Static toe settings thread60-365967

[jakemanr]

I am interested in static toe settings and how they affect vehicle stability at higher speeds. I am trying to understand the explanation given by CHagen in this thread from 2014:

https://www.eng-tips.com/viewthread.cfm?qid=365967

Part of the explanation describes how the self aligning torque builds at the tire with the increasing slip angle, and reduces at the tire with reducing slip angle, and that this acts to center the steering. I understand this, but I cannot see a difference here between a car with static toe out, or static toe in. With respect to this phenomenon they are the same.

Another part of the explanation describes "slip angle drag" which I assume is Fx generated at the contact patch due to tire slip. This is where I can see a difference between the static toe in and static toe out examples. Considering a car with positive scrub radius, and static toe in: The outside tire has the higher slip angle, and higher Fx. The Fx here generates Mz around the steering axis due to the scrub radius, and acts to center the steering. Conversely, a car with negative scrub radius would feel more "stable" with static toe out on the front axle. Have I understood this correctly? Is this really a contributing factor here?

The last sentence of the first paragraph of this explanation says:
"The point where the tire force acts moves rearward slightly and slip angle drag incurring on that side reduces the yaw moment generated by the lateral component of the slip angle. A lower yaw rate vs. steering angle gain and overall near center steering stability."

So rather than my explanation of Mz at the steering axis due to positive scrub radius, he describes Fx at the outside wheel generating Mz at the car C of G and reducing the Yaw moment.. Physically this also makes sense; I just imagine the influence to be very small here.

Can anyone shed more light on this topic?

 

[GregLocock]

Many words, arranged in sentences.

Here's how I'd look at it. Consider a car driving in a straight line. Push the front of the car to the right. With toe-in the right hand wheel now develops a bigger slip angle and pushes the car to the left, whereas the left wheel develops a smaller slip angle and stops pushing the car to the right. Net result is a leftwards force, helping to bring the car back on centre. With toe-out the right hand wheel now develops a bigger slip angle and pushes the car more to the right, whereas the left wheel develops a smaller slip angle and stops pushing the car to the left. Net result is a rightwards force, pushing the car off-centre, ie an unstable response. That is bad so the opposite must be good.

However that is just a thought experiment.




Cheers

Greg Locock


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[jakemanr]

Hi Greg, thanks for responding.

I have read your paragraph several times, but I don’t understand it. I agree with your toe in example, but disagree with your toe out example. I would rewrite your explanation like this:

Consider a car driving in a straight line. Push the front of the car to the right. With toe-in the right hand wheel now develops a bigger slip angle and pushes the car to the left, whereas the left wheel develops a smaller slip angle and stops pushing the car to the right. Net result is a leftwards force, helping to bring the car back on centre. With toe-out the right hand wheel now develops a smaller slip angle and stops pulling the car to the right, whereas the left wheel develops a larger slip angle pulls the car to the left. Net result is a leftwards force, helping to bring the car back on centre.. exactly the same as the toe in example

What am I missing?

 

[BrianPetersen]

Try a generous static toe-out setting in real life, and let us know what happens. Drive slowly at first until you get a feel for it. Prepare for skittishness.

 

[jakemanr]

Hi Brian,

I am planning to experiment a bit but I'm waiting until I have my summer tires fitted again, low corner stiffness of my winter tires masks a lot of the feel. In any case this experiment wont help me to understand why either setup behaves the way that it does. I don't know the suspension kinematic of this car..

 

[BrianPetersen]

Maybe we have to simplify Greg's thought experiment until you "get it". Step at a time. Explain at which step you don't "get".

Imagine driving down the road on wooden tires with zero compliance. Any non-zero toe angle represents the wheel being dragged sideways at the tire contact patch. Do you "get" this?

Now, let's suppose that there is a random force that gets applied sideways to the car at some height above ground level as it's driving down the road. Maybe it's a cross-wind applying a sideways force at the car's aerodynamic center of pressure in side view. Maybe it's because the road is at a slight camber resulting in a sideways force being applied at the car's center of gravity when viewed from the car's own internal frame of reference (which is what you feel as a driver). Do you "get" this concept?

Now, understand that this sideways force is trying to tip the car over a little bit. Do you "get" this? If you don't, search youtube for a video of a transport truck being blown over in a very strong crosswind. Do you "get" this now? The force being applied sideways is applying a torque trying to tip the vehicle over.

How does the car resist tipping over? By the wheels on the opposite side of the applied force incurring more of the vertical force and the wheels on the near side of the applied force incurring less of the applied force. In the above-mentioned video of a transport truck being blown over by the wind, the truck stays on the ground until such time as the outside wheels are taking up the entire weight of the truck and the inside wheels are taking up zero ... at which point they leave the ground, and then the situation goes catastrophic. This is an illustration of the wheels on the opposite side of the sideways push taking up more vertical load and the wheels on the inside taking up less. With me so far?

Now, let's imagine a situation short of that catastrophic blown-over situation but nevertheless, a crosswind or some other side force is pushing from (let's say) the right, and therefore the left wheels are seeing more downforce and the right wheels are seeing less downforce as a result, to counter the overturning torque of the crosswind.

Now ... Wheels are toed in a little. The outside wheels have more downforce on them. The increased downwards force on those wheels means the sideways component (because of the toe-in) on the outside wheels (trying to steer the car to the right a little) overcomes that of the inside wheels. So, while the external force is trying to push the car left, the various forces involve lead to the car itself trying to steer right, against the applied force. If you get the set-up right, it's stable.

Or ... Wheels are toed out a little. The force pushes to the left. That puts more downforce on the left wheel which is toed out, steering to the left a little. The car wanders to the left. Except ... the acceleration to the left itself transfers weight to the right. Then the right wheels get loaded more, and now the car wanders to the right. Except that transfers load to the left, loading the left wheels more, and now the car wanders to the left. And it ends up skittishly wandering all over the place, picking the direction of whichever side happens to be more loaded at the time. A crosswind blowing to the left will bias this towards the car being blown left.

Real tires of course do not have such on/off friction behavior, so there's not going to be a binary selection between wanting to follow the direction of the left wheel or the right wheel, whichever happens to have more weight on it at the time. But the direction will be the same. A little bit of toe-in is generally stable running down the road, and a little bit of toe-out will be more skittish, more wandery.

Some cars may specify static toe-out. They're usually front wheel drive. It may be because the design of the suspension and the various forces involved lead to compliance effects that tip the balance towards toe-in while actually running down the road, as opposed to sitting statically with no load on a test rig.

 

[GregLocock]

One curious aspect is rear axle toe settings. Usually what is good for the front axle is bad for the rear axle, yet toe-in at the rear is quite common, and demonstrably worthwhile.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[GregLocock]

I think I got the toe-out example wrong.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[Mongrel]

Perhaps a different way of looking at it... Here's how it was explained to a group of us as teenagers.
When you turn left, for example, the weight of the vehicle shifts onto the right side wheels as it rolls. The extra weight on the right wheel will make the steering force from that wheel dominant, and with toe-in, it will try to steer left - because of the extra force on the wheel that's pointing left.
The exact same goes for body roll induced by wind, as the others have said above. Basically, weight transfer amplifies the steering input from the side experiencing the extra weight.
Also used was the example of snow skis, but since it doesn't snow in Australia, that one was a lost cause, heh.
Very simplistic, of course, but this was for a group of teenagers who didn't know what slip angles and all that sort of thing were yet. Perhaps the different way of looking at it will help?