Did someone change the dyna uper part square backbone frame for a round pipe frame? Only for easthetic reason.

Please don't tell me to change my bike !!

 

For this reason, the square frame ugly.

 

I mean this part

 

Anything is possible with good fabrication, but this would be MAJOR surgery. The "backbone" is a significant load-bearing part of the chassis architecture, and plays an important role in handling and structural integrity. It's not just a place to hang the tank. The original engineers would've calculated potential loads in all directions (while allowing room for the bulky wiring harness that runs through this area), and probably settled on box section for a reason. Again, anything is possible with good fabrication... But all those same factors would need to be accounted for.

 

You're right, in fact it handle pretty good. But is it possible that the frame is overdimensioned for unusual use? like whellies and jump.

 

I agree... They probably didn't take jumping into account!

 

My '60 chopper has a round tube. If, I thinking that's a big if, you leave your motor stock, and only doing up to 75 mph, it will be fine. Bump it up to 100+ hp, go 100+, well, you might start having problems. You might be better off going with an aftermarket frame:

http://www.paughco.com/2008-catalog/...20Paughco.html

Dave

 

Cheaper and easier to bend round tube. I'd bet they had a good reason to go rectangular.

That said, I don't see why you couldn't do it. Would take some figuring on how to blend it together at the neck and seat areas, but entirely doable.

What is your concern? My stock backbone is pretty exposed (for a Dyna) and looks fine after a little bit of minor welding, grinding, and paint. It's not as svelte as a classic rigid but at some point ya gotta embrace the bones of the project vehicle.

 

Modern designers model stresses on a computer before cutting metal. Modern cruiser and sportbike frames don't look like they do by accident and often have more parts than traditional frames. Steel is trivially cheap in bulk and so is laser cutting gussets etc. The large backbone diameter is for rigidity. Harley knows traditionalist owners would prefer the old Shovel frames remain forever but they aren't nearly as stiff as they look (besides horrible rear suspension geometry).

There's at least one Dyna with a round backbone mod on Chop Cult which is a useful place to learn your options and has many swingarm chop threads. A good fabricator can do the job (this is NOT 120V fluxcore offshore wire welder work!) as can you if you are an experienced weldor but then I doubt you'd be asking the question. Downgrading to a somewhat flexier frame is no big deal but trying to run a Sporty tank (which every chop owner and his kid sister run today for mysterious reasons) will never look quite as good as on a traditional frame. If you want a traditional look while hiding more backbone you could dish a set of Softail tanks instead (dishing is very retro and most people are too lazy to do it so your scoot will be different) and gain fuel while hiding the ugly. You can also carefully remove the neck gussets and gusset the neck like a traditional chopper using the same round tube neck. It's not really a lot of work other than pulling the tank and forks then covering the engine with a welding blanket. Cover your VIN on the neck tube to protect it from spatter.

Totally stripping the bike and using a frame jig is the best method but plenty of accurate, professional welding is done in the field with inclinometers and laser levels. Do NOT make a motorcycle your first attempt at doing it that way or have your non-pro buddies do it. You can also tack weld a brace outside the area to be cut then remove it after the job is done. There are many ways to skin that cat. Find a real weldor locally and show him what you want to do.

Check your messages. BTW I suggest changing your nick to omit your email address lest bots flood your inbox with spam.
You can run an aftermarket frame if you don't mind jumping through the hoops for a special construction title and those usually still need minor fab to get what the owner wants. Go get familiar with chopping.

 

Post pics of your work, curious want to see your outcome..