Trailing arm roll center

[jayfeather]

I am designing the suspension for an SAE baja car and it is the first year our team is designing a trailing arm rear suspension. I was wondering were exactly the roll center of a trailing arm rear suspension would be. on the ground? Or does it even have one?

 

[GregLocock]

That depends on what you mean by a trailing arm suspension.

A good way to identify its likely position is to ask - where does the lateral force get reacted into a suspension memeber that connects to the body?

Another way of thinking about it is if you are after 'free' roll stiffness, is how does this prevent body roll?

FWIW many of the definitions of RCH only agree to within 50 mm or so, at straight ahead, and get worse as you go into roll.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[jayfeather]

The best way to describe the trailing arm style would be like 2 single sided motorcycle swing arms, trailing straight along the longintudinal axis of the car. the axis about which both trailing arms pivot would be completly along the lateral axis of the car. Since the trailing arm would have no camber change during ride would there even be an instantaneous center?
FWIW?

 

[PTwizz]

Another way to visualise the problem:

If you apply a lateral force to the body at roll centre height, no roll will occur.
Apply the force above roll centre height and the body will roll away from the force, apply force below roll centre height and the body will roll towards the force.

Your rear roll centre is at ground level.

Pete.

 

[GregLocock]

I bet it isn't! - in practice, I think it'll end up at around axle height.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[PTwizz]

My logic runs thus:
Using the conventional means to determine roll centre height, the projected instantaneous centre of motion of each wheel is an infinite distance away to the side. A line connecting that centre to the contact patch will therefore be at ground level (because it connects to the contact patch). Therefore the roll centre must be at ground level. This is only true when there is no roll in the body (try to draw the diagram with some roll in!).
In a more intuitive vien, any lateral force applied to the body above ground level will cause an increase in vertical load at the outboard wheel. As the wheel can only move vertically (relative to the body) the suspension will compress and the body will roll.
Force applied at ground level produces no increase in vertical load, so no roll.
Another way to look at it is as a simple system of two vertical sliding pillars, each pin jointed at the contact patch. The body can slide on each pillar. The only centres of rotation are at ground level, the pillars must remain parallel, so the centre of body rotation must be at ground level.
I can picture this in my head - when Eng-Tips installs the direct brain interface these explanations will be a doddle!

Pete.

 

[jayfeather]

Pete,

This was the first thing I drew when we were talking about the trailing arm on my team and the discussion moved to the white board. This is a definition that I have been trying to conceptualize. In the rear view plane of the car, there is a contact patch center point, and a point directly vertically above this point that lies on the trailing arm pivot axis(essentially calling this the I.C.). By cross connecting these points you would get an intersection that would be somewhere around axle height that would change during roll as I would intuitively expect.... maybe?

Joe

 

[PTwizz]

My understanding was that IC would be at an infinite distance to the side, on the basis that the vertical motion represents an infinitely large radius.

What this probably shows is that the conventional means of establishing RC height does not hold true when values are very large (or small).

I doubt that axle height has any bearing on roll centre height in this case. If the axle height was doubled, the kinematics would be unchanged.

In situations like this I often find myself reaching for the Meccano.

Pete.

 

[GregLocock]

I agree, if you have an idealised pure trailing arm the geometrical RCH will be zero at zero roll.

However every additional piece of reality that you add to the model will lift the roll centre, I suspect.

As to where the force based roll centre is, heaven knows.




Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.