hard acceleration, however, some of the power applied to the rear axle is used to compress the rear suspension which lifts up on the tires and robs traction and power. These characteristics make the vehicle neutral and keep the power and suspension dynamics independent.
You should pay close attention to how the lower link adjustments change the paths traveled by the bottom of the birdcages during chassis roll. Keep in mind that any change to the path traveled by any trailing arm will affect roll steer. For example, in illustration 2A, both the top and the bottom links move the birdcages and the rear tires rearward on the right side and forward on the left side during chassis roll.
This action will cause loose roll steer. We can reduce loose roll steer by lowering the bottom links at the chassis. You can see how this adjustment works in illustration 2B. We've lowered the bottom links to a level position and now the bottom of the right side birdcage moves forward during chassis roll instead of rearward as in illustration 2A. On the left, we have reduced the forward movement of the bottom of the birdcage.
As a result, loose roll steer is reduced. Basically, we've position the bottom links to counteract the forward L. As a result, we reduced loose roll steer.
We can reduce loose roll steer further by lowering the bottom links further as shown in illustration 2C. On the left side, the bottom of the birdcage now moves rearward until the link reaches a level position instead of forward as in illustrations 2A and 2B. Consequently, a further reduction in loose roll steer results. Some forward bite may be lost when the bottom links are lowered but the effect on forward bite is usually minor relative to the overall handling improvement that is realized by reducing loose roll steer.
Another method used to reduce the loose roll steer of a 4-link suspension is to shorten the bottom links.
Notice, in illustration 2D, how the shortened bottom link pulls the bottom of the right side birdcage forward during chassis roll more than the longer links in the other illustrations.
The bottom of the left side birdcage does lose some of its rearward movement because of the shortened bottom link. But since left side birdcages typically move down much less than right side birdcages move up during chassis roll, the overall effect, when shortening the lower links, is a reduction in loose roll steer. However, if the left rear of your chassis hikes up during cornering, loose roll steer may increase whenever both bottom links are shortened! We could reduce loose roll steer even further by combining the long bottom link arrangement of illustration 2C on the left side and the short bottom link arrangement of illustration 2D on the right side.
The preceding paragraphs should help you understand why. The length of the bottom links are dependent on the roll steer and traction characteristics desired by the chassis tuner. You should use the information in this article to determine the correct link lengths for your application. A 4-link birdcage rotates or "indexes" on the axle tube whenever the suspension moves unless both upper and lower links are equal in length and parallel to each other.
In really extreme bad conditions watching the car can tell somewhat it needs, but be aware that things are happening quicker than your eye can see. Best is a video shot at an angle covering whole car from left rear corner. A good quality video shot from left rear at an angle will tell you much when play back one frame at a time. Please try to learn as much as you can about this subject if you are tuning your own 4 link.
It can get dangerous if you go too extreme. Also remember also that after your run the 4 link adjustment and rear components are also important.
You do not want severe rear suspension unload when you back off it either. When you cross the traps you do not want to hit the brakes and experience severe rear suspension unload or if you pull a chute.
We may later get into using the actual car's center of gravity calculated at speed to determine where to place the chute tether attachment point. One other point to make. If you set your preload with a top bar do not get wild and crazy. Before you set preload with all weight in the car you should be able to easily slide the front bolt out after removing the nut. One top bar will be used to set and adjust pinion angle. With this adjustment a little can make a big difference.
If you are using the top right bar to adjust pinion angle, the top left bar will be your pre-load bar. In this example you will lengthen the top left pre-load bar to pre-load weight to the left front and right rear tires. Usually even a fraction of a nut flat can be noticed in your handling. If you are using the top left bar to set pinion angle, then the top right bar will be your pre-load bar. To jack weight in pre-loading using the top right bar you will make it shorter.
With strong and fast cars rear double adjustable shocks are a must. How do we know if we have the correct spring rates; To begin with we need to understand tha the spring in a drag car has only two functions.
Support the weight of the car up to the starting line. Give as smooth of a ride aas possible while going down the track. Possibly a third function is to support the additional weight during weight transfer.
Usually a to lb car will want between a to a rear spring on it, but that is never absolute. I know Outlaw 3, lb cars working well with springs even. You can not go by just the rear weight to guess what springs you need. Too many other thing figure in, such as the exact shock mount placement in relation to center of axle, Shock mounting angles if any, You let the car tell you what it wants and you can do that without ever going down the track.
You also need enough compresion to allow a good smooth ride down the track. Bottoming or topping a shock out is a sure fire way to unload tires and if at high speed to crash. Support car on all 4 wheels so you can get under it. Measure how much of the shock ram is sticking out of the shock. If you can't and at all the way down on adjusters, that means the spring is too stiff. If you have adjusted the lower spring adjusters as high as you can and you still can not get at least The more rear travel the shocks have available the bettter it is to tune.
At one time people were using shocks with 5 inches total travel in the rear. That does not allow much room for any error. Problem is that the longer travel shocks do require more room to mount.
I will use the 6 inch travel shock as my example; Ideal will be 1. This tolerance does allow it to work with more than one spring rate spring becuase of the lower spring mount adjustability. I did touch on it but not this much. Hope this helps you. Oh' A ladder Bar car will need to hit the tires a little harder than a 4 link car will becuase of the difference in how they work.
