TECHNOLOGY STORIES BIKE INFO, GEOMETRY ... - Vintage Trek

use a deeper, more aerodynamic rim than the rear wheel.
Mountain bike wheels have different needs
While road bikes benefit from improved aerodynamics,
mountain bikes place a greater need on wheel durability and
rigidity. They also sometimes require special configurations,
like the ability to accept a disc brake rotor. Again, Bontrager
Wheelsystems mountain bike wheels select those features
which will best create the ultimate structure.
With disc-specific wheels, there is no need for a flat
rim sidewall. This allows optimization of the rim shape to
reduce weight. Placing a rotor on the front wheel creates
an asymmetric spoke configuration that benefits from OSB
(Offset Spoke Bed), thereby reducing the required dishing
and providing more balanced spoke tension from left to
right side of the wheel. Disc wheels also used crossed
spokes, to efficiently transfer disc brake forces to the rim.
With rim brakes, Bontrager Wheelsystems incorporate
tall sidewalls so that brake adjustment is easier, and pad
wear has less effect on proper adjustment; taller sidewalls
provide increased surface for the brake pad to mate to.
Like with Bontrager road wheels, Bontrager mountain
wheels focus on balancing spoke tensions on the drive and
non-drive side of the wheel. To do this, they employ OSB
(Offset Spoke Bed) rims and special hub designs with modified
flange spacing. These features greatly reduce the tension
differentials from side to side, creating a stronger,
more durable structure. The higher left side tensions allow
more torque transfer to the left side drive spokes. They also
provide increased strength through reduced lateral wheel
flex. In other words, Bontrager Wheelsystems mountain
wheels are stronger.
Truing Bontrager Wheelsystems wheels
Most Bontrager wheels employ standard, externally
adjustable spoke nipples. The only exceptions are the
Bontrager X-Lite Carbon Road wheels, and the Bontrager
X-Lite Aero road wheels where a small aerodynamic benefit
can make the difference between winning and losing a race.
Bontrager Road wheels use PST (Paired Spoke
Technology) which require a slightly different technique
to true. In many respects, truing Bontrager Wheelsystems
wheels with PST is just like truing a conventionally spoked
wheel. Each spoke has both a vertical and lateral component
to its pulling force. As you tighten a spoke, it pulls
radially in towards the hub, and laterally out towards the
hub flange.
The difference is that on a Bontrager wheel with PST,
the lateral force is directly opposed by its 'partner', the
spoke adjacent to it. As the partner reacts to your tightening
of a spoke, there is no further lateral force applied to the
rim. Contrast that to a conventionally spoked wheel where
each spoke has two 'partners'. As you tighten one spoke,
it effects the tension, and thus the spatial position, of the
two partners. This in turn effects the next outward pair,
and so on.
When truing Bontrager Wheelsystems road wheels, PST
gives you more control over both vertical and lateral rim
deviations. If the rim is slightly out of true but very round,
you can loosen one partner and tighten the other. The rim
moves laterally, but not up or down. And since no other
Trek 2002 Tech Manual
spokes are directly affected, you're done.
Vertical deviations
With wheels built in our factory, the tolerance allowed
for vertical deviation is 0.5mm. A 23c tire with 120 PSI will
exhibit more out-of-roundness than this.
Our wheel builders use a vellum, a highly sensitive
truing stand that uses dial indicators driven by wheels
pressing on the rim. When 0.5mm passes by the indicators
on the vellum, the needles move about an inch. What looks
like a mountain on the vellum will be totally missed by
the rider, even at high tire pressures on smooth pavement.
With an egg-shaped wheel where 0.5mm height change
occurs over 1/2 of the wheel rotation, the out-of-roundness
may be invisible with a normal truing stand. If that same
0.5mm deviation occurs in a short rim section, it’s very
visible to the naked eye.
With Bontrager Wheelsystems, the same 0.5mm vertical
tolerance is allowed, but instead of an egg shaped wheel
it can show up over a very short section of the rim. In
either case, the rider will not feel it, nor will it effect the
ride of the bike. Consider the much greater magnitudes
in the out-of-roundness of a wheel. The tire will be out of
round by 1-2mm on a 23c tire, more as the casing gets
bigger. A rider sitting on the bike with that same 23c tire
at 110PSI will compress the tire by another 2-3mm. And
unless your roads are a lot better than here in Wisconsin,
the road surfaces often have 5, 10, and even 20mm variation.
A note about the "little marks" on the rims
On 2002 Bontrager rims there is a small spherical
indentation in the braking surface of the rim. This isn't a
blemish, it's a wear indicator. If the braking surface has
worn so that the indicator is no longer visible, have your
dealer replace the rim.
Technical Specifications
For detailed technical specifications, wheel building
instructions, spoke lengths, tensions, and hub maintenance
information, please refer to the Bontrager Wheel
Building Manual, Bontrager Service Manual, or cybersurf
to www.bontrager.com.
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