Paul P. Mealing

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Saturday, 15 March 2014

The physics of driving

This is quite a departure, I know, but one of my hobby-horses is how little people know about the physics of driving. Unlike our man-made road rules, the laws of nature are unbreakable, so a rudimentary knowledge can be useful.

But what prompted me to write this post was a road test I read of the new, upmarket Infiniti Q50 in EVO Australia (March 2014). The big-selling feature of the Infiniti Q50 is its so-called ‘direct adaptive steering’; a world first, apparently for a production car (as opposed to a prototype or research vehicle). It’s a totally ‘fly-by-wire’ steering system, so there is no mechanical connection between the helm and the front wheels. Personally, I think this is a dangerous idea, and I was originally going to title this post, rather provocatively, “A dangerous idea”. Not surprisingly, at least to me, the road-tester found the system more than a little disconcerting when he used it in the real world. It was okay until he wanted to push the car a little, when the lack of feedback through the wheel made him feel somewhat insecure.

There are gyroscopic forces on the front wheels, which naturally increase with cornering force and can be felt through the  steering wheel. The wheel weights up in direct proportion to this force (and not the amount of lock applied as some might think). In other words, it’s a linear relationship, and it’s one of the major sources of determining the cornering force being generated.

I should point out that the main source of determining cornering force is your inner ear, which we all use subconsciously, and is why we all lean our heads when cornering, even though we are unaware of it. It’s the muscle strain on our necks, arising from maintaining our inner ear balance, that tells us how much lateral g-force we have generated. On a motorcycle, we do the opposite, keeping our heads straight while we lean our bodies, so the muscle strain is reversed, but the effect is exactly the same.

Therefore, you may think, we don’t need the steering wheel’s feedback, but there is more. The turn-in to a corner is the most critical part of cornering. This was pointed out to me decades ago, when I was a novice, but experience has confirmed it many times over. Yes, the corner can change radius or camber or both and you might strike something mid-corner, like loose gravel, but, generally, if the front wheels grip on entry then you know they will grip throughout the rest of the corner. This is the case whether you’re under brakes or not, wet or dry surface. It’s possible to loosen traction with a heavy right foot, but most cars have traction control these days, so even that is not an issue for most of us. The point is that, if the front wheels grip on turn-in, we ‘feel’ it through the steering wheel, because of the gyroscopic relationship between cornering force and the weight of the wheel. And cornering force is directly proportional to the amount of grip. The point is that without this critical feedback at turn-in, drivers will be dependent on visual cues to work out if the car is gripping or not. What’s more, the transition from grip to non-grip and back won’t be felt through the wheel. If this system becomes the engineering norm it will make bad drivers out of all of us.

While I’m on the topic, did you know that at twice the speed it takes four times the distance to pull up to a stop? Perhaps, you did, but I bet no one told you when you were learning to drive. The relationship between speed and braking distance is not linear – braking distance is proportional to the speed squared, so 3 times as fast takes 9 times the distance to stop. This is independent of road surface, tyres and make of car – it’s a natural law.

Another one to appreciate is that at twice the speed, changing direction is twice as slow. There is an inverse relationship between speed and rate of change of direction. This is important in the context of driving on multi-lane highways. A car travelling at half the speed of another – that’s overtaking it, say – can change direction twice as fast as the faster car. This is also a law of nature, so even allowing for superior tyres and dynamics of the faster car, the physics is overwhelmingly against it. This is why the safest speed to travel on multi-lane highways is the same speed as everyone else. An atypically slow car, in these circumstances, is just as dangerous (to other motorists and itself) as an atypically fast car.

Addendum: I also wrote a post on the physics of riding a motorcycle.


Amelia Jack said...

I think if the front wheels grip on turn-in, we ‘feel’ it through the steering wheel, because of the gyroscopic relationship between cornering force and the weight of the wheel. And cornering force is directly proportional to the amount of grip. You have shared useful article regarding Driving. Keep it up.
Tyre Changers

Carl said...

When you hit the brakes, the tires grip the road better when they are rolling and not skidding.
And Anti-Lock braking Systems(ABS) are there to ensure that the wheels don't lock and skid.
Personally i feel that ABS are very good when safety is concerned, but not when you are learning to drive for the first time.
I feel its important to learn the limits of the brakes of your car or bike and learn to control it first without the ABS, in order to take the fully understand the advantage of the ABS.

Paul P. Mealing said...

Hi Carl,

I have to admit I didn't expect this post to generate so much interest, as it's the first I've written specifically about driving. Though I wrote a review of a film about Ayrton Senna, a few years back.

I've been driving for over 40 years, and I've only driven cars with ABS in the last decade and a bit. Before ABS (some) cars were fitted with a small valve that pushed more hydraulic fluid to the front wheels than the back under hard braking. The valve was simply activated by the car's natural tendency to nose-dive under braking, so the harder you braked the more braking was done by the front wheels compared to the rears. This stopped the back wheels locking up and trying to overtake the front. It was very simple, totally mechanical and very effective. It was designed to make the front wheels lock up first.

Cars with ABS still use this system, so that the ABS only works on the front wheels (or at least the two I've owned have, both French). I think ABS is brilliant, but in my car it only activates under very hard braking indeed. You can anticipate the point where it will engage, but this is dependent on the brake 'feel'. I've driven cars that have good brakes but no feel, so you've no idea how much braking you have left. A good braking system is progressive in that the pedal 'hardens' as you get closer to lockup, when the ABS intervenes. The problem some people have with ABS is that they immediately release the pedal when it activates, when you should do the opposite: press harder.

I notice you are a physicist, so you may be interested in some of my other posts. In fact, you may know their subject matter better than I do.

Regards, Paul.