Fundamental Principles of Automotive Suspension
Thanks to Pismoboy (Chris) for the picture
There are a lot of myths and confusion floating around the internet concerning torsion bars, coil springs, shock absorbers, and spring spacers. This includes the concept of preload, the function of shock absorbers, etc. People often post questions on the message boards regarding which torsion bar to use to lift a truck and how will coil spacers affect spring rate. I'm going to try to unconfuse the confused and unmyth some myths by explaining how suspension works and how various modifications affect it.
First, the basics; what do the various components do?
Shock Absorbers/Dampers: The function of shock absorbers is to dampen the up and down movement of the truck and nothing more. If a truck doesn't have shocks then it will continue to oscillate up and down for a long time after hitting a bump. Inside a shock is a piston that moves within a fluid when the shock is compressed or extended. In simple terms, there is a hole in the piston that allows the fluid to pass from one side of the piston to the other when the piston moves. Fundamentally speaking, this system wants to resist movement, thereby stopping the oscillations. The vast majority of shocks do not provide any appreciable lift. There are a few, high pressure shocks like bilsteins that do help lift a little, but nothing much.
Torsion Bars: found on the front end of early toyota trucks and 4runners, torsion bars (tbars) are what support the front of the truck. Tbars are the simplest of springs; it's just a long, round piece of steel that is twisted by the suspension arms. This twisting provides the springy action needed hold up the truck. Have you ever built one of those rubber band powered model airplanes? The rubber band acts just like a torsion bar. The spring rate of tbars is typically measured in foot-pounds per degree of rotation (lbs/deg). So, if the tbar is rated at 100ft-lbs/deg, it'll take 300lbs to rotate the free end of the tbar 3 degrees.
Coil Springs: coil springs can be found in the rear of 2nd gen 4runners, in the front and rear of 3rd and 4th gen 4runners, and in the front of Tacomas. Fundamentally, coil springs are torsion bars that are wrapped into a coil. If one were to look at the metal of the spring under compression, one would find it is being twisted. However, further explanation of this behavior is beyond the scope of this article. The spring rate of coil springs are measured in pounds per inch. So, a 100lb/in spring would need 300lbs exerted on it to compress it 3 inches.
Coil spacers: Spacers are used to inexpensively lift a truck. They are pieces of plastic or aluminum that are placed at the top or bottom of the coil spring, or on top of a coilover shock.
Control Arms: These are the "links" found in the rear of coil sprung 4runners and the A-arms found in the front of independent front suspension. The job of control arms is to locate the tire and keep the tire as close to perpendicular to the road as possible. The springs and shocks attach to the control arms (or in the case of the rear suspension, the axle) and hold up the truck.
Now that we know what everything does, lets see how they all work together.
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