Fabricated a rear toe-control link – piece of cake. With that done, one side of the rear suspension is complete, so a wheel was bolted on to check fit-up. Guess someone did something right; it fits great, with the tire offset being right where it was designed.
Realized the rear uprights were inadvertently swapped side-to-side when the first suspension arms were made. Turns out it was due to how much better they package – with one exception. It moves the top pivot nicely rearward, getting it away from where the shock wants to go, providing a good straight shot at the inboard pivots. The one bad thing is that the brake calipers also swap sides, such that the emergency brake cables point toward the rear of the car. However, after staring at it awhile, that’s an allowable compromise compared to all the benefits. The caliper, specifically the e-brake bracket, sticks out behind the caliper by nearly 7″, fouling where the shock needs to go. But, since the e-brake cables have to be custom anyway, it ends up routing rather nicely, curving inward and forward, under the drivetrain, up and over the tank, and forward to the lever.
Started building up the second set of suspension arms. Since the spherical bearing for the front rocker-arms or suspension aren’t here, I’m working on the arms that use neither.
Fabricated the spacer for the upper spherical bearing, drilled out the top tapered hole in the upright, pressed the bearing into the arm, and it looks pretty good, just have to find a couple high-accuracy bolts.
Also, the bottom arm has been corrected; stiffening plates are yet to be added at the outboard end. The lower arm is now basically non-adjustable, but it’s not an issue since the upper arm’s used to set both camber and castor.
Since the bearings for the rocker arm’s haven’t shown up, I’ll probably build one upper and lower rear arms to see if there’s any issues there. But before that, since the coated turbine housing is back, I’ll probably fire up the engine just to hear how well the turbo and muffler work to quiet it down, and to hear, for the first time, the sound of the turbo whistling! 🙂
Making some changes in the front suspension. The rod-end at the top of the uprights are being replaced with spherical bearings and the arms are being changed to aim directly at the in-board pivots. The rear pivots are changing from horizontal to vertical bolts, allowing the rod-ends to aim directly at the upright. Pictures when the first unit is built.
Spent the day working out the push-rod geometry. It means applying loads to tubes in the middle of their span, but since the spans are short and made from 1.5″ x 0.085″ square tubing, it’s not like they’re going to move much, but gussets are an option.
My car buddy, Alan, stopped by. He’s been a metal fabricator for decades, having helped to build, among many other things, the Nissan GTP cars. When he saw the mocked-up front A-arms, he said, “WTF is this?” That’s Alan, giving straight opinions – unlike internet forums where you’re not allowed to say something’s wrong with someone’s car. Anyhow, my defense was that I’d just built them the day before and that I wasn’t real happy about the zig-zag upper arm, either, and said I can do better – he said, “That shouldn’t be hard.” I gave him the Smokey Yunick autobiography for Christmas, because I think he and Smokey were separated at birth!
Built one upper and lower front A-arm to test the design. Looks like it’ll work as-is with the shock mounted outboard. Installation ratio is about 0.65, which translates into a 300 lb/inch spring to get 130 cpm wheel rate. That’s not bad, and wheel rate rises slightly in compression.
With that out of the way, it brings up the issue of inboard suspension. What I prefer doesn’t mean much if everyone else like something different… so, I’v decided to go inboard. The reasoning is that since it’s more complicated, that’s what needs the attention. The simplier outboard solution uses the same A-arms and is a no-brainer to install.
Started on the front suspension, tacked-together per the plans to see what’s going on, since a beta-builder reported trouble. If there’s a problem, it’s likely because the A-arms were modeled in CAD as point-to-point, while the real A-arms have threaded bungs at right angles to the chassis, so the tubes aren’t where they are in Software Land. The solution of which way to go, outboard or push-rod, will be answered in the next couple of days.
The front push-rod design went smoothly thanks to the Mitchell software. The design freedom of pushrod suspension is great since it allows achieving all the goals: placement, accessibility, aesthetics, and the target spring rate.
Ordered double-adjustable QA1 shocks with 6″ travel, identical units being used at all four corners. Also ordered the uber-cool Tilton
pedal set, never mind the price. Next will be the rear suspension design, starting with carefully measuring the Miata rear upright.
A long-term background exercise has been the chassis side treatment – how to make it clean, elegant, simple, yet functional. I should mention that while the car started out as a “mid-engine Seven” visually, it has diverged somewhat over time. All the various parts require small changes to the initial body shape; the end result is that the finished product will have its own unique identity – good.
Been working hard on the front suspension design which is testing my nerves – just like 12 years ago. Since I’m using Mazda Miata front uprights, I wondered what kind of numbers Mazda used. The internet revealed the stock suspension points, and even if they aren’t exact, the Miata’s setup is very interesting! What I find fascinating is how Mazda doesn’t follow the commonly held notion about keeping the roll-center stationary, not even close. But if it’s examined using force application points, figuring that the outboard tire carries most of the load, then it works. Of course, the reality is that it (the car) works just fine, as Miata owners will attest. So it’s very interesting how Mazda’s setup violates the so-called stationary roll-center rule… very interesting indeed. So, good, this means I press on with my suspension design and hopefully have some good numbers this week. Once the front suspension is done I can start working my way aft.
On the front suspension, I’ve got a good design candidate, though it’ll have to wait for the front nose; only then can the inboard suspension points be fixed. Wouldn’t help to fix them now only to find the nose is in the way.
Regarding the sportbike shocks… the more I think it through the less likely they’re going to work due to the spring rate and shock travel.
With shock travel of 1.5″ (or 2″ if I shorten the bumpstop), the 595lb/inch spring rate’s probably going to be too low. This is what happens due to all the leverages involved and the short stroke. While the springs can be replaced with stiffer ones, the spring rates end up getting ridiculously high… a shame. It’s not a closed case yet, but if you feel they can work in your project, drop me an e-mail else they’ll end up back on Ebay. If that happens, I’m right back to where I was with Kimini, facing the purchase of $300-$500 shocks, ugh, this was supposed to be low budget. Eh, what I’ll do is specify a coil-over shock of common length. This will allow builders to go “economy” or “nuts”, as their budget allows.
And so it begins. Having decided on the front tire diameter now the real suspension design begins. Using Mitchell’s excellent WinGeo3 software, I’m working through many iterations, trying to get static FAPs (Force Application Points.) What’s frustratingly familiar is how I can get static FAPs, and roll centers for that matter, but I’m not happy with the camber gain curves yet. I’m not complaining and quite enjoy the iterative process. It’s a good feeling knowing that once this step is done, the car will have a stable, predictable nature to it, just like Kimini has. And like the Mini, it took weeks to gradually settle on those elusive points in space about which the suspension will pivot.