APR wrote:
"Eiron" wrote in message
...
wrote:
You can't easily go wrong designing a bicycle, if that means choosing a
frame,
wheels etc. from a manufacturer's catalogue, and having it built for you.

A young girl died in Brisbane Australia approx 10 - 12 years ago when her
cheap imported bike became dangerously unstable at speed on a down hill
section of bitumen road. The University of Queensland undertook a review of
the bike design and came to the conclusion that the steering head angle in
conjunction with fork design led to the instability displayed at speed (the
design evidently became unstable at approx 35 - 40 Km/h).

The relationship between steering head angle and trail distance (distance
from where a line through the steering head will intersect the road to where
the tyre contacts the road) is very relevant for controllability. You should
not mix and match forks and frames if you don't know what you are doing.



APR wrote:
"Eiron" wrote in message
You can't easily go wrong designing a bicycle, if that means choosing a
frame,
wheels etc. from a manufacturer's catalogue, and having it built for you.

A young girl died in Brisbane Australia approx 10 - 12 years ago when her
cheap imported bike became dangerously unstable at speed on a down hill
section of bitumen road. The University of Queensland undertook a review of
the bike design and came to the conclusion that the steering head angle in
conjunction with fork design led to the instability displayed at speed (the
design evidently became unstable at approx 35 - 40 Km/h).

The relationship between steering head angle and trail distance (distance
from where a line through the steering head will intersect the road to where
the tyre contacts the road) is very relevant for controllability.

Specifically, trail distance is result of the relationship between head
angle, fork rake (or offset), and tyre diameter. Wheelbase and seat
tube angle also have effects on controllability, through weight
distribution and transfer. A good test of whether someone understands
these sometimes counterintuitive relationships is to ask him what
happens when a raked fork is turned back to front: does it increase or
decrease the trail? Fork material and design will also have a
resonance, and roads have their own frequencies, different in different
countries usually according to the size of trucks/number of axles
permitted; these can set up interference patterns evidenced as wheel
shimmy.

You should
not mix and match forks and frames if you don't know what you are doing.

Thanks for the well-meant warning but I've done suspensions and
steering before, even wrote a book about it. If you're interested in a
good read on bicycle dynamics, I can recommend:
David E. H. Jones, The Stability of the Bicycle, PHYSICS TODAY, APRIL
1970, pp34-40.

Andre Jute
Visit Jute on Amps at http://members.lycos.co.uk/fiultra/
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