The myth of "stiff frames" and "stiff wheels"

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Originally posted by Oarhead
Thanks to arty for the leg work and articulating it nicely.

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Seconded.



Recently I was thinking about buying a new road frame; I spent quite a bit of time reading the reviews on the Road Bike Review site. There were numerous reports about dramatic changes in the feel of the bike in question when users switched from a metallic seatpost to a carbon variety. The reviewers claimed their very stiff frame was made significantly more comfortable by this simple component exchange.

Found this on an unrelated search. I think Arty sums up much of it but to me the most glaring obvious ommission is the 80 lbs. As Arty states forces of hitting a pothole are non linear and dispersed through the frame. What he doesn't mention is that sometime during that impact the force is FAR greater than the weight of an 150 -200 lbs rider. Measurable force at somepoint may hit 5 or even 10 times that amount and this guy uses an 80 lbs. weight statically and then says 'there I told you'.

About all this proves to me is when you decide on the answer before you design the experiment you can usually come up with an experiment to confirm that answer. He should do a study on why people consider this sort of 'research' as valid or worthwhile.

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Originally posted by L7
What he doesn't mention is that sometime during that impact the force is FAR greater than the weight of an 150 -200 lbs rider. Measurable force at somepoint may hit 5 or even 10 times that amount and this guy uses an 80 lbs. weight statically and then says 'there I told you'.

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Excellent point.

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Originally posted by L7
What he doesn't mention is that sometime during that impact the force is FAR greater than the weight of an 150 -200 lbs rider.

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That doesn't make the relative contribution of the frame any more significant. The relative magnitudes of the deflections won't change no matter how much force you apply (up to the point of material failure.)

Example: the flex measured at the seat tube for all the bikes was approximately 75/1000ths of an inch, and was 0 at the head tube for all bikes. The largest difference between two frame materials at any point was 1/1000th, or roughly 1/75th of the total difference.

So let's say you put a force of 800 pounds on the bike. Assuming a linear increase, the total deflection would be 75/100ths, or 3/4 of an inch. This is quite significant, and probably about what you'd get when hitting a really big pothole. But you're still only getting 1/75th of that deflection from the frame! The rest is coming from the fork and tires.

Arty: If the frame is getting into nonlinear deflection, it's permanently deforming or failing catastrophically (as you pointed out), and we can therefore ignore it. We're not addressing the crashing or breaking case, we're addressing the "road feel" case.

Oarhead: "When you do this you are putting all sorts of forces and moments on the bike." True but irrelevant. My point is limited to this assertion:

"The perception that different frame materials or different wheel constructions cause a bicycle to feel perceptibly 'rougher' or 'smoother' is not supported by the available data."

Here's what the data does support:
Data supports the assertion that different frame materials can feel different to the rider under hard acceleration (measurable flex) and under hard cornering on bumpy roads (measurable flex).

Data also supports the assertion that changing fork material and dimensions can feel different to the rider.

Data supports the assertion that tire inflation and construction can feel different to the rider.

Data supports the assertion that differences in seat construction can feel very different to the rider.

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It's strange to see everyone get so worked up about this...like I've attacked everyone's religion. Not a *single person* has stood up and said "You know, maybe there's something to that. Maybe road bikes haven't fundamentally changed in 80 years, so we're reduced to inflating insignificant differences to the magnitude of holy war in order to differentiate our products." And I think it's telling that the arguments have all been either
a) anecdotal
or b) "well, I know this is wrong but I don't have the time or equipment to prove it."

At some point, someone's going to weld accelerometers to a seatpost and handlebars and capture a bunch of data. Until then, I'm chalking this up to the DVD effect I mentioned earlier: people are bad observers, and they really want to believe that the toy they just paid $2000 for is much better than their old one or the one their friend has.

This article should lay the topic to rest.

"Frame Materials for the Touring Cyclist"
http://sheldonbrown.com/frame-materials.html

Does anyone else have anything further to add?

