STX Rules and Setup Questions

[Racecomp Engineering]

That's from Fred Puhn's "How To Make Your Car Handle".

It's not too difficult to calculate the actual spring rate if you have a few measurements. Then you can plug in different diameters and see what you get, or change the A and C measurements for adjustable bars.

For hollow bars like our RCE set, the numerator becomes 500,000 (OD^4 - ID^4).

Don't forget about the motion ratio for the swaybars when you start looking at wheel rates. There's some info in the suspension model thread stickied in the suspension forum.

- Andrew

[DocWalt]

Originally Posted by renfield90

I would question this. Typically the fat bar helps a CS car more in transitions due to the weight transfer happening faster - my car was best in slaloms with my Hotchkis bar at full stiff, to the point where someone who was used to driving the 6 time XP national champion Lotus drove my car and said "wow, this actually slaloms really good." On the soft setting it was still very good, but not as good as hard.

Where the fat bar hurts you is in steady state transitions where the car will want to push, and you might be able to achieve a slightly higher max lateral G load by going with a smaller bar. It will, however, put power down with less power-on oversteer than the smaller bar, and so I compensated for the push by driving lines that let me get on the power sooner.

For an STX car the sway bar is less important. Whereas the stock car will have many mm of suspension compression available (which results in a large weight transfer by the sway bar), a proper STX car will only have a few. Because you can make dramatic changes to the balance of the car using spring rates, the bars should be used to fine tune that balance.

I swear by big bars as a personal suspension philosophy, but may need to go smaller once I select springs and shocks. It is mostly personal preference though and there's no substitute for trying them yourself.

I agree that what works for a CS car may not work for a STX car. Obviously we're on the same platform but a totally different setup. My "soft" STX setup is still double the stock spring rates, for example.

I'm afraid it's definitely a personal preference, like you said. I used to have the same philosophy as you regarding stiffer sway bars than springs, but I've sort of gone a bit neutral on it. There's a lot to be said about having the wheels "disconnected" so they can act independently. Coming from a FWD car that had a lot of problems putting power down because of a fat swaybar has changed my opinion a bit.

Ah well, just comes down to picking bars that I think will work for me and trying them. Yay money haha

FWIW The only reason I'm even thinking about swaybars is for balance reasons. My car is clearly a bit oversteer biased, but it slaloms like a champ. I can attack slaloms like I've never even thought was possible in a street car, lol.

[renfield90]

Originally Posted by Racecomp Engineering

That's from Fred Puhn's "How To Make Your Car Handle".

It's not too difficult to calculate the actual spring rate if you have a few measurements. Then you can plug in different diameters and see what you get, or change the A and C measurements for adjustable bars.

For hollow bars like our RCE set, the numerator becomes 500,000 (OD^4 - ID^4).

Don't forget about the motion ratio for the swaybars when you start looking at wheel rates. There's some info in the suspension model thread stickied in the suspension forum.

- Andrew

Good post. And perhaps more proof that Whiteline doesn't really care what the spring rates for their bars are.

Edit: pretty sure this makes some assumptions about the rigidity of the sway bar material?

[renfield90]

Originally Posted by DocWalt

I'm afraid it's definitely a personal preference, like you said. I used to have the same philosophy as you regarding stiffer sway bars than springs, but I've sort of gone a bit neutral on it. There's a lot to be said about having the wheels "disconnected" so they can act independently. Coming from a FWD car that had a lot of problems putting power down because of a fat swaybar has changed my opinion a bit.

Ah well, just comes down to picking bars that I think will work for me and trying them. Yay money haha

FWIW The only reason I'm even thinking about swaybars is for balance reasons. My car is clearly a bit oversteer biased, but it slaloms like a champ. I can attack slaloms like I've never even thought was possible in a street car, lol.

Getting off topic for a second, but did your FWD car have a fat bar on the drive axle? That's a big no-no in my personal suspension religion for precisely the reason you described, you can't put power down. I have the TRD rear bar for now, may step back to OEM when my suspension goes in.

What are you trying to improve about the balance? For an STX car I'd first get the steady state balance just right, then use shock settings to get transitional/slalom balance right. Adding adjustables to my CS car let me go softer on the bar to reduce steady state push while mostly retaining it's awesome slalom qualities.

[Racecomp Engineering]

Originally Posted by renfield90

Good post. And perhaps more proof that Whiteline doesn't really care what the spring rates for their bars are.

Edit: pretty sure this makes some assumptions about the rigidity of the sway bar material?

It does make assumptions about material...there can be minor differences but they are very minor for the most part.

- Andrew

[DocWalt]

Originally Posted by renfield90

Getting off topic for a second, but did your FWD car have a fat bar on the drive axle? That's a big no-no in my personal suspension religion for precisely the reason you described, you can't put power down. I have the TRD rear bar for now, may step back to OEM when my suspension goes in.

