2023 Civic Si Spring Rates, Motion Ratio, Ride Frequencies, Autocross

[eromani]

I'm autocrossing my newly acquired '23 Civic SI in GS now with the possibility of prepping it for STH. I wanted to get some data on how the car is set up stock. I started by measuring calculating the motion ratio of the front and rear suspensions, and used the published total mass and weight distributions, and did my best to estimate the unsprung mass. Hopefully this is useful for some people on this forum, and maybe others can check my unsprung weight and motion ratio numbers.

Next steps...figure out stock Lateral Load Transfer Distribution, but I need more accurate numbers first on Center of Gravity height, Roll Center height, and Sway Bar rates/MR's.

OEM Data	N/mm	kg/mm	lb/in
Front Spring Rate	27.00209276	2.753447178	154.186
Rear Spring Rate	55.90501125	5.700724636	319.226
Front Wheel Rate	25.50155572	2.599546965	145.7231756
Rear Wheel Rate	31.46229801	3.207165954	179.7845601
OEM Ride Frequencies	Hz
Front Ride Frequency	1.361044116
Rear Ride Frequency	1.899871277

Vehicle Data
Total Vehicle Weight (kg)	1339
Front Weight Distribution (%)	60.3
Motion Ratio (Front)	1.029
Motion Ratio (Rear)	1.333
Unsprung Weight Front (kg)	55
Unsprung Weight Rear (kg)	45

Front Weight (kg)	807.417
Front Single Corner Weight (kg)	403.7085
Front Single Corner Sprung Weight (kg)	348.7085
Rear Weight (kg)	531.583
Rear Single Corner Weight (kg)	265.7915
Rear Single Corner Sprung Weight (kg)	220.7915

Conclusion so far after driving it on a practice/tune event...

Front springs are a bit soft, rears are just fine. It's actually quite a good ride and handles really well in a competition environment. There is significant turn-in understeer, helped with trail braking. Mid-corner once the car sets is very neutral, and the rear rotates on lift off. Corner exit is as expected, put your foot down and pull yourself out of the corner.

What this car really needs is to reduce some corner-entry understeer with some more negative front camber. Bringing the front ride frequency up to at least 1.8 would be nice too with stiffer front springs, and balance that out with more rear bar. All of that is of course illegal for G Street class, but will be fine for STH. I'm currently maxed out on front camber (pulled the strut pins) and it's at -1 driver side and -0.9 passenger side.

So aiming for a target ride frequency of 2Hz all around, you get the following spring rates:

Springs Based on Target Ride Frequency	N/mm	kg/mm	lb/in
Front Spring Rate	58.31	5.95	332.94
Rear Spring Rate	61.95	6.32	353.76

So an STH build might look like coilovers with camber plates, 6k springs front and back, and balance with adjustable sway bars.

Thoughts? Disputes?

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[DYI01]

The cheap and easy option to up the front spring rate is a pair of OEM FK8 CTR front springs, which are 263 lb. springs. They really seem to balance the feel front to back and the stock shocks handle the rebound no problem. The increase in front spring rate definitely increases the roll stiffness and keeps the front tires planted better. An OEM FK8 20.5mm rear sway bar will also help to keep the balance. These are stealthy and cheap OEM+ upgrades IMO.

I'm currently using a Whiteline 22mm rear sway bar and It clunks on me occasionally and feels almost too stiff at times. I've been contemplating going to the OEM CTR rear bar to give a bit more of a compliant ride over uneven left/right bumps, it unsettles the car a bit too much. Otherwise the setup makes the car handle extremely neutral, a bit of understeer at the limit of the tires, lift off a bit to make the rear step out a little and power down to straighten it back up.

 

[eromani]

The cheap and easy option to up the front spring rate is a pair of OEM FK8 CTR front springs, which are 263 lb. springs. They really seem to balance the feel front to back and the stock shocks handle the rebound no problem. The increase in front spring rate definitely increases the roll stiffness and keeps the front tires planted better.

Thanks!
Will the FK8 front springs lower the ride at all? I'm liking the stock ride height actually. (I'm new to Civics, used to my Ecoboost Mustang and all of it's options).

 

[DYI01]

Thanks!
Will the FK8 front springs lower the ride at all? I'm liking the stock ride height actually. (I'm new to Civics, used to my Ecoboost Mustang and all of it's options).

It keeps stock height. I bought a set of low mileage (500 miles) used OEM springs for $75 locally on FB marketplace. Once installed it took a few weeks for the springs to settle but they sat at stock ride height once broken in. The FK8 springs go right in without any issue, they are actually easier to put in since you don't need to compress the springs to install the top hat. The FK8 springs are physically shorter than the Si springs, but the coil is much fatter.

 

[eromani]

It keeps stock height. I bought a set of low mileage (500 miles) used OEM springs for $75 locally on FB marketplace. Once installed it took a few weeks for the springs to settle but they sat at stock ride height once broken in. The FK8 springs go right in without any issue, they are actually easier to put in since you don't need to compress the springs to install the top hat. The FK8 springs are physically shorter than the Si springs, but the coil is much fatter.

Great to know! Used OEM is my favorite option. Too bad there's no OEM option to get more negative front camber (not that I've seen at least).

