Tire Modelling in Racing Sims - Pneumatic Trail
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I've been experimenting with the tire files in various rFactor mods. A line has appeared in some of the newer mods that doesn't appear some of the older mods. The line specifies a value for pneumatic Trail. Pneumatic trail (PT) is as important a property of a tire as there is. Without this line in the .TBC file, pneumatic trail in the tire is assumed to be zero.

Ever wonder why real NASCAR drivers can dip the inside tires onto the apron in a turn and not spin? The answer is the pneumatic trail of their tires. Ever wonder why real F1 drivers can race without spinning every time they get on the gas? The answer is pneumatic trail in their tires.

Pneumatic trail is defined as the distance between the centerline of the tire/wheel and the point of application of lateral force. Whenever a tire is rolling the tread is deflected so that the center of the contact patch is behind the centerline of the wheel. This trailing of the center of the contact patch adds inherent directional stability to the tire.

It's really not as hard to drive race cars as it seems to be in racing sims. Anyone who has driven a kart or car fast in real life knows the that vehicle wants to go in a straight line. Pneumatic trail adds directional stability to a tire such that the tire naturally wants to travel in a straight line. For all these years sim developers have forced us to drive on unrealistically unstable tires because they haven't properly included pneumatic trail in the tire model.

I can provide modified .TBC files for several mods that provide proper pneumatic trail response. The mods are transformed into something much more realistic.

What does this have to do with force feedback?...

Tire Properties - Self Aligning Torque
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Pneumatic trail has everything to do with force feedback!

Pneumatic trail causes the tires to want to travel in a straight line. If the tire is forced to turn then lateral force is applied to the tire. The pneumatic trail distance then becomes a lever. Lateral force acts on the PT lever causing a torque around the center of the wheel. This torque attempts to straighten the wheel back out in the direction of travel.

This force moment around the center line of the wheel is called 'Self Aligning Torque' (SAT).

The important part about Self Aligning Torque is that the torque is transmitted through the steering linkage to the steering column. The torque and can be felt by the driver's hands on the steering wheel.

So what? How is this any different from plain old mechanical trail caused by caster?

Self Aligning Torque is different because it changes in response to the load and slip angle of the tire. Torque increases as load increases. Torque also increases as slip angle increases, but only up to a point. After a certain point SAT will decrease. This point is approximately the peak slip angle of the tire.

Here is the whole point! If you can feel the Self Aligning Torque through the steering wheel, you can tell when the front tires are at their peak slip angle.

I don't want to spend any time expaining why this is important. If you don't know why it's important then start googling:

Pneumatic Trail
Self Aligning Torque
Peak Slip Angle

Nice. I did a lot of simulations of this kind of stuff back in grad school and came to a similar realization about why you can feel optimum slip through the wheel. I've been hooked on FFB ever since I found sims (GPL and then NR2003) that could properly communicate those forces.

Unfortunately the whole ISI-based series of sims (F1XXX, rFactor, GTRX, GTL) failed miserably in this regard until the advent of RealFeel for rFactor. After using RealFeel with well designed mods it's hard to take any of the SimBin products seriously. I hope RealFeel can be extended to those sims or maybe ISI will get their shit together finally...

Thanks for the "feedback", Mike :)

Suffice to say, real drivers do feel what the car is doing through the steering wheel. With proper front-end geometry, the driver can feel when the front tires are at peak slip angle, and when they are sliding.

Even power steering does not completely mask these forces.

If you don't believe me, stop in a parking lot, turn the wheel to full lock, and apply throttle. The front wheels and steering wheel will straighten out. This is caused by pneumatic trail and mechanical trail working to make the tires to turn toward the direction of travel.

Regarding the 'subtlety' of current force feedback technology, here is something I noticed last night.

I was jumping in and out of rFactor while experimenting with different PT values. I kept noticing that sometimes when entering the car, when I turned the steering wheel it would load up heavily and return to it's original location when released. Other times when the steering wheel was turned it would stay where I left it. This is with the same mod and FF settings. I thought it must be some inconsistency in the system.

Then I realized what was causing it!

Sometimes when entering the car I would have the brakes engaged, and other time I didn't. When the brakes were engaged the front wheel hubs could not rotate in respose to the turning of the steering wheel. The wheel hubs and tire contact patches stayed locked up. The tire carcass was twisting as the steering wheel was turned. When the steering wheel was released the tires would snap back into shape causing the steering wheel to return to its original position.

If the brakes are not engaged the wheel hubs can rotate and the tire contact patch can roll to a new location. The tires don't have to twist. So the steering wheel stays where it's left.

I was pretty impressed!

Nope. Keep your spring loaded wheel. Realism is the name of the game. I'm sticking with the G25 for now.

I have G25 too. Your custom FFB settings are welcome.

Great posts, Pat. I have the G25 and I'd be interested in seeing your FFB settings. I haven't tried RealFeel yet.

I haven't read the whole thread, but could you post your RealFeel settings along with FFB controller settings for some mods?

BTW Pat when you talk about optimizing the PT parameter, are you just doing it by feel or do you have some data to work with? If you are doing it by feel then how do you account for caster settings? Also you probably know this, but there is a discussion along these lines at RSC, involving the creators of RealFeel & carFactory.

Yes, Pat, excellent post and explanation. I've just started using RealFeel and would be interested in your settings, both for RealFeel and PT if available for the R2P rFactor sims.

Prior to RealFeel I've always felt SimBin did a better job of making the cars feel connected to the track, while rFactor cars to me always felt a little disconnected, for lack of a better word, even though they share somewhat of a common physics engine.