Editor’s Note: Lennard Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn.
Saw in a Paris-Roubaix tech photo gallery a bike fit jig which is used by Movistar. I could not make out the brand – I could discern a label that said “Bike Size by ????.” No hits for said product on the Internet. Any idea what brand it may be or comparable products? I have struggled for years to get the measurements on my stable of rigs identical.
I don’t know what jig Movistar uses, and many pro team mechanics have jigs to quickly duplicate rider position as they set up a new bike for the rider. Some of these are homemade jigs that the mechanics made themselves, and others are more polished versions. All of them are roughly equivalent to the “FitStik,” a device by Cyclemetrics that you may remember consisted of graduated wooden sticks at 90 degrees from each other. Quality Bicycle Products (QBP) bought the rights to the FitStik around a decade ago but has shelved it.
In my January 11, 2011 column, two of the four photos show tools by Serotta and Specialized that do what you’re looking for. The Serotta Cycling Institute X-Y tool and the Specialized Body Geometry X-Y measurement tool. You can obtain both in the USA.
However, if you’re not setting up dozens of bikes in short order, as pro team mechanics do numerous times per year, you can get the same amount of accuracy yourself with just a tape measure, if you know a very simple trick. The key is to use “stack and reach,” which is simply a means of establishing the same starting point on both bikes – the center of the bottom bracket – and taking only horizontal and vertical measurements relative to that point. In other words, you are setting up an X-Y coordinate system with its origin at the bottom bracket.
This illustration shows how stack and reach is used in frame sizing (many manufacturers now list stack and reach numbers as well as the usual top tube length, seat tube length, seat angle, etc. for frame geometry specs) as well as handlebar position. However, stack and reach can also be used for finding the relative positions of any other components on the bike as well.
Here’s how you can get the position of the handlebar and saddle on your two bikes exactly the same quickly, inexpensively, and with a minimum of frustration:
1. Stand the bike up in a corner of the room with the rear tire touching one wall and the handlebar touching the adjacent wall. The bike should be vertical and parallel to the wall which the handlebar is touching. Measure horizontally from the back wall to the center of the bottom bracket. Measure vertically from the floor to the center of the bottom bracket. You’ve now found the origin of your X-Y coordinate system.
2. To find the position of your handlebar, first measure horizontally from the back wall to the center of the bar and vertically from the floor to the center of the bar. Subtract the horizontal distance from the back wall to the bottom bracket from the horizontal distance from the back wall to the center of the bar. This is the “reach” measurement of the bar, i.e., the X-coordinate of the center of the bar from the center of the bottom bracket. Subtract the vertical distance from the floor to the bottom bracket from the vertical distance from the floor to the center of the bar. This is the “stack” measurement of the bar, i.e., the Y-coordinate of the center of the bar from the center of the bottom bracket. You can use the same procedure to find the X-Y position of the saddle relative to the bottom bracket (pick a reference point on the saddle to measure to, like the tip of the nose).
3. To set up the second bike exactly the same as the first, stand it in the same corner of the room with the rear tire touching one wall and the handlebar touching the adjacent wall. As before, the bike should be vertical and parallel to the wall which the handlebar is touching. Take the same measurements. Then adjust its saddle and handlebar positions to duplicate the stack and reach measurements you got from the first bike. Voila! You’re done, with precise accuracy, and you didn’t have to buy an expensive tool you might use only occasionally.
Feedback from last week’s column:
In regards to Adam’s note about the oft-scapegoated bottom bracket, most creaks involve, directly or indirectly, the pedal stroke. Most creaks come out only under load, for instance. In the days of three-piece cranks, the bottom bracket gained a reputation as being complex and difficult to work on, a reputation it kept even with the advent of simple modern units. Add this to the fact that to an untrained rider, nearly all creaks seem to come from directly beneath the rider, and such a rider can only jump to the conclusion that the mysterious bottom bracket has become faulty in some way.
Not sure that David is right about moment of inertia catching on. Nimble did it for years. Haven’t seen anyone else do it, but I could be wrong.
I now remember that there is another company recently that does measure moment of inertia of its wheels vs. other brands, and does inform the world about how well its wheels do in this regard: Mad Fiber. Here is my story from the first day of the Interbike Outdoor Demo two years ago when the wheels first appeared. In it, you’ll find this statement: “Hjertberg claims that the rotational inertia of his wheels is lower than any other, including the Lew-designed Reynolds RZR wheels, since the weight of his rims and spokes is so low.”
Given that rotational moment of inertia = mass x radius^2, for a spoked bicycle wheel it would seem like rim weight (+tire and tube weight) is a pretty good proxy for an actual measurement of moment of inertia. For instance, my American Classic Sprint 350′s could be built on much heavier hubs, and I bet you’d have a hard time measuring the difference in inertia. But switch to a pair of Ksyrium rims, and I suspect you’d see the inertia jump almost exactly in proportion to the rim weight. Now if we could just get wheel manufacturers to publish rim weights separate from assembled wheel weights…
I’m a physicist and years ago worked at a government aeroballistics facility where they were developing mortar shells, among other things. We had a very simple method of measuring the MOI of the shells, which would certainly be applicable to bicycle components. Basically, it was a length of music wire (~30″) suspended vertically. An item to be tested was suspended through its center of mass at the bottom of the wire. The top holder for the wire was rigged so that it could be rotated about 20 degrees against a stop and held there. The item under test would rotate back and forth oscillating: the period was measured and used to determine the objects MOI. The wire was first calibrated by doing this with a number of different aluminum cylinders for which the MOI can be very easily calculated using their length, diameter, and mass.
Feedback from the April 17 column:
I’ve been wiping my tires with my finger – no gloves for 38 years. The first eight or so were in NYC where there was a lot of glass. I can count the times I’ve cut my fingers on half of one hand; not sure why some people cut their finger and others don’t.
Back when I was 16 I did get a custom Harry Quinn, which had a much shorter wheelbase then my PX-10. On one of the first rides I did get my finger caught between the tire and the seat tube. It locked up the rear wheel. I skidded until I slowed down enough to yank it out and ride away without unclipping. Only took one time to learn.
You probably remember that sometimes people used cloth adhesive tape as tire savers.
Regarding the “wiping tires” thread, I wanted to share another alternative method. My ex-wife is from Colombia, and I used to spend some summers down there doing large group rides out of Medellin. Whenever we ran through glass or other potentially damaging debris, someone would call it out. Then virtually every rider would instantly produce a flat piece of rounded off wood – looked exactly like tongue depressors used by doctors here – and they would proceed to scrape debris from their tires rather safely and effectively. I have adopted that same idea myself, using smaller popsicle sticks. The only problems with this system is that I regularly misplace the stick after each ride, and I worry about my kids health as I encourage them to eat too many popsicles! Next time I see my doctor, I will ask for some tongue depressors…