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From: Adrian on 18 May 2010 14:38
"Mortimer" <me(a)privacy.net> gurgled happily, sounding much like they were
saying:

> OK, so stopping in 100 ft would produce a survivable deceleration. But
> would 100 ft of sand drag be enough to produce sufficient deceleration
> force to slow a 40-tonne HGV to zero so it didn't run off the end of the
> escape lane or hit an earth bank or whatever?

More than enough.

> Would the retardation force be as great as if the lorry had been able
> to slow using its brakes?

Far greater.

Allegedly 30t from 56mph...
http://youtube.com/watch?v=X70f0zzVENI
http://youtube.com/watch?v=oRFYNgbRwgw
From: Mortimer on 18 May 2010 15:51
"Adrian" <toomany2cvs(a)gmail.com> wrote in message
news:85g54uF1meU9(a)mid.individual.net...
> "Mortimer" <me(a)privacy.net> gurgled happily, sounding much like they were
> saying:
>
>> OK, so stopping in 100 ft would produce a survivable deceleration. But
>> would 100 ft of sand drag be enough to produce sufficient deceleration
>> force to slow a 40-tonne HGV to zero so it didn't run off the end of the
>> escape lane or hit an earth bank or whatever?
>
> More than enough.
>
>> Would the retardation force be as great as if the lorry had been able
>> to slow using its brakes?
>
> Far greater.
>
> Allegedly 30t from 56mph...
> http://youtube.com/watch?v=X70f0zzVENI
> http://youtube.com/watch?v=oRFYNgbRwgw

Ah right. Did the sand drag start at the point where the pale blue barrier
started? Was the black lane leading up to it just tarmac? The truck stopped
in a *very* short distance - "ni pissing" about (to borrow the phrase on the
sign in the background!).

From: Adrian on 18 May 2010 16:07
"Mortimer" <me(a)privacy.net> gurgled happily, sounding much like they were
saying:

>> Allegedly 30t from 56mph...
>> http://youtube.com/watch?v=X70f0zzVENI
>> http://youtube.com/watch?v=oRFYNgbRwgw

> Ah right. Did the sand drag start at the point where the pale blue
> barrier started? Was the black lane leading up to it just tarmac?

You can see the reaction of the cab as the actual retardant starts -
behind the barrier...
From: Man at B&Q on 19 May 2010 05:59
On May 18, 7:23 pm, "Mortimer" <m...(a)privacy.net> wrote:
> "Clive George" <cl...(a)xxxx-x.fsnet.co.uk> wrote in message
>
> news:m9WdnXgwGMjbSW_WnZ2dnUVZ7sWdnZ2d(a)brightview.co.uk...
>
>
>
> >>> There's one..  Not that I'd know either way about the physics of it all
> >>> :)
>
> >> About 100ft by my ruler. The ordinary stopping distance for a car at
> >> 56mph is well over 300ft - presumably a lot more for a heavy lorry.
>
> > And the ordinary stopping distance is actually a fairly gentle
> > deceleration on the scale of things.
>
> > 56mph, 300ft = about .35g
> > 56mph, 100ft = about 1G, which is about the maximum braking a car can do.
> > That doesn't begin to hurt the driver.
>
> > Double, triple that - still not going to be a problem. Think about the
> > films you see of car crash tests - 30mph to zero in about a metre, which
> > is 8G+. That'll give bruising, but it's survivable.
>
> > Lorry vs car makes no difference - it's the deceleration which matters.
>
> > If it was going to be like driving into a brick wall, it would need to be
> > zero length. 100ft is nothing like that.
>
> OK, so stopping in 100 ft would produce a survivable deceleration. But would
> 100 ft of sand drag be enough to produce sufficient deceleration force to
> slow a 40-tonne HGV to zero so it didn't run off the end of the escape lane
> or hit an earth bank or whatever?

Does it matter? It'll still be slower than hitting something without
the benefit of the escape lane.

> Would the retardation force be as great as
> if the lorry had been able to slow using its brakes?

If it could, then there would be no need for the escape lane.

MBQ

From: Mortimer on 19 May 2010 13:01
"Man at B&Q" <manatbandq(a)hotmail.com> wrote in message
news:63054dd2-1d84-46f6-8728-bea364c17831(a)r21g2000prr.googlegroups.com...
On May 18, 7:23 pm, "Mortimer" <m...(a)privacy.net> wrote:
> > Would the retardation force be as great as
> > if the lorry had been able to slow using its brakes?

> If it could, then there would be no need for the escape lane.

Yes there would: I said " if the lorry had been able to slow using its
brakes" (ie the normal situation). I was assuming that the lorry would use
the escape lane because it *wasn't* able to use its brakes because they had
failed.

The videos of the test show that the lorry *is* slowed very dramatically by
sand - even more so than by braking - which answers my question.


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