Re: "Catastrophic" failure of mountain bike fork lowers (Manitou Skareb Comp)



M

Marvin

Guest
Phil, Squid-in-Training wrote:
> The magnesium lowers of my 2003 Manitou Skareb Comp had begun to

crack last
> year, about 6 months before I stopped riding it. I've kept it

stashed away
> for a while until today, when I was cleaning my room and found the

lowers.

If you manage to lose an entire set of fork lowers in your room you're
very nearly as messy as I am :)

> My experience:
> From the first day I bought the bike, the front wheel had always

rubbed the
> v-brake pads when turning. I thought this was just because it was a

light
> bike (23lb Giant XTC2). But as I continued to ride, my expectations

turned
> to frustration, as the larger tire that I had put on was actually

beginning
> to rub the arch when braking hard. This wasn't really a problem

until I
> began riding more aggressively. It was at that point that noticed

that the
> wheel was beginning to get cocked to the side with the mere

application of
> weight. I knew this was out of the norm and searched for the

problem, which
> I found in the form of a crack in the fork leg that contained the

coil
> spring. New updated lowers with a beefed-up arch/leg join were sent,

and I
> installed them without a hitch. After another 2 months of use, I

broke the
> damping assembly inside the fork. Rather than fix it or pay to have

it
> fixed (I wasn't working at an LBS at the time) I left it as it was,
> spring-only. After another 3 months, I acquired my current bike

(Giant
> STP2) and a Fox Vanilla. The Skareb with the updated, uncracked,

flexy
> lowers was put aside.


Just to add my voice, I've had one of these reverse-bridge forks fail
on a customer's bike. It doesn't seem to be safety-critical since he
only noticed it when the clunking noise and fork flex really got on his
nerves and he inspected it a bit more closely. Since the legs can't
fall off and the skewer is still tight, there's no short-term potential
for a catastrophic failure. More mileage would probably twist and snap
the QR skewer, at which point you really would be in trouble, but my
customer got round the rest of a fairly tough off-road ride with no
worse problems than his fork legs walking around and crapping up his
steering (disc brake still worked fine). As soon as we removed the
wheel the lower legs twisted around quite independently and freely, but
the axle and skewer seemed to hold them enough for him to get home.

It definitely looks like a design problem, right on that sharp corner
where cornering stresses and the asymmetry between spring and damper
cartridges combine. The fact that it also seems to be quite a flexy
design in a material that doesn't like being flexed suggests that
Manitou really weren't thinking all that carefully during the design
phase.

Bottom line: yes they're a bit of a suspect design, no failure wouldn't
necessarily have resulted in a faceplant.
 
W

Werehatrack

Guest
On 11 Feb 2005 02:07:18 -0800, "Marvin" <[email protected]> may
have said:

>Bottom line: yes they're a bit of a suspect design, no failure wouldn't
>necessarily have resulted in a faceplant.


The tips of the tubes, where the QR clamps, are also mag alloy. I
suspect that they'd break off before the skewer would fail...but it
would take a while. The greater danger would be if the arch failed
during a bumpy downhill run; the asymmetric flex could put the wheel
into one tube enough to give just the added amount of unwanted braking
to produce an endo.

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