McP strut theory question: Grassroots Motorsports forum: Grassroots Motorsports Magazine

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July 20, 2011 11:22 a.m. ReverendDexter SuperDork

I'm curious what it is in either the geometry or motion of a McPherson strut that allows them to be used in both what are considered very good and very poor handling cars.

For example, I know that it's used under the fox-chassis Mustang and my understanding is that it's used under both BMWs and 911s.

I'm assuming that it has to do with things like length of the LCA, location of the upper strut mount; length of strut; geometry of the spindle; relative rates of springs and bar in comparison to each other, overall vehicle weight, and weight distribution of the vehicle, etc.

Can anyone get me further dialed in, or is this a case of "read the crap out of Race Car Vehicle Dynamics"?
July 20, 2011 11:32 a.m. triumph5 Dork

All those things plus proper design to begin with, and chassis/unibody torsional rigidity. For example, what's one of the first items added to a race car with struts? Strut tower brace. Good design is only good if there's good building to execute the design.

And, of course, the intent of the usage of the vehicle they're bolted to.
July 20, 2011 12:22 p.m. Dr. Hess SuperDork

The problem with McP struts is the lack of camber gain with body roll (bump). People compensate for that by setting the static camber excessive, like -3 degrees, so that when the body is rolling 4 degrees, the tire is only +1, assuming zero camber gain, which would still be pretty close to 0 to -1 where the tire is best. Or, -3 + 3 = 0, which would be acceptable, etc. All the rest of that stuff matters too, but it is really just maximizing the utility of a crap geometry at that point.

The advantage of the strut design is that you can build the suspension much tighter, giving you more engine room bay area or a smaller car for the same engine size.
July 20, 2011 12:41 p.m. donalson SuperDork

also you can use softer springs as the compression tends to be much closer to 1:1 (ie 250lb spring is 250lbs at the wheel)... where a dual wishbone is cantilevered... which is why you can use 1000lb springs and it still not be harsh...
July 20, 2011 12:50 p.m. Matt B Dork

While I like the direction your headed, I'd be careful to compare those cars you mentioned on a macstrut-vs-macstrut basis since they're really only used in the front. I'd bet the real differences in handling come from the rear suspensions.

IIRC, their rear suspensions are either relatively sophisticated independent multi-link setup in the rear (in the case of the Europeans) or a live-axle (Mustang, of course).

That said, I'd still like to see a comparison between the front macstrut geometry on a bmw vs. a mustang. I know Mustangs suffer badly from overlowering, as any macstrut car. However, I don't hear the same about the Bimmers. I could be wrong, but it'd be cool to see the differences.
July 20, 2011 1:16 p.m. GameboyRMH SuperDork

Cars with McPherson suspension handle best using the Go-Kart Theory of Suspension Design: Any suspension can work, if you don't let it.

Travel is the enemy of McPherson suspension geometry. The arm is somewhere near level when the suspension is at rest (ideally the arm should be hanging down from the car, but if the car's designer really didn't give a damn what he was doing, it might even be pointing up, which will give you a backwards camber curve!). Beyond a certain point away from the level position, the camber curve of a McPherson strut will go flat (camber stops changing) or even reverse itself. This is bad. So to keep the suspension in the butterzone you need to limit suspension travel (i.e. hard suspension). Longer control arms aren't a good option because that would mean even less camber gain.

So in summary, things that can ruin McPherson suspension are long travel and level/upward control arm angles at rest. Things that make it handle well are short travel and downward control arm angles at rest.

Also the fact that most of the good-handling cars with MacPherson suspensions are lightweight little cars is no coincedence. It's a case of awesome + lame = pretty good.
July 20, 2011 1:39 p.m. theenico

Caster.

Most of the good handling Mac strut cars (Porsche, BMW, etc.) have lots of caster. As such, they gain camber as the wheels are turned/steered.

Back when I had my ITB class Scirocco, I went to great lengths to get 7 degrees of caster. Custom camber plates to move the top strut mounting rearward combined with offset bushings to move the control arm forward allowed me to get this.

A side benefit is that I didn't have to run as much ststic camber as the other guys in front drive cars.
July 20, 2011 1:52 p.m. Strizzo SuperDork

well, longer arms would stretch the camber curve, so you still might gain 1.5 degrees before the curve goes flat or reverses, but it would be over a larger range of travel. then you could also have the inner LCA bushings much higher than the outers (control arm "droops" at normal ride height), but that jacks with your roll centers.

nothings free, of course
July 20, 2011 2:23 p.m. ReverendDexter SuperDork

Interesting you bring up the resting angle of the LCAs... I've never even thought about them sloping down towards the wheel as an option, and that the loss of negative camber under compression was just a given.

That makes SO many things I've read about LCA relocation finally make sense!

Can you explain what the resulting "jacking with your roll centers" would do in terms of behavior of the car? My understanding is that would raise the roll center, making for a quicker transitioning car with more "leverage" to create body roll... is that close to right?
July 20, 2011 5:02 p.m. iceracer SuperDork

theenico wrote:

Caster.

Most of the good handling Mac strut cars (Porsche, BMW, etc.) have lots of caster. As such, they gain camber as the wheels are turned/steered.

Back when I had my ITB class Scirocco, I went to great lengths to get 7 degrees of caster. Custom camber plates to move the top strut mounting rearward combined with offset bushings to move the control arm forward allowed me to get this.

A side benefit is that I didn't have to run as much ststic camber as the other guys in front drive cars.
July 20, 2011 5:06 p.m. iceracer SuperDork

iceracer wrote:

theenico wrote: Agree, I installed offset bushing in the control arm which nearly doubled the stock caster. On my ZX2SR it it was a big improvement. I could run less static camber and still get good tire temps. Caster.

Most of the good handling Mac strut cars (Porsche, BMW, etc.) have lots of caster. As such, they gain camber as the wheels are turned/steered.

Back when I had my ITB class Scirocco, I went to great lengths to get 7 degrees of caster. Custom camber plates to move the top strut mounting rearward combined with offset bushings to move the control arm forward allowed me to get this.

A side benefit is that I didn't have to run as much ststic camber as the other guys in front drive cars.
July 20, 2011 5:37 p.m. Hasbro Dork

All of the above on caster

X1/9 stock caster - 7*

Civic Si EP3 - 0*