H-arm linkage geometry

[AtomicLabMonkey]

Does anyone know how to calculate anti-lift/squat for an H-arm rear suspension? No books that I have access to provide a specific H-arm geometry example graphically layed out.

In RCVD, Milliken says to just determine the side view geometry in the same manner as a semi-trailing arm. However if you rotate the STA pivot axis around to be parallel with the vehicle centerline, as with an H-arm, the instant center would seem to be at infinity. Milliken also makes this same statement in another section about H-arms, that the side view IC is at infinity. Yet in the same sentence they say it's "difficult to obtain high values for anti features" with H-arms.

Given a side view IC at infinity, don't the anti percentages have to be zero? If theta-R is zero, anti-squat = zero.

Two A-arms that are parallel to each other would also seem to have an infinite IC, yet Olley seemed to think they would provide anti-dive if both were angled up towards long. chassis center. The literature seems confusing on this point. Any thoughts? Thanks,

 

[GregLocock]

Not oto sure what you mean by an H arm, is that a wishbone with a revolute joint at the outer rather than a spherical joint?

In reality you can't ignore compliance effects, which affect the hard points a little and the load paths a lot.

Yes, you can shift SVIC around by angling the inner arm points. Whether you actually get the modification in vehicle behaviour you were after depends on a lot of things. I have seen cars with anti squat and pro dive on the rear suspension, 5 cents gets you a dollar that if the hinge lines for both arms were shifted to lie almost parallel to the vehicle x axis the ride would be vastly improved and the performance under accel and decel would still be ok.





Cheers

Greg Locock


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

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[AtomicLabMonkey]

Description is in the attached file.

Not really interested in compliance effects at the moment, just determining the theoretical values of anti-squat/lift (if any) for an H-arm & camber link suspension. Thanks for any help,

 

[AtomicLabMonkey]

Didn't seem to take my attachment earlier. H-arms were used for example on the 90s Thunderbird rear, or the current GMC Acadia. The lower arm has 4 pivot bushings instead of the 2 + ball joint of an A-arm, and the upper link is a 2-force member with a pivot bushing on either end that only handles a component of the lateral loads.

http://us.tnpv.net/2006/WKA200607/WKA2006072159875_pv.jpg

 

[GregLocock]

OK, that's what I thought. So, the upper link doesn't react traction to any meaningful extent, so your IC is at infinity, on the hingeline of the lower arm in side view. If it were to pass through the CG you'd get no squat.

In practice, of course, things are more complex.

Cheers

Greg Locock


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

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