The new GM compact platfrom, used in the new Opel Astra and the Chevy Cruze, has a variant of the classic cheap hatchback car torsion beam rear suspension that GM claims is "new" and "better".

I doubt if anything in suspensions is truly new but their claim of "better" is based on removing the sideways location load from the torsion axle by connecting a watts linkage to each hub carrier with the pivot bracket mounted on the chassis just behind the axle centre line.

In the classic torsion beam axle it is "U" shaped and pivoted at each corner. It acts as a trailing arm for bumps etc. equal on both wheels but on roll, or one wheel bump, it twists to provide roll resistance. By careful design sufficient lateral strength is retained whilst allowing twist. The advantages are cheapness, very compact installation below the floorpan and the loads are fed into the car corners right where the sill meets the rear pillar so a high stiffness area. Also it is argued that the high toe in change of a short virtual arm semi trailing arm is avoided. Against that the inherent slight flexibilty of the beam in side loading gives a toe out on high lateral G.

The GM addition of a watts linkage should eliminate this last problem by providing very rigid load path for lateral loads without compromising any of the vertical or longtitudinal compliance or movement. I suppose that if you anchored the hub ends of the watts links below the trailing arm mounting to the hub then you could reduce the camber change in roill of the beam by setting up a counter twist vis the watts linkage offset distance. Maybe if you could angle the watts links down to extend virtually to the contact patch ( and retain a sensble RC) you might largely eliminate camber change.

If all of this does work and one can calculate the various compliances correctly to get the camber correction explained above then IN THEORY (!!) this layout could be good high performance rear axle - any thoughts.

It's a good solution, tho I'd point out that its weight and complexity is approaching that of an IRS - 7 bushes. There's a perfectly respectable IRS that uses 8, albeit by cheating.

Basically GM are playing catch-up on this one and need something technical to boast about - the usual solution to compliance oversteer is toe control bushes, which are admittedly expensive, but not as expensive as a Watts link, and then you get to reclaim your boot floor for something useful, which is the single biggest problem with the Watts setup.

I don't know about your more complex suggestions, rather like the toe control link that Opel used to turn their 1930s technology semi trailing arms into something that was at least as good as a live rear axle, lipstick on a pig is the phrase that springs to mind.

Oh well, at least if I was a GM beancounter I could see a $30 cost save dangling within reach!

Incidentally I'd be a bit wary of building a twistbeam myself, the welds have a very exciting time in terms of fatigue.

Hi Mariner. Can you post a link or an image?

http://www.richardaucock.com/wp-content/up...n-a-twist-2.jpg

Thanks Greg for posting the link, I tried to find one but failed.

I agree about stealing the real estate behind the axle line with the GM layout, it seems to me that you would add weight by trying to feed full lateral loads into the bodyshell behind the rear wheels. I can see that GM has done this with a lateral sub frame into the side rails so that is pure extra weight/cost. Of course it may be more subtle, you design a basic torsion axle layout for the cheaper models( with a heavier gauge if needed) then only add the Watts link and lateral frames for the up market/more powerful ones. That would be quite smart production design, it will be interesting to see if GM try to leave the Watts bits of the US version of the Cruze when it comes out later this year.

Thanks Greg. Interesting squashed tube forming the twist beam - overcomes the fatigue on the welds you mentioned.

Obviously a lot of engineering has gone into this thing, but a visual analysis doesn't excite me. The Watts doesn't actually provide the "very rigid load path for lateral loads" mentioned by Mariner especially at the cornering limit with inside rear off the ground or close to. At best, the Watts can only double the lateral stiffness by adding the lateral stiffness of the unloaded wheel across to the loaded wheel - effectively toeing both wheels in rather than just one. At the other extreme if both wheels were carrying equal lateral load, the Watts will contribute fully to lateral stiffness.

The lateral stiffness as such is no big deal, what is important is the toe compliance effects, which are always oversteer in the simple twistbeam, but could(and should) be understeer in this design. Dynamically this is quite a nice solution, but to my mind falls between two stools, it is $30 more expensive than the simple twistbeam, and eliminates your spare wheel well (etc), but is till more compromised than an IRS. Now, that may be the happy medium, but if your two major competitors are running IRS then you don't get to boast much. Admittedly a cheap IRS is still likely to be more expensive, probably heavier, and noisier. OTOH you have a chance of going AWD with an IRS, the design needs a tearup for AWD.

Personally I think a well sorted twistbeam is fine as it is, for that market. I rented an Astra for a couple of weeks two years ago and found it to be a pretty reasonable Focus competitor.

Beam me up Scotty..

That's odd, all I'm getting is X

Because it's a cross beam axle.

Smart arse...

How so? In my (simple) view of this, the toe compliance is a result of lateral compliance.

In plan view split the total suspension lateral compliance into two components, in front of the axle and behind. Now apply a lateral force at the pneumatic trail point. The two compliances form a seesaw, if they are correctly balanced you have lateral compliance but no toe compliance, if not then you have toe compliance AND lateral compliance. Adjusting the ratios of these compliances is how you get the desired passive steer characteristic- difficult, but not impossible with a conventional twistbeam, tricky with a semi trailing arm, and comparitively easy with a multilink or Chapman strut. The watts link provides a big stiffness behind the axle, encouraging toe-in on the outer wheel, which is understeer.

That's not to say that the lateral as opposed to steer characteristic is unimportant, but on the scale of things I'd much rather have a soft system that gives toe in than a stiff system that gives toe out.

The Watts looks to be not very far behind the axle and only transfers lateral motion to the other wheel. I would think that will only reduce the amount of toe out - not be capable of producing toe in.

The Watts looks to be not very far behind the axle and only transfers lateral motion to the other wheel. I would think that will only reduce the amount of toe out - not be capable of producing toe in.

I don't believe you have accounted for the pivot bushes at the L/R front chassis mounts - the ones with the 4 large studs protruding.

Noted those. To my eye, the trailing arm will toe-out the outside wheel - with or without the Watts.

just found this topic, the idea of the linkage is for comfort apparently. It seems that the main bush that mounts the axle will be quite soft for comfort, that would obviously cause the axle to move around quite a bit, so the linkage is there to solve that.

If you search around you can find the international patent for the system which gives a bit more detail