Monday, February 27, 2012

Springs & co.

The spring is one of the most important parts of the suspension. You might say:"well no sh*#, Antony..." Anyway, here are some details which some of you may find interesting: 

If you put a linear spring in a pressing machine. Every other single mm of compression will require an extra equal x amount of force, which is normally measured in Newtons (N). Kilogram force (Kgf) or pound force (lbf) units are also valid to measure the spring rates. Many will speak in Kgf, because it gives people more sense of what they are talking about. Remember however that 1 Kgf is approximately 9,81N. The EIBACH springs, which I have knowledge about, are measured and marked in Newtons (N).

Back in the early 2000's I had a Toyota Corolla WRC and we had EIBACH linear springs for it. On a competition EIBACH spring you can see a number code as, for example: 300-75-0055. Where 300 is the length in mm, 75 the inner diameter in mm and 55 the rate in N/mm. I measured them in our press, just for my own information. Those springs took, for the 55 example, 55 N to compress the first mm. Compressing it 1 more mm required another 55N, therefore 110N for the first 2 mm of spring compression. I noticed that almost all the springs I measured never really gave the same readings. They often varied by 1 to 3N or so. Either my press was inaccurate, or they must be very hard to manufacture identically. My guess tells me springs are hard to make identical. EIBACH does state that there is an allowed 2% tolerance which may go either way on the spring rate, which at worst might come to 4% spring rate difference between a right corner and a left corner on your car. It may be worth checking your springs before you mount them! Maybe somebody could share their experience with me?

In the mid to late 90's, the works Toyotas were running very hard springs, in the range of 50-80N/mm on the front for gravel and as much as 90-100N/mm on tarmac, if I remember correctly. Things changed drastically with the involvement of top Nordic drivers and Toyota Team Sweden (TTS) who went, together with some evolutions of the Öhlins dampers, way down to values around 45N/mm on the front. In those days, this was a revolution. I remember a particular time when, back in and around 1998, we had picked up a Toyota Celica st205 GrA from TTS and took it down to Portugal, in the Fafe Lameirinha and Cabeiciras de Basto area (Northern Portugal) to test it on smooth gravel. This Celica was on the superstrut suspension (anyone heard of this one?) as opposed to Mc Pherson. I thought the car was fantastic so we called the TTS boss and he said something like:"hey, wait your car is accidentally on snow springs, that it is not right..." I thought the car was awesome on softer snow springs (55N/mm on front). The superstrut also helped.

Celica st205 GrA ready for rally

"Superstrut" on Celica st205 GrA
Corolla WRC upright McPherson

Linear springs were used on the Fords I drove as well, except the rates were much lower than on the Corolla and Celica. By the time I had joined Ford they were using 30N/mm on the front generally, for gravel, and around 50N/mm for tarmac (not so sure anymore on the tarmac rates, although I do remember we had a rain setup where the rates went down by 5N/mm all around). On gravel the standard setting was 30N/mm front and 25N/mm rear. I asked for a 30/21 ratio for my car. This gave my car a bit more grip on the rear and hence a bit more understeer, which suited my driving style better. I once tried a 30/27.5 ratio during Acropolis shakedown. It was slow and rubbish for me, way too much oversteer. All this to say that springs are super important for car balance and sometimes you don't know if you have the right setup until you tried them all and compare on the clock. In my case, I was significantly faster with the 30/21 balance even though the 30/25 felt better.

Back to MAZDA, who's initiative (in and around 1987-1992) was to use progressive springs rather than linear. Basically the spring rates had ramps that went from softer to harder, as compression increased. I remember my father saying that they were extremely hard to figure out. He spoke of his engineers making regular custom orders to EIBACH for various rates and ramps to test on the car. I also remember that some of those desired ramps were not achievable due to material constraints. I believe this has probably evolved now (I will make a future post, discussing the basics of metal properties and how this was a major subject in the early 90's for MAZDA). Luckily we had some very good engineers and drivers for this sort of setup work. Timo Salonen and Hannu Mikkola had appropriate feeling for the suspension adjustments and how it translated to corner speed, general car balance and traction. Back in 2006 I crossed paths with an old MAZDA engineer who had moved on to MITSUBISHI during the Tommi Mäkinen years and then on to the Dakar team. I asked if he had taken the progressive spring knowledge over there and he said: "Waddaya think?"

