Autocross- SCCA EV-X Class vs. SS

[gearchruncher]

For a casual dual purpose car, 12k/20k would be a good place to start.

Dallas, any idea why all the kits out there would pick a much lower rear frequency than front? Does this help with comfort or have advantages with higher speed somehow?

The thing that led me down this path is I am struggling with power on oversteer on my setup, and I figure that I might as well improve my spring rates as I play with the setup and try and get it tuned to my driving style.

 

[gearchruncher]

I think a common misconception here is if someone is new to posting here that they are new to SCCA or autox in general. In my case neither is true and I've worked with Rick on the PAX numbers over the years as well as talked to Jason about his venture into it when he got interested.

I'm not following the logic that the EV-X nats winner should be top X spot in PAX.

Nobody said EV-X should have top spot in PAX. Just pointing out that the current EV-X PAX doesn't align with the results at various events against AS and SS results (and AS/SS are not anywhere near top PAX spot either, top AS was 51st and SS was 69th at nationals).

If you're knowledgeable about PAX and how it's determined, what makes you think the EV-X PAX is currently soft?

Side anecdote: Got to drive a stock Boxter GTS this weekend. Never driven a Porsche before. Hopped in it and ran 0.5 seconds faster on a 43 second course on my very first run than my best run in my Tesla. And the GTS isn't SS competitive.

 

[dsgerbc]

Here's what I know less:
Front Motion Ratio: 0.67
Rear Motion Ratio spring: 0.56
Rear Motion Ratio shock: 0.94

These seem low.
MPP says

1mm of perch adjustment in the front equates to roughly 1.25mm of ride height change.

And somehow the same for the rear. Which works out to about 0.8 motion ratio. I believe the rear is wrong tho. @MountainPass

RW Ohlins install instructions have a helpful table on page 15, which works out to about 0.76 front ratio, which is close enough to 0.8. The rear is about 0.64 though. I also measured front/rear arms and don't remember getting a lot of disagreement with the estimates off RW's numbers.

The rear shock motion ratio seems realistic, it's pretty close to the outer pivot point.

 

[Dallas J]

Dallas, any idea why all the kits out there would pick a much lower rear frequency than front? Does this help with comfort or have advantages with higher speed somehow?

The thing that led me down this path is I am struggling with power on oversteer on my setup, and I figure that I might as well improve my spring rates as I play with the setup and try and get it tuned to my driving style.

Ive seen this a bunch where manufacturers will take really wild guesses on motion ratios and pick springs for published weights before production cars even exist. For example, when the Evo came to america, they could have gotten numbers from the Evo7 which is the same chassis (CT9A) but they just seemed to make assumptions with higher front rates based on weight distribution. But with the rear MR the rear rates need to be 25% or more stiffer just for a square NF.

Ohlins is really bad at this it seems. On the S550 mustang their first version of the coilovers had something like a 50% stiffer front spring from OEM but a softer than OEM rear spring.

Basically, most these companies are just pushing out volume and really not dedicated to making a single chassis right. That's why its good to support people like MPP that's focused on limited chassis cause they can test, iterate, and update.

The tesla oversteer issue is likely more related to the open rear diff. On a normal AWD car the inside rear spin will cause the center to lock and push power forward to curb the oversteer. The tesla is more like a simultaneous FWD and RWD car than an AWD and kinda needs to be treated like that. This is why we ran Dans car with bias pushed forward. The bias was our tool to let us get on throttle sooner and harder without oversteer since the front seemed to have plenty of grip to pull through corners still.

 

[gearchruncher]

This is my measurement from my own car. 8.75/13 = 0.67. Am I missing something about the wheel to spring ratio in this system?

Here's the rear:

 

[gearchruncher]

This is why we ran Dans car with bias pushed forward. The bias was our tool to let us get on throttle sooner and harder without oversteer since the front seemed to have plenty of grip to pull through corners still.

I ran Dan's car at nationals with 50/50 bias and basically never experienced power oversteer, but my own car does it a lot and I spend a lot of my focus managing it. I'm trying to use this situation to learn more about suspensions and try some things out.

I'm on 275 A052's and he was on 295 RE71's, and it's concrete vs asphalt, so I'm sure some of it is that also.

 

[Dallas J]

I ran Dan's car at nationals with 50/50 bias and basically never experienced power oversteer, but my own car does it a lot and I spend a lot of my focus managing it. I'm trying to use this situation to learn more about suspensions and try some things out.

I'm on 275 A052's and he was on 295 RE71's, and it's concrete vs asphalt, so I'm sure some of it is that also.

Yeah, that's the difference of tire + concrete. Its just an adjustment tool really. At the Packwood Pro in Dans car I ran 50/50 also cause it wasn't having any issues with exit oversteer. But on our local small lot or if it gets wet Ive been pushing it forward. Literally just as a means to bias towards whichever end seems to have more grip for the day and variables.

 

[dsgerbc]

Am I missing something about the wheel to spring ratio in this system?

