General Cycling Discussion - Bike Frame Software Experiences
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10-15-10, 03:19 PM
Anyone know of an easy way to forecast how a frame will behave given dimensions and angles?
Did a Google search and came back with these:
Seems the good stuff is $$$; really, all I want to do is have a clue how a frame handles given chainstay length, seat and head tube angle, etc. Not intending to use it to make my own frame, just to analyze existing frames.
Anyone do this?
You are looking at finite element analysis, which you will have no idea how to do competently unless you have mechanical engineering experience.
10-15-10, 04:15 PM
Really? No one can give simple rule-of-thumb rules, like for example that 72 degree's is a relaxed head tube/seat tube angle, etc. That's surprising.
10-16-10, 10:03 AM
I would think that the "forecasting" has already been done by the manufacturer. The better manufacturers would have their own engineers who are instrumental in the design of the frame. The copy type manufacturers, would just copy a design (ie reverse engineering).
You're asking how it handles. This is by experiment. You need to try it out, or someone else will try it out and give you their opinion on how it rides.
Not only tube lengths, angles, shapes and thickness, but how the carbon is put together to make it all work as intended.
At the Cervelo web site, under "Engineering" there is "Technical Presentations". There you'll get some of your answers.
10-17-10, 04:53 PM
The Paterek manual at http://www.timpaterek.com/tpmanual.htm is an interesting read. I am not an engineer but have worked in the R&D field at IBM and have some natural sciences and math background. It read ok to me but there was no way to do "what if" analysis.
As a lay person my interest comes from being in the middle of specifying a custom frame build. The builder is very knowledgeable but I find myself at a disadvantage in helping guide him other than what "feels" ok in my current bikes and what doesn't. I wish I knew more about the framebuilders' art since it is over 100 years old. There would seem to have evolved a bunch of well known "solutions" and "rules of thumb". Something in between Sheldon Brown's guide http://www.sheldonbrown.com/frame-sizing.html and a finite element analysis software package.
You've said it yourself, you're not an engineer. So don't try to engineer your frame.
Those well known "solutions" and "rules of thumb" are known to your framebuilder. If you can't trust your framebuilder to use that knowledge along with his years of experience to create a frame which will fit, ride and handle in the manner you describe to him, then you need a different framebuilder.
If you need to micromanage the process by dictating angles and tube lengths--and the framebuilder doesn't tell you to take a long ride on a short pier--you need a different framebuilder.
If you you ignore that second one, and micromanage the process and the frambuilder makes exactly what you specify, you deserve the end product.
If you make it though the process without pissing off the framebuilder, and you let him use his experience design the frame, you'll get a BikeCAD drawing to sign off on. That will have all the angles and measurements that will work for the frame his experience tells him you need. If you're curious, ask him to explain his decisions to you. Otherwise, just sign it and wait for the delivery of the best frame you've ever ridden.
10-17-10, 09:19 PM
Wow tsl, that's pretty harsh. What makes you think I want to micromanage anything? I have a healthy curiosity but I KNOW I am not an expert. When I flew airplanes in days past, I always wanted to know how everything worked in the little Cessna because my skin was at play. I would never have accepted "just trust us" as an answer, nor would any rational pilot.
I understand it's your opinion and you are entitled to it. But I see nothing wrong with being curious. And yes, I trust my framebuilder. That doesn't mean I trust ANYONE blindly.
Anyone else have something CONSTRUCTIVE to add to this discussion?
When I flew airplanes in days past, I always wanted to know how everything worked in the little Cessna because my skin was at play.
Yes, but you didn't try to design the Cessna. You bought (or rented) the plane accepting the assumption that the plane's designer chose the correct airfoil profiles and whatever in order to make the plane operate safely and comfortably within its design envelope for its intended use.
By trying to decide between a 72° and a 73° (or whatever) HTA, you've crossed the line to trying to design the frame.
Curiosity is just fine, and learning how all this stuff interacts is incredibly interesting. I lurk on a framebuilder's forum for exactly that reason.
But the number one thing I learned is don't dictate specs to the framebuilder. Every single one of the big names refuses customers who try to do this, and they recommend to the new guys that they do it as well. (Unless of course you're a designer, like Gauzetti or Hampsten, who is contracting with the builder to fill the role of job shop.)
If you still want to design it yourself, use BikeCAD (http://www.bikeforest.com/CAD/bcad.php). It's probably the same software your builder uses.
Figuring out how a bicycle will handle (and especially feel) given a particular geometry is not simple. You can find rules of thumb for the effect of a particular tweak, but there is a lot going on and it is often hard to say for sure which factor will dominate.
I assume that most of those CAD tools are there to generate technical drawings and calculate all the measurements, and determine the fit to a human (elbow angle, knee extension, etc.). But as to how the geometry will affect stability, comfort, etc. I don't know...
10-18-10, 03:55 PM
Thanks bijan. It has become clear over time that frame design is more trial and error than science. The designers get some known solutions and then tweak them for the most part, or so it looks like from the buyers' point of view. I don't pretend to second guess in a professional sense, but to understand. To carry the example of a light aircraft, one chooses which airplane to fly based on how it handles - whether a Cessna, Piper, Mooney or whatever. A big part is choosing whether an airplane is high or low wing and what kind of airfoil shape it has, loading vs horsepower, etc. Nobody would call a pilot crazy for wanting to know this. And no aircraft designer would take insult from a pilot that asked either. It's about making sure one is getting what one needs. In the case of planes, NASA has great literature about airfoils, etc. and the rest one gets from the manufacturers. When a person decides to buy a homebuilt airplane like a Lancair, everyone expects the technical questions. The designers don't take offense in my experience.
Thanks for the link to bikecad, tsl. It may or may not be the right tool. I certainly don't want to build my own frame. I am more interested in "what-if" analysis.
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