I'm not sure how worked up you think I got but I do tend to be annoyed when someone takes really bad data and presents it as valid hard science. I deal with Parks Canada on a daily basis for christ sakes I don't need more of that crap in my life.

With his tiny weight this guy is measuring comparitive deflection of different materials. There is NO REASON to ASSUME that this will remain constant under much bigger loads. He offers no proof that it will and yet extrapolates results assuming that this will remain constant. What's more the load is so light and measureable differences so small that even the margin of error could account for a different feel at real life load values that he has not come remotely close to.

Afterall a 100lb person's stiff bike is a 200 pounders wet noodle that may well be exacerbated with different materials. Maybe not, we certainly don't know with this 'research'.

I don't get worked up at people trying to change or redesign bicycles but having someone trying to foist bad science on me and others is a little irritating.

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...and they really want to believe that the toy they just paid $2000 for is much better than their old one or the one their friend has.

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Actually, no. At least not for everyone. In fact, at this very moment, I'm attempting to buy a 15 year old steel road frame. Why? It was built by one of the finest frame builders to ever come out of SoCal. Ride matters to me and who builds the bike is absolutely the most important consideration. I'm also going to look into to buying back my old Medici Pro Strada, one of the best bikes I have ever ridden.

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Originally posted by Spats
Example: the flex measured at the seat tube for all the bikes was approximately 75/1000ths of an inch, and was 0 at the head tube for all bikes. The largest difference between two frame materials at any point was 1/1000th, or roughly 1/75th of the total difference.

So let's say you put a force of 800 pounds on the bike. Assuming a linear increase, the total deflection would be 75/100ths, or 3/4 of an inch. This is quite significant, and probably about what you'd get when hitting a really big pothole. But you're still only getting 1/75th of that deflection from the frame! The rest is coming from the fork and tires.

Arty: If the frame is getting into nonlinear deflection, it's permanently deforming or failing catastrophically (as you pointed out), and we can therefore ignore it. We're not addressing the crashing or breaking case, we're addressing the "road feel" case.

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What you've presented once again is static loading. That tells us nothing. As I pointed out before, when you're riding a bike the forces it experiences are very dynamic. Magnitude, duration, direction, cyclical loading, frequency, etc. All of these come into play and will generate the "feel" that everyone is talking about. Sure other components may play a part in this. I don't discount that. But the frame will still play a part. But basic engineering principles (some of which I've noted above) tell us that this test is poorly conceived and thus its results are largely meaningless.

For a good individual example, look at Galloping Gertie. A specific frequency range of wind would cause the bridge to oscillate wildly. Finally the bridge collapsed from the stress. Now I know we're not talking about failure nor a bridge. But Galloping Gertie is a classic example of the things people overlook when they are designing or analysing a system. The things we build live in a dynamic universe and thus we must consider the multitude of dynamic actions that can and will occur.

With this in mind, hanging weights from a non-moving bike doesn't really strike me as a very comprehensive test.

Look, I'm not exactly god's gift to engineering. I studied ME for my first 2 years in college. So I won't claim to have all the answers. But I do know enough to say that this test doesn't even begin to cover all that is occuring in a dynamic system such as a bike.

This thread is starting to PISS ME OFF!!!!!


GGGGRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRR!!!!!!

While I don't mind this thread dieing at all I must say I'd take a room full of nerds over one chickenshit trying to be a censor while hiding behind an alias.

When you think about it what's the worst kind of nerd? One that doesn't have the strength of conviction to identify himself with his own beliefs. Those are the chickenshit nerds that create viruses and terrorizes internet boards while remaining anonymous.

Ogre, your mama called she wants you to go back to Powder and play with the other nerds now.

If it's just a matter of comprehension I'm glad to help you with the tricky bits. The easiest thing to comprehend though, is IF YOU DON"T LIKE IT DON"T READ IT. Easy concept, you should try it then you don't have to be anonymous chickenshit boy.

Now, now. Take it easy on poor Ogre. I actually think he's funny. Also remember, he trips out merely thinking about "What if cat were spelled d.o.g.?" :D