Designing a Fixture for Fork Blade Length

Posted on November 11th, 2008 by admin under Bikes.

Fork blade lenght gauge fixture

Fork blade length gauge fixture (click on picture to enlarge)  This is one I’ve had kicking around forever. I never used it much because the 2 added barrels at the drop out ends won’t allow the fork blades to lay flat on the bed after they’ve been raked. So, I just used it as a gauge to insure that the dropouts were inserted exactly the same. I guess I’ll scrap that idea. I hate to take any fixture apart that is functional even if it is not used to it’s potential. Perhaps I’ll add some eyelets that allow the barrels to slide in and out of use. The next step is to add a cutting guide to assure a good straight hack saw cut or to mount the whole fixture to a chop saw such that it is adjustable for the forks intended use. For the math challenged, I’m told by Keith Anderson that he merely adds a centimeter to the unbent blade length, assembles the fork and bends both blades at the same time. Andy Newlands at Strawberry sent this catchy little “equation” for cutting blades prior to raking. Andy, by the way, bends both blades at the same time too. The straight blade length is equal to the “M” design distance from axle to race minus the fork crown dimension “H” (crown race to bottom of fork crown casting) plus the dimension (L-y) which is the difference in length between the chord length (bend) to the axle centerline and the straight length to the intersection of a perpendicular from the axle to the fork centerline.  L equals two times the bender radius times the radian quantity inverse sine of the square root of the product bender radius times fork offset divided by two all divided by the bender radius and y equals fork offset divided by the tangent of the same quantity inverse sine mentioned before. A table of L, y and fork offset can be developed for each bender radius and then you just determine the straight blade length from the design length M minus the fork crown dimension H plus the (L-y) value.  Once the tips are brazed, the blades are cut to length and brazed into the crown/steerer sub-assembly (pre-brazed) and then bent on my old Marchetti bender then aligned.  Works for me – may not work for you. However, once you get it down it’s pretty quick. Andy N. Thanks Andy!

Here are some links to Richard Sachs home made measurement device:

here1 here2 here3 here4 here5

Richard quipped:  “i used a chopped and channeled 1970s fixture to get this done. not fancy, but dedicated, accurate, and fast atmo.”  here6

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