Replicating trainer and outdoor bike fits

So I have two bikes - an old beater road bike and my race (triathlon) bike. I bought an extra set of aerobars and a reversible seatpost and am trying to set up my two bikes identically so I can have the beater bike live on my trainer and only ride my race bike outdoors. I have a Powertap P1 power meter, so it takes about 30" to switch it between the two bikes. This way I put less total mileage on my “nice” bike, and also always have a backup.

Using a tape measure and plumb bob, I have gotten the two fits pretty close, but not exactly. I’m having a particularly tough time getting the saddle setbacks to match using just a basic plumb bob and tape measure. Does anyone have suggestions for tools I could use that don’t cost $295?

I am thinking a basic adjustable drywall square would work, but I would need to rig something to anchor it to the bottom bracket (or floor). Any ideas?

Thanks a lot everyone!

Can you describe how you are using the tools and measuring?

I might have some suggestions, but want to avoid duplication.

Taking center point of BB using plumb bob and marking spot on floor, doing same for saddle nose, marking on floor, measuring between the two points. The bike frame tends to get in the way of the plumb bob when marking the spot for the saddle nose. Another complication is that bike A is a TT/tri bike, bike B is a road bike with a reversible seatpost, so can’t use seatpost as a reference point really.


  1. Mount your starter (goal bike) in the trainer. That will give you a solid platform that won’t move.
  2. Make sure the axles are level, if possible. If you have a 4 foot long level, you can do this easily. Otherwise, measure the axle height from the floor and match the front to the rear, using spacers under the front as needed.

I have other ways to measure, but your method will work well once you set up the bikes in a solid holder (trainer).

Thanks - sorry, I should have clarified - I am already doing this in the trainer. The issue is that the bike frames interfere with the plumb bob when finding the saddle nose, so it throws off the measurement.

Try a doorframe (or a corner of a wall). Lean the bike every so slightly and move the bike so the tip of the saddle hits the edge of the doorframe/corner. Then measure the distance from the bottom bracket to the same edge of doorframe/corner. If you have the bike perpendicular to the corner, the sideways lean of the bike won’t affect the fore/aft position of the saddle/bottom bracket. Repeat this measurement 4-5 times and take the average. You’ll be within a millimeter or two of the “correct” number.

(I hope that description was clear. I can take a picture if not.)

This is an excellent guide on setting up two bikes identically:

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