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Thoughts on front spring rates for a track-only S197?

I would like to hear thoughts on front spring rates for a track-only S197. I have JRi DA struts from Cortex and Tim didn't recommend going much past 500lbs so that's what I went with. My thought (and his) was a softer spring would let the front suspension/tire get a better set or bite vs skittering across the pavement in understeer. I have spoken to a couple of other drivers who run 800lb springs. That seems like a huge jump and the car has to react so much differently with that rate but it works for them. It seems we all agree that a soft rear spring is the way to go. Thoughts?
 
4,627
4,551
Exp. Type
W2W Racing
Exp. Level
20+ Years
Pleasanton: 1/2 way between Sonoma and Laguna Seca
I would like to hear thoughts on front spring rates for a track-only S197. I have JRi DA struts from Cortex and Tim didn't recommend going much past 500lbs so that's what I went with. My thought (and his) was a softer spring would let the front suspension/tire get a better set or bite vs skittering across the pavement in understeer. I have spoken to a couple of other drivers who run 800lb springs. That seems like a huge jump and the car has to react so much differently with that rate but it works for them. It seems we all agree that a soft rear spring is the way to go. Thoughts?
Spring recommendations are all over the place, its basically a season to taste kind of thing.
A lot depends on the rest of the cars setup, aero, roll center height, roll axis, roll steer, sway bars, tire type, driver style etc.
 
A lot depends on the rest of the cars setup, aero, roll center height, roll axis, roll steer, sway bars, tire type, driver style etc.
Yeah I realize it's not that cut and dry. Maybe we can get some additional details. I can't really explain roll center and axis since I don't measure it. I use slicks, Watts and torque arm, 35F/18R swaybars, APR wing and LS splitter.....and I drive it like I stole it!
 
165
103
Exp. Type
HPDE
Exp. Level
3-5 Years
25 min. to 1½ hrs. from Sonoma (ugh... traffic!)
Yeah I realize it's not that cut and dry. Maybe we can get some additional details. I can't really explain roll center and axis since I don't measure it. I use slicks, Watts and torque arm, 35F/18R swaybars, APR wing and LS splitter.....and I drive it like I stole it!
Which of the 4 holes do you use for your Watts center pivot? And why?
 
4,627
4,551
Exp. Type
W2W Racing
Exp. Level
20+ Years
Pleasanton: 1/2 way between Sonoma and Laguna Seca
Yeah I realize it's not that cut and dry. Maybe we can get some additional details. I can't really explain roll center and axis since I don't measure it. I use slicks, Watts and torque arm, 35F/18R swaybars, APR wing and LS splitter.....and I drive it like I stole it!
The height on the watts pivot is the rear roll center. Do you have extended ball joints up front? Which hole is the watts in?
 
Which of the 4 holes do you use for your Watts center pivot? And why?
According to Cortex, front grip is improved as you move the pivot south. Since I was running without a rear swaybar I needed all the front grip I could get. I've managed to get to a reasonable balance adding the 18mm rear bar and playing with shock high speed/low speed compression and rebound.
 
Again, which of the four holes please?
I currently use the second hole from the bottom. I will have to re-read the Cortex guide as I have been told that raising the rear roll center will increase front grip so now I am confused. I guess that's why it's good to experiment and find out what changes work for you. That will be a change I make for next year.
 
3,966
4,052
I think the general base setup is still about 650 front, (with splitter) 400 rear (with wing) "blue" bar up front, 18mm or no bar in the rear.
I have no idea what to tell you on the watts..

well, actually I do
 
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I currently use the second hole from the bottom. I will have to re-read the Cortex guide as I have been told that raising the rear roll center will increase front grip so now I am confused. I guess that's why it's good to experiment and find out what changes work for you. That will be a change I make for next year.
Raising the rear RC increases rear roll stiffness (you can view it like increasing the stiffness of a rear swaybar) - it "improves front grip" by taking away rear grip to change the car's balance. You don't really want to take away rear grip nor increase rear roll resistance much in an S197.
 
