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Tie rod force and castor effect

Tie rod force and castor effect

Tie rod force and castor effect


I'm trying to understand how forces work through the steering system and their effects on the dynamics of the vehicle.

My understanding is that the lateral force at the tyre induces a mount around the kingpin axis location (and if we only consider lateral forces, then only the castor offset matters I think?). This moment then feeds into the tie rod where

tie rod force = moment around kingpin / longitudinal offset from tie rod to kingpin axis

IF the above is correct (probably not), I presume that this force tries to reduce the steering wheel angle regardless if the tierod/rack is in front or behind the kingpin axis?

I'd appreciate any input

RE: Tie rod force and castor effect

Yes. True statement. (Of course, the lateral force at ground level acting on the mechanical trail at ground level isn't the only source of steering forces.)

The tie-rod force will be the opposite sign depending on front-steer versus rear-steer, but front-steer will need a steering box or steering rack that moves the tie-rods in the opposite direction with steering wheel movement, so what the driver feels at the steering wheel is in the same direction.

RE: Tie rod force and castor effect

Your second paragraph just made me realise that, so thank you.

So according to my equation, I can see how castor increases tie rod/rack forces for a rack thats in front of the kingpin axis . As castor increases, the kingpin moment goes up, and, the distance from the tie rod to axis reduces.

But does this mean that increased castor doesn't increase tie rod load for rack behind the the kingpin axis? Because we increase castor, so moment around kingpin goes up, but then the distance from kingpin to tie rod also increase, so the tie rod force should more or less be the same?

RE: Tie rod force and castor effect

The torque around the steering axis is the same, let's assume that the lever-arm (distance from steering axis to tie-rod outer end ball joint center) is the same length, so the force will be the same magnitude although of the opposite sign. Change to caster angle changes the mechanical trail at ground level and thus the torque around the steering axis (for a given cornering load) but on its own, it doesn't change the lever arm length from the steering axis to the tie-rod end because that's fixed by the design of the knuckle. I'm assuming, of course, that your change in caster angle is not being achieved by changing the design of the knuckle ...

Bottom line, front-steer versus rear-steer has no impact on steering wheel torque, and that's why the choice of front-steer versus rear-steer is mostly governed by packaging. Nowhere to put a steering rack ahead of the front wheel axis if it would have to slice through the space where you have a transverse-mounted engine and gearbox.

RE: Tie rod force and castor effect

But the above is true only if the angle change without moving the steering axis. I'm not sure how castor is adjusted on cars usually, but if we increased it by moving only the LBJ, that would move the steering axis too, surely? So I guess the steering axis wouldn't move if we increased caster by moving both UBJ and LBJ by same amount.

RE: Tie rod force and castor effect

Are you only interested in tie-rod loads due to lateral forces? As BrianP alluded to, you can have steer moments from non-lateral forces (bump steer for instance).

RE: Tie rod force and castor effect

You can't change the caster angle without moving the steering axis! "Changing the caster angle" = "moving the steering axis".

Like I've said many times before ... Give us "the big picture". What is the "big picture" overview of what you are trying to do?

Are we talking MacPherson front suspension, or are we talking upper and lower A-arms, or something else?

On a CAD screen, you can move the steering axis (= "change the caster angle") on the CAD screen by moving attachment points. If you want to increase the caster angle, for example, you can move the upper strut mount of a MacPherson strut back, or you can move back where the upper ball joint of an upper-and-lower-A-arm design is, either by changing the shape of the arm or changing where its attachment points are. Of course, you can move the lower ball joint of either design as well, but I'm operating on the ASSumption that you want to keep the wheelbase more or less constant. Maybe that ASSumption is wrong. I don't know. Tell us "the big picture".

In the field ... in the repair shop ... if it is a MacPherson, the usual design intent is that the caster is not adjustable. Of course, perhaps if the motivation for adjusting the caster is that the car has suffered collision or bump-impact damage, you can replace parts, or bend parts, or shim things. If the motivation is "tuning" then that's what adjustable upper strut-mount plates are for. You can move the upper strut mount forward or back to change the caster angle, or in and out to change camber and a whole bunch of other things at the same time.

Upper-and-lower-A-arm suspensions sometimes have a designed-in shim pack at one (usually the upper) of the A-arm attachment points. On those, you change shims, to move the upper ball joint back and forth (to change caster) or in and out (to change camber).

RE: Tie rod force and castor effect

The largest contributors to tierod loads (hence net steering force) are the slip and camber induced tire aligning moments. Note that as slip angle/lateral acceleration level) increases net aligning moment rises but then falls off and can go negative, even while lateral force continues to build. Caster is generally used to diminish the change in slope of the net tierod force that results from all of this. Called the "tierod load gradient", it is a key ingredient to road feel and driver satisfaction.

The most common mistake made by those playing around with caster changes is the new roll steer value that accompanies it. Front view tierod angles also have a major influence on the vehicle dynamics. In fact, many evaluators get the wrong conclusion about caster changes because the roll steer change is often more powerful a player. The biggest joke happens when brackets on the upright/knuckel are used to maintain the roll steer during a caster change by keeping the front view angles constant, but the flimsy brackets used to do this contribute an overwhelming amount of compliance steer. This is how you shovel thru all the b.s. when writers make claims about this very topic.

As to front vs. rear steer, its difficult to pass a steering shaft thru a transmission with FWD vehicles. There is also quite a benefit to pull on a chain than to push on it, if you get my analogy, hence front steer on heavily loaded vehicles (like race cars, etc. . Bad for frontal barrier, though.

RE: Tie rod force and castor effect


The big picture is trying to understand why increasing castor increases tie rod loads (or does it..?), and in general I use a double wishbone suspension to build this understanding.

Ultimately, does it depend on how much the mechanical trail changes by compared to how much the kingpin to tie rod distance changes by? And technically, we can get a decrease of tie rod loads with increase in castor?

RE: Tie rod force and castor effect

Yes, increasing the caster (e.g. by moving the upper ball joint backwards in the vehicle while keeping the design of the spindle the same) increases the tie-rod loads. I can't think of a circumstance where it would be otherwise.

Just keep in mind that there is a whole bunch more going on with respect to tie-rod loads, as Cibachrome mentioned. There's way more to it than just cornering loads acting on the moment-arm that constitutes the mechanical trail.

RE: Tie rod force and castor effect

Despo8 - please show a diagram of your example suspension and the calculations you are using to determine tie-rod loads.

I can picture in my mind what should happen, but that won't clarify the picture in your mind - so put your picture on the page and the equations you think apply.

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