Tony Raven said:At 50mph that foot gap takes 13ms to cross. It takes typically 70-150 times that to react to the
van slowing.
First of all, the gap high-speed opportunist drafters leave is usually greater than one foot
(despite what boasts people may make or what it looks like from a distance). Two, three or four feet
(or even a few yards) is more like it.
I dispute those reaction times, anyway. It takes a fraction of one second to react if concentrating
hard. It's like formation flying or motor racing. Being very close to the vehicle means you can read
what it's doing much quicker and more precisely than you can when further away, and so virtually act
as one with it, increasing the gap by inches as and when it closes by inches.
Longer reaction times are quoted for car drivers because the typical gap is greater: it takes longer
to realise that it has changed; plus concentration is nothing like so high or focused. Of course I'm
not advocating that people drive closer - because although reaction times would be reduced, margin
for error would also be reduced. (I think they should drive slower, concentrate harder and leave
bigger gaps).
Quoted message said:Conclusion, the van does not need to brake suddenly, even gentle slowing would be enough to cause
a collision.
Anyone who has tried it knows that it is easily possible to cope with gentle slowing.
I don't dispute that it's dangerous. The main dangers (assuming a relatively suitable vehicle and
road is chosen) are the lack of margin for extra hesitation (scratch nose at the wrong time and
you've had it), failing to employ good braking technique, and loosing control or breaking something
after hitting a pothole.
It is possible to out-brake an average articulated lorry on a good road bike (even if the lorry
brakes as hard as possible), but may be impossible for lighter vehicles. Drafting big lorries
occasionally is moderately mad, IMO. Drafting vans at high speed is suicidal - unless the driver is
co-operating, in which case it's only barking mad.
~PB