What's the disadvantage of small wheels?

But for brake rotors, on MTB's, the rotor speed passing the caliper fo
larger rotors is greater... n'est ce pas?


Wrote:
Jeff Wills writes:

Almost as obviously, a smaller rim offers less rim-caliper braking.

Maybe you could explain this more completely?

Let's look at this more closely. The rim passes the brake caliper at
the same speed regardless of wheels size, at the forward speed of the
bicycle essentially. Therefore, leverage is the same for small and
large wheels. What is different is the size of the heat sink, the rim
volume and area. A small wheel will heat faster then a large rim by
the ratio of size. A brake converts kinetic energy to heat and the
rim blows it away to the air. Less rim, less cooling surface and less
energy storage in the metal of the rim.



Jobst Brandt


--
Mr_Potatohead

On Thu, 19 Aug 2004 10:57:34 -0700, Peter
wrote:

wrote:

My theory is leverage.

Take two bicycles and riders that weigh roughly the same,
one with large wheels and one with wheels half as tall.

Both are rolling at 20 mph.

The rim caliper brake pads are pressing against a rim that
goes around at the same 20 mph.

For braking at this speed, the difference in the rotating
mass of the wheels is negligible--it's the much greater mass
of the bicycle and rider that we're trying to brake to a
halt.

Think of a spoke out to the rim as the lever through which
you're trying to resist the turning of the hub by squeezing
the rim attached to the spoke. The longer the lever, the
greater your mechanical advantage.

The leverage effects almost cancel out. You can also think of
the radius of the tire as the lever arm by which the road is
trying to keep the wheel turning, so on smaller wheel bikes
both lever arms are reduced in size and you still get
adequate braking force. I said "almost" above since the
brake is acting on the rim and the road is acting on the outer
edge of the tire. So if the tire is 1" wide we might have
a ratio of 9/10 for the brake/road lever ratio for a 20" wheel
while it would be about 12.5/13.5 for the 27" wheel.

However, another factor with braking, especially on long
descents, is rim heating. The smaller rim will heat up more
for the same amount of energy that's being dissipated by the
braking. I've noticed the rims on my Bike Friday getting
very hot on descents and take more care to use the front and
rear brakes equally so neither rim will get too hot.


Flip a bicycle upside down, press the crank gently, and hold
the wheel still with a finger against the spoke nipple.

Now slide your finger in toward the hub, doing your best to
maintain the same pressure against the spoke. With less
leverage to resist the turning of the hub, your finger
pressure is no longer enough to resist the wheel against the
same force.

But when braking the force trying to keep the wheel turning is
coming from the friction between the road and the tire. So
your comparison should be to put one hand on the tire and push
it backward while resisting that force with the other hand on the
rim. In that case you need essentially equal and opposite forces
regardless of wheel size (except for the small effect due to the
tire radius being larger than the rim radius as mentioned above).

Dear Peter,

Hmmm . . . it sounds suspiciously as if I was wrong.

I think that I follow your explanation about the ratio of
the brake-pad-to-the-hub and the hub-to-the-contact-patch
remaining the same regardless of the size of the wheel.

I also see your subtle point about the rim not being quite
as far out as the contact patch.

Here's where I want reassurance. (It's nice to have
engineering types handy to work things out and pat me on the
head.)

To remove the slight difference between the distance from
the hub to the rim and the distance from the hub to the
contact patch, let's use a hypothetical pressure brake that
pushes down on the tire instead of squeezing from the sides
like a real brake.

If I'm following you, a 10-pound pressure on a full-size
700c tire should produce the same braking force as a
10-pound pressure on a 1-inch diameter round rubber caster.

Right?

And the disk brake needs the greater pressure because it has
a shorter caliper-to-hub leverage compared to its
contact-patch-to-hub leverage.

Right?

Hopefully,

Carl Fogel

On Fri, 20 Aug 2004 05:10:51 +1000, Mr_Potatohead
wrote:


But for brake rotors, on MTB's, the rotor speed passing the caliper for
larger rotors is greater... n'est ce pas?


Wrote:
Jeff Wills writes:

Almost as obviously, a smaller rim offers less rim-caliper braking.

Maybe you could explain this more completely?

Let's look at this more closely. The rim passes the brake caliper at
the same speed regardless of wheels size, at the forward speed of the
bicycle essentially. Therefore, leverage is the same for small and
large wheels. What is different is the size of the heat sink, the rim
volume and area. A small wheel will heat faster then a large rim by
the ratio of size. A brake converts kinetic energy to heat and the
rim blows it away to the air. Less rim, less cooling surface and less
energy storage in the metal of the rim.



Jobst Brandt


Dear Mr. P.,

I think that Peter and Jobst are right and that I was wrong.

The leverage is from the brake-pad-to-the-hub versus from
the contact-patch-to-the-hub.

With rim calipers, the ratio remains roughly the same, no
matter how large or small the wheel is.

But with a separate rotor, the brake-pad-to-hub leverage is
much smaller compared to the contact-patch-to-the-hub
leverage, so disk brakes need to squeeze harder.

Sorry about misleading you.

(And even sorrier if this is yet another goof.)

