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Discussion Starter #1
Once again been reading reviews. The smart has Didion suspension which in essence means that the frame and tires stay essentially flat on the pavement, while the cabin and cockpit do the tilting. A mainstay of early race cars and a good way to make a compact simplified suspension.
I've yet to see a single reviewer identify the kind of suspension used.:(
 

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The car has a Dedion tube a "Glorified swing arm" with shocks & coil springs which is not part of the unitized frame..
The front end has struts... Pretty basic stuff...

I should add there is no sway bar in the rear, only in the front so the body does tend to roll rather easily...
 

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IMO - There's nothing too revolutionary about a DeDion tube. It's actually a really simple design (that's ironically a bit complicated).

But by today's standards, it's kinda outdated.

There aren't many current cars out there that use a DeDion tube.

Fully independent suspensions and magnetic suspensions is where it's at. :)
 

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I'd substitute "cheap" for "weak".

Have you ever loaded anything on the tailgate? The rear suspension crouches down at the very mention of weight. :D

Wonder if Renault/Mercedes went for a better setup for the Twingo/fortwo.
 

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A 'friendly' guy at a convenience store I frequent is fond of relating his experience with driving a smart when he was stationed in Europe. He's so very fond of saying he always felt the car was going to roll over when he went around curves.

Odd, regardless of the car's misgivings, never once have I had the sensation of impending roll over. And I do tend to corner aggressively.

No question in my Dragon experience, the tires break traction long before any possibility of roll over, barring hitting an obstacle. And, I've got tires intended for sports cars.
 

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Definitely agree there. It might "feel" like it's rolling because of it's high COG, but darn the thing just sticks to the ground like a magnet!

The tyres will go long before you get on two wheels! :D
 

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Discussion Starter #9
I think the "high center of gravity" is dependent upon how much the tridion rollbar/cage weighs above the center line in relation to the amount below said centerline. It's steel after all. The rest seems to be incidental in terms of weight above vs. weight below. If there's enough steel riding high, it might make a difference.:)
 

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And to say I haven't had this car up on two wheels would be a lie. My "tyres" are low profile, the roll is transferred to the body. Slide or roll, which do you prefer? The laws of physics say something has to give. Even on a rain soaked parking lot, my car chose the roll.
 

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ALL cars with normal (spring based, non active) suspension designs "tilt" in turns. The question is "how much".

The front is a totally conventional MacPherson arrangement with antiroll bar. The rear deDion axle has no antiroll bar, but the roll center (the point at which body-roll-inducing forces act) is at the height of the chassis end of the links at the back that locate the axle side-to-side, and that's pretty high up.

The deDion axle, in general, is not a bad way to support the back end of a car like this. It keeps the tires straight up relative to the ground no matter what the bodyshell does above it. There is NO independent suspension that can do this! It doesn't have the bad side effects of a traditional rear axle drive car in which there is a heavy differential bouncing around up and down back there. The unsprung weight is little different from IRS. All that is "good".

The "bad" things about the standard suspension are generally that the springs are too soft, the dampers are hard, and there is not enough upward travel in bump, particularly at the front, which leads to the front hitting the bump stops a lot. Higher-rate springs would reduce bottoming and reduce the pitching during acceleration, braking, and turning. I'm sure there will be a price paid in normal ride quality when you are rolling straight down the road.

The other "bad" thing about the rear suspension has to do with those lateral links at the back. Essentially, this suspension design binds up when it goes too far away from its "design" ride height. The lateral links swing in an arc which wants to pull the back ends of the U-shaped axle tube inwards towards each other. To accommodate this, either the bushings have to flex some (which means the back end of the axle isn't located laterally very stiffly ... bad) or the axle itself has to bend (which pulls the wheels into toe-out, particularly on rebound ... bad). I've had the car get unsettled when going over a sharp crest. It makes sense. This unloads the suspension, which pulls the wheels a little into toe-out, and the arc that the links swing in "steers" the rear axle a bit, and that adds up to the back end getting squirrelly.

