22mm F / 16mm R swaybar change in handling
 

Problem I'd like to correct : a bit of mid-corner understeer. That being said, I don't want to do anything drastic to hurt the street-ability, so was considering sway bars.

Looks like most of the swaybar kits (RCE/Eibach/Whiteline) are 22mm/16mm rear equiv bars.

Whiteline says this is a 123/167% increase in front stiffness and ~0/29%/65% increase in stiffness over my 2020 stock bars of 18mm/15mm as I understand it.

The large increase in front diameter vs the pretty small increase in rear stiffness makes me think these sway bar kits would maybe increase understeer but I know there are a lot of other factors like motion rato/etc.

The 22/16 seem to have come out before the 2017 refresh moved to a 15mm (vs 14mm) rear bar. Is that still a good size combo with stiffer than stock (and square) springs?
(I'm running RCE SS1s w/330 lb/in F/R about 1" lowered from stock and 245/40-17 RS4 tires on 17x9 wheels. No other real changes. Alignment ~ -3.6/-2.4 camber with near zero front toe, tiny bit of toe in at the rear. Tire wear is good - they wear down evenly across the surface, but I know wear isn't necessarily equal to grip).

22/16 kit, or maybe just adding a 16mm rear bar instead and leave the stock front?

Thanks in advance of advice.

Looking at the RCE strut on their site, it looks like it might have less bump travel than OEM. That's more likely the source of your understeer than sway bar diameters. Everything else in your setup would make for a slightly loose car. Fix for that is raising the car to get it off the bump stops or adding camber plates with lower stack height. Our camber plates would add bump travel and fit the KW/ST made shocks that RCE offers. Adding more front bar would make it tighter than it already is.

Separate note, make sure you have OEM toe setting in rear. That's what we run. Too much rear toe in can cause mid corner and turn in push.

I seem to remember RCE saying that bump travel was one of the things they tried to keep - with the camber plate having relatively low stack height. Maybe Andrew @Racecomp Engineering can chime in.

I'm toward the upper end of the ride height that I think would be decent to run with these - the helpers are completely compressed and there is a decent amount of "preload" on the springs. Worried going much higher would end up hurting droop travel and or/requiring several hundred pounds of load before the springs start moving in a bump that causes full extension. So not sure what to do about ride height. I know others are running these lower than I am.

The fix for mid corner understeer on my car was going stiffer on the front bar. I was already running the Perrin adjustable 22/16 setup. Going to the stiffest front bar setting helped get rid of most of it. I am on 7kg spring setup square.

At a 1 inch drop on RCE SS1 coilovers you have more bump travel than stock...overall stroke is about 10mm less than stock but much more of it is bump travel vs droop at that ride height. Should be over 3 inches of bump. The camber plates on our RCE SS1s use a recessed design similar to 949's version to maximize bump travel. Our original RCE camber plates we released for GD chassis Impreza around 2006 also used a similar design, though the set on the SS1s is slightly different and made by KW.

What kind and what size tires are you using?

- Andrew

Mentioned buried in the first post, but 245/40-17 RS4s on 17x9 ARC8 wheels.

Interesting. Seems counter-intuitive to me given what I know about sways.

What are the details of your suspension? (how low, etc). What tires are you using?

Was going to look this up but the garage link just reports "HTTP ERROR 500"

If the shock setup has enough bump travel and you have a decent amount of camber then the focus should be on adjusting weight transfer towards the rear. The RCE setup uses the same spring rate front and rear. we have found that works too.

Rake is the next thing to play with. Raising either end will raise roll center which effectively raises roll stiffness. So raising the rear has a similar effect to increasing rear sway bar diameter. I'd try that first. As a general rule, we try to stay within about 1/4" of OEM rake on any platforms we work with. Roll center axis is baked into the chassis and it's usually not a good idea to fight it. If raising rear ride height doesn't bring the balance back, then try a 16mm rear bar.

Sorry, missed it!

Agreed, some rake can help. I would consider adding stiffer springs all around...7 kg/mm works well on SS1 and is only a small change in ride. Could also try a 20/16 swaybar kit.

- Andrew

What is OEM rake for these cars? It's been a while since my car was stock. I may have that measurement, but I'll have to check my notes.

I thought so too but it worked. I talked to a couple others before trying it.

My setup:
-1.25 ride height from stock
RCE T2 7kg springs
Verus camber plates, stock rear mounts
-3.6 front camber, -2.4 rear, zero toe.
White line anti dive kit adds .5 degree of caster.
White line roll center correction.
22mm front sway full stiff, 16mm rear sway on soft.

@Racecomp Engineering, @949 Racing - any comments on NoHaveMSG's experience going stiff/soft on a 22mm/16mm bar set to cure understeer? Apparently he's not the only one.

As to the recommendation of a 20/16mm bar combo (or the more common 22/16) - can anyone explain why this wouldn't jut add more understeer? It apparently works, but that's a +2mm on the front (or +4mm) on soft but only +1mm for the rear. Seems like that would just stiffen the front and add understeer. Soft/hard would allow +2mm/+2mm combo I guess.

Thanks for the info!

My understanding is it has to do with the poor camber curve of the Macstruts.

Mac struts don't have a camber curve. It's so small that you can consider it zero.