In order for the rear of our race cars to steer, there must be vertical motion. For most dirt cars using the traditional four-link rear suspension system, the motion can be created and controlled through several means. There are three basic methods that can cause rear movement on a dirt car. One is the use of lift arms, or lift bars, second is the use of the angled panhard bar with the left side mounted to the frame and the right side much lower and mounted to the rear end. Third is the use of the left rear four link angles that cause the LR wheel to be pushed up under the front mounts lifting the LR corner of the car.
The extent and use of the four-link angles in all of the above depends on what the racers goals are and somewhat the track conditions that must be dealt with. There are a lot of different settings involved with the rear four-link suspension and most racers are overwhelmed with the complexity. So, let’s examine how the angles affect rear steer and learn when and how much steer we need to use.
Explaining The Four-Link System – The parallel 4-link rear suspension is common to Dirt Late Model, Dirt Modified and Sprint Cars. The original purpose for using the 4-link suspension was to have a suspension system that produced very little, even zero, rear steer as the chassis moved vertically. Racers, being true to their nature decided to experiment with various angles in the 4-link and found that zero rear steer might not be the most efficient way to go under all circumstances.
Today, we have various schools of thought on where to position the links on a 4-link system and many more theories on why. Let’s examine what happens with the various changes and look at the big picture to try to understand what is really happening to our cars. Even though the current trend among top Dirt Late Model racers is to minimize the steering characteristics of the rear suspension, there are times when even these teams must get a little more radical.
Basic 4-Link Designs – There are two basic designs of the 4-link rear suspension, the “standard” 4-link where both links are forward of the rear end axle tubes and then there is the “Z” link where the top link is rearward and the bottom link runs forward. Both of these designs can be positioned to produce near zero rear steer and by far the most common is the 4-link. The 4-Link design can be made to produce somewhat more rear steer than the Z-link.
One of the big differences between these systems is ability to create weight jacking and forced loading of the left rear tire that is common with the 4-link system whereas the Z-link cannot. As we introduce more rear steer which moves the left rear wheel forward, the angle of the bars is such that the wheel is forced down and is trying to lift the chassis, so load is transferred onto the left rear tire. This loading is desired by some in order to produce more bite under dry and slick conditions.
If you look at the way the car is situated when the rear is severely steered to the right, the left rear tire is pointed more to the middle of the front tires and is driving from the middle of the car. Because of the added loading of this tire, most of the rear weight is on this tire. The right rear tire is helping to locate the rear of the car, but does little to drive it off the corners.
Both of the suspension types are usually attached to a bird cage that may or may not be locked to the rear axle tube and where the rear end may be free to rotate. A separate structure is attached to the rear end to control rotational movement of the rear end upon acceleration and braking. This could be a “third” link or pull bar, similar to that used on a three link suspension, a lift arm that runs forward and is attached well in front of the rear end or a combination of several systems.
If the bird cage link brackets at both sides were mounted solid to the rear axle tube, then as the car rolled in the turns, there would be a significant amount of binding going on because the bird cages would be trying to move different amounts and possibly in different directions. The suspension would be trying to twist the rear end as each axle tube would be rotated differently.
If we change the angles of the links so that one side of the car produces more fore/aft movement at the bird cage, we cause that end of the rear end to move in a direction that will cause the rear of the car to steer away from straight ahead. This is called Rear Steer and most of what is used for dirt racing is steering to the right.
Under some conditions, rear steer is less desirable, especially on hard and tight dirt tracks that act more like asphalt than dirt. Rear steer on dirt tracks that are slick is not only acceptable, but could be useful under certain conditions.
Understanding Rear Steer – To even begin to understand how the car will rear-steer and to what extent, we first need to completely understand the movement of the chassis and what causes this movement. The chassis mounting points of the 4-link and Z-link will move vertically as the car transitions into and out of the turns and even down the straightaway and the amount of movement dictates the degree of rear steer.
As a chassis rolls in the turns, three basic things can be happening overall: 1) the left side of the chassis may move up while the right side may move down, 2) the right side may move down and the left side may stay near that static location, or 3) the left side may move up and the right side may remain unchanged. With the same set of suspension link locations at each side, a car may well produce very different rear steer characteristics from each of the three scenarios.
A 4-link can be made to produce varying amounts of fore and aft movement of each end of the rear axle, depending on the combined angles of the links. By starting at a neutral setting for the links, meaning that for a certain range of movement up or down, the axle will not move fore and aft, we will show you how we can produce axle movement.
If, on a 4-link, we move the chassis mount for the bottom link up, then as the chassis moves up, the rear axle will move more forward. On the Z-link, we see the same effect for the bottom link. The opposite is true if the chassis moves down. For both systems, the axle would move to the rear. That is exactly why we need to know which way the chassis is moving at each side of the car under all conditions.
Knowledge of the extent and direction of shock travels will come in handy as we plan out our rear geometry. We can translate shock movement to suspension movement. Using either shock travel indicators or data acquisition will tell us what is really happening. I don’t see wide spread use of electronic data gathering on dirt cars, so some mechanical device must be used to help us understand our true movements.