Raising the rear RC increases rear roll stiffness (you can view it like increasing the stiffness of a rear swaybar) - it "improves front grip" by taking away rear grip to change the car's balance. You don't really want to take away rear grip nor increase rear roll resistance much in an S197.
I just checked and I actually have the pivot set second hole from the top. I agree that rear grip is not something I want to sacrifice but I was not using a rear sway bar and the front just didn't seem responsive. I have since added an 18mm bar and made adjustments on both the front and rear JRi's and the car feels much better. The feel confident in the rear getting on the power out of turns but I may lower the pivot and see how it changes the balance just for my own knowledge.
 

Dave_W

Cones - not just for ice cream
455
458
Exp. Type
Autocross
Exp. Level
20+ Years
Connecticut
One thing to note on moving the Roll Center is that it's "like" changing roll stiffness but it's actually changing the roll moment (lever arm between CG and RC). Increasing the roll moment (lowering the RC so it's further from the CG) means more weight transfer happens through the shocks & springs, which takes time - and you can control the timing of that with your shock valving.

I think the "front doesn't seem responsive" complaint is usually a call for more low-speed compression damping on the front - it "supports" the front end better during turn-in. If you fix it with roll stiffness, you can end up with steady-state understeer. Just my understanding.
 
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@docs302 thank you for bringing this up. Im learning from this thread. I think Drew got down to around a 500 spring front spring. Rear im not sure. I believe a 250. Pm him I believe he ran a similar set up as yours prior to the Cortex SLA .
 
I think the "front doesn't seem responsive" complaint is usually a call for more low-speed compression damping on the front - it "supports" the front end better during turn-in. If you fix it with roll stiffness, you can end up with steady-state understeer. Just my understanding.
I believe that's exactly what I did with my struts and it did make a big difference in getting the front to stay on line. What I also found is concentrating on how I drive.....using techniques like lifting and trail braking and being focused and precise, made a big difference on making the changes work.

@docs302 thank you for bringing this up. Im learning from this thread. I think Drew got down to around a 500 spring front spring. Rear im not sure. I believe a 250. Pm him I believe he ran a similar set up as yours prior to the Cortex SLA .
As Fabman said earlier......shock and spring rates are all over the place and often subjective. It's funny reading the opinions of drivers and engineers who have completely different theories on set up. I know of two friends with 800lb front springs on an s197 yet if you look at Andrew Aquilante's set up or Cortex, they seem to prefer more weight transfer and compliance.. I saw a thread where I believe Terry from Vorshlag was criticizing Andrew Aquilante and the pic of him lifting that front tire a foot off the ground powering out of a turn. Telling a pro race winner and multiple time Nat Champ that what he is doing is wrong is pretty silly but those are the two camps and how they think. I think a 500/250 spring works great. That's what Cortex recommended for a track-only car but I went with a 300 rear. Lot's of variables to consider.
 

Grant 302

basic and well known psychic
Lol. Domestic Power’s why you “don’t run stock suspension” thread is one of my favorites.

This is a great topic, but one which still seems to me has nothing in it, and is perhaps a bit beyond your skill and level of knowledge. My apologies for being a bit harsh there.

My suggestion is to learn a little more about geometry and the adjustments above that are clearly misunderstood in your misc. posts above.

One thing to note on moving the Roll Center is that it's "like" changing roll stiffness but it's actually changing the roll moment (lever arm between CG and RC). Increasing the roll moment (lowering the RC so it's further from the CG) means more weight transfer happens through the shocks & springs, which takes time - and you can control the timing of that with your shock valving.

