Started today with no idea what to work on, and chose the cowl. The dash frame took all day to get just right but it has to be since several other aspects of the design count on it. I worked exclusively off the manuscript’s Cowl Chapter… which now needs work based on what was learned today – which is good for you future builders. The hot humid weather didn’t help – I’m beat.
The gas tank is tentatively done, in CAD at least. By moving one side a little, capacity is now 16 gallons! This is enough for just about any drivetrain builders might want to use and gives the option of cutting it down if not as much capacity is needed. Still waiting for the fuel level sensor, and have to place the various external hose fittings, but it’s getting there.
I had a discussion with a race car designer who feels that the flat diffuser is better. The Katz book (figure 6-39 if you have it) shows that downforce increases with a diffuser angle of up to 12 degrees, while drag actually drops at angles up to about 4 degrees, then increases up to 12, but remains lower than if there was no diffuser. McBeath, in, Competition Car Downforce, discusses curved diffuser design but doesn’t provide data (which is understandable since aero design doesn’t transfer easily between vastly differing cars.) I don’t have a big-ass wing helping to suck air out through the diffuser, though the Katz data does not either which is good for my application. So, since Katz has actual quantifiable data, and because Kimini felt very stable at high speed, I’m going back to the flat design.
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.
The front suspension pickup points have been chosen, and next is working out the pushrod and shock setup. I’d like to make the shocks a feature, visible from outside, but it’ll be a challenge to miss the driver’s feet, steering shaft, pedals, master cylinders, radiator ducting, etc, etc. Checking brake master assembly clearence though, shows there won’t be any choice but to use a bottom-pivot assembly. There’s simply not enough vertical space for an overhung type, even with remote cylinders. One perk of a floor or firewall mount is that I don’t have to snake the steering shaft through the pedals, or avoid putting the shocks there.
At the Cars and Coffee event were two really cool cars. One was an original Cobra, not that big a deal around here, but this one had a polished aluminum body, just like chrome and very impressive. The other was what appeared to be an unrestored Toyota 2000GT. I never thought I’d get a chance to see one in person; they’re just beautiful cars, very elegant. It’s a shame they were never imported; they could have outsold Datsun’s Z car but I think they were too expensive. A real shame. Interestingly, it was parked next to a Ferrari Daytona, allegedly the inspiration for its styling.