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CB750 Rat Racer

November 20, 2013
By roccitycafe in Uncategorized

Ever hear the expression: “The cobbler’s kids have no shoes”? The more I work in the custom bike world, the more I find this to be true, ask any builder and I think you’ll get a chuckle and nod if you mention this idiom. When your entire day, week, month is dedicated to building awesome creations for your customers, it’s hard to find time at the end of the day to make something cool for yourself.

This spring, I happened upon an ad on craigslist for a 1977 CB750F for a price I really couldn’t ignore. I already had one mind you, but that one is my daily rider, “old reliable”, “grocery getter”, you get the idea, and it was not to be molested in any way which would take it off the road for any amount of time! I quickly bought this bike, justifying the extra CB750 in the garage by telling anyone concerned that “for the price I got it, I could just resell it come summer for at least twice what I paid”. Though this may have been the case, the truth was I just needed something I could play with for myself, I had my rider, so I could just go nuts on this one.


Rather than do a “full teardown” build, I decided to point in a direction with this bike, I wanted something fast, I wanted something fun, ergonomics was in, cosmetics was out, I didn’t want to shell out tons of bucks to do it either. Fortunately my day job happens to be building custom bikes and parts, so as far as the fabrication and machine work went, I could do all that myself. This formula turns out to work really well if you’re okay with spending a lot of time working on something.


The first thing I went about on this one was to replace the front end and brakes, I never liked the feel of damper rod forks on my other 750, and the old disc brakes weren’t all that confidence inspiring either. I happened upon another craigslist find of a ZZR1200 front end, wheels, fairly new tires and both sets of brakes for about what new tires would cost, so I jumped on that. I had a feeling I’d be up for some fabrication to make everything work, but I really didn’t have any idea as to the extent. Since the front end didn’t come with trees, I got to designing some of my own. I didn’t like the idea of converting ZZR trees to the CB750 frame because newer bikes use less rake and less offset to get the same trail, putting a low offset triple on a high rake frame like the CB750 would result in a bike requiring heavy wining and dining before even suggesting flipping into a turn, so that just wouldn’t do. I fired up the Bridgeport and started carving up some aluminum that would mate the new forks to the old frame and maintain the same trail. This took some figuring out on the computer, but after a little head scratching I was able to come up with a design that did it, machining the parts took about a day, by the end I was walking around ankle deep in aluminum chips.


With the front end done, the rear wheel was looking lonely, so I set about attempting to shoehorn it into the swingarm. As can be expected, it was nowhere close to fitting, between being too wide and nothing else lining up, it just wasn’t going to work. I’ll spare you the carnage, but after another 20 hours of designing, welding and machining, I had an alloy swingarm and modified sprocket carrier that would put the wheel exactly where it needed to be. This, of course, made apparent the fact that the oil tank was the wrong shape to accommodate a reinforced swingarm like the one I had just made. Again, nitty-gritty aside, I built a new oil tank.


With all that work behind me, I finally had a bike that was ride-able, ugly as hell, but rideable. Of course, ride-able and enjoy-able are two different things. With the stock seat and tank, there was just no way to ride this thing the way it wanted to be ridden, it was like having a trailer hitch on a Porsche. I figured changing up the seat and ditching the stock pegs would help for sure, and being not too shabby at fiberglass, I decided that would be the quickest route to a nice seat, after a few days I had something workable. The pegs were knocked out on the Bridgeport and Rockwell lathe, along with the levers welded together with stainless tube, all told about 3 hours of work, ugly as hell but definitely what I needed, good grip, folding, and in the right place.

The tank, on the other hand, was another story.


Riding this bike with the proper bars, foot controls and seat made the tank feel like it was trying to perform some kind of operation on me which I would rather avoid. Fire up the torch! The tank I designed really only had three requirements, I needed at least 4 gallons, it had to fit me, and if possible, I would like to shave some lbs. off the stock weight. After another few days of pounding aluminum, I had something that didn’t leak. At this point in the late summer, having put only a handful of miles on this beast, that’s really all I was hoping for… hold gas, no leak, good. While doing the tank, I also got the bug up my nethers to build a quarter fairing, not sure why, but I had a spare windscreen from a build years ago and decided somehow that this bike should be it’s new home. Fortunately the fairing didn’t take all that long so I told myself it was well worth the extra time.


