BigGreen,
So 1 SB rolls and that is enough to say SA and IRS the cars will tip at the same speed? Please note the SB in that video was nearly Flat (while sideways at speed) with the right rear bottomed out and then the front right hit the curb sideways inducing the roll. You could roll a modern day F1 car that way.
Why did VW and other manufacturers evolve their suspensions that you still think as equal? One guess is the cars were starting to generate more speed and were able to roll too easy and it started to become safety issue. It wasn't much of an issue with my 36hp '57 but later it was in my other cars.
In my race experiences with a stock SA you have a lot more oversteer from the rear jacking which promotes the up on 2 wheels or the eventual roll. I never seemed to have these type experiences with IRS.
This is another animal but I have to say with a modified SA (Z-bar, Shimmed Z-bar, limiting straps, flop stops, camber compensator - used in different configurations and sometimes with larger torsion bars) I was never able to generate anywhere near the cornering ability as with an IRS although the modified SA was safer (I'm trying to get back to the original topic of this thread)...
"So 1 SB rolls and that is enough to say SA and IRS the cars will tip at the same speed?"
- I said a car with the same track width and CG height will tip at the same speed. Simple enough, and nothing really to do with the video.
By the way, where did you get the impression the 1303 in that movie is going faster than any other car there ? I don't think it is.
I also said an IRS equiped beetle will roll a lot more than a swingaxle car. And that's not good for resisting a rollover.
Body roll makes any car far more likey to tip. So in that respect an IRS car is actually more dangerous and will tip over sooner. In fact an IRS Beetle rolls exactly twice as much as a swingaxle beetle. Can you explain why that is ?
"Why did VW, mercedes, fiat, renault & porsche give up on the swingaxle ?"
- I'd say they did that for better straight-line stability on freeways. Not for better cornering, like yourself and so many VW tuners seem to think.
About camber compensators and Z-bars. They affect the spring rates and affect the sprung roll resistance.
Think of a simple situation with SA, no droop or bump limiters, the rear z-bar unloaded when the car is at rest. When the car is cornering at steady state, the outside rear tire tries to jack the car up because the rear has so much roll resistance. This makes the rear try to lean into the inside corner (like a motor boat). The Z-bar transfers load from the inner rear wheel to the outer, making the body lean more (reverse of a roll bar), thus making the rear less stiff. With a looser rear the car should oversteer less.
You can think of this like putting a spring and a reverse spring, the combined spring rate of both springs added up is higher than with just the spring, but with the reverse spring the total spring rate of the system is less.
In shortness, a z-bar makes the rear looser in roll and affects the balance of the car. It also makes the rear stiffer in bumps.
I think it's weird that two solutions have been offered to SA rear: one is to stiffen the rear in roll with a sway bar, and the other is to loosen it in roll with a z-bar/camber compensator. Even more weird is that both of these can probably be made to work.
BTW. Brent: do you want to clarify why you think a modified SA is safer than an IRS? I've always felt the opposite.
Sorry if I implied somewhere that a modified SA is safer than an IRS. That is probably a whole different argument.
The 2 SA solutions that you talk about have been tried by me and I find the one with a sway bar better for me. I think most people that are not racing or driving hard will be happy with a stop limiter or straps AND the Z bar. This will make the car much safer.
Most word of mouth hardware talks, even with racers, is suspect because you don't know what other factors played into the good or bad experience they had with a certain setup. And too many reports are opinion and theory, not analytical step-by-step development experience.
I've been developing and debugging Porsche and VW racers since 1970, with exceptional drivers, at all kinds of tracks and competitions. That's from parking lot slaloms to the Daytona 24 hour.
Here is what I have found that you can take to the bank. Always look at what the Porsche factory did to solve or minimize a problem. They spent lots of money and had really good engineers work it out. To make swing axles survivable in high performance situations, they softened the rear torsion bars to add body roll, and added the camber compensator to control the axles and supplement the softer torsion bars.
1. Swaybar on swing axle: I put an ADDCO 19mm front and ADDCO 19mm rear sway bar on my hotrod 1968 Bug in 1971. I had tried limiting straps and flop stops, shimming the z-bar, everything. The straps worked, but made the whole car jerk when the axles violently hit the limits! The flop stops usually worked loose. The Z-bar shimming was okay, but the factory Z-bar was too thin to really do well.
Surprisingly, the 19mm rear sway bar acted as a suspension travel limiter and fought to keep the swing axles level. It also made the Bug hyper in responding to initial steering inputs, but never induced wild oversteer.
I drove that car to mid-Mexico and back, at max speed on desert roads, in wild traffic and abrupt manuvers, all on Polyglas tires and Sears heavy duty socks.
I wouldn't put a sway bar on a swing axle now, but not because I had a bad experience with one. I found that all things being equal, a camber compensator works well, and does not change steering responses. I liked the quick steering effect, but that's not for everyone.
2. Tendency of swing axles to put the car on it's roof: A swing axle, again all things being equal, will contribute to a rollover faster than an IRS. But lots of conditions have to be right in the road surface, speed of the car, type of tires, and how the driver copes.
A great driver can handle a swing axle when things get touchy. An average driver can't. Everything the average driver does by instinct works against him in a swing axle, especially braking.
The big event that begins a flip is how a swing axle causes single tire toe-in as it moves up or down. More toe means a self-steering effect, trying to pivot the car (nose moving right as the tail moves left in a right hand turn), and the rear center of gravity goes up. A swing axle trips over its own rear wheels.
Low-pivot swing axles and limited axle travel greatly minimize this effect, but Mother Nature is against us. Dr. Porsche put the front roll center at ground level (low) and the rear midpoint of the transaxle (high). That's one thing that makes the swing axle feel unstable at high speed.
Porsche dropped the swing axle in the Auto Union P-Wagen in the late 1930's, 550 Spyders in the mid-1950's, and pushed swing axle development as far as it could go in the early 60's 356B Carrera 2. That car had a 130mph top end and it was a handful. But Porsche had the 911 on the drawing board, so why fix it?
IRS's can flip too, but it takes more work to do it. A fast-sliding IRS car will either hit something while going sideways, or skip and bounce high enough to lever its center of gravity too high.
A conventional "sway-bar" AKA "anti-roll-bar" does its work by transferring springrate from the inside wheel to the outer one when cornering. This adds load to the outer tire, making it more prone to reaching its adhesion limit (translation: bigger rear swaybar=loose, bigger front one=push).
The stock Z-bar is linked to the axles in opposition - it actually reduces the load on the outer tire...and the aftermarket transverse-leaf "camber compensator" accomplishes the same end. Perhaps not the best answer for maximizing skid-pad numbers, but it does help keep the shiny side up.
Because the VW trailing-arm front suspension has a roll-center axis at pavement level and the rear roll-center axis is always going to be higher (approximately at the height of the pinion shaft) the roll-center axis must be along a line pitched down towards the front of the chassis. Body movement in a corner pivots about that axis and there is NOTHING that you can do to prevent it - tweaking of spring & shock rates, sway bars/CC's, are all Band-Aids. If you're only after good auto-cross performance and don't give a damn about street roadability, you can get a swingaxle to work, but IMO IRS is the only way to go for a street-driven car. http://www.shoptalkforums.com/viewtopic ... 3d3700326d
