Evaluating / Designing Suspension (KSport Examined)

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so let's talk about the velocity plot above. there are a few things to look at. first, there are two phases to most automotive dampers: low and high speed. low speed generally is for handling while high speed generally is for bumps. low speed is usually a damper shaft velocity of about 0 - 4 inches / sec and high speed is over 4 or 5 inches / second.

the 'knee' in the plots above shows that the ksport's damping curves change from low to high speed slopes at 2 inches / sec. this is a little early.

without getting into discussions on transmissibility, etc, let's just look at the slopes. generally, the compression slope should be about 2/3 (.67) of total damping and the rebound slope should be about 3/2 (1.5).

the min plot has no knee in rebound. the slope multiplier is only 1.08; this means that the 'min' setting is overdamped in compression compared with rebound. the damping ratio is a comfy 0.5.

mid has an obvious knee at about 2 inches / sec. the multiplier is 1.41 and the damping ratio is 0.78. this is a pretty good curve. the only thing i'd change would be to move the knee to 4 inches / sec.

max has the same knee at 2 inches / sec. the multiplier is 1.73 and the damping ratio is 0.96. this is overdamped all around, leaning heavily toward rebound.

the middle setting looks like a good choice here.

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now let's look at the high speed damping. this is the slope to the right of the 'knee' in the velocity plot. ideally, we want a lower damping ratio of around 0.2 - 0.3 to reduce discomfort over high frequency bumps at speed.

the min setting gives us a damping ratio or 0.24, but it's a little underdamped in rebound compared with compression.

the mid setting has a damping ratio of .25; still biased against rebound

finally, the max setting has a damping ratio of .26 with an even greater bias against rebound.

so what would the ideal damping curve look like for this spring rate on the front of a boxster? well there is no true 'ideal'; it's all compromise.

for a streetable track car, i'll go with a low speed damping ratio of 0.7, high speed of 0.2, and a knee at 4 inches / second. look at the chart below for a comparison of my 'ideal' front shock curve for my 400lb springs to the 'mid' setting on my ksports.

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there's another damper performance indicator called hysteresis. basically, hysteresis is a difference in how the shock behaves based upon conditions. if a shock produces 200lb rebound force at 4 in / sec, but only 170lb rebound force at the same velocity immediately after hitting a bump, we have hysteresis.

essentially, hysteresis is degradation of performance that occurs during use resulting from internal construction. this can be caused by sticky seals, cavitation, or a host of other issues.

the following chart is a velocity plot of my front damper at 'max' setting. the outer-most lines are while the shock is accelerating; the innermost are while the shock is decelerating. the difference is hysteresis. it's in the neighborhood of 6%; not great, but not bad. probably average. for the price, i'd say it's pretty amazing actually; i expected much worse.

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Summary (Fronts Only!!)


the dampers curves are reasonably close to what i've calculated are the 'correct' values for this car with my chosen spring rate of 400lb/in. the minimum setting is underdamped and the max setting is overdamped; this is good, it means that the 'middle' of the curve is where we want to be.

by my calculations, for these springs, setting my front dampers between 18 - 24 should be about as close as i can get to my ideal.

look for a similar analysis of the rears shortly.

[RandallNeighbour]

holy crap, batman! That's some serious technical talk, replete with illustrations!

After you do the write up on the rear coilovers, put all the cookies on the lower shelf for this village idiot and type something like:

"Turn the knobs all the way to the right and then click back to the left X times for comfy street use and Y times for DE's..."

You're one smart feller.

[grantsfo]

Guess my question would be how long will K Sports hold those specs? Thats why I went with Koni tube coilovers in the KW's I installed. I just havent built up courage to go down market to try anything else other than Koni or Bilstein tubes. And its nice to have great Koni support here in the US.

It is cool to see a cheaper alternative being tested however! Keep up the good work!

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Originally Posted by grantsfo

Guess my question would be how long will K Sports hold those specs? Thats why I went with Koni tube coilovers in the KW's I installed. I just havent built up courage to go down market to try anything else other than Koni or Bilstein tubes. And its nice to have great Koni support here in the US.

how long they'll hold those specs is anyone's guess. time will tell...

as for US rebuilds? KSport USA has a shop in arizona that does rebuilds in like two days for around $100 per corner. they will custom valve as well, which is nice.

i think ksport still has some kinks to work out, but overall these things look a lot better than i thought they would. more to come.