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Rec.Bicycles Frequently Asked Questions Posting Part 3/5
Section - 8c.8 Wheels for Heavy Riders

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Date:    Fri, 25 Jul 2003 00:08:48 -0700

Some heavy riders get poor service from mainstream wheels.  Common
durability problems include wheels that go out of true and broken
spokes.  Common strength problems include wheel collapse broken rear
axles, and broken ratchet mechanism.  A ``better'' wheel improves
durability and/or strength.

Variations in wheel use and budget make it hard to make general
recommendations.  However, here are some things that can help build a
stronger or more durable wheel:

 - A stiffer rim improves wheel strength and spoke durability by sharing
   the load among more spokes.  Rim stiffness is increased by using a
   wider rim and is also improved by using a deeper rim.  It is clear
   that a wider rim will build a stronger wheel; however, a deep rim is
   radially stiffer, which shares the load among more spokes and thus
   makes the wheel laterally stronger.  Thus, using a deep-section rim
   can improve lateral wheel strength and spoke lifetime, and can also
   reduce the frequency of wheel re-truing.

 - All other things equal, a heavy rim is stronger.  It is also stiffer,
   and rim stiffness improves wheel strength.  However, rim shape has a
   dramatic effect on stiffness, and many heavy rims do not have a good
   shape for building strong or durable wheels.  In particular, many
   heavy rims are not very deep.  For many uses, a lighter deep-section
   rim builds a better wheel than a heavier but shallow rim.

 - For ``dished'' wheels, use a rim with offset spoke holes.  The rim
   should face so the holes are as close as possible to being centered
   between the flanges.  Although offset rims move the nipple position
   only a few millimetres, with highly dished wheels the change is a
   substantial percentage of the dish.  Reduced dish improves spoke
   bracing angles which improves wheel strength; allows higher tension 
   on the low-tension spokes which reduces the rate of re-truing; and
   may allow higher overall spoke tension, which improves wheel
   strength.

 - On front wheels, use hubs with wide flange spacing.  Wide-spaced
   flanges may be as much as 1cm wider than standard flange spacing, and
   some aerodynamic hubs space the flanges as much as 1cm closer
   together than standard hubs.  Wider flange spacing improves the spoke
   bracing angle and thus improves lateral wheel strength.  It may
   also reduce the rate at which wheels need to be re-trued. 

 - Center the rear hub by using a narrow sprocket cluster.  Wide
   clusters use space on the right and thus push the flanges to the
   left; such asymmetry is called ``dish''.  Dish hurt the bracing
   angle of the spokes coming from the flange which is closer to the
   center line.  Dish also forces different left and right spoke
   tension.  The spokes with lower tension are more likely to go slack
   under load, which weakens the wheel and make truing more frequent.
   The spokes with higher tension may pull the nipples through the rim
   before the rim's compressive load strength is reached, thus limiting
   overall spoke tension for the wheel.  Reduced overall spoke tension
   further weakens the wheel and makes retruing yet more frequent.

   Narrower clusters reduce dish.  As of 2003, Five-sprocket and narrow
   six clusters are largely unavailable. 6-sprocket and narrow seven
   clusters are available, but mostly in lower-priced products.  Some
   are good products, but some may be less durable.  Eight and
   nine-sprocket clusters use the same spacing; they get nine in the
   same space by using thinner material for the sprockets and chain.
   Nine-speed equipment has a reputation for breaking and may be
   unsuitable for riders who already have component durability
   problems.

   It is sometimes possible to build a 7-speed hub by bolting a
   replacement 7-speed body on to a new 8-speed hub.  Some people 
   retrofit 8 out of 9 sprockets of a 9-speed on to a 7-speed freehub
   body; but doing so may cause poor reliability due to the thinner
   sprockets and chains. 

