Mountain Bike Reviews Forum banner

1 - 20 of 25 Posts

·
Registered
Joined
·
978 Posts
Discussion Starter · #1 ·
I got my bike back from the LBS after shock and fork service and they said the shock's air sleeve was severely damaged. Some deep grooves were carved in the side of it, just where the sag would put the piston bushings. They couldn't find any dirt that would have caused this, so why did it happen after just 2 years? (this was the 3rd service I had done on the bike, so it's not like this is the first time the shock was serviced) I think I put something like 4500-5000 Km (2800-3000 miles) on this bike in the time I owned it, so is that the mileage you'd get from the Monarch shock? Or is the LBS right and it is due to the odd angle at which the shock is actuated due to the linkage design of the Element? The force used to actuate the shock is definitely not in-line with the shock body. It is not an expensive part, so I will be getting a new air sleeve, but I'm curious.
 

·
chips & bier
Joined
·
1,597 Posts
The force actuating the shock is always in line with the shock body: the upper and lower mounting points offer rotational freedom, so they don't allow any (vertical) bending moment to be transferred to the shock. If the linkage is somehow misaligned you might have a bending moment on the shock in the sidewards direction, but that would probably mean the shaft is REALLY worn, not the air sleeve.

Could you show us some picks when the shock gets back?
 

·
Registered
Joined
·
978 Posts
Discussion Starter · #3 ·
The linkage is not in-line with the shock body, so any force applied will be composed of a force that is in-line and a force that is not in-line. See my paint drawing (don't laugh). Of course, the linkage will take some of the unwanted force, but not all. This results in a bending force that acts vertically on the bushings inside the shock. So it's not so much a problem with a misalignment of something, as it is the geometry of the linkage.



The lower bushing acts as a pivot point, forcing the upper bushing to be pressed more firmly against the air sleeve wall on the inside. Also without dirt, this will wear the air sleeve. At least, according to the theory of the LBS :)
 

·
Super Moderator
Joined
·
6,776 Posts
Well-drawn, OP! My 999 RSL suffered the same exact problem...non-linear shock stroke = premature wear. Bike has since sold....:madmax:
 

·
Registered
Joined
·
978 Posts
Discussion Starter · #5 ·
Aw cr#p. It is a fine bike, that's for sure. You should go for it. The air sleeve will cost about 30 euro I think, so it's not very expensive. I got 3000 miles out of it, that's 0.01 euro per mile :) Running cost on the chain/cassette is much higher.
 

·
Super Moderator
Joined
·
6,776 Posts
Aw cr#p. It is a fine bike, that's for sure. You should go for it. The air sleeve will cost about 30 euro I think, so it's not very expensive. I got 3000 miles out of it, that's 0.01 euro per mile :) Running cost on the chain/cassette is much higher.
I am perfectly happy with my 2012 Trek Superfly 100 Pro. I was able to build it just as light as the Rocky Mountain Element 999RSL.
 

·
chips & bier
Joined
·
1,597 Posts
I've seen this misconception show up a couple of times on the interwebs and Zonoskar's LBS (and the both of you) are still wrong ;-)

Well drawn... but....you're almost there. There's no resultant force acting perpendicular to the shock's centreline. Why? Because the shock has a pivot on both ends. It's the carbon link (with all the bushing wear...) that carries that perpendicular load and keeps the seatstay end rotating in an arc. That's in part why the bushings there get such a beating.

I could refer you to a mechanics textbook (and sound like a complete @$$) but here's a thought experiment: imagine away the seatstay and link, so only one end of the shock is connected to the frame (front triangle). What happens if you push upward or downward on the loose end of the shock? It rotates around the front bushing assembly. It's no different with stays and link connected, except that it's the link pushing on the shock, not your finger.
 

·
Registered
Joined
·
978 Posts
Discussion Starter · #8 ·
You're probably right. I eyeballed the decomposition of the forces on the linkage and thought there would be a small perpendicular force. I still believe there will be such a force since you never can have a perfectly aligned and friction-less suspension design.

