CAD model for shorter cranks
After jumping into a thread about short cranks (which seems to be a topic of heated discussion here), I decided to create a simple CAD model to help me understand the ways that shortening cranks can affect other aspects of seating position.
The model is NOT good enough to prove or disprove anything, and it doesn't say anything about how shortening cranks influences mechanical properties. And that wasn't my goal anyway. I went to shorter cranks because of knee pain, not to squeeze out a few more watts or achieve a more aero position over my Albatross bars. What the model DID do is help me more clearly see relationships in terms of fit.
It also helped me decide that I didn’t need a new extra long setback seat post, which is expensive. So if it saves you some money, cool! However, I am not responsible if you buy a Paul, Tall and Handsome because of what the model says, only to find that your real world experience is not what the model predicted!
Why? This is a "simple" model of a complex system. There are a ton of reasonable assumptions and simplifications built into the model. But while individually reasonable, I have not and don't plan to test what they do in concert with each other. So remember, the model is super precise because it's CAD: you can get measurements to the 100th of a millimeter; but this precision can fool you into thinking that wrong answers are right (ie, I make no guarantee or warranty of accuracy!).
How the model works:
The model lets you compare two crank lengths with your choice of other dimensions constrained. Still other dimensions can be allowed to vary (and will be calculated for you based on the constraints you choose). You can constrain different things, depending on what you are interested in learning. For instance, you can constrain set back and see how much you would need to raise your saddle. Or you can figure out how much further back you need to push the saddle so that you don’t have to raise it at all.
Measurements you will need, with caveats:- Saddle setback is the actual set back from the seat post axis to the point where the sitz-bones make contact with the saddle. I simplify and say this is also the location of the hip joint (when in reality it is probably a bit higher, but “simplify”).
- Distance from sitz-bone contact point to pedal axle at lowest point of the stroke. This is based on the “slightly-bent-knee” theory of saddle height, but just measure whatever you have and like on your bike, regardless of how “bent” the knee is here. The measurement forms a radius of a circle which is all possible saddle positions to retain this extension. The model simplifies and uses the same number for both crank lengths, even though it will change a mini amount because the angle of this measurement is changing. And since the angle also changes when you move along the arc by sliding the saddle back and forth, the measurement is only valid within a reasonable range of possible setbacks (but this is constrained by your saddle and post anyway). Related, this measurement is only an ok assumption for a normal range of seat tube angles, maybe between 71 and 75 or so. Super slack seat tube angles necessitate a different pedal reference point because the difference between leg extension at the lowest point and the furthest point of the stroke is too great.
- Point on your foot over the pedal axle to ankle, measured horizontally on the floor, this point on the “ankle” to tibial tuberosity, tibial tuberosity to hip joint.
- Seat tube angle. This is critical, so guesstimate with care.
- Crank lengths, of course.
Using the model:- It’s CAD, so it's not super user-friendly, but it is not hard to learn the basics. You interact with the model the way you interact with any CAD model, by setting “constraints” (lengths, angles, or relationships) and letting the model solve other variables for you. Some of the variables already have constraints, and you can change or remove these. You can add constraints to things that don't have them. And/or you can grab points and move them, which pulls other connected points along with it while trying to retain the “constraints.” Be careful because the model might move points you think are fixed when you change other constraints (eg, the pedal position for exploring knee-pedal position may change, and you have to reset it)
- You can read measurements on non-constrained variables by clicking on the line segment that represents the variables and looking at its properties.
- The model currently doesn’t solve for the saddle height measured from the BB along the seat tube with the shorter cranks. It will tell you the original height and the change, but it was too cluttered to do all three. Just add them up if you want to know this.
- It’s in SolveSpace, a free and open source multiplatform (Linux, Windows, Mac) 2 and 3D CAD application. It’s available free here, https://solvespace.com/download.pl. Why SolveSpace? Because. Please just accept it! If you want to port it to SketchUp, feel free, but see below!
- The model will copy certain dimensions that remain constant, (eg leg dimensions), so they are the same for both cranks.
Here is the model. Because the forum only accepts certain file types, I changed the file extension to .txt. SolveSpace files are text files in the first place, but you will need to change the extension to .slvs to open it. Please keep in mind that I am not a veteran CAD user, so the model might not work flawlessly or elegantly--I learned how to use CAD in order to build this model. And again: I make no warranty or guarantee that the model is accurate.
I am posting it here with a Creative Commons 4.0 Attribution-Noncommerical-Share Alike license. More info here https://creativecommons.org/licenses/by-nc-sa/4.0/
I welcome comments, questions, or modifications, but please post the file if you change stuff, so we all can see it. The next post will have some things I learned with screenshots.
Last edited by Frkl; 02-04-22 at 04:40 AM.
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