Bike Forums - Curvy geometry vs. straight geometry

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Quote:

Originally Posted by Scooper (Post 17908871)

I'd love to see some finite element analysis images showing stresses in straight vs. curved geo frames.

That would be nice, except the curved tubes are only one characteristic of a frame. The way the whole frame is designed and built will dictate the stresses throughout the structure, not just the external appearance of the tubes.

I thought about it, but I modeled a fork in solidworks and it was so annoying that I pushed the entire frame idea down the list

What about forks? is there any difference curves vs. straight? I guess the answer is similar

Quote:

Originally Posted by maltess2 (Post 17909392)

What about forks? is there any difference curves vs. straight? I guess the answer is similar

There is a very well regarded frame builder named Doug Curtiss who uses "banana" curved seat stays and supposedly the curved shape helps dampen road vibrations and acts as sort of a shock absorber... which says nothing about strength of course.

Quote:

Originally Posted by maltess2 (Post 17909392)

What about forks? is there any difference curves vs. straight? I guess the answer is similar

Dave Kirk did some sophisticated testing on straight leg vs. curved leg forks when he was at Serotta. Here's what he posted about that testing:

Quote:

Originally Posted by Posted by Dave Kirk, 05-07-2005

Hey Henry,

I built a number of different forks with the same blades and crowns. In some cases I built matched pairs of forks with a unicrown to eliminate that as a factor.

The forks were tested on a testing rig that could load the fork in a variety of ways. The steerer was held rigidly and an air cylinder pushed on a fake axle clamped in the dropouts. The angle that the piston pushed could be adjusted so that it was "horizontal" or inline with the wheelbase (as if running into a wall) or it could be set to push straight up from the bottom as if the rider was landing from a hop. It could also be adjusted to simulate the load path the fork would see if the rider where to run into a small bump.

Along with this rig that was a load cell that told me how much force the fork was seeing and a displacement gage that would tell how much the fork moved.

All this was hooked up to a plotter so we could graph the results.

We could run the test for anything from ultimate strength to fatigue. It was pretty cool to turn it up high and just shear forks in two! Nothing is more fun than breaking expensive stuff.

I don't recall the numbers as it was years ago but I can say that in the "hit the wall test" the curved fork and the straight blade fork had the same displacement with a given load. The "landing a hop" test showed a small difference between the two....the curved fork having a greater displacement at a given load.

The "hit a bump - load path test" showed a significant difference with the curved fork having a good bit greater movement.

All the lab stuff is cool but it really matters how it feels on the road. We did blind tests on the road with the forks covered so you couldn't see them (not easy to do by the way) and compared the curved vs, straight fork. Some folks could tell the difference, others couldn't. We also asked the subjects which fork they would rather ride - A or B. As I recall no one liked A (straight) better.

So that is my experience. If you don't like my testing methods or results I respect that. I won't argue about it. I feel confident in my results. If you don't that's cool too. The cool thing about all of this is that so many things work so well. There is no one "right" way. I just get frustrated when people call things fact without testing to back it up.

Thanks for your time,

Dave

then there's a mix.. offset parallel to steerer tube, at the fork crown, and a little bend in the fork blades , combined ..

I did so on a robustly made Touring frame (pictures on Film, not digital)

I'm an engineer, but I recognize that aesthetics are important.

In frame tubes I can really only argue about basic structural and material principles - I'm not a frame designer nor a structure-oriented ME. I think I got the basics right in my posting, but Unterhausen may well be correct. Perhaps the properties of the tube are less important than the emergent properties of a fully-joined frame, so I was not addressing the right question.

As far as the OP's original statements about fracturing of curved frames, I dunno. Anything could have been involved from real differences between tube designs, systematic errors in tube building or in frame assembly, or undiscovered damage to tube inner structure, in the case of a composite tube. Or poltergeists.

Wanna talk about EMI, magnetics, system safety in automotive, or power electronics? Glad to, but those are not very useful in a frame builder forum. An overall critique of curved tube frames is not what I tried to provide.

With diamond frames the straight tubes are more efficient in most cases. It's basically a truss, and straight sticks rule in most trusses.

Could one come up with a situation where a curve in these elements was an improvement, probably, but I have to doubt that there is any serious engineering going into it, mostly just guesses and marketing. I am talking tubes here. There is so much design flexibility possible with something like carbon, I would imagine almost anything is arguable, though you then get into a question of whether there are nay points in it, and other than cool factor it probably gets tough to prove an actual advantage, even if theory is reasonable.

Aesthetics, there are certainly some attractively curved frames, most of which are not in the realm of the purely practical, like beach cruisers, and such. There are some serious folk, for the most part into the diamond shape, who are trying to introduce curves into the top tube and seat tube, a swoop look. It normally doesn't look good. I'm not being conventional it just seems proportionally hard to pull off a graceful curve with the amount of curvature they are willing to use and the ratio of top tube to seat stay. Some of these even award winning efforts are lumpy if the tubes are viewed alone. But if someone makes lit look good, why not.

Can a curved stay give a better ride? Maybe. Rob English has stays that are I think 3/8 by .28 wall. I think I got those numbers off a build of his. So the standard size is so much stouter, maybe it can be bent and there will be a difference as is found in forks, according so some.

Forks are structurally very different, and the idea is to actually bend them, while a conventional diamond frame is meant to be rigid. Different situation different rules.