Damn - Looks like the top shock bolt is broken (again)

[jdgretz]

Started to take off the Hyper Pro to get it rebuilt before the San Diego get together.  The bolt head turns but the threads on the other side don't    Had this bolt break once before in 2013 - about 50,000 miles ago, but that time the bolt broke right where the threads started.  Hopefully it will be about the same place so I can get the bolt out and remove the shock.  If it's further up the bolt, I'm not sure how I will get the shock out.

*sigh*

jdg

[Huzo]

Shit !
Is that on your '07 Norge JD? That's gotta' be a bugger!
Do you reckon you should get a higher grade aircraft bolt or equivalent ? Mine'll be getting pulled down upon return to Oz, and I'll be checking that out quick sticks.
It was ok when I put the Nitron in a year ago, but after hearing your tale, who knows?
PS. Not over torqued ?

[pete roper]

If you are still running the original shock Spring I'm not surprised. It was woefully inadequate even for solo work. Remember the bolt is essentially made to work in tension, not in shear. If the shock keeps topping out and hammering the bolt in shear I'm not surprised it has failed. If the cause wasn't addressed the first time history will repeat.

Pete

[Huzo]

If you are still running the original shock Spring I'm not surprised. It was woefully inadequate even for solo work. Remember the bolt is essentially made to work in tension, not in shear. If the shock keeps topping out and hammering the bolt in shear I'm not surprised it has failed. If the cause wasn't addressed the first time history will repeat.

Pete

Would you expand on the tension/shear comment Pete ?

[Sasquatch Jim]

  Tension means pulling it apart.  Shear is when it takes sideways force.  Screw it.  out.

[sidecarnutz]

Pete's making perfect sense here!

[pete roper]

If the spring and damper unit is properly sprung and valved it will perform the function for which it was designed, absorbing the energy imparted to it by *Bumps* and turning it into heat that can be easily dissipated and in doing so preventing shock loadings. That's why they are known as 'Shock Absorbers'.

If the spring rate is inadequate though the entire travel of the damper rod will be used up and at the very end of its travel the second line of defence, the bump-stop, a rubber collar around the damper rod will come into play acting as a cushion to prevent the piston or clevis on the other end of the rod simply whacking into the damper body and effectively turning the entire innit into a solid object acting like a bloody great drift that simply hammers on the mounting bolts at either end of the shock and imparts shear loadings on them.

Unfortunately the bump stop can only work to a limited degree and for a limited time. If you look at shocks on most early Norges and Griso's in particular you'll find the bump stops are usually flogged to pieces by the constant bottoming out of the damper rods. The result is continual hammering on the mounting bolts which are designed to take a loading in tension, (Lengthways.) and not in shear, (Sideways.) something will, inevitably, give in the long term and in this case it is usually going to be the bolt!

A spring is essentially *Battery* for kinetic energy. It stores it as it is extended or compressed from its at rest position and then releases it as it returns to that point. The energy is dissipated by forcing oil through valves within the damper body and turning the kinetic energy into heat. This prevents the spring from simply 'Pogoing' and speeds up and smooths out the dissipation of the unwanted and wasted energy.

As a kid you may remember putting your thumb over the end of a bicycle tyre pump and pumping like crazy and finding that the end of the pump near your thumb getting really hot? Same principle. The air is acting as a spring and storing the energy of your frenzied pumping but not having any decent way to dissipate it it simply turns the energy imparted to the spring, (The air inside the pump tube.) into heat which it then imparts into the pump tube, piston and rod by conduction and hey-presto! The pump tube, (Most noticeably.) heats up!

With the shock absorber, (And forks.) on your bike the extra energy just gets turned into heat and transferred by conduction from the oil within the shock to the shock body and then radiated. By putting a heavier spring on more energy can be stored in the spring meaning less has to be dissipated. If it can't store enough, (The spring is too soft.) then it will dissipate some of that energy by beating the ever-lovin' piss out of the mounting bolts! Or, in the case of forks, the steering head bearings.

Pete

[jdgretz]

If you are still running the original shock Spring I'm not surprised. It was woefully inadequate even for solo work. Remember the bolt is essentially made to work in tension, not in shear. If the shock keeps topping out and hammering the bolt in shear I'm not surprised it has failed. If the cause wasn't addressed the first time history will repeat.

Pete

Nope, HyperPro with a spring rated for my weight plus a bit for two up.

