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Cortex Torque Arm and Watts: An Honest, Thorough, Track Based Review

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Fabman

Dances with Racecars
6,519
8,154
Exp. Type
W2W Racing
Exp. Level
20+ Years
Pleasanton: 1/2 way between Sonoma and Laguna Seca
My car understeered like a pig with no rear sway bar on the cortex setup. It was potentially faster and easier to drive, but was no fun whatsoever. I could see no rear sway bar working in a car with coyote power, but with only 290whp I couldn't throttle steer the car how I wanted.
This is when you move the watts up...
 
303
372
CA
What I'm not hearing here is what hole is the watts in? It makes a HUGE difference which hole its in as that is where the roll center is and that has a major impact on the handling. Do yourself a favor, move it all the way to the top and run a session then move it all the way to the bottom and do a session. Then season to taste...the experience is eye opening.
Good call, this was next on my list. Wanted to try adjusting with sway bars first. I have the bellcrank on the 2nd lowest hole from bottom. I intend to take advantage of the extra grip from the lower rear roll center so I want to keep the bellcrank on the lower half of the diff cover.
 

Fabman

Dances with Racecars
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8,154
Exp. Type
W2W Racing
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20+ Years
Pleasanton: 1/2 way between Sonoma and Laguna Seca
Good call, this was next on my list. Wanted to try adjusting with sway bars first. I have the bellcrank on the 2nd lowest hole from bottom. I intend to take advantage of the extra grip from the lower rear roll center so I want to keep the bellcrank on the lower half of the diff cover.
None of the s197's I race with use a rear sway bar including me.
Most run the watts in the top and big rear springs. I have been in every hole and had springs from 450# to 250# in 25# increments and 3 different sway bars....this is how you find your happy spot. I ended up with the watts in the bottom, 300#'s and no bar. That's my happy spot. Go from one extreme to the other and it amplifies the effect so you know exactly whats happening then the in between adjustments have more meaning. Small changes are harder to discern especially at first. But once you know, you know.
 

Fabman

Dances with Racecars
6,519
8,154
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W2W Racing
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20+ Years
Pleasanton: 1/2 way between Sonoma and Laguna Seca
Good call, this was next on my list. Wanted to try adjusting with sway bars first. I have the bellcrank on the 2nd lowest hole from bottom. I intend to take advantage of the extra grip from the lower rear roll center so I want to keep the bellcrank on the lower half of the diff cover.
Don't confuse side bite with forward bite...these are two related but separate things.
You can improve one and worsen the other. It is then that a combination of adjustments may be needed...this is why I say go to the extremes and learn what does what first.
You can learn more in one good test day than some will learn in a lifetime because you will KNOW what does what....IF you are willing to do the work.
 

Dave_W

Cones - not just for ice cream
984
1,277
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Autocross
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20+ Years
Connecticut
Key point to remember for tuning with roll center height - changing the distance of the roll center from the center of gravity will change how much the car rolls, but it does not change the actual amount of lateral load transfer for a given lateral g-load.

Moving the roll center up - closer to the center of gravity - means more of the lateral weight transfer happens through the suspension links and less through the anti-roll bar, springs, and dampers. Moving the RC down & further away from the CG means more of the lateral load transfer happens through the ARB, springs, & dampers. Therefore, changes in your shock settings, ARB & spring rates have more affect with a lower the RC. But, the amount of load they're resisting (the elastic lateral load transfer) is increasing as well. And changing the RC-to-CG distance on one axle can also change how the other axle's load transfer happens.

If the CG and RC are at the same height, then the stiffness of the springs, ARBs, and dampers have no effect whatsoever on handling balance (at least on a perfectly smooth track).

This article has good info on roll center:

And this article really gets into the details with lots of formulas, but even if you skip over the math it gives some really important insights.
 
125
112
Exp. Type
HPDE
Exp. Level
Under 3 Years
Ann Arbor, MI
Key point to remember for tuning with roll center height - changing the distance of the roll center from the center of gravity will change how much the car rolls, but it does not change the actual amount of lateral load transfer for a given lateral g-load.

Moving the roll center up - closer to the center of gravity - means more of the lateral weight transfer happens through the suspension links and less through the anti-roll bar, springs, and dampers. Moving the RC down & further away from the CG means more of the lateral load transfer happens through the ARB, springs, & dampers. Therefore, changes in your shock settings, ARB & spring rates have more affect with a lower the RC. But, the amount of load they're resisting (the elastic lateral load transfer) is increasing as well. And changing the RC-to-CG distance on one axle can also change how the other axle's load transfer happens.