Long story short, new (old junk) flatslide carbs, sandbent exhaust and *some tuning, this turned out to be a fun bike to ride, tucking into the fairing and opening the throttle is quite the experience indeed. I was initially surprised at how even a primitive fairing like mine would shed so much wind resistance. The high speed stability of the bike is also quite satisfying, my daily CB750 at high speed almost feels like a sneeze would topple her into an unrecoverable tank slapper, but with the newer suspension and rubber, along with a slightly stiffened frame, at speed, it’s almost boring. Fortunately maintaining the stock geometry keeps this bike fairly nimble for diving into corners.


On the note of new ideas, I’ve begun putting together a CB750 parts engine for this bike, I’ve become a little bored with the stock engine performance i.e. I’m not at all scared of this bike. Using the Bridgeport and a nice old Lisle hone, along with some Yankee ingenuity, I’m cobbling together a 900cc mill out of the CB750 husk and some other “junk”. I’ll keep you all posted!

*hours on top of countless hours of frustration and smelling like gasoline

Gas welding a fender

October 5, 2013
By roccitycafe in diy, fabrication, metalworking

cafe racer fender

I’ve been gas welding cafe racer parts for a few years now, and although I use tig mostly, gas welding is a great way to get started in metal fabrication. In fact, if I weren’t making parts for a living, I’d probably go with gas because its more portable than a tig, and the basic setup is thousands of dollars less. You will need better ventilation in your shop though, as gas welding puts out a lot of fumes and flux isn’t good for your lungs, where a clean tig weld doesn’t put out fumes at all unless you accidentally dip the tungsten.

Gas welding requires a cleaner metal edge than tig, tig will work fine with a fresh cut as long as there’s no grease on it, but gas welding really requires a bright shiny edge. I like to use a deburring tool to cut a chamfer on the edge of each piece I’m joining, this gives better weld penetration and also ensures a clean weld area.

Use some clamps to line up the edges for tacking, usually I’ll clamp one edge to start a tack, then tack down the weld line without clamps, as I can easily hold the parts in place once one end is tacked together.

I’m using a #1 welding tip for this job on 14ga 3003 aluminum. Since almost all my parts are the same alloy and thickness, I don’t need a lot of different equipment. Common aluminum welding/brazing flux works great, just mix it with water until it’s dissolved, I use about a teaspoon per ounce of water. One of the tough parts about gas welding is getting the flame the right heat and the right mixture. On a #1 tip, I like to have roughly a 10″ flame, it’s a good balance of enough heat to get the weld going and not enough to burn through too quickly. The mixture is determined by looking at the cone, or the bright blue flame coming right out of the torch, there are really two sections of the cone, the main cone and a tail, the tail is longer than the main cone and as you increase the O2, it will get shorter. For a good neutral flame (what you want for gas welding) the tail shouldn’t extend out beyond the main cone, so increase the O2 just until the tail meets the main cone, don’t go further or you’ll have an oxidizing flame.

Flux is best applied to a hot edge, so it doesn’t run away from the weld area. I give the weld edge a few passes with the torch and then brush the flux on, it should be hot enough that the water boils off immediately and leaves the white residue of the flux on the edge. Once the edge is coated, the parts are ready to weld.

For filler, I’m using .062 1100 tig rod, some people really like to use .030 welding wire, but I find this really too thin for my tastes, it’s not stiff enough and tends to melt before hitting the weld puddle. The filler rod also needs to be fluxed for a good weld, so what I do is just set the torch on the bench facing up, and hold the flux brush to one side, then run the rod across the flame so the brush rubs against it after it’s heated by the torch, any way you figure out how to coat the rod works just as well though.

