I can't comment on how strong BMW structures are, but I am 100% positive there is quite a bit of flex in the 2015 GS structure. The body flex surprised me as I pull into an angled driveway or gas station ramp. Also, I can definitely feel the floorpan flex and shudder over bumps. The LS460 was a little better in those regards.

 

The only way to really measure the stiffness of a car is to put the car on a test jig and twist it, but also destroying it in the same process.

A second example of the same car is required to measure the bending stiffness on a jig, but this second example will be destroyed in the process too, just like collision testing.

Seat of the pants feel?
We own 2IS, 3.5IS, 3.5GS & 4GS.
I reckon 3.5IS feels stiffest.
Then 4GS.
Then 2IS.
With 3.5GS the weakest.

Why?
I'm not God, but maybe the newer models are heavier with thicker gauge metal and/or taller box sections, hence stiffer?
More importantly, maybe the compact IS's feel stiffer than the midsize GS's because the shorter wheelbases & narrower tracks have less flexing?

Normally, the longer the wheelbases and the wider the tracks, the greater the flexing.

I would need to have a G20 long term to really give a seat of the pants opinion.
The factual way to do it, would be to put half a dozen (6) G20's on a test jig, and bend/twist the life out of them...
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Per Car and Driver

2017 LEXUS IS 350 F Sport RWD Roadholding, 300-ft-dia skidpad: 0.84 g
2016 BMW 340I RWD M Sport Roadholding, 300-ft-dia skidpad: 0.91 g
2020 BMW M340I RWD Roadholding, 300-ft-dia skidpad: 0.96 g

 

These tests are not accurate unless done back to back on the same day.

For example, Car and Driver have the bigger & heavier 2013 GS350 F Sport pulling 0.89g on the skidpan.
Normally, the heavier the vehicle, the less the centripetal speed.

PS
Another example.
Car and Driver magazine stats.
2017 Honda Civic Si valued at $24k is 0.88g.
2021 Honda Civic LX base valued at $22k is 0.93g!
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Not to mention the tire difference. Those 2020 BMWs come with Michelin PS4S (I think the 2016's came with Birdgestone RFTs? But still also summer performance tires), whereas the IS comes with those questionable Bridgestone Turanza tires.

 

Your mileage will vary with that assumption. Tire specs have a lot to do with the skidpad along with drivetrain layout and center of gravity. Fact is, the IS/GS/RC platforms have great handling feel but doesn't mean they are the best handlers at the limit. its evident comparing track times of the cars and following the reviews of the cars.

 

Yes, it is difficult to measure, and one may be quicker through the slalom [speed of change of direction], but the other may pull more g's on the skidpan [grip].

For example, generally go karts are quicker through the slalom, but the big heavy Formula 1 cars pull more g's.
Likewise, an MX-5 may be quicker through the slalom, but a big heavy Aventador will pull more g's.
Because both the Aventador and the F1 cars are both long and wide, their Polar Moment of Inertia is higher, making them less agile.
However, their low profile C of G to width ratio allows them to resist roll, to resist weight transfer to the outside wheels, to grip on all four wheels, to pull more g's.

They'd have to all be tested "back to back" on the same day, at the same location, with the help of Correvit testing equipment.

True, for a combination measurement of both performance and handling, the track times are great.
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Wrong! 2020 M340i comes with 4 different tire options unless you do custom order and choose non run flats summer performance tires. Dealers rarely order that configuration.

1. 18 inch run flats all season standard
2. 19 inch all season run flats optional which is what I had and has now swapped out to Michelin PS4S
3. 19 inch summer fun flats optional
4. 19 inch Summer non run flats Michelin PS4S optional.

Run flats are terrible, either way, G20 is superior to the Lexus in driving dynamics. That was easily apparent last year May when I still had the IS200T and drove the G20 330I M Sport. I was ready to dump my 3IS on the first test drive. 3IS 200T has a lousy motor, the worst of the competition and to add insult to injury there's no decent tunes. The turbo lag in my IS200T F sport was so bad it was a road hazard especially when a quick maneuver or merging was required. I hated that car with a passion and because of that engine swore to never settle for a 4 cylinder again.

Consequently, although the B48 was superior I decided to get the M340I. I put 44k miles on the 3IS so I know how it drives, it was a terrible car unless you want to drive it like a regular appliance which is antithesis for a compact sports sedan. I don't miss that Lexus, even my spouse's RX350 is superior in its power delivery.

 

True, both the old 2012-19 F30 328i and the IS200t have more lag.
The 2019-26 G20 330i is the latest in turbo technology with less lag, but some lag still remains, and overall, like any turbo motor, the G20 330i feels artificial in a turbo way.

