800W Q128c commuter build

[molybdenum]

My motor is indeed a Q128c, but I didn't think the holes were obviously too large. I've been riding on this wheel build for a year and a half, and I also periodically check/adjust spoke tension.

I saw your post, but had nothing to contribute regarding hole size; it's ironic that my problem may have stemmed from the subject of your inquiry. As for the holes, I'd have to measure them carefully, as there is no documentation regarding these. I know from previous reading that the original MXUS 3000 V1 motors had holes that were way too large, and when fitted with brass washers, riders were able to put many KW through them, seemingly without incident.

I stopped by Grin yesterday and purchased some custom cut Sapim 13/14 stainless steel spokes to fit my wheel. These are the same spoke I originally used, and the motor is laced into an Alex DM18 rim. The folks at Grin also thought it unusual that my spokes would fail at the elbow but didn't attribute it to the hole size. The spokes broke off near but not on the elbows in every case. The shear points were no where near the area making contact with the hole. I inspected the spokes around the broken ones or signs of stress, but saw nothing obvious. It is possible that larger holes cause excessive bending at the elbow when the spoke is stressed by impact, and that the weakest points resulting from this bending are at the tops of the elbows?

I have some washers lying around with ~13 Ga centers. I may give them a try, but for now, I'd prefer to avoid a total re-lacing job.

They broke at the elbow? This is often a sign that the spoke hole diameter is too large for the spokes. What diameter are the holes and which spokes are you using?

Coincidently, I was wondering about the same thing in relation to this motor just a few days ago.

 

[-dg]

My motor is indeed a Q128c, but I didn't think the holes were obviously too large. I've been riding on this wheel build for a year and a half, and I also periodically check/adjust spoke tension.

The spokes broke off near but not on the elbows in every case. The shear points were no where near the area making contact with the hole. I inspected the spokes around the broken ones or signs of stress, but saw nothing obvious. It is possible that larger holes cause excessive bending at the elbow when the spoke is stressed by impact, and that the weakest points resulting from this bending are at the tops of the elbows?

It sounds like the wheel may not have been pre-stressed / stress-relieved when it was built. This is a step after tensioning the wheel of briefly overtensioning each spoke individually to it's yield point. The concept is that the spoke when installed is curved near the elbow and where it crosses other spokes. This curve means that as the wheel rotates and the spoke becomes the one nearest the ground it unloads so the curve tightens and relaxes once per wheel rotation. Over time this will cause fatigue failure of the spoke. Usually this will be at or near the elbow. By pre-stressing we force the spoke to actually bend sharply at the angles so the spoke is then a series of straight sections with kinks at the bends.

To pre-stress a wheel lace it up and tighten it to about the finish tension and roughly true it. Then put a lever between pairs of spokes and twist so as to stretch the spokes. I use an old crank arm for this, but have used a long wrench handle too. See http://www.sheldonbrown.com/wheelbuild.html for details. You can use your hands, but if you are building more than one wheel you will have sore hands. It's a bit of a scary process the first time

.

A correctly built wheel will run tens of thousands of miles with minimal retrueing.

 

[molybdenum]

To pre-stress a wheel lace it up and tighten it to about the finish tension and roughly true it. Then put a lever between pairs of spokes and twist so as to stretch the spokes. I use an old crank arm for this, but have used a long wrench handle too. See http://www.sheldonbrown.com/wheelbuild.html for details. You can use your hands, but if you are building more than one wheel you will have sore hands. It's a bit of a scary process the first time

.

A correctly built wheel will run tens of thousands of miles with minimal retrueing.

I broke the spokes coming home from work Friday; I had a car turn an aggressive right into my path so I took the center line and my rear wheel hit a deep rut at 40+ kph. On a brighter note, I've put about 15,000 km on this wheel before breaking anything and I've hit some nasty stuff before. My wheel is heavily dished, which doesn't help matters. Interestingly, all four spokes broke on the [strike]higher[/strike] lower tension disc rotor side. The elbows point in on the rotor side and out on the cassette side to help ameliorate some of the stress differential.