The shock had started feeling a bit springy, so I was going to pull it off for a rebuild.  Last time I had it rebuilt was about 50,000 miles ago.  Probably should have pulled it a few miles earlier.

About to head out to the garage to see if I can pull the head out and see how much of the bolt is still in there.  Could be interesting getting the remainder out if it's broken in the sleeve in the middle of the top of the shock.

Shit !
Is that on your '07 Norge JD? That's gotta' be a bugger!
Do you reckon you should get a higher grade aircraft bolt or equivalent ? Mine'll be getting pulled down upon return to Oz, and I'll be checking that out quick sticks.
It was ok when I put the Nitron in a year ago, but after hearing your tale, who knows?
PS. Not over torqued ?

When I changed it the last time, I went to 10.9 bolt.  I don't think there is much better - maybe Titanium but those can be brittle, so I'm not sure that would be the way to go.

Torque values are 50Nm which is about 37ft lb.  That shouldn't have been an issue.

jdg

[pete roper]

The 10.9 designation is once again showing its Tensile strength, not its strength in shear which is the issue.

[jdgretz]

The 10.9 designation is once again showing its Tensile strength, not its strength in shear which is the issue.

Suggestions for a replacement?  I'd prefer not to have to go through this again.  Perhaps replace every 30,000 miles?

Standard Guzzi part AP8152398?

jdg

[Wayne Orwig]

Makes you wonder if a better bolt will just move the point of failure to something a LOT more expensive.

 

[jdgretz]

Well, the bolt head is approximately 36.4mm long (original is 80mm), which means the break is just about dead center    At this point, I'm not real sure how I'm going to proceed.  It looks like if I take off the rear fender, I may have access to the spacer that I can then cut in pieces and take out a slice at a time until I can slide the shock off the remaining bolt and then use pliers to unscrew the remainder of the bolt.  Ignore the green tint - that's an issue with the lighting.





Any other thoughts or suggestions are certainly welcome.

jdg

[jdgretz]

If I remove the rear wheel with the bike on the center stand, do I need to support the swing arm assembly since the shock is no longer connected to the lower connecting links?

It may be easier to get a sawzall to the spacer from the rear, rather than taking off the rear fender and going in that way.

jdg

[Huzo]

Thanks Pete for the expanded comment.

[SED]

If the spring and damper unit is properly sprung and valved it will perform the function for which it was designed, absorbing the energy imparted to it by *Bumps* and turning it into heat that can be easily dissipated and in doing so preventing shock loadings. That's why they are known as 'Shock Absorbers'.

If the spring rate is inadequate though the entire travel of the damper rod will be used up and at the very end of its travel the second line of defence, the bump-stop, a rubber collar around the damper rod will come into play acting as a cushion to prevent the piston or clevis on the other end of the rod simply whacking into the damper body and effectively turning the entire innit into a solid object acting like a bloody great drift that simply hammers on the mounting bolts at either end of the shock and imparts shear loadings on them.

Unfortunately the bump stop can only work to a limited degree and for a limited time. If you look at shocks on most early Norges and Griso's in particular you'll find the bump stops are usually flogged to pieces by the constant bottoming out of the damper rods. The result is continual hammering on the mounting bolts which are designed to take a loading in tension, (Lengthways.) and not in shear, (Sideways.) something will, inevitably, give in the long term and in this case it is usually going to be the bolt!

A spring is essentially *Battery* for kinetic energy. It stores it as it is extended or compressed from its at rest position and then releases it as it returns to that point. The energy is dissipated by forcing oil through valves within the damper body and turning the kinetic energy into heat. This prevents the spring from simply 'Pogoing' and speeds up and smooths out the dissipation of the unwanted and wasted energy.

As a kid you may remember putting your thumb over the end of a bicycle tyre pump and pumping like crazy and finding that the end of the pump near your thumb getting really hot? Same principle. The air is acting as a spring and storing the energy of your frenzied pumping but not having any decent way to dissipate it it simply turns the energy imparted to the spring, (The air inside the pump tube.) into heat which it then imparts into the pump tube, piston and rod by conduction and hey-presto! The pump tube, (Most noticeably.) heats up!

With the shock absorber, (And forks.) on your bike the extra energy just gets turned into heat and transferred by conduction from the oil within the shock to the shock body and then radiated. By putting a heavier spring on more energy can be stored in the spring meaning less has to be dissipated. If it can't store enough, (The spring is too soft.) then it will dissipate some of that energy by beating the ever-lovin' piss out of the mounting bolts! Or, in the case of forks, the steering head bearings.

