Bearing diameter theory

Squid-in-Training said:

Let's say that we have gigantic bearings, about the size of a rim for any
given rim size (huge balls, not a series of small balls). My impression is
that rolling resistance would be huge.

Weight would be huge, and it wouldn't be certified for high rpm (it'd
probably be certified to about half a kilometer per hour or so :P ). I'm
not so sure rolling resistance would necessarily be huge when lightly
loaded (assuming you could somehow make the balls weigh about what a
regular ball weighs rather than ten times the whole bike causing the
system to be fairly highly loaded just on its own weight).

Quoted message said:

Now, if we take the bearing diameter and cut it down continuously, we would
have less and less friction owing partly to the relative bearing tolerances
and partly to the torque exerted by the friction force = F*d.

1. At what point do the returns diminish for a decreasing bearing size?

About when the diameter gets too small to support the load of a typical
bike. 'Round about a quarter inch or just under.

Quoted message said:

2. If we could produce well-toleranced, hard-as-diamond nano-whatever
bearings and axles, would the friction decrease to zero as the bearing
diameter limit approached zero?

Zero-friction bearings don't exist, not even fluid dynamic bearings or air
bearings. The size of the axle needed to support half the rider's weight,
even if it were made of monocrystalline diamond, would limit the bearing
size to not-all-that-small.

Quoted message said:

3. How would you adjust such a thing? With a microscope? 😉

Loupe, I'd say.

Jasper

Squid-in-Training said:

Let's say that we have gigantic bearings, about the size of a rim for any
given rim size (huge balls, not a series of small balls). My impression is
that rolling resistance would be huge.

Large bearings exist and don't have particularly high rolling
friction. The main losses are from pushing aside the grease and from
accelerating the mass. You can take care of the first by using lighter
grease, or oil, and the second one goes away at constant speed.

The more important factor becomes dirt in the bearing plus the mass of
the bearing. In the end, there's really no compromise made when
choosing bicycle bearing diameters. You just pick the size that fits
your design the best.

-
-----------------------------------------------
Jim Adney [email hidden]
Madison, WI 53711 USA
-----------------------------------------------

Squid-in-Training said:

Let's say that we have gigantic bearings, about the size of a rim for any
given rim size (huge balls, not a series of small balls). My impression is
that rolling resistance would be huge.

Large bearings exist and don't have particularly high rolling
friction. The main losses are from pushing aside the grease and from
accelerating the mass. You can take care of the first by using lighter
grease, or oil, and the second one goes away at constant speed.

The more important factor becomes dirt in the bearing plus the mass of
the bearing. In the end, there's really no compromise made when
choosing bicycle bearing diameters. You just pick the size that fits
your design the best.

-
-----------------------------------------------
Jim Adney [email hidden]
Madison, WI 53711 USA
-----------------------------------------------

Squid-in-Training said:

Let's say that we have gigantic bearings, about the size of a rim for any
given rim size (huge balls, not a series of small balls). My impression is
that rolling resistance would be huge.

Large bearings exist and don't have particularly high rolling
friction. The main losses are from pushing aside the grease and from
accelerating the mass. You can take care of the first by using lighter
grease, or oil, and the second one goes away at constant speed.

The more important factor becomes dirt in the bearing plus the mass of
the bearing. In the end, there's really no compromise made when
choosing bicycle bearing diameters. You just pick the size that fits
your design the best.

-
-----------------------------------------------
Jim Adney [email hidden]
Madison, WI 53711 USA
-----------------------------------------------

Jim Adney said:

Squid-in-Training said:

Let's say that we have gigantic bearings, about the size of a rim
for any given rim size (huge balls, not a series of small balls).
My impression is that rolling resistance would be huge.

Large bearings exist and don't have particularly high rolling
friction. The main losses are from pushing aside the grease and from
accelerating the mass. You can take care of the first by using lighter
grease, or oil, and the second one goes away at constant speed.

The more important factor becomes dirt in the bearing plus the mass of
the bearing. In the end, there's really no compromise made when
choosing bicycle bearing diameters. You just pick the size that fits
your design the best.

