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Discussion Starter #1
hey guys, i've been hovering around the forum for a while now, but i just signed up. i looked through the thread with all the bikes, there are some great rides out there! I have a 1975 Yamaha RD250, which im doing a cafe style restoration on. I plan on replacing the seat with a smaller, flater one. i was wondering how on some bikes, especially hondas, the frame at the tail is curved at a 180 degree u bend setup. Is this a factory setup on honda's? how would i be able to modify my frame to have the curved rear? Here is a pic about what i mean.
*if this bike belongs to anyone on the forum, i hope it is okay to use, if not let me know!*

thanks for any help!
 

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Most Japanese bikes do not have that curved frame tail - it is usually custom fabricated to "finish" off the look. The look is copied from british and european bikes which used a ubend end of the frame to attach a stock rear fender.

if you look at this pic of a stock norton commando you can see the rear frame loop is used to attach a fender, a light, and a license plate bracket:



my only guess as to why it became a cafe thing is that brit bikes were the first cafe racers and if you build a seat around that loop it looks good and provides extra suppot and mounting.

The problem with the pic of the honda you posted above is that with brit bikes the engineers measured for tire clearance, but hack jobs don't always and I am willing to betcha on that little cb the first thing that hits when the suspension compresses is the tire against that rear hoop.
 

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Geeto - spot on. The guy who had my Ducati previously shortened his loop and it was rubbing on the tire at times...I just bent and welded a new section to restore it to the original length.

Ring - I don't have much experience welding or bending and it was pretty damn easy. If you wanted to add a longer loop, just take your time with the bending...use a hydraulic press and then just heat the metal with a torch and manually tweak it for the finishing touches. Again, take your time and let the metal cool down on its own...don't dunk it in water. Repeatedly cooling and heating metal too fast will make it less durable.











Cheers,
AK
 

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Discussion Starter #4
guys, thanks so much for that info! i was afraid someone would be snotty to me, especially because this was my first post. I really appreciate the help, you are great! I am going to try to work on the bike in school as i go to high school at a tech center in upstate new york. Thanks again!
 

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A metallurgical note.

For a reasonably low loaded part such as this rear loop, be sure to get a low carbon seamless tubing. Something like 1020 / 1018. Don't worry about stuff you may hear or read about more exotic materials or Ni-Cr-Mo steels, particularly if you are hot working and welding. Low carbon steels will have plenty enough strength, and they will not be as metallurgically affected by heat as many higher strength materials.

Even so, a good practice is to locally temper your welds at about 500 - 600 degrees F after they are welded. (Heat the weld and a small area around it to 500 - 600 degrees F, and let it cool slowly). This will toughen the material and remove any possibility of brittleness at the weld.
 

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Wow...talk about synchronicity! I just got the same info from an old welder buddy, he does my welding when it needs to be strong AND pretty.....I can do strong. Heh. Crazy part is....it came almost verbatim.

As for rubbing, take make a strut rod that measures what your shocks do when compressed and temporarily bolt you rear end in place. Then work up your loop. As noted...it's pretty simple.

The bike in the pic does not rub according to the owner. I asked about it during his build.
 

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quote:Originally posted by Swagger
The bike in the pic does not rub according to the owner. I asked about it during his build.
dude must not weigh very much (or be a midget). I can see he doesn't carry passengers since there are no pegs for them (or frame mounts) but if that is a cb450 (which it looks like) I can tell you from setting up the LSR chassis that the tire will hit the frame in weird places before the shocks bottom out.
 

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I should go into the frame loop bending business. I have the machine to bend those loops in about 30 seconds. Although I can only bend 1" OD and larger right now. I should get the dies to do 3/4" OD. Ohhh, and 7/8" OD so I can do handlebars :)
JohnnyB
 

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Hey Johnny - I'm sure you know, but the smaller dies you gotta watch out you don't pinch the metal and create a fold instead of a bend on the inside. I had to start with a big ol 5" die and work my way down or else the bar would pinch on the end of the die.

Anyhow - I'm sure you know what's what.

Cheers,
AK
 
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