Spokes

[weh8127]

Today I discovered a broken spoke on the rear wheel of my SC rig, the rear wheel if the tug. Other spokes on that wheel are loose enough for me to feel some movement with my fingers. The wheel is a standard BMW steel rim laced with stainless spokes, about 3500 miles solo and about 1000 miles with the car.

I know that stainless is generally more brittle than carbon steel and that the stainless spokes are a little thicker than standard chrome spokes. Was it a mistake to use stainless spokes for a SC rig? Is it wise to replace that one spoke and re-true the wheel or should all be replaced? What about the front wheel?

The only other time I had a broken spoke was on a BSA single in 1972 but this is my first experience with side cars. All comments welcome. TIA

[skychs]

I had the same issue with my R60/2 side car. I removed the tire, replaced the two broken spokes with SS then trued the wheel. No issue since.

In my case I had a couple friends who liked to fly the sidecar putting even more stress on the spokes/wheels.

[Wayne J in MA]

The threads on spokes can be formed by one of two methods. They call be rolled or cut. I read a post recently on one of the forums warning about cut threads being a potential problem, the sharp cut is easier to break under side loads. This could definitely be an issue with the forces of a sidecar. You might want to look at your threads.

Wayne

[weh8127]

https://www.cranepointllc.com/blog/roll ... resistance.

I found this online and its very interesting but none of the usual suppliers mention thread type in their spoke description, so there is no way to choose. The only choice offered is chrome or stainless. Is one better than the other for the loads imposed by a sidecar?

[weh8127]

http://www.stones-custom.com/index2.htm

Well, I just spoke to the people at Stone's Cycle in Northboro, MA. They do a lot of custom wheel building and are happy to work on vintage bikes (the owners personal bike is a BMW). When one spoke breaks they say it screws up the rest so its best to replace all and stainless is superior to chrome in their experience. They use Buchanan for wheel parts. Guess Ill go there.

[Flx48]

Hi Bill-
One can look at the spoke to see if it has rolled or cut threads.
A cut thread will have a major diameter the same or less than the spoke diameter; the wire grain has been cut, which is why the spoke is now weaker, being only as strong as the minor thread diameter.
A rolled thread will have a major diameter that is larger than the spoke diameter, because the thread is rolled, (forged) and the major diameter is raised compared to the spoke diameter, and because the grain is left continuous it retains its strength.
I don't think any reputable manufacturer would cut a motorcycle spoke thread.

When stainless steel spokes first came on the scene, ('70s-'80s?) there was concern over their suitability, but they've proven to be reliable.
Most ss spokes are supplied in larger diameter than the stock steel, Buchanan's (the spoke benchmark people) are 9 gauge, while the /2 stock Berg Union spokes are the smaller 10 gauge. (or the metric equivalent)

Certainly possible you had a spoke with a metallurgy problem, but it seems unlikely.
And you would probably remember if the wheel took a big curb hit, or other abuse.
I would probably first suspect the spokes weren't tight enough, or they were not set after the wheel was laced.
Remember also that in sidecar use spokes need axial, as well as radial strength, so things must be "righter."

The spoke plates have a pair of slots in them to accept the pair of raised edges on the bottom (beveled) side of the spoke head, these want to be aligned together during assembly, they help prevent the spoke turning with the nipple during tightening/loosening.
When the spokes are initially tightened, and before the wheel is trued, the spoke heads want to be set with a hammer and punch, insuring solid deep mating between spoke head and spoke plate recess.
If not done, it can result in loose spokes.

The instruction manual's list of things to do every 4k miles (check steering head bearings/swingarm bearings/clean carbs/ check points/etc) includes checking spoke tightness.
For me, a byproduct of only using rubber tubes is the necessity of checking tire pressure weekly.
And a byproduct of checking pressures weekly is it gives me the opportunity to get a regular good look at tire condition and to go all xylophone on the spokes for a sound check on their tightness.
Best-
George

[weh8127]

Right you are, George. I found the bad spoke while checking tire pressure.