Interesting wheel failure

As part of my ongoing adventures in building bike trailers to tote my
boats, I modified one to take an electric motor.

On my initial run, after riding a few hundred feet on level road, I
tried a fairly steep hill to see how much torque I had. I estimate that
the total load (bike, trailer, rider, batteries) was around 500 lb, and
the hill grade 5-10%. The good news was that there was enough torque to
climb the hill without pedaling. The bad news was that 4 spokes snapped.

The wheel I used was a rather poor quality 26" one off of a junk bike.
The motor is rated at 500W peak, with an internal planetary reducer
(~10:1) with a 9T driving sprocket. The driven sprocket was 34T. I
geared this low intentionally (~8 mph top speed) specifically to assist
with starting and hill climbs.

The interesting thing was that the spokes mostly failed at mid span,
showing necking, indicating that it was purely tensile overload, one
spoke snapped the nipple. Interesting, because I've never seen spokes
fail in that mode with normal cycling, obviously the motor provides more
torque than I could hope to.

I'll try again with a better wheel and a bit higher gearing -- and I'll
pedal. I guess I should have really spent more time analyzing the forces
involved, since getting the supporting frame around the motor and wheel
stiff enough to not deflect under such large forces is not trivial. It's
given me renewed appreciation for the elegant minimalism of bike frames.

On 31 July, 14:52, Peter Cole wrote:
As part of my ongoing adventures in building bike trailers to tote my
boats, I modified one to take an electric motor.

On my initial run, after riding a few hundred feet on level road, I
tried a fairly steep hill to see how much torque I had. I estimate that
the total load (bike, trailer, rider, batteries) was around 500 lb, and
the hill grade 5-10%. The good news was that there was enough torque to
climb the hill without pedaling. The bad news was that 4 spokes snapped.

The wheel I used was a rather poor quality 26" one off of a junk bike.
The motor is rated at 500W peak, with an internal planetary reducer
(~10:1) with a 9T driving sprocket. The driven sprocket was 34T. I
geared this low intentionally (~8 mph top speed) specifically to assist
with starting and hill climbs.

The interesting thing was that the spokes mostly failed at mid span,
showing necking, indicating that it was purely tensile overload, one
spoke snapped the nipple. Interesting, because I've never seen spokes
fail in that mode with normal cycling, obviously the motor provides more
torque than I could hope to.

I'll try again with a better wheel and a bit higher gearing -- and I'll
pedal. I guess I should have really spent more time analyzing the forces
involved, since getting the supporting frame around the motor and wheel
stiff enough to not deflect under such large forces is not trivial. It's
given me renewed appreciation for the elegant minimalism of bike frames.

Test torque (including radial load) the replacement wheel looking for
uneveness in tension.

On Jul 31, 9:52*am, Peter Cole wrote:
As part of my ongoing adventures in building bike trailers to tote my
boats, I modified one to take an electric motor.

On my initial run, after riding a few hundred feet on level road, I
tried a fairly steep hill to see how much torque I had. I estimate that
the total load (bike, trailer, rider, batteries) was around 500 lb, and
the hill grade 5-10%. The good news was that there was enough torque to
climb the hill without pedaling. The bad news was that 4 spokes snapped.

The wheel I used was a rather poor quality 26" one off of a junk bike.
The motor is rated at 500W peak, with an internal planetary reducer
(~10:1) with a 9T driving sprocket. The driven sprocket was 34T. I
geared this low intentionally (~8 mph top speed) specifically to assist
with starting and hill climbs.

The interesting thing was that the spokes mostly failed at mid span,
showing necking, indicating that it was purely tensile overload, one
spoke snapped the nipple. Interesting, because I've never seen spokes
fail in that mode with normal cycling, obviously the motor provides more
torque than I could hope to.

I'll try again with a better wheel and a bit higher gearing -- and I'll
pedal. I guess I should have really spent more time analyzing the forces
involved, since getting the supporting frame around the motor and wheel
stiff enough to not deflect under such large forces is not trivial. It's
given me renewed appreciation for the elegant minimalism of bike frames.

Maybe with better quality spokes you can get the
hub flangies to fail. Or maybe shear off the driven
sprocket.

Once you've hooked it up to a 48 spoke steel hub
you can start "analizing" like mad, maybe pull a car
up a hill? See if you can fold up one of the stays?

I love destructive testing.

Leo Lichtman wrote:
"Peter Cole" wrote (clip) The interesting thing was that the spokes mostly
failed at mid span, (clip)
^^^^^^^^^^^^^^^^^^^^^^^^^^
When you started going up hill, were you also encountering bumps? If the
load was bouncing, you could have had two or three times the stress compared
to a smooth riding surface. If that is the case, you might help the
situation by adding suspension to the trailer. This doesn't necessarily
have to be --maybe support the boat in a couple of slings, that can stretch
a little.

