The Chopper Compendium

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.

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

Thanks. I guess i need to read back through it again.

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.

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

Thanks Gary. Probally will step up on tubing thickness just because. I'd rather have over kill than not be thick enough.

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.

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.

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

Hi, I am in the process of laying out a conversion for my 2000 Heritage Springer. Being an FLSTS, the brake is on the left side. I would like to add another brake to the right side by duplicating the left side. What I'm asking, is if anyone knows if the fixed tubes are solid or hollow, if hollow what is the wall thickness ? I am also interested in how they were originally manufactured - pressed, brazed, or welded to the cross bar.
Thank You

main legs are hollow . old HD tech was brazed with reinforce tube . the modern version , you could ask on modern HD forum

No joy on the modern forums. Thanks, for the reply.