Self aligning torque and steering feel

[Jersey Tom]

So, if pneumatic trail and SAT, everything that gives you feedback runs out (or decreases) BEFORE you reach peak Fy, how are you going to know when the Fy is?

When the car stops responding when you turn the wheel - you're out of Fy. Simple as that. There's no magic making the steering feel give the driver some "ah ha! this is the optimum point!" feedback.

[Erunanethiel]

So, if pneumatic trail and SAT, everything that gives you feedback runs out (or decreases) BEFORE you reach peak Fy, how are you going to know when the Fy is?

When the car stops responding when you turn the wheel - you're out of Fy. Simple as that. There's no magic making the steering feel give the driver some "ah ha! this is the optimum point!" feedback.

Then why do people not like uncommunicative steering wheels? Like Porsche's

[Jersey Tom]

So, if pneumatic trail and SAT, everything that gives you feedback runs out (or decreases) BEFORE you reach peak Fy, how are you going to know when the Fy is?

When the car stops responding when you turn the wheel - you're out of Fy. Simple as that. There's no magic making the steering feel give the driver some "ah ha! this is the optimum point!" feedback.

Then why do people not like uncommunicative steering wheels? Like Porsche's

You would like to have steering feel that makes it stupid easy for the driver to feel out the limit of the tires - doesn't mean it's possible.

Beyond that, the steering system (and feel) of a modern car is much more involved than just pneumatic and mechanical trail. You could do a whole Masters or PhD thesis on it (and I know guys who have).

[RR98ITR]

This seems a reasonable place to ask if "Mz" contributed by mechanical trail is a bad thing in the context in which it's being discussed in this guys blog: http://fsae1000.blogspot.com/2008/11/al ... -joke.html . IE - is the price we pay for increased "feel" a corresponding decrease in what we get out of Fy?

Scott

[RR98ITR]

Is it me? If this was the FSAE board at least I'd get Claude Rouelle all up in my grill with "WhoTFareYou?"

I hope it's not me. I don't mean to come off as impatient or appearing to feel entitled to "the answer" or anything like that. But I did come here for higher brain power than I've been able to develop on this subject.

All the texts don't quite point the way on this, and my own engineering and math education is represented now by a residual sense of smell which I trust only to limited resolution.

I think I Know that a working tire gives an SAT, which we can characterize as the product of Fy and PT, and which we can feel thru the wheel. Leaving aero out of it, and so fixing Fy, and then adding MT, there would seem to be more "something" as registered at the steering wheel. Is everything coming thru the steering wheel equivalent - ie if one "resistance" is an understeering moment, is any other additive resistance likewise an understeering moment?

I've seen fast cars with lots of caster and mechanical trail, and fast cars with lots of caster and reduced MT via offset, AND fast cars with very little caster or trail.

Anybody got a clever way to think this thru? Surely Ron Tauranac worked this out long ago?

Scott

[Lycoming]

I think I Know that a working tire gives an SAT, which we can characterize as the product of Fy and PT, and which we can feel thru the wheel. Leaving aero out of it, and so fixing Fy, and then adding MT, there would seem to be more "something" as registered at the steering wheel. Is everything coming thru the steering wheel equivalent - ie if one "resistance" is an understeering moment, is any other additive resistance likewise an understeering moment?

I'm not really sure what you're trying to ask here, but...

Yes, there would be something registered at the steering wheel; in the situation described above, you feel a greater torque attempting to center the steering wheel back to neutral steer. Understeer/oversteer is more related to yaw moment on the whole vehicle as opposed to self aligning torque through the front tires, so I don't really get what you're trying to say here. I don't think that you really feel understeer through the steering wheel; you perceive understeer as tracking on the outside of your desired path.

If you're simply referring to the direction of the torque felt at the steering wheel... forces and torques are both vectors, and there are rules for summing vectors that you should be familiar with if you wish to discuss these kinds of topics.

I've seen fast cars with lots of caster and mechanical trail, and fast cars with lots of caster and reduced MT via offset, AND fast cars with very little caster or trail.

Well, sure, although remember that castor also affects other things, like camber change as a function of steer angle. If you think there's reason to be concerned about steering effort being too high, there are ways to deal with that with things like gear ratios at the steering rack, or power steering.

[RR98ITR]

People discuss caster mostly in terms of inducing negative camber as you turn the wheel. Also in terms of weight jacking, though less often except in the case of karts. And of course stability and "steering feel" at the Fy limit. But in all of this discussion it's always as if caster is "free" except for steering effort and that dismissed where steering is assisted.

Yep, I know that yaw moment is required to get a car around a corner. And I take it that Mz is detractive from that. Hence the question: additional mechanical trail feels like the tires aligning torque thru the steering wheel - how much like it is it? IE - they both feel like "not want to turn" thru the wheel, but then there's only so much you can get thru the wheel and so we have to reconcile hands, ass and eyes.

So, IF they are the same, then there is a cost to increased caster...that nobody expresses an awareness of. Fine if you've got the power to add a little more front downforce, but less so if you can't afford to ignorantly throw away the very thing you thought you were working for.

Scott

[RR98ITR]

Just wanted to resolve my "contribution" to this thread. With some kind help I've determined that this is me:

http://www.flickr.com/photos/mytravelph ... 2804386413

The attendant forces caused by mechanical trail are resolved thru the steering and do not equate to an understeering moment on the car.

Unlike Alfred E Neuman I do feel shame. "How DOES he do that?"

Scott

[bl79]

I thought Tune to Win is fairly explicit about this unless Im misunderstanding the question. I think this is a fair summary:

As maximum lateral grip is approached PT drops to zero. PT+MT dictate steering "weight". If PT is dropping towards 0 this is feedback to the driver - the wheel gets light around peak grip. The important take away here is that if your PT is 1% of your total steering trail then it reducing to 0 is useless. Instead if you select your MT with some care you will still be providing a fairly large difference in steering feel to the driver making it easier to feel out the limit.

[Greg Locock]

If you don't have any MT then on centre feel is non linear, the steering wheel will feel loose. If you have too much MT then you will find it harder to pick the drop in cornering stiffness as you approach saturation, but for general driving the wheel effort will increase nicely with Fy .

tie rod force = FY*(MT+PT)/SteeringArmLength*some angle stuff.

PT starts high at FY=0 and drops to 0 at saturation. That equation pretty much defines the issue.

In terms of numbers a typical production tire has a PT of 60mm at 0 Fy, 30 mm at typical 0.3g handling, and typical front suspensions will have MT of 10-35 mm. If you have a Pacejka model or real data then PT=MZ/FY. Typical fully saturated slip angle is say 10 degrees.

[Erunanethiel]

Up!!

So we can say that steering feel is misleading?