Rear Torsion Suspension - How does it work?

[cunning plan]

Okay, so I understand the front, the leaves are held in place and work with / against each-other to provide spring.

However, I am really not sure how the rear works? :?

So the torsion arms go into the splines in the rear axle and the other end connects to the spring plate. BUT what is IN the axle? What makes it 'spring'?

Instead of the axle getting bigger or staying the same size to house a spring mechanism, it actually gets smaller, almost like there are two axles.

:sign0009:

 

[dubdubz]

The rear is in 2 sections

the torsion bars are just that - independent.
The splined torsion are secured at the middle with splines in the same way the springplate has splines

the front also operates on each tube don't forget - they are tied of course at the hubs.

 

[yachty]

The torsion arm is the spring mechanism. It twists under load and then springs back.

 

[cunning plan]

The rear is in 2 sections

the torsion bars are just that - independent.
The splined torsion are secured at the middle with splines in the same way the springplate has splines

the front also operates on each tube don't forget - they are tied of course at the hubs.

The torsion arm is the spring mechanism. It twists under load and then springs back.

:shock: Ahh thank you, that explains it. I am pretty amazed that the metal 'rod' can be twisted like that without breaking :shock: I presume it is a composite of metals?

I guess it is not too different from a coil spring where the metal has been cast in a certain position and wants to stay in that position, which then provides the spring mechanism. 8)

 

[Trikky2]

I guess it is not too different from a coil spring where the metal has been cast in a certain position and wants to stay in that position, which then provides the spring mechanism. 8)

Absolutely right. A torsion bar could be thought of as a straightened out coil spring. Lots of cars used them including for example, the front of Morris Minors, 70's Simcas and all round on the Citroen Traction avant (Maigret car). On the front of these cars the torsion arms ran along the front chassis rather than across it of course, because the engine had to be accomodated.

Their advantage is smaller wheel arches (no macpherson strut to accomodate) whilst their disadvantage is they weigh more and the costs are higher than a coil spring/strut setup.

 

[Haveacamper]

We must not forget the influence the trailing arm has on th wheel travel...

let us consider the vehicle at stock height without a trailing arm. when the wheel travels up, the wheel travels in an arch with the arch length created by the spring plate. the wheel will obviously rotate about the torsion bar which has a linear stiffness. at stock height, as the wheel travels up it will also intially travel backwards until the springplate is horizontal. although only by a small amount, this does change the wheel base between the front and rear wheels. Without the trailing arm, there would be zero camber change as the wheel travel up. an example of this is if you picture the front trailing arms as 1 springplate, when you lower the front of your bus the camber angle remains unchanged.

the introduction of the trailing arm provides the camber change with vertical wheel travel. how? well, as we know, the trailing arm is mounted to a cleat on the outside of the torsion tube/chassis by means of a bush. the axis at which the trailing arm rotates is slightly offset from the axis of rotation of the springplate (which is the centerline of the torsion bar). this offset, in rotation means that when the wheel travels the vertical direction, it also travel inboard at a predecribed motion. this is dependent on the position of these axis and the properties of the bushes used in the springplate covers and the trailing arm. if being pedantic, this setup is actually a semi trailing arm. this setup will also provide a level of toe change with vertical wheel travel.

I hope this doesnt confuse you more. if you want me to clarify anything just let me know.

you know notice a level of enthusiasm. This is because i have recently started working at Ford of Australia, modelling/designing suspension systems.

on a personal level i would love to get modelling our campervans to see exactly how good the "original" setup was compared to all the aftermarket suspension mods, from lowered buses, horseshoe plates, adjustable springplates, dropped spindles, coilovers, and the Red9Design wishbone suspension. none of which have actually quantified these "improvements".

NaFe

p.s. Trikky2, not to mention they are generally good over a large weight range similar to leaf springs

 

[Trikky2]

NaFe

p.s. Trikky2, not to mention they are generally good over a large weight range similar to leaf springs

True

Nice post Nathan - always enjoy a good read from a fellow geek :lol:

 

[Haveacamper]

hehe. thanks. i'm learning a hell o fa lot really quickly here.

the most difficult thing i will stuggle with modelling the camper is getting all o fthe geometry...so all of it lol

 

[cunning plan]

Very interesting! arty0051:

I watched a video on YouTube showing spring-plate adjustment before I attempted mine and it showed the importance of the trailing arm position on the spring-plate as it has an effect on the camber and suspension geometry in general.

Although I am now putting a brand-new spring plate on, so marking the plate doesn't help. :|

 

[Tristan71]

If your removing your spring plates be careful they are lethal there under a huge amount of twist as soon as they come off that lip ....... Bang

 

[cunning plan]

Thank you for the warning! Everyone reading should take note of this, but my bus was already lowered and there was literally no tention at all :?

I just unbolted everything and slid it out! :shock: