the Quick `n Dirty on - Fort Collins Bike Co-op

the Quick `n Dirty on - Fort Collins Bike Co-op
the Quick ‘n Dirty on
This is one of the most difficult concepts in bicycle
mechanics to convey. Why? There is very little science on
the biomechanics of cycling and there are so many
individual preferences on bicycle fit.
There are several key measurements that set the basic framework for
bicycle fit. In reference to road bikes (and with a particular focus on
racing) Keith Bontrager suggests the three important measurements are
the 1) saddle height, 2) saddle to bar distance, and 3) fore-aft saddle
position. There are additional measures (bar height compared to saddle
height, bottom bracket height, stand over height, wheelbase, head tube
angle, chainstay length, and front center) which determine how a bike
will handle under certain conditions. We can’t cover all the details –
preference on these additional parameters is largely gained from
experience riding many different bikes.
1)Saddle height
2) Saddle to bar
3) Fore-Aft
Saddle height is solely determined by leg length, set at a distance that
allows a slight bend in the knee during the furthest extension of the
cranks. It is affected by the seat tube length and seat post extension.
The saddle to bar distance should account for torso and arm length as
well as the desired riding posture (more aero or more upright). It is
affected by top tube length, stem length, seat post offset, and saddle
rail positioning. Fore-aft saddle position is the most debated measure
of the three – some bike fitters swear by certain measures related to
femur length or K.O.P.S. (to be discussed later). Fore-aft saddle
position is affected by the seat tube angle, seat post offset, and
saddle rail positioning.
the Quick ‘n Dirty on
Notice that the basics were missing a few common fit
measures – like stand over height, so what’s actually
Bontrager’s three basics relate primarily to pedaling efficiency and the
fore-aft distribution of gravity on the bike, which affects comfort,
aerodynamics, and cornering. While the three basics control how a bike
rides, he assumes we can cover the necessities.
1) Stand over
2) Seat tube
3) Top tube
4) Bottom bracket
5) Chainstay
6) Wheelbase
7) Front center
8) Head tube
Stand over height is probably the single most important measure for
urban and mountain biking. You don’t want to endanger the family jewels
during unexpected dismounts! Seat tube length is the frame property that
controls stand over, and along with seat post extension, affects saddle
height. The top tube length similarly is the frame component of the
saddle-to-bar dimension, further refined with seat post offset and stem
length. The bottom bracket height, chainstay length, wheelbase, front
center, and headtube angle affect how a bike “feels.” Longer chainstays
and longer wheelbases feel more stable compared to shorter ones. Shorter
chainstays are stiffer than longer ones. Steeper head tube angles are
twitchier than slacker ones, but extremely slack angles can feel floppy.
The front center is a second order measurement related to frame size,
head tube angle, and fork length. It is a good one-figure-measure of how
a mountain bike feels.
the Quick ‘n Dirty on
So, you’re helping someone pick out a bike at the co-op.
You aren’t custom making a frame and you don’t have the
selection to shop around for perfect fit. What can you do?
1) Raise or lower the seat post
Raising or lowering the seat post is the most basic adjustment to fit
someone to a bicycle. Aim for a height that allows near, but not full
extension of the knee while at the longest reach of the pedal stroke.
Make sure not to exceed the minimum insertion line!
2) Raise or lower the stem
Most quill stems have room for 1-3” of vertical adjustment. Loosen the
retaining bolt on the top of the stem, unseat the wedge, and adjust
the vertical position of the stem. Watch for the minimum insertion
line! Clamping stems (threadless) have room for vertical adjustment if
there are headset spacers present on the steerer tube. Unfortunately,
the request is usually to raise the stem, in which case the spacers
would have to be above the stem, which is rare (there is an adapter).
3) Move the saddle fore-aft
There are a few reasons to adjust the fore-aft positioning of the
saddle. One is to reduce or increase the saddle-to-handlebar distance
to accommodate a person’s reach. Another reason is to adjust the
saddle-to-bottom bracket relationship in order to better balance the
rider’s center of gravity over the bike. Moving the saddle fore-andaft can also accommodate different pedaling positions.
4) Swap the stem
The first component change is often the stem. If the user complains of
back or neck pain, straining to reach, wrist or hand pain, or leaning
too far forward, then a shorter stem or a stem with more rise may be
the proper solution. Less often, a rider will complain that they feel
cramped while riding. A longer stem, or a stem with less rise (or even
some drop to it) may be the correct solution. For clamping stems
(threadless) with rise you can always flip the stem upside down to
turn that rise into drop!
5) Swap the seatpost
Seatposts come with varying offsets (the position of the clamp
compared to the axis of the seat tube). If someone complains of a long
reach, switch to a zero offset post. If someone feels cramped, pick a
post with more offset, which will place the saddle further back.
6) Swap the handlebars
Handlebars can vary from flat, to drop, to riser, to mustache, to
apehangers. Switching handlebars can do a lot to change the effective
reach and rider posture. Be aware of brake and shifter requirements
when considering a handlebar swap – there are some constraints.
the Quick ‘n Dirty on
This is a commonly accepted practice, so I feel that I
must teach it, but many suggest this is an over
simplistic method that is coincidental, in it’s fit at
Knee over pedal spindle or K.O.P.S. is a method of bike fitting aimed at
optimizing pedaling efficiency. The positioning of the rider is adjusted
so that the tibial tuberosity (the bump below the knee) lines up with
the pedal spindle when the cranks are in the three o’clock position.
The rider position is adjusted by selecting a
bike frame with an appropriate seat tube
angle and a seatpost with the correct offset.
The riders position can then be fine-tuned
with fore-aft saddle positioning. K.O.P.S.
claims to optimize biomechanics, but what it
really does is place the rider’s center of
gravity (CG) over the pedals, which is
important, because applying force to the
pedals is how a rider should provide input to
the bicycle. If the rider is too far back,
pedaling forces will drive the rider back off
the saddle, requiring constant input from the
rider to pull himself back over the saddle.
To far forward and the opposite would happen.
In the middle the rider’s gravity balances
the pedaling forces with little input.
K.O.P.S. is probably most relevant for competitive road and crosscountry mountain biking, where pedaling efficiency matters. For
recreational riding, comfort, posture, and bike handling may trump
K.O.P.S. In gravity-oriented mountain biking, some bikes are designed
with blatant disregard to K.O.P.S., placing the riders CG further back
so that the bike is balanced when descending a steep slope.
When will K.O.P.S. affect you? Most bicycles are designed with a seat
tube angle and equipped with a seatpost that should accommodate a wide
range of rider sizes. The most common K.O.P.S. issue arises when using a
much longer (or much shorter) fork on a mountain bike than what the
frame was designed for. Adding length to the fork “jacks up” the front
end, slackening both the head tube angle and the seat tube angle, and
moving the rider’s CG further behind the bottom bracket. This position
may work well for descending, but riders typically complain that it
feels sluggish and low power in the flats and while climbing. This may
be remedied by moving the saddle fore, but this also changes the saddleto-handlebar dimension which may make the rider feel cramped.
the Quick ‘n Dirty on
Stems and seatposts are the first parts to change to make
a bike fit a rider better.
Short + rise
Quill stems can be
raised or lowered by
loosening the quill
bolt. There are many
quill stem styles
meant for different
bike fits and styles
of riding.
Clamping stems have
limited adjustment.
Generally, if the
reach isn’t right
go for a new stem.
There are
adjustable styles
and vertical
adapters to get
more rise.
Just short
Long + drop
= racing
Short + long
Seatpost style and fore-aft saddle
positioning can move the rider
closer to or further from the bars.
Select a zero offset post for less
reach or a setback seatpost for
more reach.
the Quick ‘n Dirty on
Handlebars. There are so many options, but don’t get too
excited about something new until you make sure your
controls (shifters and brake levers) are compatible.
Flat handlebars are most common
on mountain bikes and
commuters. Compatible with
cantilever and V-brake levers,
thumb shifters, trigger
shifters, stem shifters, and
down tube shifters.
Riser bars are just a variation
on the flat handlebar. Same
Drop bars are most common on
road and cyclocross bikes.
Compatible ONLY with drop-style
brake levers, bar end shifters,
integrated road shifters, stem
shifters, and down tube
Mustache bars are most common on
older 3-speed cruisers. Compatible
with some, but not all mountain
and road components depending on
placement relative to bends. Must
wear tweed.
Alt bars are the it thing for
retrogrouch mountain bikers and
hipsters. Extra hand positions,
ramming spikes, gadget attachments,
Aero bars and bar ends are never
cool. Just kidding. Aero bars may be
warranted for road time trials or
triathlons. Bar ends aid in climbing
and add hand positions. Also utilized
for the classic “I know this is
probably a bad idea but this hook
will hold my groceries.”
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