Pete

   Great explanation - thank you.

[rodekyll]

Chances are that the threads have no tension with the bolt head off.  That means you might be able to use a screwdriver to catch some broken burr and turn it out.  Also, a long left-handed drill bit might spin it out pretty easily.  If you can get to the other end of the bolt you might be able to use a normal direction drill bit on it.

[jdgretz]

Chances are that the threads have no tension with the bolt head off.  That means you might be able to use a screwdriver to catch some broken burr and turn it out.  Also, a long left-handed drill bit might spin it out pretty easily.  If you can get to the other end of the bolt you might be able to use a normal direction drill bit on it.

Can't get to the other end as there is frame or other metal piece there that the bold almost nudges up to.

If you look closely at the photo, you'll see that it is a clean shear - nothing for a screwdriver to catch on.  The left hand drill bit is a possibility.

Todd suggested a small amount of JB Weld may do the trick.  Heavy emphasis on small amount.

jdg

[rodekyll]

A tiny rubber disk as a wad and a dowel on a left-handed bit might do it, too.  I've used pencil erasers before, but they can be crumbly.

[ozziguzzi]

If you are still running the original shock Spring I'm not surprised. It was woefully inadequate even for solo work. Remember the bolt is essentially made to work in tension, not in shear. If the shock keeps topping out and hammering the bolt in shear I'm not surprised it has failed. If the cause wasn't addressed the first time history will repeat.

Pete

At the risk of..........

If the bolt sheared, why would it not be likely to shear at the back of the bolt head or at the other end -inside surface of the nut , rather than in the middle (where the shear action would be less)

Just askin

[Huzo]

I know that if you drill an appropriate size hole, you can drive a Torx bit in and then attach a 6mm socket to the Torx. Don't drill the hole too big though, or you'll expand the bolt in the hole, then you'll be rooted !

[Chuck in Indiana]

At the risk of..........

If the bolt sheared, why would it not be likely to shear at the back of the bolt head or at the other end -inside surface of the nut , rather than in the middle (where the shear action would be less)

Just askin

Agreed. That seems like a strange failure to me.

[pete roper]

Oh it's strange all right but the fact it's happened twice would seem to indicate it's a causative failure rather than just a random and unlucky fault with material for example.

I too would expect a failure to occur at the radius of the shank to the head of the bolt but since it hasn't failed there you need to expand the search for a cause. While I've seen several significantly bent top mounting bolts I've not seen a snapped one but in this case it's happened twice. The one obvious change is the custom built shock and to impose a bolt-snapping force on the mount the only thing I can see that might contribute is if the shock keeps bottoming out and the hammering is going to effect the bolt in shear.

I may be completely wrong but it's the most logical explanation to me from half a world away......

Pete

[Chuck in Indiana]

On the first break, you said..  but that time the bolt broke right where the threads started.
Naturally, that is what you would expect.

There's something I'm obviously not understanding here. That looks like a clean break right in the middle of the bolt. *Not* at any stress riser. <scratching head> I looked at the shop manual on line and didn't see anything about the shock mount. The parts list shows a bolt. Period. The sketch shows threads half way up or farther on the bolt.. but it just may be a sketch and have no resemblance to the real bolt. Is there a tapped mount on the frame where the bolt goes through the shock and bushing and just screws in? Maybe the bolt is bottoming in it and not letting the screw fully torque down the bushing? Curiouser and curiouser.

[TimmyTheHog]

Is it possible for the OP to post a picture or 2 of the section view where it got broken off?

but if I have to speculate, I suspect the bolt surface had been already been externally chipped or simply a "small deep dent".

If it is so, the bolt can actually start cracking internally right at the tip of those dents (normally you can't see those crack because they are usually micro fractures starting within the body of the bolts) and until there are not enough area internally to withstand the force exerted by the shock...

that will be when the bolt shears

[pete roper]

On the first break, you said..  but that time the bolt broke right where the threads started.
Naturally, that is what you would expect.

There's something I'm obviously not understanding here. That looks like a clean break right in the middle of the bolt. *Not* at any stress riser. <scratching head> I looked at the shop manual on line and didn't see anything about the shock mount. The parts list shows a bolt. Period. The sketch shows threads half way up or farther on the bolt.. but it just may be a sketch and have no resemblance to the real bolt. Is there a tapped mount on the frame where the bolt goes through the shock and bushing and just screws in? Maybe the bolt is bottoming in it and not letting the screw fully torque down the bushing? Curiouser and curiouser.

