I know my dimensions and I bring a tape measure, plumb bob and some tools with me.
But I am silly about stuff like that.
You could make a ghetto plumb with a big nut from the hardware store and a piece of string.

I read a tech article by Leonard Zinn online at Velo News a couple years back which discusses establishing an X Y coordinate system to transfer fit measurements from old bike to new. I turned that into an Excel spreadsheet so that with the first bike measured, all I need to do is find the X and Y of the bottom bracket of the other bike. With that date entered, the spreadsheet calculates the measements for height and position of saddle and handle bar.

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You can get close by measuring x,y. Put rear wheel up against a wall and measure horizontal to BB, saddle nose and then brifter hoods. Measure vertical from floor to BB and hoods. You subtract out BB dimension for distance from BB to use with rental. Seat height can be a challenge with differing saddles. From BB to top of saddle thru center of rails usually works ok. Seat angle can be measured using a smartphone app or winging it.

This method should work with only a cloth tape packed. It assumes wall and floor are perpendicular which is probably rare. But you'll probably get closer than many rental shops.

You can use the Stem Calculator to find your current X/Y position, then use the Bar to HT calculator to work backwards to the Roubaix geometry. Both bikes have Stack and Reach on the ST database. The saddle height and fore/aft should be the exact same, or close to. You may also want to measure from the tip of the saddle to the hood trough as the bar reach and hoods might vary slightly depending on the components. Of course, this is to get exact measurements. You can probably get away with a less exact way, and still enjoy riding your rental bike.

having done this at least a half-dozen times ... you don't need to worry about x/y coordinates, geometry, spreadsheets or even a plumb bob. Bring a tape measure and allen wrenches.

And a spare stem or 2. In my experience rental shops are not too keen about swapping stems. They are almost always short, for me anyway, so I bring a spare 110 and a 120.

At home ... measure your saddle height relative to center of BB. Measure your bar height relative to BB height (ground to bars then subtract ground to center of BB). Measure your saddle setback from center of BB. You can do that with a plumb bob or by standing the bike against a door jam centered around the BB. Measure tip of saddle to center of bars (at the stem)

Then on the rental bike set saddle height (adjust to any crank arm length diff), then setback, then bar height (take out or add spacers) then saddle to center of bars (change stems if needed).

Rather than a plumb bob to measure where the BB is, I use a level and mark my top tube with a piece of tape. I then make all of my forward/backward measurements from there.

Saddle height is measured from the center of the BB to the top of the saddle through the marking.

Bar height is measured the same was as described earlier.

jaretj

Thanks All.
Good info. I will measure, and hopefully have some good rides this weekend in Boulder, Co.

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2017 Cervelo P2
2017 Cervelo S2
itraininla.com
#itraininla

Um yeah!!!
ok I tried this. Now I know why "I" need to pay a bike fitter and my DIY construction at home is always crooked...

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2017 Cervelo P2
2017 Cervelo S2
itraininla.com
#itraininla