The dirt track swing arm suspension, is it dead?

I received an email several weeks ago from a faithful reader regarding what some of the basic principles around a dirt track swing arm suspension. The most general rule of thumb is that they are a very forgiving type of suspension. The rear springs are mounted on the lower trailing arms, which create a motion ratio for the rear springs separate from the motion ratio associated laterally when figuring wheel rate.

Motion ratio

The nice thing about this motion ratio on the left side is that when the car rolls over on to the right side springs, the left rear spring does not unload as fast or as much as the axle is dropping. It will essentially hold a little more wedge than if it were mounted on the birdcage. The bad thing about the springs being mounted on the arms is on the right side. As the car rolls on to the right rear, the swing arm pivot is lowering at the same time the shock mount is lowering. There is less spring support in body roll because both points are moving. Because of the motion ratio on the arm the spring is compressing a little, but the actual rate the axle feels is a fair amount softer than the actual spring rate.

There are some questions about bar angle on swing arm suspensions and it seems with the testing we have done there is a tradeoff between the bar angles in the back and the angle of the pull bar, if your car is running one. The more pull bar angle there is the less effective the rear trailing arm angles are. While we are on the topic of pull bar angle, 18 to 20 degrees is pretty standard, but it is dependent on several different factors. My rule of thumb is that if you step on the gas and the rear end jacks up and pulls off of the springs, there is too much pull bar angle in it. Ideally you want anti-squat in your car, a slow progression would be about the best.

Bar angles

The trailing arm angles and lengths really depend how instant and how much anti-squat you want to run in your car. If you are someone who likes a large amount of instant traction and a large amount of anti-squat then shorter rear trailing arms and more angle will help you achieve that. I personally believe that too much anti-squat with a swing arm car won’t keep the car running on the springs and lap times will suffer in the slick. Longer rear trailing arms will also be very forgiving in the rough muddy conditions. There is no perfect solution for the length of the rear trailing arms it’s really about driver feel.

There are exceptions to every rule and every general theory. On cars that have to run on a tire rule, it seems, a little shorter bar or a little more angle help the weight ‘hit’ the tire faster and aid in generating  heat into the tire a little quicker. Be cautioned though, the traction can run out on the straight too quickly if this is taken to an extreme. The best rule in racing is not if one is good two is better. Two could just make it easier to find your way to the back instead of going forward.

Pull bars

A little needs to be said about pull bars as well, since it seems that is the most common torque reaction devise in the swing arm style car. The longer the bar, the longer it pulls down the straight. The shorter the bar, the more instant traction, but the sooner it will run out on the straight. The biggest problem with pull bar length and angle is the clearance you have to mount it in the car. As you put more angle in it, the closer it gets to the drive shaft and the bottom of the frame. It is all a compromise. If you want to run a very long pull bar and want to run a lot of angle in it, you will probably have to offset it on one or the other side of the drive shaft.

Creating a new suspension or tuning the one you have often requires a lot of tradeoffs, so I would recommend designing a lot of adjust ability into the car of don’t be afraid of wrecking the paint when you have to cut and redesign it in the middle of the year.

Til next time,