Re-engineering the Shift Spring

[Chuck]

I have some experience with springs..

[Scud]

I have some experience with springs..

 

Do you have a spring on hand that you could measure and use their online calculator? If not, I could post some measurements. I think that with your experience you might make a better decision than I would. I'm kind of limited to: "Here's a spring. Can you make me one just it, but better?"

[Lucky Phil]

What about a Titanium spring, like GP bikes have? Save some weight:)

 

Ciao

[Lucky Phil]

I have some experience with springs..

I have some experience with piano wire Chuck. Used to use it to hold the wing and hori stab leading edges on on the C130 Hercules and gear doors on the A4G Skyhawk.

 

Ciao 

[Chuck]

 

I have some experience with springs..

 

Do you have a spring on hand that you could measure and use their online calculator? If not, I could post some measurements. I think that with your experience you might make a better decision than I would. I'm kind of limited to: "Here's a spring. Can you make me one just it, but better?"

 

Yeah, I do. It's used, though.  Actually, "just like it but better" could very well be the answer. Higher spec wire, maybe better coil winding machinery. I don't have access to a spring winder any more, and making them on a mandrel on the lathe is a PITA.

I'll make that today's project.. if I get time. Quite a bit going on right now. I have a new pup out in the shop and oiler nozzles to make.

[Chuck]

What about a Titanium spring, like GP bikes have? Save some weight:)

 

Ciao

Great idea! After all, we know Guzzis are all about lightness. 

[Tom M]

Swooshdave was looking for more broken spring pictures.  Here's mine.

 

I've had three springs break.  I believe two of the factory supplied springs broke as shown in the attached picture.  I also had a Cannon Racecraft (?) spring, which was made from a heavier wire, break at the tightest bend.  I don't have that picture any more.

 

I'd be happy to buy an alternative to the factory part if you guys come up with something.  I can help with design and fabrication of any parts other than a spring if needed.

 

 

[Scud]

 

What about a Titanium spring, like GP bikes have? Save some weight:)

 

Ciao

Great idea! After all, we know Guzzis are all about lightness. 

 

 

So I see this is partly in jest... but the guy did mention "exotic materials." I said "I'm not that worried about cost, what would it take to make it out of some exotic material?" Then he said that the exotic materials only have benefits in much thicker springs than the one we need. In his opinion, piano wire is the best available material.

[Chuck]

Ok, here is some preliminary data.

The stock Guzzi spring wire is 2mm dia.

Spring rate/degree is 45.1 Nmm.

Torque max is 1036.8 Nmm.

Max safe travel 22.9 degrees.

Running the numbers on .005" bigger in diameter:

Max safe travel 20.91 degrees

.005" smaller..

24.04 degrees.

 

Interestingly, common music wire is only good to 300 degrees F, while stainless 304 is 570 F. 

So. Before we can go any farther, we need to know how many degrees of rotation the preselector uses..

[Scud]

Thanks Chuck. I can measure the rotation tonight. Let's see... when did I last use a protractor for anything? Do I have one? Can I find it?

[Chuck]

Just for giggles, I ran the numbers with 2 active coils instead of 1 and got 48 degrees safe rotation. Don't know if there is room for that or not.

[Scud]

The spring has 1.5 active coils. A thinner spring might be part of the solution. And extra coil would take us to 2.5. I'll take a bunch of measurements tonight. I fit a much thicker wire (the one I made out of solder) - so maybe a thinner wire with an extra coil would fit?

 

A thinner spring brings a concern about it being able to hold the arm in place. From what I can see, the arm does not require a lot of tension to be held in place. Braking will flop it forward and sometimes land it in the right position. That's how I was able to get a few shifts on the way home after my recent spring-break (it sounds like more fun if you call it Spring Break). But when I found 3rd I was close to home so then I left it in 3rd.

 

I also noticed that when I pre-emptively replaced the spring on the Scura that the original was significantly weaker than the replacement, which broke shortly thereafter. I put the original, weaker spring back in and it's been fine since  -  but maybe that's because there's a spare under the seat.

[luhbo]

About 20° I assume. Waiting to hear from you, sounds interesting.

Another thing you could discuss with your spring guy: as such springs see some severe torsional stress it would make probably a huge difference for the expectable lifetime if you remove the U-bend at the outer end, as this U-bend eliminates any torsional freedom at this part of the wire. I guess that without this U the overall stress in the whole wire could be remarkably lower. As your solution in this direction interferes with the other nearby spring you alternatively could try an additional small pin as a boss on the long lever (pin, hole, weld point) or a bent tab out of the lever (unless the whole lever would fail in this case).

[luhbo]

What I forgot to mention: an additional winding will make the spring even softer (with thin wires) and/or enlarge the safe travel (for a given diameter).

 

What I also wanted to mention: Piano or Music Wire stands for nothing usable in this context. It stands more or less for drawn, blank wire only. It's no indication for hardness or anything else, especially not for a certain technical suitability. One piano wire can give you a reliable spring, next time a different piano wire will leed to the known scenario discussed here.

[Chuck]

What I forgot to mention: an additional winding will make the spring even softer (with thin wires) and/or enlarge the safe travel (for a given diameter).

 

What I also wanted to mention: Piano or Music Wire stands for nothing usable in this context. It stands more or less for drawn, blank wire only. It's no indication for hardness or anything else, especially not for a certain technical suitability. One piano wire can give you a reliable spring, next time a different piano wire will leed to the known scenario discussed here.

^^^^^Where did you come up with that?

 

Uhhh, there are standards for piano wire that have to be met. For instance for 304 stainless wire..

 

 

• Tensile Strength: 205,000 psi
• Specifications Met: ASTM A313, ASTM A555

•  
•  

 

If there were no standards, making a spring would be a crap shoot.