rear twist axle 1

[zerodof]

Can anybody tell what are the key structural compliances/attributes required in the concept design of twist axle.

What modelling tools are typically used to model the compliance/kinematics/stress as I usume Multi body simulation ( linear code) is not the appropriate because of large displacement.Any ideas/suggestions.

 

[GregLocock]

I think the early ones (eg VW Polo) were designed using hand calculations, with the strength verified by a variety of strain measurement tests.

Although using a linear FE code will give slightly incorrect results, essentially they are linear springs, so you'll get pretty close to the right answer.

Essentially you have two trailing arms connected by a torsional spring, and a bending beam (To give camber stiffness). The torsional properties of sections are fairly easy to calculate, and so is its bending strength.

Having said all that, I wouldn't design one for my own use. The fatigue issues associated with the welding of the beam to the arms are quite serious.


Cheers

Greg Locock

 

[zerodof]

Thanks Greg for the valuable info. What is the difference between roll steer and toe steer(in deg/mm) and under what condition I should measure this.

Do you think longitudinal and lateral compliance is also important for this type of suspension.What is the effect of these characteristics over the ride and handling. In other words what rear kinematic characteristics are most influential on R&H of the vehicle.

 

[GregLocock]

Toe steer is the change in toe as the suspension moves in jounce. Roll steer is similar, except that one wheel moves up as the other moves down.

The difference may be negligible or significant, there's no easy answer. It all depends on what you want.

The longitudinal compliance is more of a harshness/road noise requirement, I think R&H mostly just work around it. One obvious problem is that you'll get significant steer if one wheel moves backwards with this type of susepnsion.

Lateral compliance, particularly toe compliance (ie toe per unit lateral force at the contact patch) is very important.

You also need to think about camber compliance and camber in ride and camber in roll.

I don't have set numbers for any of these, I work to vehicle level targets for the understeer budget, and specific manouevres, and some other tests, rather than having rules of thumb for each particular value.

However, basically if the ride and roll steer curves are out then you will have to be very clever elsewhere. Rear suspensions should be quite neutral, I think.



Cheers

Greg Locock

 

[zerodof]

I can understand the lateral compliance but not the camber compliance. What exactly you mean by camber complince in ride and roll.
Due to large geometrical non linearity in the twist or bending of the centre beam do you think the complince should be taken at the maximum wheel travel, as compliaces comes out different at the start of the jounce and at the end of the jounce.

 

[GregLocock]

I didn't

Camber compliance is the change in camber due to a lateral force at the contact patch.

Camber change in ride and roll is analagous to toe change.

I wouldn't worry about where you measure compliance, it'll only change slightly (I think) with suspension travel.


Cheers

Greg Locock