alt

Juicing it up!

/in /by

Well, you decided it is time to “upgrade you car” with a few bits and pieces. While this article is not meant to cover what you can and cannot do to move from Stock to Production to Improved, etc… it will cover some relatively easy and somewhat inexpensive changes you can make to help improve your times.

Suspension Change Order and Basics: How about some negative camber? Most notably, camber adjustment is one of the first things people change in order to allow the car to use its tires more effectively. Beyond that, there are a number of things you can do. The general order of changes for tuning a suspension system are:

  • Spring rates and ride height
  • Adjust alignment
  • Select sway bars
  • Optimize alignment
  • Tune shocks
  • Fine tune alignment

Many drivers go through this process and then find themselves back at the first step. This is normal and is all part of tuning.

The car’s suspension geometry and sprig rates determine the car’s basic tendencies. From this point the alignment can be adjusted to make the tires work more efficiently. The testing that determines the best alignment at this point also gives a good indication as to the correct sway bars to be used on the car. After the “correct” sway bars are on the car, the alignment will likely need some adjustment. With the springs and bars selected, the shocks can be tuned to help the car through slaloms.

Getting into the details:

Ride height most notably affects the center of gravity. A lower center of gravity reduces weight transfer. Be careful, lower is not always better. There are several things that weigh in when choosing ride height: center of gravity, tire clearance, camber and effect on body roll.

A lower center of gravity is good. The car should be as low as possible without compromising other characteristics. Tire clearance is necessary and the car should be high enough so as not to allow the tires to rub the fenders (think about where the tires versus fenders will be under a heavy turn (compression)).

Camber describes the camber gain or loss at the wheel due to suspension travel. This is a function of the geometry of the suspension arms and struts. Normally you would want the wheel to gain some amount of negative camber as the tire is moved upward into the fender (bump direction). The chassis usually rolls toward the tire, causing the tire to lose camber relative to the ground. The gain associated with the bump motion reduces the apparent camber loss caused by the body roll. Similarly, you would like the wheel to lose some negative camber when the tire is moved in the rebound direction or downward. In the end, the change in camber helps keep the tire flatter on the pavement during cornering. Lastly, the ride height of a car affects how much of the chassis will roll in a corner.

Spring Rates need to be selected to control body motions and still allow the tire to follow the pavement. Ever feel like the car is chattering across the road when hitting some bumps in a turn? Amongst other reasons, stiff springs can cause this result. In the end, the springs need to be stiff enough to keep the body roll in check, but soft enough to comply with the bumps.

Two pieces of information are very important to the selection of spring rates; how much the spring moves when the tire moves (motion ratio) and the weight supported by that axle. The third piece of information that is often relevant is the amount of lateral grip that is expected. More grip means the need for a higher spring rate to control body roll.

Sway bars are another way to combat body roll. Sway bars are not as effective as springs in combating body roll, but they, both front and rears, are used to tune the car.

Most cars with a stock suspension, do not have enough camber adjustment to accommodate the needs of an AutoXer. There are a number of ways to gain more camber adjustment and each has side effects. Best leave this to you local specialist to go over this with you as this an article all unto itself.

Shocks, shocks and more shocks. Lots of choices here. Generally shocks are there to damp unwanted chassis and wheel motions (hence the optional name; dampers). Without the shock, the wheel and tire will bounce for a long long time. This bouncing causes constant pressure between the tire and the pavement to vary wildly, and with it, the size of the friction circle (refer to an earlier article to learn about this) will vary wildly.

The sprung mass of the car (chassis) and the unsprung mass of the car (wheel, tire, brakes, etc..) are connected by a spring. The body motions and the road surface cause the the two masses to oscillate relative to each other. Damping absorbs the energy of the oscillation, stopping the motion and returning the system to equilibrium.  So what can the “wrong” or improperly set shocks do? A severely overdamped spring causes the tire to skip across the pavement. A spring that is close to being critically damped performs as desired. In reality, you generally keep the system slightly over damped to give yourself a car with quick response. So what if the pavement is different at each venue? Adjustable shocks can be used to allow for changes in surface roughness and variations in either the mass or spring stiffness of the suspension system.

Well,  I hope this gives you some idea of how AutoX’ers (and DE’ers) are modifying their suspension to enhance performance or to just simply dial out some unwanted handling behavior.

Special thanks to Andrew Howe for the sound and understandable advice.

Tom