Re-engineering the Shift Spring

[swooshdave]

How about a selection of broken spring pictures, surely there must be plenty of examples. Are they all breaking in the same place?

[Chuck]

How about a selection of broken spring pictures, surely there must be plenty of examples. Are they all breaking in the same place?

In this thread alone, there are pictures of the spring breaking in different places. That hints at a metallurgy/heat treat problem to me. A simple spring like this should be able to be wound from music wire, and be bullet proof, I would think.

[Scud]

 

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If you look carefully in the above picture, you can see where the two springs almost meet. This is with the lever in the downshifting position.

But that's not the end position, is it? In the end positions the bump resp. the step would have contact with the small excentric pin, the lock pin as you could call it.

 

 

There is some travel remaining to end position. It was hard to hold the levers in place and take the photo at the same time. The discovery here, at least for me, is that the little right-angle part of the spring cannot be larger - because it would interfere with another spring.

[Scud]

I've seen breaks in two positions - personally and in reviewing other threads. I think the most common is in the coil (as in my picture on the first page). Next is that the small straight piece breaks off at the 90-degree bend. That's what Czakky described and what I experienced very shortly after installing it on my Scura.

 

The break on the coil was on my LeMans, which has the correct size boss for the coil to go around.

 

Anyway, I just heard back from the spring place. They can make 50 springs at $6.00 and 100 @ $4.00 each.

 

I just talked with the representative. He said they would make it from piano wire. Apparently, there is another, slightly cheaper material that is commonly used, but it is weaker than piano wire. I'm going to mail him a broken spring and a new one.

 

He thinks he may be able to use a slightly thicker wire, but has to have the sample in hand before promising anything.

 

I don't want to get over-optimistic, but I think I smell a solution.

[luhbo]

A thicker wire might break easier. I think that the thickness is limited by the travel the legs make, so I'd try a thinner one.

[Chuck]

Piano wire would be good. I think I smell something,too.. oh, it's just a smellfie. 

I'd go with the spring manufacturer's recommendation.

Piano wire, music wire.. same same.

https://www.mcmaster.com/#piano-wire/=1a5kz24

[Scud]

When I mentioned the possibility of using a thicker wire, he said that would reduce how far the spring could bend (and therefore would break easier). I just ASS-u-med thicker would be better, but maybe thinner would be more durable. He also said that a weak spot is created anytime a tool holds the wire and a bend is applied. That's why we are seeing breaks most frequently at 2 specific locations.

 

But maybe just using the highest quality raw material (piano wire) will solve it. It's easy to imagine a strapped-for-cash Moto Guzzi production manager choosing to downgrade the spring material to save a Euro a unit. "Hey we can save €10,000 this year by using material X instead of piano wire..."

 

While I am getting a free "Spring 101" class, he's pretty clear that he doesn't give advice or make recommendations... the company makes springs to specifications, not to applications. The customer does their own engineering.

[swooshdave]

 

How about a selection of broken spring pictures, surely there must be plenty of examples. Are they all breaking in the same place?

In this thread alone, there are pictures of the spring breaking in different places. That hints at a metallurgy/heat treat problem to me. A simple spring like this should be able to be wound from music wire, and be bullet proof, I would think.

 

 

There is actually only one picture of a broken spring in this thread that I can find. It's good to gather the data on where the spring is breaking (if more than one location) before solutioning.

[swooshdave]

 

While I am getting a free "Spring 101" class, he's pretty clear that he doesn't give advice or make recommendations... the company makes springs to specifications, not to applications. The customer does their own engineering.

 

Either there is a quality issue, a design issue or both. For a 15+ year old motorcycle sometimes it's easier to re-engineer (which typically means over-engineering) a solution.

[docc]

Mine broke within s few mm of scud's shown on page one.

[luhbo]

Spring Is Sprung - another nice old times read from 2004

[Chuck]

This might be instructive..

https://www.acxesspring.com/torsion-spring-calculator-instructions.html

[swooshdave]

This might be instructive..

https://www.acxesspring.com/torsion-spring-calculator-instructions.html

 

That helps but you also need to know what material and tempering you want.

[Chuck]

 

This might be instructive..

https://www.acxesspring.com/torsion-spring-calculator-instructions.html

 

That helps but you also need to know what material and tempering you want.

 

One of the things you can calculate on a torsion spring is music wire.. that's what Scud is looking at.  Thought it might give an optimum dia. etc.

[swooshdave]

 

 

This might be instructive..

https://www.acxesspring.com/torsion-spring-calculator-instructions.html

That helps but you also need to know what material and tempering you want.

One of the things you can calculate on a torsion spring is music wire.. that's what Scud is looking at. Thought it might give an optimum dia. etc.

If that’s even the right material to use.

 

 

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