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Fork Vibration Comparison

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Old 01-10-19, 02:58 PM
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TiHabanero
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Fork Vibration Comparison

Compared the vibration of two carbon forks I have to two steel forks I also have. The carbon forks both vibrate at a higher frequency than the two steel forks. I wacked them against the work bench and listened to them as though they were tuning forks. The carbon forks ride harsher than the steel forks and I wonder what the difference in frequency is telling me. Any ideas?
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Old 01-10-19, 03:17 PM
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fietsbob 
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Another use for an Oscilloscope.. ?

Or there are vibration sensing tuners if you want to have a note name for the vibration,... based on 440 Htz A ..

And octaves , halves and doubling those numbrers (Stringed musician's gear.. )





...

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Old 01-10-19, 03:18 PM
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79pmooney
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Tuning forks,. Not an expert here but this is basic structural resonant frequency, Pitch goes up with stiffness, down with weight. Your CF fork is lighter and stiffer.

And yes, I have observed the relative discomforts on rough roads many times. At Cycle Oregon, those of us riding traditional bikes have learned that we need to expect the rapid slowdown when the pavement turns rough. THose slowing are on CF bikes.

Now, steel forks can also be too stiff. My Mooney's original fork was designed to be stiff enough touring. Too stiff all the rest of the time. I never felt "planted" going around corners on steep, rough descents. The new fork was made from the same blades, but using the narrow end of the taper instead of the stiff end. Much less stiff and a far better, more secure ride. (I wish I knew where my old fork is. I'd love to compare the frequency with the new next time the new is off the bike. New - as in 1984.)

Ben
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Old 01-10-19, 03:21 PM
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79pmooney
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Originally Posted by fietsbob View Post
another use for an Oscilloscope.. ?
Or a tuner. I could document my chromemoly forks with my chromatic tuner. (I don't know it it would work with 531. Manganese moly.)
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Old 01-10-19, 03:39 PM
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what note on the piano ?
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Old 01-10-19, 04:04 PM
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Whacked a carbon fiber fork against a work bench? Why do I think this is a joke?
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Old 01-10-19, 04:57 PM
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TiHabanero
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No joke, Milton. I pulled the carbon fork after 2 years and replaced with steel. The thing was just too stiff for my liking. I have a carbon fork from a Giant Defy as well, and that thing is just as bad.
Steel is heavier and more dense, hence the lower frequency. Make sense as it will move slower than the lighter, less dense carbon.
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Old 01-10-19, 05:19 PM
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I think materials which are light are able to be light because they have high strength. and strength and hardness of a metal are directly proportional (cf ductility, which is inversely proportional to strength). So a lighter material like carbon fiber, because it is stronger, will be stiffer, more brittle, less ductile. And that means less flex, which means that any bump in the road will be more directly transmitted to the rider, as opposed to steel which will "flex". I'm not a bike expert but I know a little about metals.
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Old 01-10-19, 05:36 PM
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You don't think attaching a wheel and frame to the fork would make that more applicable to riding?
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Old 01-10-19, 11:15 PM
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Carbon can be made in a far wider variety of temperments than steel or aluminum. The most comfortable bike I've ridden was full carbon, the least comfortable jumpy nervous feel-every-bump bike I rode was also full carbon (a scott).

My personal experience has definitely been that bikes with carbon forks and aluminum frames have been a much less buzzy ride than the full aluminum bikes I rode before them. (My full aluminum bike was a cannondale touring bike so not a stiff racing machine). 'course could be just better bike design in newer bikes as well.
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Old 01-11-19, 06:56 AM
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Originally Posted by TiHabanero View Post
Compared the vibration of two carbon forks I have to two steel forks I also have. ... I wacked them against the work bench and listened to them as though they were tuning forks.
I just want to pipe in to say what a novel idea that I had never thought of doing before
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Old 01-11-19, 07:51 AM
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I'm not sure if whacking a CF fork or CM fork on a work bench has much to do with how much you can tell about anything. It might if you stuck a hub and axle in it and secured it with a skewer. One way you have two pieces of material standing alone that allows vibration, when secured with a hub you actually have a piece that represents a rectangle.
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Old 01-11-19, 08:34 AM
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Not an expert here, so just speculating. I don't think the only question is the frequency of the vibration--you don't actually feel that directly. The vibration of the fork is felt by you primarily through your hands, correct? So the real question is how efficiently does the fork communicate this vibration to the handlebars--to the extent that frequency is a factor in this, I would think it would be more of a question of the relationship of the vibration frequency of the fork, the stem and the handlebars. Really, though, I would think that amplitude of the vibration is likely a bigger factor than frequency.

