2 doors vs 4 doors

Does anybody know if there has ever been testing to see which configuration is faster on a track? Coupes usually offering slightly less weight while 4 doors offer a slightly stiffer chassis.

I am obviously only talking about cars that are offered in both configurations such as the 3 series.

I always thought the 2 door was the stronger/stiffer chassis.

Maybe you're right, I've heard that the e36 M3 4 door is stiffer than the two door. It seems like rally cars (which need to be really stiff) favor 4 doors as well, or they do in recent history at least.

4 doors have a pillar between the doors, 2 doors have these huge honkin' doors and then the holes they fill cut into the side of the car.

4 doors tend to be stiffer. M00t point once you put a cage in and seam weld everything though...

In reply to HiTempguy:

Yeah I figured that, I'm more talking about cars that were straight off the showroom floor. Like an e90 M3 vs. an e92 M3.

HiTempguy wrote: 4 doors have a pillar between the doors, 2 doors have these huge honkin' doors and then the holes they fill cut into the side of the car. 4 doors tend to be stiffer. M00t point once you put a cage in and seam weld everything though...

It depends if the 4-door is the same length & wheelbase as the 2-door. Quite a few of the 4-doors are longer so this may negate the 4-door being stiffer. In the older American cars, typically the 2-door was the more rigid chassis for the most part.

It seems to depend on exactly what model - sometimes the 4 door is lighter and sometimes the 2 door is. An older American car example: With Dodge Darts, the 2 door post model is the lightest and probably stiffest, followed by the 4 door - and then the 2 door hardtop is the heaviest and probably least stiff of all the fixed roof notchback designs.

My original Neon ministock car was a 2 door, and when it got wrecked, I replaced it with a 4 door. The 4 door felt stiffer, and would lift the rear wheel higher with the same suspension setup.

The old car had a lot of laps, and the firewall was starting to crack, so it might have just been age, too.

I think in the case of the E36 that the 2-door was stiffer longitudinally and the sedan was stiffer in torsion. Not that it was real significant.

What WAS significant was the extra inch or so of headroom for a helmet in the sedan, and possibly some aerodynamic advantage to the slightly lower roofline.

A lot of the stiffness would depend on the 2 door design. If it is a 2 door sedan with a B-pillar it should be similar to a 4 door. If it is a 2 door hardtop with no B-pillar it will not be as stiff.

Another thing to consider depending on the intended use is the Coefficient of Drag. For instance the early Focus sedan (4 door) has a better Cd than the hatchback (2 door).

HiTempguy wrote: 4 doors have a pillar between the doors, 2 doors have these huge honkin' doors and then the holes they fill cut into the side of the car.

4 doors have 2 big holes on each side, with a skinny pillar in the middle. I'd rather have the 2 door for stiffness, but I'm not an engineer.

ppddppdd wrote: I think in the case of the E36 that the 2-door was stiffer longitudinally and the sedan was stiffer in torsion. Not that it was real significant.

My 325i seems to be pretty flimsy. The sedans came standard with fixed rear seat backs with a metal support behind. Mine has the optional split folding seatbacks which the coupes all had. The standard sunroof probably doesn't help either. I am sure a 328i without the sunroof or the folding rear seats would be the stiffest.

Joshua wrote: Maybe you're right, I've heard that the e36 M3 4 door is stiffer than the two door. It seems like rally cars (which need to be really stiff) favor 4 doors as well, or they do in recent history at least.

I thought rally cars were most often 4 doors because the popular awd cars were 4 doors.

2 door hatchback vs 2 door with trunk will change things too. Hatchbacks have a huge opening in the middle of teh body while the rear shelf in cars with trunks tend to add some stiffness.

I'd also guess that a lot would depend on whether both body styles were intended from the initial design. If the car was originally intended to be just a 2 door, but became really popular and someone decided to add a 4 door version, I'd bet that the 2 door is way stiffer. But if the car was originally a 4 door and then they converted the 4 door platform to a 2 door, the compromises in the 2 door design may limit it's stiffness.

Or I could be grasping at straws. It wouldn't be the first time.

Bob

monark192 wrote:

Joshua wrote: Maybe you're right, I've heard that the e36 M3 4 door is stiffer than the two door. It seems like rally cars (which need to be really stiff) favor 4 doors as well, or they do in recent history at least.

I thought rally cars were most often 4 doors because the popular awd cars were 4 doors.

I also thought this was a homologation thing, not a "which is actually better from an engineering standpoint", and for some reason I can't fathom, new car buyers buy cars with more doors.