😖
 
This is a great topic, but one which still seems to me has nothing in it, and is perhaps a bit beyond your skill and level of knowledge. My apologies for being a bit harsh there.
Yes Grant, definitely a bit harsh. Not everyone (including me) can discuss the physics behind suspension design and how that translates with movement of all the pieces and variables involved but I typically get more out of discussing this stuff with the less knowledgeable and skilled and taking their real-world experiences than listening to the engineers trying to out do each other with their knowledge of yaw rates, moment arms and force vectors. If that's your thing than by all means go at it.....I never make a comment about how useless that stuff is to me and probably many on this site reading it. I just accept that I don't understand all of it but it's education and many will want to dig in and try. As a physician and educator, I am always talking to those who are also "professionals" but may not have my experience and knowledge. If you were on a medical forum with me and you chimed in with a simplistic understanding of some subject matter I might post a response to help educate...or just not say anything. What I wouldn't do is feel the need to say "I'm so above everyone's knowledge on this subject that I am getting nothing out of the discussion and would recommend you go back and educate yourself on the subject so you can post something that is at my level of interest.....or don't post at all"

Am I being harsh? I have never been one to challenge someone else's comment on this site but I decided to make an exception. Not sorry.

I will say that I appreciate the feedback I've received from everyone.....keep it coming.
 

Grant 302

basic and well known psychic
Nice. 😁

I wasn’t thinking you needed to really dig in to the technical parts. I just mean enough to understand the adjustments on your own car and what they do, before trying to have a discussion on spring rates for track only cars.
 

Dave_W

Cones - not just for ice cream
455
458
Exp. Type
Autocross
Exp. Level
20+ Years
Connecticut
Sorry, I should have said "more weight transfer goes through the springs & shocks as opposed to the suspension links." In theory (as I currenty understand), if CG and RC are in the same location, the body doesn't roll in a turn, but there's still the same amount of weight transfer (remembering that weight is a force, not a mass) determining the dynamic weight/force on each tire / contact patch. Which means that 100% of the force of the weight transfer in that case is going through the suspension linkages and not the springs/shocks/swaybar. When you move the RC away from the CG, you start shifting more of that force through the shocks/springs/swaybars, and you can then use those items to change the Total Lateral Load Transfer bias front-to-rear to adjust handling balance. Keeping in mind that shock adjustments only have an effect when the suspension is moving (if the shock piston isn't being moved through the shock fluid, it's valving doesn't matter).

In my thinking, and I'm still coming to grip with this line of thinking (pun intended), given the same spring/swaybar roll stiffness so the same steady-state balance, stiffening the shocks will mean the car will take longer to change from one steady-state to another (e.g., straight to turn, turn to straight). And during that transition period, changing relative shock stiffness on certain corners lets you affect the TLLT during the transition differently from the steady-state TLLT. But this is starting to get into things like moment of inertia of the rolling chassis, second- and third-order distance/time derivatives of acceleration and jerk, and outside my current knowledge. I probably need to buy the Milliken & Milliken book.
 

Grant 302

basic and well known psychic
I’m not needing an apology from anybody. But I hope you appreciate the issue of brevity when explaining things that just take more time and effort. Your previous post is more likely to be misunderstood. The second might be too complicated for your intended audience.
 

Norm Peterson

Corner Barstool Sitter
939
709
Exp. Type
HPDE
Exp. Level
5-10 Years
a few miles east of Philly
In my thinking, and I'm still coming to grip with this line of thinking (pun intended), given the same spring/swaybar roll stiffness so the same steady-state balance, stiffening the shocks will mean the car will take longer to change from one steady-state to another (e.g., straight to turn, turn to straight). And during that transition period, changing relative shock stiffness on certain corners lets you affect the TLLT during the transition differently from the steady-state TLLT. But this is starting to get into things like moment of inertia of the rolling chassis, second- and third-order distance/time derivatives of acceleration and jerk, and outside my current knowledge. I probably need to buy the Milliken & Milliken book.
Not sure I agree. Damper forces being velocity-sensitive peak before the displacement-sensitive spring and bar forces peak, so at some point in the roll motion you've approximated the total steady-state force balance even though the motion hasn't ceased. IOW, maybe it all gets close enough to having taken a set at some point.


I know, getting into a time history approach is maybe a bit heavy on the theory, right?


Norm
 

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