Saturday, October 31, 2009

Rackufacture: Karate Monkey Rear Rack, Part 1

This is gonna be a really long post. Pretty dry, too. If you're not too into the rack building thing, this would be a good one to skip. There, you've been warned.

My first project is finally underway - it's a rear rack for the Karate Monkey. It's a pretty challenging design for a first attempt and as a result, I've been having trouble making any progress. I think I've been a little overwhelmed - it's all new ground and you think something is gonna work one way and then it doesn't, or some way you had planned to do something takes way too long and you have to figure out something different. And then there are the mistakes. Anyway, it's been really slow and with only a coupla hours at a time I've not been able to get in a groove and I've been getting frustrated and pissy. What I needed was a good solid chunk of time to lock myself in that shop and really focus in on what I was doing and get some shit figured out.

So I pleaded my case to Patty last night and she agreed to let me go on an OCD bender. I walked into the shop at eight o'clock last night and walked out 8 hours later majorly wiped but feeling like I'd finally got over the hump and had this project out of park and into drive.

I picked this as my first rack because it's what I need right now. I want my Karate Monkey to be my primary winter bike, but I need a better way to carry things. I got these new waterproof bags a coupla months ago and I've been hanging them off of this rack that clamps to the seatpost. It's the best solution I was able to find in the way of off-the-shelf racks . . . more traditional styles don't work well because they hinder access to the rear disc, which I need if I have a rear flat. I have no complaints about the construction of this rack, as it is rock-solid, but it places the center of gravity of your load so high that it really goofs up the handling of the bike.

My plan is to build a rack that drops that COG way down. (This drawing shows the top hoop only, which will support the panniers. There will be some struts that run down to the dropout, but I'm still working on how I want to do these.) Click the drawing, or any of the pictures for that matter, for a bigger view.

One thing that happens when you drop the bags down is that they move closer to the arc created by your heel as you revolve the cranks. So you have to move the load farther back to provide clearance, which is not all that desirable. I've tried to balance those two things with this design.

Since I'll be using my Lake boots for a good part of the winter and they're my largest shoes, I used them to establish the arc. I measured from the center of the crank and used 15" as my heel travel radius.

I ordered a coupla piceces of foot-square 4130 sheet - one .09" thick and the other .125". My first job was to make some tabs for bolting the rack to the frame.

The tab in the center of the picture below is my first attempt. You wouldn't believe how much time is wrapped up in this little dude. I used the .09" material and it just looked too thin and geometry sucks and it just made me generally unhappy. I'm not a perfectionist, but I do need at least decent work from myself and I knew I was in for a do-over. The other tabs shown in this picture are waterjet-cut tabs that I purchased from Alex. They're gonna be the bomb. But I'm convinced that making tabs is an essential part of my rack-building education, so I need to do it the hard way the first few times.

I decided to get a little more serious this next time around. I scribed the layout on the sheet and then cut them out of the sheet with a hacksaw. Heh, nice straight cut.

Then I clamped them into a set of vise grips and roughed them out on my bench grinder.

Next, I took them down close to their final shape using files. Lots of filing, lots of paying attention. I left just a little extra meat on them for final cleanup.

Finally, I cut them out of the "dogbone". The dogbone idea turned out to be extremely helpful, just because they are so small and the size of the combined tabs was much easier to lay out, handle and clamp in the vise.

The rack design is intended to take advantage of the cantilever brake bosses on the frame, which are not being used, since I'm running discs. My original plan was to make the top hoop in one piece. I would make the u-bend first and then the tighter-radius bends at the ends that attach to the bosses. But I scrapped the whole thing when I crimped those bends. Except that after sleeping on it, I decided to save the "u" and splice some bent ends onto it. This decision was made easier by the fact that would need another full (4') length of tubing to start over and I didn't have any more left. The whole crimped bend thing actually turned out to be a blessing in disguise, as you will see below. This is the u-bend just before I crimped the ends.

I cut a coupla short lenghts of tubing for the ends and set about to cut notches in them to accept the tabs. I've already refined my method for doing this quite a bit and probably have a ways to go, but here's where I'm at right now. First I measure and mark the depth of the cut on both sides of the tube. Then I mark a vertical cut line on both sides.

