Here is the best answer you're ever going to get.
Do both unless you're tracking your car and very specifically are tracking your car and are trying to optimize your car to the max.
From what i've read via VWRacers FB page: The car feels nicer with the front bar but they lost a tiny bit of time with it. Front oem bar paired with aftermarket rear seems to be the best balance.
This is just what i've read. Maybe they're entirely wrong. I don't know.
Me personally I do track but I don't currently do anything competitive. Just track nights.
Id say do both because the front bar makes the car feel like a go-kart.
Personally I love the way the car drives with it vs before. More fun.
Be careful of who you take advice from. Most are slow or sponsored or both.
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I've done actual testing to the best of my abilities - it's unreasonable to swap sway bars on the day of an event, so I did the next best thing: I tried my 26mm FSB or stiff vs soft on same day, and was very surprised to find the stiffer setting being notably faster (1+ sec). It showed higher lateral Gs in every corner at VIR, and higher min corner speeds as well.
As others have alluded to it likely depends on overall setup - you can add roll rate with springs or sway bars or combo of both. I'm not saying a FSB will for sure make you faster, but it did for me and I have data to show why I'm confident that is the case.
Me and
@Mini7 go back and forth with our data. I'm not sure what exactly he was running at the time (tires, spring rates, etc), but comparing his min corner speeds vs my min corner speeds at VIR seems to be a pretty strong argument to try a front bar. Or stay on stock springs.
Jose is by no means slow - and I'd argue is a much better+more consistent driver than I am overall.
In a nut shell most of my corner speeds are faster, primarily in all the lower speed corners. He's got a couple mph on me in South Bend and Hog Pen.
Overall times cannot be directly compared because IS20 for me and IS38 for him.
We can compare within the Catalyst app, but essentially he's only got 0.18 sec on me all the way from the start line to Oak Tree (despite starting the lap out at +15mph over me at 136.9mph vs 121.5mph). He does carry 2mph more through South Bend at 91.4 vs 89.7mph though.
He puts a ton of time on me down the back straight (approx 2.8 sec due to power alone - 147.6mph vs 131.1mph), I gain back 0.6 sec through Rollercoaster and entering Hog Pen, where he does carry more speed (71.4 vs 68.3 for me) and then rockets down the front straight for an overall 3.17 sec lap time difference overall.
My car has it's biggest advantage through the T4/5 complex - min 49.3 vs 41.2mph. I peeked at his "optimal" lap data, and it looks like his best was 45mph through there if cherry picking from other laps for benefit of the doubt.
Videos of the laps in question:
Me, IS20 GTI with the Koni shocks and stock springs + F/R sway bars:
Jose, IS38 GTI with 529 Coilovers and a slew of other things, but with stock FSB:
Hopefully he will be able to chime in as I know he's got a front sway bar going in. I'll be really interested to see his results.
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I will add that the front bar WILL push a bit when the tires are cold. Once up to temp though it helped me a lot. Driver ability also will play a role. You have to be able to adapt to the setup and potentially change some inputs to get the most out of any setup because you're changing the shape of the friction circle. With the front sway bar set stiffer, you can "lean" on the outside tire a bit more because it's not losing [as much] camber in turns. If you're still doing ALL of your braking in a straight line, then yeah you won't be getting all the benefits of the added roll stiffness.
26mm H&R front bar on stiff:
26mm H&R front bar on soft:
Braking is down(negative), accel is up(positive). Left turns are left(negative), right turns are right(positive). I tried to circle around all the data "hits" possible for a rough idea of the max friction circle. The top one was comprised of a shorter session so the # of hits scale is different (we're only interested in where hits can be generated at all for this), but you can clearly see where with the bar setting being the only thing changed - it allows you to more effectively multi-task on corner entry.
So yeah. If you can't change your driving to take advantage of a change, then yes you'll be slower. This goes for anything that changes the friction circle. Some tires multi-task better than others (especially Hoosiers). The above was all on Kumho V730s for reference.