Topic: Keeping rear inline when hardtailing

[NastyBobber]

Hey guys i was just wondering if there is a technique to keeping the rear hald of the bike perfectly inline with the front when i cut the back of my frame off and weld in my hardtail. Im building the hardtail from scratch it is not pre fabbed or anything. Should i build some sort of jig for the whole bike to keep it in line or is there a better proven way. BTW im hardtailing a 76 CB750. Thanks

[greenjeans]

Going to need a jig of some sort.   Gotta keep things inline.

[NastyBobber]

Hmm yea but i wonder how i should do it, i mean if i sat here and actually thought about it i could certainly come up with something. Im just wondering how other guys did it with there bikes. Its not even bike specific really. Any ideas guys?

[faux fiddy]

Hmm yea but i wonder how i should do it, i mean if i sat here and actually thought about it i could certainly come up with something. Im just wondering how other guys did it with there bikes. Its not even bike specific really. Any ideas guys?

A friend used to set frames up clamped to a bridgeport to bend them into shape. Some things aren't even perfectly straight from the factory. He is able to use the machine to measure head tube perpendicular to the rear axle.

[Kong]

If you have to ask you really shouldn't be the one doing the work.  A frame is a very serious thing.  Your life, maybe the lives of others, depend on it.   That said I can tell you what you are actually doing here.  You are building a basket case for the next owner, one that will come without a frame.

[andy750]

If you have to ask you really shouldn't be the one doing the work.  A frame is a very serious thing.  Your life, maybe the lives of others, depend on it.   That said I can tell you what you are actually doing here.  You are building a basket case for the next owner, one that will come without a frame.

Well said Kong.

[NastyBobber]

Iv been welding for years and was just wondering what the best way is to keep things in line?! im not some hack doing shady work here?!? I have never hardtailed a bike and was curious about the correct way of doing it. Im not going top pay someone else for something i could easily do myself. All i need to know is how i should keep the rear in line with the front? ill probably just end up fabbing a jig.

[Skunk Stripe]

I think part of this equation is how are you hardtailing it? You could just go the simple route and put on struts which would allow for quick return to stock if you do not like it. Maybe try that first to make sure you like a cb with no rear suspension. You would not have to worry about a jig or anything then.
Where you going to kong the frame too? No reference the kong the naysayer poster.
http://konga750.blogspot.com/2008/08/konging-frame.html
http://www.hondachopper.com/garage/konging/konging.html
I would probably read the hondachopper first but the other one seems to have better pics.

[johnrdupree]

You'll need a jig.  Do a Google image search for examples of how other folks keep everything in place.

It sounds like you want to have the wheels (or at least axles) in place, cut the parts you don't want, and weld the hardtail bits in.  This limits you to a stock wheelbase, though.  If you want to stretch it, I would imagine that involves creating a rear axle carrier that you can position where you want and a ton of measuring to make sure it's square to the bike.  Engine mounts may be your friend in that case.

Good luck, and post pics.

~john

[Hinomaru]

NastyBobber, Here's some CB750 hardtail links that might be useful:

http://www.hondachopper.com/garage/frame_alterations/frame_alterations_1.html

http://www.hondachopper.com/garage/csc_frame_jig/csc_frame_jig.html

http://www.tcbroschoppers.com/store/index.php?main_page=product_info&products_id=8

http://www.cycleonemanufacturing.com/id227.html

http://www.kingcustomcycles.com/home.html

[splitt]

I'm not an expert at building frames, but I have hard tailed a few choppers.