Neverthless the combination of KAYABA dampers and progressive springs allowed the MAZDA to have better traction than the competition in the late 80's and early 90's. KAYABA's gas-cooled damper meant that the damper did not overheat and lose its effectiveness as easily as before (when dampers overheat, they basically become air pumps until eventually they seize altogether, then you're screwed). Thus the damper controlled the spring travel with effectiveness. Traction was the key to the car's success and it was attained thanks to the softness on the first few centimeters of spring travel which then became as hard as necessary for rough terrain. It showed especially on slippery surfaces where Timo, Hannu and Ingvar's skills together with the MAZDA's traction had the edge over the competition's raw, less-controlled power. Unfortunately, as soon as there was good grip, the horsepower disadvantage was a real problem.

An interesting note I'd like to point out is that back in the late 80's and early 90's the popular concept was to let the spring do the work. The damper was merely there to keep it under control, discreetly, if you see what I mean. The philosophy later changed completely. The shock absorber became king. Thanks to advances in technology, dampers became so "bullet proof" and effective that, in a way, springs became secondary. To exagerate a bit you could say that, nowadays, they just hold the car up... here is a little anecdote to illustrate my point about the suspension revolution:

My father often test drove the MAZDA after Timo and Hannu. They would determine a setup and if they had doubts they'd ask him to drive and give his opinion. Although he wasn't competing anymore at that time, he had a pretty good idea of what the car could do. 12 years later he rode with me as I tested the Ford. His reaction to how the Ford "swallowed" bumps and stones was interesting. Driving the MAZDA, you often had to "navigate" around hazards whereas for FORD the motto was "just go".  

On some modern rally car uprights, you will notice a second smaller spring accompanying the main one. This is usually a "helper spring". Below is a clear pic of the helper:
Spring with helper on top
You can see a hint of one (in red), sitting above the yellow spring in the following pic:

EIBACH linear spring on Celica st205 grA

On the following pic you can see a small white spring under the main one. I believe it is a helper as well.   
White helper spring on MINI upright
The helper is used to keep the main one always aligned perfectly and tense during lift off. It has an insignificant spring rate and will be at full coil-bind on normal driving position, therefore it has no effect in terms of car suspension.

There also exists "tender" springs:

Spring with tender on top
Tenders have a significant spring rate and are not at full coil-bind on normal driving position. I know the tenders exist in linear or progressive specification. They have an effect, at least on the start of suspension movement until they reach full coil-bind, at which point the main spring works on its own. Their use, in a 2 spring system, significantly complicates matters. I am unaware if they are used on top rally cars. Maybe someone could fill us in on this.


  1. The spring is one of the most important parts of the suspension. Its very informative and interesting article.all the points are very useful. Simple but very effective writing. Thanks for sharing such a nice post.
    Compression springs

  2. Hello Antony, thank you again for your informative writing.
    I rally only at a very "clubman" level with an old Group2 car, but tried to measure my springs, too. two years ago, I borrowed a pressing machine for a couple of days and found it very difficult to get repeatable results even from the same spring. Have to add that I dont use racing springs with machined flat upper/lower ends, but standard road car shape which hardly fitted the available tables of the machine. So the first 1 or 2 centimeters of compression it was impossible to record any reading. With further compression the readings became more repeatable. What I did not manage to do was to measure a progressive spring. I recognised the progessive force but have no idea how a spring manufacturer would call the figures - I think there is no industrial standard on this, any manufacturer measures maybe to his own standard and hence you find no forces for progressive springs to compare.
    At least I could assort "pairs" of springs together out of that what was available in my garage. It isn´t of big use anyway as clubmen cannot run a back to back test of various suspensions with exact time taking. Its more about finding some to feel comfortable with. Some which give a forgiving handling which helps on the stage as myself is rallying rarely and such the challenge is rather to avoid time-consumptioning mistakes than bring up possible corner speed or traction to the ultimate.

  3. Do you calculate the ride frequencies and things like that when you choose the spring rates?

  4. I have never witnessed it personally but it would make sense that it is done.

  5. If one compares a LONGER spring to a SHORTER spring both having the same wire diameter, same number of coils and same spring diameter, the LONGER spring is more likely to buckle and demonstrate lateral instability than the SHORTER spring.
    Compression spring manufacturers