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?

 

[gannet]

Maybe because the Z4 was designed as a roadster first so the extra weight on the coupe is the Tin roof? Whereas on other conversions they are designed as coupes and the extra weight in the roadster is down to the extra chassis stiffening they have to do to compensate the lack of a roof?

Just guessing mind you :?

 

[Wondermike]

Sounds pretty much spot-on to me

 

[GP20]

The magazine you read is wrong. The Z4M coupe is merely 33lbs (15KG) heavier than the Z4M roadster, not 220lbs or 100kg.

I could be mistaken, but I've read the extra weight is due to reinforced drive-train and rear axle. I'm not sure the tin roof itself adds any more weight than the soft-top (canvas and metal frame) and soft-top motor combined.

 

[enfield]

The Coupe is way more rigid and if memory serves me correctly - the suspension is different to make the most of the rigidity and that is where the weight comes from....

or GP20 could be right. I agree that the weight difference is around 15kg.

 

[MonsterZ4]

The cars you have mentioned all have fabric soft tops. I doubt a 206cc, Focus with tin-top, Merc 500sl or any car with metal folding roof fair as well in the weight stakes?

I like a bit of meat on the bone anyway! My VX220 weighed the same as a bag of Crisps (without me added), and used to get blown all over the road!

 

[laser4596]

Most cars are designed as a coupe/hatchback first and then later in product cycle a convertible is bought out. When this happens the car has to be strengthed along the sills and across the car due to the removal of the roof, usual with steel. This adds weight.

The z4 on the other hand was designed as a convertible from the offset. When the coupe was introduced they wont of removed any of this strengthening but they have put a metal roof on the thing. Metal weighs more than fabric so......

 

[GP20]

Metal weighs more than fabric so......

Metal weighs more than fabric. But does metal weigh more than fabric plus another big block of metal that gets the top moving? I think some people who have done the CF roof conversion have mentioned that the stock roofs (aluminum I assume?) are actually not that heavy.

 

[takedown8]

Ahh! GP20 is right again!

I got the number wrong. The roadster is 15kg lighter than the coupe, but I guess I was still wondering why it's actually lighter when all of those other cars I mentioned are significantly heavier (usually at least 100kg) in the ragtop form.

The response that the Z4 was built as a roadster first actually makes a lot of sense to me. I just looked up the Porsche Boxster and the Cayman since the Cayman was built on the Boxster platform, and the weights are almost identical for the two cars!

BTW,I edited the original post to make it 15kg lighter instead of 100.

 

[GP20]

Yeah either 15kg or 100kg, your question is 100% legit as it still holds true that most convertibles still weigh 100~200lbs more than their coupe counterparts. Prime examples being the 350Z, or the E46 M3. Good discussion subject nontheless.

 

[Curtis]

The coupe has two extra side windows and a large rear window which would account for some of the extra weight plus that extra bit of bodywork extending back from the roofline.

 

[takedown8]

It's too bad, because I think that from now on I will always want a convertible which, for most models, is gonna mean a heavier vehicle. Strange how I always thought ragtops were lighter.

 

[Rick Hunter]

It's too bad, because I think that from now on I will always want a convertible which, for most models, is gonna mean a heavier vehicle. Strange how I always thought ragtops were lighter.

It's because of all that wind blowing through your hair while driving.

 

[RustyZ4]

It's too bad, because I think that from now on I will always want a convertible which, for most models, is gonna mean a heavier vehicle. Strange how I always thought ragtops were lighter.

It's because of all that wind blowing through your hair while driving.

Not in my case its not