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Simple bike chain guard made from large chain ring

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Picture of Simple bike chain guard made from large chain ring
I built an inexpensive chain guard for my bike.  Getting your pant leg or shoelaces caught in the chain while riding is at the very least annoying, and can potentially be very dangerous.  The usual pant clip does keep your pant cuff out of the chain, but does not keep shoelaces safe.

I ride a fairly heavy bike, and realized I never used the largest gear on the front derailleur.  I decided to replace it with a smooth plate, which acts a chain guard.  I made this at TechShop.
 
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Step 1: Mount the chain ring on the lathe

Picture of Mount the chain ring on the lathe
The existing big chain ring was a 44 tooth one, but I never rode fast enough to shift up into that range.  I found a 48 tooth gear in my parts bin, and decided to repurpose it.   

Since I had access to a metal lathe at TechShop, I decided that was the tool I wanted to use.  I used the four jaw chuck on the metal lathe, since this crank set uses four spokes to locate the gears.  It is important to center the part when using a four jaw chuck, hence the dial gauge on the left in the photo. Centering is done via small adjustments to each of the jaws, measuring the runout between adjustments.  This was by far the longest step in the process.

Make sure the chain ring is held in tightly, but not too tight or the gear will flex out of round.

Step 2: Cut teeth off of chain ring

Picture of Cut teeth off of chain ring
ChainGuard-2.jpg
ChainGuard-5.jpg
Move the cutter in close and turn on the lathe.  Slowly slice the teeth from the outside of the chain ring.  It will be pretty noisy initialy, as it is an interrupted cut.  When the sound becomes uniform, you know you have cut all the teeth off.  Stop the lathe, inspect your work, and unmount it when you are satisfied with the cut.  I cleaned up the edge with a small file. 
EGiR2 years ago
This is very nice, to be sure. An elegant solution.
I happen to read this as my chain guard just fell off. I always use the highest gear, but there are bolts for a crap plastic chain guard. I will see about making a new one and whether there is an instructable there. Thanks for sharing. (ps - is the Tech Shop a Hackerspace of sorts?)
Yes, the techshop is a hackerspace of sorts. Aren't those awesome? I remember having the idea when I was a kid that people should share tools like they share books. Comforting to know I'm not the only one who feels that way. At the time it never occurred to me that the knowledge shared would be even more valuable than the tools.
Tommi Potx2 years ago
hmmm, nice; now where did I put my lathe?
seraphim_722 years ago
As a machinist your setup made me cringe. One hard spot or hard aluminum drag and you would have sent a literal saw blade off that lathe at high speed. A better setup would have been either a faceplate so you could take advantage of those four bolt holes (and other parts) or a smaller lathe (or 4 jaw chuck) with the jaws reversed and the body of the gear laid flat against the face of the chuck. Always safety first!
justalf2 years ago
And don't forget to adjust your front derailleur's stop point so it won't change on the the big ring as it will just slip off it probably.
Samimy2 years ago
wow man ! this is definitely something i really need ! my jeans are all oily because of the chain.
my bike chain has got cover but even that cover gets oily and black and turns my jeans to trash.
camp6ell Samimy2 years ago
(removed by author or community request)
BradMartinson (author)  camp6ell2 years ago
Right - but rolling up the pants leg does not save the shoelaces from the chain - that was my primary motivation. That, and an excuse to use the lathe :-)

Also - I plan to remove the front derailer at some point, since I don't use it. And while I'm at it I will shorten the chain, since it's a bit droopy.

thanks for the comments!
P.S. Oh yeah, and wouldn't it be easier to just shorten your shoelaces?
You should know that the shifting pattern on a bike with multiple chainrings is not a simple line: 1Front-1Rear,1F-2R, ..., 1F-6R, 2F-1R, etc. On my ten-speed, for example, the lowest gear ratio is 1F-1R, and the next higher is 1F-2R, but the next higher after that is actually 2F-1R, on the big chainring. After that comes 1F-3R, then 2F-2R, and they continue alternating until the two highest gears, which are both on the large chainring. Your more modern bicycle may not be exactly the same, but there is NO chance that your gears are in a straight line like you say.

This is a very detailed article on bicycle gearing, which you may not want to read: http://www.kenkifer.com/bikepages/touring/gears.htm

This, however, is a simple calculator so that you can find out which gain ratios your bike has and which order you should shift between them in: http://sheldonbrown.com/gears/
This fellow's web site has a wealth of information to help with other cycling projects as well.
Another solution is to use a pants strap. Added bonus is that it has a reflector on it as well.

Personally I don't see the use in this, as I use my outer gear, but to each their own.

You'd also need to adjust/disable the front derailer so as to not accidently pop the chain out over that I would think.
sprinks2 years ago
Be very careful with these, these are not meant to act as true chainring guards. They will not protect shins should you slip from pedals. A thin piece of metal (~3/32" thick) like this trimmed chainring with enough force behind it can do alot of damage. I've had one nasty experience with a similar setup previously used. True chainring guards are just barely larger OD than the OD of the chain on the chainring it's intended to protect.
mysss sprinks2 years ago
That's an excellent point. More importantly, a real "bash guard," which is what this instructable produces, is made of much flimsier metal, so it would probably bend rather than cutting you as deep as this one. More importantly, I think they usually have a rolled-over edge to prevent cutting you as well.
zumbob2 years ago
интересная штука
derte842 years ago
It's perfect for converted fixed bike! Having 2 chain ring is not so nice looking but this is better looking AND functional.. thumb up!