Does this sound right to anyone who actually knows anything about engineering?
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Old 01-11-19, 03:07 PM
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I find the vibration differences interesting, that's all, and know in some way it is connected to the stiffness of the fork. Of course when the axle is installed the legs don't vibrate at the same frequency, that is understood. Add to it the tire, tube, spokes, rim, and you have a whole different animal when it comes to what is felt at the handlebar.
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Old 01-11-19, 04:37 PM
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Originally Posted by TiHabanero View Post
I find the vibration differences interesting, that's all, and know in some way it is connected to the stiffness of the fork. Of course when the axle is installed the legs don't vibrate at the same frequency, that is understood. Add to it the tire, tube, spokes, rim, and you have a whole different animal when it comes to what is felt at the handlebar.
Completely agree with the above. Also the castor aspect, not sure what it's called but the bow of the fork would have to change a lot of the characteristics of the way it rides also .

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Old 01-11-19, 07:01 PM
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Originally Posted by TiHabanero View Post
I find the vibration differences interesting, that's all, and know in some way it is connected to the stiffness of the fork. Of course when the axle is installed the legs don't vibrate at the same frequency, that is understood. Add to it the tire, tube, spokes, rim, and you have a whole different animal when it comes to what is felt at the handlebar.
I think frame geometry (head angle etc.), trail and other factors play a larger role. Add 2" tires at relatively low pressure and i bet the fork material becomes almost meaningless compared to other factors.
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Old 01-11-19, 07:31 PM
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From what I have experienced, head angle has no affect on what is felt at the bar. Fork material/design and the rest of the kit certainly do.
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Old 01-12-19, 09:48 AM
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Originally Posted by TiHabanero View Post
From what I have experienced, head angle has no affect on what is felt at the bar. Fork material/design and the rest of the kit certainly do.
You measured vibration on 2 different bicycles with the exact same fork, same wheel, tire and remainder of the bike and only the head angle was different between the two setups?
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Old 01-12-19, 10:28 AM
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You can think of the fork as a filter with one of its functions to dampen unwanted vibrations from the road. When you tap the fork and listen to it you're measuring the resonant frequency of the fork. Vibrations at the resonant frequency will receive little dampening from the fork and be transmitted to your hands. I don't know what vibrations from the road look like but I believe fork design is a compromise between transmitting enough feedback to provide crisp handling vs dampening out unwanted vibrations.

I suspect Carbon transmits a broader spectrum of vibration which allows for crisper handling. Steel will dampen more road noise but also will be less responsive to steering inputs.

Personally, I prefer a stiffer fork which will allow crisp handling when you need it and use different tires to handle rougher roads if necessary. With a noodly fork there is nothing you can do to improve handling.
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Old 01-12-19, 10:42 AM
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Thru the stem cable hanger , on an all aluminum AlAn cyclo-cross fork looked clean, but set up a lot of vibrations, fore and aft when front brakes were applied,
which a similar bike, same maker, but a road frame with side pull brakes, did not do..
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Old 01-12-19, 10:45 AM
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So the fork has a dominant resonant frequency. As mentioned above, a function of stiffness. Higher stiffness (Young's modulus), higher frequency. Also inversely a function of mass along the fork (actually, the second moment of the area of the fork blades, times the density of the material. So a larger moment, lower frequency.

Shouldn't really be an issue unless you happen to hit pavement at a speed where the bumps are hitting at exactly the resonant frequency of the fork. The fork wouldn't reject that movement, and you'd get the thing bending and flexing a lot. Some of us have had the experience of pushing a street sign back and forth at its frequency. Impossible to move much in one direction, but if you get it resonating, you can almost shake a sign out of concrete. Or, remember the videos of the 1940 Tacoma Narrows Bridge collapse. I think that, with a CF fork, the resonant frequency is so high you'd never experience this type of resonance and extreme flex.

Otherwise, the dominant thing affecting ride is stiffness.
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Old 01-12-19, 11:42 AM
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Never banged a fork against a work bench but my experience is as follows...

I have three bikes, all with reputations for quality:
All CF road bike (18 pounds - 23mm tires),
Aluminum road bike with CF Fork (23 pounds - 25mm tires)
All aluminum (commuter) with aluminum fork (34 pounds - 32mm tires).

Running (as close as I can) 75 pounds in the front tires and 85 pounds in the rear tires over the same terrain, The all CF bike is the smoothest, followed closely by the aluminum bike with CF fork and, easily the harshest ride is the aluminum bike with aluminum fork.
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