Road safety advert

[smorodina]

In the recent advert for road safety that tells you how long a car takes to stop at 35 mph. The front wheels have locked and the rear wheels are turning normally. Am I wrong in thinking that as the weight of the vehicle is transfered to the front when it brakes, that the corresponding decrease in weight on the rear axle would make the rear wheels lock up before the front?

[sddsddean]

The answer to this is 'depends'. Presumably the car doesn't have ABS or the wheels would still be turning. So can we assume this is an old car as most modern cars have ABS as standard. If so, it may have disc brakes on the front and drums on the back (lots of modern cars still do as well). As a disc brake is much more efficient than a drum brake it will 'lock up' much easier. This is probably the answer to your question. Weight distribution won't come into it.

[Hippy]

Sorry to be a party pooper, but the special effects boys have been at it again. The front wheels of the car are not in conatct with the road, but the car is running on small castors just inboard of the real wheels. A small smoke charge provides the illusion of the skid.

Almost all of the scene is a fake, including the slo-mo effect. The real action in the shot was slower than reality, and the camera was speeded up very slightly (less than would have been necessary for true slo-mo of a real 35mph car). This gives a dreamlike quality. The little boy is a manakin and the out of focus flying bird across the foreground is a clever wire suspended touch.

All of the actors in the scene, the woman pedestrian and the man on the bike for example all have their eye-line and body twists focussed on the impact. It all adds up to a very powerful twelve seconds. I suspect that it took days to put together!

[PhilLew]

If the situation was real and not done with camera trickery, the car could have been rear wheel drive and was still in gear when the brakes were applied :-)

[sddsddean]

Not doubting you for one second Hippy, but how do you know? Inside information or are you one of the unsung special effects boys?

[Eddiecurrent]

Most cars have a pressure regulated valves either in the wheel cylinders or on the brake lines that feed the rear brakes. These valves prevent the rear brakes from locking hence the rotating wheels, no special effects, no castors etc. If the rear wheels locked under braking this could cause the vehicle to perform a rear end skid, pretty fatal unless you can handle that type of thing.