GSX750 chopper#2, tank and wheel mounting, seat rails...

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Prof
Founder, Choppers Australia
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GSX750 chopper#2, tank and wheel mounting, seat rails...

Post by Prof » Tue May 17, 2016 10:51 pm

As mentioned we are using a late model Sportster tank. It needs the filler cap moved to the top of the tank, the petcock shifted to the lowest point, new mounting brackets and some panel beating. That means heat and sparks which combined with a petrol tank means meeting your Creator real quick unless some precautions are taken. Don't fool around with empty fuel tanks and sparks.

Tank needs to be thoroughly washed out with detergent. Make sure there is NO fuel smell as washed out tanks have blown up and cut the unfortunate fabricator to pieces due to fuel still held in the seams!!
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Where I can, I initially fill the tank with water while cutting of welding. Car exhaust can also be run into the tank to expell oxygen and prevent ignition. Before the first cut, I play a flame into the tank with it tucked behind the steel workshop door...
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Petcock on this model tank is mounted at an angle, so it needs to be cut out with the angle grinder and a small blade...
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Filler presents the same problem. A hole saw cannot be used because of the offset 'unleaded only' filler hole...
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Now for some panel beating. Tank is marked against frame. Not sure how this will go as the existing folds are very angled and the seam runs through the middle of the new edge we have to make. I use heat and a small ball pene hammer. There is a bit of a trick to getting this area set down with a minimum of crinkles and cracking but not easy to explain with out diagrams. Maybe next time!...
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Basically finished. It has gone well except for the tear you see on the left and a small rip right at the rear of the tank. I'm very pleased however as this was quite a difficult job...
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Back of tank now sits down as it is supposed to. Front is lifted a bit over an inch...
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Now to set up the frame to take the rear of the tank. James has agreed to a hidden rear mount. Frame needs to be well boxed in this area as the steering head will want to push the rear of the backbone down where it meets the two tubes. A 4mm plate is shaped and welded across the rear of the backbone to provide a larger weld area at the junction. Space is left underneath for hidden wiring to coils and headlight to run through the backbone...
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Bottom of box section is cut and bent to shape...
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And welded in from the inside...
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Pattern for top plate now made...
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Marked out and the curved sides cut using a small left over blade (bottom blade compared to full size blade still on the angle grinder...
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Curve is bent by hand in the vice...
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And tacked just enough to hold it in place while we mark it and then pull it off to cut a hole...
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I need to weld the tears in the tank, so weld in the petcock hole while I am at it. Nail welded to the piece allowed me to hold it in place to tack it...
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Now for a solid piece of 3mm plate in the bottom of the tank to hold our mounting bracket...
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In place...
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We can now begin fabricating the actual rear mount. Measuring to find centre...
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The tank will have a 5mm tab that locks in under the box section. Marking out a square hole in the box section...
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Cut out square holes by drilling a hole in each corner and cutting up to them with a small angle grinder blade...
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Cold chisel (another one of those forgotten old school necessities) out the waste piece of plate...
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Square file to clean up the corners. Old blunt files can be bought at swap meets and 'sharpened' back to new in acid...
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When you use the file, only put pressure on the forward stroke...
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The under side of this box section needs to be reinforced with a piece on 30 x 30 x 3 flat bar. Always cut on the horizontal. Practice the skill of cutting to within the last half mm so the piece doesn't fall. You can then just twist if off...
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Reinforcing bronze welded in place...
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Tank needs to clear the frame by 2-3mm. We use some blue tack to work out out spacing. This determines the thickness of the rubber cushions that will support the rear of the tank and protect against vibration...
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A bit of measuring to allow for a 5mm strip of rubber on the tank tab and we can bend up the tab adn tack it to the tank...
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Everything double checked and final welding done...
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Now for the front mount. We use a piece of tube whose inner diameter is a couple of mil larger than the outer diameter of the back bone. Extra diameter allows for a thin piece of rubber (inner tube) between tank and frame. Use a dividers to get a nice curve...
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Burning the paint off the front of the tank to allow for welding. Looks cool don't it!..
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Final clean up with wire brush and sand paper. The tank sits 30mm above the backbone, so we make a filler piece to weld into the gap...
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Bike level is double checked and then level used to get the tank in the right position...
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James needs to be able to get an Allen key into the front mounting bolt, so we use a set square to make sure it will clear front of tank...
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Front tab now welded to filler. It protrudes a couple of inches into the tank and that is welded in as well...
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Piece of inner tube punched out for the mounting bolt...
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Tank again levelled and marked out for location of mounting bung...
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Hole position is punched, centre drilled and then progressively enlarged with the array of drill bits seen here. Progressive drilling reduces likely hood of drill bits wandering...
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Threaded bung. Notice that the thread beings 10mm down, so bolt can't be cross threaded. It has a thin lip to prevent it falling into the hole we have just drilled. If you look closely, you can see that the high area around the hole has been hammered down a couple of mil so the bung doesn't interfere with the tank tab. Long bolt in picture is screwed into the bung prior to welding to make it easier to eyeball it and keep the bung square to the frame...
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Rubber cushion at the rear of the tank, just forwards of where the tab contacts under the box section so that...
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The front of the tank sits up 10mm and must be pulled down by the mounting bolt. This will keep the rubber mounted tank firm and not wobbly...
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Bolt, stainless washer and rubber washer (inner tube also) pulls the tank down perfectly...
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Not as simple as a couple of bolt holes drilled through a couple of steel ears, but the tank is rubber mounted so should last a lifetime and it sure looks a lot neater and allows for much easier cleaning.

