fork blades need aligning
 

old steel mtb fork. one drop out is a little in front of the other. maybe 4mm. is there an easy way to get them even?

in 09 dannoxyz posted this "Easy way to fix the fork-blades is to remove the fork from the bike, hook the dropouts facing up underneath a door, and bend the fork over a mandrel (a big split piece of firewood works well). Grab the top of the fork blade that needs bending back and push down to increase its rake to match the other one. If you need pictures, I can work something up."


but I cant seem to figure it out exactly. anyone know how this works or another method?

thanks!

Straightening out a fork is more art than mechanics, and you'll find very few shops if any these days that will straighten out a fork. You may have them help order you a new fork.

A fork needs to be aligned over a number of dimensions.

First is to insure that the forkends are separated a standard distance for the front axle spacing. Most modern front axles have 100mm spacing. Easiest way to do this is lay the fork against a non-abrasive surface, and either push down on the other arm carefully, or pull firmly while having someone else stepping on the bottom fork blade until you sense a tiny amount of yield. Measure again and repeat if necessary. Better to bend a little at a time than to try and get it right on the first shot and overshoot.

Second, the forkends must be aligned parallel to the level of the fork crown. The easiest way is to have a fork alignment jig that will secure the forkends in something like a quick release axle. But this must be a very strong clamp and the clamp must not flex from side to side. The other option is to find something like the gap under a heavy duty door. You can also sandwich the fork tips between block of wood and some steps/stairway to a porch for example. Anything than can hold the fork tips and prevent side-to-side twist. Next, take a steel bar wrapped in tape (to protect paint on the fork) or 2x4 wood stud about 4 - 6 ft long. Slide the bar or stud under the blade that needs to come up (peek down the steerer), and over the other fork blade. Pry firmly and gently. Remove, check alignment (place on flat surface and look from side, or if the fork tips are level, measure the blades for level near the crown.

Third, with forkends bottomed out, the steerer should be centered on the mid-line that bisects the fork. You will need some sacrificial headset or mechanism that goes over the steerer and protects the tube from ovalizing or thread damage while we apply lateral force on that fork. Ideally, you'll have an alignment table with level fork attachment mechanism that is VERY SECURE. Holding the forkends in the clamp, and using a long cheater tube over the steerer and protection mechanism, pull on the steerer tube sideways until it centers on the mid-line.

Lastly, the fork curvature or offset angle at the crown should be set to achieve the intended trail. This is best done with a rounded block of wood under the fork that is high enough to raise the fork off the table. Usually, the fork is laid concave down on the rounded surface and force applied to the fork crown and upper fork blades. For straight blade forks, a straight block of wood the same height as the fork tip mount will work and force applied to the steerer tube.

This only works for steel forks. Don't try it on aluminum or CF forks and never on fork with some type of epoxy bonded steerer.

Park discontinued the tip alignment reference shop tool .. it offered a T where the axle is in the dropout.

Older shops may have one still..

The Park "FT-4."

This is what it looks like, if you spot one in the back of your LBS:

http://www.os2.dhs.org/~john/park-ft4.jpg

this sounds like it might work. but I'm not exactly sure I'm picturing it right. how do you hold the fork tips while still allowing the pry bar to push one forward?

Old school mechanics kept a piece of granite, marble, or a glass pane to check forks for squareness.

Lay the fork on a flat surface with the crown, or blades just below the crown against the corner, and the tips on the surface. If all is jake you'd have 4 point contact, like an OK table. OTOH if the blades aren't squared up, it'll rock like $5 card table. Note the bold If, this isn't a bullet proof test because there are other variables, like front edges on dropouts that aren't perfect. But it's a reasonable place to start.

Next you need to check for side to side deflections, and/or whether one dropout is higher than the other. Here's a way to start. Mount a QR axle into a 2x4 so you can clamp it above a bench vice, or use one of these. Put the fork on making sure to seat fully to the top of the dropouts, and with the steerer leaning on the bench. Mark the position, repeat with the fork flipped. A good fork will end up in the same place.

Unfortunately if the fork fails, it might be a high dropout, or a deflection of both blades to one side. It's very hard to separate these because they cause similar effects, but sometimes a good eye can tell you. Yo help your eye, put a straight from dropout, past the crown and out past the end of the steerer. Measure the horizontal distance at the end of the steerer. Compare to the same test on the other blade.

When finished cross check on that sheet of granite.If you don't have any flat glass, you can do this in a cemetery, the same way I had to in Vermont after a crash 40 odd years ago.

A length of 2x4, weight lifting bar, hockey stick, ect will act a a good enough bending bar to twist the drop out ends of a fork back to being parallel to the crown area. Often this will slightly spread the drop outs wider then they were before. I've posted a few times in the last year how to check fork alignment. With a good wheel, a bench vice and some slats of wood it's not hard, just takes a focused mind. Andy.
PS- If the bike tracks straight then don't wory too much about this, except for making the drop out faces parallel to each other so you don't wear the bearings prematurely.