It depends on where the frame is cut for the hard tail. I put a cycle one mfg. hard tail on my cb750. It attaches it to the front frame rails right forward of the lower rear motor mount bolt. I first cut the lower frame rails & backbone, then attached the motor to the front frame section using the front motor mounts. I slid the slugs attached to the hard tail section into the front frame section, then I installed the rear motor mount bolts loosely. I triangulated it by putting a 3/4" pipe through the axle slots then duct taped the pipe in place at the front of the axle slot. I measured the pipe 24" out from each axle plate & marked the pipe. I placed a piece of tape on the top of the neck & marked the center after placing a straight edge on top of the backbone. I then measured from the front center of the neck to the two 24" marks on the pipe. I adjusted the frame until both measurements were equal, then tacked the hard tail to the frame. I removed the motor & welded the hard tail in place. I would weld the slugs in place by welding one rosette weld, then going from one frame rail, to the other frame rail, to the backbone, then back to the first frame rail so the temperature would stay even to keep the frame from warping.

If you would like to find out from those who build frames & hard tail sections, ask in the Q&A over at www.hondachopper.com for more experienced opinions.

[faux fiddy]

Other than what I mentioned above, (jig of sorts)  what I know is from this site and others. Search for Kong, Konged, or 'Konging."


http://forums.sohc4.net/index.php?action=printpage;topic=34720.0

http://forums.sohc4.net/index.php?topic=65169.0

[754]

If you are an experienced welder , building a jig should not be a problem, just think it thru..

 Whatever you use to hold your frame. I would work off the engine mount holes being level, with engine out, and work from there. Keep in mind, unless you have a very rigid fixture, frame may require minor tweaking after welding.
 Something tells me you may not be ready, I mean you have not even mentioned what you want to build off of, ie table , or I beam, etc.  There is many ways to do it IF you FULLY understand the task at hand.

 If you need a lot more info than has been given, you may not be ready for this...

[splitt]

If you read about the frame manufacturers back in the 70's, some of them simply had the frame layout drawn out on a sheet of plywood with a center line down the middle. They would then clamp the tubing to the plywood before tacking it together. They placed stepped cones with a hole in the center on the top & bottom of the neck, they placed a broomstick with a point ground at one end through the hole in the cones. After they wiggled the tacked together frame around to line up the neck with the center line of the plywood they would weld it up solid. That piece of plywood was there only jig. They built thousands of frames this way. I could never decide whether this was ingenious or just scary.

I have also seen articles about hardtails built using a stock swingarm to line everything up. I'll see if I can find the article on how to do that.

[splitt]

I found that article here:

http://www.network54.com/Forum/152868/message/1062039794/How+to+chop+a+japbike

[754]

Nasty Bobber, What do you have in mind so far
 How much stretch?, how much lower at axle??..Raked by rear section or not.. or other means of rake?

 Give us a bit of your plan..

[NastyBobber]

Wow guys thank you very much, iv got more than enough info here to early get this job done. I plan on doing it most likely within the next few weeks. Depends if i have time due to work and many other projects going on right now. As far as my plan i am going for a semi low and long bike. not sure if i want to rake it yet but i will decide that when i start mocking things up. This was just one thing i was curious about and basically the only thing i really needed input on. I will most likely stretch the bike 3 or so inches and drop it possibly 2. Like i said i will find out all that when i have the bike sitting in front of me in the mock up stage.

Thank you guys very much for all of your help with this, all of your input really helped me out. cant wait to get this show on the road. By the way its a 76 CB750 if you guys were wondering.

[NastyBobber]

The way the guy does it in that article with the magnetic angle finder seems like a pretty accurate way to do it also. I like it

[OldSkul]

Or you could go with a Cycle one harrdtail. Its built in a jig and uses the engine case for alignment.
I'm running 5 inches of stretch.
 Alignment on my jig was a waste of time once its bolted to the engine its square.
 Making brackets and fitting the gas tank, oil tank footpegs are so much harder.

Oh and the whiners saying you cant do it need to read the Worlds Fastest Indian, that "Hack" was doing stuff modern engineers and fabricators couldn't touch.
Talent and perseverance rule!

[NastyBobber]

Haha yep crazy stuff goes down out there just to get a bike on the road and looking decent, Back in the day every guy that chopped his bike was practically riding a deathtrap haaa. Those guys botched there setup together and ran it. I will look into that Cycle one hardtail your talking about. Thanks