 - Center the rear hub by using a wider dropout spacing.  Centering has
   the benefits listed above.  Common dropout spacings range from 120mm
   to 135mm and some tandems use 140mm and winder.  If you have the
   luxury to select the frame, get a wider spacing, but beware of
   possible heel clearance and crank width issues with wider spacing.
   If you have a steel frame it may be possible to spread the stays by
   5mm.  Beware that spreading requires special tools and skills to
   avoid frame damage.  Aluminum, titainum, and carbon frames cannot
   typically be spread without damage.  Note that simply ``stretching''
   the frame to fit over a wider hub may cause gradual frame damage.
   Spreading the frame requires a longer hub axle.  Many hubs are
   offered in various widths but make sure you have the right parts in
   hand when the frame is spread.

 - Use a rear hub with a tandem-rated ratchet mechanism.  Many
   ``racing'' quality hubs are designed for low weight and are no
   stronger -- and are sometimes weaker -- than mainstream components.

 - Use a rear hub with an oversize axle.  Freewheel hubs are available
   with steel axles of 17mm.  Freehubs are available in two general
   styles, see the FAQ ``Cassette or Freewheel Hubs'' section.  The type
   labeled Hugi/Campagnolo needs a much larger axle; the type labeled
   Shimano/SunTour does not need as large an axle.  Most modern (2003)
   freehubs have adequate axles even for heavy riders, but some older
   ones are inadequate and new designs often bring new weaknesses.

 - Use a large number of spokes.  Most sizes of wheels can be built up
   to 48 spokes.  Note that there is less selection in high-spoke-count
   hubs and rims, and parts often cost more.  The benefit of many spokes
   is partly limited by the rim strength: a large number of spokes may
   require lower tension on each spoke to avoid collapsing the rim.
   Thus, a very strong rim is required to realize the full benefit of
   using many spokes.

 - Use high spoke tension.  High spoke tension improves radial and
   lateral wheel strength, improves spoke durability, and reduces the
   rate at which spokes loosen and let the wheel go out of true.
   Maximum spoke tension varies from rim to rim and, unfortunately,
   makers do not typically publish a recommended tension.  As of 2003,
   I have seen only one rim which listed spoke tension.  Thus, tension
   must be discovered in the manner described in _The Bicycle
   Wheel_ [Brandt].  Although this procedure allows you to set tension
   for a given rim, it is done as part of building the wheel, which
   keeps you from choosing rims based on strength.

 - Stress-relieve the spokes.  Spokes are prone to break unless they are
   stress-relieved after wheel building.  Stress-relieving is described
   in _The Bicycle Wheel_ [Brandt].  Stress relieving is also summarized
   in the wheels section of the FAQ (``Stress Relieving Spokes'').

 - Use swaged (``butted''; thinner in the center) spokes.  The elbow and
   threads take high loads and should be of thicker material; the center
   should be slightly thinner so most stretching takes place in the
   center section, thus reducing elbow and thread failures.  Using
   swaged spokes also reduces rim cracking at the spoke hole.

 - Choose spokes according to the type of wheel failure.  If spoke or
   rim eyelet durability is a problem, use lighter spokes.  It may seem
   odd to solve breaking problems by going to a lighter spoke; but
   spokes are run at 1/2 their yield strength or less, so do not break
   from overload.  Instead, they fail from gradual degradation caused by
   repeated stretching and relaxing of the metal.  Lighter spokes cause
   the wheel load to be shared among more spokes, reducing the loads on
   each spoke and thus improving spoke lifetime.  A similar mechanism
   can damage rims at the eylet, and lighter spokes can therefore also
   help reduce rim damage.  Note that using more spokes and a rim with
   greater radial stiffness also helps spoke and rim bed durability.

 - If wheel collapse is a problem, use thicker spokes.  Note that very
   thick spokes (2.3mm) will not fit in some hubs.  Beware that using
   thick spokes may hurt spoke and rim bed durability.  Note that using
   more spokes and using a rim with greater radial stiffness and greater
   lateral strength and stiffness can also reduce wheel collapse.

