Crossing Over: Bike Fit and Cyclocross

As #crossiscoming hashtags start to appear with more and more frequency, it is a good time to reexamine your bike fit.  Let's dive into the finer points of setting up your CX bike as you head into the season.  


Cyclocross can trace its origin to the early 1900's. In the winter months, cyclists would race from town to town, off road, over fences and through streams on their road bikes as a way to stay fit in the off season.  The bikes developed along with the sport - from touring bicycles adapted for off-road use to the highly specialized cyclocross bikes we see today.  Bikes are designed to accommodate the demands of the discipline.  Races force riders to dismount, remount, brake hard, accelerate hard, ride through varying terrain - grass, mud, sand, and pavement.  The way you use the bike during a cyclocross race and a road race are different. It would follow then, that your position on the bike should also be specific to the discipline.  Just as a crit racer and a touring cyclist have different positions to match their riding styles, there is value to using the right tool and using the right tool correctly.  That is where bike fit steps in.  The goal is to create a relationship between you and your bike that is specific to your sport - your strengths - and your limitations.

 
(c) Mat Street

(c) Mat Street

 

Unfortunately, the common way that riders approach cyclocross fit involves matching your road bike saddle height and setback and then raising the bars and shortening the stem.  While this is not a terrible place to start - higher and closer bars give the rider a more upright position which takes weight off the front end to aid in control- there is a lot of room for improvement from this basic method.  What follows will break down the factors missed with this common practice of matching and adapting road fit to setup your cyclocross bike. 

Factors missed:

Assumed proper road bike fit

All of the above recommendations are predicated on the fact that your road setup is “optimal”.  “Optimal” can be defined as a position which encourages proper posture, works within your natural range of motion and that you are stable, powerful and balanced.    

Saddle choice

There are a few practical considerations to make when choosing a cyclocross saddle.  First and foremost should be riding posture.  The saddle you choose should enable comfortable anterior tilt of the pelvis (think rolling the hips forward) to enable a straight back.  Saddle choice and back health are closely related. With the violent nature of the efforts in cyclocross as well as the bumpy terrain there is an even higher risk of back pain.  A good saddle should also allow for safe remounting.  Most will find that a flat profile saddle allows the rider more surface area to land on while jumping back on the bike.  Keep in mind that different saddles put the rider in a different spot in space, so even if you match the height and setback to your existing road setup, you might be positioned differently.       

Saddle fore/aft

CX racers often find themselves in and out of the saddle as they accelerate out of a corner or up an incline.  A forward adjustment to your saddle's position on the bike may make the act of getting out of the saddle smoother, quicker and easier.  Standing up out of the saddle is similar to getting out of a chair.  It is much easier to stand up out of a chair from the edge of the seat compared to the back of the seat.  If we apply this principle to the bike fit, a forward adjustment to the saddle in relation to the pedals would make the act of getting out of the saddle easier.  Over the course of a race, you may do this dozens of times.     

Mountain vs. road pedal stack

In general mountain bike pedals have more stack than road pedals.  The difference between a Shimano Ultegra road pedal and Shimano XT mountain pedals is about 5mm.  All things constant (shoe sole thickness, insoles, socks and saddle) this would mean that to maintain the relevant body angles that comprise your seat height (ankle and knee angle at the bottom of the pedal stroke), we would need to raise your saddle height by 5mm when you change from an Ultegra road pedal to an XT mountain pedal.     

Cleat position

Cleat position can also affect the relevant body angles in respect to saddle coordinates.  Since the distance between the saddle and the pedal can be made up by ankle angle and knee angle and the position of the cleat on the foot defines the lever length, moving the cleat towards the heel will require a lower seat height (all things remaining constant) compared to a more forward cleat position.  Picture how your knee straightens out if you were to put your heel on the pedal vs. being clipped in.  There may be some good reasons to move your cleat position on your cross bike compared to your road bike.  

