Re-engineering the Shift Spring

[Chuck]

Ah, one other thing. I was thinking that if we solved the breaking at the coil by whatever method.. why is it breaking at the bend on the short end? I wonder what minimum bend radius on music wire should be.. so looked it up. 1 1/2 stock thickness. The one I have is .078" as nearly as I can measure, so is well *under* the minimum bend radius of 3 mm. (.118") That could explain it.

[luhbo]

The question I had is whether the mentioned standards (while I didn't know there are any at all for "Piano Wire") are enough to consistently make reliable springs.

If "Piano Wire or better" is sufficient for that, then it's ok, sure. But for sure as well, a drawing like that would not pass my desktop.

[Chuck]

The question I had is whether the mentioned standards (while I didn't know there are any at all for "Piano Wire") are enough to consistently make reliable springs.

If "Piano Wire or better" is sufficient for that, then it's ok, sure. But for sure as well, a drawing like that would not pass my desktop.

Piano or music wire definitely has standards. All I've been saying is detailed in my ancient (naturally)   copy of the mechanic's bible. The composition of the alloy, and how it is manufactured. There is a whole chapter on making springs.  

This stuff isn't rocket science. It's been known before the turn of the last century.

2017-11-08_12-50-01 by Charles Stottlemyer, on Flickr

[luhbo]

I actually found a ASTM A228 standard. And a warning not to use Piano Wire of too high tensile strength because of a risk of delamination etc. etc.

Long story short: ask the spring supplier what exact sort/grade of piano wire he used for his springs.

[Chuck]

Scud was right, it has 1 1/2 active coils. My bad..

That changes the Guzzi spring to a safe travel of 30.48 degrees. This is using the ASTM A228 piano wire.

[Scud]

It's hard to get a good measurement with the spring installed, but I estimate that the spring is at 22 degrees at rest when installed. Springs on the towel approximate the installed position.

 

 

It relaxes slightly on an upshift:

 

 

This looks like the spring might need to travel 38 degrees for a downshift.

 

 

So... I'm no engineer, but what if the 90-degree tab was a little further along the bend. The current active coil is 1.5 turns, or 540 degrees. If we put the 90 degree bend at 550, that gives us 1.53 active coils. But more importantly, it cuts 10 degrees off the installed position, which would bring the downshift travel within spec.

[Chuck]

Well, now at least we know why they are breaking at both places. Normally Guzzi engineering is good (except for 4V valve trains)  Maybe the same guy designed this spring, too?  

I take it there is no room to wind another coil on it? 

[Scud]

 

I can fit another spring in there, but it is tight. It binds when I move the arm. The spring wire would have to be a lot thinner to allow 2.5 coils and still move freely. So the question is... how much pressure is needed and could a thinner spring do it?

 

What did you think of my idea to extend the coil by 10 (or even 15) degrees? As a point of comparison, the good springs I have removed had fatigue beyond that point anyway and they were still working.

[Chuck]

 

 

What did you think of my idea to extend the coil by 10 (or even 15) degrees?

Here's the spring I preemptively replaced at about 16000 miles. As you can see, it is already at 12 degrees deformation from over travel.  It would have broken sooner or later. Probably sooner.    I would say that even at 15 degrees, we would be operating at the limits of the spring. If we are going to go to all this trouble (fun, actually) we should come up with something that won't break in normal service. I'd like to see a good safety margin. 

How about stacking some feeler gauges in there to see the biggest diameter wire we can use? Then we can plug 2 1/2 coils in the calculator and see how much torque it would have with the smaller diameter wire.

2017-11-09_10-39-44 by Charles Stottlemyer, on Flickr

 

My gut feeling is if we can't get another turn, we should at least explore the extension spring.. even though it would involve considerably more development to come up with something the home mechanic could do.

 

[Scud]

Thanks Chuck - that's how my pre-emptive replacements looked too. It seems that one potential solution is to stay with the existing thickness, but manufacture it so it looks like that - slightly more than 1.5 coils. That would keep the downshift travel under the limit you calculated. If the current downshift travel is 38, and the maximum safe travel is 30.48, increasing the coil length by 12 degrees would cut the downshift travel to 26 degrees. The fatigued springs work perfectly, until they break. There are no symptoms or advance warning.

 

Another is to use thinner wire and add a coil - but maybe we could add 7/8ths of a coil or something like that. My thinking is that if the thinner wire is weaker (and less likely to break) that it will also allow more travel. Using some more travel in the at-rest position would increase the tension - possibly close to what the current spring does, or at least what a fatigued spring does.

 

I can measure that space tonight, but in effect we already have a measurement. It is exactly 3 times the thickness of the spring. The third wire (in pic above) goes in like a tight feeler gauge.

 

And this is kind of fun. But it would not be any fun at all if this was not a spare tranny and I was anxious to get a bike back on the road.

[swooshdave]

Is this issue isolated to the V11s? Has anyone looked inside a later six-speed box and see if it’s the same? Looking at where th springs are breaking seems to confirm the incorrect boss/spring relationship. In the examples provided have you confirmed the boss or spring size? Fixing a problem sometimes requires looking beyond the parts in your hand.

[Scud]

I've confirmed the boss size on mine. My experience opening transmissions is limited to the V11. Here's a link to parts diagram of a Griso transmission. Totally different.  http://www.harpermoto.com/parts-by-motorcycle/2000-up-moto-guzzi-motorcycles/griso-v-ie-1100-2005-2008/gear-box-selector-en-griso-ie-1100.html

 

[Chuck]

Ok using 1.52 active coils, the rate is only lowered fractionally @33.22 N-mm/degree

Safe travel is 30.89 degrees.

I'll be back with what smaller wire could do.

Edit:

Ok, I'm back. Using .071" wire with 2 1/2 active coils, which gives (in my opinion) enough clearance to not bind.. and is readily available.

Rate is 20.68 N-mm/degree

Safe travel 49.51 degrees. 

That sucker wouldn't break.. if it is strong enough to do the job. That would be 1.8 inch lbs per degree compared to 2.9 with the 2mm spring.

What do you think?

[Scud]

Sounds worth trying. I don't think that the lever needs much pressure at all to be pushed back into position. I'd be willing to invest $300 from my Play-Pal account on 50 springs. 

 

Fine-tuning idea follows:

 

I estimated travel for downshifts at 38 degrees. The extra coil gives us 48 degrees range - but carries the potential risk of a too-weak spring. What if we use half of the 10-degree excess available travel for preload?

• 2.5 coils is 900 degrees.
• Therefore, 895 degrees would be 2.48 coils. I reckon that would give 5 x 1.8 inch/lbs as pre-load to compensate for the weaker, thinner wire. Does that make sense - or is that too small of a difference to mean anything? The at-rest spring would appear a little more open - the opposite of the fatigued spring in your earlier picture, which is 12 degrees closed.

If I understand this correctly, these are the pressures applied by the two springs when installed:

• Stock (1.5 coils):  2.9 in/lb x 22 degrees = 63.8 in/lb
• Thinner (2.5 coils): 1.8 in/lb x 22 degrees = 39.6 in/lb
• We could get to 48.6 in/lb with 895 degrees of coil (5 degrees extra preload), or to 57.6 in/lb with 890 degrees (10 degrees extra preload)

Gosh, this is more analysis than I signed up for... but I want a spring that won't break.

[Chuck]

I want a spring that won't break, too.  Let's go with the 895 degrees. I'll split the cost with you so if this is a bust we each only lose one fiddy. I think it's worth the gamble. 

Oh.. I don't suppose they'd make a couple so we could test to destruction before the order? If we *knew* they would work, we could order more than 25.