Transferring Bike Fit

I just got a new gravel bike and hoping to transfer over a similar fit to my road bike. I have set up to my best of the ability but know fore/aft could be improved. Having ridden it on single track including some steep descents, I am very happy with my handlebar position and stem length.

I have all the measurements of my road bike fit along with each individual bikes complete geometries. I am hoping to be able to correctly run a ratio to calculate out what the new saddle position should be. Where should I begin in order to correctly determine my position.

There are so many different variables introduced by frame, handlebar, and saddle geometries that I find it easiest just to ignore all of it and measure everything relative to the bottom bracket.

On the “source” bike, find the measurement between the centre of the bottom bracket and where you put your hands. This is your reach. Note that some places on the internet would say the reach measurement would be to the end of your stem, but this ignores the fact that different handlebars can put your hands further/closer to the bottom brackets. your hand position is ultimately what you care about.

Next, measure the vertical difference between the centre of the bottom bracket and the top of your handlebars where you put your hands. The easiest way to do this is measure the distance between the floor and your bottom bracket and the floor and your handlebars and then subtract the two. This is called your stack.

To transfer it over, just adjust the new bike to match your stack and reach measurements as closely as possible.

Saddle height is easy to transfer, since as long as you’re using the same pedals, shoes and crank length, it will be the same on both bikes. Saddle setback is harder since different saddles have different lengths and I find my default position on different types of saddles is slightly different. I’m not sure if this is the “right” way, but I measure from the back of the saddle to my hand position on the source bike and then transfer that to the new bike, but only once I’ve already set up the stack, reach and saddle height. I find that gets me in the ballpark for saddle setback and then I make small adjustments from there.

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Here is my common reference for a handy chart to record, and then transfer your setup from one bike to another.

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A quick/dirty method, which has served me well:

  1. Grab a length of string
  2. Tie a knot in it near the end
  3. Tape that knot to the center of the bottom bracket.
  4. Tie another knot at the center of the saddle (or the tip if that’s easier)
  5. Tie a third knot at your most common hand position (e.g. the hoods)
  6. Now complete the loop/triangle back at the bottom bracket.
  7. Cut off the extra string.

Now you have a “loop” of string, with three knots in it, and that loop can create the exact relations between the bottom bracket, saddle, and hand positions.

When you want to set up a new bike, repeat the process by taping the bottom bracket knot in place, using the 2nd knot to get the saddle height, then pulling the “triangle” tight and setting your hand position where it needs to be.

Best thing about this technique is that it requires zero measuring/leveling tools. Added benefit is that the string packs down to zero space so you can carry it with you (e.g. when traveling and using a rental bike.)

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I have found this site works very well for what you need --> http://www.bikegeo.net/

It doesn’t consider handlebar reach, stack height and width though. That, you need to factor yourself.

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How about taking 2 pictures of both bikes from same distance and overlay them in, say, some photo editing software, even powerpoint can do it. It would require to make photos semi transparent to see both bikes simultaneously. Then align photos horizontally (say take skewers as reference) and align bottom bracket for main reference…

Just a thought

Not really good enough if you want something closer to mm accuracy. You’d have to absolutely nail position relative to the camera to even have a decent chance. The parallax effect of the single point camera view will lead to distortion.

It would take a tripod for the camera and exact positioning of the bikes base on a common reference (rear tire m\would be easiest). It would work for ‘close counts’ start, but short of actual tape measurements.

I am working on my own sheet to address some of the issues I see in other one. It is not complete, but I figured I’d share for now.

https://docs.google.com/spreadsheets/d/144xqVbAt5X3CBgJvJalb0kgWgggPHDDRjpMyznc53Kw/edit?usp=sharing

I bought a veloangle at the end of last year and used that to mirror my post-bike fit position on one bike to two others. There is a web app that costs money that converts the polar coordinates (angle / radius) to x-y coordinates. I found the useful in the initial setup but now am happy just to have the polar coordinates to mirror fit across multiple bikes. It helped me identify some issues with bikes I had built up and ‘adjusted’ at the fitting shop that were not set to the right positions. It let me get sufficiently close that I cannot feel differences in my seated position relative to the pedals.

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That seems like a hellishly complicated to get a bunch of simple dimensions.
Measure all the verticals from the floor and subtract to get the differences you need and with a wall behind the bike measure from the wall to all the same points to get the horizontals, subtract to get the differences you need, not sure what all the fuss is about.