If James wants to put some time into it, the front tab can be moulded into the front of the tank and the back bone moulded to the top of the tab is level with the frame and looks almost a fixed part of the bike.

Truly righteous classic choppering I'd say...
Chopit'nrideit... Prof

Bearcx
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Location: Gawler, Sth Aust
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Re: James GSX750 chopper #2, tank mounting...

Post by Bearcx » Wed May 18, 2016 5:29 pm

Wow, Andrew, That's a lot of work for a tank mount. Agreed, it will look sweet, and last forever.
Would it have been easier to cut the rear out, and make a new piece, instead of hammering and heat.?
The brave may not live long, but, the cautious do not live at all.

Prof
Founder, Choppers Australia
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Joined: Sat Oct 22, 2005 3:54 pm
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Re: James GSX750 chopper #2, tank mounting...

Post by Prof » Fri May 20, 2016 3:17 am

Not as much extra work as you might think Bear. Area had to be boxed in anyway. Rear mount had to be centred. Rear of tank still needed panel beating and reinforcing and a tab of some sort made. Plus a threaded bung would have had to be machined and welded in, plus rubber top and bottom.

So now onto the rear end. Original shockers were too long and not able to be shortened. James bought some off the net and they did not have a clevis and the bottom plus spring rate seemed to be for a very much heavier bike.

I suggested a set of CB750 K series shockers which are easily procurable as after market items and can easily be shortened. I had a good second hand set on hand so we went with them.

Once dismantled the thread on the plunger rod is lengthened (see white arrow) and then an inch cut off...
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Three quarters of an inch is cut off the spring...
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... and it is then heated and tapped down to give a nice end. The last couple of inches of the top coil will then have a progressive flat ground on it...
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To work out where the guard needs to sit and how far back the rear frame tubes can go we mount the shocker on the swing arm and extend it to full extension and mark the spot. Wheel is brought right forwards on the axle plates (not yet done in this pic). This is where the wheel will be when you put on that new chain and needs to clear the guard. Then we collapse the shocker and pack the wheel up on extra blocks until the top mount meets the mark at the rear frame tube. This is where the wheel will come on full compression when you hit that big pothole the councils so obligingly create just for us bikers...
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Slugs are made for the extensions to the rear frame tubes. These are slightly larger diameter to fit the existing tubes. Tubing we are using to extend the frame is slightly thicker and the two pieces are cut from an old swing arm I have on hand...
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While we are at it we cut back the rear fender; an FX rear guard. One side is cut and then a paper pattern made. Dirty fingers do the marking and it is then cut and traced onto the other side...
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New lengths of tube are cleaned up and slipped over the slugs...
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Until now we have been using a CB500 wheel, but now we install the FX rear wheel we are going to use. James wants a classic chopper with spoked wheels. We are using a CB750 Honda front wheel and the HD rear hoop. Main reason for using FX wheel is that James wants to keep the disc brakes. The GSX runs a 48 tooth rear sprocket as does the FX. The 16" wheel is smaller diameter than the GSX wheel and we can get a 46 Toother for the FX wheel which will gear the GSX up a little... which for a cruiser is beneficial.

