Wednesday, February 12, 2014

Vehicular Anorexia

I am always obsessing about weight savings. Maybe it's a side effect of owning a Lotus. Perhaps I am haunted by the ghost of Lotus founder Collin Chapman who said "Power makes you faster on the straights, lightness makes you faster everhwhere." (I wish he would haunt my driving skills a little more and my wallet a little less!). I seem to find inordinate joy any time I can save weight on my Lotus and revulsion deep in my gut any time I have to add weight.

So I'm sure you know where this post is going... :)

This winter I'm looking to shave another dozen or so lbs off of my already featherweight car bringing it close to 1850lbs with 1/3 tank of gas. That 1/3 tank probably weighs 15 lbs or so, which means even if I'm out of gas I will be 30lbs over the minimum weight for my racing class, but here's what my mania has produced to achieve the latest reduction.

The Battery

Batteries are heavy. They often contain lead and lead is heavy. Long ago I switched away from the stock battery which is a 29 lb. monster. That size is mostly needed to support the drain on the system caused by the vehicle alarm system. The little 4 cylinder 2ZZ motor in the elise definitely does not need that kind of capacity to turn over. So 3 years ago I installed a cut-off switch and a 7lb Braille battery. Great! 22lbs saved! Seems good... but not good enough! (I did mention that I might be possessed didn't I?). Enter the Antigravity Battery brand  XPS SC-1.

I've known for some time that lithium batteries are much lighter, but the products available have generally been too expensive. The SC-1 advertises as 12 oz, but my scale says just under 16 oz. If it works I'm not going to complain. The question is... will it start? Too cold to take the car out so I haven't installed it yet. But if it does the job, I get to save 85% of the weight vs the Braile B106 I'm using now, and it's cheaper too! All I need to do now is figure out how to mount it... I'll make that a separate post.

Lightness Added: 6lbs

The Wing

Ever since I built it I knew that I had added a tragic amount of weight when I added the wing. It adds and estimated 25lbs, much of it at the very top of the car, raising the center of gravity. about half that weight is in the supports and attachments to the deck lid. The wing itself weighed 13 lbs. (excluding mounting bolts) when I weighed it a few weeks ago. The construction was entirely 1/8" 1x1 Aluminum Angle, 090 plate aluminum, a 1-1/4" wood dowel for the front edge, steel angle brackets and aluminum rivets with a contact paper skin. I'm actually quite pleased with the contact paper, and the aluminum rivets are probably as light as i can get without a tig welder and a lot of practice time.

After observing the wing in action I believe the 090 aluminum end plates, the wood dowel and the aluminum angle are all over built, or just poor material choice. These are being replaced by much lighter components. The angle is being replaced by 3/64" 1x1 angle (available at Home Depot).

The front rod and end plates are being changed out for carbon fiber. Carbon fiber is not cheap, but I found a discontinued size I can use directly at a steep discount. The end plates won't be as big as I would like, but they will be a little bigger, and the cost is about 1/3 the cost of buying a larger piece in a standard size and cutting it down.

There's a bit of a problem in that I need to have good supports for this CF tube at the front and I need to butt join it to the end plate efficiently. Luckily, I have a friend who just bought himself a Maker Bot 3d printer, and he pointed me to the free OpenSCAD program which renders 3D objects defined in it's own programming language. Since I've got my own software consulting business, this programmatic style is perfect for me. Here are the supports rendered in OpenSCAD

And here are the inserts for the end of the tube (view from inside tube)

So what sort of weight savings does this buy me? Well here's how it works out (orange values are estimates):

endplates 3982402
front rod 7841551
fastners (all)4504501
center supports1301302
total (g)5,9262,995
total (lbs)13.046.59
saved (lbs)6.448250.54%

I haven't accounted for the printed parts yet, but they are very light and will probably be offset by a few of the steel angles that I was able to trim down. About 5 oz. was also removed by trimming corners and overhangs from the bracket that mounts the wing to my deck lid.
Lightness added: 6.75 lbs.

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