By on October 18, 2012

A comment several articles ago on quantifying ride evaluation (the ‘ass-o-meter’ as one commenter put it) lodged in my brain. It reminded me that I never expanded on the additional data acquired during this magazine test three summers ago. I’ll do that here, add a few words on the timed track testing and then give you hungry readers some enticing graphs, to balance out my verbose ramblings (wink wink!).

In June 2009, Grassroots Motorsports Magazine, based in Ormond Beach, FL, contacted me about providing an FCM coilover package and also coming out for the tire/suspension evaluation day using staff writer’s Tom Heath’s 1994 R-Package Project Miata on their regular test course, a go-kart track in nearby Ocala, GA. The overall timed results and related driving commentary are detailed in that article, telling part of the story. You can download it from or read it on our website (PDF) (link: ) .

Space constraints meant that I didn’t get to share all of the data gleaned from that day. My intention with this post is to provide more of that data (vertical acceleration and shock dynos) to illustrate some of the key elements of suspension tuning.  For these tests, I utilized a speed bump in their parking lot, storing data on my laptop via our Signal Quest +/- 5g 3-axis box.

The timing of building/delivering our suspension was quite tight as they had a narrow window to do the testing, plus I’d be traveling right after the 4th of July weekend. Not much of a vacation that year! I finished valving/assembly the morning before my flight and checked it on the plane with me. Luckily, TSA let it through (leaving a ‘we’ve inspected this package’ note).

Having a closed circuit, a test driver who knows his car and and two full days was a dream come true. Tom’s Miata had a bit of oomph, delivering about 240 whp through its turbo-charged 1.8L engine, but it was still riding on the stock R-package Bilstein shocks. Bilstein had provided a brand new PSS9 adjustable setup, while I brought along an FCM Elite 3140 coilover suspension. I chose spring rates that would provide similar roll stiffness to the PSS9 setup, while also taking into account variables like the combined weight of the vehicle and its occupant. We used 425/300 lb/in spring front/rear, compared to the 342/342 on the PSS9 and ~190/110 of the stock R-package springs, which also used strongly rebound-damped Bilsteins. The R-package cornered on the ~240 lb/in bump stops very actively and sat very low from the factory, constantly pulling into the rubber bump stops like a race coilover from the factory.

Two items not mentioned in the GRM article were the time constraint and the weather conditions. We would have two days to perform all tests, including a few different tire widths/compounds on the stock suspension. Then, we’d leave the 225 NT01/15×9 wheel combo on for evaluating the three suspensions: R-package, PSS9 and FCM. For the second constraint, we had showers to contend with throughout both days. Rain would frequently douse the track, requiring us to wait until clouds cleared so the sun could dry the asphault sufficiently to test again … before the next shower.

The initial tire tests plus stock suspension evaluation took most of the first day. As the second day started, the Bilstein PSS9 went on and got 4 sessions of 5 laps each. It was immediately apparent the rear spring rate bias was too strong (or too soft in the front). Each session we made a change, e.g. cranked up the front damping (went to setting number 5 on the 9 way adjustable shocks), softened the rear and even removed the rear sway bar to tame the tail. It was late in the day when we called the PSS9 tests a wrap.  I had to rapidly swap in the FCM suspension and pray the rain stayed away long enough to get some good runs. With clouds looming yet again, I set the heights to 11.75/12.0”, a bit lower than the PSS9 up front but same in the rear. No corner-balancing or alignment was done prior to any tests. We left the dampers at the nominal ‘best grip/ride’ setting of 2.5/3.0 turns F/R (out of 6) from full stiff and sent Tom out.

He only managed 1 test session of 5 laps with our FCM setup before the sprinkles started. It was already 4:30 so no chance for additional testing and no time to play with damper values which I’d really wanted to do. C’est la vie!  Tom still managed to average a better time than either of the other two suspensions! But what’s anecdotal evidence without numbers to back it up?