 

[absolude]

The cheap and easy option to up the front spring rate is a pair of OEM FK8 CTR front springs, which are 263 lb. springs. They really seem to balance the feel front to back and the stock shocks handle the rebound no problem. The increase in front spring rate definitely increases the roll stiffness and keeps the front tires planted better. An OEM FK8 20.5mm rear sway bar will also help to keep the balance. These are stealthy and cheap OEM+ upgrades IMO.

I'm currently using a Whiteline 22mm rear sway bar and It clunks on me occasionally and feels almost too stiff at times. I've been contemplating going to the OEM CTR rear bar to give a bit more of a compliant ride over uneven left/right bumps, it unsettles the car a bit too much. Otherwise the setup makes the car handle extremely neutral, a bit of understeer at the limit of the tires, lift off a bit to make the rear step out a little and power down to straighten it back up.

This is the kind of mod I like the most!
How is the ride on rougher roads? Is there a big difference? Any bouncing?

 

[DYI01]

This is the kind of I like the most!
How is the ride on rougher roads? Is there a big difference? Any bouncing?

The Si ride is already pretty firm, the FK8 springs don't make that huge of a difference in ride quality, but it's definitely firmer. I can tell the car feels way more even though, stock the front felt much softer than the rear over bumps. 90% of people likely wouldn't feel the difference in my car vs a stock one. The stock FK8 springs are still a progressive rate spring, so its gonna feel soft at the top of the travel and farther it compresses the stiffer it gets. The OEM dampers handle the spring rate without issue, the rebound damping is on point IMO.

 

[absolude]

The Si ride is already pretty firm, the FK8 springs don't make that huge of a difference in ride quality, but it's definitely firmer. I can tell the car feels way more even though, stock the front felt much softer than the rear over bumps. 90% of people likely wouldn't feel the difference in my car vs a stock one. The stock FK8 springs are still a progressive rate spring, so its gonna feel soft at the top of the travel and farther it compresses the stiffer it gets. The OEM dampers handle the spring rate without issue, the rebound damping is on point IMO.

Sounds great!
I already have the Type R rear sway bar waiting to install.
Didn't know they were progressive. Interesting.
Found a set of Type R springs locally. Guy is saying they were on the car for a few thousand miles.
Still thinking...

 

[FL5]

Hey @eromani , Honda of America Racing Team is currently running our 2022 Si in STH. Feel free to ping me with any questions about setup. Currently reasonably competitive. Will you be at Solo Nats this year?

 

[eromani]

Hey @eromani , Honda of America Racing Team is currently running our 2022 Si in STH. Feel free to ping me with any questions about setup. Currently reasonably competitive. Will you be at Solo Nats this year?

Would love to talk about your STH setup! I'm only doing local events, running GS. Only thing I have so far is tires. Aug 27 will be my first event with the SI. The car is a few weeks old!
Eric

 

[eromani]

The FK8 springs go right in without any issue, they are actually easier to put in since you don't need to compress the springs to install the top hat. The FK8 springs are physically shorter than the Si springs, but the coil is much fatter.

@DYI01 does the FK8 spring become unperched at full droop if you don't need a spring compressor to get it in?

 

[DYI01]

@DYI01 does the FK8 spring become unperched at full droop if you don't need a spring compressor to get it in?

No, there is still preload on the spring once the top nut is seated completely. I was able to push the top hat down by hand to get the nut started without any compressors.

 

[OGGsr]

What can you modify in STH?
@FL5 Love your car and I remember seeing the HART 6th gen Civic Si at VIR back in the early 2000's

 

[eromani]

What can you modify in STH?
@FL5 Love your car and I remember seeing the HART 6th gen Civic Si at VIR back in the early 2000's

There's a handy guide from SCCA here: https://www.scca-classifier.com/a/index.html
Has all the allowed modifications and classes for your chosen vehicle.

 

[BCinMD]

This thread is relevant to my interests!

Just picked up my 23 Si back in May and have run it in my first autocross events ever over the summer - the second was just this past Saturday at Summit Point in WV.

My performance on Saturday was, to say the least, disappointing. Last in street class, over 12 seconds slower than the fastest car (a heavily modded ND Miata)...and most embarrassing was the 2005 Toyota Corolla that finished two places above me . To be fair, he at least had performance wheels and tires on the car, and some kind of free flowing exhaust...but still, getting beat by an almost 10 year old Corolla is pretty humiliating.

I'm still bone stock, including riding on the standard Goodyear all-seasons. Needless to say, the car is awful when it's sliding / under-steering around every corner. I also realize that as the driver, I have A LOT to learn about the sport, driving technique, etc. (My 21 y/o son, who autocrosses a slightly modified NB Miata, told me that the "driver mod" is the most important!)

But I also know that if I want any chance to stop finishing close to dead last, I need to make some changes. Here is what I'm thinking:

1. lightweight wheels and summer performance tires (looking at Konig Dekagram 18 x 8.5 wheels; not sure about tires yet, but slightly wider 245/40 w/ ~300 tread wear as I want to get a couple years out of them if possible - open to suggestions?)
2. rear sway bar (thinking about SiriMoto 20mm)
3. rear motor mount (but I think this will push me out of Street Class into STH so may not want to do this)
4. high volume air intake (PRL)
5. exhaust (looking at the MAP catback "street" version)

Ultimately I need to decide if dropping around $3500 on the above mods is worth it. I'd hate to spend all that only to find out that my skills on the autocross track are just not going to improve enough to take full advantage of the upgrades.

Going to follow this thread and excited to learn more about autocrossing the Si.

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