If you look at the rear suspension design, the center pivot of those arms is higher than the axle ends. Lowering the rear of the car by an inch or so would put them close to level which ought to reduce the axle-steering and binding effects, in addition to lowering the overall center of gravity a smidge ... it's on my wish list ...

As for the next generation model,

When they jump it off the one-side ramp, when the car lands, the rear wheels stay upright (obviously after the one that jumped off the ground lands again) and that means it's using an axle of some sort, since no independent design can do this.

BUT ... I'm hoping that they locate the axle differently to give better geometry. There are a couple of spots where if you stop the video, you can get a (blurry) view of the undercarriage, and I'm not seeing those lateral links, which suggests that it might be a different design. Getting those links out of there would free up some room, too.
 

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Go Faster, I disagree with a point you made... The Dedion tube ends are pushed out as the rear springs compress, causing toe in on the rear wheels... The tube ends are pulled together as the body raises (spring extension)... The center pivot point for the lateral arms is higher than the connection to the ends of the DeDion tube... So as the car flattens out (lowers)the arms will too, pushing the tube outwards...But all in all its fairly minimal....
As for body roll the aftermarket offers stiffer anti-roll bars for the front and there is a kit to put one on the back...
When I built the new subframe and suspension for Frankensmart I had the opportunity to make the new Dedion tube with 2 mounts to the frame about 3' apart since it was a square bottom U shape instead of the half round factory shape with one mount that allows the body to roll...
With the 2 mounts spread apart the body and Dedion tube would stay parallel at all times...But I chickened out and went with a single mount...Wished I hadn't now...
 

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Yes, you're right, I cut the explanation short in the interest of simplicity. The first little bit of suspension compression leads to toe-in (if the axle flexes) because the lateral links are slightly angled down at stock ride height, but unloading the suspension leads to toe-out, and that's the situation where I've had the back end get squirrelly.

Another side effect of the links having a slight down-angle is that cornering forces will give a slight jacking effect. Given the soft-ish spring rates, this will raise the back end during cornering by a small but likely non-trivial amount (Bad). Yet another side effect is that if the outside wheel goes over a mid-corner bump or dip, the movement of that link at an angle will steer the whole rear axle assembly a little bit (Bad). All of these little effects might not each do much on their own but add up all of these little "bad" effects, particularly with the short wheelbase that magnifies their effect, and you get unsettled behaviour when cornering on bumpy surfaces.

(It was never designed to be a sports car ...)
 

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Mine is lowered, wedgies and has a rear sway bar stock. Handles fine, only time I can feel it get squirrely is accelerating through a dip simultaneously turning hard right or left. Most cars would be unsettled doing this. I get on a freeway everyday that requires me to turn left at the light onto the onramp and there is a dip I have to go through. I've had the traction control flash even though it's warm and dry. One of the back tires must get a little air as the suspension unloads quickly. It's an uphill ramp and I have to be on the gas hard to keep up. I go up and over a hill right by my house with many twisties everyday and have no problem passing people in the corners. See Little Mountain Drive in the Google Earth attachment. The curves are sharper than they look.
 

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The wall thickness of the Dedion tube is rather thin...Before we started on Frankensmart we were fooling around with a 2x4 on top of a floor jack and raising the stock Dedion tube up and down (with the lateral arms connected) thru its travel and measuring the tips of the Dedion tube... its moves in an out quite freely...:D
 

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answer to "Center of Gravity" question:

The engine and transmission and fuel tank are all below the floor in the back.

The battery is under the passenger footwell.

The highest center of gravity in the car are the passengers.

I have seen smarts involved in every possible kind of accident and (in my experience), they have never turned over on their own, unless acted on by an outside force (running into or being hit by something else)?
 

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In the days of the muscle cars the CG was usually around the heigth of the camshaft... I'm guess'n the CG of a smart is somewhere between the upper & lower floor pan of the car... just my .02
 
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