Double ball joint suspension - missing link?

[NoahLKatz]

I never paid much attention to these until now but have become interested after reading that they can improve the ride/handling tradeoff.

A conventional A-arm is a structure, but the 4-bar resulting from replacing it with two links is a mechanism, where a longitudinal force/motion would tend to rotate the spindle.

Isn't a 5th link required to account for the added DOF?

Even with zero scrub radius it would seem that a one-wheel bump would cause steering kickback.

 

[GregLocock]

They aren't the easiest thing in the world to design, but no, they can have perfectly normal steering and handling. First generation X5 is an excellent example (note that they went back to an A arm afterwards). My recommendation would, in general, be to steer clear of them unless you are desperate to get the steering axis located inside the tires cross section top and bottom, ie reduced KPI and scrub or if you've got some truly horrible packaging problems with the rack.

So the story goes - get the steering axis in a relatively neutral place (ie centrepoint steering). This gives smaller tie rod loads. This reduce the load on the rack and allows you to use a lower torque EPAS. At the time EPAS was not really suitable for big fat heavy cars, so that was the only way to get EPAS in for them.

Here's an interview with one of the team

https://www.carsales.com.au/editorial/details/turning-away-from-macpherson-struts-24476/

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[NoahLKatz]

Thanks for the link, though he didn't really say anything specific enough to increase my understanding.

Upon first reading about these, I expected that there would be a 5th link to restrain the new DOF and that one end would have a rubber pushing to allow wheel recession, but that wasn't borne out.

I read several places that the benefit is uncoupling of suspension parameters which can improve the ride/handling tradeoff, but none said how.

As to my specific questions, is anything I said incorrect?

If not, I'm still puzzled.

 

[GregLocock]

At any given time the two arms form a virtual A arm, managing the migration of that focus point is a large part of the problem with the K&C of these things. Note that BMW didn't get it 100% right, at full lock when parking the steering goes over centre, which is not a surprise and delight.

Bushing design is important, at least with the architecture I am familiar with which is basically a lateral link, and a leading link. The body end bush of the lateral link is practically a ball joint, in properties, the front arm body end bush is a complex hydrobush, and is inherently fairly soft, for impact harshness, but snubs at higher loads.

As such there is a lot of precession in impact/ single wheel bump, but as the tie rod is parallel to the lateral arm, that 4 bar mechanism minimises toe changes.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?