RubeRad

I have a compact double on my commuter, and I haven't shifted to the big ring for like a year. I have a 3D printer and am thinking about replacing the big ring with an ornamental 'bash guard' (and also ditching the shifter and mech)

How tight do chainring bolts need to be (given the force is always orthogonal to the tightening direction), and would that crush plastic?

(For those that know 3D printing, I only ever use PLA, which is not as hard as other materials such as PETG or ABS)

Andrew R Stewart 

Transferring bolt torque to actual surface pressures (to see if the PLA can hold up) is way beyond me. Park lists about 8-14 NM of torque for steel bolts Torque Specifications and Concepts | Park Tool

My approach would be to file/sand off the teeth from a worn out ring and avoid the cool factor of 3D printing. Andy

RubeRad

I'm way too cool for that

JanMM

Guessing that incorporating steel washers and threadlocker into the assembly might maybe help hold things together without breaking.

I used a jigsaw and file a couple of decades ago to turn a worn big ring into a chainguard for a single ring setup. Definitely not ornamental.

70sSanO

I would never use plastic and then torque chainring bolts.

In a pinch, I have used an old bike as a poor man’s lathe in turning an old chainring down to a bash/chain guard. All it takes is a file and the desire to keep turning the crank by hand. Also, a towel over the spindle to help keep the shavings out.

John

csport

We can actually do what Andy says: assuming the threads are perfectly lubricated (i.e. the bolts can rotate with no friction in the threads), the work done in one full rotation of a bolt is 2*pi* torque. For a metric 8 x 0.75 chainring bolt the compression force is
(work done in one rotation)/(thread pitch) = 2*pi*torque_in_Nm / (0.75 * 10^-3 m) = 850 kgf * torque_in_Nm.

For a 1 Nm torque this is 850 kg force. Are we getting the number right?

Of course there is also friction between the bolt head and the washer/bash guard.

dsbrantjr

That is just the resting force. To the force exerted by torqueing the fastener you must then add the force exerted by pedaling.

70sSanO

I’m thinking this thread is more about just wanting to use a 3D printer. The outer ring has been there for years, no real reason to remove it even if the FD and shifter are removed.

John

Crankycrank

If you're inner and out rings are fastened by the same bolts and as mentioned by others already I also think using a plastic ring, which has to support enough torque to keep the inner ring stable, is going to risk cracking. You could always try it and keep an eye out for cracks and check the tightness of the bolts but there are enough cheap metal types available on Ebay or elsewhere to make it worth buying one and as also mentioned already you could just keep the outer ring as is or file down the teeth for the same effect. Examples here. I just used a search for 110 BCD rings but use whatever fits your cranks if different. bicycle 110 bcd chainring bashguards - Google Shopping

RubeRad

You have seen deep inside my soul

Kapusta

I support this idea on principle. 3D printing bike parts is just awesome.

As long as the female part of the chainring bolt fully supports the inner ring and passes through the bolt hole on the crank arm, you don’t need an insane amount of bolting tension to keep everything in place under pedaling load. Also, the plastic ring is not being subjected to the pedaling torque.The torque on the chainring is being transferred to the crank through the female half of the chaining bolt that passes through the crank arm spider and the chainring.

That said, the only plastic bash I ever had used metal sleeve inserts in the bolt holes.

If you do this, I would try to make it so that a wide washer could go between the head of the chainring bolt and the plastic bash so that you can tighten it to spec.

And since this is being 3D printed, it is important that this be a cool looking design.

All that said, you can get BBG bash guard for about $22 shipped.

Post pics!

70sSanO

So I guess my filing while turning the crank doesn’t fit the criteria.

What if I splatter paint it as I’m cranking it?

John

Kapusta

That would be fine if you don't have a 3D printer.

RubeRad

Yeah, I've seen BBG, and they're cheap enough it almost eliminates the desire to 3D print one.

Whatever happens, I will. But not only am I cheap, I'm also lazy. So it might happen in a week, it might take a few years.

Geepig

If you are going to remove the big sprocket then surely all you need is a set of tubular spacers the same thickness as the sprocket and enough clearance on the 3D printed guard to fit on the spacers?

Kapusta

If I am reading this right, I had the same though you did, initially. The issue I see there is that unless the plastic guard was a snug fit over the spacers, it would rattle.

I guess it depends how precise the printing is.

Crankycrank

If you really want to use the 3D printer what about just creating a thingy that snaps on to the outer ring in any kind of creative design you can think of? No fasteners needed.

RubeRad

That's not a bad idea, just a precisely-sized plastic dish with a slightly recessed rim so it can snap onto the big ring teeth

Kapusta

Maybe heat the chainring with a torch and set it into the plastic dish.

gsa103

But, but....I have a hammer (3d printer), everything must be a nail!
+1 for BBG, I bought a nice anodized one for my Yeti.
And yes, I'm also a 3d printer addict (FDM & SLA). Definitely experiment with PETG for functional prints. It's way less brittle than PLA.
I've printed mounts for bike lights, protectors for shipping, and a bunch of other things for bikes.

Kapusta

I belonged to a Maker Space for a while and my favorite saying was “Why buy something for $20 when you can make it for $30?”

RubeRad

That's amusing, but if I made something, it would cost me maybe 50c in materials (and $200 in time, but only if my leisure time is billable same as work, which is not realistic)

RubeRad

I've printed a bunch of bike stuff, about half the stuff in my thingiverse 'bike' collection, as well as a beer-cap stem-cap

Kapusta

Yeah, right. Start factoring in the cost of the printer, cost of maintenance, the number of times a job gets botched, and your time both on the job and on the 3D printer in general. That BBG bash starts to look cheaper.

There is nothing wrong with it, I just find if funny. I've been there, I get it. I did a ton of work on a laser cutter, and I liked to tell myself the stuff I made only cost me the cost of the wood. Sounds, good, anyway

But as you said, this is leisure and gratification. Its all good.

RubeRad

Lucky for me, I won the printer in a raffle, and I'm too cheap to maintain it. If I botch the job twice, I'm out $1.50. As for my time, it's hobby time I get enjoyment from (otherwise I wouldn't do it).

How much does it cost me in time when it takes me 2 hours to change my own bar tape instead of paying for a professional bike mechanic to do it right first time in 30 seconds?