How to match spring and shock rates?

[gsx_hooptie]

Hey guys,

I've often read about the importance of mating springs with appropriate shocks, but how do we decide what's within the boundaries?

For example, if we're talking about Eibach Prokits with rates of 330/160 (taken from http://www.prostreetonline.com/pso/pages/techtips/showarticle.asp?articleid=8), I'd imagine we're supposed to adjust or revalve shocks to get close to this number. What tollarance do we have to get the right number? I doubt the adjustments on KYBs or Tokicos are accurate enough to be too close. Just as an example, would 300/150 be close enough that you wouldn't notice they were off?

Also, what over/understeer effects could you get from different shock/spring rates?

Also, how far off do spring rates have to be from shock rates before you blow a shock?

Lastly, I searched around for a few minutes and found the stock spring rates in the FAQ, but not the stock shock rates. Is there a site that lists the different shock rates, or if they're adjustable, the range of rates?

Thanks for the help, trying to learn.

 

[DG-FNR]

Sadly, the answer to all these questions, especially the ones related to balance, is "it depends".

The way we set up cars is to pick a spring rate based on natural frequency, then set a damping force to obtain a desired damping ratio, trim the weight transfer distribution with the sway bars, and then test the hell out of it to see what really happens out in the real world.

It's a constant, never-ending process of predict, change, test, validate.

You allso want to be REALLY careful about what you read out on the net. Most of it is bullshit. Some of it is ballpark-reasonable. Very little is very accurate.

Quick example - that article you linked listed 1G AWD spring rates as 146/140. I have tested a set of 1G AWD springs on my electronic spring rate tester, and got 136/135. Now does that mean 146/140 is bullshit? Not necessarily. I expect a fair amount of rate swing on cheap springs, and the springs I tested had been well used. 7% error is more than I like to see on a spring (I fail springs with 3% error) but that's actually better than a lot of cheap aftermarket springs I've tested.

Or maybe there is a difference in spring rates year to year for 1Gs.... lots of possibilities.

In any case, to work out natural frequencies, you need corner weights, unsprung weights, motion ratios, and spring rates. When you've worked that out, you can then work out the shock force needed to produce various damping ratios. If the damping ratio works out to be between 0.6 to 1.0, you're probably OK as a starting point.

Of course, you also need to know what the dyno plot looks like for the shock....

And then when it's all said and done, a lot will depend on how you drive the car, the tires you use, the surface you are on, the sensitivity of the driver to the setup.....

It ain't easy. If it was, everybody would do it.

DG

 

[jtmcinder]

Lastly, I searched around for a few minutes and found the stock spring rates in the FAQ, but not the stock shock rates. Is there a site that lists the different shock rates, or if they're adjustable, the range of rates?

You have to really search around to find the damping forces for shocks; they are rarely published. In a dream world, you have full force-by-velocity plots for all settings for all candidate shocks, but you can at least get some information if you really search hard. At a minimum, you want the compression and rebound at some low velocity (e.g., 1 to 2 inches per second) and at some high velocity (e.g., 7 inches per second).

The main problem when you come at this from a text-book point-of-view is that most talk about damping ratio as a single thing. That's not wrong -- because damping ratio is a single thing -- but it is misleading in that you can achieve the same damping ratio an infinite number of ways when you think about rebound and compression separately (which you really need to do). You want a large majority of the damping to occur during rebound, not during compression. One way to think about this -- assuming that you understand the basics of transmissibility; see http://dsmtuners.com/forums/showthread.php?t=183081 -- is that having low compression damping makes the suspension have a lower damping ratio during bump, so more of the high-frequency bumps can be absorbed and not transmitted. Meanwhile, lots of rebound damping keeps the low-frequency swales and such from being transmitted. The car rides nicely and handles well.

This is why shocks that change rebound and compression at the same time are not optimal unless you are extremely lucky or have them revalved. You want just enough compression to keep you off the bumpstops and then leave it there while you tweak the handling with rebound.

Backing up, however, I agree with Dennis -- see http://dsmtuners.com/forums/showthread.php?t=169955 -- that you should decide on springrates before getting into the shocks. My guess, however, is that almost any springrate you decide on will be best matched with Koni Sports, assuming you go with off-the-shelf parts.

- Jtoby