Tacking is pretty easy, just apply the flame to the edges of the metal parts and once a shiny melt area starts to form, dab quickly with the filler, remove the flame and you’re good. Welding is done pretty much the same as tig, keep the torch about 15 degrees off vertical and apply filler to the leading edge of the weld puddle. Starting the weld is the same as tacking, hold the torch so the main cone is about 1/8″ off the surface, a weld puddle should form quickly, as it does, dab the filler rod into the front edge of the puddle (hold the filler about 15 degrees off HORIZONTAL, this keeps it from melting before hitting the weld puddle). Back the torch tip off the work about 1/4″ and advance down the weld just a bit. Move the torch tip back to 1/8″ off the metal and repeat. This is the basic technique, the rest is just fine tuning, you’ll notice that the weld starts to get hotter the longer you’re welding a part, so generally the torch tip is moved a bit further back as you go. This also happens as you get closer to an edge, most of the time, I don’t approach an edge from the inside, but will stop about 1/2″ short and weld the edge from the outside in.

Once the weld is done, a quick cleaning with a stainless brush takes the flux off, and the weld is ready for planishing, usually with a gas weld, the metal is soft enough that it can be hammered back into profile with the part, so no grinding is necessary or all that recommended. If the weld still hangs above the profile after planishing, filing the excess is okay.

By roccitycafe in fabrication, metalworking

I took some time to document building an aluminum fender for a bike I’m working on. I make quite a few of these parts but usually don’t have time to record the process. I start with a piece of 14ga 3003 aluminum, cut to about 20″ and 4-5″ wide, depending on the style I’m going for.

The fender is a combination of two curves, the main curve to match the diameter of the wheel, and a tighter curve to match the profile of the tire. To get this shape the center of the fender needs to be stretched and the sides shrunk. Since the shrinking is minor, I’ll use a dolly and soft slapper to do it, rather than a tuck shrinker which is for more dramatic shrinking.

I start by drawing a center line and two other lines each about 3/4″ from the edges, the center line is for reference and the two other lines are so I have a guide to where shrinking and stretching part ways, I could do this by eye now, but doing the lines is habit and probably not bad practice. In theory, the lines off the edge mark metal which isn’t either stretched or shrunk, so technically it’s not moved. That’s a good way of thinking about it.

I start off by hammering the center portion into a shot bag, and then going right to the dolly to bring the sides in with the soft slapper. Right away the fender starts to take shape, it’s a pretty basic form so the only challenge really is to keep things even. I use the wheel to smooth out the hammer and shrink areas, then do a check on the tire. One more round of hammering the center and lightly shrinking the edges gets the curves where they should be. Another go in the wheel, and a second pass with a large radius anvil from side to side smooths the fender out. I use a ruler to make sure the edges are spaced evenly, some light working against the dolly brings them in where needed. The final check is done against the tire and the fender is ready for sanding and polishing.

Where to get this part

Building an alloy tank (2)

September 27, 2013
By roccitycafe in Uncategorized

Doing some more work on the tank, this video shows the top panel of the tank mostly completed and test fitted on the buck. In the video I use the same process of tuck shrinking, stretching and planishing on the wheel to form the side panels to match the pattern buck. To join the panels, I’m using clamps to fix them in place while I mark a line to cut. It’s much easier to shape your panels with extra material and then cut them to match before welding than it is to try to match a precut panel to another one. Still photos are used to demonstrate the tacking, straightening and welding process to join the top three panels together. Care must be taken to avoid welding an entire seam without tacking and straightening first because as the material heats up and the weld shrinks, the panels may never line up by the time the weld is finished. The last part of the video and some stills show the process of joining in the knee sections, in my bullet style tanks, I make the knee section as a separate piece and weld the seam because it gives a sharp line to the tank, alternately the knee section can be formed directly from the side panel if a smoother transition is desired.

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Building an alloy tank

September 27, 2013
By roccitycafe in fabrication

I got a lot of questions about the basics of metal forming in regards to the manx tank build I’m illustrating. Unfortunately I didn’t take that many pictures of the actual forming processes while building that tank, so I took and edited some video of the actual forming basics for another bullet cafe racer tank I’m working on. The shape is different, but the same methods apply.