Another thought: with a gross weight of 500 lb, a few lb extra weight in
the wheels will not be noticed. Look for some wheels with cast
construction, or perhaps fabricated carbon fiber disk wheels.



A clarification: the trailer (plus boat & batteries) was probably ~250
lb, the bike & me the other 250.

On 31 July, 19:25, wrote:
Leo Lichtman wrote:
The interesting thing was that the spokes mostly
failed at mid span, (clip)
^^^^^^^^^^^^^^^^^^^^^^^^^^
When you started going up hill, were you also encountering bumps?
If the load was bouncing, you could have had two or three times the
stress compared to a smooth riding surface. *If that is the case,
you might help the situation by adding suspension to the trailer.
This doesn't necessarily have to be --maybe support the boat in a
couple of slings, that can stretch a little.
Another thought: with a gross weight of 500 lb, a few lb extra
weight in the wheels will not be noticed. *Look for some wheels with
cast construction, or perhaps fabricated carbon fiber disk wheels.

Just a moment! *If spokes do not rattle from overload, they won't
unscrew. *Because spokes only loosen from loading, bumps and heavy
loads can only un-tension spokes when they are in the tire to road
contact patch of the tire. *Spoke tension does not increase when the
wheel supports a load.

That's because your wheels are ****e.
Well made bicycle wheels have the magority of their spokes increase in
tension. Remove the bows from your spokes and retest and you will
have some idea of a well made bicycle wheel.


*Loading is not going to cause spokes to fail
at midspan which would be fatigue failures, and fatigue failures occur
at the highest stress points that are not at midspan unless some
physical damage occurred there.

There is something oddly amiss in this failure analysis.

Jobst Brandt

On 31 July, 21:49, Peter Cole wrote:
Leo Lichtman wrote:
"Peter Cole" wrote (clip) *The interesting thing was that the spokes mostly
failed at mid span, (clip)
^^^^^^^^^^^^^^^^^^^^^^^^^^
When you started going up hill, were you also encountering bumps? *If the
load was bouncing, you could have had two or three times the stress compared
to a smooth riding surface. *If that is the case, you might help the
situation by adding suspension to the trailer. *This doesn't necessarily
have to be --maybe support the boat in a couple of slings, that can stretch
a little.

Another thought: *with a gross weight of 500 lb, a few lb extra weight in
the wheels will not be noticed. * Look for some wheels with cast
construction, or perhaps fabricated carbon fiber disk wheels.

A clarification: the trailer (plus boat & batteries) was probably ~250
lb, the bike & me the other 250.

You need to torque test the wheel if you intend it to drive yourself
as well as the trailer. You are putting the wheel to a use for which
it was not created. Testing for radial load is adequate when using a
wheel for its intended use. You have a high torque to radial load
ratio and so the wheel needs to be made torsionally stiffer to survive.

On Jul 31, 2:25*pm, wrote:
Leo Lichtman wrote:
The interesting thing was that the spokes mostly
failed at mid span, (clip)
^^^^^^^^^^^^^^^^^^^^^^^^^^
When you started going up hill, were you also encountering bumps?
If the load was bouncing, you could have had two or three times the
stress compared to a smooth riding surface. *If that is the case,
you might help the situation by adding suspension to the trailer.
This doesn't necessarily have to be --maybe support the boat in a
couple of slings, that can stretch a little.
Another thought: with a gross weight of 500 lb, a few lb extra
weight in the wheels will not be noticed. *Look for some wheels with
cast construction, or perhaps fabricated carbon fiber disk wheels.

Just a moment! *If spokes do not rattle from overload, they won't
unscrew. *Because spokes only loosen from loading, bumps and heavy
loads can only un-tension spokes when they are in the tire to road
contact patch of the tire. *Spoke tension does not increase when the
wheel supports a load. *Loading is not going to cause spokes to fail
at midspan which would be fatigue failures, and fatigue failures occur
at the highest stress points that are not at midspan unless some
physical damage occurred there.

There is something oddly amiss in this failure analysis.

Jobst Brandt

if a spoke unloads (presumably you're talking about the one at the top
of the wheel unloading the most) due to weight, what's keeping the hub
off of the ground? I'm just ASSuming that the spoke opposite that
one must be loaded in tension, yes? and since the tensions must sum
to zero otherwise the hub would be moving relative to the rim (OK, it
does, but not enough to really matter) ASSume a radial spoked wheel,
evenly tensioned. You introduce a 150 lbf compressive load between
the bottom of the rim and the hub. so you've unloaded the bottom
spoke along that axis, and correspondingly increased the tensile load
on the top spoke along that axis. So tension on a spoke CAN increase
above the tension introduced to it when the wheel was built due to
load. or am I missing something?

nate

On 31 July, 22:15, N8N wrote:
On Jul 31, 2:25*pm, wrote:



Leo Lichtman wrote:
The interesting thing was that the spokes mostly
failed at mid span, (clip)
^^^^^^^^^^^^^^^^^^^^^^^^^^
When you started going up hill, were you also encountering bumps?
If the load was bouncing, you could have had two or three times the
stress compared to a smooth riding surface. *If that is the case,
you might help the situation by adding suspension to the trailer.
This doesn't necessarily have to be --maybe support the boat in a
couple of slings, that can stretch a little.
Another thought: with a gross weight of 500 lb, a few lb extra
weight in the wheels will not be noticed. *Look for some wheels with
cast construction, or perhaps fabricated carbon fiber disk wheels.

