Recently we discussed understeer and oversteer (you can still catch it online, plus the importance of understanding the road rules, in the Life & Style section by clicking on my name in one of my stories) so let's discuss the concept of load transfer and its influence on traction because understanding this (and having practised in a training environment), can come in handy in an emergency.
You may have heard this referred to as weight shift, but that's not quite the right term. The weight isn't really moving in relation to the rest of the vehicle so much as the load is being transferred between the tyres.
That matters because it's all about the grip we can get from the tyres, as well as which tyres are actually providing the grip at any given moment, but we'll get to that.
Many factors affect tyre grip, but the one we're looking at here is the downward pressure on each tyre. Think of it like opening a new jar. You need grip on the lid to open it, and that requires applying more pressure by squeezing it tighter. Same for a tyre. If it's pushed down a bit firmer onto the road, it offers a bit more traction.
Bumps, crests and road camber also have an influence, but you can only observe and allow for those factors, not control them (do observe them though, they're important).
Anyway, on level ground that downward pressure will be the result of three factors, two of which are set by the car and one of which you have some control over with the wheel and pedals. Those are, the weight of the vehicle (which won't be the same measurement at each corner even at rest, but we can discuss that another time), any downforce that may be produced when moving swiftly enough (and hopefully there's no lift, but that's a possibility and another thing we can discuss later), and load transfer.
You have control over load transfer because it occurs when we decelerate, turn, or accelerate. These will apply more downward pressure to some tyres, and less on others, thus affecting the grip of each one.
Specifically, deceleration will give the front a bit more downward pressure, and as long as you haven't locked the front brakes you'll get an increase in front grip, but it will come at the expense of decreasing the rear grip.
Accelerating will put more downward pressure on the rear tyres and reduce it at the front, transferring some grip rearwards.
Turning puts more downward pressure on the outside tyres and less on the inside tyres.
If you're accelerating and turning, that will put the most downward pressure on the outside rear and the least on the inside front.
When staying within the limits of grip, this can all be helpful to some degree, because the tyre you're asking to do the most work (for example, the front left tyre when slowing and turning right) does get a little more traction from this load transfer effect.
However, the change in speed and direction can easily place more demand on the tyres than the change in grip can deal with, resulting in oversteer or understeer.
The combination to be most careful of in an emergency is when you decelerate and turn. That puts more downward pressure on the outside front and less on the inside rear. This (along with their weight distribution and other factors) partly explains the lift-off oversteer that some cars have. But most significantly, a true Scandinavian flick in rallying uses this effect with the foot brake (not the handbrake), along with the pendulum effect you get from swinging one way and then the other. This is why turning is more difficult to control when emergency braking. And once you start sliding, the load transfer gets harder to keep track of too, because it actually shifts in relation to the direction of momentum, not in relation to the vehicle body (something else we can discuss further another time).
One thing you can take from this is don't always blame the car, it might just be the driver. The other is the importance of proper facilities for advanced driving instructors so more people can safely learn and improve their skills for emergency use. Failing that, you can also practise these principles at home with a good sim rig (although some titles will be better for this than others).
Sam Hollier is an ACM journalist and a motoring fanatic who builds cars in his shed in his spare time.
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