tobold.org

correct • elegant • free

△ Cam.misc △

Cycle & pedestrian railway bridge, council comment? ▻

Safer driving and cyclists

In article <3805ku$58f@acorn.acorn.co.uk>, William Turner
<wturner@acorn.co.uk> wrote:
>Sounds like my car! However, I can see the original posters point, but it
>is the fact that you _are_ in a low gear which causes the problem. Low gears
>are designed to provide acceleration, ie a large increase in speed for a small
>increase in throttle.

Dodgy physics alert! :-)

Low gears are designed to provide high torque at the wheels, which they
obtain by trading in rotational distance.  So a low gear gives a small
increase in road speed for a large increase in engine speed.

Now, if engine speed is directly proportional to throttle position, this
suggests it should be easier to control the road speed in a lower gear.

They're not proportional, of course, because there's an opposite (and
equal :-) effect I haven't taken account of.  The gearing reduces the
reactive torque from the wheels to the engine, giving a large increase
in engine speed for a small increase in petrol flow.

So far as I can see, the effects of the gearing should balance out.

Perhaps what makes it easier to control road speed at higher speed is
that overall drag is much higher.  You need to burn more petrol to
maintain the higher speed, so there is a small increase in road speed
for a large increase in petrol flow.

>Now, to move to another position, you have to overcome the initial static friction,
>which is lower than the dynamic friction. [...]

I suggest you invest in a can of WD-40, which will sort out most of your
friction, both dynamic and static :-).

>BTW: Cyclists find it more difficult to travel at low speeds too, so I can't
>see why they are saying that cars should find it easy!

But that's for a completely different reason.  A bicycle (or motorcycle)
uses the gyroscopic effect of the wheels to help maintain balance.
At slow speeds, the gyroscopic forces are lower, and it is harder to
balance.

IMHO, anybody who can't adequately control a car at anything above
walking speed should not be driving.

Tim.
--
Tim Goodwin        | "The past contents of this file
Public IP Exchange | is now obsolete." -- u.nzl.1

Original headers:

From: tim@pipex.net (Tim Goodwin)
Newsgroups: cam.misc,uk.transport,uk.misc
Subject: Re: Safer driving and cyclists
Date: 19 Oct 1994 18:30:22 +0100
Organization: PIPEX, 216 Science Park, Cambridge, England
Message-ID: <383l3e$pt3@pipe.pipex.net>
References: <CxtJny.7xs@festival.ed.ac.uk> <3805ku$58f@acorn.acorn.co.uk>

△ Cam.misc △

Cycle & pedestrian railway bridge, council comment? ▻