Things are busy this week and next so it’s a bit slow on updates. It’s one of those deals where by the time I get home, take a shower and eat, it’s late and I don’t feel like doing anything!
However, the parallel trailing arms have been modeled in WinGeo and nothing bad has appeared. I’ve had a couple readers express concern that moving away from A-arms is a bad idea. What’s driving the suspension change is packaging; one reason is that there isn’t enough room behind the drivetrain to have A-arms with a base wider than 10″ which I feel isn’t strong enough. I understand the concern but feel trailing links aren’t much different than A-arms when it comes to toe control. No one bats an eye when they have to dial out bumpsteer at the front yet everyone freaks when it’s the same situation at the back – it’s the same thing.
In the 1960’s, F1 cars commonly attached the rear toe-link to the lateral link, so this was tried first – it was terrible, the wheels toing in 0.440″ over the full travel! The reason is because as the suspension moves into bump the trailing links pull the upright forward. Since the toe link is attached to the inboard pivot point, the upright and lateral link rotate forward as one about the inboard pivot, leading to really bad toe-in. The trick is to have a dedicated toe-link not attached to the lateral link, resulting in the much better control.
I was reminded that I got in trouble using trailing links once before, with Kimini. However, in that case I purposely designed in rear toe-in in bump. Older and wiser, I know better now and will design in a very small amount of toe-out.
Things are changing – again. After a buddy reviewed the chassis he asked, “Why didn’t you use simi-trailing arms for the rear suspension like on Kimini? It’s a nice compact layout and allows long links.” Um…
It’s a combination of things: the forward links can’t go too far outboard because they hit the wheels; they can’t go too far inboard because they hit the drivetrain; even in-between they hit the intercooler on one side and the alternator on the other; I wasn’t sure, due to the simi-trailing nature of the arms, if the camber and toe curves would be polite. Perhaps it was the workload of getting the chassis into the computer that kept me from spending the time on it. Or, perhaps I just got tired of dumping so much time into such a small part of the chassis with little to show for it. Well now that the chassis is (more or less) done it’s being redone – again.
The trailing links can be made parallel to chassis centerline if they’re attached part way along the lateral links. It’s not the best engineering decision though because of the bending loads imparted into the lateral links – unless they’re made really strong. This might be an acceptable place for engineering compromise because it solves a number of packaging problems all at one time: it greatly decreases the amount of “stuff” behind the drivetrain; the lateral links can be made as long as I want; because it’s then true trailing arms (parallel to chassis CL), there’s no toe-steer going on; it also frees up more space for the muffler, an necessary component that easy to forget. The shocks/springs can package fairly straightfoward, but again, only if they’re mounted slightly inboard of the lower pickups. Since the trailing links pick up from this area of the lateral links already, it’s probably okay – if it’s strong enough. I’ll run the numbers, but since Kimini had to use 1.25″ tubing for the lower tube – without the spring load – it may go as large as 1.5″ OD square tubing, due to the separate large vertical and longitudinal loads. Overkill? We’ll see what the numbers say.
So if it sounds like I’m talking myself into redoing it again, well, yes. Better this way than to build it from steel and then become unhappy with it. I wasn’t real happy about it before, and my buddy’s comments pushed me over the brink of redoing it.
In other news, I think I found a bug in Sketchup. Recall I’d been bitching about how sometimes tubes don’t “snap” to where they’re expected to. I just caught it red-handed; it was easy to see this time because it was in a clear area of the drawing. Clicking on a node I wanted to connect to, it looked like it connected fine, but it was very slightly off. Zooming way in showed that it was snapping to a point in thin air, ignoring the valid node right near it. Huh? Selecting the area confirmed that there’s nothing there and yet it wanted to connect. Sigh, going to have to keep a close eye on that.
A major chapter has closed – Kimini was shipped to her new owner last night.
As this is my first full-blown CAD project, I get to learn all the quirks like everyone else, like CAD’s wonderful ability to see everything with such exact precision. The snap-to feature is something I’ve been using all along and taking – too much – for granted. Move a tube to a junction and it “snaps” to the node, as it should. However, had I zoomed in close, I would have seen that it does indeed snap to a node… just not necessarily the one I intended! So where several tubes come together; the situation’s ripe to have an added line be “slightly off” due to snapping to something nearby. The real damage (timewise) is that later tubes carry along the error, building errors the further along I went. My payment for being lax is to have to go back through the entire chassis and confirm each node is truly a singular point. I also noticed that the fuel tank frame doesn’t quite fit… huh? Sigh, oh well, another thing that I thought was finished isn’t. Add it to the list… but it’s coming along, quirks and all.
That aside, the last of the tubes are in place, minus the rack and radiator mount; now it’s a matter of figuring out how to optimize all the tubes. After that each tube will become its own component and get the end profiles cut; it’s not hard, just very tedious. After that, everything gets dimensioned – that’s the fun part – then it’s time to start cutting steel! The idea is to build the car from these plans, adding notes and corrections as I go along, so that by the time the car is built there’s a known-accurate set of plans. Unlike with Kimini, it means that if Midlana ever gets bent, there will be accurate drawings to make new parts.
Since no one likes the footwell reinforcement (I admit I wasn’t sure about it myself), I took it out for now. All this stuff is subject to constant change as it progresses; even after it goes to steel there will no doubt be further refinements as the chassis solidifies. Per a suggestion from Dean, and remembering how I handled Kimini’s floor, part of the floor is going to be double-skinned. This prevents the driver’s legs from dropping out if the floor panel gets ripped off in an accident. (This is what happened to race car driver Bob Bondurant, so, as with Kimini, it’s called a Bondurant panel.) This may change slightly, maybe using smaller diagonals, but you get the idea.
