New way of measuring bike position

Discuss light weight issues concerning road bikes & parts.
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Joined: Mon Jul 01, 2019 12:36 pm

by andmic

Hi everyone

As an avid road cyclist I’m quite interested in bike position, but I find the ways to measure this position to be often not telling enough or too complex. That’s why I want to contribute to this forum in a first post giving you my own technique of measuring and comparing positions on a bike. In my opinion, this technique is quite simple and elegant, but at the same time comprehensive. I’m wondering what you guys think. Does it seem useful, or is it rubbish? I used a photo of my own bike, a Ridley Helium SL, to illustrate the technique.

I consider there to be four important measurements when it comes to bike position.

The first one is the length of line a, measured from the bottom bracket to the tip of the saddle. This differs from normal saddle height in that it incorporates saddle setback.

Then there is the length of line b, measured from the tip of the saddle to where the handlebar attaches to the stem. This measurement combines reach and saddle to handlebar drop.

Beta then is a measurement that gives an indication of the angle between the upper and lower body, and is largely determined by how flexible the rider is.

In my opinion, these first three measurements can give a good indication of the personal position of a rider. The fourth measurement, alfa, doesn’t tell much about the position in itself. Instead, it shows how tilted this position is. This last measurement is less bound to the individual rider, but is nevertheless important to understand the aerodynamics of ones position.

That’s it!

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by TobinHatesYou

Sorry, but no, your technique is based on an arbitrary point as people 1) don’t sit on the nose of the saddle and 2) saddles vary in length.

Also even if we normalized everything against one specific saddle, angle Beta is meaningless because it doesn’t tell us about the actual physiological details of the rider. Does the rider roll his pelvis forward? Does he arch his back? Long torso? Long arms? Does he roll his shoulders? Does he bend his elbows in the drops? Etc.

by Weenie

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by zefs

If you are just comparing positions on the same bike to get to a conclusion, then this method of measurement would work fine in my opinion. Thing is that fit will be different when the terrain changes, so might be a good idea to do the tests on the same terrain when doing fit changes (e.g test 2 different positions on a flat route, then the same changes on hilly route etc).

In the end I've found that the best way is to go by feel and make changes based on what issues you are experiencing and if they are related to fit.

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Location: Geneva, Switzerland

by basilic

Most people first position the saddle correctly, then the handlebars (relative to the saddle position).
Your a and alpha are both important for the saddle position, and b and beta for the bars.

You have 4 measurements. Alternatively, you measure saddle height, setback, tip of saddle to bars (your b), bar drop, also 4 measurements. It seems to me that angles are difficult to measure with any precision, whereas distances are easy.

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by cyclenutnz

You might find interesting

If I were to resolve road bike measurements down to 4 it would be
1. Setback at 80mm width of saddle
2. Distance from top of pedal to 80mm wide saddle point

Those measurements accomodate different saddle and crank lengths

3. Stack of the top of the bars from BB
4. Reach to the curve of the bars from BB

And those two points define exactly where your hands end up allowing for different bars (but not different crank lengths)

The best candidate for a 5th point would be saddle angle. - 3D Motion Capture and Frame Finder Software

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by wingguy

I don’t understand the point of this system. What is it actually for?

You can’t use it to set up a bike, because you’d have to be juggling two tape measures and a protractor at the same time. You can’t use it to find a bike, because the measurements are meaningless against a geometry chart without some deep maths being involved.

So when you take these measurements from your bike... what then? What are you actually doing with them?

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by jih

Instead of the saddle tip, the part of the saddle where the sitbones contact

Instead of the tops, the part of the hoods where the hands rest most of the time while riding

by Weenie

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