I use something called a tuck shrinker to shrink edges, some people like to use an an edge shrinker, but hammering a tuck more effectively shrinks than a mechanical shrinker. Stretching the center is done with a hammer and shotbag. Lots of people new to metalworking are under the impression that the english wheel is the workhorse of the shop, but most of the shaping is actually done with the first two tools. The wheel is really a finishing and fine shaping tool.

I’m using a wooden buck I made years ago for this tank, since then I usually make a foam buck because it’s not really necessary to have something as sturdy as wood when you’re just using it as a shape reference. Wood is certainly nice when you need to clamp panels in really complex shapes or where really critical accuracy is required, but for most alloy tanks, getting things within half an inch or so is perfectly fine.

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By roccitycafe in Uncategorized

I had a customer who ordered a Phoenix tank for his CB750 ask about installing an oil tank to replace the ugly stock unit, not completely knowing how much work it would be on this particular bike, I shot him a number and he told me to go ahead!  After fabricating the top half of the tank, I set about coming up with a way to fit 2-4 quarts of oil in there too.


alloy phoenix cafe racer tank for Honda CB750


I had to build the tank in a big U shape to clear the backbone of the frame, so it would need two oil drains as well, otherwise one half would always stay unused.  Because I was building the tank on an empty frame, I never thought about clearance to the engine head…


alloy tank with integrated oil



Turns out the oil feeds would need to be installed on the inside of the tank, and fitted with 90 degree connectors to route the lines back along the frame instead of just straight down.


phoenix cafe tank


So I removed the weld in bungs and installed some custom machined ones where they’d fit best.


cb750 alloy tank


Then I finished up the bottom of the tank.


alloy phoenix tank



custom cafe tank



cb750 oil tank


And the top…


building alloy tank


And then sanded and polished the tank…


alloy tank


Then fitted the seat I also made for it.



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Building an alloy Manx tank

September 10, 2013
By roccitycafe in alloy tank, cafe racer, handmade, metalworking

Going to do a little writeup on building an aluminum tank, recently I took some in process photos as I built a manx tank for a customer’s cx500 cafe racer.

manx tank for a honda cx500
First I like to start with a pattern, foam is fine to use as a template, especially when it doesn’t require an exact copy, but you’re just working out a shape.  It helps to figure out mounting and clearance issues with the bike before banging out any metal too.
manx alloy tank
Depending on your style, you can start forming any part of the tank first, I usually like to start the sides on manx types first, because the top is mostly flat and a lot simpler.  I like to be able to put each side next to each other to guarantee symmetry. 
Since the pics were just in progress pics, I’ll detail more of the actual forming process in the next few days’ posts. 

By roccitycafe in cafe racer, fiberglass, handmade, seat, upholstery

 When the pan is all padded, I use it as a guide to cut the vinyl covering, I could just make patterns and stitch each cover the same exact shape and size, but if the trimming on the foam or fiberglass is off just a little bit, the cover won’t ever look right.  It takes more time this way, but it guarantees a top notch result.

Since I use a flat fell type of seam for durability, I need to create a trim line offset from the edge of the foam by a little more than usual, I like to use about 1/2″.  This is where the vinyl will be trimmed and where I’ll line up the edge of the skirt piece.

To stitch the skirting to the main seat piece, I use transfer tape to stick the pieces together before actual stitching, this keeps things from moving around and ruining the straightness of the seam.  The first stitch is done on the backsides of the vinyl, then it’s folded over and the next stitch is done about 1/4″ from the first seam.  This makes the skirting hang perpendicular to the main seat piece without any bunching.
Next, the back piece gets stitched to the main seat piece, I use transfer tape again to lock it in place so nothing bunches up while stitching.  This piece can be cut extra large because it’s trimmed to match the back piece of foam.  
Once the back piece is stitched, the whole cover is temporarily put on the foam and the back piece is pinned to the back foam, this gives it the shape it will take on when the pad is done.
To make the back skirt piece, I use two pieces of transfer tape, one to fold the first 1/2″ of vinyl over on itself, and the next piece on top of that fold.  
The rear skirt is then stretched over the back piece of vinyl and pressed so the transfer tape holds it in place, then the pins are removed and the piece is unfolded and stitched. 

