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While thinking about my 3-wheel WWII fighter-inspired car ideas recently, I may have inadvertantly come up with some interesting suspension ideas. Unencumbered by having to fit under bodywork, I let my mind jump around to the rediculous. In this case, function would follow form as I was trying to envision a way to represent wings with the front suspension arms on the F4 Corsair inspired car. Think of an inverted McPherson strut with an extension coil spring instead of a compression spring. The pivot point of the supsension would be near the top of the bodywork, so the roll center would be much higher than the CG and with only 1 wheel in back, I can't see body roll being an issue at all.

The other idea that came from that was an 'X-wing'-like deal which would be like a double wishbone setup except that the upper suspension arms would meet the lower point of the spindle and the lower arms would meet the upper pivot mount of the spindle (forming an 'X' in frontal view). This should change dynamic camber to always be negative, with more negative camber given as the roll increases.

Or I could be tired and these ideas suck, but right now I really want to get to work on building 1:18 scale models.

Steve

Well, I've never seen, in serious practice, a vehicle with it's roll moment a negative value. That'd be interesting, to say the least. I would imagine that the effects of the roll moment would still come into play. I mean, you still have a large leverage arm, it just happens to be negative. I'm attempting to envision the design, and the best I can come up with would be taking a McPherson strut suspended car, flipping it over, and having bigass wheels to work with it (though their effect will be negated, as in your scenario, they wouldn't be existant). Obviously, the rear wheel would be some type of swing-arm design (for durability sake).

So what would happen in my imaginary vehicle? With the roll center higher than the center of gravity, I'm envisioning two things. Suspension movement, and it would appear that exactly the opposite of a regular McPherson strut assembly would apply. Camber would go positive under compression (where in a standard setup, camber would go negative).

Secondly, as the vehicle turned, there wouldn't be body roll, exactly, but it WOULD want to pick up the inside tire as it tried to suck up onto the rear wheel and the outside wheel. I actually was thinking about cornering on a motor-trike, when I was riding home the other day. They don't have the ability to lean into a turn like I do, and a sharp maneuver would put them on the front and outside rear wheel. Simply turn that around, and the same basic principal would apply.

Now, if you did a double wishbone setup, with one wishbone an equal distance above the center of gravity as the lower link, then you would have a fairly well planted vehicle with the capability of giving a relatively compliant ride, no compromise of camber. This would locate the roll center to almost exactly at the center of gravity, and if that were the case, you would have no leverage for body roll. And lateral movement would have the body pushing against the links directly. The downside, is mid-turn bumps, you'd go negative with roll moment, and increase the likelihood of roll (as the suspension compresses, the center of gravity lowers but the roll center remains constant). To counter that? Set the roll center to be slightly lower than the center of gravity (to give yourself some cushion), and run a slighly more aggressive spring rate.

That might work. But might not. There are ways to acheive no body roll without compromising bump absorbtion. One method was used on a military vehicle years ago. I can't even remember the name of the truck, but it had ZERO body roll while turning, but could stuff a tire in the wheel well without changing anything. It used hydraullics and sensors. As one wheel was compressed, the one diaganol to it was forced down. So in a turn, the outside front would force the inside rear up, and the outside rear would force the inside front up, and they would cancel eachother out. Kind of cool.

As for the second idea, I dig it. Sounds like the cross-over IFS used on some pre-runners, designed for incredible travel. But with an extra link. You could simply add caster to acheive more dynamic camber, but with your idea, you could have progressive camber based on suspension movement. As a wheel is forced down, it's forced to have more camber. Did I follow that correctly?

Good ideas! The first one and the second one combined would kick some ass!

Good point about the + camber change, which I forgot to mention, that's the concern that lead me to idea #2, which would add - camber as cornering loads increase. Sounds like I explained it well enough and, yes, the rear would be a swing arm motorcycle shaft drive rear end. Thing is, these ideas would likely be overkill for these cars as they would be really damn light, however, I thought they were interesting enough to share with people who may appreciate it (or shoot them full of holes). My family is no help as I might as well be talking to my sister's blind rabbit about this kind of thing.

I heard of the vehicle you're talking about and can't recall what it was either, or maybe I'm just thinking about the Citroen WRC's semi-active roll bars. I know there was the Mercedes Carver that used hydraulics and sensors to lean the tires in, but that's too complicated a solution for me.

Steve

I definately appreciate the innovation and idea, your brilliant with this kinda stuff and I figure my constructive criticism will only aid you in further designing those negative aspects out of a system.

I'm sure you could come up with a way to easily conquer the camber thing. Hell, even on a small scale, flipping the control arm to the upper end, and having the mounting point down low might not be a bad idea if you could figure out the camber issue.