True side view IC for Mcpherson Strut 1

[JakubMech]

Just after a comfirmation in regards to the true side view instant center for a mcpherson strut suspension system.

The IC point needs to lie in the wheel center plane, projecting the lower control arm to the plane is the same as with a SLA system.

However for the strut top, is it projected onto the plane by a line normal to the strut axis when viewed from the front? This would mean that if there was say +15 deg KPI, then the line would be from the strut top at +15 deg to the horizontal when looking from the front, until it hits the wheel plane.

Jakub

 

[crseng]

Luckily Jakub its not that complex. In the side view simply extend the lower control arm line in the plane of the wheel center. At the top of the strut draw a line normal to the strut center line axis in the side view to intersect the lower control arm line. The intersec is the longitudial view IC.

the from view works the same for the later IC. Connecting the Lateral and Long. IC's gives you the IC axis of the wheel.

chris

 

[JakubMech]

Thanks crseng,

Im ok with the basic idea of the side view IC, I was just wonering about being absolutly correct in the position of the IC.
As you know, the lower control arm inner points are projected onto the wheel center plane then projected back to intersect the strut top line (forming the side view IC). I was wondering if anyone projects the strut top point to the wheel center plane first then back to the intersection, as with large spindle lengths and KPI values the strut top can be some distance from the wheel center plane.
I actually did a quick calc and found that projecting the strut top to the wheel center plane first (with a line normal to its axis in the front view), then projecting back to get the IC yielded little difference with the quicker way.

Jakub

 

[NormPeterson]

For “absolutely correct” positioning of the IC’s you DO need to work your strut upper attachment projections in a plane through the strut top that’s perpendicular to a line through the strut top mounting and the ball joint in the control arm.

This plane is analogous to the plane defined by an upper control arm (chassis pivots and the upper ball joint) in an SLA suspension (except that the strut plane must be perpendicular to your caster and SAI, while the plane of an SLA UCA need not be), and its intersection with the plane of the control arm (defined by the chassis pivots and the ball joint) is the instant axis of the wheel.

So unless you have a rather unique case with either zero caster or zero SAI this plane will necessarily not be parallel to any global axis. FWIW, I see a difference in the SVSA length computed by the two methods as being a little over 6% for a SAI of ~15*, a strut length of ~24”, and a strut top located ~6” inboard of the wheel center plane.

Norm

 

[JakubMech]

Thanks NormPerterson,

Thats exacly what I did, transfered the strut top ball joint to the wheel center plane by following a line perpendicular to the strut in the front view.

This was for a vehicle with about 16 degrees KPI, 4 degrees castor, and doing some quick clacs It seems the difference I got between the two methods is about 5%, just as you quoted.

Thanks again,

Jakub

 

[NormPeterson]

Caster also has a small effect on your IC calcs, since that parameter typically moves the strut top out of the transverse vertical plane through the front axle. And since that's also the longitudinal location where the line perpendicular to the strut in front view intersects the wheel center plane, you need to either project that point back to the front axle centerline along a perpendicular to the caster angle or calculate the intersection based on the longitudinal offset of the strut top from the front axle centerline. At least this is a very small correction, on the order of 1%.

Norm
I keep forgetting that I can't edit on this forum . . .