Self aligning torque and steering feel

[Erunanethiel]

Why does SAT decrease before the peak Fy? Doesn't it mislead the steering feel, since it the lightens BEFORE the grip runs out?

[-Felix-]

Because the pneumatic trail decreases towards the SA where peak FY occurs. This helps very much in terms of steering feedback, because the SAT and thus steering wheel torque decreases and gives the driver the feel that the front axle gets "light" and loses grip. So its good that it happens BEFORE the grip runs out. What use would be a lighten steering AFTER grip runs out? I'd already know by the fact the car is sliding towards the wall that there is no grip anymore. And I am sure, that should be avoided..

[Erunanethiel]

Because the pneumatic trail decreases towards the SA where peak FY occurs. This helps very much in terms of steering feedback, because the SAT and thus steering wheel torque decreases and gives the driver the feel that the front axle gets "light" and loses grip. So its good that it happens BEFORE the grip runs out. What use would be a lighten steering AFTER grip runs out? I'd already know by the fact the car is sliding towards the wall that there is no grip anymore. And I am sure, that should be avoided..

How does pneumatic trail help to the steering feel?
Thanks for the answer

[Jersey Tom]

In Pacejka's Tire & Vehicle Dynamics there are some good visual examples of why aligning torque will always fall off before you get to max lateral. As Felix mentions, it is because pneumatic trail decreases (as more of the footprint is sliding equally).

Don't forget that pneumatic trail is one [often] small part of steering feel. You can make steering torque follow Fy by just throwing mechanical trail at it. And once you throw power steering (as in all production cars now) in on top of it you have an entirely different animal.

[Tim.Wright]

this is covered very well in Pacejka's book but i will try to explain it here.

SAT arises because the resultant force of the lateral cornering forces in the contact patch do not pass through geometric centre of the contact patch.

This is because the lateral forces build up due to lateral deflection in the tyre carcass and this builds up from zero the the front of the contactpatch until it reaches a mximum near the rear of the contact patch. This inbalance of forces in the front and rear of the contact patch generates a moment about the centre of it. This is your SAT.

However, there is a limit to the amount of lat force a tread element can supply and when you reach it, the tread element starts sliding. When the tread element is sliding you provide less force then the non sliding elements which are stuck to the ground because dynamic friction is typically less than static friction.

What does this have to do with the SAT? Well this sliding starts at the back of the contact patch because there is more force there as described above. Recall that the lat forces were higher in the rear of the contact patch but near the limit they start sliding so they produce less and less force until you get to a point where the lat forces in the rear of the contact patch only just balance out the (negative) SAT coming from the front and here you have zero SAT.

[Erunanethiel]

this is covered very well in Pacejka's book but i will try to explain it here.

SAT arises because the resultant force of the lateral cornering forces in the contact patch do not pass through geometric centre of the contact patch.

This is because the lateral forces build up due to lateral deflection in the tyre carcass and this builds up from zero the the front of the contactpatch until it reaches a mximum near the rear of the contact patch. This inbalance of forces in the front and rear of the contact patch generates a moment about the centre of it. This is your SAT.

However, there is a limit to the amount of lat force a tread element can supply and when you reach it, the tread element starts sliding. When the tread element is sliding you provide less force then the non sliding elements which are stuck to the ground because dynamic friction is typically less than static friction.

What does this have to do with the SAT? Well this sliding starts at the back of the contact patch because there is more force there as described above. Recall that the lat forces were higher in the rear of the contact patch but near the limit they start sliding so they produce less and less force until you get to a point where the lat forces in the rear of the contact patch only just balance out the (negative) SAT coming from the front and here you have zero SAT.

How about the steering feel? I meanwhen you don't get SAT you won't need much torque to turn the steering wheel which means bad feedback. What is pneumatic trail and what ddoes it have to do with steering wheel

[Jersey Tom]

Pneumatic trail is Mz / Fy. It is the gain of how much SAT you get per unit Fy. It is unavoidable that it will fall off to zero as you get to max Fy. But again, the designer can put in mechanical trail in the suspension to get whatever overall gain of steering torque they want.

[Erunanethiel]

Pneumatic trail is Mz / Fy. It is the gain of how much SAT you get per unit Fy. It is unavoidable that it will fall off to zero as you get to max Fy. But again, the designer can put in mechanical trail in the suspension to get whatever overall gain of steering torque they want.

What is Mz?sorry I am new

[Lycoming]

Mz is another word for self aligning torque; Moment about the Z axis in the usual coordinate system.

[Erunanethiel]

this is covered very well in Pacejka's book but i will try to explain it here.

SAT arises because the resultant force of the lateral cornering forces in the contact patch do not pass through geometric centre of the contact patch.

This is because the lateral forces build up due to lateral deflection in the tyre carcass and this builds up from zero the the front of the contactpatch until it reaches a mximum near the rear of the contact patch. This inbalance of forces in the front and rear of the contact patch generates a moment about the centre of it. This is your SAT.

However, there is a limit to the amount of lat force a tread element can supply and when you reach it, the tread element starts sliding. When the tread element is sliding you provide less force then the non sliding elements which are stuck to the ground because dynamic friction is typically less than static friction.

What does this have to do with the SAT? Well this sliding starts at the back of the contact patch because there is more force there as described above. Recall that the lat forces were higher in the rear of the contact patch but near the limit they start sliding so they produce less and less force until you get to a point where the lat forces in the rear of the contact patch only just balance out the (negative) SAT coming from the front and here you have zero SAT.

How about the steering feel part?

[olefud]

http://www-cdr.stanford.edu/dynamic/bywire/tires.pdf

[Jersey Tom]

How about the steering feel part?

Could you be a touch more specific?

[Greg Locock]

Ignoring various bits of geometry which don't alter the basics much

SteeringWheelTorque= (mechanical trail+pneumatic trail) * latacc*vehicle mass*Front axle weight proportion/steeringarm length*steering gear ratio* a bunch of constants

[Erunanethiel]

How about the steering feel part?

Could you be a touch more specific?

How would you be able to feel the tires losing grip?

[Jersey Tom]

How about the steering feel part?

Could you be a touch more specific?

How would you be able to feel the tires losing grip?

If you're trying to do it with pure pneumatic trail effect I suppose it would be when all the steering torque goes away. But I don't think you'll find many or any cars with zero mechanical trail or power steering involved.

More obvious way of knowing when the tires are saturated is when you're turning the wheel the the car isn't responding... likewise can be seen in recorded data, if it's available.