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

Discussion in 'Car Information and physics updates' started by Alex Hodgkinson, Mar 19, 2021.

  1. Alex Hodgkinson

    Alex Hodgkinson Sector3 Developer

    Jun 5, 2017
    +1,839 / 0 / -0
    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.

    Attached Files:

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  2. GooseCreature

    GooseCreature Well-Known Member

    May 30, 2015
    +664 / 0 / -0
    As clear as mud! :D
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  3. R.Noctua

    R.Noctua Well-Known Member

    Dec 12, 2019
    +91 / 0 / -0
    I didn't understand anything, but it sounds interesting. :(

    What's in it for us?
  4. ravey1981

    ravey1981 Well-Known Member Beta tester

    Apr 15, 2018
    +751 / 0 / -0
    More realistic vehicle handling. As Alex's suspension modelling has become more complex/accurate then these things start to really matter.
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  5. Maskerader

    Maskerader Well-Known Member

    Oct 6, 2019
    +148 / 0 / -0
    Self-centering forces on the wheel, in the cars mentioned in the patch notes, should be now closer to real life.
  6. nickh158

    nickh158 Well-Known Member

    Jan 31, 2015
    +64 / 0 / -0
    My head hurts. I need to go and lie down now.
  7. geoffers

    geoffers Active Member

    Jul 11, 2015
    +36 / 0 / -0
    What I don't understand is how the contact patch centre can be anything other than directly beneath the centre of the wheel on a level surface. I can see it could be on a slope.

    On second thoughts, is it to do with the deflection of the tyre, if so it changes with tyre pressure, is that correct?
  8. Thomas Jansen

    Thomas Jansen Sector3 Developer Beta tester

    Apr 5, 2018
    +552 / 0 / -0
    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.
  9. Maskerader

    Maskerader Well-Known Member

    Oct 6, 2019
    +148 / 0 / -0
    Upd: When the car is stationary, the contact patch should be below the axis, but when braking and turning, the car pushes on the rims, the rims push on the rubber, and the contact patch lags behind because the rubber is flexible and it sticks to the track surface.

    That's for RWD cars. I suspect a FWD car accelerating out of the corner applies different forces on the front tyres.

    You can watch this video for more explanation on pneumatic and mechanical trail how they are done in ACC:
    Last edited: Mar 20, 2021