Translating Fit Coordinates

May 28, 2015

 

By: Ben Waite

 

A poor bike fit equals poor fit coordinates.  Fit coordinates are where your body contacts the bike.  The first step of this process is to get a bike fit from a skilled and experienced fitter.  

 

After your fit, you will have a set of fit coordinates that will need to be translated onto your existing or new bike. This is a crucial part of completing the fit process.  Getting this wrong can negate everything you've done up until this point. 

 

Here are a few tricks that can make this process easier for you.  (The example is specific to a TT bike but can be applied to a road bike, as well.)

 

Armpad Stack & Reach

 

Historically, a lot of bike measurements are done with respect to saddle position. However, the preferred method is to decouple all cockpit measurements from the saddle because there is an inherent tolerance stackup and measurement error. 

 

Similar to the new convention of sizing frames with ‘frame stack’ and ‘frame reach’,  ‘arm pad stack’ and ‘arm pad reach’. These are measurements from the BB to the armrest pads where stack is the vertical measurement and reach is the horizontal measurement.

 

1. Arm Pad Reach: Reach is typically more difficult to dial in so set this up first. This could require changing stems or hardware on your aerobar.

 

There are a few methods to measure fit reach, but I prefer using a simple laser level lined up with the center of the BB.

 

 

You place your measuring tape at the back of the pad and your fit reach is where the laser intersects the measuring tape (try to get this within 2-3mm).

 

 

 

 

2. Arm Pad Stack: After the reach is set, move on to the stack. Most time2, this can be achieved through headset spacers or risers on the aerobar. If possible, use risers over headset spacers as risers typically have an aero profile. Keep in mind that risers will move the arm pads and extensions while keeping the basebar in its current position. Where a rider prefers their basebar comes down to persoanl preference.

 

3. Arm Pad Width: Pad width is a large contributor to comfort on the bike. After the arm pad reach and  stack are set, dial in the arm pad width. This should be simple adjustment by sliding the pads inboard or outboard. However, some integrated aerobars make it difficult and may require adapters or different hardware.

 

4. Saddle Height: Saddle height is arguably the single most important fit coordinate on your bike. It is imperative you verify your saddle height is correct. Remember that saddle height is dependent on crank length and pedal system. If you have multiple bikes, you need to take this into account.

 

When measuring saddle height, you'll want to specify where on the saddle you're measuring to. With contoured saddles, you can get drastically different measurements by measuring more towards the front or rear of the saddle. For example, my fit sheet specifies a saddle height of 75.3cm measured 12cm rearward of the tip of the saddle with 165mm cranks and Look pedals.

 

5. Saddle Setback:  This usually involves sliding the saddle along the rails. As it is measured in reference to the bottom bracket, you can measure it similar to fit reach in the photo above if you don’t have access to a laser level.


After setting saddle height and setback, always double check the height and fit stack by verifying the drop. This is a good final sanity check to show that both the saddle and cock pit are in the correct relative location.

 

Your bike fit coordinates are an essential part to your training. You wouldn’t go on a tempo run wearing a shoe that is 3 sizes off so you need to be just as responsible for verifying your bike is set up correctly. It is always a good feeling when you jump on a new bike you just built up and it feels exactly the same as every other bike you have been on.

 

 

 

 

Ben

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