Repentantly,

Carl Fogel

wrote:

Hmmm . . . it sounds suspiciously as if I was wrong.

I think that I follow your explanation about the ratio of
the brake-pad-to-the-hub and the hub-to-the-contact-patch
remaining the same regardless of the size of the wheel.

I also see your subtle point about the rim not being quite
as far out as the contact patch.

Here's where I want reassurance. (It's nice to have
engineering types handy to work things out and pat me on the
head.)

To remove the slight difference between the distance from
the hub to the rim and the distance from the hub to the
contact patch, let's use a hypothetical pressure brake that
pushes down on the tire instead of squeezing from the sides
like a real brake.

If I'm following you, a 10-pound pressure on a full-size
700c tire should produce the same braking force as a
10-pound pressure on a 1-inch diameter round rubber caster.

Right?

I hesitate to use the word pressure here since that gets us
into all sorts of complications with coefficients of
friction between the brake pad and the tire, areas involved, etc.

But if both brakes create a tangential retarding force of 10
lbs on the outer edge of the tire, then the wheels (and attached
vehicle) will stop equally fast in both cases. But the small
rubber caster may melt first and prematurely end the experiment.

And the disk brake needs the greater pressure because it has
a shorter caliper-to-hub leverage compared to its
contact-patch-to-hub leverage.

Right?

Yes, with the same caveat about pressure.

"BruceW..1" wrote in message
m...
There are some pretty cool and lightweight collapsible bicycles on the
market, like this one at 18 lbs.:
http://www.dahon.com/heliossl.htm

And of course there are much nicer collapsible bikes:
http://www.betterbicycleco.com/allegro.htm
http://www.ritcheylogic.com/bab_home.htm

This boils down to a basic physics question. Is there a disadvantage to
riding smaller wheels (like 16")... that spin faster?

In the history of bicycles, wheels started out really big. They are
much smaller now.

Of course chuckholes would be a bigger problem with small wheels, but
assuming smooth roads is there a disadvantage to riding smaller wheels?

Thanks for your help.

Assuming smooth roads takes care of the main issue. Small high pressure tires
transmit more vibrations and bumps to the rider than larger wheels and tires. I
have a 20" wheel folding bike that is pleasant to ride because it has a
suspension fork and a suspension seatpost.

http://cheg01.home.comcast.net/r20.html

Here is the latest incarnation with less weight and fewer gears:
http://cheg01.home.comcast.net/20_light_1.jpg
http://cheg01.home.comcast.net/20_light_2.jpg
http://cheg01.home.comcast.net/20_light_3.jpg
http://cheg01.home.comcast.net/20_light_4.jpg

wrote in
:
/snip
Let's look at this more closely. The rim passes the brake caliper at
the same speed regardless of wheels size, at the forward speed of the
bicycle essentially.

Jobst Brandt

snip/

The rim passes the brake caliper at approximately _twice_ the forward speed
of the bicycle.
Otherwise, you are correct.

wrote:
My theory is leverage.

I'll throw in my 2 bits for leverage, but not just leverage for the
brakes: diminished leverage and rotational speed for the hub (and lower
chain speed for the same road speed). That would increase friction
(although it's probably a small penalty).

I'm curious if the greater acceleration in tire at the contact patch
(due to the smaller circumference) would increase rolling resistance as
well.

Bill

IK writes:

wrote in
:
/snip
Let's look at this more closely. The rim passes the brake caliper at
the same speed regardless of wheels size, at the forward speed of the
bicycle essentially.

Jobst Brandt

snip/

The rim passes the brake caliper at approximately _twice_ the forward speed
of the bicycle.
Otherwise, you are correct.

The top of the rim moves at twice the forward speed relative to the ground.
The brake is moving at the forward speed relative to the ground.
The top of the rim moves at the same speed relative to the brake as the
brake moves relative to the ground.

ik@ik writes:

Let's look at this more closely. The rim passes the brake caliper
at the same speed regardless of wheels size, at the forward speed
of the bicycle essentially.

The rim passes the brake caliper at approximately _twice_ the
forward speed of the bicycle.

Correct. The hub moves forward at the speed of the bicycle, the
ground contat stands still and the top of the wheel is twice as fast
as the hub/bicycle. I need to keep that mental picture in focus.

Otherwise, you are correct.

Thanks,

Jobst Brandt

writes:

Larger wheels roll more smoothly over surface irregularities
than small wheels. It's not just chuckholes, but much
gentler bumps and dips. Jobst Brandt has suggested that the
suspension on the small-wheel Moulton is not so much a
convenience, but a necessity to take care of the rougher
ride. And the Moulton wheels have gotten larger over the
years, not smaller.

I believe that was to achieve an industry standard tire size.
The suspension is certainly a necessity on a small wheeled Mouton
(I commute on one), however, the result is much smoother than
a large wheel, unsuspended bike.

A fainter advantage is that a smaller wheel has less mass,
so it accelerates with less effort. Outside of sprinting,
this is negligible.

The suspension more than compensates for that effect 8-(. I used to
occasionally take the Moulton to the weekly training crit; it was a
hell of a workout. Actually, Moulton wheels aren't any lighter than
most 700c wheels, but the wide tires and suspension give a good grip.

Joe