God Hjulstämning Alla!
Jag tar mig friheten att saxa följande ur min egen
velomobilseminarieartikel fr 1998, inget är
omodernt ännu tycker jag och hoppas att någon ger
mig feed-back. Hälsningar Anders Brage.
"Where does energy go in small wheels?
Small diameter wheels are prone to stumble over
obstacles. On soft ground, like a lawn or a gravel
faced pedestrian path, they dig steeper holes to
come out of than larger wheels do. I think this is
the main reason for big wheels. A folded bike with
big wheels however is to bulky. Bike wheels have
for long been preferably large, until sir Alec
Moulton combined a small wheel diameter with a
suspension. The Dunlop invention of the pneumatic
tyre gave rise to a new type of rolling resistance
that is to day fairly often misinterpreted, though
the Michelin brothers did invent a good solution
to one part of the problem with their radial
carcass tyre.
A plea for good bicycle tyres have been heard
before and is still to be emphasised for small
diameters.
One major problem is said to be the low priced
import giving developers no margin between the
cost for materials and ready available tyres on
the market. I think there is a technical approach
with patents and profits awaiting for a truly
dedicated developer.
The rolling resistance when no power is
transmitted, as measured by Ian Sims Australia,
give to faint a difference between tyres, to make
up for the sensation of power drain when going
uphill, changing from a racing bicycle to a
mountain bike or a small diameter wheel bike. Does
size really matter this much? I think not, because
the sensation is the same if only tyre pressure is
lowered.
The missing link may be the power transmitted from
the drivetrain through the pneumatic tyre to the
ground. It has to be transformed into stresses in
the tyre carcass, and in my opinion, these
stresses tend to return the deformed cross-section
to the toriodal shape. Meaning, the tyre
cross-section flattened towards the ground, is
deformed back to a round shape by the stressfields
in a diagonal carcass, producing a redundant
work-out equal to squeezing a tennis ball. This
can be designed away in three ways.
· One being a radial orientation of the filaments
together with circomferential filaments building
up the carcass, whereby the tractional filaments
are separated from the filaments deformed by the
ground.
· The other, being a more wide shape of the tyre
cross-section, seen on modern racing motor bikes,
with a rim Width to tyre Height ratio slightly
above one.
· Hence Wrim/Htyre >1. (equ 3)
This could also be accomplished by simply putting
an ordinary tyre on to a wider rim, which however
is hard to find. This ratio has a value close to
one for very thin racing push bike tyres and less
than 0.4 formountain bike tyres. I think
proportion matters more than size uphill. I have
not been able to test this
hypothesis.
· Third, the rubber compound, inherited from the
car tyre industry, where damping is favoured,
couldbe switched to a blend with a greater
bouncing factor, leaving more of the deformation
energy to be elasticly recoverable.
A possible all composite wheel has been made, to
show the that it could be done. Soft spokes from
Kevlar in Polyurethane matrix broke, after that
they had been repeatedly bent at the rim.
The picture (finns på liggisthemsidan) shows a
prototype all composite filament wound wheel, when
loaded across a cable in a softened state, by
closing the hubs to each other. Climbing
capability enhanced by the elastic deformability
and an inherent suspension that gives a minimal
unsuspended weight and good dynamics."