In recent weeks our discussions have touched on some skidpan teaching techniques, brought things back to basics with descriptions for oversteer and understeer, and we expanded on those with an explanation of how load transfer affects tyre grip.
If you'd like to read those first, here are the links:
Fundamentally, a pneumatic tyre is just a sturdy, multi-layered balloon.
It is therefore very dependent upon proper inflation, and it is also flexible, especially under the weight and forces applied by the vehicle that it is helping to hold up.
Modern tyres have multiple layers. To quickly put these as simplistically as possible, there's an inner lining holding the air in, there are a number of layers making up the tyre's construction (typically including steel belts for the last 40 years or so), and then there's the tread.
Meanwhile, the sidewall can actually be thought of as a separate part, because it has a flexible makeup of its own (its central and outer layers are significantly different to the treaded section) and that flexibility is what we're going to talk about.
When you see just how severely a tyre gets warped out of shape as it's subjected to forces like turning sharply, it's quite astounding.
Looking at the fronts because they do the most work to change the vehicle's direction (because the fronts have to initiate the turn, the rears just have to follow), and as our earlier discussion on load transfer explained, the momentum change of slowing and turning puts more downward pressure on the outside front tyre.
In a 1400kg race car for example with say 760kg over the front axle, so about 380kg on each front tyre at rest, that cornering load on just the outside front can easily exceed 1000kg. Admittedly, street tyres won't have enough traction to achieve such high load transfer numbers the way a racing slick can, but the principle is the same.
Delving into this further, that load isn't just in the vertical plane. For as long as a tyre can cling onto the road, a substantial load transfer is also sideways relative to the tyre.
Importantly, this distortion also affects the total traction budget, because after you've gained a little extra grip from load transfer via the extra downward force adding some traction, you could start losing that grip again almost straight away as the distortion from sideways load means the tread is no longer in proper contact with the road surface.
The way race cars deal with this warpage of the tyre is to get aggressive with the suspension angles when they're setting up the car.
One angle is called camber, and it's negative when the tops of the front tyres are closer to each other than the bottoms when the car is static. Another angle they use is positive castor, and that tilts the wheel and tyre in a similar manner to camber, but only when the steering is turned. What each of these angles achieves is to counteract that sidewall warpage on the outside front.
However, in a street car it's not practical to dial in such aggressive angles. Excessive camber means the inside portion of the tread is the only thing touching the road most of the time. That causes excess heat on the freeway, excess wear all the time, and it reduces the amount of rubber on the road when braking.
Castor is the better solution but you still can't go nuts with that because it adds load to the steering componentry. At the very least, the power steering system will run hotter.
So here's where we circle back to having the correct tyre pressures. Way too low and the tyres can distort so badly they break the bead (which means they separate from the wheel) and suddenly deflate. But way too high is also quite dangerous, especially coupled with excess heat (and the pressure climbs further as they get hotter as well).
So, have you checked your tyre pressures with a reliable gauge recently? You'll certainly wish you had halfway into your next emergency manoeuvre, that's for sure.
Sam Hollier is an ACM journalist and a motoring fanatic who builds cars in his shed in his spare time.
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