Springer leg tubing

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Swamprabbit1
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Springer leg tubing

Post by Swamprabbit1 » Thu Nov 27, 2014 7:59 pm

As springer legs get longer in length. Does the tubing diameter go up in size or does the tubing wall thickness increase? Is there a rule of thumb for tubing to length? Thanks guys.
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hansgoudzwaard
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Re: Springer leg tubing

Post by hansgoudzwaard » Thu Nov 27, 2014 8:39 pm

Gary W Has a section on that topic on his article about stringers on chopper handbook.

Swamprabbit1
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Re: Springer leg tubing

Post by Swamprabbit1 » Thu Nov 27, 2014 11:15 pm

Thanks. I guess i need to read back through it again.
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gww25
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Re: Springer leg tubing

Post by gww25 » Fri Nov 28, 2014 9:46 am

You can use one of the online deflection calculation programs to compare different combinations of tubing for various lengths compared to a 'known' set of forks. I used to build to specific length using different walls (some very thin) but it became such a pain in the ass to make a bunch of different sized lugs that nowadays i just use .25 wall for almost everything. For the skinny little retro forks, once they get past 6-over, I use 1x.313 and 1.125x.313
People not familiar with tube (or bar stock) fork legs expect them to be 'stiff' and are pretty surprised to see how much they flex. I try to select something that has about .08 max deflection at 150 pound load for any given length so you can use that for a starting point. The real limiting factor for bar or tube legs is the yield strength of the material as that determines how much the legs can deflect and still return to being straight after the load is removed. This is the reason some builders use 4130 but even those who do still have failures so it's no guarantee against having the forks over-stressed. If your using DOM make sure it's 1026 and not 1020.
I have friends who've been riding on 48-inch long forks made from .188 wall Dom for thirty years with no problems and I also know of guy who bent a set of solid 4130 legs that were only 36-inches long.
Just for your info both Dick Allen and Sugarbear designs use solid 1.25-inch 4130 no matter what the length but that doesn't limit deflection as all steels, even 4130, deflect the same under the same loads but yield strength determines how far the deflection can be before permanent deformation.

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gww25
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Re: Springer leg tubing

Post by gww25 » Fri Nov 28, 2014 10:11 am

By the way here's a link to a pretty good video that shows how much legs can flex. It's a long vid and it takes a while to get to the 'riding' part. The other interesting thing is that even though both of these Sugar Bear front ends look different and have different rocker designs both bikes are set for for Zero trail geometry. (this is what Al says, but I've measured some of his front ends and found that most are really set up for 1-inch of trail).
https://www.youtube.com/watch?v=GEbGK1X9OJg

Swamprabbit1
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Re: Springer leg tubing

Post by Swamprabbit1 » Fri Nov 28, 2014 11:18 am

Thanks Gary. Probally will step up on tubing thickness just because. I'd rather have over kill than not be thick enough.
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gww25
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Location: Murphy, Texas
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Re: Springer leg tubing

Post by gww25 » Fri Dec 12, 2014 7:26 am

Springer leg material has always been a shot in the dark as the 'normal' engineering calculations just don't seem to apply. Remember that both Freddie and Mondo and Jeri still use 1.25 o.d. by .125 wall 4130 for forks up to 30-over. A lot of small shop builders use 1026 with .125 wall as well. If you do the calculations these tube specs will not work yet they're working just fine in reality. If this spec works then why does Sugar use solid 1.25 yet if you compare Sugar's to Mondo's side by side on the highway both sets of forks behave and flex virtually identically.
I'll post up the calculations for various configuration when I have time later this weekend.

Swamprabbit1
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SELF INTRODUCTION: I'm from Arkansas. Farm equipment mechanic. Love working on bikes with my 7yr old daughter. Single dad. Doing it all on my own. Good standing member over on the Honda rebel forum and TCU forum. My first chopper build was a Honda Rebel 250. Looking for a big brother to it. A Honda Rebel 450. I'm wanting to build a girder for the 250 and was told that this is the place to come for knowledge and help.

Re: Springer leg tubing

Post by Swamprabbit1 » Mon Dec 15, 2014 10:51 am

Thanks Gary. This will be going on a small bike. I will be building another Rebel 250. I know this is not the big HD's like everyone else builds. But they are fun to ride and go unnoticed in many circles. Because they are little Rebels. Which makes it even more of a reason for me to build them.
Again thanks.
Through the Gates of Hell for a Wounded Marine.
Devil Doc

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Re: Springer leg tubing

Post by jbfrmca » Wed Jan 06, 2016 12:44 am

https://www.youtube.com/watch?v=qM6zhH-0-fs
very cool seeing it flex and seeing the springs in action.

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