IRS suspension

[Andy White]

Hi Eduardo,

Your rear suspension is swingaxle, not IRS. The 901 box is designed for use with cv joints and driveshafts therefore it can only be used with IRS. To fit the IRS you'd have to weld inner mounts to the chassis to fit the double jointed trailing arms. This gives you an advantage as it restricts the amount of camber you have on the back wheels, reducing tyre wear, improving road holding and traction. Have a look underneath any 1302/1303 and you will see the difference.

Good luck,

Andy.

Originally Posted by Andy White

Your rear suspension is swingaxle, not IRS.

Just to be thoughly confusing, both the swingaxle ant the trailing arm suspensions are IRS.

[GS guy]

I think Eduardo's issue is using an IRS style transaxle with his mid-engine Spyder layout? The VW style IRS uses a 4-joint axle with rigid trailing arm that locates the rear outer stub axle and wheel. The typical Spyder layout uses a flipped swingaxle (flipped ring gear inside the trans) to allow running mid-engine, with a simple trailing link and coil-over shock (or torsion bar and springplate as on the Beck?). The fact that the axle is also connected at the transaxle and itself provides as a stressed suspension supporting member allows the simple mid-engine arrangement. Same as used on the original Spyder.

To run an IRS style transaxle - such as the Porsche - requires a total re-work of the rear suspension with upright that totally supports the wheel, managed by links and coil-over shock. However, I have seen one guy that fitted custom drag-car style U-jointed "swing" axles to a mid-mounted 914 transaxle, the type normally used on T2 trans drag car conversions. These were custom adapted tothe 914 trans. Made for a pretty clean set-up and according to the owner worked great.

Jeff

[G-force]

Guys, the IRS setup has exatly the same number of suspension joints as a swingaxle.
The only difference is that IRS pivots from the front of the tranny rather than at the diff (swingaxle). So IRS behaves more like a trailing arm suspension than a swingaxle, that's all.

Originally Posted by Andy White

IRS gives you an advantage as it restricts the amount of camber you have on the back wheels, reducing tyre wear, improving road holding and traction.

The only advantage is better straight-line stability and generally more predictable handling. IRS doesn't corner better nor offer any more grip than the swingaxle.
(they wont believe that !)
(I know they wont, but somebody had to say it anyway)

Originally Posted by GS guy

The VW style IRS uses a 4-joint axle.

No it doesn't, it uses only 2 joints just like a swingaxle. The inner joint locates in a different position further forward that's all.

Originally Posted by GS guy

To run an IRS style transaxle - such as the Porsche - requires a total re-work of the rear suspension with upright that totally supports the wheel, managed by links and coil-over shock...

All Porsche 911 with the 901 box used basically the same IRS semitrailing-arm design as a VW Bug. The only (minor) difference is the inner joint of the 911 semitrailing-arm sits closer to the centre-line of the car, that's all really.

So there's no reason a 550 kit-car couldn't use the suspension from any air-cooled 911 (other than the 964) or from a VW Bug.

[GS guy]

G - seems like you're confusing suspension pivot points with drive axles.
A swingaxle drive axle only has 1 pivot point - at the differential. The suspension pivots use that as one of the pivot points, and the 2nd at the torsion bar - as you described.
While the swingaxle and IRS designs share similar numbers of suspension pivot points (at 2 as you stated), the IRS semi-trailing arm has the advantage of Significantly better camber control through the full suspension travel - at the expense of added weight and complexity. It also provides a controlled amount of camber gain in roll, along with a lower roll center. The reason it can't be used in a car like the Spyder is you'd have to locate the inner forward pivot point inside the engine! You could, however, adapt a 914 trailing arm design and use the IRS style trans - again at the expense of added weight and complexity.

There's a reason all auto manufacturers got away from using the swingaxle suspension (with some rare exceptions - like the Ford twin I-beam pickup front suspension) - roll center is too high and too difficult to control the huge camber changes while maintaining decent suspension travel. It certainly is less expensive and much simpler to build from a manufacturers cost perspective!
However, in certain specific applications (smooth pavement and very limited suspension travel) it can be made to work well - especially with added features like using a Z bar or ZRS design.
Jeff

Color me ignorant, but what is ZRS?

[GS guy]

Originally Posted by vwdevotee

Color me ignorant, but what is ZRS?

It's a swingaxle based suspension that uses zero roll resistance at the rear, but makes the swinger behave much like a solid axle car. It allows the rear-chassis to roll without resistance - but at the same time keeps both tires planted flat on the ground - regardless of roll angle. All vehicle roll resistance is controlled up front, with a standard sway bar. SOP on current formula Vee designs, though it was created back in the late 60's (or maybe before).

[G-force]

Originally Posted by GS guy

G - seems like you're confusing suspension pivot points with drive axles.

No, sorry bud, I'm not confusing anything. Thanks for the 'lecture' though, it was interesting...

The swingaxle has exactly the same number of joints as the IRS semitrailing-arm suspension, and I've never said otherwise.

Likewise the number joints in the transmission has nothing to do with the suspension geometry. A simple fact which most people on here seem to have missed, yourself included.

Originally Posted by GS guy

The IRS semi-trailing arm has the advantage of Significantly better camber control through the full suspension travel - at the expense of added weight and complexity.

Not true at all. It just reduces camber changes to 1/3 the value of a swingaxle when travelling in a straight line. This is good for stability but doesn't improve cornering ability at all.

Do some research, you'll find the semi-trailing arm design doesn't make the car corner better, just gives a more linear response and therefore a more predictable ride and better straight line stability that's all.

The IRS car will have more body roll and likewise more positive camber than the swingaxle in the same corner, since the suspension on a swingaxle car is not affected AT ALL by body roll. That's a simple fact of the laws of physics that most beetle 'experts' really don't seem to realise.

In fact body roll causes positive camber on the IRS semi-trailing arm design. And some negative camber on the swingaxle.

BOTH cars suffer from jacking GS_guy, and this also causes more positive camber in the corner. The semi-trailing design just suffers 1/3 the amount of jacking that the swingaxle gets.

So at the end of the day if you work out the math you find they generally have the same amount of positive camber in the same corner, and neither car has more grip.