Now, to the backstory of speed bump testing. Here are the R-package accelerometer graphs, followed by shock dyno plots, then the PSS9 and FCM. In each run, Tom drove 10mph over the bump, which had a trapezoidal shape and sharp vertical edges. While you’ll notice the accelerometer didn’t read exactly 1.0g, the baseline was consistent for each test.




The softer springs on the factory R-package helped mute the impact of the speed bump but the chassis was jacking up and down even before the speed bump was encountered (oscillations visible in the data from time 80-82s, then 84-86s). The upward acceleration was mild (~0.3-0.4g), but followed by a stronger downward pull (0.6-0.7g) on the descending edge. Again, note the oscillations and ‘ringing’ in the data which is due to literally bouncing off the bump stops from the excessive rebound damping. The overall time scale on this graph is DOUBLE that for the PSS9 and the FCM setup, indicating how long it takes this setup to stabilize after a disturbance. Peak to peak vertical g values are between 1.4 to 0.1g, over 1.3g of vertical acceleration. The result is an extremely harsh and jittery feel.



As we see here, there is much less rebound with the PSS9. The sharper vertical acceleration is likely due to proximity to the front bump stops. With the PSS9 setup at the recommended Bilstein height (approx 12” front and rear, which I felt was too low for the shock body length and their longer bump stops, 40mm vs. our 16mm), there is less jitter but higher vertical acceleration values. Some ringing still occurs after the speed bump (for about a second), showing the system is underdamped. There may be some influence from the rear frequency being 25% higher than front.

Peak to peak vertical acceleration is between 1.45 to 0g on the first part of the bump and 1.5 to 0.38g on the second half, so still between 1.1-1.4g.

Two features jumped out at me: First, the front and rear suspension both stayed closer to 0.2 g in rebound stroke while the PSS9 went below 0g on the front  The PSS9 rear was better-behaved than the front and softer than the FCM rear. Second, the FCM setup had essentially zero ringing (before or) after the bump compared to the PSS9 and certainly the hyper-packing R-package. With 425/300 spring rates, the rear was about 5% higher frequency than the front. Peak vertical g’s at this height are about 1.5-0.2 and 1.4-0.2 or between 1.2-1.3g.

From the data, I felt the car with FCM suspension was too low in the front so I added 1/4” of ride height, to 12/12 front/rear. Tom settled the suspension and repeated the speed bump test:

This was truly sweet, and a great illustration of how important ride height can be to your ride comfort! Essentially 0.5 g of acceleration both upward and downward, with a small bias in the rebound direction on the rear, and perhaps a bit too much high-speed bump. I’d call this a neutral-jacking setup that promotes smooth settling, grip and aids confidence at the limit. A speed bump is a special case but it can show how a suspension will respond to sudden inputs or mid-corner bumps that are apt to throw a vehicle unnervingly off-line.

The whole experience with GRM was really gratifying, if stressful. I really enjoyed the camaraderie around cars and many areas of life. Amusingly, the picture of me for their article was from behind, head buried in the wheel well. An accurate depiction of where I spent most of my time! Now, fast-forward three years and I can see some revised damping, even stiffer springs and other changes to net more speed and perhaps comfort as well.

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3 Comments on “Supension Truth #4: Once Upon a Bump...”

  • avatar

    Just curious- in your coilover kit, why don’t you use an independent height adjustment which changes the shock body length while leaving the spring perch alone? As is the case with the Tein Flex.

    • 0 avatar
      bumpy ii

      Those systems are more difficult to adjust the height on without removing the shock to spin the bottom mount, since the preload in the spring is fighting you as you try to twist the shock body.

      That setup also requires the entire shock body to be threaded ($$$) versus using Ground Control sleeves, and the charging nipples on the shock body would probably have to be left off completely.

      • 0 avatar

        Yes, as bumpy ii said, extra cost and lost droop travel as JIC and Tein setups experience which is detrimental to the suspension’s overall functionality. Adjustable lengths IMO are unnecessary amd evem a bad idea once you have a good sense of how long the shock body + bump stop need to be for that vehicle. We do use a Schrader valve for a majority of builds (that will change in the near future with our new shock assembly machine).

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