Slip Angle

Read the title.  Raise your hand if you know what I’m talking about.  Anyone?  Anyone?  Bueller?

Low slip angle- Notice how the front tires are only slightly angled.

Slip angle is a technical term which is defined as the angle difference (commonly measured in degrees) between the direction a tire is rolling and the direction a tire is traveling.  If a tire has zero slip angle, it is rolling straight down the road.

Any non-zero slip angle implies that there is a cornering force.  Drivers on a typical weekday commute make their trip with entirely unremarkable slip angles – generally so small that the driver doesn’t notice.  Rally drivers, on the other hand, operate with very high slip angles – the tires are often more sliding than rolling.

For the engineers out there, Paul Haney does an awesome job of breaking down the interaction between tire and pavement.  His book, The Racing and High Performance Tire, is a must read to fill out one’s automotive engineering education.  He covers many topics, but the fundamental concept is how slip angle affects lateral grip and drag.  His focus is tires, but the discussion on slip angle is exquisite.

When a wheel is turned, the tread and carcass of the tire deforms.  Every tire behaves a little differently (and surface type and condition plays a huge role in this behavior), but the fundamentals of static and sliding friction are important.  At small slip angles, the rubber is primarily in static friction (transverse to the rolling direction).

As the slip angles increase, a transition is made until the entire contact patch is in sliding friction.  Somewhere in the middle is the right amount of slip angle.

Coefficients of sliding friction are lower than that of static friction, but vector math can prove that the slip angles that provide almost all static friction don’t generate much lateral force.  Initially,  you turn the wheel more and the car turns more sharply.  We’re still well within a commuter’s normal experience.  At some point, however, that effect starts to mute.

High Slip Angle- Take a look at the angle of the front wheels. That's not the fastest way.

After the “magic” angle, turning the wheel to a sharper angle will decrease the cornering force.  Lots of novice drivers give the wheel this much input… when they start to straighten the wheel they are surprised to find the car turning more sharply.  In this scenario, the lateral force component of the vector is relatively high but the total vector size has shrunk.  Often this condition is confirmed by the steering wheel going light in the driver’s hands.

I’ll follow this up later with more discussion and some force diagrams, really getting into the nuts and bolts of how tires work.  For now, I’m going to make Davin find two photos to illustrate high and low slip angle situations and hope that the readers will post some questions to be answered.

One thought on “Slip Angle

  1. Very happy to see some slip angle discussion! Maybe I’ll see if I can still get my hands on the Formula SAE Tire Testing Consortium data (helps when I still advise for my old team… and the GF is on the team, and I am the only one who knows were the CD with all the electronic files still are…)

    I am interested to see which angle you approach this from. As an engineer that has spent a large amount of time staring at tire data and sims to pick suspension parameters based on slip angle and optimal tire behavior. Usually this ended with the me telling the other drivers to forget what they think, they’ll get what I tell them they need and be fast with it. I also always said I would change any of my designs if they could give me a better one AND beat my lap times with it. Never happened 🙂 I think an understanding from a driver’s point of view would be a very good thing though.

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