This is good information. I read Sheldon Brown's wheelbuild a couple years ago but somehow missed the pre-stress stuff. I guess it can be done at any time. I had extra time today, so I repaired and re-trued the wheel. I have a bunch of extra spokes in case I lose more.

Interestingly, when the 4 spokes broke, the wheel was considerably out of true and when I replaced and tightened them, things were mostly corrected. This suggests to me that these spokes may have been taking an inordinate amount of the strain, which is my bad for not re-tensioning more frequently. I made sure all the spokes were around the same tension with the wheel true and with no obviously loose or overly tight spokes.

 

[-dg]

This is good information. I read Sheldon Brown's wheelbuild a couple years ago but somehow missed the pre-stress stuff. I guess it can be done at any time. I had extra time today, so I repaired and re-trued the wheel. I have a bunch of extra spokes in case I lose more.

Interestingly, when the 4 spokes broke, the wheel was considerably out of true and when I replaced and tightened them, things were mostly corrected. This suggests to me that these spokes may have been taking an inordinate amount of the strain, which is my bad for not re-tensioning more frequently. I made sure all the spokes were around the same tension with the wheel true and with no obviously loose or overly tight spokes.

Yes, you can pre-stress at any time. Also new spokes should be done too. I think generally spokes fail from fatigue not overload. It is the cyclic variation in load that kills them. It sounds like the ones of yours that broke may have been loose, not tight so the variation was a larger fration of the overall stress. See Jobst Brandt's excellent book The Bicycle Wheel for a better explanation.

 

[1N4001]

all four spokes broke on the lower tension disc rotor side.

Your heavily dished wheel may be to blame for this too. The disc side spokes will have a fairly low tension. When the wheel is exposed to high loads, the low-tension side may lose tension completely, which wreaks havoc across the wheel. You might want to increase the tension across all spokes so the disc side does not fall too low.

I think generally spokes fail from fatigue not overload. It is the cyclic variation in load that kills them.

This is true. Unintuitively, butted spokes are less prone to breaking and fatigue, due to being more flexible. Continuously thick spokes are stiffer and tend to break rather than bend elastically.

Check out Sapim's checklist if you don't believe me. I quote:

It is not correct to think that a thicker spoke makes a stronger wheel.
A stronger wheel is obtained by using a thinner (more flexible) spoke

 

[molybdenum]

Thanks everyone for the excellent suggestions,

At first, I just assumed the higher tension side would fail and even had to edit my post as I didn't put much thought into it until after hitting the submit button and reviewing my schematics.

I can see how lower tension can lead to problems, and lead to a hammering effect when going over bumps; although spoke tension was periodically normalized among the spokes, I'll bet everything had gradually loosened over time. I've done my best to stretch everything out in a rudimentary hand pre-stressing and I've tightened up both sides to maintain the dishing but place better tension over the disc side. And finally, I've re-trued the wheel and it is out by less than 0.5mm.

I'm lucky to be riding at all this week, as good quality Sapim spokes just happened to be available at my neighborhood Grin. I recommend anyone reading this invest $10.00 in some good quality spares cut to size to avoid downtime while waiting for parts, or at the very least, an extra tear-down to replace stopgap spokes with good ones. For spares, I have everything - spare motors, controllers, batteries, LCDs, throttles, tires, tubes - but spokes? Nope.

Today's ride was good, and the wheel handled well. It was unusually hot for May in this part of the world. I bought a few "safety orange" T-shirts at Michaels Crafts for $3.00 each just in time for the 30C+ weather. Now, drivers can see me coming a mile away and I can be cool 8) on my work commute.

 

[molybdenum]

Hello ES,

Been a few years, and I'm still riding this build daily. 80k km and several drive trains/tires later, the Q128c motor and electronics still work as they did day one. The frame is starting to show its age, flexing like a noodle, so I'll be delving into old threads trying to gather information for a new build. I'm still interested in a minimalist ultra stealthy ~500W system. I'll be looking into a small geared rear hubmotor and converting a robust hybrid as the roads are rough and my daily commute is over 30km.