Technical Split-beam Front Axle - Am I nuts?

Bought a new e100 long wheel base van in 79 ,put 118 k miles on it, 3 sets of tires , hauled everything from concrete blocks to kids , wife loved it , she liked it cause she cast up higher than the cars , high enough to see everything , never ever had a complaint about handling..

 

I have driven my twin beam '92 from new. 131k miles and tale end of 4th set of tires. I thought it handled pretty good for a truck even at 85mph in its mid-life..I can't imagine where the toe is going but the truck tracks strait with out any correction from the steering wheel. Relegated to around town use now. All good but I didn't design nor build it at home from bits and pieces.

 

There are two basic problems with swing axles at the front.

The first is bump steer. The wheels as viewed from the front travel in arcs, in other words the front track changes with suspension travel. The "Fat Man" steering linkage shown on page one of this thread shows the correct way to get around that.

The second problem is much more serious, swing axles produce a very high front roll centre, and there is no way around that.

Swing axles are at the back in this picture, and that is bad enough.


The infamous swing axle jacking forces can cause a front wheel to "tuck under", lift the front of the car very suddenly, often rolling the car. Just like a pole vaulter digs in his pole and vaults into the air. Its bad enough at the back, at the front swing axles are deadly.

The usual fix is to greatly limit front suspension travel, but the whole thing is still pretty unpredictable.

Now the truck I beam suspension has the swing axles almost the full width of the vehicle, and they cross over. That is very different to cutting a beam axle in half and having the pivot points in the middle. Very long swing axles can sort of work, very short swing axles do not, especially if the pivot points on the frame are high up.

Having two central pivot points set very low on the frame is an improvement, as it lowers the roll centre height. You can get away with that on a light weight race car, and have only maybe a couple of inches of ground clearance at the front. Race tracks don't have the problem of concrete curbs or speed humps though.

 

Im guessing this^^^doesnt happen if your traveling at the posted speed limit and driving in a reasonable fashion .....vw , Porsche, tempest , corvair , newer Buick SUV all use swing axles successfully and many more I know nothing of ..

 

Porsche have not used swing axles since the 356. Any Porsche later than that uses semi trailing arms with outer CV joints, and sometimes with an upper camber link. Designs vary quite a lot depending on year and model, but none have swing axles these days.



Corvair certainly had rear swing axles, and it is the single biggest reason it gained the horrible reputation Ralph Nader gave it of being "unsafe at any speed". Even VW gave up on swing axles about 1970 I think it was, and went to semi trailing arms like Porsche.

Don't know anything about the Buick SUV, but I bet its not a true swing axle. They would not dare do it these days. If one ever crashed they would be sued into oblivion.

Can you locate a picture of this newer Buick swing axle rear suspension ?

Well no, unless you try to go around a corner, or avoid hitting someone that cuts in front of you or stops unexpectedly.

Its only when you suddenly have to depend on having good brakes, tires, steering, and handling to avoid an accident, that having a half way decent vehicle really matters.

 

As a young person (66-69) there was a stretch of winding road where folks tested their" road racing " skills , the least beaten car for a year or so was an older fella with a turbo charged corvair , he could out corner just about anything ( except a Daytona Ferrari ). A fellow I rode to junior college with had the tempest with swing axle , he went into a curve way too hard and the rear jacked up causing the car to do a couple spins , the one and only time he pushed it too far ! The Buick may not be a true swing axle but the tires droop just like a vw up on the lift. BTW , IMO the biggest problem with corvairs was lazy owners not servicing them properly / on schedule ..

 

Yup.
That is the problem exactly.
Swing axles feel pretty good right up to the point of instability, where jacking forces take over, then it suddenly becomes uncontrollable.

Now you can do all kinds of things like lowering the ride height, fitting super stiff springs, limiting suspension travel, and it might go pretty well. It may slide before it reaches the point of instability. Its all good. Hard suspension, zero body roll, a real race car that vibrates the fillings out of your teeth.

As Colin Chapman (of Lotus fame) once said. "The only way to make a really bad suspension design work is by preventing it from working". What he meant was by making a bad suspension super stiff so the suspension cannot move, an awful suspension geometry design may be slightly less bad.

If you are a suspension tuner and a red hot racing driver, and know what you are doing it can work. If you are Joe average in a basically softly sprung road car with some body roll, it might also feel pretty good, until one day you go past the limit of stability, and out of control, and crash the car.

Modern cars are designed to have gradual terminal understeer and just sufficient body roll, so that a typical driver becomes frightened long before control is actually lost. The driver backs off and there is no problem.

What you don't want is a vehicle that feels completely stable, then suddenly does something dramatic and unexpected that cannot be recovered from, even by a very alert and skilled driver.

 

I've also seen of Flemke-ish variant, where there is only one spring but it is mounted to the chassis via a pivoting yoke / bracket. The adjuster screws still hit the top of the springs in the same place, but the entire spring / axle assembly pivots as a unit. Cranking on one bolt would let you adjust spring rate, but cranking down on one bolt while loosening the other would let you change ride heights / tilt the chassis over the axle, too. I don't have a photo handy, sorry.

 

"Even VW gave up on swing axles about 1970 I think it was, and went to semi trailing arms like Porsche." A Z-shaped equalizing torsion bar, a sort of pro-roll bar, was added in 1967. Mercedes-Benz had employed the same principle in the form of a horizontal third coil spring on some models a decade earlier.

The later-type semi-trailing arm rear suspension was introduced only on some VW models in 1968.

(As for semi-trailing arm IRS, those were first used by Lagonda and Lancia, simultaneously in 1947. I do not know if they knew what each other was doing.)

 

These Z bars or camber compensators were the opposite of an anti roll bar. They supported the weight without adding any roll stiffness. Same with the 3d coil spring on the Benz low pivot swing arm rear suspension. The object was to reduce roll stiffness, the opposite of an anti roll bar. They helped tame the tail happy handling or oversteer tendencies of these swing axle cars.