Do wagons have a stiffer chassis than sedans?

Do LGT Wagons and Outbacks have a more rigid body structure than their sedan counterparts?

I know on the previous generation of BMW's 3 and 5 series, the wagons were stiffer than the sedans, but don't know if that was a typical or atypical result. It seems that wagons would, since the "cage" of the upper body extends further, making it more "boxy".

I know almost all sedans are stiffer than their convertible counterparts for the same reason.

Just wanted to know if anyone had data or theories on this.

Don't have any specific data.
But the fact that the wagon has greater structural depth than the sedan should make a difference, as long as the various pillars are pretty rigidly attached. I am guessing the trunk opening in the sedan might reduce torsional stiffness, but not really too sure.
ICY/Phoenix raced an LGT wagon in Grand Am ST class a few years ago. I have always wondered why, was it stiffness, better weight distribution, aerodynamics? I am sure it was not weight, since the wagon is about 80 lb heavier (I think) than the sedan. Anyway, now they race a Spec B.

There is no metal L-R behind the back seat in the wagon. Sedans would have some sort of bracing behind the backseat and the rear deck as well acting as stiffeners.

True. The sedan has the cross bar at the base of the rear window, obviously not there on a wagon.

Volvo race the 850 wagon when they first came out in the BTCC mainly to piss off the other companies and it also had better aerodynamics. While wagons have a little more drag than a sedan, it also generated less lift. Since it doesn't want to naturally get airborn, you pretty much don't need to add a wing or spoiler, both which generates a lot of drag. The other thing was the mechanics liked it because they have more room to work on the inside even with the roll cage.

As far as torsional rigidity, it's hard to say unless you have the vehicle completely stripped down and analyze the construction.

So for a wagon, which would be the most beneficial? (not concerned about bling)

1) A cross bar that goes across the rear-seat footwell... like this:
http://www.nengun.com/do-luck/rear-cross-bar

2) A rear strut bar like this:
(Bottom Picture) http://www.laile.co.jp/products/suba.../s86103ta.html

3) or something in the undercarriage like the first picture here, or any of the other product locations?

http://www.laile.co.jp/products/suba.../s86103pb.html

Resistance to torsion and bending can't really be told without the numbers, but I will bet there is a significant bit more structural bracing (about 60 lb) in the wagon than the sedan. But what you really should worry about is if you have that beautiful massive sunroof on the wagon because that will definately reduce torsional stiffness since you're taking out all that material and relocating it to non-optimal locations.

And thats how I justify leaving my roof-rack cross braces in despite better aero if i remove them...

a rear strut bar will not give you any benefits. the rear struts are mounted on the floor hence your entire floor will already serve as a bar. by just looking at it, you would need to brace that big open space between the rear seat and cargo space...maybe going diagonal from the floor to the roof?

try a front strut bar combined with cusco under chassis braces F & R.

Originally Posted by Waxiboy

a rear strut bar will not give you any benefits. the rear struts are mounted on the floor hence your entire floor will already serve as a bar. by just looking at it, you would need to brace that big open space between the rear seat and cargo space...maybe going diagonal from the floor to the roof?

try a front strut bar combined with cusco under chassis braces F & R.

I thought the consenus was that the rear bar was probably worth it in the wagons, but the front bar wasn't, since the mounts are so close to the fire wall. On the rear, they are bridged by the floor at the bottom, but the top of the strut mount is were the flex can be reduced with a brace. Sedans have a cross memberr below the rear glass, while the wagons don't.

folks seem to think the lower braces are worth it, from what I have read. ????

Unless you are racing on the track and have really limited suspension travel, it's really not going to matter. If you are racing on the track, a full roll cage would stiffen the entire car.

Originally Posted by 05LGTLtd

On the rear, they are bridged by the floor at the bottom, but the top of the strut mount is were the flex can be reduced with a brace.

The top portion of the rear strut is already mounted on the floor, hence the entire floor is your brace.

I have the Cusco F & R lower braces and it made my wagon stiffer. When I hit dips in corners it's much more stable and it got rid of some of the squeaks. I also have KW V.2 coilovers, AVO RSB brace, Super pro poly LCA bushings, and whiteline ADJ. F&R swaybars.

Originally Posted by sebberry

I was jacking up the rear of the car from the passenger side (just below the center of the rear door) and both sides came off the ground... there was no flex in the body...

Of course that isn't a scientific test, but it shows you how rigid the wagon is...

Originally Posted by Waxiboy

The top portion of the rear strut is already mounted on the floor, hence the entire floor is your brace.

That and the fact that the way the rear suspension is designed the strut tops see ZERO lateral forces.

All the lateral forces are localized in the subframe. The suspension mounting point at the rear strut top is just to keep the car off the ground.

You could cut out the entire floor of the cargo area and localize the strut tops laterally with bailing twine and it wouldnt make a diffrence in lateral tortional rigidity. Its just the way the suspension is designed, to only place an upward force on the unibody with all lateral forces localized to the subframe.

Agree with Roby. Strut top hard points pick up vertical loads ONLY. However, such loads can still impart torsion on the overall space frame of the car, which will result in deflection, which may alter the suspension geometry somewhat.
I am not sure that simply bracing these points to each other will increase the overall stiffness that much.

Either the sedan or the wagon is a pretty complex spatial frame structure, pretty tough to say what exactly is happening, and how to make it better, without knowing a lot more. It would be interesting to see some original design data.

Anyone want to perform a 3d finite element analysis?