Let's talk about tyre slip angle.
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You can read the other pieces I've written on oversteer and understeer, load transfer, tyre distortion, the road surface and the lack of linearity, and more, first if you want to.
Here are the links to make it easy:
The weird thing about tyre slip angle is the term infers that the tyre is beginning to lose traction, whereas it's actually just getting to work to provide the grip you're asking of it.
Yeah, it's being stressed, but just like writers on a deadline, undergoing this kind of stress is when they get the most work done.
First off, it may be helpful to think of a tyre as a colony of millions, perhaps billions, of tiny elastic bands, each one attempting to hook itself onto any of the similarly-tiny surface imperfections of the road.
This is why old smoothed road surfaces offer less grip, as do dusty surfaces.
This is also why a tyre under more vertical (downwards) loads can get more traction on a given surface.
It's also why tyre distortion affects grip, but the thing you need to wrap your head around is that this tyre distortion initially increases grip. Partly because it is accompanied by those forces pushing it onto the road, and partly because that distortion also means (if you have the right suspension geometry and setup) more of those bands are actually called into service at this given moment (ie. more tread surface is touching the road under some conditions).
Too much distortion, or too much lateral or longitudinal load (with not enough downwards load to compensate), and that is when actual slippage (oversteer, understeer, wheelspin or brake lock-up) starts to occur.
To give a definition for a tyre's slip angle, this is the difference between the angle a tyre is pointing (measured against the direction of vehicle momentum) versus the angle at which actual traction is being provided.
If say, a front tyre is pointed (steering) 25 degrees to the left, but the grip they're providing (assuming the rear is also gripping for this simplistic example) results in an actual change in direction of 20 degrees to the left, we'd say the slip angle in that moment for that tyre is 5 degrees.
Importantly, different tyres work best at different slip angles. They will have a range of a few (or maybe several) degrees where grip is high, and a slip angle where grip peaks.
Road tyres generally don't have a very big ideal slip angle before the grip starts going away and you begin sliding. 3 or 4 degrees might be in the ballpark of where they peak, but each tyre is a little different.
Race tyres for tarmac have larger slip angles where they work best, perhaps as much as 10 degrees for peak grip. Again, this varies by tyre (and the manufacturer usually knows what the best range of slip angles is from testing).
Tyres used in drifting are the hardest ones to wrap your head around. They can still provide usable traction with slip angles up to 40 degrees even when they're spinning way faster than the road speed. This is not ideal for lap times of course, but they still give more than enough traction for skilled drivers to work with in drifting competitions.
Wet weather significantly reduces this slip angle for all tyres (because those tiny bands all have a much harder time clinging on). In fact, it's hilarious watching amateur drifters in the wet, because the slip angle has become so narrow that their usual techniques just make the vehicle face backwards in relation to its momentum.
On dirt, tyres also offer usable traction at angles and rotational speeds surprisingly different to where they are pointing, and how quickly they're turning. Partly because the grip is low to begin with, and partly because the wheelspin is sweeping away loose trash to get to a better surface underneath.
Slip angle will also determine the best design and setup for the steering geometry, so we'll discuss that in future. Spoiler alert: you don't always want the inside front tyre turning a bit sharper than the outside front tyre (hint: load transfer comes into it).
Sam Hollier is an ACM journalist and a motoring fanatic who builds cars in his shed in his spare time.