Wheel is centred using a plumbob (red arrow)...
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Space on sprocket side is measured with a calipers...
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Calipers is measured with a verniers and a spacer machined up out of aluminium...
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Right side is now measured and a temporary spacer made from ally also. Because James is not present while we are doing this work, I am not sure what rear brake caliper he will choose, hence the temp spacer...
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Now for bending our rear tube extensions. Each are packed with sand with a wooden block tapped into the open end and then bent using heat. Sand must be perfectly dry and the plug tight. Thin tubing bends easily and keeps its round cross section. A spirit level is used to ensure both sides are of equal height. Takes a bit of tweeking to get both bends the same but patience is the game here.

In this pic you can see the wedges we have used to centre the guard. Up until now we have not welded in the rear frame extensions. Now we can locate them evenly each side of the guard and weld them in...
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We could machine up some threaded top shocker mounts, but I have a piece of CB500 (from my original '74 CB chopper) rear frame with just what we need, so Jake cuts the mounts off...
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... and machines up a couple of collars which are then notched on the mill to fit onto the underside of the rear frame tubing...
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Now for careful measuring. Then a piece of half inch square tube clamped in place. Mounts will be placed against this when welding. It will keep them square and also allow us to line them up so they are horizontal...
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Mounts tack welded in place...
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James plans to spend some time here next week and we will then set up seat rails to his satisfaction. We also need to work out what he wants for sissy bar mounts. Once these things are worked out set rails, rear guard mounts and remainder of rear frame can be fabricated.

While this has been going on, Jake has been setting up the CB front wheel. We'll put up some pics of that next...
Chopit'nrideit... Prof

Prof
Founder, Choppers Australia
Posts: 5625
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Re: James GSX750 chopper #2, tank mounting...

Post by Prof » Mon May 23, 2016 10:12 pm

Forgot to show you how we pulled Jame's forks apart to put in the slugs. A lot of the later 80's Jap bikes had the forks topped with a alloy plug retained with a spring clip.

How do you get them out without copping a plug at 100 mph in your eye?

Here's how we do it...

Tool consists of a piece of angle iron with a piece of plate welded across one end (white arrow). This has a slot machined it for different diameter forks. A movable threaded plate (green arrow) is held in place by the piece of half inch UNF threaded rod that is screwed into the end of the forks to free up the plug and spring. Angle iron and fork tube are pulled tight together in the vice...
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With the plug pushed in, the retainer can be grubbed out with a couple of sharp objects (sharpened spokes in this case). The threaded rod can then be unscrewed to release spring pressure...
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Ok. Back to the front wheel.

We are replacing the GSX mag with a K series CB750 front wheel. They both have same axle size. The CB hub is wider than the GSX so we can't use the speedo drive, but I will suggest that James uses pushbike style wireless speedo. Small and no cables to uglify the front end.
Both brake rotors are the same diameter and very close hole pattern. However, the CB rotor is almost twice the thickness of the GSX rotor which will make it difficult to centre the wheel. So we opt for the GSX rotor...
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The centre hole has to be enlarged to fit the CB hub...
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The holes also have to be widened by about 2mm...
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GSX rotor on CB wheel...
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With the wheel centred, the rotor is 2.5mm too far to the left, so Jake mills that amount off the brake lugs...
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You can see the milled section in this pic where Jake is fitting the the caliper...
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Final check on wheel alignment...
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Spacers are now measured up (see how we did the rear wheel) and machined out of stainless. Left side first...
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... then right side...
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Wheel now set up.

