Mind Exercise: Fastback Miata or S2000 (theoretical) why's and why nots

A friend and i were just having a conversation about various convertibles vs. fastbacks (and cars that were one or the other, or both) and which were/are better for track, daily-driving, or whatever. So with the Miata being the best affordable car in the 2-seat RWD category, it's the obvious one to make a comparo of. Or coudl just do it with a theoretical S2000 as another car that was vert-only.

Put aside effects on sales or profitability for Mazda. Let's assume this in a financial vacuum.

Put aside "but the Miata has always been a roadster, so it has to stay that way" and other purist arguments. Let's assume that the Mi

Put aside what would physically look pretty to my/your eyes.

What would be the physical/practical/performance pros and cons of a fastback Miata (think the shape of a 944 or 2nd gen RX, let's say)? I'll start with a few offhand:

Pros of fastback:

- Interior cargo space for gear/track tires?

- Likely reduced drag coefficient vs. RF/vert/hardtop - just a guess.

- More rear weight bias?

- Less chassis bracing required?

Cons of fastback:

- Additional total weight?

- More rear weight bias?

- Hotter inside due to larger greenhouse?

- More compact rear suspension design (i.e. strut?) to accommodate more cargo space?

I can't really think of very many "practical" differences other than those. What else?

The arguments could be made for, say a vert BRZ/86, I guess. The real-world comparison here is the older 944/968 and RX-7 standard vs. vert offhand. 

I think you covered it. More security?  Less rain getting in?  Better safety potentially?  It may not actually be heavier.  Less bracing required I suppose?

I'm guessing the convertible version won't be any lighter, and might be heavier due to extra chassis stiffeners needed with the lack of a roof.

Another nod in favor of the fastback/coupe is a strong potential for less interior noise at speed.

The convertible may also be excluded from some tracks/clubs for motorsports events due to safety concerns.

I knew I bought that FR-S for a reason...

In theory and with all else being equal, it should be lighter as a coupe. But if a car is intended as a convertible from the outset, that can be minimized - the F-Type vert is only 45 lbs heavier (1.3%) than the coupe. The old RX7 is an example of what happens when you have to convert a coupe into a ragtop later.

Aerodynamically better, definitely. Miatas with a hardtop are better than soft tops with the roof up. It may be quieter on the highway, but coupes can boom if you’re not careful. Still, it should be improved. 

In a car the size of a Miata, you may have trouble packaging a set of track tires under the hatch without veering into clownshoe proportions. Small cars don’t get massively bigger just because they grow a hatch.  You’d also have a significant packaging problem with the fuel tank - I think a move to rear struts would almost be required if you want any cargo space, as that would let you drop the tank down where that bulky multilink rear lives. 

The top would have to be tall enough to allow drivers to wear a helmet. That means a higher roofline than the current RF in my case, although I can wear one with a slightly modified seat in a hardtopped NA.  

RF; keep the top up

Miata looks so good as a fastback.

In reply to buzzboy :

Needs a wing on the rear to balance things out.

Keith Tanner said:

  You’d also have a significant packaging problem with the fuel tank - I think a move to rear struts would almost be required if you want any cargo space, as that would let you drop the tank down where that bulky multilink rear lives. 

 Irish might remember the gas tank in his GT6 and how drastically different that placement is to the Spitfire the car is based on. 

I'm still waiting for the RF to grow on me.  Hasn't happened yet.

In reply to buzzboy :

S2K fastback isn't too shabby either

I don't understand the love for fastbacks. I think it looks goofy on both cars. Not to mention they both look like they increase the interior volume by about the size of a can of tennis balls.

Plus - you cannot beat the interior volume of a convertible...

I'd be driving a fastback ND right now if they made one. I didn't fit in the RF and I can't run the soft top in rallycross.  I've almost-bought two different S2000's as well - I'd have snagged one 8 years ago if there was a full tin top version.

So I'm in a BRZ even though I think the ND is a better car in almost all the ways. 

The RF with our roll bar actually has increased storage capacity - you have access to the roof storage space. It's not room for race rubber but you can put long things in there. We can easily modify the stock car so you can do the same without a bar. Someday I'm going to start pulling roof panels to see if I can increase headroom in there. It's not lighter than the convertible, it's 100 lbs that's mostly on the rear wheels.

