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?

news:3F9F81FF.B2F54D3E@worldnet.att.net...
> 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.

news:3F9F81FF.B2F54D3E@worldnet.att.net...
> 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!

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

news:3F9F81FF.B2F54D3E@worldnet.att.net...
> 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=-

=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 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

> 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

> 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 jobst.brandt@stanfordalumni.org

On Wed, 29 Oct 2003 12:40:50 +0000, Chalo wrote:

> 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

An extreme example of that were the old ('60s) bikes used for motor-paced racing on a track, with a
24" front wheel and 700c rear. They had reverse rake on the forks in order to have enough trail to
be ridable. The use of the small front wheel was to allow the rider to get closer to the motorcycle.

--

David L. Johnson

__o | I don't believe you, you've got the whole damn thing all wrong. _`\(,_ | He's not the kind
you have to wind-up on Sundays. --Ian (_)/ (_) | Anderson

The Moultons compensate for the small wheels with full suspension. Moulton was a pioneer in
this regard.

Even my unsuspended Bike Friday NWT, though, is comfortable enough to
me. Granted, it got more so after I bought a Brooks saddle with springs, but it was good even
before this, not a signifcantly harsher ride than my unsuspended 700c bikes.

I also have a Dahon Boardwalk D6, 20 inch wheels, no suspension. This is Dahon's cheapest
multi-speed bike. It's very comfortable, pretty much like any low end comfort bike with 26" or 700c
wheels. The low gearing and relaxed geometry mean it's best suited for laid back recreational rides,
but the wheel size isn't an issue.

sggoodri@mindspring.com (Steven Goodridge) wrote in message
news:<99edd02a.0310291132.3f6ee52c@posting.google.com>...

> > 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

In article <8b4b7de4.0310291240.30548f5a@posting.google.com>, Chalo <chumpychump@hotmail.com> 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

Making too much sense, Chalo. Gotta stop doing that.

David L. Johnson <david.johnson@lehigh.edu> writes:

>> 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

> An extreme example of that were the old ('60s) bikes used for motor-paced racing on a track, with
> a 24" front wheel and 700c rear. They had reverse rake on the forks in order to have enough trail
> to be ridable. The use of the small front wheel was to allow the rider to get closer to the
> motorcycle.

The use of the reversed fork was to get even closer to the motorcycle than just the small wheel
allowed. After this became standard practice, the rules were set and the roller on the motorcycle
was extended to not allow the driver to get any closer to the bicyclist. The whole get-up is absurd
if you look at it closely. The motorcyclist sits rigidly upright so far back that he needs long
handlebar extensions with which to steer.

Jobst Brandt jobst.brandt@stanfordalumni.org

jobst.brandt@stanfordalumni.org wrote:
> 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 jobst.brandt@stanfordalumni.org

Roller skating has nothing to do with it. I have just measured the rolling radius of a 26" tire at
24.5" and the rolling radius of a similar profile 20" (406) tire at 19" then calculated the the
height of the tires in each case above the next peak if they were being ridden on coarse agregate
roadway with the peaks 0.5" apart, which is rough enough in anybody's language. The 26" tire will be
.005" above the next peak and the 20" tire will be .0066" above the next peak, which in the whole
scheme of things is insignificant.

The above is armchair theory at its best. The proof of the pudding is in the eating. Having had the
experience of riding an aluminium alloy framed 26" wheeled tandem and owning a 20" wheeled tandem, I
am of the opinion that there are so many other factors to consider that the ride quality and
efficiency of a 20" wheeled bike versus a 26" wheeled bike each shod with the same type of tires is
so far down the list as to be not worth consideration.

Like others before us, my wife and I are so impressed with our 20" wheeled BF tandem that we are
considering scrapping our "good" solo bikes in favour of 20" wheeled solo bikes. We have only days
ago returned from a tour of the UK on our 20" wheeled tandem, where we commenced riding in the
Orkneys and ceased in Cornwall. But what would we know about it, we are both over sixty and
shouldn't be messing with bikes should we?

Alan.