Wednesday, March 21, 2012

Suspension Series Part 1 - Shocks and Springs

Selection of coil-over springs.

To avoid confusion, I want to be clear that I am not a suspension expert.  I am preparing my MR2 for NASA HPDE (High Performance Driving Events) with the intention of working toward my time trial and competition license.  Over the last couple months I have been reading articles and books about suspension design and tuning so that I can make my car as good as I can.  The more I learn the more I will share.

It seems that a lot of people confuse the purposes of the shocks and springs.  The spring should be determining the “stiffness” of the suspension not the shock.  That is not to say that their functions aren’t interrelated and as such it is very important that they complement each other in order for them to be effective.  Primarily the function of the spring is to keep the wheels in contact with the ground.  In short the springs allows the wheels to move vertically over bumps and into divots to maintain contact with the road.  The spring is actually the load bearing component of the suspension and is a boundary between what is referred to as “sprung” weight and “unsprung” weight (see future articles).  The shock’s, or the damper’s, sole purpose is to control the motion of the spring.

When a spring is compressed and then released, it will bounce or oscillate.  The same can happen to an unrestrained spring in a car and in such a case you will see the wheel literally bouncing after it hits a bump or pothole.  It’s not very often you will see this on the street, but I have seen it a few times.  The damper is designed to control the spring to prevent this from happening.  Ideally, when a wheel encounters a bump it will travel over it causing the spring to compress.  As the bump tapers off, the wheel will follow the profile of the bump back down to the level surface of the road and then its vertical motion will cease.  In other words the natural tendency for the spring to oscillate will be limited to one up and one down motion also known as a cycle.  This is achieved by a properly designed and matched set of springs and dampers. 

Another way to think of this relationship between the spring and the damper is to consider this system separate from the car.   If the spring is compressed then released it will want to bounce up and down a few times.  As I described above, paired with the right damper under the same test, the spring will be compressed and when released will rebound back to its uncompressed length and stop.  Now imagine that we replace the spring will one that is much stiffer but we don’t change the damper.  When we compress the spring we will need much more force to do so.  The spring is now storing or absorbing much more energy and when we release it the spring will rebound with a lot more force.  Now the spring is too strong for the damper and it cannot be controlled.  The spring will oscillate a couple times more than we would like it to because the damper is being over powered by the spring. This is an example of an under-damped car.  Now if we instead swapped out the spring for one that is much softer than the original, then this condition would be called over-damped.  When the spring is compressed and released the spring will rebound slowly; too slow to react to changing road/track conditions.  In a car, imagine hitting a bump and the spring can’t compress because the damper is to stiff.  This artificial increase in spring rate isn’t beneficial to the handling of the car because even minor bumps will be  

Adjustable Shocks

Considering the above examples, if you get stiffer springs for your car but use the stock dampers, then you may be under-damped depending on how much stiffer your new springs are.  This is precisely why adjustable struts/shocks/dampers are available.  You can buy adjustable dampers and throw on a new set of springs and tune the damper to them.  If you decide you need to change out the springs, then you can quickly make adjustments.  Some racers change their spring rates to suit specific tracks or in the case of an Auto-x’er they may change out their springs for different lot surfaces.  Theoretically, once you have set the dampers to the correct stiffness for your springs, you shouldn’t have to change them, but that isn’t so in the real world.  Different tracks and surfaces will need fine tuning.  However, it’s not likely your adjustments will vary much from the theoretical “ideal.”  Even adjustable dampers have a specific working range.  If you choose springs outside that range then you will need to change them out. 

One major drawback of adjustable dampers is that the adjustments can be very inconsistent and aren’t usually repeatable. Dennis North, a highly successful Auto-X’er has tested thousands of shocks and repeatability.  Unless you pony up big bucks for Penske’s, the adjustability is pretty much useless.  No two shocks from the same manufacturer are alike.  Sometimes adjusting them a little stiffer has the complete opposite effect and vice versa.  His suggestion, for the serious yet budget conscious racer, is to get Bilsteins which are re-valvable. This means you can disassemble the shocks fairly easily, change out the internal valves which control the compression, and rebound to tune the shock.  Obviously having to disassemble the shock is not as convenient as an adjustment knob, but it is more customizable. 

Another drawback of most adjustable dampers is that they will only adjust rebound, compression, or both simultaneously.  Compression is how much resistance the damper has when the spring/damper is being compressed and rebound is when the spring/shock is extending.  If you can only adjust one you might find yourself under/over damped in compression but OK in rebound or vice versa.  Higher end dampers will allow you to adjust both independently but they also tend to be quite a bit more expensive.  So the end of the story is, if they aren’t going to be consistent, it probably isn’t worth spending the extra money.

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