What size Front sway bar are people running for autocross?

746Watts

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Met a guy the other day in cam. Car was setup with a watts, front and rear bars, 3600lbs, moton coilovers, 550lb/250lb springs, 315 bfg. What really blew my mind is the guy said he was running a 38mm front sway bar on the 5th setting. He was adamant that this was the only setup stiff enough for the s197 and that all other bars are too soft. My thoughts is that this just takes away mechanical front grip front too stiff a sway bar spring rate?
For reference that bar is 300% stiffer than 2011+ bars, (and 340% stiffer than 3v bars).
My whiteline front bar (33mm, 4 hole) is around (using my own calculator from assumed values): 149%-211% stiffer.
The general eibach bar (35mm 3 hole) gets to 194% stiffer.
In reference to above: 100% value equals a stock 2011+ FSB. ex: 149% equals 49% stiffer. Just to clear things up since people do things differently.
What is everyone elses experiences with FSB settings for autocross?

Edit: To add also for comparison Sam strano's bar only ranges from 108%-147% which is honestly one of the softest bars i've seen.
 
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noldevin

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I believe the s197 front end is kind of backwards when compared to the standard "more sway bar or spring rate = less grip" rule.
It easily rolls out of negative camber, which I guess is far more detrimental to grip than extra stiffness.
 

Norm Peterson

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I believe the s197 front end is kind of backwards when compared to the standard "more sway bar or spring rate = less grip" rule.
It easily rolls out of negative camber, which I guess is far more detrimental to grip than extra stiffness.
The S197 seems to have a decent amount of roll stiffness to begin with, somewhere around 2.5°/g for the later GT's with the 5.0 to maybe 3°/g for the earlier GT's. Which puts you close to the land of diminishing returns (if not in it already), so getting much below 2°/g is going to take a large increase in bar rate.

Especially if you run stiffer springs and are trying to keep the ratio of bar to spring (contributions to roll stiffness) somewhere near where it is in stock form . Which probably isn't a bad idea, actually.


Somewhat better than a WAG - if you can get the suspension roll stiffness up to the point where roll is down to 1.9°/g (suspension spring and bar stiffnesses only), the car will visibly roll about 2.5°/g due to unequal side to side tire deflections. The higher figure is what you'd base your static camber setting on.


Norm
 

noldevin

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I'll agree that these cars have surprisingly decent roll rates for what they are. But it does seem like most people who track and autocross like to run a stiff front and soft rear, and many of them bring up the camber curve as part or most of the reason. Whether their reasoning is right or not, I don't know, but the results are definitely positive regardless.
I'll be trying a big front bar soon to see what it does for me. Obviously, springs are a better way to add stiffness, but I'd rather retain some ride quality if I can. Dual purpose car ftl
 
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Norm Peterson

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The front suspension roll stiffness is slightly bar-heavy (about 55% for my '08), which isn't a bad place to be. Ignoring the (severely) limited options available in SCCA's 'Street' category of autocross, you're probably best off to increase both spring and bar stiffnesses instead of trying to do it all with only one of those. Without stiffer springs, you'll lose front tire contact patch and caster under hard braking if you're at a ride height that's any good at all for camber gain in bump.


Norm
 

modernbeat

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In my testing, for Stock/Street class where you are stuck with all sorts of things you can't change, the big bar works. But, it comes with compromises. The soft spring and big bar combo is the recipe for ripping stuff up and breaking things.

Once we got real spring rates on the car, and were able to balance it, we were satisfied with the moderate Whiteline bars. If you are maxing out a bar, you should be looking at a different spring.
 

frank s

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^^What sort of things were you breaking with the large bar/soft spring combo?

Good question.

I have the 5.0 convertible and was running the H&R 26mm rear bar, standard front bar and springs/dampers. The effect of the fat rear bar was not as remarkable as I had expected; nice sense of quicker turn-in, but still a degree of understeer in enthusiastic cornering.

Changed to Steeda's 3-hole, 1 3/8 front bar in the middle hole, and found a little oversteer, just off neutral. I'm remembering the statement of a "stock" Corvette driver who told me (early in my Mustang phase) that he tried everything, nothing worked until he put on the fattest front bar he could find, when it all came right. I didn't believe him, but there might have been something in what he said.
 

746Watts

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Once we got real spring rates on the car, and were able to balance it, we were satisfied with the moderate Whiteline bars. If you are maxing out a bar, you should be looking at a different spring.

What is considered "real" spring rates? The 550/250 combo or the 800/350 combo?
 

ddd4114

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But, it comes with compromises. The soft spring and big bar combo is the recipe for ripping stuff up and breaking things.

Once we got real spring rates on the car, and were able to balance it, we were satisfied with the moderate Whiteline bars. If you are maxing out a bar, you should be looking at a different spring.
I'd have to agree with this from a road racing perspective, and I imagine autocross is somewhat similar. If you're limited based on class rules, then of course you're stuck. I have the freedom to use any spring/bar combination in my class, and I just use sway bars as tuning tools. I try to do the majority of my handling adjustment with spring rates. I'm using fairly stuff 800/325 lb/in springs now, and that isn't even enough. However, that's with a lot of weight, 315mm Hoosier R7s, and aero. The car is lowered a decent amount, so the front roll center is far below ground, and as stated the S197 camber curve isn't great. It's easy to wear the outside edges of tires without a lot of stiffness and a lot of negative camber.

Once I started using good dampers (MCS), I found that adding a lot more spring rate really didn't hurt ride quality that much. You definitely notice that the car is stiffer, but it isn't much less comfortable. I originally had Koni Yellows on the car, and they were brutal on the street even with moderate spring rates. I think the least comfortable setup I've had is Koni Yellows and basic lowering springs. There was so much body movement yet so much damper stiffness, it was nauseating. To Jason's point, while the S197 seems to be a bit more accommodating of stiffer sway bars, a lot of other platforms, like older Hondas, will destroy subframes with stiff sway bars unless you add supporting structure to the mounting points.

When class rules aren't a limitation, I think the only time the soft spring / big bar combination is best is NASCAR (or similar). However, that's for aerodynamic reasons. When taken to the extreme, it also needs anti-dive geometry to match or the nose of the car will get torn up. In my testing, I've found that the car seems to be faster with more spring and less bar (both with similar overall stiffness), but I can't really explain why. Without being able to explain with data, more bar and less spring seems to make the car more easily upset in transitions. Furthermore, using stiffer springs over bars will provide more pitch stiffness (which is pretty terrible for the S197 stock), and it can be tricky to get shock damping right with a soft spring / big bar setup (possibly the cause of the issue I had).
 

Norm Peterson

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In my testing, I've found that the car seems to be faster with more spring and less bar (both with similar overall stiffness), but I can't really explain why. Without being able to explain with data, more bar and less spring seems to make the car more easily upset in transitions.
More bar means less independence. Maybe also that on the inboard side the spring and bar forces are acting in opposite directions has something to do with it.


Norm
 

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