Calculating suspension/chassis loads

[fastback33]

So you have a load going into the tire, through the upright, then through both a-arms? How do you know if it is torsional, axial, etc.? Just point me in the right direction, no need for a full explanation.

[Jersey Tom]

Huh??

Each tire generates 3 forces and 2 moments.

[fastback33]

Huh??

Each tire generates 3 forces and 2 moments.

So it is a force in the x,y,z axes, and moments acting through where?

Never mind, I don't fully understand loads going into the suspension. Where can I learn more? Unless you're bored, and want to explain it to me, I was trying to give you guys an easy way out.

[Jersey Tom]

Linear forces in X, Y, Z. Moments in X and Z. Can make the argument for My from rolling resistance, but most of the time you think of My being applied to the tire from the brakes or engine.

Read up on tire forces and moments. For the subject of just suspension loads, how they get generated isn't all that critical. Once you know the "black box" forces and moments... just a static (or dynamics / D'Alembert) problem.

[fastback33]

Do you have any references/suggested books? This is one thing i've been very curious about for a long time but, never really had the thought that there would be a devoted book towards this particular subject.

[mep]

Try to break it down to a force system.
You said you don’t need a full explanation just the right track to go so I give you a quick drawing of a static case.

You see one of the tires and the body of the car viewed from behind.
The car does not drive, so you just lift the body.



The wishbones and the push rod just takes either push or pull forces because they can rotate around the bearings on their ends.
The static load from the tire gets transferred by the wheel into the upright.
The point of attack might be a bit of centre so a little moment arises.
This is balanced by the top wishbone.

The vertical force on your tire is compensated by the vertical force of the pushrod.
But this creates a horizontal force also, which has to be compensated by the lower wishbone.
The same forces appear on the body of the car just in the other direction.

That is the way you have to go, when you want to know all the forces.
The next things you have to do is looking from the top on the suspension and figure out the forces.
Then you can put cornering-, accelerating-, and breaking forces to your wheel.

[Jersey Tom]

Do you have any references/suggested books? This is one thing i've been very curious about for a long time but, never really had the thought that there would be a devoted book towards this particular subject.

It's simple statics, man.

[The_Man]

It's simple statics, man.

Thats a nice cover.

[speedsense]

Do you have any references/suggested books? This is one thing i've been very curious about for a long time but, never really had the thought that there would be a devoted book towards this particular subject.

Excellent study on this very subject, and others:

Race Car Vehicle Dynamics by Milliken (SAE book)

[riff_raff]

fastback33,

Jersey Tom pretty much told you everything you need to know. It's a simple statics problem. Each joint in the suspension has a potential of 6 degrees of freedom: 3 translational and 3 rotational. If the joint has rotational freedom along an axis, it cannot transmit torsional forces. If a joint has translational freedom along an axis, it cannot transmit axial forces.

So establish the degrees of freedom at each joint, and then do a simple vector analysis to figure out the forces that result. The limits of the forces will mostly be a function of the instantaneous traction produced at the tire contact patch. The transient dynamic loads will be higher than static loads, so be sure to use a conservative factor of safety. Also, be sure to account for any joint frictions in your analysis.

Good luck.
Terry

[fastback33]

In all honesty, I was expecting something, a bit more...complex. Thanks guys. Now for a bit mroe complex, maybe..

So under braking and acceleration, the loads going into the uprights/a-arms would be torsional?

The reason I ask is that, during the FSAE competition, a lot or some of the teams' designers had a lack of understanding of how the forces worked within the suspension/chassis system, and it kind of baffles me that it is this easy when so many people didn't take it into account.

[The_Man]

How the loads travel form the tyres to the chassis is mostly dependent on the geometry of your suspension. The loads are carried by the 2 control arms, the tie rod and the spring/push rod. How much force is going through which component is determined by the geometry if we ignore compliance.

For example in braking the design anti dive in the system decides the amount of load through the arms and the amount of load that goes via springs and that eventually decided the forces on uprights. Their nature torsional or tensile is again dependent on the mounting points and the geometries.

[Jersey Tom]

In all honesty, I was expecting something, a bit more...complex. Thanks guys. Now for a bit mroe complex, maybe..

So under braking and acceleration, the loads going into the uprights/a-arms would be torsional?

The reason I ask is that, during the FSAE competition, a lot or some of the teams' designers had a lack of understanding of how the forces worked within the suspension/chassis system, and it kind of baffles me that it is this easy when so many people didn't take it into account.

Dude, it's all the same. Doesn't matter if it's cornering or braking or accelerating or some combination. It's a combined load. There's always some combination of forces AND moments acting at the tire/ground interface.

These get transmitted through the wheel and hub into the upright.

The a-arms are ideally axially loaded bars (though not necessarily always the case, e.g. with flexure joints). They're the simplest element you learn in statics. That combination of bars has to resist the forces and moments at the tire.

The concept is dead simple. I guarantee any FSAE student that's made it through statics knows this stuff.

The absolute magnitude of the forces involved are what give people trouble. To do it accurately you have to know your tire states as you go through a track. That's not necessarily easy. Most students don't even get an accurate measurement of their Fz at each corner.

[fastback33]

This is the first time, i've even tried to design a suspension system, so thanks for bbearing with me guys. Is there any possible case where the loads going into the tires can increase the loading into the suspension links, by a serious magnitude? I would think not, but, nothing is ever impossible.

[The_Man]

This can possibly happen if your suspension reaches its physical limit. That is the suspension geometry or a physical interference hinders the movement of the control arms. You should avoid this in the range you expect the suspension is going to move.