Spring Rate Mechanics.

Life can be tedious enough already without a harsh ride. So what’s this all about?

Text & Photography: NICK BUTLER

All of us have seen, used, or are using super-short coil-over shock absorber units. This minimalist movement started when the pillars of American rodding decided it would be real neat to shrink the most vital component of a car’s suspension down to a size that makes a Roman chariot look like it has suspension. Since then followers of fashion have been paying dearly for the privilege. If your politics dictate that if “they” do it, it must be right and you must follow, save yourself some time and read no further. If, however, you believe there might be a better way this article may give you food for thought.

Before we look at how and why we set up a suspension system, let’s remind ourselves of the absolute fundamentals. We need suspension to cushion out bumps and to prevent wheels leaving the ground every time the vehicle hits a bump or dip in the road. If suspension settings are too stiff neither of these requirements are satisfied (most of us have seen Fl cars balancing on three wheels when they get a puncture - their suspension is very stiff), whilst if the settings are too soft we need a lot of suspension movement (and corresponding ground clearance) to stop the axles bottoming out on their bump stops. Body roll also becomes a problem.
Ignoring the intricacies of geometry for the moment, what the suspension sees when it hits a bump is a vertical acceleration directly proportional to the size of the bump and the speed at which the car hits it (large bump at high speed = large acceleration). The spring’s job is to absorb that acceleration by turning it into potential energy. It does this by compressing, then as the acceleration is reduced or removed it extends back to its normal length and thereby releases that stored energy by restoring the vehicle to its normal ride height. The shock absorber’s job is to slow the whole process and stop the car oscillating up and down.

Our suspension must therefore compromise between:
1. too soft or too hard.
2. Not enough or too much roll resistance.
3. Overall suspension characteristics.
Our Fl car analogy can he further extended by comparing it with a mid-’60s American sponge wagon. Both represent no-compromise approaches to two opposed requirements. The job of the suspension on the Fl car is to make the car corner as quickly as possible and at the same time restrict the change in height between the underside of the car and the road surface for optimum ground effect. Because suspension travel is restricted so much the spring rates are extremely high in order to prevent bottoming out as much as possible. Why does this matter if driver comfort is of no consequence?
Simple, every action has an equal and opposite reaction. If the suspension hits the stops hard the car will leap off the ground and if that bump happens to be on a corner, the next time the wheels contact terra firma the car might already be in the scenery. The uncomfortable ride that results from the above is in direct contrast to the Chevy cruising down to the local burger bar. Its marshmallow suspension settings can be viewed as follows:
1. Plenty of travel to absorb large bumps without transferring energy through to the passengers.
2. Soft springs allowable because of large amount of travel.
3. Plenty of ground clearance to give full rein to the suspension.
4. Soft acting shock absorbers to prevent artificially hardening up the springs.
5. The anti-roll bars must be equally soft to prevent the independent suspension units tying together too much, hence -
6. Body roll when cornering.
7. Pitching occurs over transverse ridges or when coming to a sudden stop.
We don’t need to use our imagination very much to realise that the suspension settings in the two cars give one that corners on rails but is damned uncomfortable and one that glides along is if it were airborne but responds badly to changes in direction

1. Are you going to run your car super low? (ie, 2 to 3” ground clearance), if the answer is yes you will need high spring rates to prevent bottoming out and bump stops set to limit the suspension movement in order to maintain some ground clearance at full bump.
2. Are you committed to running super short shock absorbers? If the answer is yes then again you will have to run high spring rates in order to prevent the unit bottoming out over the smallest of bumps.
3. Do you want more than 3” to 4” suspension travel? If yes, then you will need a shock absorber with a corresponding amount of travel unless you incline the unit at
an angle or build mechanical advantage into the system. More on this later.
4. Do you want a comfortably soft ride? Then you will need relatively low spring rates along with adequate amounts of travel to allow the suspension to work off the bumps.
5. Do you want to reduce bump thump (The tremendous crash felt if you hit a pothole)? If you are using coil-over-shocks you won’t reduce it because the bushes in the shock eyes have to be very stiff to withstand the constant compressive load they experience from the moment you fit them (some shockers are fitted with Rose bearings which give the harshest ride of all).

