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  #1 (permalink)  
Old 01-20-2019, 11:06 AM
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Aligning the tail for the rear axle mounts

Hi everyone

I've formed the outline of a plan in my mind, and I thought I should check it out with some more experienced builders first. I don't know any chop builders in real life, so I thought I should ask here.

I have an old (1995) Japanese 4-inline-cylinder bike. I've had it for some time, and it has rather high mileage but it gets immaculately maintained. It also gets thrashed quite a bit. But I know that all of the specs are right, because I rebuilt the head, I do regular valve clearance shimming, I use nothing but the best motorcycle synthetic oil and change it frequently, etc. In short, in the garage the bike gets spoiled, and on the road it gets ridden very hard.

I know my way around this engine reasonably well. I know which parts wear out soonest, etc. The biggest issue with these reliable bikes is the fact of (new) parts unavailability. You just can't get new engine internals any more, anywhere. So I got myself a spare (used) engine, which I have yet to dissect and analyse.


1) 4-cylinder old Japanese bike with good engine, good clutch, good absolutely everything.
2) Spare engine for that bike, which most likely has a lot of good parts inside it.


To make an old-school hardtail chop, using everything I've got, with a completely refreshed engine that's good for another 100K miles.


Make a frame out of the existing frame and pieces of suitable steel pipe, which allows the standard rear wheel to slot in at just the right place. In other words, getting the back end all to line up.

Can get (if it turns out I need them)

New big-end bearings and new cam chain and new full gasket set and new individual gears (direct from Japan as these are standardised parts).

Can't get (will have to resort to picking whichever's better from the parts I already have)

New pistons, new piston rings, new cylinders, new head, new valves, new cam gears, new camshafts, etc. Most engine parts no longer available new.

Can do

Usual engine stuff: measuring, cylinder honing, valve lapping, head skimming, etc.
Cutting and welding steel pipe.


[See diagram]:

1) With bike on centrestand, weld (using a SMAW inverter-welder) one end of a piece of temporary pipe to the point along the swingarm just above the hole where the axle goes, and the other end to the subframe.
2) Do the same as (1) on the other side.
3) Remove swingarm spindle. The swingarm stays in place because it's welded there.
4) Take an angle grinder to the swingarm, cutting all of it off, leaving just the ends
where the axle goes suspended up there from the subframe. The remainder of the swingarm with its end sawn off should drop out.
5) Disconnect the shock, etc.
5) Weld the actual final rear frame pieces.This means cutting appropriate sections of steel pipe and welding from the axle mounts (which used to be parts of the swingarm) suspended in space to 2 places on the frame, completing the classic hardtail chop look.
6) Cut off the subframe.
7) Cut off the temporary pipe.
8) Complete (subject to checking for integrity/strength by a qualified engineer (not me).

What do you guys think? Is this a good or a silly way to ensure the existing wheel, chain, etc. will line up? Thanks in advance.
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  #2 (permalink)  
Old 03-14-2019, 12:17 PM
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A lot of questions to unpack.

First pipe and tubing are completely different.
Pipe is measured by the ID (inside diameter)
Pipe can be + or - .025 thousandths because it is used for construction.
Tubing is measured by the OD (outside diameter) and is + or - .002 thousandths!

Big difference.

You are not telling us what motorcycle you are using. It matters.
For instance a Honda CB 750 has a frame that is only 1/16" thickness tubing.
So making a sub frame means you will be using .120 wall tubing.
You will have to be very careful to not use a grinder on the stock frame tubing.
Only use a scotch brite pad to clean up the tubing you will be welding.

IF you are going to weld the swing arm in place it could be a problem.
There are bushings or bearings and if they get worn the frame could start having movement and eventually crack.
All bushings have som clearance.
I think it would be better to remove the swing arm and weld it.Just connect your sub frame to the main frame directly.
Take the center stand off of the bike and get the bike on a level surface so you can make accurate measurements.

IT is sort of a waste to cut the swing arm all up and just save the axle holders.
Waste of gas and waste of time. You can get axle plates from cycleonemenufacturing for $20.00.

Forget that "getting a qualified engineer" business. IF you could find or afford that person
you would be wealthy.

Go online and see videos of frames being modified to find out how to do this part of your project.
Find our what you need.

So to sum up. Get a flat build table or build one. Get the frame on it and clamp it down.
Make your measurements. Make something to hold the rear axle plates.

You can order pieces of steel DOM tubing online at Ebay . 9 foot pieces or shorter can be delivered by UPS. Make sure to clean the tubing before welding.

You must have the rear wheel and tire mounted so that you can know the height of the rear axle position.

Last edited by donald branscom; 03-14-2019 at 12:22 PM..
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  #3 (permalink)  
Old 03-14-2019, 07:49 PM
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Bike Year, Make, Engine: 2004 Ultima Chopper
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That project seems very doable. I went through the pipe thing when I was making the Shovel frame below..I used concrete reinforcement rod instead of pipe to make the initial jig..It is very easy to work with and easy to correct mistakes, or re bend if necessary..Once you have the hard tail made out of the rod, and is spot welded to existing cradle and lined up..then all you have to do is tack weld or clamp your tubing and bend around the rod using your can't go wrong as long as your jig is right..Reinforce your welds with some cool looking plating at stress points..forget the qualified engineered bs..The strength
is in the backing plates anyway.

The axle plates shown below , just 1/4 plating cut out with the torch..the trick in strength is the reinforcement pieces, and also gives it a cleaner rounding wound do the same where attached to the frame. Get some threaded rod and a piece of pipe to go in between your axle plates , same distance as your swing arm or according to the size of tire you want to run....and to level it up when you weld your plates to the hard tail, weld in as one piece..or forget about the threaded rod and pipe, and go ahead and mount your wheel in the plates with spacers and then weld to the hardtail..using a level vertically it would have to come out right that way..I think you mentioned that above...then finish off with some scrolly looking reinforcement.
all of the above would have to be done to the jig before you even started with your tubing or you will end up taking a year and starting over many times until you go broke....Once you bend a piece of pipe, that's it.

2004 Ultima 127 Chopper
1975 Harley Davidson Shovel head Chopper
Shreveport Louisiana

Last edited by Shovel52; 03-14-2019 at 08:43 PM..
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  #4 (permalink)  
Old 04-26-2019, 08:03 AM
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Can you tell us the make and model of the motorcycle and motorcycle engine.

Is there a reason you are not telling us?
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