Angles? The motion ratio isn't a static number, since the arm won't be horizontal most of the time, while the coilover/spring angle isn't exactly vertical either. I did measure those at some point with about 2" drop, but could not locate the measurements.
There could also be other front suspension peculiarities. It's not a true rigid A-arm, there are two ball joints on the bottom. Maybe the hub is also tilting a hair in response to one link moving up/down. It'd be nice to put together some freeware CAD model of the front/rear suspension...

But given that MPP and RW's front numbers are pretty close, I'd take them over the raw FLCA hole length ratios.

 

[dsgerbc]

It'd be nice to put together some freeware CAD model of the front/rear suspension...

Was searching some free CAD models, but came across this 3d scan of the front (and rear for that matter) suspensions. These are for a RWD, but still somewhat useful.
You can see there's a significant inclination of the front shock. Probably 15ish degrees, which isn't enough to turn a .69 arm ratio into a .8 motion ration. So, there's has to be something else going on. I'll try to measure the motion ratio directly once I'm swapping my OEM suspension back on for winter.

 

[gearchruncher]

Wouldn't a tilt on the shock reduce a motion ratio, not increase it?

 

[dsgerbc]

Wouldn't a tilt on the shock reduce a motion ratio, not increase it?

Vertical shock: 1mm spring movement => 1/0.67 ~ 1.5mm of wheel movement
Shock at 20 degrees tilt from vertical : 1mm spring movement => 1*cos(20) /0.67 ~1.4 of wheel movement.
1/1.4 ~ 0.71. So, the number 0.67 cited above, turns into 0.71, closer to the MPP/RW number but not all the way.

It's not the motion ratio as usually defined but the inverse of it, but since GC started calling it that I used the same definition.

update: I think I may be confused. brb.

 

[dsgerbc]

Nevermind, drew a diagram, did some math. Looks like I should've been dividing by cos(20) instead.
So, the motion ratio as defined by GC does indeed go down with coilover incline, further away from MPP/RW numbers. Now I'm confused and need to do some more math. Maybe the spring perch offsets -> lowering metrics MPP/RW give don't correspond to motion ratios somehow, not sure.

 

[gearchruncher]

So I just swung my front suspension. I measured the hub relative to the wheelwheel, and the spring length.
I got a 0.60 ratio. The spring moved 28mm when the hub moved 47. This isn't super precise because even one mm makes a difference, I'd guess it could be 0.58 to 0.62 or so.

And hey, look... 0.67 (the arm ratio) times Cosine 20 (the spring tilt) = 0.62!

I'm pretty convinced that the front is well under 0.7, and thus a spring on 900 lbs should be 17 kg/mm for a 2.0Hz response.

Off to see if I can figure out the rear.

 

[dsgerbc]

So I just swung my front suspension.

At what ride height? It matters. MPP explicitly say that the effective motion ration will depend on bushing stiffness etc.

Also, what do you have for the compression rod bushing?
I planned to do the same while swapping tires on for an event tomorrow, but run up against a time constraint.

 

[gearchruncher]

The sweep was from full droop to about normal ride height. I'm well aware the angles mean the ratio is not the same through the whole arc. I was just checking if it was near what my other measurements showed. Like you say, technically bushings could make a difference too, but hard to believe it would be 0.05 or 0.10 difference.

I have whiteline compression bushings (solid poly with a caster offset)

 

[dsgerbc]

Can someone school me on tires? I had a weird event today on well used RT660s. Ambient air in low-50Fs and overcast. Tires at my regular 31-32psi (which worked well during summer) = undriveable, sliding every where and plowing badly under braking. Even did an unrecoverable 180 for the first time ever, scaring some course workers. Pumped to 35ish for the last run hail mary = tons of grip, coned a way an FTD in a modest field of drivers.

I was using tire blankets, but it's unlikely that they finally let me build some heat by the final run making the pressure change just a coincidence - the grid was a mess and there was like 5-8 minutes of slowly rolling back to my spot after a run. So tires were losing most of the post-run heat anyway - not much to preserve with blankets.

What gives?

 

[SK360]

My RT660’s are well used as well. 2nd season on them and I’m officially done with them forever. I’ve never been able to find pressures I was happy with, most recently I’ve settled on 36F/34R

Today was the last of the season. A frustrating event due to rain that seemed to happen just before my runs, the tires I mentioned above and frankly shitty hungover driving, whatever.

First season since I got this car with no FTDs. Anyone around recent nationals should recognize some names on the results page there, the talent pool in the area is deep.

I’m not sure what the clunking in the video is either, certainly hoping it’s something I forgot about in my center console lol

 

[bmwmkc323i]

If anyone is interested in Yokohama A052 in 275/40/18's with 5/32 tread remaining, please DM me. The wheels I just bought have these tires and I have no plans to do any tracking and need regular street tires.

 

[Rodney!]

Looks like the 3 was moved to AS for 2024... Commence whining from the Corvette people!

 

[gearchruncher]

Wow!

Makes me kind want to bring my car down to a max AS config and see how much it slows down from EV-X. The Model 3 would benefit a lot from shocks and a rear sway, which you could run in AS. Lack of camber would not be great, but I'm not sure that would give up 1%, which is the general gap between an EV-X car and an AS car. Could be a very interesting 2024.