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Spreading Forks

Old 09-24-16, 08:09 PM
  #1  
byrd48
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Spreading Forks

Hi,

My bike has 96mm fork dropouts. I have been using 100mm hubs for a few years with no issues, except for the hassle of having to spread the forks when installing the wheel. I used a dropout alignment tool tonight and spread the forks out 3 times to 115mm, and every time, they snap right back to 96. Should I leave it in place overnight? Is there a better way?
Thanks!
Jon
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Old 09-24-16, 08:17 PM
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time isn't a factor, but strain


you have to exceed the elastic limit to get permanent set

e.g., cold-setting rear triangles you spread/relax, spread more/relax, etc., until you find the elastic limit.
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Old 09-24-16, 08:19 PM
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Steel has a property called spring back. It's the same as you flex anything elastic. First you're working within the elastic range where it flexes but will return to it's original shape. Beyond the elastic limit, it'll bend permanently, but the same spring back will remain.

So if you want to spread your fork 4mm, you have to 4mm beyond the elastic limit, then when you let go it's spring back, but end up the same 4mm away from where it started.

It's hard to feel when you're working with a screw that way, so I prefer to use a gauge, and bend by hand where I can feel the point at which it's bending. I know because it requires progressively more force to flex it, then suddenly it moves withour needing more force.

I also prefer to bend each blade independently half the distance, gauging on the other. If you bend both together, the weaker one will bend while the stronger one is still in flex mode, so you end up with one blade doing all the bendiing.
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Old 09-24-16, 10:42 PM
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Thanks, I used the frame tool because I felt like it would be easier to spread both sides evenly. It seems that the elastic point is beyond the point the tool can move the forks. I certainly could slide pipes over the blades, but how do you keep them aligned to center?
Thanks!
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Old 09-24-16, 11:00 PM
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Originally Posted by byrd48 View Post
Thanks, I used the frame tool because I felt like it would be easier to spread both sides evenly. It seems that the elastic point is beyond the point the tool can move the forks. I certainly could slide pipes over the blades, but how do you keep them aligned to center?
Thanks!
No pipes, no tools, unless you have the strength of a 5 year old, you can do this with your bare hands. If you have a vise, that'll help, but it's not needed.

Take an axle and bolt it to one dropout so it just reaches the opposite side, and that's your gauge. Now brace up the fork however you can, or I prefer to carefully brace the fork on the floor and stand one foot on it and grab the upper blade near then end and gently lift. Be careful here, you're stronger than you think, so when you first feel the fork give, STOP. Your reflexes are probably about 1/8" movement slow which is perfect.

When you do this think back to your childhood and breaking turkey wishbones. As you probably learned back then the one holding it farthest out wins. Same with the fork. Stand half way up the blade, and pull the moving one from the end.

Once you've moved one side half the distance. Reset the gauge and move the other.

BTW I forgot to mention, if working on the floor, it's easier to set the gauge in the upper blade, so the other can lay flat on the floor.
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Old 09-24-16, 11:07 PM
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Thanks!
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Old 09-25-16, 04:57 AM
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I've used Sheldon's method a few times and it worked fine. Best to check if all is in line when finished and he gives a way of doing this too.
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Old 09-25-16, 09:46 AM
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All Thread rod and several nuts, and some wrenches, will exert the outward force even easier.


jam together a pair of nuts to hold the rod from rotating , then unscrew a separate nut to press outward from between the blades/dropouts.



./.

Last edited by fietsbob; 09-25-16 at 09:51 AM.
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Old 09-26-16, 03:19 PM
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Follow FBinNY's instructions. The threaded rod method risks unequal bending.


You have a fork, not forks and it has fork ends, not dropouts.
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Old 09-26-16, 09:28 PM
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It was used successfully, in a much greater spred to put a Hub motor in a Brompton

See Folding bike archives ..

He took the 74mm spread out to 10cm.
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Old 09-26-16, 10:05 PM
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Whenever I need to spread frames or forks I first establish the alignment base line. With a fork if I find that the fork is splayed to the left side I will bend out the right blade first. Then recheck the splay/alignment and drop out width before the next bending.


I figure if I'm bending metal I might as well do as little as possible to both end up where I want to be WRT alignment and axle fit. Andy.
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