The bolt doesn't have a nut Chuck. It screws into a threaded bush of some sort welded to the LH side of the upper mount. The bolt has a plain shank and is threaded only slightly longer than the depth required for the bolt head to a but the mount on the RH side. I'll try and get some pics of the upper mount on the stripped Griso frame at Michael's workshop if I remember in the next couple of days. Feel free to nag if I forget. :D

Pete

[jdgretz]

Oh it's strange all right but the fact it's happened twice would seem to indicate it's a causative failure rather than just a random and unlucky fault with material for example.

I too would expect a failure to occur at the radius of the shank to the head of the bolt but since it hasn't failed there you need to expand the search for a cause. While I've seen several significantly bent top mounting bolts I've not seen a snapped one but in this case it's happened twice. The one obvious change is the custom built shock and to impose a bolt-snapping force on the mount the only thing I can see that might contribute is if the shock keeps bottoming out and the hammering is going to effect the bolt in shear.

I may be completely wrong but it's the most logical explanation to me from half a world away......

Pete

Yeah, that's bugging me as well.  For some reason I don't have the mileage marked down as to when I swapped the shock, but the first failure happened somewhere around 30,000 miles and I know I didn't change the shock before 12,500 miles on the bike, so that would have been something less than 20,000 miles for the shock.  This one has almost 50,000 miles on it, so I pushed the recommended rebuild mileage about 20,000 miles too far (must have overlooked the recommended rebuild mileage as I don't remember reading, or being told that, but it's now on the web site).

I don't think I ride it that hard. and I have not heard of anyone else having Hyper Pro shock failures, so I don't know.  Changing to another manufacturer is certainly an option, but would it be of any benefit?

On the first break, you said..  but that time the bolt broke right where the threads started.
Naturally, that is what you would expect.

There's something I'm obviously not understanding here. That looks like a clean break right in the middle of the bolt. *Not* at any stress riser. <scratching head> I looked at the shop manual on line and didn't see anything about the shock mount. The parts list shows a bolt. Period. The sketch shows threads half way up or farther on the bolt.. but it just may be a sketch and have no resemblance to the real bolt. Is there a tapped mount on the frame where the bolt goes through the shock and bushing and just screws in? Maybe the bolt is bottoming in it and not letting the screw fully torque down the bushing? Curiouser and curiouser.

The shock mount is simply a U-shaped bracket welded to the frame.  Clear hole on the insert side and a threaded boss on the other. I think the bracket can be seen on the Frame I image, just behind the bracket where Pin 9 goes through the frame.  The twin ears pointing toward the rear are, I think, the shock bracket.  I don't have the photos of the bracket being out of square from the first time, nor do I still have the old bolt.  Thought I had the photos of the first failure, but not sure where.  I'll have to look for those.

Is it possible for the OP to post a picture or 2 of the section view where it got broken off?

but if I have to speculate, I suspect the bolt surface had been already been externally chipped or simply a "small deep dent".

If it is so, the bolt can actually start cracking internally right at the tip of those dents (normally you can't see those crack because they are usually micro fractures starting within the body of the bolts) and until there are not enough area internally to withstand the force exerted by the shock...

that will be when the bolt shears

I'll get the bolt cleaned up a bit and see if I can get a good photo or two of the end of the bolt.

jdg

[TimmyTheHog]

I'll get the bolt cleaned up a bit and see if I can get a good photo or two of the end of the bolt.

jdg

Thank!

when cleaning it, don't sand blast or grind off anything...just simply a rack wipe clean would be the best.

[jdgretz]

OK, pictures of the end of the bolt.











jdg

[pete roper]

Looks like it's sheared just at the end of the threaded part which would make sense as the cut thread would be a stress raiser.

[rodekyll]

It's a very well behaved break.  Organized, perfectly perpendicular to the shaft, burrless, and very clean.  If I had an employee like that, I'd promote her -- except for the total failure to do the job.

Do you have another identical (unmolested) bolt to compare it to?  It's got a wonderful tooled roll to the broken edge that suggests it broke at the end of the threads, which is a major stress riser in any direction.  I don't see any signs of the shaft having shifted or having worked for any length of time while loose.

How is the fit of the shock to the mount?  Any slop?  Shock eyelets the right size?

I see Pete just said it better.  Goodonya, Pete!