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Rear twist beam questions

Rear twist beam questions

Rear twist beam questions

My questions concern rear twist beam axles used on fwd vehicles. On a perticular car that I am currently working on the twist beam has four(?) mounting points; two at the front of the beam onto the chassis and one behind each stub axle onto the dampers. The coil spring is in between the two. The car in particular is an Opel Kadett/Holden Astra manufactured approx '92.
What proportion of the suspension loadings do the spring and damper share? It looks like the damper does not take hardly any loads or is that dependent on the valving of the damper? ie, more restrictive valving = firmer ride therefore more load transfered through the damper? Would fitting a brace between the two damper towers have any effects and what would they be?
I also have an opportunity to purchase a book by miliken, any good?

RE: Rear twist beam questions

Buy the book. It is about $A 220 new. It is very good.

The springs and the shocks work in different ways, so there is not a fixed ratio between the loads in them. On a smooth road almost all the load is static, through the springs. On a rough road the split is more even - full compression on a spring is probably around static load+1600N, say 4000-5000N, whereas the max shocker load is probably 1500N, dynamic. Just to confuse things the jounce bumpers can probably take at least 5000N on top of that.

Having said that I'm a bit of a cynic when it comes to rear suspensions, I'm not sure that bracing the rear towers would actually improve the handling noticeably, compared with bracing the front towers, which tend to be weak in torsion since the engine gets in the way of the structure you'd like to have. One trick is to put your car on a 2 post hoist and measure the deflection between the tower tops as you lift the car off its wheels - one particular vehicle had 6mm of deflection in that test.



Greg Locock

RE: Rear twist beam questions

Thanks Greg.
The books second hand and cheaper than $220, so a bargain I think!!
I'll do the deflection test and see what happens. Problem is I would not know what an "acceptable value" of deflection would be. As the car is now 10 years old I suspect it my be more than when new due to joint fatique etc.

RE: Rear twist beam questions

Well it is such a simple mod I guess there's nothing wrong with suck it and see. There's no acceptable number as such - I was just surprised to see such a large deflection on a production vehicle. I suppose 1 or 2 mm would be fine. When we braced the car I think we made it 3 times as stiff, so that would be 2mm.


Greg Locock

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