I haven't driven BMW's 3.0 in-line six twin turbo for 7 years, but last time, heaps of lag and a flat spot down low.
I can understand why certain people like Frog98 love their GS F & LC500's and atmospheric engines.

Only the electric turbo can spool to 70,000 rpm in just 200 milliseconds can totally eliminate turbo lag.
However, BMW probably discontinued their electric turbo development to focus on electrification.

One of the reasons why Toyota Motor Corp has equity of $200B versus BMW AG's equity of only $69B, is because TMC can't be bothered developing ICE's much further.
The ICE is nearing its end, and everyone is focused on electrification, hence Lexus have no further major ICE developments unfortunately.
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NA is always best but the B58 has almost virtually zero lag, I encounter lag in less than five times and that was in comfort mode. You need to drive that car, it begs you to drive like a hooligan, rev happy, pins you in the seat, triple digits comes on in a blink. It's bonkers, I am happy I chose the B58, the hype is real and the transmission along the with torque vectoring LSD from the M2 makes it really shine.

 

I think we are discussing several things at cross purposes here.

1) Skidpad is mainly a function of traction which is mainly due to the type of tires used. Not a very good measure of chassis.
2) Slalom imo is a good summation of the overall chassis/tire/suspension/powertrain combination of a car
3) Track times can be deceiving. A track with mainly fast straights will favor a straight line rocket but with a poor chassis/suspension but a technical track with many curves will favor the car with the better chassis/handling but relatively poor powertrain.
4) EXE46 is well qualified to comment on his 3IS handling vs the G20 since he owns both cars but no one can really credibly comment on the handling/chassis of the 3.5IS vs the G20 3 series since none of us has test driven or measured the handling characteristics of the 3.5IS. And it bears noting that the 3.5IS chassis is heavily revised from the 3IS after Lexus did extensive tuning of the IS on the Shimoyama test track.

 

Some excellent points.

 

Base IS300 AWD but I like the new blue exterior and caramel interior combination :

 

God, that fake airvent on the bumpers is awful. They didn't even try to make it look real, like with some squiggly lines. just a piece of black smooth plastic

 

I test drove another friend's G20, but non-M Sport.

Low front seats, but tremendous front shoulder room.
Low rear seats, but very narrow in the rear, though leg room and foot room average.

Incidentally, sat in 5 Series.
Bit bigger than 3 Series alround.
Higher seats, but much wider than 3 Series too.
5 Series seats higher in the back than 3 Series, but still bit low compared to 4GS.
5 Series much wider than 3 in the back.
5 has more rear foot and legroom, however I would have expected 5 to have much more rear legroom than 3.
That's why 3 has 94 cu ft EPA, while 5 only has 99 cu ft EPA; an improvement of only 5 cu ft.

C Class is alround soft.
G20 is a little firmer, but superb springing and damping.
G20 slightly firmer than C Class, but G20 superbly sprung and damped for superb ride on 225/45R18 Hancook Korean non-asymmetric luxury tires.
Our IS200t is too firmly sprung, and even more firmly damped, and never as quiet cruising as the G20 either.
Lexus 2.0T has the most unpleasant and artificial torque curve amongst these three turbos, and IS200t has worst NVH of the three motors.
G20 much better suspension than old F30; old F30 was firmly sprung but softly damped, such that it would bob up and down on bumpy corners.

On the question of handling, G20 non-M Sport not as sharp handling as ageing IS200t non- F Sport chassis at all, but the IS200t is too firm and slightly loud cruising, while G20 has a superb ride and NVH.
In a previous post, it was unfair for me to compare my optometrist friend's G20 M Sport to my wife's IS200t non-F Sport.

In my experience, the bigger the turbo, the more the BHP, but the greater the lag & flat spot, as it takes the larger heavier turbine more time to "spool" up.
Old F30 had a lot of lag, and a big flat spot down low before the turbo would spool up.

Lexus needs to make major reworked 3.5IS softer & more refined just like the rest of their lineup lately, and Lexus 2.0T needs to be updated and tweaked too.
Lexus should only use firm suspension on their F Sport trim.

3 Series packed with little minor electronic gadgets.
3 not intuitive like iPhone iPad, nor Lexus; for the very first time in a new 3 Series, I did not know how to start the engine - then I could NOT figure out how to put the tranny into Drive; I knew how to release the Parking Brake though.
G20 needs an experienced person to teach us what to press and how to operate.
So many gadgets may break down over time.

If I had to choose between friend's 2020 G20 non-M Sport and wife's 2017 IS200t non-F Sport, I would take the G20 any day.
Overall, G20 a great car to be leased.
Else just buy it, and pay for a few odd electrical repairs here and there...
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