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A PHYSICIST WRITES . . .
(June 2006)
This month my mind is on friction, the force that your tyres use for gripping the road as you steer or accelerate or brake or negotiate a hill. A dull topic? Not if you stick with me! Friction (in physics and in motoring) behaves like this: when two surfaces are being pushed across one another they grip together, resisting the force that is trying to make them slide — but only up to a limit.
This maximum grip depends on (1) the state of the two surfaces, and (2) how much downward force is pressing them together. So the heavier your car is, the more tyre grip will be available to brake with. But don’t forget you will need the extra braking force because the car is heavier. This is why the Stopping Distances in the Highway Code apply equally to all vehicles (in theory).
What you ought to worry about is item (1), the state of your tyres and the road. This can make the grip vary from almost as much as you will ever need, down to almost zero. I needn’t go into details, I’m sure. But I do want to try to explain how it is that your four tyres won’t always be gripping the road equally.
Think first about braking: this effectively transfers some of the weight of the car from the rear tyres to the front. Result? There will be more grip available at the front and less at the back. Your braking system should allow for this by automatically restricting the pressure applied to the rear brakes — otherwise you would be at risk of a rear-wheel lock-up and a skid.
Next, think about steering: on a tight bend, the outer two tyres (front and rear) are bearing the greater load and therefore supplying most of the sideways grip that you need to get round. Now suppose you make the mistake of braking sharply (while on the bend): I don’t believe any braking system is able to allow for a left–right difference in grip, like it can between front and rear. If one wheel locks up, extra stress will be put on the other tyres. [Footnote: even though I mention ABS (Antiskid Braking System) below, I forgot that it does of course detect and allow for left-right differences in grip when it’s preventing individual wheels from locking!]
If two or more wheels start to slide, you will be in real difficulties. The point to remember is that the available grip between each tyre and the road has to cover both the steering force and the braking force. Hence the rule: don’t brake while you are cornering. And the more slippery the road surface, the sooner you will get into trouble if you do...
Which reminds me that I didn’t say why my mind is on friction and tyre grip. It’s because I’ve just fulfilled a vow I made here more than a year ago, by spending a morning on the skid-pan at Castle Combe! It was three hours of terrific fun, plus expert instruction and the invaluable experience of what different skids feel like and practising how to escape them.
OK, the peanut-shaped track seemed narrow and we never went beyond 30 mph or second gear, but how else can you learn about skids safely? We switched regularly between a rear-wheel-drive BMW 3.18 and a FWD Rover 2.6 — the main problem was that their reverse-gear positions were on opposite sides of the lever.
First the twelve customers were put into the cars three at a time and told just to go out and skid! Every few minutes we changed driver and after that we switched cars. It was slightly scary that the two cars were always on the go together ... fortunately the instructors were on hand to wave us round and round one half of the track when the other car was temporarily stuck at the far end.
Then we went inside for a session on skidding on a bend and how to correct it: foot off the gas, clutch down [unless you have Traction Control] and try to point the front wheels in the direction you’re sliding. Above all, don’t think of braking before you’ve recovered your grip on the road. It’s all about remembering to do the correct actions rather than the instinctive ones.
Back outside to put this into practice. All the while I was getting more used to driving on the treated surface, also the different ways in which the RWD and FWD cars tended to slide on it. For the next exercise an instructor took the Rover (plus each trio of nervous passengers) fast straight down the course and then braked hard to start a four-wheel skid — all steering lost! Next he showed us how you can get back enough steering to avoid an obstacle ahead by using on-off or ‘cadence’ braking [not to be tried in a car fitted with ABS].
Then it was our turn to try. It wasn’t always easy to remember to steer at the same time as pumping the brake. But apparently, owners of cars with ABS also tend to forget that it gives them a similar ability to steer during emergency braking.
The final event was a time-trial in the BMW: once around the slippery track in each direction, requiring a tricky 180° turn after the first circuit. The plan was that each of us would drive the course carrying the next person on the list as passenger. My mistake was to go out first, before I’d fully taken in the route to be followed. I didn’t mention that Mrs S was also enjoying the day’s fun and games, did I — her mistake was to be second on the list!
She had grasped the route to start with, but unfortunately my performance confused her totally for her turn. Still, between us we gave the other ten a good chance to work out what to do themselves. As for our own timings ... I’d say they were a tribute to our most careful and cautious driving under difficult conditions.
Peter Soul
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