If the CG and RC are at the same height, then the stiffness of the springs, ARBs, and dampers have no effect whatsoever on handling balance (at least on a perfectly smooth track).

This article has good info on roll center:

And this article really gets into the details with lots of formulas, but even if you skip over the math it gives some really important insights.
I guess this is one of the reasons for Test 'n Tune track nights at Gingerman and others. I think my order, doing one thing at a time and adjusting up and down to help with rear exit throttle might look like this, any thoughts:

1) Watts link adjustments
2) soften rear shocks
3) soften rear sway bar
4) stiffen front sway bar
 
Very appropriate thread as I’m trying to determine what spring rates to buy for my cortex coilover 3v. Currently on 400f/200r with eibach front bar and cortex rear bar.
Anyone have information on the cortex rear bar? I don’t hear it talked about much, if at all.

9A14A130-2663-468F-91F6-905BB9571536.jpeg

C419329E-45C1-40F8-A997-5EF73B617DCF.jpeg
 

Fabman

Dances with Racecars
6,519
8,154
Exp. Type
W2W Racing
Exp. Level
20+ Years
Pleasanton: 1/2 way between Sonoma and Laguna Seca
Very appropriate thread as I’m trying to determine what spring rates to buy for my cortex coilover 3v. Currently on 400f/200r with eibach front bar and cortex rear bar.
Anyone have information on the cortex rear bar? I don’t hear it talked about much, if at all.

View attachment 75055

View attachment 75056
500/300 is a good combination with a 18mm rear bar and watts in the second from the bottom hole. The fast spec mustangs around here run 600/425 and no rear bar with the watts in the top hole. Very efffective combination as well. Different way to skin the cat. In the end It’s a season to taste sort of thing. The cortex comp sway bar is roughly the same rate as the 18mm bar when set on the softest setting.
 
500/300 is a good combination with a 18mm rear bar and watts in the second from the bottom hole. The fast spec mustangs around here run 600/425 and no rear bar with the watts in the top hole. Very efffective combination as well. Different way to skin the cat. In the end It’s a season to taste sort of thing. The cortex comp sway bar is roughly the same rate as the 18mm bar when set on the softest setting.
I appreciate the response and suggestions. I’m just a hpde guy running slicks but I have a 3 day event in June so I’ll test a few settings as mentioned in this thread, thanks!
 
303
372
CA
303
372
CA
Key point to remember for tuning with roll center height - changing the distance of the roll center from the center of gravity will change how much the car rolls, but it does not change the actual amount of lateral load transfer for a given lateral g-load.

Moving the roll center up - closer to the center of gravity - means more of the lateral weight transfer happens through the suspension links and less through the anti-roll bar, springs, and dampers. Moving the RC down & further away from the CG means more of the lateral load transfer happens through the ARB, springs, & dampers. Therefore, changes in your shock settings, ARB & spring rates have more affect with a lower the RC. But, the amount of load they're resisting (the elastic lateral load transfer) is increasing as well. And changing the RC-to-CG distance on one axle can also change how the other axle's load transfer happens.

If the CG and RC are at the same height, then the stiffness of the springs, ARBs, and dampers have no effect whatsoever on handling balance (at least on a perfectly smooth track).

This article has good info on roll center:

And this article really gets into the details with lots of formulas, but even if you skip over the math it gives some really important insights.
Well I guess I'm going to be "that guy" and open a can of worms, as I've been told the exact opposite by a chief engineer at an unnamed mustang suspension company, take a read:

  1. Cornering grip is a function of tire contact patch shape, area, camber angle and lateral weight transfer. Wider wheels and tires give a better contact pattern shape and along with lower air pressure, more contact area. A lower rear roll center height gives more grip through reduced lateral weight transfer from reduced jacking forces. The only downside to this is that the lower the roll center, the slower the lateral weight transfer occurs. This needs to be balanced against the roll center height of the front suspension, so that they both transfer weight at the same rate. When people install Watts links, they are almost always raising the roll center in the rear. This reduces grip from increased jacking forces, but causes the lateral weight transfer to happen faster. To the inexperienced driver this feels confidence inspiring, so they think the car handles better, even though it has less grip.

At this point I don't know who or what is right, as fabman suggested I will play around with the different bellcrank positions to see how the handling is affected.

Look at the Kenny Brown rear end packages. All of them focus on lowering the rear roll center. I understand that it's all a "package" (ie, lowering rear roll center and changing nothing else along with it wouldn't necessarily be beneficial). But from the things I've read, it seems like lowering rear roll center and then adjusting the car around that is "technically" the way to make the most lateral grip. I'm new to suspension theory so I haven't had the chance to test this yet though, so I'm very eager to hear other s-197 guru's experiences.
 
