Interesting topic, and one that provides great fodder for speculation, since the systems involved cannot easily be separated to assess their individual contributions/effects.
mickeg opined...
I disagree, I think that when there is a sharply angled curve at the bottom of the fork that the shape of the fork acts a bit like a mini-swing arm. But, I do not want to get into a long argument on this, that is my opinion.
...and I agree but for somewhat different reasons. Speaking as someone who has sliced and sectioned a number of forks, I can say they do not remain constant in either wall thickness or section width/shape/diameter except for some straight-bladed forks which do remain constant in all those dimensions. Generally, as a result of the rolling and shaping process, straight and curved blades become smaller in diameter but at the cost of thicker walls when rolled down at their tips. Similarly, things differ up-top, with blade sections being either generally round or oval, and these all affect the actual flexibility of the blade along its length and bend/chord (or lack of it, in the case of straight blades).
For curved forks, placement of the bend really does change the flex characteristics of the fork, but more as a result of section width and shape and response to forces in the direction of travel.
Speaking as a hobbyist framebuilder, a factor commonly overlooked in comparing the flex of curved versus straight fork blades is the matter of blade length. Even if both kinds of forks have identical rakes (whether at crown or somewhere mid-blade) that result in the same offset (and trail, when combined with a given head angle and wheel/tire outside diameter), curved blades must be longer to get to the same place and will therefore flex along a greater length.
Fork crown design has an effect on fork flex and microvibrations as well. When a fork crown is inserted inside a fork rather than capping the blades, it has the effect of shortening the blade length -- or butting the upper section so massively as to change the fork's flex characteristics to a large degree. I've built forks that "felt" different but were identical in geometry and blades, but differed in crown style or construction (i.e. fabricated/stamped versus forged).
Steerer dimensions and head tube length and diameter probably have as much effect on microvibrations as other factors.
In my experience (and sadly -- except for generalities -- all we have is our own experiences and opinions, which are valid for us but devilishly hard to separate from myriad other factors in play. Because so many factors differ, ceteris parabis is out the window), the steerer, stem height/reach/construction, handlebars, and handlebar covering as well as the amount of rider weight placed on the handlebars -- and where -- are factors at least as large as fork rake and construction when it comes to feeling microvibrations in the hands and even body. The larger threadless steerer on my Nomad is much larger in diameter than the threaded one in my favorite randonneur bike, and the quill stem on the rando bike visibly flexes -- and the handlebars are a different width, bend, and even wall thickness than those on the Nomad. I could surely feel a difference when I replaced the conventional headset locknut on the rando bike with a taller one (Sugino High Column Nut
http://bmxmuseum.com/forsale/160507 ) that sleeved and supported more of the exposed length of quill. Compression-wrapping Grab-On foam grips with one or more layers of tape surely damped microvibrations by the time they reached my fingertips. Surely, some rubber brake hoods are more comfortable than others in this regard for me.
Tires (including tread design and pressure) play a huge role in the propagation or damping of microvibrations, as does the road surface. So do hub dynos. I have identical SON28s on my Nomad and most-favored rando bike, though wheel diameters and tires sizes and pressures differ. The one on the Nomad gives me no trouble, while the one on the rando bike produces such disturbing vibrations as to make riding the bike extremely unpleasant when using it at some speeds and unbothersome at others -- apparently from magnetic eddy currents producing vibrations that are transmitted through the hub to the axle and then through the fork to the handlebars where I have my hands. I blame some combination of fork design combined with my particular handlebar and stem. So far, my "solution" has been to ride at speeds above or below where the vibrations are most noticeable, but it is a nuisance on an otherwise viceless bike.
Micro-vibrations and their source(s) depend on many factors, but the degree to which they bother probably comes down to the most variable factor of all -- the individual rider.
Best,
Dan.
