Control arm pivot axis

[JohnSerkaian]

I've got general questions regarding front suspension control arm geometry and I'll do my best to clearly explain them: Ideally, one would assume that the pivot axis of the front control arms would be somewhat parallel to the longitudinal axis of the vehicle. (At least that's what much of the research assumes.) Vertical adjustments would be used to manipulate anti-dive characteristics but the axis of the arms, looking down from the top of the vehicle, would have the pivot axis parallel to the vehicle axis. However, some of my vehicles splay the horizontal pivot axis so much it doesn't seem to make much sense from a geometry standpoint. Two of my post-war vehicles splay the arms in a semi-trailing arm fashion so the tie rods work off centered pivots just behind the lower control arms. (The center pivot is properly positioned at the intersection of the LCA pivot axis. Typical examples - 1961 Corvette w/kingpins, 1955 Thunderbird w/ball joints.) My late model GM vehicles with front-steer steering systems use lower control arms with the rear pivot points splayed outward - opposite of the trailing arm approach of the post WWII geometry. (It almost seems like GM tossed out "proper" geometry to make plenty of room for their clunky starter motors and exhaust pipe routings.) Two examples were my 1977 Caprice and 1978 Firebird (same suspensions) which rode and handled fine in a variety of street and track driving conditions. So.... What are the reasons early IFS systems splayed the control arms in a semi-trailing fashion and why did GM go to the other extreme with some of their modern IFS systems? I can see some possible logic behind the trailing-arm concept for reduced ride harshness but I can't see how any deviation from a parallel axis could improve handling. Anyone out there that can shed some more light on this topic? Thank you.

 

[GregLocock]

I'd be interested to hear as well.

I notice that some relatively modern IRS like the Lincoln LS use upper arms that are almost semi trailing - I've never seen the point. Is wobbling the castor angle about so helpful? I suppose you can then do some fancy things with the toe curve and so, but really, we chase linearity, not fancy curves.

The Tbird IFS, is a very 'square' design, which agrees with my prejudices.





Cheers

Greg Locock

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

 

[NormPeterson]

Deviations from the (plan view) parallel axes change the caster gain curve (and camber while turning). In the GM instances, I think this works opposite to the effects of the anti-dive, for a little more and slightly increasing understeer as the wheels are steered further from straight.

That's the way I read what was happening in the rather similar front suspension on the '79 Chevy Malibu I used to autocross. That car worked OK in the larger radius sweepers but felt disproportionately clumsier in most slaloms.


Norm

 

[GregLocock]

Wow, you and I posted simultaneously.

I've just thought of one very good reason not to use semi trailing upper arms - typically in a rubber bush the axial rate is less than 25% of the radial rate. As you make the upper arm into a semitrailing arm, the toe, camber and lateral stiffness of the suspension (which are all worth worrying about) start to 'see' the axial rate of the upper arm bushes.

Cheers

Greg Locock

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

 

[NormPeterson]

Indeed. Right off, I had the oddest feeling that I'd hit the 'submit' button twice.

Something else was going on in the US domestic car market between the two 'eras' mentioned in the original post, that being the near universal switch from unbelted bias ply tires to steel-belted radials.

If I'm not mistaken, GM also played some odd games with the front view instant centers during this time (placing them outboard of their own wheels, letting the camber gain go the 'wrong" way, and pushing the geometric roll center below grade) in some models.


Norm

 

[GregLocock]

My guess is that they decided that other things ($$) were more important than textbook handling.



Cheers

Greg Locock

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

 

[Fabrico]

Off-road racing has been-there-done-that too. There are two significant problems and one disappearing advantage to a semi-trailing LCA. Nowadays they are either parallel or just as many if not more, are wider in the rear or “semi-leading".

The first problem is that despite other efforts, dive is nearly impossible to control. A semi-trailing setup not only fails to significantly counteract dive, it all but feeds it. While certainly not a cure, a semi-leading control arm slightly increases anti-dive at as the front end goes down.

The second problem was that even with chassis and control arm reinforcement, and use of semi-rigid bushings, hitting bumps with a semi-trailing arm causes the trailing angle to increase. This causes wobbly steering. At nearly all times, a semi-leading arm transfers considerably less lateral leverage from bumps to the chassis.

For off-road at least, one of the most tangible measures of ride harshness is vertical wheel rate over bumps. The disappearing advantage to a semi-trailing lower control arm is that it does not reduce wheel rate unless it is pointed downward. From level on up, vertical wheel movement is just as fast, or faster, than a double wishbone setup. From level on up, a forward facing, or semi-leading LCA, provides a slower vertical wheel rate than a trailing arm.

The chassis of a higher powered off-road car or truck can spend a considerable amount of time nose up at anywhere from 5 to 10 degrees, not unlike a boat. As far as castor, with a semi-leading arm setup, when the lower control arm is down the front of the vehicle is usually up. For a majority of the time, the castor increase from vehicle attitude cancels most of the decrease from the LCA moving back. During braking there is a decrease in castor not only from the arm moving back but also from the vehicle nose down attitude. At times of high traction (read panic!), the compounded negative castor angle seems to be quite advantageous for maintaining control.

Not sure how this relates to old street cars and the fact that "bumps" and wheel travel can be subjective, some of it may be a clue.

 

[JohnSerkaian]

Thanks all. This has been a nagging question I first asked many years ago to my college (GMI) professor, Simone Mola, after I read some of Maurice Olley's research papers on IFS and IRS systems. The rest of the Automotive profs didn't have an explanation, either, so I thought someone reading this forum might have been around when these or other suspension systems were developed to know the "logic" behind the decisions for such drastic deviations.(Mola did provide some first-hand information on GM's penny-pinching that resulted in the early Corvair swing-axle IRS. He, and others, had to pitch their files containing some damaging analysis that GM didn't want getting into the "wrong" hands.) So... I'm still curious if anyone out there can shed some additional light on this control-arm pivot axis topic. Thanks again.

 

[Fabrico]

You might check into GM's technical journey with the front end of the first Monte Carlo's. A lot of it was based on theories from Mercedes.

Another oddity in moderate numbers is Audi's split upper arm with two upper ball joints.

You may not find a definate answer as there are still dozens of signicantly different front ends who's makers all think are swell; and a dozen, or so, of those who think their's is the best.

 

[GregLocock]

Oh, I can see why Audi used a double ball joint upper arm. It allows you to move the steer axis outboard, into the tire. This reduces KPI and gives you the ability to set the scrub radius exactly where you want it.



Cheers

Greg Locock

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