If you are also expermenting with power adders etc. Know this, no matter how crazy and backwards it wiill sound. The more you increase power to the rear wheels the less suspension action you want front and rear. Fortunately now there are several computer programs to aid in plotting 4 link adjustments, and those who are using them are getting good results if they can just understand the program and how they work.
That is the easiest way for a novice. Long or short bars. Longer in my opinion is better, But years ago Larry Kopp running out of Wayne County Speed Shop showed up with an Oldsmobile Firenza with a different cutting edge range of ideas all on the same car.
He had 4 link bars that were quite a bit longer than the normal 20 - 22 inch bars, and the car did not work very well. I do not think it was because of bar length. I think it was that the complete over all set up was so radical that other parts of his suspension were working against him.
They ran that car only one year. I have so many photos of that car. I do think several of the ideas on that car were really good but bottom line is it could qualify but could not win. They went to a more conventional car the next year and began winning again.
Amongst everything else they had the rear coilovers operating in a configuration like a rocker arm like a Can Am car. The higher it is, the greater effect it has. By theory the further forward the Instant Center is the more the front will lift and the further back the less it will lift.
As I said that is just theory that in extremes just doesn't work. Weight Transfer does still work by the front rising but in most cars today we do not want the front to rise on it's suspension any more than it is necessary. I promise you that when the front tires are off the track you have all the weight transfer you can handle. Spend more time on making the rear to work right. Once you have that you can do some extra fine-tuning on the front if you need it.
Most True Bigger tire cars don't. Ed I have just mentioned some of the beginning basics here. Hope I helped some. Did that answer some your questions or did I get too far off the topic? When you do get a good suspension baseline on your car I imagine that the point of actual instant center will be either right above the ground, a few inches above, or it could even be buried several inches into the ground and behind your rear engine plate, probably somewhere around the rear of the door and near the ground or between the ground and your frame height.
Time will tell. The measurements I told you to write down about from the point of balance to the rear axle and to the front axle you can use to figure what percentage of your wheel base is forward of the point of balance and what percentage is to the rear of it.
Once you know these two percentages and you know the vehicle weight you will then know the actual percentage of weight on the rear and on the front and the amount of weight on each end of your car.
Not unless you want a mph wheel stand when you change gears. What is right for your car is what you find that works. Don't be surprised if some of this sounds different. Maybe we will get even deeper into this if you all wish for us to do it? Ed Do understand that I am not a Chassis Man and only work with chassis of my customers that are local. For years I did my own too. My experiences came as a racer in what was at one time considered to be a high horsepower car. I did go through an Alston Chassis Clinic years ago, but was mostly in disagreement with them the whole class.
I had a car that was a handful at the time and It did not take me real long to learn the things to not do. I also was blessed to have at that time in my life some friends in Warren Johnson and Larry Olsen that helped me along quite a lot. That car had a tendency to pic the front end up when I shifted to high gear at around to mph.
Although not real high, I can promise you that loosing sight of the track at mph plus is just not a fun type of entertainment. BIG PROBLEM with it was the torque and power was violent even just on motor and the rear to front weight distribution was asinine and set up too much to the rear by my ignorance.
It was very wheel stand prone and initially also wanted to do a barrel roll like those of Charlie Buck and Animal Jim. In a leaf spring suspension, the leafs themselves must perform 2 functions. First, they hold the rear axle in the car. They prevent both forward and aft movement and minimize pinion angle change during suspension travel. Secondly, while they are doing this, they also support the load of the vehicle. This is a compromise when compared to link type suspension where the two functions are isolated.
Also, an OEM vehicle that has had thousands of hours of development time behind it will operate within a predictable range of suspension travel, leaf springs do a very adequate job. When the operating envelope is changed… lower ride height, more horsepower, different weight distribution, maybe a trailer, the leaf springs cannot be expected to perform as intended if the operating parameters are changed.
To clarify, picture a de-arched leaf that cannot compress. With 4-link suspension, the two functions of locating the rear axle and supporting the vehicle, are isolated.
Springs take care of supporting the vehicle — which leaves the links free to deal with locating and articulation. With a leaf spring rear suspension, softening the spring rate can cause other problems such as side to side flex or axle wrap when the axle tries to twist the leafs out of the vehicle. The rear of the vehicle is where you will actually see the most ride quality improvement.
This is because you sit closer to the rear end and any load changes fuel, passengers, luggage, trailer will be supported by the rear suspension. For those customers who are looking for ride quality… start with the rear end. It really comes down to where there is the most room. A forward position will offer slightly more travel and can sometimes offer better ride quality. A rear position can offer slightly more load capacity. Any spring, coil, leaf, or air, will perform better if placed farther apart under the chassis.
Keep in mind these performance differences are quite small and that the real criteria should be available space in your particular vehicle. We typically try to place the lower bars level at ride height. The upper bars should also be level, or slightly down at the front if need be.
This configuration will provide stable handling and braking characteristics. It is important to get the bars exactly the same from side to side to avoid unpredictable handling problems.
It is also extremely important to make sure the Panhard bar is level at your highway ride height. This will minimize side travel of the rear axle induced by the arc of the Panhard bar going through its travel. Obviously there are precise formulas for placement of the 4 link bars to maximize certain performance criteria, but these performance differences are quite small on a road car.
By the way, did we say NO?! If you want to see this effect for yourself, get a sheet of pegboard and a couple of yardsticks… simulate the scenario for yourself. The second effect of running the upper bars backward is completely screwed up handling dynamics.
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