Veloster rear suspension
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From: Floating around the AUDM
Vehicle: X3 Sprint, S-Coupe Turbo
Veloster rear suspension
I've been looking at the rear suspension of the Veloster with interest. It's nothing revolutionary but I thought I'd share my thoughts on the matter... Here are some pictures of the rear axle. I simplified it with a top-down view in paint.
and with wheels, springs, and dampers
It is called a 'twist beam' rear axle. The large beam that runs across the car acts as both the axle and a swaybar - that is it will actually twist to resist one wheel bumps and body roll. This is all pretty regular stuff. Now the part that is perplexing me: the rear pivots are not perpendicular to the centerline of the car.
This is done because the pivot bushings are locating the axle, ie they are what holds the axle to the car. If they pivoted horizontally (ie the same way as the hood pivots), then there would be nothing to stop the axle scooting sideways from under the car under cornering load. The bushings have to point sideways a little bit so that they are capable of resisting cornering forces.
Buuuut...
THIS MAKES FOR SOME PRETTY NASTY FORCES ON THE AXLE.
I drew some more diagrams to help explain what I mean. Imagine that a t00ner installs lowering springs on their veloster, or has a heavy load of cold air intakes in the trunk. The veloster's bum is now sitting two inches lower than it was to begin with. The axle pivots around its pivot points and is resting closer to the floor of the car. Now consider the fact that the axle's pivot points are attempting to follow a different curve than what the axle has actually taken... what now?
The green arrows are the arc that the axle follows - the red lines arrows are the axes about which the axle is trying to follow. That difference in paths simply creates a force on the axle. But that force has to go somewhere or else the axle will asplod - the pivot bushes twist in their mounts and take the load. So in the lowered veloster, the rear axle and its mounts are being permanently stressed. But that is not all. Let's take a look at the suspension from side on:
The purple and blue arms are the trailing links. At standard ride height, they are roughly paralell to the floor, as you can see in the first picture. Now imagine that the veloster takes a left hand turn. The body rolls to the right, and the outside suspension arm compresses. Both suspension arms follow the same arc, so the axle remains paralell to the center of the car and is pulled forward by the shorter arms. Now take a look at this lowered veloster:
Because the axle is already tucked up under the car at rest, we get some funky geometry going on. When you start to corner and the body rolls, the outside suspension arm compresses even more. Now what matters here, is that the inside wheel actually moves backward: the suspension arm that was previously compressed has to move back to its fully extended position before it can compress again. This translates to initial roll understeer (the rear axle is steering into the corner) that will possibly then transition into roll oversteer, depending on how low the t00ner has set the ride height.
Not to forget that the lowered car is torturing its mounting bushes ALL OF THE TIME. The lowered car is trying to squash the axle together ALL OF THE TIME. Make of that what you will.
Some other points to note about this twist beam rear suspension:
-The rear sway bar is welded into place, inside of the twist beam itself. This will make changing it extremely difficult - you better trust your welder...
-The shock absorbers are going to be absorbing lateral force as the axle shuffles sideways under cornering: they will also have to bend when you encounter roll steer. Not only that, but they are mounted inboard of the springs to save space. This means that they aren't getting enough velocity to function as well as they should be.
In summary: the rear suspension of the Veloster makes a lot of compromises in its performance for the sake of interior space and simplicity of construction. This is NOT a suspension system you want on a sports car. It will be difficult to overcome the intrinsic weaknesses in this design if and when anybody wants to safely improve the handling of their car.
and with wheels, springs, and dampers
It is called a 'twist beam' rear axle. The large beam that runs across the car acts as both the axle and a swaybar - that is it will actually twist to resist one wheel bumps and body roll. This is all pretty regular stuff. Now the part that is perplexing me: the rear pivots are not perpendicular to the centerline of the car.
This is done because the pivot bushings are locating the axle, ie they are what holds the axle to the car. If they pivoted horizontally (ie the same way as the hood pivots), then there would be nothing to stop the axle scooting sideways from under the car under cornering load. The bushings have to point sideways a little bit so that they are capable of resisting cornering forces.
Buuuut...
THIS MAKES FOR SOME PRETTY NASTY FORCES ON THE AXLE.
I drew some more diagrams to help explain what I mean. Imagine that a t00ner installs lowering springs on their veloster, or has a heavy load of cold air intakes in the trunk. The veloster's bum is now sitting two inches lower than it was to begin with. The axle pivots around its pivot points and is resting closer to the floor of the car. Now consider the fact that the axle's pivot points are attempting to follow a different curve than what the axle has actually taken... what now?
The green arrows are the arc that the axle follows - the red lines arrows are the axes about which the axle is trying to follow. That difference in paths simply creates a force on the axle. But that force has to go somewhere or else the axle will asplod - the pivot bushes twist in their mounts and take the load. So in the lowered veloster, the rear axle and its mounts are being permanently stressed. But that is not all. Let's take a look at the suspension from side on:
The purple and blue arms are the trailing links. At standard ride height, they are roughly paralell to the floor, as you can see in the first picture. Now imagine that the veloster takes a left hand turn. The body rolls to the right, and the outside suspension arm compresses. Both suspension arms follow the same arc, so the axle remains paralell to the center of the car and is pulled forward by the shorter arms. Now take a look at this lowered veloster:
Because the axle is already tucked up under the car at rest, we get some funky geometry going on. When you start to corner and the body rolls, the outside suspension arm compresses even more. Now what matters here, is that the inside wheel actually moves backward: the suspension arm that was previously compressed has to move back to its fully extended position before it can compress again. This translates to initial roll understeer (the rear axle is steering into the corner) that will possibly then transition into roll oversteer, depending on how low the t00ner has set the ride height.
Not to forget that the lowered car is torturing its mounting bushes ALL OF THE TIME. The lowered car is trying to squash the axle together ALL OF THE TIME. Make of that what you will.
Some other points to note about this twist beam rear suspension:
-The rear sway bar is welded into place, inside of the twist beam itself. This will make changing it extremely difficult - you better trust your welder...
-The shock absorbers are going to be absorbing lateral force as the axle shuffles sideways under cornering: they will also have to bend when you encounter roll steer. Not only that, but they are mounted inboard of the springs to save space. This means that they aren't getting enough velocity to function as well as they should be.
In summary: the rear suspension of the Veloster makes a lot of compromises in its performance for the sake of interior space and simplicity of construction. This is NOT a suspension system you want on a sports car. It will be difficult to overcome the intrinsic weaknesses in this design if and when anybody wants to safely improve the handling of their car.
