# FEM: How to Design weight-distribution in bike prototypes?

Inferio! No-one can understand this bloated question here, now more focussed.

Suppose you are doing a new bike. How can you be certain that some critical point won't get too much force and the right parts gets the force that is then again supported by the ground? Of course, everyone here has played the wonderful bridge construction -game and knows that weight-distribution requires proper analytical reasoning. I think there are some multi-variance-something-methods to investigate it. Anyway, just skim-read a book and they give many different formulae (apparently numerical approximations) for different kinds of attachments (or welding points or anything you call it). So I don't try to underestimate the hardness to design proper weight distribution. Any design hints welcome.

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...the answer has certainly something to do with FEM -modeling, that is how industries create aircraft etc... not sure though how it is used in this contex. – user652 Jan 8 '12 at 10:32
Basically, a bike frame is no different from a bridge. Any mechanical/civil engineer would instantly recognize a half-dozen different different approaches to analysis, depending on the precision needed and the tools at hand. As a first approximation consider all joints to be hinged -- this is the fundamental approach to truss bridges, and it permits reasonably accurate solutions with pencil and paper. – Daniel R Hicks Jan 8 '12 at 14:09

I'd guess that a bike is more difficult than a bridge because, with a bike:

• The transient ('shock') forces are relatively larger
• The transient ('shock') forces are more difficult to predict.

Some design hints might include:

1. Over-engineer it (make it stronger than necessary)
2. Trial and error (fix it if it breaks)
3. It's better to fail slowly/gracefully than catastrophically/dangerously (e.g. better to blow a tire or break a spoke than to collapse a wheel).
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What do you mean by weight distribution? Most of the mass is the rider, and the rider moves.

Bicycle construction is very experimental. Analysing dynamic forces is complex, partly because most of the mass is only loosely coupled to the bike. With most mechanical structures the loads are more predictable, for a bicycle you only have to look at flatland bmx riding to see what a bicycle has to handle. A simple case to think about is the rider leaning into a corner. The forces are not vertical with respect to the bicycle, but at an angle. So you have side loads on the wheels and frame.

The usual approach for commercial bikes is to overengineer and test. Manufacturers of racing bikes sponsor riders partly to have crash test dummies to ride experimental bikes. They keep making them lighter until something breaks, then add a bit back. Or not, they might just choose lighter riders or replace the frames more often.

For homebuilders you can deliberately build light and see what happens (as I did with One Less Ute)

Simulating the loads in software will be complex and I think you'll end up trimming your Monte Carlo parameters to the point where you're not getting a very representative sample (or you're buying a lot of supercomputer time). The extremes that I have done on a rigid diamond frame bike include: dropping ~3m onto a gravel road at ~40kph horizontal speed; riding down a twisty gravel road (~10cm high to low points) at 60kph with 15kg on the front panniers and 30kg on the back; riding 450km of sealed road with 20kg front, 40kg rear, 20kg backpack. I weighed about 80kg when I did those, the bike weighed less than 20kg.

My approach is to assume that the entire weight of the bike + load can be applied to one wheel with the bike anywhere from horizontal to vertical. Then allow for some bouncing. I try to get roughly 60% of the weight on the rear wheel when the ride is seated and there's no extra luggage for normal road use. Load bikes don't obey those guidelines at all.

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