By angling the hacksaw blade, you can cut one side at a time, which makes it much easier to make the cut on line.

Then I use a cutoff wheel just slightly thinner than the tab to widen the slot.

I take it to its final width and square the bottom edge with a file.

One final step is to file a bevel on each side of the tube and then it is ready to accept the tab. I copied this tab style from Alex. Maybe at some point, I will try some different styles/designs, but for starters, I think it's smart to use something proven. Plenty challenging for a newb.

Here it is brazed up and finished off.

After making sure I would have enough bender clearance, I decided to bend the tubes after installing the tabs. The bend angle is trial and error and I used a layout to check the angle.

I needed a dowel pin to hold the join the bent ends to the U. I'm using 3/8" x .035" tubing, and 5/16" tubing won't fit inside. So I cut a 3" long piece of the 5/16" and cut a slot down the length. Then I cut it in half and squeezed the diameter of each piece down in the vise so they would fit inside the 3/8". Another stolen idea, this time from Alistair.

Here are the ends being fit into the U. This arrangement lets me fine tune the angle of the ends and would not have been possible had I not ended up making the top loop in sections.

The canti bosses have 10 mm female thread. The canti post threads into the boss and has a 6 mm internal thread. I was able to cut the pivot tube off of the post and file the end flat.

Here is is being threaded into the boss. This is what the rack tab will bear on.

Here it is, bolted up and fit into place, which left me very tired and very happy. The end of the U extends farther past the fender than I like, so I will trim the legs to bring it in. This is another thing that would never have been possible if I hadn't crimped those bends. Guess that turned out to be a pretty good thing.

I've used up so much 3/8" tubing on test pieces and mistakes that I don't have enough left to finish this rack. Another order is on the way and should be here early next week. Can't wait to get my hands on it.

Thursday, October 29, 2009

Rackufacture: Hand Bender Do's and Don'ts

Education ain't cheap. Just ask any college student. I paid my dues tonight.

I've been making good progress on a rack for my Karate Monkey and was excited to get home tonight and make a little more. I knew exactly what my plan was: Take the u-bend from last night, clamp it to the bench, engage the bender and then "roll" it 13 degrees to get the bend in the right plane. The setup went just fine.

But when I went to bend it, halfway through the bend, I felt the pressure on the handle give, which is a sick feeling and means your bend has crimped. F@#K. That was a whole 4' length of tube that just went into the toilet.

Since it was already trashed, I decided to bend the other leg and that one crimped too! DOUBLE F@#K!!! And more importantly WTF! These benders do a great job and I couldn't figure out what the hell had just happened. Here's the difference between a normal and crimped bend.

I ended up making a buncha bends and burning a buncha tubing to get this figured out, but it was worth it. The deal is that, in order for these benders to do a good job, the "shoe" has to keep the tubing in tension as it pulls the tubing around the die. I thought I was doing that in the arrangement above, but the body mechanics required to do that bend make it extremely difficult to maintain tension. And if you don't maintain tension, the tube separates from the sidewall of the die and bad things happen.

I just noticed that your eyes glazed over. Sorry. And that was a sucky explanation, I know.

Condensed down, don't bend anything the way I tried to unless you want to throw it in the garbage. Clamp your bender in a vice and do it that way instead. Money.

And speaking of money, here's how I spent mine tonight.

But wait. Yeah, I hear ya. There's some interesting shapes in that pile. Hmmm.

Tuesday, October 27, 2009

Rackufacture: Homemade Tube Bending Dies

I've been spending money like a drunken sailor. Trouble is, I don't have any kind of respectable buzz to show for it and now I'm broke. I watched Debra Winger and Richard Gere in that one movie so I know how it works, and I'll bet you a million bucks there's a boatload of real sailors who have uttered that exact line.