If you don't think the RF looks good, you are dead inside.

buzzboy said:

Miata looks so good as a fastback.

Is that the Autokonexion fastback? Too bad that it was apparently quite poorly made, with people spending as much on getting it to fit as they did for the fastback itself. Autokonexion had less-than-stellar build quality, which largely contributed to them no longer existing.

In reply to Keith Tanner :

Sorry Keith, I must be dead inside.   I find the drop-top ND much more attractive than the RF.    Stock, it looks like it's on stilts, and even lowered a bit,  the car looks very awkward to me from some angles.  I thought it looked killer in pictures, but much less awesome in person.     Besides, I'd rather have a convertible than a targa.   

That was a very brave admission, Joe. You're in a safe space here.

I think the RF looks good, but for me, it doesn't make much sense. 

Although I'm sure it would be a better DD with extra noise insulation and such.

We played a little with the Mumpkin’s fastback. 

We consistently found 4 mph faster top speed in the quarter mile with the top in place compared to open top. 

There was a LOT of interior space. Much more than I thought.  A well designed version could easily haul race rubber. 

The headroom was limited with a helmet, but that was not because of the height. It was because of the width.  The Mumpkin’s fastback top tapered from the beltline to the roof line, and made helmet space tight on the left side. 

The car looked good. Sorry Joe!! 

You can already carry a full set of race rubber in an NA*

* as long as they're 10" wheels for a classic Mini

z31maniac said:

I think the RF looks good, but for me, it doesn't make much sense.

The Manic Miata is a '96 with high-back Corbeau semi-racing buckets and a big fat dual diagonal rollbar sporting covered padding.  I never drive it with the top up.

I sat in an RF with the targa and window retracted, and it felt almost identical - just more comfortable.

Duke said:

z31maniac said:

I think the RF looks good, but for me, it doesn't make much sense.

The Manic Miata is a '96 with high-back Corbeau semi-racing buckets and a big fat dual diagonal rollbar sporting covered padding.  I never drive it with the top up.

I sat in an RF with the targa and window retracted, and it felt almost identical - just more comfortable.

Yeah, but I'm not a fan of the weight penalty or dramatically increased complexity. 

Once they get to the dealers I'm going to go check out both. 

The RF mechanism is basically the same one as the PRHT that's been around for a decade, so we have a pretty good idea of long-term reliability at this point.

Keith Tanner said:

You’d also have a significant packaging problem with the fuel tank - I think a move to rear struts would almost be required if you want any cargo space, as that would let you drop the tank down where that bulky multilink rear lives. 

 

This is what I was thinking with the possible downside reference of having to package the rear suspension into something more compact. But didn't think abou the gas tank. I guess the solution on a Miata would be to make it lower but longer, into the area where the bottom of the current trunk is? I'm thinking of the interior arrangement (and fuel tank) of a 944, which is probably a foot longer than a miata.

Robbie said:

I don't understand the love for fastbacks. I think it looks goofy on both cars. Not to mention they both look like they increase the interior volume by about the size of a can of tennis balls.

Plus - you cannot beat the interior volume of a convertible...

I'll agree I don't like either of those two grafted-on fastbacks, but that's mostly because they're limited by not actually being built with the car, so have structural/support requirements for support, whereas a factory-built Miata or S2K variant would be able to, for instance, have continuous c-pillars from the fenders, probably resulting in a wider and more accessible hatchback area (again, the 944 or RX7 are good examples of this). I'm not a fan of the "skinny hatch" in most "custom" vert-to-hatchback conversions, personally.

drsmooth said:

In reply to buzzboy :

Needs a wing on the rear to balance things out.

“If ya like it then you shoulda put a wing on it...”

I generally prefer coupes.

They can be stiffer, torsionally, if the designers take advantage of the roof - my Z4M coupe has twice the stiffness of the convertible.

Coupes usually have better aerodynamics and therefor slightly better fuel mileage.  My Solstice coupe has 5 mph on top end on an identical convertible.  Both of my coupes weight withing 20-30 lbs. of the open cars, which have to add top motors etc.

They are better for relaxed long distance touring, and if they are styled properly, they also look better.