Thread: Articulation

Originally Posted by wagoo

John,Something you might like to calculate and discuss.
I was just rethinking a statement on my earlier post, where I said I could not visualise any forces either torque or gravity induced that would cause the wristed arm to unload the suspension on that corner, and I still believe that to be true, the arm is passive aside from for/aft location of the axle But when looking at it from another angle, what forces on an unhinged radius arm contribute to stability? RAs are all about antisquat/antidive.When the front axle is pulling, the torque reaction forces from the axle housing impart a downward force at the radius arm chassis mounting points. The wristed arm on the left can not pull down on its side, wheras the normal radius arm would pull down on the right, so the vehicle should theoretically lean to the right,depending on how much tractive effort the front axle had to contribute to climb a steep offcamber right hand down gradient.Propshaft torque should also contribute to right hand lean. So although not borne out by my earlier tests, I can imagine the vehicle being less stable than with standard RAs in that scenario than the opposite one, where the steep off camber puts the vehicle in a left hand down attitude.In this instance the right side radius arm would pull the right chassis rail down, once again assisted by propshaft torque, but the left wristed arm wouldn't pull its side down which should lead to a more stable situation,compared to standard RAs.
So, is it swings and roundabouts ? And are there any disadvantages compared to an offset(either side) 3 link? And is there a case for putting the wristed arm on the diff side to counter propshaft torque?
Wagoo.

For front radius arms, assume we are looking at any (left/right, or normal/hinged) arm from the right side of the vehicle - then vehicle forward direction is left to right.

During forward acceleration front radius arms will be in tension (in order to pull the vehicle from their chassis mounts). To resist anti-clockwise rotation of the axle housing (reaction to torque at the tyres) normal radius arms will pull down on the chassis mounts. If one arm is of the hinged type then the normal radius arm alone must supply practically all of the torque resistance (greater pull down on the chassis mount that side) - hinged radius arms can resist a little rotation by virtue of the resistance to rotation provided by the flexible bush, but this can be ignored for practical purposes.

The hinge joint of hinged radius arms is usually (when made from a stock radius arm) a considerable distance behind the axle housing. Given the direction of the tension force acts along a line from the chassis mount to the hinge, the tension tends to resist the torque on the axle housing when the axle is articulated so the side with the hinged arm has drooped down while that with the normal radius arm is near the bump stop. When the axle articulates in the other direction, the tension in the hinged arm can (depending on suspension geometry) add to the torque acting on the axle housing, and the normal arm then has to provide more resistance (greater pull down on the chassis mount that side).

During axle articulation, if either type of arm is inclined upward from chassis to axle mount (or hinge joint), there will be a component of this tension acting upward at the chassis mount and downward at the axle mount. This will be opposite when the arm is inclined downward - this component will then subtract or add to the downward force on the chassis mount by the normal radius arm.

Just a little trick-haven't seen it in this thread yet so apologise if its already covered.
When raising the rear suspension(like I have on my defender) the A member pulls the top of the diff forward causing more severe tailshaft alignment and the more severe the angle of the A frame gets compared to the tailshaft the worse this gets and the more backwards and forward movements occur with suspension travel. Adjusting diff/tailshaft alignment using adjustable radius arms doesn't change this effect (but does save unis)as the radius arms are so much longer than the A frame.
I found a good compromise was to machine up 4 spacers and use them to raise the A frame ball joint 50mm above the standard mount position on top of the diff.

Originally Posted by farmport

Just a little trick-haven't seen it in this thread yet so apologise if its already covered.
When raising the rear suspension(like I have on my defender) the A member pulls the top of the diff forward causing more severe tailshaft alignment and the more severe the angle of the A frame gets compared to the tailshaft the worse this gets and the more backwards and forward movements occur with suspension travel. Adjusting diff/tailshaft alignment using adjustable radius arms doesn't change this effect (but does save unis)as the radius arms are so much longer than the A frame.
I found a good compromise was to machine up 4 spacers and use them to raise the A frame ball joint 50mm above the standard mount position on top of the diff.

good idea, but doing so will raise the roll center and rasie the antisquat %, which has already been raised buy lifting the vehicle with longer springs....what you can do is make an adaptor that rasies the arm portion of the A frame and leaves the ball and fitting at stock postion. you can also make adjustable length trailing arms and dial the pinion angle back in....

Originally Posted by uninformed

what you can do is make an adaptor that rasies the arm portion of the A frame and leaves the ball and fitting at stock postion. you can also make adjustable length trailing arms and dial the pinion angle back in....

Hey I always have done this to relieve the angle on the ball joint.
It's been a while since I brushed up on my knowledge of suspension geometry but I believe it changes nothing with regards to the suspension geometry or characteristics.
While it seemingly changes the angle of the upper a frame links the points of interest in calculating geometry are all still the same, yeah?

Originally Posted by uninformed

good idea, but doing so will raise the roll center and rasie the antisquat %, which has already been raised buy lifting the vehicle with longer springs...

I prefer the method of spacing down the a frame mounts at the chassis as opposed to spacing up the ball joint mount itself.
Thus lowering roll center and increasing anti squat which seems to work effectively on rovers with the square rear cross member.

Originally Posted by Grimace

Hey I always have done this to relieve the angle on the ball joint.
It's been a while since I brushed up on my knowledge of suspension geometry but I believe it changes nothing with regards to the suspension geometry or characteristics.
While it seemingly changes the angle of the upper a frame links the points of interest in calculating geometry are all still the same, yeah?

correct, the bushes and ball joint center remain in stock location so nothing changes geometery wise


I prefer the method of spacing down the a frame mounts at the chassis as opposed to spacing up the ball joint mount itself.
Thus lowering roll center and increasing anti squat which seems to work effectively on rovers with the square rear cross member.

While it would increase the antisquat, it would not change the roll center as the ball joint center is still in stock location. I would think raising the AS on a lifted vehicle which will already have its AS raised and if running bigger tyres raised again even more, not be the best idea.....IMO

cheers,
Serg

Just a thought.

with a 2" spring lift does it tension the sway bars? would 2" block under each mounting point bring them back to 'standard'

Originally Posted by blitz

Just a thought.

with a 2" spring lift does it tension the sway bars? would 2" block under each mounting point bring them back to 'standard'

No the swaybars are only tensioned with bodyroll/articulation not spring lift, but lowering the mounting points will prevent the swaybar links from running out of travel and overcentreing.
Wagoo.

Originally Posted by wagoo

No the swaybars are only tensioned with bodyroll/articulation not spring lift, but lowering the mounting points will prevent the swaybar links from running out of travel and overcentreing.
Wagoo.

Agreed - I removed my rear swaybar from my 5dr D1 after fitting a two inch lift as I was finding that the swaybar links would rotate 180 degrees and then bind against the rear spring seat and tear out the rubber bushes. When doing my research on lifts I never heard mention of this happening before so I guessed it was just my setup. I always planned to put in a custom spacer, but now I'm used to the extra on-road roll and appreciate the extra axle movement when off-road, so probably won't bother.