If you are wise enough to fit separate shocks and springs then shock eyes can be selected with big soft rubber bushes which give your suspension the best chance of absorbing that huge impact that occurs in a fraction of a second. Whilst on the subject of bushes it’s worth remembering that the big boys (GM,Ford etc.) spend literally millions on the design of these little rubber bushes. Some of you may remember an in phrase back in the early ‘70s - ‘void bushes’. As a result of their enormous investment to improve passenger comfort Ford realised that they could “tune” the frequency response (and absorption) rate of suspension bushes by moulding in little pockets. If you look at modern day items you’ll notice a plethora of pockets and bump ridges which go to make the bush soft in normal use andharder in extreme conditions.
Some are even directional in that they are softer in one direction than another.
In the light of this overwhelming evidence, can even the toughest guy on the street not swallow very hard when he elects to heave out “those old fuddy-duddy rubber bushes” and pushes in a pair of his favourite coloured plastic ones?
Spring rates are generally referred to in lbs per inch. Typically a front spring on an average street rod may be 400lb/in. However, if we incline our favourite coil-over shock absorber unit we will affect the spring rate seen by the wheel. Likewise if the shock lower eye fixing is placed inboard of the outer wishbone/spindle swivel joint, then this mechanical advantage will reduce the rate seen by the wheel. This reduction in rate is also accompanied by an increase in the amount of travel at the wheel relative to shock absorber movement. Some sharp talking salesmen will say that’s all you need to do to make a short shock absorber unit (typically with just 2” travel) work. What they hesitate to point out (even if they know) is that the inclining trick produces a geometry pitfall that requires very high spring rates. Additionally this set-up will at most give you a linear compression of the spring with wheel movement. This is not ideal. Why? Remember we learned that large compressions of the suspension are the result of hitting large bumps at high speeds. The ideal way to resolve these impacts is with a rate which increases and thereby delays or prevents the car from hitting the stops - the worst thing that can happen. Production cars frequently use progressively wound springs which bunch up as they compress. At the very least the spring is mounted vertically so the rate remains the same rather than decreasing. The two other significant pitfalls are that firstly, the linear rate syndrome does nothing to resist body roll in cornering and secondly, over loading of springs can occur.
This leads us onto safe stress:
Every spring has a maximum safe stress that cannot be exceeded if it is to recover to its original length over its expected lifetime. For example, the allowable working stress for a coil spring made from 1/2” diameter chrome vanadium wire is around 100,000 lbs./sq.in. under average service conditions. The formula for calculating the stress is:

Load x mean coil diam.
0.393 x (wire diam to power of 3)

Let’s run some average figures through this formula and see there is still more gloom associated with the All American Dream set up. Assume our small block powered car weighs 2500 lbs with 60% over the front wheels, i.e. 750 lbs. per front wheel. Now build in the short shocks at an angle and inboard of the outer swivel to give, say, 2:1 advantage (2” of wheel movement gives 1” of shock/spring movement).
Our 400lb/in. spring with 8 coils and an inside diameter of 2 1/4” has a wire diameter of around 0.43”. So for every inch of compression (produced by 400lbs of load) the spring will experience 34,304 lbs/sq.in. stress. Our maximum safe stress is exceeded as we pass 2.92” compression


To support the weight of the car the spring must compress:

750x2 3.75”

We have therefore exceeded the safe stress of the spring before the car has even started to roll.
The accepted remedy as our over stressed spring begins to settle (permanently compress) is to fit ever stiffer springs. This is all very well if you want stiff suspension with minimum travel but if you don’t, what next?

If we look back at the stress formula you’ll notice that if you decrease the load and/or increase the wire diameter you’ll decrease the stress in the spring. This is the key to softening up suspension. Before diving head first into yet more theory, take a break and think of your daily beater - Escort, Astra, Golf - they all use McPherson strut front suspension. The spring acts directly with the wheel (1” wheel movement = 1” spring movement) The spring will also be a lot larger than our coil-over shock spring in diameter. The resultant spring rate will be low - typically less than 300 lb/in. The travel will be high - typically around 6”. And what have you got? A nice comfortable ride. Yet again compromise confronts us. The larger springs and longer shock absorber units are bulky and therefore more difficult to package neatly. This may not matter on your fat fendered Ford, but on an open-wheeled
roadster it certainly won’t look racy. To make the springs direct acting you have to mount them as outboard as possible (i.e. closer to the wheel). This again does nothing for the neat package devotee.
Open-wheeled cars confront our obsession with neatness. As one of our customers said recently, “I know it (short, inclined coil-over unit) doesn’t work as well but I want it ‘cos it looks good.”
For years I’ve taken a keen interest in race car trends knowing these projectiles represent the forefront of handling and design. By mellowing out some of the more extreme elements, principles of their suspension design have been incorporated in many Auto Imagination projects, in most cases without adding to cost or complexity, but merely packaging the same components more sensitively. It would seem, at last, that other street rod enthusiasts are starting to look beyond the accepted style boundaries. Two cars I judged at the excellent American Speed & Custom Show had inboard cantilever suspension systems enabling the builder’s to unclutter their corners and choose optimum spring sizes,
rates and geometry settings. By so doing they certainly received full points for innovation and clean looks and, if they did their sums correctly, have been rewarded by sweet handling cars too.

Now you’ve started to understand why the various elements of suspension affect ride quality so much, you can choose your compromises wisely and benefit from a free handout in the comfort department. Even the smallest changes really do have significant pro’s and con’s. It makes so much sense to select the parts on merit, not just on how trendy they happen to be. If you’re building a full-bodied car there can be no excuse for not building a suspension that gives you a comfortable ride. Even with the bulk of larger springs and shocks, with thoughtful packaging the whole assembly can be tucked behind inner wheel wells. The bottom line is, surely no one can excuse exchanging spring rates for comfort.