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Making my own parts
I mentioned on a separate post that I am designing and manufacturing my own components. I'm working on a 40t chainring and a 175mm crankset, though I may go with 170 since that's what I'm used to. If you happen to be an engineer, I'd like some feedback on my designs, so all the dimensioning is included. I will use 7075 Al for the chainring and hopefully the same or 6061 for the crankarms.

Thanks in advance.

This is the 40t chainring

The drive-side crankarm. I back-figured the chainline so the 1/4 chainring above lines up.

The non-drive crankarm, the same size just without the boss for the chainring.
Is the chainring going to mount to the crank just via a single bolt?

Are you OK with clearance between the crank arms and the chainstays on the frame?
Wow this is really cool! Always a great appreciation for an invention on paper, always get a second opinion which you are. Keep designing my friend you are on the right path! Smile
Good maintenance to your Bike, can make it like the wheels are, true and smooth!
(08-19-2012, 11:12 PM)DaveM Wrote:  Is the chainring going to mount to the crank just via a single bolt?

Are you OK with clearance between the crank arms and the chainstays on the frame?

Yes a single bolt connection is the plan. My coworker has made some BMX cranks using the single point drive and he says it works well. This design is for a fixie. I haven't explored the clearance exactly but I based this loosely off of my 2- and 3-ring cranksets which are straight shanks.
I think the crank arms look a quite slender, around the square taper in particular, it looks like there should be a bit more "meat". In addition, I assume that you'll make the crank arms from billet, whereas most crank arms are forged, which makes them stronger, you might therefore have to make them more massive to get a similar strength. However, I'm not an engineer, maybe you are and have done the sums. Big Grin

I think the one bolt connection might be a bad idea, perhaps you could key the centre as well, make the hole in the middle of the chainring square and with a square part on the right crank arm to fit into it, or possibly a splined/toothed hole in the chainring and something on the crank arm to fit into it.
Hey capner! I remember your original post and thought it very cool!

My credentials? I graduated college in 1985 with Associate Degrees in Industrial Technology (lathes, mills, surface grinders, welding, sand-casting, etc), Metalurgy and Drafting - major in Gunsmithing. But the gov wanted to keep track of my underwear size if it changed so, I turned to my other love. Bikes.
I've often had to resort to fabricating my own solution to a problem on older bicycle restorations but it is often cost-prohibitive for the customer. I sometimes do it just for the Love of it. I've had my share of failures, buddy. What you are attempting is admirable but, if you'll curtail your enthusiasm for a moment - I may save you some heart-ache. I know that I sound like a know-it-all. Not.

The first problem that I see? No problem with your choice of AL to use for the crankarms. The problem is that they need to be cold-forged. To machine them from billet for your own personal experiment - it's Your calf, not mine! Please don't go there if you intend to actually go beyond proto stage.
Even then, with both 6 or 7000 alloys, I would anneal them as both tend to be brittle under 'sudden impact' lateral loads such as a rock or curb strike. As you probably know, annealing aluminum and brass makes them 'tougher' as opposed to 'softer' as would apply to iron alloys such as steel. The 3 metallurgical charachteristics are Toughness, Hardness and ?? You can do it!
Foregoing any treatment, I am inclined to agree with xerxes on the thickness of the crankarms. I am totally in-tune with shaving grams but...

A 40 T ring with no spider. Oh boy. I'll have to sleep on that one but, you're not building a BMX racer for a 90 lb kid, either. Hmmmm. I would probably lose all of the cutouts and opt for a solid disc.

Curiosity question - What are the (4) .26" dia holes on the 4" dia plane? That would appear to be the logical place for bolting to a spider.

PS - Engineers drive trains. Why would you want thier thoughts?? Wink

Wheelies don't pop themselves. (from a QBP fortune cookie)
Sorry for being late to the party - the Mechanical Design Engineer has arrived Smile

The single pin drive will work, IF AND ONLY IF the center of the chainring is constrained radially and axially. A Ø1.125 diameter surface is not going to constrain a Ø1.250 diameter hole in anyway.

The single pin needs to be snug in the tangental direction, and loose radially.

Why the mix of inch and metric -use metric like most of the world.

