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Suspension frequencies

Old 07-07-2017, 08:38 AM
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Suspension frequencies

Hey club,

I'm in between suspensions and trying to figure out where to go. While it's easy to just go buy a thing, I want to science the **** out of it.

So, can you kind folks go out and measure the front and rear suspension frequency of your car and report back with your spring/shock/swaybar configuration?

Mine:

Stock bars, Koni shock full-soft, Eibach Pro-Kit spring: 2.0Hz front, 1.6Hz rear.

How to measure: use a phone app like Physics Toolbox to record accelerometer data, place the phone on the trunk or hood and begin rocking the suspension up and down for 30 seconds or so. It will settle into its natural frequency. Count the number of peaks in the log over 15-20 seconds, figure out how many peaks that is per second and that's the number. Repeat at the other end of the car.

I've reading/watching Shaikh from Fat Cat Motorsport and would like my next suspension to follow his principles, but trying all the setups gets really expensive.

The idea is that rear frequency should be higher than the front, which should be true on a stock RX8, not true on my current setup.
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Old 07-07-2017, 09:40 AM
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How did you determine that rear frequency should be higher than front?

Pretty much everything I've seen except the OE setup (according to FCM's spreadsheet) has front and rear the same or front higher than rear.
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Old 07-07-2017, 11:08 AM
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Originally Posted by NotAPreppie View Post
How did you determine that rear frequency should be higher than front?

Pretty much everything I've seen except the OE setup (according to FCM's spreadsheet) has front and rear the same or front higher than rear.
Shaikh's whole Flat-Ride thing is that rear should be higher frequency so it can catch up to the front quickly and is out of phase. I don't know for a fact that the OE setup has this, but the Miata does so I would assume Mazda used similar philosophy on similar suspensions. That's sort of why this thread, to get a read of reality from different setups.
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Old 07-07-2017, 08:37 PM
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I think it really depends on the car. If all out racing is what you are going for then the flat ride principal isn't 100% exact but it should be ball park. If you run the car with a lot of damping the flat ride doesn't matter as much, as you turn the flat ride matters more since the car is going to oscillate more.

Flat ride is having a freq about 10-15% higher in the rear than the front. The frequency of the springs determines comfort. I have added a chart of spring frequency and comfort for you to view.

A lot of the calculations are approximate for motion ratio's and might not be exact. I suggest sticking around the stock spring rate split percentage. You can vary a little from it and still feel like the car resembles what it was originally designed like especially if you maintain stock sway bars. I tried a 11/11 and 8/6 on my rx7 where the 8/6 is basically the same split as the stock spring rates and the 11/11 is what came with the ohlins DFV stock. the 11/11 is a lot stiffer and has flat ride but got damn it feels a lot stiffer than the 8/6 set up which glides over bumps at high speeds. the 11/11 set up the rear would feel like its bouncing violently over bumps at high speeds and the car felt like it wonders a little when the speeds get above 100mph. The softer rate springs feel like glue in the same spots on the same street and I honestly don't even notice the bumps are even on the road. the 11/11 is a for sure flat ride, the 8/6 is a stock bias split and obviously softer. the 8/6 is in the 1.5 hz range and the 11/11 set up is in the 1.8-2.0 hz range.

the set ups do feel and behave differently. the 11/11 feels great when hammering it in the low gears and taking turns fast, the 8/6 feels soft in comparsion, the 8/6 is amazing at high speeds where the 11/11 feels sketchy and skates. 8/6 you can ride for days in the 11/11 you feel a whole ton of bumps and cracks in the road.

I ran stock shocks on swift sport springs on the rx8 and that is a frequency of 1.34 F 1.17R and it felt like the rear would bounce up and down a lot while the front was stable. I ran good times in autox and the car was grippy and stable at high speeds. The car was comftorable but I felt like I needed more rear spring to better balance the car.

my current set up is 2.1F and 1.89R hz and it feels great. it feels like the car is supported and the car rides well, just a little stiffer. its the Ohlins DFV set up with 9KGF/ 6KG rear. I think the balance of the car is pretty damn good as well with stock sway bars. the car might exhibit a hair under steer which is good because it forces me to stay on the throttle through corners all the time and through slaloms which is fun and have a little safety net. you can also drive fast into corners and trail brake without fearing a total rotation of the rear under hard braking. it feels very good.

Now if you use flat ride you will need to tune the car a little different I think. you will probably end up with rates around 9KG front and 7KG rear depending on how flat you want to go but that gets you higher ratio rear than stock. I suggest perhaps staying with the stock front sway bar and dropping down to a miata rear sway bar and see if that better balances the car out. If you disconnect the rear bar you can feel like the car doesn't want to rotate/transition well in the slaloms.

at least that has been my experience.


Last edited by lOOkatme; 07-07-2017 at 08:43 PM.
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Old 07-07-2017, 09:07 PM
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Another thing to consider.

shocks. Some shocks have more compression damping than others and you need to look also at the rebound damping.

some use compression + spring like the stock shocks and others use less compression and more spring like the ohlins DFV and many other set ups.

When evaluating shocks I think many people overlook the transition between compression and rebound damping. you want the shock to freely move around 0 IPS. you don't want large forces here and you want the forces around 0-1 IPS to be gradual, not harsh. like you want to catch an egg falling from the sky you want this transition to be smooth and gradual. you pass through this section of the damping ALL THE TIME. you hit a big bump you pass through it, small bumps pass through it and little oscillations.....pass through it. you want the tires to be able to move somewhat freely up and down and soak it up, not move the car. The real good shocks move with little force, the cheaper ones like to stick and ramp more quickly. you don't want to hit a big bump and go from 0 to 250lbs force in a matter of 2 IPS. many coilovers do this on compression and its going to be harsh IMO. you want a slower ramp with controlled mid speeds and a blow off high speed.

Ohlins do a good job IMO if you look at their shock dyno's, stock is alright but ramps a little more aggressively, other coilovers fail pretty bad at this IMO. way too harsh of a ramp which the tires and grip won't like at all.



notice how almost all the settings have a smooth transition and little force under 1 IPS.

I always run my settings around 7-12 from full hard, it misses the harsh ramp up but is super controlled.

tein flex. those harsher settings are god damn harsh and will not let that wheel stay on the ground. settings 8-16 are useless IMO on the rebound side.




there are other shock dynos and look at that 0-1 IPS and the transition between compression to rebound being smooth.

attached stock shock dyno for viewing pleasure.
Attached Thumbnails Suspension frequencies-chart_rx8_oe_tokico_fr_rr.jpg  

Last edited by lOOkatme; 07-07-2017 at 09:12 PM.
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