I have been reading up on the rear suspension of the new Fiesta, and I *think* I get it…..
As my Clio has a twist beam as well, I though there might be lessons to be learned here (though development was for road use mostly)
design parameters
first some stuff I found on the web to provide some context:
everything is copy /pasted, I’m not that smart…
One of the drawbacks of a twist-beam suspension is the location of the bushings. They are lower than the wheel center and therefore the wheel moves forward in jounce. This is undesirable for good impact harshness.Moving the bushing upwards usually difficult, as it would conflict with the vehicle’s structure. To get a decent level of impact smoothness the bushings should be soft.
Another drawback of a twist-beam suspension is behavior during side loading, especially cornering. The lateral loads translate and rotate the whole axle due to lateral compliance in the front bushings and due to structural compliance of the twist-beam metal structure.The rotation causes an unwanted toe-out movement which implies an over steer effect. Common solutions to correct this issue are complex reinforcements, stiffer bushes or an additional Watt’s linkage. However, such measures add cost and can generate other issues such as weight /NVH increases.
The challenge was to find a solution which reduced the gap to the functional performance of independent rear suspension.
The solution should be easy to install and to replace.
The solution has to find a way to separate ride comfort and dynamic agility without sacrificing comfort for agility.
A modular approach fitting into each program and the use of existing parts was desired.
All bold emphasis is mine.
Analysis
How do I see the problems in relation to my 182?
The bushes are lower than the wheel center
No easy way to fix this really. Moving the mounting point up involves ‘a bit’ of hacking and welding.
Lateral compliance in the front bushings
We know how to fix this…Powerflex or Pure Motorsport bushes to the rescue!
Structural compliance of the twist-beam metal structure
Pure Motorsport rear beam kit anyone?
As we see, pretty much everything is available to correct the drawbacks of a twist-beam rear axle.
As to having an ‘over steer effect’ not being undesirable. It is the uncontrolled aspect of it that is (remember this is road-car engineering).
Geometry changes
To decrease the Toe out ‘over steer effect’and increase the predictability of the handling, they found that inclining the spring towards the center of the car helped with redistributing the lateral load on the rear twist-beam.
“during cornering the outboard spring is compressed and the inboard spring is de-compressed. In total more lateral load is created to the corner outer side fighting against the lateral load coming from the tire and reducing lateral compliance.”
The problem is, that this decreases boot space as the top spring mount moves towards the centerline of the car.
The REALLY impressive part of what the Ford engineers did, was in the spring. They inclined the top and bottom windings opposite relative to the center line to create an S shape. This angles the spring force, obviating the need for spring inclination towards the center of the car.
So how does this relate to my Clio?
Since the Clio has nearly the same distance between the shock and spring, Replacing it might just work Although not knowing the rate of the Fiesta’s spring is a large unknown factor. I cannot say if it would be a good idea to use it as a direct replacement. (car weight etc will of course also be a factor)
For a track car with a rear coilover setup, or when luggage space is of secondary importance over handling, things are different.
The only limits are imagination and funds.
A new bracket that turns the mounting point 90deg. The use of a spherical bearing to allow for rearwards inclination of the coilover and cutting and shutting the upper mounting points inwards might well accomplish the same thing.
combined with stiffer/solid bushes and maybe stiffening the beam itself might give more control over geometry changes leaving the stiffness aspect to the spring/shockabsorber.
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