Why are the wheels big? Why not tiny wheels?

For what reason are bicycle wheels 26-inches/700cm? What would happen
if the wheels on an upright were small like the wheels of a recumbent?
Please explain the geometry and the physics!

on a mountain bike, how are you gonna clear obstacles?
"Kolaga Xiuhtecuhtli" wrote in message
...
For what reason are bicycle wheels 26-inches/700cm? What would happen
if the wheels on an upright were small like the wheels of a recumbent?
Please explain the geometry and the physics!

You mean like Bike Fridays that are 20" x 20" or Dahons that may be even
smaller?

The 'geometry' and 'physics' are trivially different unless you've a
specific question.

My riding partner rode 1200 miles with me this spring, to Florida and back,
on her Friday Air Glide while towing Friday's hardcase trailer.


"Kolaga Xiuhtecuhtli" wrote in message
...
For what reason are bicycle wheels 26-inches/700cm? What would happen
if the wheels on an upright were small like the wheels of a recumbent?
Please explain the geometry and the physics!

=v= Bigger wheels get more forward rolling momentum, so it takes
a little more work to steer them.

=v= I find that with 20-inch wheels (such as on my Bike Friday),
are actually pretty nice for everyday city commute riding, since
I have to do a lot of maneuvering.

=v= If the wheels are *too* small, you get practically no
forward momentum, so every anomaly, bump, or gum wrapper on the
road makes the front wheel turn, and you end up working hard
just to keep the bike going forward. This was one of the big
problems with my first folding bike.
_Jym_

P.S.: Here's a handy photographic reference:

http://www.bikesummer.org/2003/photo...hp?PhotoID=491

The Bike Friday I'm holding has 20-inch wheels. The woman in
front of me is riding an early model Dahon, the very model I
used to own, with the difficult front wheels. (Newer Dahons
with 20-inch wheels are much better.)

I have a Bike Friday (NWT) and have noticed 3 traits of the smaller wheels.
They are very quick. You can really accelerate on them. They make for very
responsive steering. Some would say too responsive, almost twitchy. You can
really over steer and go down in a turn if you aren't careful, ask me, I know.
Last but not least, they do transmit more road shock than bigger wheels.

Andy

I forgot to mention, after several years of the Bike Friday being my main ride,
when I look at my old bike, it reminds me of one of those old antique bikes
with the one HUGE wheel. I'm just not used to it anymore.

Andy

"Kolaga Xiuhtecuhtli" wrote in message
...
For what reason are bicycle wheels 26-inches/700cm?

They seem to offer the best handling for non-smooth roads and trails. The
higher rotational inertia of larger wheels provides some steering damping,
and a nominal amount of gyro effect (though this is apparently not crucial
for balance).

What would happen
if the wheels on an upright were small like the wheels of a recumbent?

This is what Dr. Alex Moulton spent many years researching:

http://www.alexmoulton.co.uk/

Please explain the geometry and the physics!

Small wheels deflect off of obstacles at a sharper angle, causing a harsher
ride.

I couldn't find any websites with good equations for you. Do a little Google
searching - there's plenty of information about bicycle physics to be had
(though some of it is bunk, as I just found out!).

Cheers,

-=Barry=-

Kolaga Xiuhtecuhtli wrote in message ...
For what reason are bicycle wheels 26-inches/700cm? What would happen
if the wheels on an upright were small like the wheels of a recumbent?
Please explain the geometry and the physics!

Smaller wheels absorb less road shock, unless the tires are inflated
to lower pressure, which increases rolling resistance.

The recent advancement of affordable, low-weight shock absorber
components now make high-pressure small-diameter bicycles more
practical and comfortable. The Moulton bicycles are an excellent
example of fast, comfortable, high-quality bikes with small diameter
wheels.

Steve Goodridge

Kolaga Xiuhtecuhtli wrote:

For what reason are bicycle wheels 26-inches/700cm?

Because those sizes are approximately the largest that will work with
small frames and still allow basic frame design principles to apply.

What would happen
if the wheels on an upright were small like the wheels of a recumbent?
Please explain the geometry and the physics!

Small wheels rise and fall more abruptly upon hitting bumps, and
descend much more deeply into holes than larger wheels do. Damping
out the additional vertical motion of small wheels uses rider energy
and causes vibration-induced fatigue.

Small-diameter tires flex more deeply to establish a contact patch
than large-diameter tires do. This causes significant extra rolling
resistance. The shorter contact patch of a small wheel's tire gives
up some amount of traction compared to a larger wheel with a similar
width tire, and accelerates tread wear.

Most of the shortcomings of small bicycle wheels can be remedied at
least somewhat by the use of higher tire pressure along with effective
suspension. Traction and wear shortcomings will remain, though, and
are exaggerated by higher tire pressure.

Small front wheels reduce steering trail dimension, which is
proportional to wheel diameter. Within normal limits, trail has a
steering-stabilizing effect correlated to its length. To hold this
value at a normal figure with small wheels, a slacker head angle or
shorter fork offset must be used, both of which tend to increase wheel
flop (the tendency of a bike's steering to "fall" into a turn under
the force of gravity), so one way or the other you wind up with a bike
that requires more rider intervention to keep under control than a
large-wheeled bike does.

Small wheels serve to maximize wheel strength and minimize bike weight
and physical size. Larger wheels reduce rolling resistance and
increase rider comfort and steering stability.

Chalo Colina

Kolaga Xiuhtecuhtli writes:

For what reason are bicycle wheels 26-inches/700cm? What would
happen if the wheels on an upright were small like the wheels of a
recumbent? Please explain the geometry and the physics!

It's not physics. You can test this by roller skating on a freshly
chip sealed paved road or for that matter on a dirt road. That ought
to be convincing, considering that bicycles are commonly used to ride
on such surfaces regularly.

Jobst Brandt