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Side Load Influence in McPherson suspension.

Side Load Influence in McPherson suspension.

(OP)
Hi guys.

I'm new to this forum and considered some of the things I've read this is a valuable source of knowledge.

My name is Tiago, I'm a Mechanical Engineer from Brazil and I'm currently developing a study for my specialization course in Automotive Engineering about the side load influence over the wear and leakage of the struts in a McPherson suspension.

I'd like to know more about the influence of this side load in vehicles to answer some of my proposed questions on my Final Paper. For example:

- How does it affect the handling and comfort of the vehicle?

- How does it affect the life cycle of the strut? (I understand that this phenomena increases the dumping force and causes unnecessary friction to the inner tube)

- What kind of compensation should be done by the spring? Should it be 50% of the force? 100%? 10%?

- Can it be compensated, at least partially, by the geometry angles? How? Which ones?

If you could answer me in order to help me address these questions and enlighten me in which books or some paper to seek this information, I'd be very pleased.

Thank you very much in advance.

RE: Side Load Influence in McPherson suspension.

A lot of your questions about the geometry could be answered if you draw free-body diagrams of the strut with the spring at various positions (centered, off-center, etc).

Keep in mind that it is one thing for the forces to be more-or-less balanced when driving straight ahead, but it all changes when you drive fast around a corner, so one way or another, the strut has to take up significant lateral loads.

Motorcycles have (essentially) a strut front suspension without the benefit of a lower control arm to guide the bottom of it, and it works. Numerous attempts have been made to "solve" this "problem" over the years, and every attempt has had disadvantages far greater than the "problem". They use guide bushings with antifriction coatings on the inner and outer fork tubes with the mating surface ground very smooth (and some have specialized coatings such as DLC or TiN to reduce the friction and increase the wear life even further), and the whole deal runs submerged in oil. The design of the forks separates the upper and lower guide bushings by as far as possible to reduce the moment loading on them. The newer and more expensive designs are "inverted" - the inner, thinner tube is inside the outer, upper tube - to increase their stiffness.

I haven't had a MacPherson strut apart, but one would think that the same concepts are applied.

RE: Side Load Influence in McPherson suspension.

Bear in mind that few motorcycles exceed 20000 (to be generous) miles between suspension rebuilds, and their rods are significantly bigger in diameter, and their damping loads are less, and the piston to top seal distance is greater. Obviously the bending moment is much greater on a motorcycle, of the order of 3kNm, whereas on a car we try to keep it to significantly less than 100Nm .

I don't know of any generally available papers that go into this, sadly.

Cheers

Greg Locock


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