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Mad Hatter

Gotta go Faster
5,237
4,227
Santiago, Chile
500/300 is a good combination with a 18mm rear bar and watts in the second from the bottom hole. The fast spec mustangs around here run 600/425 and no rear bar with the watts in the top hole. Very efffective combination as well. Different way to skin the cat. In the end It’s a season to taste sort of thing. The cortex comp sway bar is roughly the same rate as the 18mm bar when set on the softest setting.
Dido on de 500/300 spring set up. tried the 600/350 setup, but did not like it much and really ruled out curbing the car. No rear bar and with some fine tuning on the front bar, you can get a easy to drive car that rotates on demand and hooks up like crazy on corner exit.

Understeer can also be induced with too much throttle too soon. I had a problem with that for a long time. Also do you have a Torsen? Didn't see that mentioned. Not having one give you understeer on corner entry and oversteer on exit.

I did not say the W@tts word because BS1 might be watching.
 
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Dave_W

Cones - not just for ice cream
984
1,277
Exp. Type
Autocross
Exp. Level
20+ Years
Connecticut
A lower rear roll center height gives more grip through reduced lateral weight transfer from reduced jacking forces.
True for independent suspensions, but (I believe) a solid axle does not produce jacking forces, at least in the same magnitude as an independent suspension.

The only downside to this is that the lower the roll center, the slower the lateral weight transfer occurs.
"the lower the RC" is technically "the further the RC from the CG" - which means more of the lateral load transfer is going through the spring/shocks/ARB, and subject to the timing of the shock valving to reach steady-state roll angle, as opposed to going instantly through the suspension links. If RC=CG, then all lateral load transfer happens through the suspension links, it happens instantly, and there is no "roll" in the unsprung mass (but there IS load/weight transfer).
This needs to be balanced against the roll center height of the front suspension, so that they both transfer weight at the same rate.
If the car is balanced to start with, yes. Playing with the RC-CG distances front-to-rear changes the timing of load transfer / roll on each axle, and can be used to tune transient handling (or allow the shocks to have a bigger/smaller affect on transient handling). See Figure 14 in the second link I posted.
 
6,362
8,186
Well I guess I'm going to be "that guy" and open a can of worms, as I've been told the exact opposite by a chief engineer at an unnamed mustang suspension company, take a read:

  1. Cornering grip is a function of tire contact patch shape, area, camber angle and lateral weight transfer. Wider wheels and tires give a better contact pattern shape and along with lower air pressure, more contact area. A lower rear roll center height gives more grip through reduced lateral weight transfer from reduced jacking forces. The only downside to this is that the lower the roll center, the slower the lateral weight transfer occurs. This needs to be balanced against the roll center height of the front suspension, so that they both transfer weight at the same rate. When people install Watts links, they are almost always raising the roll center in the rear. This reduces grip from increased jacking forces, but causes the lateral weight transfer to happen faster. To the inexperienced driver this feels confidence inspiring, so they think the car handles better, even though it has less grip.

At this point I don't know who or what is right, as fabman suggested I will play around with the different bellcrank positions to see how the handling is affected.

Look at the Kenny Brown rear end packages. All of them focus on lowering the rear roll center. I understand that it's all a "package" (ie, lowering rear roll center and changing nothing else along with it wouldn't necessarily be beneficial). But from the things I've read, it seems like lowering rear roll center and then adjusting the car around that is "technically" the way to make the most lateral grip. I'm new to suspension theory so I haven't had the chance to test this yet though, so I'm very eager to hear other s-197 guru's experiences.
Go with Fabman....
 
30
33
Exp. Type
HPDE
Exp. Level
Under 3 Years
Maryland
I have a quick question to throw in here for you guys running the cortex torque arm… will that torque arm work with the Boss 302 quad exhaust? It looks like the side pipes may run into where the front of the torque arm mount is bolted on to the body.
 
6,362
8,186
I have a quick question to throw in here for you guys running the cortex torque arm… will that torque arm work with the Boss 302 quad exhaust? It looks like the side pipes may run into where the front of the torque arm mount is bolted on to the body.
I have to be honest here, the Boss 302 platform and especially the LS and R models were some of the best handling cars ever built. They are still knocking off Pirsches and BMWs at national events even today, and they had to be legislated out of existence to make way for the GT4 spec cars. So, why would you want to significantly change one of those cars? You have all the tools you need already on it.
Fabman just likes to build things, which is his excuse.
 

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