 - Use brass (not aluminum) nipples.  Aluminum nipples also break more
   often, especially at high spoke tensions.  Brass nipples allow
   periodic adjustment with less chance of wheel damage.  Aluminum
   nipples tend to sieze and gall.

With thse considerations in mind, here are some specific
recommendations:

(1) Fix the existing wheels.

In my experience, most wheels are under-tensioned, even those built by
many reputable shops.  Many problems with existing wheels can be solved
by simply truing the wheel, raising the spoke tension to an appropriate
level for the rim, and by stress-relieving the spokes.  A conventional
wheel can then give good service for many heavy riders.

Unfortunately, manufacturers do not rate rims for spoke tension (I have
seen only one rim that was marked or rated), so it is necessary to
gradually overtension the wheel and then back off, as described in _The
Bicycle Wheel_ [Brandt].

Beware that many mechanics are unfamiliar with tensioning and stress
relieving procedures, or claim familiarity but do not perform them
correctly.  Thus, while there are also many mechanics do it right, you
cannot assume the mechanics know what they are doing.  Familiarize
yourself with the tensioning and stress-relieving proeceudres, then
interview the mechanic who would repair your wheel, and ask them to
explain in detail how they determine proper spoke tension and how they
stress-relieve the spokes.  If they deviate from the standard procedure,
there is a good chance they are missing something important.

Note also that a shop or particular mechanic may have a history of
satisfied customers and yet still build wheels with low spoke tension
and/or improper stress relieving.  This can occur because low tension
and lack of stress relieving are less important for average riders, so
such wheels may not lead to customer returns.


(2) Use a deep-section rim.

The simplest and probably cheapest change is to use a deep-section rim.
Here, ``deep'' means at least 30mm.  A deep-section rim allows you to
reuse your existing hub and/or buy inexpensive ``mainstream'' hubs, yet
a deep-section rim builds a wheel which is dramatically stronger and
more durable than a wheel built with a shallower rim.

Deep-section rims also give better rim brake cooling, which may be
important for heavy riders in hilly areas.  Painted rims should be
avoided if cooling is a concern; color anodizing does not hurt cooling.

Fortunately, deep-section rims are available from several makers in most
common sizes (20", 650C, 26", 700C).

For dished wheels, given a choice between a deep rim and a rim with
offset spoke holes, I do not have data about which is better.  My
intuition tells me that an off-center rim is probably more imporant the
steeper the dish.  So, for example, an off-center rim might build a
stronger wheel for a 10-speed cluster in a 130mm dropout, while a
deep-section rim might build a stronger wheel for an 8-speed cluster in
a 135 dropout.


(3) Use more spokes; use lightweight spokes.

All other things equal more spokes builds a stronger and more durable
wheel.  However, a deep-section rim is of sufficient benefit that if you
are forced to use a shallower rim in order to get more spokes, it may be
the same either way.  With many spokes (e.g., 48), it is relatively easy
to overload the rim while the tension of each spoke is still low.  A
stiffer rim allows a greater spoke tension, so a deep-section rim still
helps to build a strong wheel.  Using more spokes allows the use of
lighter spokes, which increases spoke and rim bed durability.


(4) Avoid ``trendy'' solutions.

When discussing strong wheels, special techniques often come up.  For
example: asymmetrical lacing, straight-pull spokes, paired or crossed
spokes, alternative spoke materials, and so on.  For any given weight of
wheel, these approaches have not demonstrated measurable stiffness or
strength benefits in any tests I know of.  In addition, they are
typically only available in low spoke count wheels anyway.  Finally,
trendy solutions are typically more expensive.  Thus, the best value
wheels are typically built using standard components.


[Brandt]
    Jobst Brandt, ``The Bicycle Wheel''.  Avocet Press; 3rd edition,
    October 1998.

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Previous Document: 8c.7 Wheel Bearing adjustment
Next Document: 8d Tech Chains

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