James came up today and we built the seat rails, rear guard mounting and gusseted the steering head area.
Chopit'nrideit... Prof

Butcher
Posts: 26
Joined: Thu Jan 09, 2014 9:50 pm

Re: James GSX750 chopper #2, tank mounting...

Post by Butcher » Tue May 24, 2016 9:38 pm

Far out !!!!!!!!!!!!

Prof
Founder, Choppers Australia
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Joined: Sat Oct 22, 2005 3:54 pm
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Re: James GSX750 chopper #2, tank mounting...

Post by Prof » Tue May 24, 2016 10:03 pm

Before we can put the chopper on the ground, we need to stabilise the rear end with the seat rails. A good height from the ground at the rear is 480 - 520mm. We set try these at 500mm and check the angle we will end up with... 10 degrees. About right, more than 15 degrees and the fornt of the rails put pressure on the rider's inner thighs...
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Measure the length...
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We'll make them the same diameter as the rest of the frame. Often seat rails are made of smaller diameter tube, but I want to keep as much strength into this very stretched frame as I can...
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A final check on the width of the rear frame tubes and also to make sure they are centred. While we are at it, we double check shocker mount width. Here measuring shocker mount distance on the swing arm...
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The top mounts are about 10mm narrower, so we correct this by bending the tubes out slightly, cold, and double check they are centred...
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James goes ahead and cuts the seat rail tubing and shapes it to be a close fit where they joint the existing tubes...
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Seat rails are tacked. Important to line up the edges...
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Once first rail is tacked in place, we use a spirit level to make sure the second will match...
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Rear cross member measured and cut out of some 50 x 12 rectangular tube I keep specially for this purpose. It looks great because it has curved edges...
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Now forward mount for rear guard can be fabbed out of some 25x5mm flat bar. Top edges curved using for neatness, looks and for easier cleaning. Once welded in, scriber is used to mark position of holes. James also makes some threaded plates our of 8mm thick round bar to be welded on inside of guard as captive nuts...
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A second crossbar will join the frame at the top shocker mounts. Here guard is being centred. White arrow shows an existing hole we will use...
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Crossbar welded in place. It was drilled before welding. Another captive thread is tacked under the guard here as well. We are using stainless M8 button heads...
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That the rear end for now. But we have a look at where the sissy bar will sit. We need to organise a neat and strong mounting set up... something to sleep on...
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While James has been working on the rear end, I have been setting up gusseting for the headstem area. First tidy up the down tube junction with a curved gusset... jus the inside one for the moment. James is very keen to get the chopper on the ground before the his fiance arrives at 6pm. Carboard pattern...
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Three mil steel plate marked out...
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Once cut and smoothed plate is tacked in place making sure it is even. Ruler is run across the down tubes and I make sure both gaps are equal...
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Now for a curved gusset from the tank to the down tubes gusset we have just made. This gusset will be a bit different to the norm. Part of the original gusset still exists so I decide to do a square rather than round gusset out of three mil plate and flat bar. An equal reason for doing it this way is to lessen the visual effect of the very wide down tubes meeting the steering head. Width of gusset will be 30mm. I bend up a piece of wire to get the bottom curve of the gusset. It didn't show in the photo, so I have marked it in with a dark line.
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Bent wire allows me to produce the right curve and also to cut the flat bar to length...
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Flat bar being bent in vice...
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Cut to correct width and a chamfer put on the inside edge ready for welding...
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Tacked in place...
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Cardboard pattern made to match.. Arrow shows where thin cardboard has been pressed against the curved flat bar to get the curve...
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Three mil plate cut out with angle grinder and small cutting disc to get around the curve...
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A second one is made and both are tacked in place. Tacks are done gradually so we don't put any stress on the steering head. Will be fully welded later. Tacks will prevent any further movement...
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Here you can see width of gusset and how it reduces the visual effect of the wide area...
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We got the bike on the ground a couple of minutes fefore Celeste arrives. James is stoked and she approves as well...
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A couple more views...
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Rear view. Shockers will transmit a foward force (red arrow) White section of rear frame tube will tend to bend forwards and we will need to add some gusseting between tubing and guard to strengthen this area.