The piece is then folded over and the second stitch is done, extra care has to be taken here because any mistakes will cause the back of the seat to be uneven. 
The cover is then checked against the foam, when pulled tight it should be even and have no bunching anywhere. 
The cover is then riveted to the base and pulled taught as it’s riveted.  Any excess material is trimmed off and hot melt adhesive can be used to flatten the creases against the fiberglass base.  
All ready for installation!

By roccitycafe in cafe racer, fiberglass, handmade, seat, upholstery

I’ve been upholstering my own seats for a few years now, and when looking back at the first ones, I’m a bit impressed at how they look now.  Not blown away mind you, I’m a novice at best, but I think I can share some techniques with beginners that may help them along the way.

The style of pad I like to do is a separate upholstery unit, in my mind, it makes for a much cleaner look on the bike and is a lot easier to remove for underseat access than the snap cover type.  This type is shown on my personal bike, yeah, I actually ride on the parts I make!

The first step in my process is to make a fiberglass base pan for the pad, I use an original seat for the mold, and wax it so the glass doesn’t stick to it, a couple layers of chopped mat is stiff enough for this purpose.  When it’s all cured, I remove it and trim to the shape I want, I use templates for the seats I make all the time, it saves some measuring and marking time and ensures everyone’s getting exactly what’s in the picture.

Next I drill out holes for 1/8″ rivets along the edge of the pan, I find rivets work best to hold down the material without starting tears in it.

Depending on the firmness desired, I make a base layer of at least 1/2″ of high density closed cell foam, similar to yoga mats, or those floor covers for work areas.  Spray adhesive or contact cement work well to hold the foam to the fiberglass base.  The next layer(s) of foam should be a lower density type that adds a bit of cushion and lets the sewn material move a bit, it looks better than using only high density foam throughout.

With the foam layers adhered to the fiberglass base, I use a bandsaw, or a hand razor saw would work as well, to cut the foam using the edge of the fiberglass as a guide.  This makes sure the foam is straight along the sides and won’t make the covering look lumpy.

Next, and this is a very important step, (I’ve had people tell me some of the “really famous” builders don’t do this and have problems with the covers tearing!), I cover the fiberglass edge in either a few layers of masking tape or gaffer tape.  This keeps the abrasive fibers from rubbing holes in the vinyl covering… surprised that some people never think that far ahead.

tomorrow – sewing the cover

By roccitycafe in cafe racer, fiberglass, handmade, seat

Ok, everyone wants it, here it is.  Here’s how you do it, the definitive guide on making your “cafe racer” seat.  If you really actually truly follow these steps, and really actually listen to what I’m saying, don’t skip steps, don’t half-ass bits of the process because it was cheaper or you didn’t have that tool, etc etc, you’ll wind up with a nice seat that doesn’t use 4 cans of Bondo.

Invariably, yes, it is cheaper and easier to buy a ready made seat, and frankly, the only reason you should do it this way is if you want your own style, or are a serious massochist.  
tools to build a cafe seat
The first thing you need is tools, shown here are a can of spray adhesive, a flexible tape measure (used more as a contour gauge than a ruler), coping saw, small drywall saw, rasp, contouring sanding block, ruler, xacto, tape measure and some pens.  
laying out a seat design
And the first step is DRAW THE DAMNED THING!  If you’re one of those youtube generation kids, you’ve stopped reading by now and are just scrolling through the pics, so have fun with that.  If you’re still tuned in though, drawing the thing is the first step to a good design, sit on it for a day or two as well, you’ll be amazed how many changes you make because of things you didn’t consider, and how many changes you’ll wish you made if you just start slogging fiberglass at something without thinking.  
cafe seat patternmaking
When you’re really happy with the design, print it full scale, or if you drew it full scale, use it to trace out negative templates, these will guide you and help keep your part symmetrical.  I like to use melamine from the hardware store, but chipboard or scrap boxes work too, just a bit flimsier.  
seat templates
As far as materials, I use urethane foam from the hardware store (it’s the yellow stuff, not the pink or blue).  The reason for this is that it’s easy to carve and doesn’t dissolve in contact with fiberglass.  Most of the videos you see on how to do this tell you to cover your foam in plastic and mold release it.  This is absolutely the worst thing to do, you want your fiberglass to stick to and follow the shape of your foam, not lay on top and slide down the sides because of the plastic and release agent.  When you’re done with the seat, you don’t pop it off your foam anyway, you tear the foam out with the back of a claw hammer, you’re done with it, there’s no point in trying to save it.  
laying out patterns
carving foam for the seat
building the seat shape