So I thought I was ready to launch into rack building but then I started actually planning my first rack which is rather, uh, non-conventional and realized there were gaping holes in my plan and that I needed to figure out. More specifically, I need to bend some tubing at a way different radius than my benders bend at. Like I said, the Rackufacture budget is tapped. I've seen lots of bending ideas on the interwebs, but they would all cost me something and I needed to make do with what I have. My dad grew up on a farm in the Depression. I know it must be in my DNA somewhere.

Luckily, I have some rad tools and some metal scraps laying around and didn't have to dig quite that deep. This is a cold saw that makes the fastest and cleanest cuts you've ever seen, but throws mean, nasty, sharp metal chips from hell. They can get lodged in the soles of your shoes and tear the piss out of your hardwood floors or linoleum and then your wife beats you with a broom until you go back outside. I hate these eposides, so I only get the saw out if I really, really need it (which I did tonight) and then I always take extra time to sweep up and watch where I step.

After some cutting and welding that is too boring to convey, I ended up with these.

Then I found a piece of pipe with an inside diameter just a little bigger than the OD of 3/8" tubing, which would be 3/8". I found a countersink bit that was just barely big enough to smooth out the inside edge of the pipe.

This particular piece of pipe happened to be welded, which means that it has a longitudinal weld seam that could possibly mess up my big plan. But I am just barely smarter than the pipe and made a mark that coincides with the weld so that it stays on top and doesn't mess with the bend.

Here we are (by that I mean you and I, in this interactive environment), clamping the tube to the die and getting ready to bend.

Here we are, with our hand on the tube, ready to bend.

Here is our end result. I think it came out pretty good.

Except for this one flat spot, which I am blaming on you. I've enjoyed this collaborative experience, but I think I would prefer to go it alone from here on out. Probably best not to use me as a reference.

Sunday, October 25, 2009

Rackufacture: Destructive Testing of Brazed Joints

Mind-numbing stuff coming up. Worse than ever. This is gonna make the last post look like Girls Gone Wild. This is as bad as it's gonna get . . . maybe if you focus on that, it will help get you through. I hear that super-boring shit can actually enrage a person. I hope that doesn't happen. Let's jump in and get this over with.

One last thing I needed to get a sense of before I start putting racks together is how strong the brazed joints are.

This is a makeshift tab joint. (The tabs in these pics are mild steel and normally you would use 4130, but mild steel is what I have right at the moment.)

This next pic shows how hard is is for a newb for fill in the ends of the tubes on each side of the tab. Brazing parts with different thicknesses takes a little practice. Of which I could definitely use some more.

I put the tab end in the vice and slid a cheater bar over the tube and put some hurt on this dude.

The brazed joint held up just fine - the tab base metal failed. (Note that this tab is only as wide as the tube.)

I took a cut through the joint - wow, looks like I used enough brass. That silver strip in the center is the tab.

Speaking of brass, but on a side note, this is about as far down as you can burn your brazing rod before you burn the piss out of your fingers. Take it from me.

Next up, same test, only with a tab that's wider than the tube.

This time the tab was stronger than the tube. The joint still held up fine.

Next I wanted to test some coped tube joints.

I wacked the ends off of the "T", stuck it in the vice did the same cheater bar routine.

The joint let go this time.

It wouldn't be much fun without a little torsion testing.

See how the tube has ripped right at the vice jaws. (To be fair, there's a massive stress riser there, but still, that's a pretty strong joint.)

And finally, I sectioned some "T" joints. This pretty much explains why you clean the inside of the tubes - the brass flows inside and adds strength to the joint if it has something to hang onto. The two on the far right are a little closer to what you are shooting for than the others.

The remnants of my extravaganza.

After the smoke cleared, I wanted to do one last thing and that was to make some dies for flattening tube ends. This is an alternative to brazing on tabs and I think I'm gonna be using this method at times. You know the stress riser thing I was talking about earlier? Well, the rounded corner of the dies are supposed to prevent that.

So thanks for hanging. Or maybe I've lost you and I'm just talking to myself. Either way, I'm as ready to be done with this as you are. It was all good but now it's time to build.

Re-entering the atmosphere as we speak. Ahhh, I can smell the oxygen - the kind that's not bottled! See you earthside.