With oversize bearing this, and oversize bearing that, are there material
gains to be had? Where's the limit? Not only that, but oversize axle this
and that for hubs? Hub axle stiffness is not an issue. Crank stiffness,
maybe, but not hub axles.
--
Phil, Squid-in-Training

Jim Adney said:

Squid-in-Training said:

Let's say that we have gigantic bearings, about the size of a rim
for any given rim size (huge balls, not a series of small balls).
My impression is that rolling resistance would be huge.

Large bearings exist and don't have particularly high rolling
friction. The main losses are from pushing aside the grease and from
accelerating the mass. You can take care of the first by using lighter
grease, or oil, and the second one goes away at constant speed.

The more important factor becomes dirt in the bearing plus the mass of
the bearing. In the end, there's really no compromise made when
choosing bicycle bearing diameters. You just pick the size that fits
your design the best.

With oversize bearing this, and oversize bearing that, are there material
gains to be had? Where's the limit? Not only that, but oversize axle this
and that for hubs? Hub axle stiffness is not an issue. Crank stiffness,
maybe, but not hub axles.
--
Phil, Squid-in-Training

Jim Adney said:

Squid-in-Training said:

Let's say that we have gigantic bearings, about the size of a rim
for any given rim size (huge balls, not a series of small balls).
My impression is that rolling resistance would be huge.

Large bearings exist and don't have particularly high rolling
friction. The main losses are from pushing aside the grease and from
accelerating the mass. You can take care of the first by using lighter
grease, or oil, and the second one goes away at constant speed.

The more important factor becomes dirt in the bearing plus the mass of
the bearing. In the end, there's really no compromise made when
choosing bicycle bearing diameters. You just pick the size that fits
your design the best.

With oversize bearing this, and oversize bearing that, are there material
gains to be had? Where's the limit? Not only that, but oversize axle this
and that for hubs? Hub axle stiffness is not an issue. Crank stiffness,
maybe, but not hub axles.
--
Phil, Squid-in-Training

Squid-in-Training said:

With oversize bearing this, and oversize bearing that, are there material
gains to be had? Where's the limit? Not only that, but oversize axle this
and that for hubs? Hub axle stiffness is not an issue. Crank stiffness,
maybe, but not hub axles.

Oversize bearings and axles aren't about stiffness, but about strength.
Broken axles can happen, therefore we need bigger axles. Something like
that. The oversize external bearings like the ones we get on cranks these
days still use the same or smaller ball size, but in a much larger form --
which means a lot mor balls, which is good for durability and load bearing
capacity.

Jasper

Squid-in-Training said:

With oversize bearing this, and oversize bearing that, are there material
gains to be had? Where's the limit? Not only that, but oversize axle this
and that for hubs? Hub axle stiffness is not an issue. Crank stiffness,
maybe, but not hub axles.

Oversize bearings and axles aren't about stiffness, but about strength.
Broken axles can happen, therefore we need bigger axles. Something like
that. The oversize external bearings like the ones we get on cranks these
days still use the same or smaller ball size, but in a much larger form --
which means a lot mor balls, which is good for durability and load bearing
capacity.

Jasper

Squid-in-Training said:

With oversize bearing this, and oversize bearing that, are there material
gains to be had? Where's the limit? Not only that, but oversize axle this
and that for hubs? Hub axle stiffness is not an issue. Crank stiffness,
maybe, but not hub axles.

Oversize bearings and axles aren't about stiffness, but about strength.
Broken axles can happen, therefore we need bigger axles. Something like
that. The oversize external bearings like the ones we get on cranks these
days still use the same or smaller ball size, but in a much larger form --
which means a lot mor balls, which is good for durability and load bearing
capacity.

Jasper

Squid-in-Training said:

With oversize bearing this, and oversize bearing that, are there material
gains to be had? Where's the limit? Not only that, but oversize axle this
and that for hubs? Hub axle stiffness is not an issue. Crank stiffness,
maybe, but not hub axles.

I completely agree. Bearings should be chosen to match the size of the
axle and hub you have in mind. Once you've done that you need to check
to make sure they can handle the load. In the case of bicycles, the
load is never a problem.

There's really no frictional reason to push for larger or smaller
bearings.

-
-----------------------------------------------
Jim Adney [email hidden]
Madison, WI 53711 USA
-----------------------------------------------