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Old Streetfighter Motorcycle Forum Post 01-14-2018, 12:14 AM   #1
sebwiers
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Default Crash Bars

I'd guess these count....

++++++++++++++++++

This is a crash cage I built for my custom / modified motorcycle.
The album at https://imgur.com/a/VR3WH has more information about design and construction of the suspension.
The album at https://imgur.com/a/7kaBh has some pictures of it in rideable form (with the tank on etc).

The cage protects the engine case and controls from damage if the motorcycle falls over, whether parked or in motion, and has plastic sliders so that the metal tubing won't dig into the ground and flip the bike over into a tumble if it is sliding down a paved road at high speed. The initial reason for building it was I plan to try some indoor flat track and outdoor ice track riding this winter, and it seems pretty likely I'll end up sliding across the track (at low speed) at some point. Ideally I'd like to never have to use my kickstand and just drop the bike like a kids bicycle when parking, but gasoline would leak out of the carbs if I do that.

The cage is just wide enough to protect cases (in this case that means 18" inside width). Some plastic sliders stick out just as far as my knees (no impact on lane splitting) and (just barely) keep handlebars from touching ground, making the overall outside width about 24". Ideal width will depend on what bike the bars go on, and what sort of damage you need to protect against. These should protect against crashes on pavement and other mostly smooth surfaces, but might not be wide enough for off road protection where the surface you fall on could have rocks poking up, etc.

Obviously those measurements are potentially different if you are building one for your motorcycle, but they are required in r/DIY subissions on reddit. Any mid sized inline 4 probably is about the same width, although some older Hondas have abnormally wide lower cases.

I built all the parts for this crash cage (as well as all other modifications to the bike) at my local hackerspace, TCMakers - http://www.tcmaker.org/blog/



First thing I made was the mounting plates. They are cut from 3/16" bar stock using a metal cutting chop saw. The dimensions are not really important, I just made them big enough that there was room to fit a support pipe more than 1 inch away from any bolts. They will get trimmed down to something less bulky once the crash bars are welded in. The important part to get right was to make the bolts holes match up with the bolts. Ideally you would use a step drill for these holes, but a regular drill bit works. Use a center punch or the drill will wander and the distance between the holes will be off. I still needed to hit up 2 of the 4 plates with a round "rat tail" file to elongate one of the holes to get them to fit.



Originally the main bar was 6 foot, with bends centered on 18" intervals. This was meant to go wide enough to protect my handlebars, but looked horrible. Here it is simply positioned leaning against the engine, with a block underneath and a cross bar resting in place, to get in idea for fit.

The bends were made using a Harbor Freight pipe bender - its literally made for bending pipe like this, but can cause trouble for "real" structural tubing (the dies won't fit right). That's one reason I went with cheap pipe. The other reason is that I don't want the crash structure to be stronger than the parts it is bolted to - the whole point is for the crash structure to get damaged INSTEAD OF the main structure. Plus, cheap pipe is, well, cheap. I got mine for free actually.

The bender and methods I used are essentially the same as the ones shown in this video - https://www.youtube.com/watch?v=lTAU3GRyIvY



I narrowed the bar down by chopping sections out of the pipe. It takes some care to do this while keeping the center bend in the right spot. If doing it again, I'd just make the three sections from separately bent lengths of pipe instead of from one bar, because there's no way I was gonna get the width right anyhow.

To join them back together, I drilled holes through the pipe and then slipped smaller diameter bits of pipe inside the joint and welded it up. This is called "slugging" the pipe. The pipes were aligned and held in place using a chunk of plywood cut to 18" wide, just to keep them straight.



After the width was correctly set and the bars ground down smooth, I joined the upper ends of the pipe to the upper cross bar. I welded it in as one solid piece to make aligning the ends easy, and then cut an amount out of the center (using a hack saw) equal to the distance between the outer faces of the mounting plates (in this case, 9.5"), marking the lines to cut by measuring out half that distance (4.75") from the center of the top cross pipe. The structure was then fit back on the bike to tack weld the top tube to the plates, and removed again for full welding. Its important to have the structure on the bike when tack welding, and make the tacks solid enough the parts stay aligned when doing the full weld - there's no way to line these parts up with simple measurement. For production welding, you would use a jig, but for a one off, whatever you are fitting to typically serve as your "jig".

