Why is the roadster lighter than the coupe?

[takedown8]

Just looking at my 2008 Car Buyer's guide and noticed that the Z4 roadster and M roadster are around 15kg lighter than their coupe counterparts, but the opposite is true for most other soft top convertibles when compared to their coupe counterparts. Just some examples:

- 650i soft top convertible is around 200kg heavier than 650i coupe
- Aston Martin Vantage convertible is heavier than Vantage coupe
- Audi A4 convertible is heavier than A4 sedan

There are many other examples where soft top convertibles are heavier than the coupes.

Can anyone explain why the Z4's are the exception?

[Starscream]

because it doesn't have a roof!

Duh!





























Seriously though IDK,

[takedown8]

Originally Posted by Starscream

because it doesn't have a roof!

Duh!

Seriously though IDK,

[swedish_z4]

I thought the weight was equel between the coupe and roadster..

[takedown8]

Originally Posted by swedish_z4

I thought the weight was equel between the coupe and roadster..

Nope. Coupe weighs more.

[mazzawi]

maybe they change something in the other car bodies to compensate for the loss of stiffness because there is no roof ? just a guess

[vicj]

It's because the car was originally designed as a roadster. Most Vert's are derived FROM a coupe, so when the roof is removed, additional support brackets and cross members have to be installed to maintain rigidity. In the Case of the Z, all the cross-bracing and reinforcement is still in the coupe, with the addition of the steel top. Thus, more weight, not less.

[takedown8]

Originally Posted by vicj

It's because the car was originally designed as a roadster. Most Vert's are derived FROM a coupe, so when the roof is removed, additional support brackets and cross members have to be installed to maintain rigidity. In the Case of the Z, all the cross-bracing and reinforcement is still in the coupe, with the addition of the steel top. Thus, more weight, not less.

Nice answer! I got a similar answer in the other forum. BTW, it's 15kg lighter, not 100kg. My mistake! Still lighter though.

[Dammmittt]

Originally Posted by vicj

It's because the car was originally designed as a roadster. Most Vert's are derived FROM a coupe, so when the roof is removed, additional support brackets and cross members have to be installed to maintain rigidity. In the Case of the Z, all the cross-bracing and reinforcement is still in the coupe, with the addition of the steel top. Thus, more weight, not less.

That's not entirely true. The Coupe version has a lot of different suspension and body parts that add stiffness that isn't there in the roady. Plus the addition of the roof supports and steel roof, makes the coupe heavier than the roady. The Z4 is also small compared to most cars, so that the retractable roof isn't as big and the machinery needed to retract and erect the roof isn't as large.

[takedown8]

Originally Posted by Dammmittt

That's not entirely true. The Coupe version has a lot of different suspension and body parts that add stiffness that isn't there in the roady. Plus the addition of the roof supports and steel roof, makes the coupe heavier than the roady. The Z4 is also small compared to most cars, so that the retractable roof isn't as big and the machinery needed to retract and erect the roof isn't as large.

But Dammmittt, if what you're saying is true, then shouldn't the coupes be heavier in general? I was just bringing up the fact that most of the ragtops that I've seen are significantly heavier than their coupe counterparts.

[Hawkeye]

It could just be because guys weigh more than girls. The cars probably weigh the same, the driver is the difference

edit: This generalization excludes Rick Hunter's "girl"

[ovrkll]

Originally Posted by Dammmittt

That's not entirely true. The Coupe version has a lot of different suspension and body parts that add stiffness that isn't there in the roady.

As far as I know, the suspension and body parts are identical, except for the addition of the hard top and the parts to support it. I really do not see a noticeable weight difference besides the need to weld on a hardtop.


My 2 cents.......

[Dammmittt]

Part numbers are different...

[Dammmittt]

Originally Posted by takedown8

But Dammmittt, if what you're saying is true, then shouldn't the coupes be heavier in general? I was just bringing up the fact that most of the ragtops that I've seen are significantly heavier than their coupe counterparts.

Nah...the machinery and supports that go into the retractable roof make it heavier...and I'm assuming someone out there uses Coupe Aluminum for roofs...

[VoiD]

rear hatch glass must also weigh a ton.

[rc'Z]

Originally Posted by Dammmittt

Nah...the machinery and supports that go into the retractable roof make it heavier...and I'm assuming someone out there uses Coupe Aluminum for roofs...

Engineers assuming again...........

[pigbat]

Originally Posted by VoiD

rear hatch glass must also weigh a ton.

I think we have a winner

[vachss]

One of the design "features" of the Coupe that I've never liked, and that adds to its weight and betrays its Roadster origins, is the presence of the rear bulkhead all but completely blocking access between the passenger compartment and the back hatch. While I'm sure this adds to the rigidity of the frame, a coupe designed from scratch wouldn't have this annoying blockage or the added weight.

[Dammmittt]

Originally Posted by vachss

One of the design "features" of the Coupe that I've never liked, and that adds to its weight and betrays its Roadster origins, is the presence of the rear bulkhead all but completely blocking access between the passenger compartment and the back hatch. While I'm sure this adds to the rigidity of the frame, a coupe designed from scratch wouldn't have this annoying blockage or the added weight.

That is very annoying...

[Dammmittt]

Originally Posted by rc'Z

Engineers assuming again...........

Without assumptions...engineers couldn't do much...

[veryhuman]

Originally Posted by VoiD

rear hatch glass must also weigh a ton.

+1

And some from the two side glass behind the door.

[vicj]

Originally Posted by Dammmittt

That's not entirely true. The Coupe version has a lot of different suspension and body parts that add stiffness that isn't there in the roady. Plus the addition of the roof supports and steel roof, makes the coupe heavier than the roady. The Z4 is also small compared to most cars, so that the retractable roof isn't as big and the machinery needed to retract and erect the roof isn't as large.

Not true. The coupes and roadsters are IDENTICAL in suspension parts and "other" body parts, whatever you mean by that. The spring rates, sway bar thickness, etc. etc. are all the same. The weight difference is simply the weight of the roof and glass. BTW, it's a steel roof, not aluminum. Again, this is a coupe derived from a roadster platform, not the other way around, as is usual.