So now that you’ve read our tech talk on selecting a shock, you now need to match the correct springs to them for the type of driving you will be doing (daily driven road or track use).
First thing you will want to know is how much does each corner of the vehicle weigh. Lets say a GMC typhoon weighs 4k lbs without a driver. On the scales we see 1200lbs on each front corner, and 800 on each rear corner. (example weights only) Clearly the front will require a higher spring rate than the rear.
Springs are typically rated in pounds per inch. So, a 500 lb spring will compress 1 inch for every 500 pounds applied. Fairly straight forward.
Now, we can determine what springs to use on each corner. So we call Einstein and he reminds us that a 500lb rate spring requires 500lbs of force to compress one inch. Our example rear corner weight is 800lbs. Ok, easy enough, 800lb corner, 800 lb spring…..not so fast. This would only be true if the spring is mounted vertically at the point of the tire’s contact patch. This would mean the shocks would be mounted perpendicular to the ground directly above the tire, find that suspension out there and I’ll give you a cookie. Typically they will be mounted inboard and at an angle. So now we need to know how much does the shock compress for 1″ of upward tire travel. Being inboard on say a front A Arm will mean that distance is something less than 1″ and require a spring stiffer than our 800lb corner weight implies. The suspension essentially acts as a lever overpowering the force of the spring. To complicate things further the angle of the shock plays a part, the further from vertical you get the less the shock will travel for the same 1″ of suspension travel, this amplifies the leverage provided by the A arm in the example above. All these factors come together into what is called shock MOTION RATIO.
So what happens if the spring installed is too stiff? It would seem obvious, as you hit a bump or the road surface changes the suspension doesn’t move. The spring will not absorb the energy of the suspension’s upward travel. The energy is instead transferred into the vehicle causing an overly firm, or very harsh ride. Proper compression and rebound settings are your friend here. You can dial in the ride quality (and to some degree traction) according to the road surface and the style of driving your doing at any given time, ie autocross vs drag racing. The opposite can happen if an overly soft spring is selected, it will collapse too easily, or bottom out (coil bind). This is highlighted in tech talk #1.
A common occurrence is that a worn out shock, poorly designed/poorly chosen shock, or improper compression/rebound adjustments will lead you to think your springs are too soft. In turn, you order stiffer springs which create a whole different problem (see above). The shock valving is the big issue, and it is far easier to turn a few clicks into the shock to dial in your ride than swap out springs. The key concept to remember here is that shocks and springs are a system and both must be selected properly.
You cannot properly select a spring/shock combo unless you know the MOTION RATIO. Having XYZ vehicle that weighs ABC pounds is not enough information, as an engineer we need to know how much each shock moves for 1″ of suspension travel and that can very well change depending on the manufacture and design of the suspension being used.
Stay tuned for more topics, and if you have a suggestion for topic to cover let us know in the comments.