Effect Of Panhard Bar Angle – A direct influence on chassis movement in the turns is the J-bar or panhard bar angle. If the bar is mounted more parallel to the ground, then it will have little influence on the vertical location of the chassis in the turns. If there is a lot of angle in the bar with the left end mounted on the chassis and higher than the right end, then as the car turns left, the bar angle will have a jacking affect causing the left side of the bar to want to ride up over the right side of the bar. This movement would raise the entire rear of the car.
Under those conditions, if the car rolls, and we know it does, and the whole chassis rises up, as we too can visually see, then the right side links may well remain in their static locations producing near zero rear steer at that side. On the other side of the chassis, there will be a combined vertical movement of the front mounts of those links to where the lift associated with the bar angle will be combined with the roll lift so that together these two effects can produce a large amount of forward movement of the left rear wheel. This movement pulls that end of the axle forward and the rear end will steer to the right.
Upon acceleration off the corners, the rear end will be driven forward and if the forward mounts of the links are higher than the rear mounts, there is a further movement of the chassis to a higher level through a jacking affect.
If we just look at the way the car steers, we might conclude that this is not a very good idea. On asphalt this would produce a very loose car that would be un-drivable. But if we look at the whole picture, including the aerodynamics of the body, we start to see why our lap times may be improved by doing this on dirt, especially on a very dry slick race track.
Side Force Flat Plate Aerodynamics – Winged Sprint Cars will generally run at an angle to the direction they are moving through the turns. The tall side plates on the wing catch a lot of air and will produce a lateral force that is the opposite of the centrifugal force that tries to take the car to the wall. Long story short, the aero force counteracts the lateral G-force and helps the car go faster through the turns, just like having more tire grip.
Another good example of this are the unlimited asphalt Late Models that sometimes run at Kalamazoo Speedway in Michigan trying to set the world record short track lap speed. They run very large panels on each side similar to the ones on a sprint car, only bigger, and those produce the same effect of countering the lateral forces using the air.
The combined effects that raise the whole rear of the car also put the rear spoiler higher into the wind stream and that can produce more aero downforce at the rear. This helps give us more traction to provide better bite off the corners.
Necessity Of Making Changes – If the track has a lot of grip, then we need much less rear steer and the associated aero help and so we make changes to our rear links so that less rear steer occurs. We may even benefit from creating opposite rear steer in small amounts, to the left, to gain more rear traction, just like we do on asphalt. The operative word here is CHANGE. We must be willing to make changes and a more thorough understanding of what happens with each change will make it easier to do with better results.
What happens at many dirt tracks is that the track is wet and tacky as the day starts out. A car that is jacked up and rear steered to the max just won’t get through the mud as well as one that is more level with all four tires on the ground. We can position the links so that there is very little rear-steer for these conditions.
As the track dries out and becomes slicker, we may need more rear steer and rear jacking to get the rear spoiler up into the air stream for more rear downforce and to drive the left rear tire into the track. The rear steer has more effect on the angle of the body related to the direction that the car is traveling and an aero side force helps pull the car to the left to keep it from sliding. Putting more angle in the J-bar is warranted now.
Measuring 4-Link Rear Steer – You can measure your rear steer in your shop. I have had racers I know do this with their NE big block Modifieds. This is an easy process and can be accomplished in one evening at the shop.
Put the front and rear link mounts in the “neutral” adjusting holes, or the middle holes for the range of adjustment. Then set the car at ride height and take a measurement level to the ground from the rear wheel rim on each side to the front say 50 or 60 inches. Put tape on the body and mark the distance.
Then jack up the left side in one inch increments and take measurements and record the height change and measurement. Go as far as you need to go to replicate the movement of the chassis on the track you run on. The difference in your measurements from the original static measurement is the rear steer amount. It will be fore or aft.
Go then to the right rear and raise and lower the chassis in one inch increments again recording the results. This corner of the car will either be lower or higher through the turns depending on your overall setup.
You can move the front and/or rear mounts up or down to see how the left and right wheels move with chassis movement. This process will teach you much of what you need to know about your car’s rear steer. Find and mark the holes that will produce Zero Rear Steer. Then chose holes that will steer the rear end the amounts that you think necessary to deal with slick conditions and anything in between.
Creating Setup Sheets – Once we fully understand how link angle changes produce rear steer at each side, we can make helpful adjustments at the track as the conditions change. We need to plan out which changes to make and be able to do them fast with little effort. A setup sheet that shows which holes to mount the links to for each set of conditions would help the crew make fast changes. If you don’t want your crew to know why you are doing things (secrecy is an asset at times) then just number the different sheets and tell them to set the car to sheet “3”, period.
Because we need to adjust other parameters on the car for changing conditions, we can include spring rate changes, shock changes and fifth and sixth coil changes as well when we make up the setup sheets. Once we develop our setup sheets, as we race the car, we can tweak the numbers according to the results.
The process of dialing in the car to the conditions using our setup sheets may take a few races, but at least we will have a plan that takes us in a positive direction. Remember that tacky tracks require less rear steer than when it goes dry and slick and plan accordingly. This will take much of the confusion out of the process of making changes when the track conditions change.
Sources:
Beak Built Chassis
(828) 478-4400
Facebook.com/beakbuilt
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