Reader Brian reminded me that there are many race organizations, not just US-based SCCA and NASA. While it’s impossible to meet all the rules everywhere, he brings up a good point. It doesn’t hurt to review rules from elsewhere and see what’s easy to implement. I realize many people won’t race their cars in officially organized events – I’m not. But because these organizations learned the hard way about what survives crashes and what doesn’t, it pays to follow their guidelines. An accident at 40 mph with an SUV on the street can be just as bad – or worse – than a single-car crash at 100 mph on a racetrack.
I discussed the future fabrication of the turbocharger exhaust manifold with my race car fabricator buddy, Alan. For fun I mentioned possibly using Inconel and was surprised when he didn’t laugh. He said it really is the best material to use, build it and forget it, if it’s in the budget. It is not, at $200 per U-bend, I’d be spending about $1200 just for tubing! No, can’t go there, but I will use 321 stainless. At $40-50 per U-bend it’s still expensive but much less so.
Several people have written questioning whether the upper chassis is following SCCA and NASA rules; specifically, how the roof tubes cross instead of running parallel to the centerline of the chassis. I don’t think I’ve broken the rules but would appreciate input if you feel otherwise, especially if pertanent rules can be quoted. The reason the roof tubes cross is to make it easy to get in and out. I really don’t want a true boxed-in cage, where getting in and out resembles an old episode of “Dukes of Hazzard”, never mind having a girl get in wearing a dress! I noticed with disappointed amusement that NASA, like the SCCA, allows using 1.375″ OD “unobtainium” tubing.
So let me know; it’s trivial to correct now; much tougher later on, never mind the liability of having a bunch of books in circulation saying to do it wrong…
One of the consequences of designing a car is often not knowing dimensions of components without first having them in-hand. Other times it’s guessing about what will fit where and placing orders before all the details are known – that doesn’t work sometimes. Such was the case with the floor-mount Tilton pedal assembly. With perfect hindsight, there is indeed room for an overhung top-pivot design, which fits better, is more ergonomical, and costs much less. The good thing is that it makes the overall project that much less expensive to build and fits together a bit better. Hard to argue with that, just wish I hadn’t been so quick to order the wrong part but so it goes.
I’ve been exchanging ideas with reader Dean, who’s comments have helped make the chassis look a bit less dunebuggy-like and a bit more sports-car-like. Never mind the ugly nose, I’m too busy working on the rest of the chassis to create a proper render. Note that the side windscreen tubes now bend directly into the roof instead of around the windscreen as before. The foot of the windscreen frame now extends forward all the way to the floor. I also triangulated the bulkhead around the knees, though it remains to be seen if it’ll interfere with the drivers legs or feet. I hope not because it hugely stiffens a bulkhead that’s normally left open on most cars.
What’s still half-baked is where to run the coolant lines. At the moment I’m back to running them down the center and if that doesn’t work out it’s back to outside the chassis. My buddy Ron suggested using large 3″ angle for the lower outside frame rails and tucking the coolant lines inside, covering them with a radiused cover. That would look really sweet but there isn’t 3″ to give away outboard of the seats – things are that tight. I can’t justify widening the car 6″ just for coolant lines so if down the center doesn’t work it’s back to along the outside of the chassis, covered, ala Cobra side exhaust-style.
Kimini is officially sold; the trailer has already left for the new owner’s home in Utah and next week Kimini will make her journey inside a proper transporter.
Blunting the impending emotional loss is the arrival of a Garrett twin-scroll GT3071 0.78 A/R turbocharger, also an emotional item, and admittedly not a necessary component, never mind this early in the build. Its purpose is to serve as an emotional goal, to look forward to hearing it whistling someday 🙂
The emergency brake system is done. Next is determining shifter, dash, and steering wheel (and steering shaft) placement. This naturally flows into dictating the cowl dimensions. It’s all but been decided to use aluminum for the cowl rather than messing around with adapting an existing – but wrongly sized – composite cowl. (The term “cowl” refers to the assembly that houses the dash and extends under the windscreen base, meeting up with the hood.)
I knowingly swapped the Miata rear uprights side-for-side to bias the upper pickup point toward the rear, making it more of a straight shot to the upper pivot points. What I didn’t realize was what I was doing to the emergency brake actuators… now pointing toward the rear of the car… doh! Fixed, it’s an awkward reach for the upper A-arm, but oh well.
The gas tank is done, complete with hanging brackets, baffles and hose connections. Since it’s triangular-shaped, not too much capacity was lost by shortening it a couple inches, improving cable routing for the shift cables and emergency brake. It does lower capacity to about 13 gallons but that’s still a decent size and, hey, it lowers the CG a bit, too. Now I know how much work I avoided by not having an emergency brake on Kimini, it’s a pain. The good news is that a stock Miata emergency brake lever and cables assemblies are used, no modifications necessary.
Had the day off so good progress was made on the gas tank, either aluminum or stainless, seen here in its protective frame. This fits up behind the seats from below, fastening with only two bolts – pretty sweet I think. Sketchup says it’s 14.23 gallons which is a good size. Still to be added are internal baffles, doors, filler, vent, etc, etc.
Reader Dean suggested running the coolant lines along the ouside of the chassis instead of through the chassis tubes. I’m good with that as long as it looks okay. Maybe something like those muffler covers on Cobras but smaller. The trick is finding something that requires little to no work; maybe perforated stainless sheet rolled over a form. I’m open to ideas