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

[eromani]

For the MR, did you calculate this yourself or find a source for it? Never seen MR as being over 1, it should be under 1 AFAIK. Then wheel rate is Spring X MR^2. The math isn't mathing for me here. Even if the MR should be flipped, .971 F and .667 R, those don't add up to the wheel rates listed. Front is in the ballpark regardless since its a strut car and the MR is essentially 1, rear is where I'm more curious.

https://robrobinette.com/Suspension_Spreadsheet.htm

I measured the MR myself. I'm used to doing motion of wheel to motion of spring, hence >1. So if your wheel rate equation uses <1, take reciprocal. The wheel rate calculator I'm using is Wheel Rate=Spring Rate/MR^2. If you're using WR=SR*MR^2, then take the reciprocal of my MR.

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[SlowAP2]

I measured the MR myself. I'm used to doing motion of wheel to motion of spring, hence >1. So if your wheel rate equation uses <1, take reciprocal. The wheel rate calculator I'm using is Wheel Rate=Spring Rate/MR^2. If you're using WR=SR*MR^2, then take the reciprocal of my MR.

Perfect! TY so rear is .75 for how I would view it. Saved me some time here

 

[eromani]

Perfect! TY so rear is .75 for how I would view it. Saved me some time here

Feel free to verify my motion ratios though! I would love another set of eyes on it.

 

[AB_22Si]

Just wanted to contribute to this thread since I autocross my car regularly. I’m running in STH right now and I seem to easily run first in class at my local events against various Focus STs, golf GTIs and WRXs. I’m running 245 RT660 tires on Enkei rims 18x8 +35. I have also been using the RV6 rear sway bar on the stiffest setting which is around 1300ft lbs since I’m still in stock suspension. The car went from very turn in understeer to almost too much oversteer which I really like. The RV6 rear bar provides and incredible amount of rotation at the expense of a very harsh ride. I just installed a set of SPC adjustable lower ball joints to add more negative camber to the front for next months event.

 

[eromani]

Just wanted to contribute to this thread since I autocross my car regularly. I’m running in STH right now and I seem to easily run first in class at my local events against various Focus STs, golf GTIs and WRXs. I’m running 245 RT660 tires on Enkei rims 18x8 +35. I have also been using the RV6 rear sway bar on the stiffest setting which is around 1300ft lbs since I’m still in stock suspension. The car went from very turn in understeer to almost too much oversteer which I really like. The RV6 rear bar provides and incredible amount of rotation at the expense of a very harsh ride. I just installed a set of SPC adjustable lower ball joints to add more negative camber to the front for next months event.

How camber were you able to get with the ball joints?

 

[AB_22Si]

How camber were you able to get with the ball joints?

Front is at -1.1 now the ball joints give you -1 degree of camber

 

[SlowAP2]

Front is at -1.1 now the ball joints give you -1 degree of camber

Do you have the alignment pics pulled on the top strut? 1.1* feels low if the ball joint adds 1*. You should be .5-.7 with the pins pulled so ~1.5* with the ball joint I would think.

 

[eromani]

Do you have the alignment pics pulled on the top strut? 1.1* feels low if the ball joint adds 1*. You should be .5-.7 with the pins pulled so ~1.5* with the ball joint I would think.

Yea agreed. I have my pins pulled and I'm at -0.9. So with ball joint it would put me at -1.9. Still not great...but better than normal lol.

 

[AB_22Si]

Do you have the alignment pics pulled on the top strut? 1.1* feels low if the ball joint adds 1*. You should be .5-.7 with the pins pulled so ~1.5* with the ball joint I would think.

Yes I pulled the pins and asked the alignment shop to move the strut tower. They showed me how much you get and it’s like .1 degree of adjustment it’s not much so I didn’t bother with it. I’m going to have to look into coilovers for more camber adjustment

 

[nsr_productions_]

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?

Good info. Thanks for sharing!

 

[ValkenDnB]

Glad you're getting to autocross the car!
Here is my perspective on a few things...

1. (Fact, not perspective) To stay in street class, you have to stick with the same wheel width, and within 1" of OE wheel diameter. So you can do 17x8 and put 245 or 255 tires on. 300tw is a pretty good place to start.

2. I would try to stay in GS for a while until you are driving beyond the capabilities of your car and need more. So I wouldn't do the motor mount yet. Order of importance for autocross is tires, wheels, suspension (springs/shocks, anti-roll bars, alignment), then bushings/bracing etc. and I wouldn't touch those until you have a SOLID understanding of what and why to change, both as a driver and on the engineering side.

3. I ran my SI bone stock on all seasons for a practice event and noticed a few things. A lot of corner entry understeer, but very correctable with trail braking. Mid corner, balance is great. Able to rotate the car with throttle adjustments, no understeer. Corner exit understeer, but that's both expected from FWD and a good thing (makes it easier to drive faster). The good thing about the stock all season tires is you can drive at the limit at slower speeds and gives your brain plenty of space to analyze the car's behavior and how to adjust driver inputs. In other words, if you need to work on your driving, it's easier to do so on all-seasons.

4. If you want to go to STH eventually, then you basically need 200tw tires, a way to get 2 deg or more of negative front camber, stiffer front springs, then stiffer rear sway bar. The easiest way, but not cheapest, is coilovers with camber plates. This would make your car relatively competitive locally, but IMO the civic SI is very competitive in GS, less so in STH.

5. If you don't care about being competitive and just want to build yourself a better handling car, then by all means go to STH and have fun.

Others feel free to chime in and disagree.

Can you do a type r motor mount as a stealth upgrade to stay in class? It’s stiffer than OEM Si but looks OEM.

 

[eromani]

Can you do a type r motor mount as a stealth upgrade to stay in class? It’s stiffer than OEM Si but looks OEM.

Technically...no i think

 

[SlowAP2]

Can you do a type r motor mount as a stealth upgrade to stay in class? It’s stiffer than OEM Si but looks OEM.

In GS no, in STH yes.

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