C[omp]ute

From: andrew cooke <andrew@...>

Date: Tue, 16 May 2017 22:26:48 -0400

I've built three wheels using the 2-leading, 2-trailing pattern.  Last
week I damaged one of these wheels.  Here's my thoughts on what I can
learn from that.


When I started building wheels I wanted to use a distinctive spoke
pattern.  I was aware of the recommendations to use only Two-Cross
(2x) or Three-Cross (3x) designs, but it was difficult to find hard
data about the risks of anything else.  I settled on the 2-leading
2-trailing (2L2T) pattern because (1) it seemed conservative (not that
different to 2x), (2) worked with a 32 hole hub, and (3) I found a pro
wheelbuilder who had built some, and who felt it was OK for casual use
(not for "real downhill", but I wasn't planning on that anyway).

And my experience has, until now, born that out.  All the wheels have
stayed remarkably true and given no problems (two have around 4,000km
on them, each, mixed road and off-road).

However, this weekend I looked down and noticed a slight wobble in my
rear wheel between the chainstays.  When I examined the wheel I found
that one spoke was completely loose - the nipple was 3/4 unscrewed.
With more study (as I re-trued the wheel) I realised that the rim near
that point had dented "inward" (towards the hub) by several mm.  This
had allowed that particular spoke to go slack, leading to the loose
nipple and wobble.

This could, of course, have happened to any wheel, and before I look
at problems specific to the lacing pattern I will give the generic
details:

  o The rim was an Easton (now RaceFace) Arc 27 (650b).  This was a
    fairly new nodel when I bought it, with few reviews (that I
    remember).  Now, however, many of the reviews on ChainReaction are
    negative.  So the rim itself may take some of the blame.

  o The tyre pressure was low (22 psi in a 2.4 Conti XCross).
    However, the tube did not have an snakebite puncture - likely
    because it was latex.

  o I believe the damage occurred while descending El Arrayan.  That
    trail is easy enough to reach decent speeds, but has frequent
    rockier sections that can be violent.  Particularly on the rear of
    a hardtail.

Together these suggest that the rear hit a rock at speed, with low
pressure in the tyres, causing the rim to give.  A common scenario.


What does this have to do with 2L2T?  The loose spoke was one of the
uncrossed spokes in the design.  This has two implications:

  1 If the spoke had been crossed then possibly it would have stayed
    under enough tension to keep the nipple from coming loose.  This
    is the advantage of crossing - that it gives the spoke additional
    movement before becoming slack.

  2 The 2L2T design is (simplifying somewhat) "pulling the wheel
    square".  This implies that it has stronger (corners of the
    square) and weaker sections (the sides).  The damage occurred in a
    weaker "side".

Personally, I am inclined to believe the first of these may be
important, but the second seems to be a stretch.


Given all the above I am planning to build a replacement (using the
same hub).  I will use a Stan's Arch rim (Mk3 is 26mm internal width,
which is almost the same as the Arc) - this appears to have a better
reputation (but again is fairly new...).  I will also run the rear at
23psi.  Finally I will probably use a 2x pattern (2x rather than 3x
since the hub holes are likely marked at that angle).  I'm not sure
it's needed, but:

  o In all honesty, the pattern isn't noticeable once the wheel is on
    the bike (a few people notice the steel frame; no-one has ever
    commented on the lacing).

  o Point (1) above (crossing could have saved the nipple from coming
    loose) seems like a significant gain if this happens mid-descent.

  o I'm starting to think that the argument "you're not doing crazy
    stuff so it's OK" doesn't hold so much weight.  First, one tends
    to push limits (eg lowering pressure until the rim is damaged).
    And second, I suspect my riding is more extreme than the majority
    of riders (after all, most bike owners hardly ride), even if I
    tend to focus on the people who are better than me.

While I'm not convinced the lacing pattern caused the failure, it
could have contributed and it's annoying having the uncertainty.
Going with a standard pattern will make any future damage easier to
understand.

Andrew