This is a simplified explanation of a few of the terms used when selecting or discussing shocks and springs. I am no expert, this is just some of the info that I have picked up during the suspension modifications to my cars and which I commonly throw into discussions to make it look like I know more than I do.
"Buying springs, well you just buy the 'best' brand don't you??"
A common misconception, but just because a manufacturer makes good quality springs and makes a set to fit your car that doesn't mean they are correct for what you want (you do have an idea of what you want, don't you?).
Assuming the springs are actually made for your model (for some cars (the ST165 being one) it is very difficult to get springs at all, let alone ones which are well suited), then the main aspects of spring choice are installed length and spring rate.
Installed length will determine how high the vehicle sits. Most performance springs will lower the car somewhat to improve looks and drop the centre of gravity. You can go as low as you want till you run into tyre clearance and driveshaft /suspension arm angle issues. However lowering beyond a certain point will not improve handling, but make it worse due to screwed up suspension geometry (for example CRX's running -4° camber).
Spring rate determines how the spring (and hence vehicle) behaves. Harder the spring, harder the car. Spring rate is historically recorded in pounds (or, more correctly, lbs/in), although Japanese catalogs use kg/mm. To convert, simply multiply kg/mm by 55.88. A selection of GT-Four spring rates are given on my suspension modifications page, and show the wide variation used in what is basically the same car.
Many springs are linear, in that the load on a spring increases with a linear rate as you depress it. Some aftermarket springs nowadays are progressive, which means as you load it they get stiffer much quicker. At low deflection (like cruising a motorway) they are quite soft, but when you hit hard bumps they compress the 'soft' section and get into the stiffer section, giving you better performance but with a more comfortable ride. No guarantees they will always do this in a predictable manner however...
When choosing springs it is very useful to know your starting point, to allow a comparison with what you are buying. To find this info you can resort to magazines, press reports, some suspension catalogs etc. Unfortunately it is unlikely manufacturers will know. At worst remove a spring and get a good suspension shop to test it for a few bucks.
But what spring to choose? Well the effects of changing spring rate are quite complex and beyond what I want to do here. There are many books and webpages on this subject out there who can tell it better than me...
Well here we go. For an entertaining afternoon of listening to highly biased discussion go to any car meet and ask what company makes the best shocks. Like the subject of tyres, car make, beer, whatever, everyone has an opinion and not necessarily based on fact and/or experience!
To allow some comparison between shocks you can have them tested on a shock dyno. There are two types, those which clamp onto the car and are used to simply test the damping characteristics of the shock without giving full graphed results (a high tech 'bounce' test), and those which give the results discussed here. They do this by cycling the shock through a wide range of speeds and recording the opposing forces graphically. A shock service outlet or race-prep shop is likely to have a shock dyno (locally both the Koni and Bilstein dealers have them) and it can be used to show exactly what damping force the shock applies.
Whereas springs apply more force the further you depress them, shocks apply more force the faster you depress them. The faster you move it, the stiffer it acts. The are two characteristics of importance, bump valving (shock compression) and rebound valving (shock extension).
My lay-mans way of understanding is that bump affects how stiff the shock is when going over bumps (duh!). By that does the car bounce up hard when going over a gutter (too hard) or don't you even feel it ( too soft?). This affects the way the car sits when cornering and during transitional movement. Rebound adjustment affects what happens after you've gone over the bump. Too soft and the car wallows and feels unstable, too hard and it gives a harsh, kidney jarring ride.
There is a standard method of rating shocks. The force (in kg) the shock exerts when travelling at 0.52 m/s is expressed as a pair of numbers (bump/rebound). For example the ST165 GT-Four front shock is (100/35). This is the terminology you will see in most Japanese and European shock catalogs. While this allows you to compare shocks, the comparison is only at that one speed (which is however a good guide, it makes it clear that some Japanese suspension kits are mega stiff!).
This rating is only good for shocks with linear damping characteristics. Through specialised internal valving some shocks have good damping characteristics at low speed, but only a little more at high speed. Too little damping at low speed (as in a perfectly linear shock) allows Cadillac style wallow at speed. The German Bilstein page discusses some of these aspects. The picture below shows a typical shock dyno plot, showing the rates before (red) and after (blue) rebuild. This shock is the rear competition spec Bilstein of a Team Toyota Italia GT-Four (refer my suspension page for a photo). Bump is the bottom half, with rebound at the top. The maximums at 0.52 m/s (graphed as 52.5 cm/s) are given a rating of (233/116) for the red (pre-revalve) shock line. The graph also shows the different angles to the lines, which show the shocks to be quite stiff even at low speed, which did give a harsh ride on the road.
The two lines on this graph appear very similar, however at the first velocity line from the left it is apparent that the blue line force is almost half the red line force. What this means is that the ride is much softer and doesn't have the harshness it did. When I revalved the shocks I also went from 250lb to 200lb springs. The car is much smoother and has lost the 'bounce' that the rear did have.
Basic idea here is that higher numbers are stiffer shocks, and with stiffer rebound characteristics you need stiffer springs (which oppose the rebound force).
Which comes down to the main point of this page. For best results your spring rate must complement your rebound rate. No point fitting ultra-stiff springs with soft worn-out shocks, as you'll bounce all over the place.
And don't just run out and buy Tokico and HKS springs (for example) because you have heard that they are the "best". If the HKS springs were designed for HKS shocks you may not get the ideal match.
Ask what the rates are, and spend your money with a bit of background knowledge and research.
"But no-one makes the shocks in the rates I need!"
There are a few options here, including 'adjustable' shocks and rebuildable shocks.
There are several brands out there which claim to be fully adjustable. Many of these are race shocks and can be adjusted with either external adjusters or by removing the spring and turning the shaft (some Koni). These adjustments are limited within a predetermined range set by the oil viscosity and internal valving. It is not intended to use this adjustment to make major damping changes to fit the shock to a vehicle it was not intended for.
I mentioned Tokico Illuminas because they claim to be fully adjustable. However, the two damping characteristics (bump and rebound) can only be adjusted together (both stiffer or both softer), hence they do not give the same control as a truly fully adjustable shock. They are definitely not the final word in shock design, as their high failure rate testifies to!
The common top adjust Konis are adjustable, but for bump only. To adjust rebound you need to modify the shock to make it rebuildable.
Bilsteins are one of the few affordable fully adjustable shocks which are widely available for Japanese cars. While there is no external adjustment, they come ready to dismantle to allow revalving at low cost (about NZ$60, US$30). Hence they give infinite control over bump and rebound throughout the speed range as required. But yes, obviously, you have to remove the shock to do so but this may not be an issue if you have your rates correct. Also, if you throw enough money at them they can be made externally adjustable for both bump and rebound, but that is beyond my budget... :-)
As you can guess I'm a Bilstein fan. This was basically forced upon me as few manufacturers produce ST165 GT-Four shocks. Bilstein (www.autolign.co.nz) could give me what I wanted within 24 hours. If they were good enough for almost every successful rally car over the last 30 years, they will do me...
Simplistic explanation of a few characteristics - www.bilstein.com
More in-depth discussion - www.bilstein.de(note that they seem to have removed the good stuff from the English site temporarily)
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