Mechanical trail, pneumatic trail: What are they and why they are important.

Some of you eagle-eyed people have questioned the mention of mechanical and pneumatic trail in the latest patch notes. So, before misinformation spreads like Chinese whispers I will dive in and explain what these two things entail.

Pneumatic trail

This describes the distance between the physical centreline of the tyre at ground level and the location about which the forces take place. In other words, it's how far the tyre's actual active footprint is from it's centreline.

If we had a tyre with the contact patch which was at the physical centreline, it would be where the red dot is below. This would give zero pneumatic trail:



To compare, the red dot below is showing the contact patch centre is slightly behind the tyre centreline. This distance is the pneumatic trail.



As this distance increases, a force called self-aligning torque is created which works to pull the tyre and therefore steering, straight. Pneumatic trail is at it's maximum when the slip angle is zero and decreases as slip angle increases. It also increases with vertical load.

Mechanical trail

This describes a physical design trait of the front suspension design, when viewed from the side.
The image below shows axis about which the front tyre rotates when steered. Looking at the point where the axis meets the ground, there's an obvious gap between that point and the physical centreline of the tyre. This measurement is the mechanical trail:



Now take note that I have not mentioned caster here. That's because in initial stages of vehicle design, mechanical trail can be treated entirely separately of caster angle.

The example below shows that we can have zero caster angle but still a large mechanical trail value:



Whereas this example shows what it would be like if we had a high caster angle but zero mechanical trail:



So why do people think of mechanical trail and caster as two and the same? Well, once you're beyond the initial stages of steering design, you can't adjust one without affecting the other. That's because to change caster you have to change where the outboard joints of the suspension are relative to each other. That will always affect mechanical trail, unless the car is especially designed to not allow that. However it's not something I've ever seen in the real world.

Mechanical trail, like pneumatic trail affects steering forces in a self-centring manor. It's not dynamic in that suspension movement doesn't change it. However as the tyre is compressed vertically by various forces the intersection point moves closer to the road which will serve to decrease mechanical trail.

 

I think it could maybe be worded slightly differently, it's not necessarily the contact patch itself that moves to the rear, but the 'center' of where the tire forces are generated is slightly rearward.