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CAD model for shorter cranks

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CAD model for shorter cranks

Old 02-04-22, 04:12 AM
  #1  
Frkl
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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.
Attached Files
File Type: txt
crank length 2022-04-02.txt (41.5 KB, 4 views)

Last edited by Frkl; 02-04-22 at 04:40 AM. Reason: Formatting
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Old 02-04-22, 04:23 AM
  #2  
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Some interesting things I learned

Here are some things the model helped me see. When I refer to fit relationships such as knee-over-pedal, I’m not necessarily endorsing them, just examining what happens to them. You probably have to click on the screenshot to read it.

1. My time would have been better spent riding. Here is the model showing 175mm and 150mm cranks with a 70mm setback, and 73 degree seat tube angle. The differences are mathematically not that much. However, the feel is significantly different. Hence, to figure out what short cranks do, go riding, not calculating.



2. Shortening the cranks does not mean raising the saddle the same amount as measured along the seat tube. This is intuitively true because the circles of possible saddle positions converge to either side of the vertical line through the bottom bracket. The difference is slightly less (here 23.75mm instead of 25mm), because some of the change is accounted for by the fact that moving the saddle up also moves it back in relation to the pedal at the lowest point. But, I think that's important: I believe that I can feel changes of 1mm in my saddle height, and I know I can feel 4mm (the difference in sole thickness between my two regular pairs of shoes).



3. It is possible to deal with the entire crank shortening by pushing the saddle back without raising it. In this case, I would need to double the setback to about 137mm. Not recommending this, even if it’s possible, but you could. More realistically, you can counteract some of the effects that short cranks will have on reach and drop in relation to the handlebars with different combinations of saddle height/setback changes.



4, Shorter cranks don’t necessarily push the knee forward of the pedal, especially if this wasn't a problem with the longer cranks. I was always convinced they would, so I burst at least one bubble here. In this case, with a constant setback of 70mm, shortening the crank by 25mm will cause my knee to move back 21mm once I’ve adjusted the seat post up the necessary 23.75mm.



5. If I really want to maintain the same knee over pedal relationship, then I would need to move the saddle up 21.94mm and back an extra 10.4mm.




6. In fact, even with 10.56 mm cranks (random nonsense small number), the knee remains behind the pedal. Seeing this was why I decided against buying an expensive seat post with extra long setback. I decided on a much cheaper option with 1cm more set back, see no. 5 above.



7. Bikes with slacker seat tube angles will require less change in saddle height than bikes with steeper seat tubes. This makes intuitive sense, of course. Here are three screenshots for 73, 75, and 71 degree seat tubes, all else constant. What is interesting here is that each will have different ramifications for reach and drop in relation to the handle bars, but that is beyond the model right now.

73:




75:


71:


Hope this was interesting!

Last edited by Frkl; 02-04-22 at 04:51 AM.
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Old 02-04-22, 07:55 PM
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IMO setting setback by KOPS is somewhat random due to varying body proportions - what we're really trying to do is achieve balance between saddle and bars, little weight on the bars. I think KOPS works fairly well with 170mm cranks. That's a first approximation, then we move the saddle fore and aft to adjust for balance. Crank length doesn't really matter at all. What matters is one's fore and aft relationship to the bottom bracket, i.e. where one's foot is at the bottom of the stroke. That determines where one's butt is OOS and the general balance of the bike.
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Old 02-05-22, 04:47 AM
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Originally Posted by Carbonfiberboy
IMO setting setback by KOPS is somewhat random due to varying body proportions - what we're really trying to do is achieve balance between saddle and bars, little weight on the bars. I think KOPS works fairly well with 170mm cranks. That's a first approximation, then we move the saddle fore and aft to adjust for balance. Crank length doesn't really matter at all. What matters is one's fore and aft relationship to the bottom bracket, i.e. where one's foot is at the bottom of the stroke. That determines where one's butt is OOS and the general balance of the bike.
Hey, thanks for the comment. I thought you made some very good points in the other thread about the importancr of gearing in the whole system, so it's great to get your input here.

The model does not depend on KOPs, and I know KOPs is just received wisdom that is kind of arbitrary. The model has a built in variable for examining the relationship, but the model doesn't constrain this variable by default. it is a result of everything else by default. You could constrain this to vertical or a particular angle if you absolutely want a particular set up, though.

I have read that some argue that center of mass position is more critical, and it wouldn't be difficult to create a torso variable and hang a vertical line from the center of mass, if that is something that people are interested in.

I personally always like my knee behind and not over the pedal because I like to always have some component of my pedal force pushing me back off my hands. So I am not wedded to KOPs. I focus on knee-pedal above because, as I said, I was always convince that shortening cranks would destroy the knee offset I like, and I wanted to "test" this conviction. I was wrong, I'm happy to admit, because I forgot about the vertical component of the saddle height adjustment.
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Old 02-05-22, 02:44 PM
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One thing I would say relating to saddle height is that we should not automatically assume that shorter cranks require a higher saddle height. It all depends how optimum saddle height was determined in the first place. One experienced UK fitter suggests starting with the same saddle height when fitting shorter cranks.
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Old 02-07-22, 02:47 AM
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Frkl
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Originally Posted by PeteHski
One thing I would say relating to saddle height is that we should not automatically assume that shorter cranks require a higher saddle height. It all depends how optimum saddle height was determined in the first place. One experienced UK fitter suggests starting with the same saddle height when fitting shorter cranks.
Absolutely. I read the link you had posted to this approach.

The model doesn't say anything about what you have to do, of course. The model is just a tool to think with, and one has to be very aware of its limitations: it shows the set of mathematically solvable solutions that maintain a particular measurement (as I set it up, contact point to pedal axle). This is not necessarily the best or only measurement to start with, and as you say, there is nothing that says that this has to remain constant at all, but the model needs to start with something.

One could choose another measurement to start with, and that would be interesting. Back angle would be one thought... It is also possible in the model to unconstrain absolutely everything but body dimensions and the frame angle, but again, this won't tell you what you must do or that one solution is better than the other. It will just show you what is mathematically possible given your leg lengths and the seat tube angle. The model will even let you bend the knee the wrong way, if you really want to

But as I said in No 1. above, the one thing that really jumped out at me from the model is that it doesn't make too much sense to overthink this. There are so many variables, as well as the option of not changing anything but the crank length. And we aren't even getting into the front half of the bike yet! It makes the most sense to find a personal solution by riding.
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