Information on 06 Mono-Skid's Front Pivot Arm

One thing that constantly puzzled me was why was only the outside 1/2 of the bracket the area that kept on failing?

I could see slightly higher loads going through it if I consider the primary load twisting through the suspension, but it would only be very slightly higher. Close enough that the inside 1/2 of the bracket should at least be showing some cracks forming (especially after the outer half had already cracked most of the way through).

When I took things apart and inspected them carefully I found something very interesting.

Have a look at these bolts paying attention to the shank diameter.

In the above picture, the upper bolt is the one that attaches the bottom of the pivot arm to the rail assembly. The lower bolt is one used to hold the idler wheels in place.

The upper one is defined as a reduced shank bolt, with the shank diameter the same as the minor thread diameter (bottom of the thread). These bolts are optimized for providing clamping loads with reduced stress concentrations and mass. They are not generally used in shear loading conditions (in this case it is used in a double shear configuration).

The lower bolt is a standard bolt. The shank diameter is the same as the major thread diameter (outside of the thread). This bolt is essentially generic and is commonly used all over the place, including single and double shear loading conditions.

Have a look at this picture of the lower pivot arm bushing beside the lower pivot arm bolt.

Measuring the diameters I found:

bushing ID: 0.400"
standard 10mm bolt shank: 0.387"
shank from bolt used on the sled: 0.356"
thread diameter of bolt used on the sled: 0.387"

This means that right off the bat, there is 0.013" clearance between the thread and the bushing on one side and 0.044" clearance between the shank and the bushing on the other side.

The way this bolt is installed, the nut is on the outside of the sled and the bolt head on the inside.

This means before the inside 1/2 of the pivot arm lower bracket (bottom of pivot arm) even sees anywhere near the same load as the outside 1/2 of the bracket something will have to give by 1/2 of the difference in clearance or 0.0155". While this may not sound like much, this means the outside 1/2 of the bracket will always see higher loads than the inside 1/2.

To make matters worse, this smaller shank diameter also directly affects the way the bolt fits into the bracket itself. As the bolt gets constantly loaded in a common direction, the head of the bolt will slowly walk (slip) in the direction of the cyclic load (unless it's extremely tight). This means that not only is there the 0.0155" extra clearance between the shank and the bushing, there will be up to 0.0155" additional clearance in the direction of loading for a total of 0.031". If the bolt was slightly loose or slightly misaligned when tightened up this could allow the 0.031" effective clearance right away.

With the outer half of the bracket is seeing much more load than the inner half of the bracket it makes sense that it would fatigue over the miles and eventually break where mine did.

The question remains - if a proper bolt is used would the arm have failed in the first place?

My educated guess is the loads would be greatly reduced, likely enough to stop the failures. Unfortunately this is still a guess.

Now I have to make the decision - do I beef this arm up anyway ("uglifying" a very nice paint job on a new arm and paying for the welding plus paying to have it powder coated) or do I install a couple of proper bolts for this application and ride - hoping it won't fail again and checking it regularly for cracks.


I suspect this potential problem exists with all 05 and 06 mono-shock skids so you might want to consider changing these bolts on your sled.

ReX thanks for the great information.

Hey rex,
Have you run this by anyone at Yamaha? I know I have delt with their customer service and it seems like some people don't get anywhere. I blew up my viper twice and a 600 twin once and they were great to me. I don't know if there is a tech department but I would be curious as to WHY they used those bolts at that location in the first place. I have 2500 miles on my 05 and I don't see any signs of crackng, But I want to update it anyhow since I DON'T want to have to deal with this on the middle of winter!

Let us know what you end up doing.

I guess I just don't get why they would run a reduced shank bolt in there to begin with. Makes me want to swap them out no matter what.

When the first set of bushings were changed on my 05 skid (somewhere around 2000 miles), there were no signs of cracking either.

Around 3300 miles I noticed very small cracks in the pivot arms forming (same place as shown in the first post in this thread).

Each time the bushings are replaced the problem loading described above occurs. When the bushings start to wear the 0.0155" - 0.031" comes out of the bushings first.

This means that the most likely time for cracks to start and grow is when the bushings are new.

Likely a lot of us (myself included) took it easy for the first 1000 kms or so - allowing the bushings to wear and help to equalize the loading before any aggressive riding occured.

I haven't talked to anyone at Yamaha about this. I expect they would be very none responsive if I called (although I'm sure they are reading it here).

I can't think of why - other than simply an oversight in engineering, a production part change the slipped through, or simply their bolt supplier is providing reduced shank bolts for that length (as their "standard bolt").

So did you....or are you going to put regular bolts back in there in place of the reduced shank?
I wonder if the 06's have the same bolt in there. I just dont get why they did that.

Since I installed a set of wider idler wheels (6502 bearings) I had the stock bolts that hold the front pair of idlers on available.

These are the perfect length for the full diameter shank to interface all the way through.

I also beefed up the pivot arm so it added some thickness. With the stock arm a pair of washers could be added under the nut.

ok...so you used the old stock bolts from the wheels and put them in place of the reduced shank bolts in the suspension?
hmmm..thanks again Rex....

Pivot bolts

ReX that is an interesting observation, and would make a lot of sense. The design requires the proper torque and clearence to maintain holding power. The reduced shoulder of that bolt would only leave you with a pinch fit that would loosen and cause fatuiqe when the bushing wore the paint off of the arm ears. If the bolt was the proper thickness as you suggest it would not matter if it was a little loose as it would aply the load evenly across both ears.

ReX you very well may have it, Yamaha changed the bolt on the '06. The bushing, or collar is the same as is the lock nut, good find.

I'm not sure if they changed this bolt (they did change some other bolts for example the upper bolts on the rear pivot arm).

My skid is supposed to include all of the 06 updates.

You are right the upper rear mount bolts were changed, but no other shaft parts were changed. Yamaha made several slight changes for whatever reason, I look forward to checking some of the changes to see why. I disassembled my pivot arm and was suprised to see how poorly designed it is. First, the holes in the arm are much larger than the bolt selected, this could allow a misalignment during assembly and movement if it loosend up. Second, the bolt is too short, this allows the pivot arm to ride on the threads instead of the shoulder. I think the design should have the pivot arm riding on the bushing, not the bolt. The last item that caused concern is the location of the idler wheel in relation to the pivot bracket, it would seem that this could transmit shock loads (flexing of the rails) to the mount. I hope to get the bugs worked out I love the overall design and simplicity.