Cyclocross, has much different efforts involved compared to most road riding.  Sprinting out of corners, standing for short efforts and lots of accelerations.  For these demands, recruiting the fast twitch muscle fibers of the gastroc (main belly of the calf) is beneficial.  The calf is a powerful muscle which is great at developing a lot of force for short periods of time.  In order to encourage the calf firing, the general consensus is that the cleat should move forward on the shoe, in the direction of your toes.  Moving the cleat this way will also help by reducing the amount of toe overlap with the front tire.  

One potential drawback to moving the cleat forward is the increased likelihood of foot numbness as pressure is moved closer to the ball of your foot which can lead to compression and inflammation of nerves and joint tissue in the metatarsal head.  This is less of a concern in cyclocross compared to road cycling where pressure is more constant and prolonged on the pedal.

Crank Length

There has been a recent trend of racers of all disciplines changing to shorter cranks. Triathletes and road racers have found some benefits which can be valuable in cyclocross. There are a few reasons why shorter cranks may be an advantage:

Aerodynamics. Triathletes first discovered that they could sustain a lower, more aerodynamic position, could run better off the bike and spin faster with shorter cranks.  Shorter cranks can open up the minimum hip angle and allow a lower torso angle.  This same logic has spilled into the world of road racers who have also found that pedaling in the drops can be more efficient with a shorter crank. 

Efficiency. There is less likelihood of being inhibited on the upstroke with shorter cranks.    If your driving leg also has to lift the opposite leg around the top of the pedal stroke, you are losing power.  Shorter cranks have been tested to reduce minimum wattage (the resistance of the leg coming over the back side of the pedal stroke), therefore increasing efficiency.

Pedal strike. In cyclocross and crit racing, shorter crank puts the crank arm higher off the ground at bottom dead center which reduces the chance of pedal strike on objects or through corners.

Toe overlap. By shortening the cranks you have a better chance at avoiding toe overlap with the front tire during slow speed turning.

Obviously in a sport like cyclocross where you are rarely going faster than 20 mph the aero benefits are not what you are after.  Instead, pull the potential benefits of efficiency, toe overlap and the definite benefits of more clearance to the ground and a shorter crank length sounds like a good idea.  

A commonly held (mis)conception about crank length is that a longer crank gives the rider a torque advantage leading to more power production.  The crank is one of the three levers used to propel your bike. The others are the wheel size and the gear ratio.  Picture your bike in its hardest gear - let’s say 53-11. Now picture trying to turn that gear with 145mm cranks vs. 180mm cranks.  The 180mm cranks would undoubtedly be easier to pedal.  If bikes all had the same gearing, you could change the crank length to make that gear feel easier or harder.  In reality, you have many options with the gearing on a bike.  Typically when a rider reduces their crank length, they find themselves in a slightly easier gear with a slightly faster cadence, at the same power output.

You can calculate how changing your crank length will affect your gearing with Sheldon Brown’s “Gear Calculator” (http://www.sheldonbrown.com/gain.html).  This calculator takes into effect crank length and spits out “gain ratios”.  I recently shortened the cranks on my CX bike from 172.5mm to 165mm.  To maintain nearly the same range I went from a 40t on the 172.5mm cranks to a 38t on the 165.  

An accurate concept of crank length’s effect on power production requires looking at the whole system.  The results of a 2001 study show that a test group of cyclists maximal power differed only 4% across a crank size range of 145mm to 220mm (http://www.recumbents.com/wisil/MartinDocs/Determinants%20of%20Maximal%20Cycling%20Power.pdf).  Interestingly, maximum power was produced on the 145mm crank.  Crank manufacturers save money by reducing the size offerings of a particular crank. Some only offer 3 sizes (170, 172.5 and 175mm), but this does not mean that everyone should find their ideal crank size in a small 5 or even 10mm window.  6 foot 6 inch Ryan Trebon rides 177.5mm cranks on his cross bike.  While this seems long, if we perform a calculation based on the same proportions, a 5 foot 8 inch rider would use 155mm cranks.    

 
Ryan Trebon talking bike fit with ACME. (c)Guru

Ryan Trebon talking bike fit with ACME. (c)Guru

 

Pedaling mechanics

The power file of a cyclocross race is a violent looking thing.  Below is a power file from Ryan Trebon.  Try imagine the constant accelerations, sprinting out of corners, powering through mud, sand and steep hills which elicited this type of power.  A good amount of time is spent at low cadence and high wattage and accelerating out of the saddle.  