I'm using is a Millermatic 252 Mig welder, but almost any welding process and machine would work - pipe this size is easy to weld, since its thick enough not to burn through quickly, but thin enough for cheap "house plug" welders and small torch sets to handle.

To get a good fit up for the weld, I coped the upper end of the guard tubes. I followed the suggestions in this tutorial and cut the coping by hand using an angle grinder. https://www.youtube.com/watch?v=mHBanmfn_CA
For this 90 degree joint, the coping was very easy.

I wanted the outside ends of the cross tube left open so I could put threaded inserts in them to mount sliders or other things. If you don't plan to do this, simple miter cuts (a 45 deg angle on each pipe end, to make a 90 degree joint like a picture frame) or any other sort of joining would do.




Test fitting - the whole thing is only slightly wider than engine, and you can see the result of the "coped" weld and open tube end. The center bend under the engine is low enough to protect my pipes on some bumps, and might serve as a mounting point for a skid plate. Most such guards do not go that low, and would probably be a simple rectangular shape (only 2 bends and upper cross bar).



The main purpose of the test fitting was so that I could fit the lower bracing bars. I had a couple 6-8 inche lengths of pipe left, and with the ends chopped at 45 degrees (using a metal chop saw) they fit the lower plates OK and reached the cage around the point where the bend was - seemed a good point to brace against impact. I just held these tubes in place against the lower plate and roughly marked how they should be cut to fit the pipe (upper length, lower length, front and back length), then cut them with an angle grinder and coped them with a half round file. Neither was a perfect fit but they were pretty good - one was fairly snug, the other off by maybe 1/8". A gap like that is easy to bridge when welding.
Once the lower support tube ends were coped to fit the cage tube, they were tacked in place on the bike. Then the entire structure was then removed for welding & finishing. I wanted these welds to be STRONG, because when the bike falls over, the cage first touches the ground right where the lower support joins the cage.




After welding, I cut down the mounting plates, used a wire wheel to clean up a bit, and coated it with linseed oil mixed 1:1 with paint thinner (mineral spirits). When dry, this inhibits rust pretty well, and is super easy to touch up if I need to modify some part of this down the road. In case you wonder what it looks like when dry, the front fork of the bike and other "raw steel" parts have the same finish, and its been outside for a few months.
I also put some tube nuts in the ends of the upper tubes. Tube nuts are basically bits of internally threaded tube (bungs) that slide into another tube to be welded in place. To hold them, I drilled some holes in the tube and "plug welded" them; easiest welds ever! These give me threads in the ends of the tubes to bolt things to - normally sliders, but also potentially lights, camera mounts, prop weapons, etc.



I made the sliders on a Bridgeport from a chunk of (nominally) 2 inch round Delrin (acytyl plastic). This is a common sort of "machining plastic" and is also what commercial sliders are typically made from. The notches make them easier to grip for installation & removal, even letting you drive the rotation with a hammer and punch. I could even make a custom pin wrench if I find I'm swapping em in and out a lot.

The center holes are blind drilled at 7/16" and tapped for 1/2" x 20 threads (to match the tube nuts I had). I just used a center finder and eyballed lining up the drill with the center, rather than getting fancy and drilling the holes on a lathe or other precision work.
To make a notch cut pattern like this in a cylinder, line up the mill end on the center line of the cylinder and cut as deep as you like into the side using whatever will end you like. Then rotate the cylinder so the notch you just cut is against the MOVING jaw and do it again, and one more time, cutting to the same dpeth each time. That gives you 3 identical notches separated by ~90 degrees. To do the last notch, you need to clapm the cylinder with the two poosing notches against the vice jaws. This results in the centerline of the cylinder shifting towards the fixed jaw. Rather than doing complex math based on circles to figure out how far I needed to move the table to get the mill end back on the centerline of the cylinder, I just moved it by half the difference between the actual diameter of the cylinder and the spacing between the jaws with the notches clamped against the faces!
This process is probably NOT going to result in exact 90 degree notch spacing, but its fast, can give you some cool looking parts, and is accurate enough to fool the eye. I figure its good enough for something meant to be ground up by pavement.
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