What are you trying to improve about the balance? For an STX car I'd first get the steady state balance just right, then use shock settings to get transitional/slalom balance right. Adding adjustables to my CS car let me go softer on the bar to reduce steady state push while mostly retaining it's awesome slalom qualities.

It had fat bars at both ends. Fat bar at the front to prevent the horrible body roll... I should have done springs instead, but that was well before I had a clue what I was doing.

I'm trying to avoid a stiffer rear bar for exactly the same reason.

I want to make it a touch more neutral, it's a tiny bit biased to oversteer. I've been adjusting dampers here and there but I need to make an actual effort at testing things at my local test and tune in a week or so. I did go slightly stiffer at the front vs. rear and it definitely felt a little more stable. I've also got to play with tire pressures, I can definitely drop the rear a little bit as the tire rollover had some room to go before the compound changes to the sidewall rubber.

[toeout]

The problem with Puhn's formula is that assumes completely perpendicular endlinks, which does not happen at least not in all adjustment holes.

[Silverspeed]

Originally Posted by GeorgeJFrick

== REAR ==
Rear Sway Bar OEM 14.0mm Adjustable: No
ADJ (Megan) 19.0mm Adjustable: 2 Way Bushings: Thermoplastic Vulcanizate $155.00
Godspeed 18.0mm No $80.00
ST 17.46mm ??? $136.00 51228
Whiteline 16.0mm Adjustable: 3 Way inc locks. 2115 BSR53Z
Whiteline 18.0mm Adjustable: 3 Way inc locks. $197.00
Hotchkis 19.0mm Adjustable: 4 Way 22445R
Tanabe 18.0mm No $215.00 TSB166R
Eibach 19.0mm Adjustable: 2 Way $240.00 82105.32
TRD 15.8mm ??? $220.00 PTR11-18130
Perrin 16.0mm Adjustable: 3 Way $197.00
Perrin 19.0mm Adjustable: 3 Way $197.00
Perrin 22.0mm Adjustable: 3 Way $230.00
Perrin 25.0mm Adjustable: 3 Way $240.00
Cusco 16.0mm ??? 14.00mm avail? $240.00 965 311 B16

Add to this list.... 2008/2009 WRX 16mm bar... Can be picked up on the cheap , $40 in my case from Craigslist. Non-adjustable, yes, but seems to work well as compliment for upsized front bar.

[kandiflippin]

Are quick latch tabs authorized on a front bumper? I have a RB front bumper that I am going to swap out with a stock bumper just for the events and figure the quick release would save me alot of time.

[renfield90]

Originally Posted by kandiflippin

Are quick latch tabs authorized on a front bumper? I have a RB front bumper that I am going to swap out with a stock bumper just for the events and figure the quick release would save me alot of time.

I would consider latches a legal comfort and convenience mod.

Be sure that it is attached very securely though. Bumpers go through a ton of abuse when they hit a cone, you wouldn't want it to tear off and get lunched under your car. You wouldn't high five someone if you were driving past them at 45mph, right? Well a cone weighs more than your hand, and on cold mornings they are stiff and not very pliable.

[kandiflippin]

Originally Posted by renfield90

I would consider latches a legal comfort and convenience mod.

Be sure that it is attached very securely though. Bumpers go through a ton of abuse when they hit a cone, you wouldn't want it to tear off and get lunched under your car. You wouldn't high five someone if you were driving past them at 45mph, right? Well a cone weighs more than your hand, and on cold mornings they are stiff and not very pliable.

Good to keep in mind, I still secure the bumper under the hood as well and for the races I will make sure to secure it on the bottom as well. The only reason I even went with the quick latches is because of that one bolt on each side of the bumper that is impossible to get to.

[smg1138]

For those of you on coilovers, what kind of ride height are you running in STX? I'm on RCE T2's and am dropped 1" Front and 1 1/4" Rear from stock height. That gives me basically zero rake angle right now. I'd like to go a bit lower, but don't want to screw up the suspension geometry. What are you guys finding to be the most ideal ride height?

[Kostamojen]

Originally Posted by smg1138

For those of you on coilovers, what kind of ride height are you running in STX? I'm on RCE T2's and am dropped 1" Front and 1 1/4" Rear from stock height. That gives me basically zero rake angle right now. I'd like to go a bit lower, but don't want to screw up the suspension geometry. What are you guys finding to be the most ideal ride height?

I had a friend with a WRX that was serious into Autox and had his WRX lowered 2" with his coilovers... He complained how shitty the car drove, so I sugfested he raise the can an inch. This fixed his geometry and the car handled great after that.

I would say 1" is probably ideal especially since you cant use roll center kits in STX.

[smg1138]

Originally Posted by Kostamojen

I had a friend with a WRX that was serious into Autox and had his WRX lowered 2" with his coilovers... He complained how shitty the car drove, so I sugfested he raise the can an inch. This fixed his geometry and the car handled great after that.

I would say 1" is probably ideal especially since you cant use roll center kits in STX.

Yeah, I know. I looked at installing a Whiteline Roll Center kit, but my local SCCA rep told me it was illegal for STX.