 
Trebon's power file from Lieven World Cup 2012.

Trebon's power file from Lieven World Cup 2012.

 

From tests performed at ACME, we have seen a correlation between pedaling dynamics and cadence.  Below is a Retul scan illustrating some changes that happened between a 56rpm cadence and a 103rpm cadence at the same power output.  Column 1 shows the measurements at slow cadence and column 3 shows the fast cadence numbers. Both scans are of the same side of the body (left).  These scans show that at the lower cadence the rider moved back on the saddle, dropped his heel, ankled more and extended his knee more. A lower cadence requires more torque (and force) for a given wattage, the rider seems to respond by doing everything possible to get more extension from his legs.  A bike fit should take into account the demands of the discipline.  In triathlon bike fits, we are concerned mainly with how rider and bike interact during sub-threshold efforts. In cyclocross fits, we simulate the efforts you might see in a race and adapt the fit appropriately.   

 
Retul uses infrared sensors to 3D map a rider's position on the bike.

Retul uses infrared sensors to 3D map a rider's position on the bike.

 

CX vs. road frame geometry

Bike fitters are concerned with contact points - saddle, handlebar and pedals positions in space.  To map out these points we use an X and Y coordinate system. Picture your bike in front of a large sheet of graph paper,  we use the center of the bottom bracket as the origin and each contact point is mapped out using its distance from the origin in the horizontal and vertical aspects (x and y coordinates).  Fitters commonly use frame stack and reach to describe the point in space of the top center of a bike’s headtube in relation to the bottom bracket.  These coordinates are valuable because they will impact where a rider’s hands will end up given a set amount of spacers, stem length and angle and handlebar.  If we want to get someone’s hands on the hoods of a bike in a particular point in space, we can take their frame stack and reach and then figure out the correct combination of spacers, stem and handlebar to get to that point.  We can also do the same with saddle setback and height off the bottom bracket - essentially the x and y coordinates of where you saddle is in relation to the bottom bracket.  Throw on the right length crank and your contact points are where they should be.

 
An illustration of how stack and reach change with the size of a bike.

An illustration of how stack and reach change with the size of a bike.

 

Frame stack and reach provides a simplified way of looking at frame geometry for fitters.  Stack and reach define a relevant reference point that explains how the bike will fit a rider and provides a simple way to compare different bikes.  It ignores other measurements that can be potentially misleading for comparing two bikes.

A common piece of (bad) advice I have heard is to size down your cyclocross bike size compared to your road bike. “If you ride a 56 road bike, get a 54 road bike.” Let’s use a real world example and see how this goes.  Let’s say you were riding a 56cm Cannondale Caad12.

Following this advice would put you on a 54cm Cannondale CaadX.  The stack and reach chart below illustrates the difference between those two frames.  A 56cm Caad12 has a stack of 567mm and a reach of 393mm.  A 54cm CaadX has a stack of 554mm and a reach of 379mm.  This puts the front end of your cross bike 13mm lower and 14mm closer in than your road bike with the same cockpit.  While the reach may get you close to where you want to be, it is pretty rare to want a cyclocross fit that puts you lower in the front end compared to your road bike. It is usually the other way around.  So in this scenario you would be forced to have add a stack of stem spacers which not only doesn’t look cool but may exceed the permitted maximum advised against by fork manufacturers.  Enve states “The spacer stack between the top of the upper bearing and the bottom of the stem must not exceed 40 mm (this distance includes the headset dust cap or tapered spacer on top of the bearing).”  

 
The difference in relevant sizing information between a 56 Caad 12 in the top right hand corner and a 54 CaadX in the bottom left hand corner is substantial.

The difference in relevant sizing information between a 56 Caad 12 in the top right hand corner and a 54 CaadX in the bottom left hand corner is substantial.

 

Just like road fit, cyclocross fit is specific to the discipline.  The old way of setting up your cyclocross bike makes lots of little assumptions.  A proper approach should take into account the demands of the sport and integrate these considerations towards your bike fit.  It is worth some consideration.