1800 LBS? Hmmm... Walk me through this

I know it seems that I start a lot of my threads with math problems, but this one is baffling me too.

The FFR roadster has a wheelbase of 90 inches. The frame is steel wrapped in aluminum and the body is fiberglass. Engine is a heavy V8 in most cases. No roof, no side or rear windows, basic carpeting, and minimal frills inside. Total weight around 2200LBS depending on the setup.

The FFR WRX has a wheelbase of 95 inches. The frame will be steel and probably have the same type aluminum panels. Body is fiberglass. The engine is a far lighter H4. Targa roof, side windows and rear window, probably basic carpeting inside with minimal frills. Total weight 1800 LBS.

So we have a bigger car with a roof and more glass that is supposed to weigh 400 LBS less than the roadster? I know the engine is lighter but where else can you “add lightness” that won’t include composites that will make the price skyrocket? :(

I’m not saying that it can’t be done, and I trust Dave not to make an estimate with doing some calculations beforehand, but how is this going to work? :confused:

The transmission is also the rear end

I was actually going at it from the other direction:

Midlana, a steel space-frame mid-engine project with a turbo Honda power plant just weighed in at a little less than 1500lbs. It's not complete, but all the big heavy stuff is on, I think.

An Elise is 1900-2000lbs. I'm torn there because it's got an aluminum structure, but it does have a lot of other stuff too.

A Miata-based Westfield Seven-clone is 1300lbs wet, give or take. Not much body to speak of, but not 500lbs less.

So, naively, I'm hoping it may be possible to sneak this thing in under 1800lbs. I think the design criteria will show how this can all work out.

My suspicion (and just hypothesizing here), is that the structure of the Roadster is heavier than the new design because it has to deal with not only the weight of that massive engine, but the torque from it, and more importantly, the length of the chassis between the engine and the diff. Since engine torque is trying to twist the chassis along that length, the chassis must be sufficiently strong between those points, which means weight. Since the engine and diff are combined in the new design, the twist-length (my word) is negligible.

This falls into the category of - make one part of the chassis lighter or less powerful, and another part can be lighter too.

One of the design goals of the original FFR Roadster was to maintain a fairly original frame. That think is overbuilt and barely notices a 460 CI All iron big block. Since the target worst case powerplant (the haeviest part of either car) is 75 percent lighter, every structural component can be reduced dramatically with no loss of strength. Next, the ****** Cobra frame off which the FFR roadster frame is heavily based was designed with a calculator and eyeballing in Britian in the 60s. The FFRWRX will be designed by soaftware that can determine stress and loading at the molecular level in 3D.. 1800 pounds here we come. For reference, what was the original weight target for the FFR roadster?

The live axle rear end with links/panhard bar compares to the axles of the WRX with the transaxle as one unit comparred ot the trans and rear end and driveshaft of the Cobra. In addition to what was mentioned above. Aluminum flat4 compares to iron V8. Sure, theres an intercooler and turbo, but they're not much. Even 2000lbs would be great in this little rocket. But, Dave and crew must have an idea of weight if they've already got the chassis designed.

...and since we haven't gotten specific enough, the fully assembled h4 engine rings in at around 220lbs. I have a bare longblock in my garage and I can move it around on my own (no headers/exhaust intake etc. About 180lbs bare). The 4.6L V8 weighs over 400lbs not including transmission. The subie 5mt also weighs close to 100 lbs less than a T45 I believe.

The reduced engine bay and transmission mounting framework needed for the lighter-weight drivetrain alone makes up a significant portion of the weight difference between the two models.

Maybe instead of all the aluminum panels they'll use a fibreglass tub instead?

Though would that actually save weight?

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Originally Posted by subyrod

But, Dave and crew must have an idea of weight if they've already got the chassis designed.

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I'm wondering if they haven't already built one less the bodywork to be honest. If they have the chassis designed, why not?

I think it is certainly doable, as noted the frame weight should be less as its not being designed for big blocks and the like, but also the CAD design and space frame technology should allow for more triangulation of lighter weight trusses as opposed to relying on two massive 4 inch tubes.

Also, I don't know that I would expect a top, rear and side windows to be included in the 1800lb goal. Factory Five has said repeatedly roadster, I am sure there will be top options available for it, but the base weight goal I expect to be on the roadster.

-Scott

Let us not forget that weight is always a very dynamic metric. I've seen 3,000 pound 240Z cars, and I've also seen sub 1700 pound 240Z cars....

The devil's in the details.

I am surprised at the rather high weight also. For reference a V8 Brunton Stalker only weighs 1460 lbs wet, and thats a full size american V8!

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Originally Posted by t3steve

I am surprised at the rather high weight also. For reference a V8 Brunton Stalker only weighs 1460 lbs wet, and thats a full size american V8!

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Keep in mind that just because it says so on the website, does not automatically make it true. Also keep in mind that it's essentially a 4-wheeled V8 motorcycle. No doors, no windshield, no windows, most likely no heat or AC installed, no roof, no carpet, no stereo........

1800 lbs is a very realistic goal. Whether or not the project come in at that number depends on lots of factors. How much of the donor car is used, A/C, power windows, removable top, interior features. Hell, even wheel choices can add 40 + pounds. To build a lightweight car you have to be diligent in every component choice, or it can get heave in a hurry. The basic layout & drive train lends itself to a lightweight design. We really don't have enough details yet, but i agree this is a very important parameter in the design.