Just a moment! *If spokes do not rattle from overload, they won't
unscrew. *Because spokes only loosen from loading, bumps and heavy
loads can only un-tension spokes when they are in the tire to road
contact patch of the tire. *Spoke tension does not increase when the
wheel supports a load. *Loading is not going to cause spokes to fail
at midspan which would be fatigue failures, and fatigue failures occur
at the highest stress points that are not at midspan unless some
physical damage occurred there.

There is something oddly amiss in this failure analysis.

Jobst Brandt

if a spoke unloads (presumably you're talking about the one at the top
of the wheel unloading the most) due to weight, what's keeping the hub
off of the ground? * *I'm just ASSuming that the spoke opposite that
one must be loaded in tension, yes? *and since the tensions must sum
to zero otherwise the hub would be moving relative to the rim (OK, it
does, but not enough to really matter) ASSume a radial spoked wheel,
evenly tensioned. *You introduce a 150 lbf compressive load between
the bottom of the rim and the hub. *so you've unloaded the bottom
spoke along that axis, and correspondingly increased the tensile load
on the top spoke along that axis. *So tension on a spoke CAN increase
above the tension introduced to it when the wheel was built due to
load. *or am I missing something?


JB's a fool.

N8N wrote:

if a spoke unloads (presumably you're talking about the one at the top
of the wheel unloading the most) due to weight, what's keeping the hub
off of the ground? I'm just ASSuming that the spoke opposite that
one must be loaded in tension, yes? and since the tensions must sum
to zero otherwise the hub would be moving relative to the rim (OK, it
does, but not enough to really matter) ASSume a radial spoked wheel,
evenly tensioned. You introduce a 150 lbf compressive load between
the bottom of the rim and the hub. so you've unloaded the bottom
spoke along that axis, and correspondingly increased the tensile load
on the top spoke along that axis. So tension on a spoke CAN increase
above the tension introduced to it when the wheel was built due to
load. or am I missing something?

Only about 10 years of rbt.

On Jul 31, 2:15*pm, N8N wrote:
On Jul 31, 2:25*pm, wrote:





Leo Lichtman wrote:
The interesting thing was that the spokes mostly
failed at mid span, (clip)
^^^^^^^^^^^^^^^^^^^^^^^^^^
When you started going up hill, were you also encountering bumps?
If the load was bouncing, you could have had two or three times the
stress compared to a smooth riding surface. *If that is the case,
you might help the situation by adding suspension to the trailer.
This doesn't necessarily have to be --maybe support the boat in a
couple of slings, that can stretch a little.
Another thought: with a gross weight of 500 lb, a few lb extra
weight in the wheels will not be noticed. *Look for some wheels with
cast construction, or perhaps fabricated carbon fiber disk wheels.

Just a moment! *If spokes do not rattle from overload, they won't
unscrew. *Because spokes only loosen from loading, bumps and heavy
loads can only un-tension spokes when they are in the tire to road
contact patch of the tire. *Spoke tension does not increase when the
wheel supports a load. *Loading is not going to cause spokes to fail
at midspan which would be fatigue failures, and fatigue failures occur
at the highest stress points that are not at midspan unless some
physical damage occurred there.

There is something oddly amiss in this failure analysis.

Jobst Brandt

if a spoke unloads (presumably you're talking about the one at the top
of the wheel unloading the most) due to weight, what's keeping the hub
off of the ground? * *I'm just ASSuming that the spoke opposite that
one must be loaded in tension, yes? *and since the tensions must sum
to zero otherwise the hub would be moving relative to the rim (OK, it
does, but not enough to really matter) ASSume a radial spoked wheel,
evenly tensioned. *You introduce a 150 lbf compressive load between
the bottom of the rim and the hub. *so you've unloaded the bottom
spoke along that axis, and correspondingly increased the tensile load
on the top spoke along that axis. *So tension on a spoke CAN increase
above the tension introduced to it when the wheel was built due to
load. *or am I missing something?

Yes you are missing something. The loss in tension of the bottom spoke
is due to rim deflection (compressive load) at the contact area so
there is no corresponding increase in tension of the upper spoke.

Phil H

Phil H