James wil be back on Thursday and we will set up the final section of the rear frame rails to carry the sissy bar (neat little solution will be presented to James) and then we'll make the seat base. Once his seat is made, we can set up the forward controls.
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Chopit'nrideit... Prof

Bearcx
Posts: 1898
Joined: Thu Oct 18, 2007 12:31 am
Location: Gawler, Sth Aust
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Re: James GSX750 chopper #2, tank mounting...

Post by Bearcx » Wed May 25, 2016 6:01 pm

Great stuff, Prof. Love all the details, and the photos. helps everyone out. Chopper is looking nice and low.
The brave may not live long, but, the cautious do not live at all.

Prof
Founder, Choppers Australia
Posts: 5625
Joined: Sat Oct 22, 2005 3:54 pm
Location: Willunga, South Australia
Contact:

Re: GSX750 chopper#2, tank and wheel mounting, seat rails...

Post by Prof » Mon May 30, 2016 11:44 pm

James was up on Friday and while I finished off the rear of the frame, he fabricated the pilot's seat and began setting up the electrics section... which we will show in a third thread.

So, the rear frame tubes need to run a bit further back for a final guard mount and to support the seat and sissy bar. Frame tubes are quite a large diameter... about 35mm OD. This will look very bulky as a finish to the rear end.

So I use some of the old tube that is 27mm OD. Still pretty bulky finish to the bike as James wants to sometimes run the bike without a sissy bar. Went to bed with the problem and had the answer the next morning... slice the last 90mm of the tube in half with a couple of threaded bungs for attaching the sissy bar.

First step... slicing the tube...
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Tubes sliced and length double checked...
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1" flat bar now shaped on the anvil to create the flat on the tube...
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Sits in nicely and then curve the end...
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Cut a few wedges and hammer surplus tube over to fit the curve and gas weld so we can easily fill the gaps...
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Frame tubes now need to be squashed with a G clamp...
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From the side the tubes now both line up...
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Looking from the top, we will line up the outside edges...
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Mark in the location of the threaded bungs. Note use of two set squares to line up both tubes at the same time...
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Marking centres with odd leg calipers...
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Jake turned up the M8 threaded bungs. Holes drilled and countersunk on flats so we can have a flat area for the sissy bar to sit against...
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James knocked up a pair of slugs from hollow bar. Hollow bar is used because solid bar will not provide any extra strength and it is better to have similar welding thickness of welded parts...
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Slugs are shaped on linisher to fit the oval cross section of the larger frame tubes and are thinned on top side to fit the curve of the frame tubes...
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Accurate set up requires a number of checks. Firstly tubes we are welding on have to follow the lines of the main frame tubes. Some threaded rod is used to get and to maintain correct distance. The flats also need to be vertical, so we use large washers and pull the nuts up tight to keep them square. I use thin threaded rod (M6 in M8 holes) so it can easily be removed once everything is welded. Otherwise we would have to unthread one of the nuts the full length of the rod; not going to waste that time..
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Frame tubes need to be centred, so after checking that guard is centred we measure from the hole in its centre to each tube...
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Threaded bungs and ends of frame need to be square to the centreline of bike. This is done by measuring to the centre of the threaded rod off the rear of the rocker boxes on each side...
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Finally, the frame tubes need to be level. A couple of straight edges allow us to accomplish this...
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Tack weld on top of each join and check. Tack weld on the bottom and check. Weld on each side and check...
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When James comes back in a fortnight we will drill a hole in the cross brace to line up with the existing hole in the guard. Then before welding it in, we will look at how to neatly make a couple of points to locate the passenger seat.

This chopper will have a seating set up similar to my shovel... a King & Queen seat, divided into three parts... a quickly removable rider seat (for access to electrics) a passenger seat (with a tool space underneath) and back pads on the sissy bar. That way when out on the road with his camping gear giving him a super cool back rest, he can access his fuses and tools without pulling everything off... and James plans to be doing some travelling on this machine...

More on all this when James comes back for another couple of days of work on a chopper he is rapidly falling in love with!
Chopit'nrideit... Prof

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