27 Apr 2008

This week was spent working on the Google Sketchup chassis drawing. Google Sketch is simplistic and has bugs but looks like it’ll do fine. The chassis renderings are far from done but they give some idea of the direction I’m headed.

The SCCA rule book led to the overall layout; in fact, it’s pretty much an SCCA rollcage with some wheels attached! SCCA rules state that for a car less than 1500lbs, 1.375″ x 0.080″ DOM tubing can be used. Great – if this size existed in the real world. Oh I’m sure that it does, somewhere, but I’ve been to about a dozen metal supplier sites and no one has it. Since the car’s supposed to be easy to build it means no hard-to-find tubing or impossible-to-locate businesses to bend it. There’s a fine line between engineering perfection and actually building something. (One of my mechanical engineer buddies is always recommending some ultimate material or part to me. I found out the hard way that while his ideas are great, what he recommends is typically¬†expensive, impossible to find – or both – but I digress.) Unless I can find a source of the smaller tube – and someone with the proper tubing bender – it looks like it’ll have to use 1.5″ throughout.

This brings up the question of wall thickness. In terms of stiffness, 1.5″ x 0.065 is better than 1.375″ x 0.080, and it’s lighter, but for a roll cage this may be a bad idea. Roll cage tubing may be exposed to large sideways point loads – think of a car rolling over and a tube striking a rock mid-span. Thin-wall tubing may display the “Pepsi-can syndrome”: high strength in compression, tension, and torsion, but collapsing like paper if the wall is dented even slightly.

Why all the angst about roll-cage tubing and SCCA rules? Because I don’t want anyone getting hurt due to me cutting corners to save a few pounds. If you want to go to thinner wall material, that’s your decision, but I won’t recommend it. If someone says that they don’t need a roll-cage because “that’s for racing, I’m only driving on the street”, I think it’s doubly important to have one. Meeting up with a car weighing 2-4 times as much makes for a really bad day; an accident is far more likely to happen on public streets than during a trackday event. Granted the speeds will be lower on the street but it’s more likely that there’ll be a collision, instead than going off the race track and rolling the car. Plus, unlike many Locosts, Midlana’s roll-cage is the chassis.

Some may look at it and say that there’s too many tubes. I haven’t yet spent time seeing if I can delete some of them, but even now it appears that removing even one will weaken the overall structure. The windscreen is present because most states demand one, so may as well use roll cage tubing for the frame (allowing easy mounting of side wind-deflectors.) If it has a windscreen, may as well connect the front hoop to the main hoop, too. The tubes on the roof are in an “X” to allow getting in and out easily. I haven’t counted, but I think Midlana has far less tubes than a Locost chassis.

Then there’s that big square tube on the floor, what’s with that? That’s a torque tube, adding – get this – 5000ft-lb/deg of torsional rigidity to the chassis (I’m double-checking this… I may have the value wrong). This is way more than any Locost and it’s only possible because the engine’s not in the way. It connects the front suspension sub-frame to the main hoop, where triangulation extends it back to the rear suspension. The center tube also serves to run coolant lines, mounting the shifter and emergency brake handle, plus throttle, emergency brake lines, and shifter cables to the rear. This frees up the side tubes to be used primarily for side-intrusion protection – and styling – instead of torsional control.

Okay, about the pictures. Like I said, it’s far from complete; the nose cone’s left out because it’s really hard to create in Sketchup. The chassis is detailed on one side only hence the lack of tubes on the far side. Also, there’s a number of different ways the side tubes can be run which really change the look. I fully rendered one that looks pretty good but would like comments on all of them. Driving the design are the two curved side-panels: the small ones inside the front wheels exhaust radiator air into the low pressure area. The large ones lead into the engine compartment for the oil cooler, intercooler, engine inlet, and general engine compartment cooling. I’m also counting on the curved panels to differentiate the car from looking like a dune buggy. The rear treatment I like, looking a lot like a 1930’s hot rod. Louvers can be added to break up the curved panel, and to help cooling (though they’re about $5 each!). The two boxes behind where the seats will be are the gas tanks; there’ll be a liquid-tight panel between the tanks and the seats. The torque tube necessitated splitting the tank into two units.

The last pictures show what it looks like without windscreen – quite striking I think. However, the reality is that most states require a windscreen, and being buffeted by wind can get tiring, never mind getting hit in the head by pebbles or large June bugs! As said before, the builder is going to be fully involved in the decision making process, no cookie-cutter plans here. Let me know what you guys think, as unlike with Kimini, it’s important to get input. If everyone dislikes the same thing, changes will be made. Overall I’m pretty happy with the layout, which overcomes nearly all of the shortcomings that I feel Locosts has. However, I don’t like that the driver’s eye still has a line-of-site view with the front tire contact patch. This means sand or rocks kicked up have a straight shot at said driver’s eyes – I may offer side panels to prevent that. Anyhow, let me know, good, bad, or otherwise.