adding material to the seat
forming the shape of the cowl
Here’s where your guides come in, you can use them to rough in the shape as you stack up the foam layers, then use them to guide your rasping and sanding to finish the shape.  
cafe seat final shape
Final carving has to be done carefully, because the final foam form is what determines your end shape, if the foam isn’t straight, or smooth, then you’re boned.  The only way you’ll recover from that is by grinding massive high spots down through the foam, reglassing, grinding again, and adding massive amounts of bondo to the low spots and adding a ton of weight and time in sanding.  99% of the fiberglass seat diy builds I see skip the step of getting the foam really smooth and straight, and go straight to slogging fiberglass.  Then with the magic of editing, they turn 28 hours of sanding, grinding, rebondoing and swearing into a minute’s worth of “finish work”.  Balls I say… this is how you really do it.  
glassing the seat
The next step, once you’re absolutely positively sure your foam is good, is to begin glassing.  You don’t start glassing with mat, or cloth, or whatever, you start with surfacing veil, the really fine thin stuff that barely adds any thickness to the part.  The reason is because you first want to create a nice shell surface to add thicker stuff to, a shell that will help thicker stuff stick and keep you from distorting the foam underneath as you apply the main glass layers.  
wetting out the fiberglass
first round of fiberglass
once you’ve got a layer on, there are usually a few spots that don’t stick all the way, so sand down the whole part to knock those off, and apply a second layer of veil, the second layer will lay down perfectly now.  
adding more glass to the seat
laying out cloth for the seat
And now onto a tacky but hard second layer of veil, you start applying your cloth.  I don’t like building one-off’s with mat because it really doesn’t lay down in a consistent thickness, doing it with cloth will save you hours in finishing time.  A critical step is to really carefully plan how you trim your cloth, make sure it doesn’t fold over anywhere, and where you overlap pieces, make sure the next layer skips the overlap, that way you’re not creating any high spots that you’ll have to grind off later.  
shaping the fiberglass
If there are any areas that don’t want to lay down, use some tin foil to hold them to the part, it peels off easily once the glass is cured and will keep you from having to spend more time grinding out bubbles and reglassing. 
filling the fiberglass
now that you’ve got a glassed part, hit it with the sander to knock down any high spots or burrs, and apply a SKIN coat of bondo, a skin coat is basically running the squeegee across the surface so the filler only fills the low spots, there should still be spots of fiberglass visible, and that’s okay because your glass conforms properly to the foam model.  
smoothing the seat shape
Once the bondo is cured, hit it with the sander again, you’ll see a mix of glass and bondo on the surface, and that’s fine, then apply another skin coat, this time use the curve of the squeegee to match the surface of the part, and do your best to make a consistent thickness layer of filler over the whole part (like, 1/32″, not 1/4″).  Hit the second layer with the sander again and if you’ve done things right, you’ll wind up with a straight smooth part, strong and light.  

adding primer to the cafe seat
I personally use Duratec surfacing primer next, it’s essentially a gelcoat but has better sanding properties, spray the whole part evenly, you can use a cheapo HF spray gun to do it.  The part should have a very light ripple, the stuff never lays completely smooth, but the ripple can then be sanded out with a sanding block and some 220 paper.  
final sanding of the seat
Any spots that are still low after the sanding can be skinned with filler, as they’re probably only a hair’s width below regular surface.  Now you’re ready to remove the foam, mount up the seat and prime&paint.