So where do you think the wear comes from? The wear is real and visible.
 

·
kabouterclan member
Joined
·
499 Posts
Where is the damage exactly? Shaft of can? Top/bottom or left/right? That's not clear to me.

Some thoughts:
Theoretically, Eric is right. In real life: if you tighten the damper , there will be some resistance to rotation in the bushings. Resulting in small lateral forces.
The damper is a telescope, with a quite unfavorable diameter/ length ratio. That’s prone to kinking / setting (dunno the right English term for “schranken.”

However, with loads and loads of dampers running trouble free, I don’t think that’s the problem.

About misaligned frames: If that were the case, the damage would be on the left or right side of the shaft, not the top and bottom.
 

·
chips & bier
Joined
·
1,597 Posts
That perpendicular force will be negligible, however. The shock angle really isn't an issue. Also, shock bushings generally don't bind as they wear, but develop radial play, so that force will become less and less as the bushing gets a bit sloppy.

Regarding buckling I don't actually think that's the problem, either. There's perpendicular external load at all on the air can or shock shaft, and the compression force on the shock is aligned almost perfectly in the centerline of the shock. I think you'll find a shock actually still has quite significant bending stiffness (would need to calculate that to be sure, though...).

Zonoskar, can you post pictures of the innards of the shock? I'd be interested to see if the wear is symmetric. I suspect this is a seal or bushing issue.
 

·
Registered
Joined
·
978 Posts
Discussion Starter · #12 ·
I'll see if I can get some pictures tonite. I must say that the shock bushing on the frame side doesn't rotate smoothly. It takes quite some force to rotate it slightly.
 

·
chips & bier
Joined
·
1,597 Posts
The frame side bushing gets way less rotation than the other one, so it wears a lot less. Out of the box the bushings require quite a bit of force to turn by hand, but should feel okay if you have the front end of the shock in the frame.
 

·
ups and downs
Joined
·
15,592 Posts
Could this be related to the internal shock lubrication of the bushings, and the ability of lube to be distributed evenly if the bike remains on its wheels most of the time?
 

·
Registered
Joined
·
978 Posts
Discussion Starter · #15 ·
The damage is less than I thought, but still there are 2 small scratches near the hole that connects the negative and positive air chambers. On the opposite side, there are some lighter scratches. I hope it is clear, it was very hard photographing the inside of the cannister.

Notice the light 'band', but I suppose this is normal.


Closeup of the 2 scratches.


The shaft is undamaged (as far as I can see).


On the bike. The hole is not at the side, nor at the bottom. So maybe there was some burr or something that caused the scratches?
 

·
chips & bier
Joined
·
1,597 Posts
Thanks for the pic!

I would think this is a shock issue, not a frame issue. It's an oddball place to get damage. Where shocks have out-of-centerline loading on them you typically get slow wearing down of the anodization, to the point where the air seal no longer seals in air, but not scratches.
 

·
Registered
Joined
·
221 Posts
It does look like a seal lubrication issue. I haven't had my monarch apart yet but I used to regularly take my float apart on my Jekyll every couple of months to grease the seals.
The anodizing should be harder wearing than that though, RS forks put up with a lot of crud IME with no signs of that kind of damage, perhaps you got a dud air can?
 

·
Registered
Joined
·
978 Posts
Discussion Starter · #18 ·
The shock is now officially dead. I had to refill it twice during yesterday's 40 km ride. Both times the pressure dropped from 190psi to about 80psi. Surprisingly, the Element keeps riding brilliantly, even with the shock at half the pressure and about 60% sag.
 

·
Registered
Joined
·
221 Posts
That's a bummer, is it getting a rebuild or are you getting another shock? Would be interested to hear how another shock compares if you go that route.
 

·
Registered
Joined
·
978 Posts
Discussion Starter · #20 ·
I'm getting my shock rebuilt with a new air can. A new shock is too expensive and upto now, I liked the monarch shock.
 
1 - 20 of 25 Posts
Top