In addition to Xerxes and RobAR's excellent comments:
* the section between the chain ring teeth and the triangular openings looks too shallow, the ring will tend to deform into a rounded square - think four lobed bio-space.
* the tooth profile will result cause a great deal of noise - look at profiles on commercial parts.
* it is advisable to make the pedal holes threaded - make sure the hand of the thread is correct.
* if you want to make the cranks this flimsy, consider titanium instead of aluminum.
* remember bh^3
* if you want the arms that thin; make the cranks 150mm long like BMX bikes
Wow those is quite a lot to absorb.

Nigel- Thanks for pointing out my drawing error with the fit between the ring and crank. My redraw will match the cranks to that 1.25 and beef them up some in the thickness as well. The pedals will be threaded, I just haven't drawn it in. In terms of the toothform, I used an online tutorial for drawing a sprocket, and my coworkers seconded its a lot of trial and error since we're not designers. Any tips or resources to ensure a good tooth? Can you also clarify what you mean about the fit of the pin? Would I benefit from adding a towel pin or just a close fit clearance hole?

RobAR- In terms of the strength of the material, cost is a factor since its a personal project mostly as well as an introduction into the trade. The 4-bolt circle is a preliminary spider setup, and in the sense of this design, they are backup in case one blows out, and looks cool.
A correction to the last post of dowel, not towel (stupid autocorrect).

I did some tweeking to the initial crankarm design. The width is bumped up to 1.25 and increased the thickness by 0.150. I also added a 20* detail for looks and a touch of "aero".

"Can you also clarify what you mean about the fit of the pin? Would I benefit from adding a towel pin or just a close fit clearance hole?"

The Ø.260 holes in the chainring should be slots, with the long axis conicident with the center of the chainring. You are not going to be able to locate the pin (or screw or whatever) perfectly. The pin is to transmit torque from the crank to the chainring, there is no force through the pin in the radial direction (axis of the crank); all the force is perpendicular to the crank. Thus you want the pin to be able to float in the chainring radially, while being held to a very tight tolerance tangentally.

Any slop in the tangential direction will result in noise and a loose drivetrain. When you stop pedalling there will be a thunk as the rear wheel drives the chainring, and it starts pushing against the crank. And it will thunk again as you start pedalling and the crank starts pushing against the chainring. It is MUCH worse on a fixie. Any slop will result in fatigue failure of one or more of the following: the pin, the crank arm, the chain ring.

You are showing a threaded hole, assuming your going to use a bolt as your pin, make a bolt with a conical feature to take up the tolerance between the bolt and the chainring, and put a chamfer on the slot in the chainring to match up with the cone on the bolt, there should be a counter bore in the threaded hole to provide clearance for the cone feature on the bolt so that the cone does not control the axial positioning of the chainring, but only provides the drive.
"Any tips or resources to ensure a good tooth? "

only to look at and measure some commercial parts. The chainrings on my bikes appear to be involutes http://en.wikipedia.org/wiki/Involute Though I have never measured to confirm.
I think I get what you mean. I'm not up on engineering lingo too much so that's why I ask so many questions. I assume figure A is what you mean with the lead-in chamfer matching something like a flathead socket-head. I mentioned the combo of bolt and dowel pin because in my trade, tool making, it is common to use dowels at close tolerance (+/- 0.01mm [0.0007in]) for holding positions, especially when they get CMM-ed for approval. I'm guessing that may be overkill for this application.

A is correct. Using flathead (senior moment last night when I said cone) takes up the tolerance, so that there is no slop. With a cylinder, there is ALWAYS a small amount of slop.
I'm loving it! Nigel - I have blue and white striped Engineers cap that I was gonna send you but, you made my head hurt so I am wearing it Smile. It only fits Cone Heads, anyway.

Capner - I believe that your new Section View is much better as per Nigels points. But, don't forget that if this is a spider-less design as you've indicated, you will need a spacer(s) between the ring and the arm to position it laterally. Or did my head get lost? Flexy.
Let's all try not to 'reinvent the wheel', please. Wheels have been just fine since they were reinvented on the 1970's television series of Planet Of The Apes. Same goes for this round thing. The closer we can stay to Industry Standards, the better in the long-run. Agree?

You have obvious skills in design thoughts and AutoCAD but, you've yet to formally introduce yourself or tell us if you're the boss, or if you are working for a boss that doesn't mind you day-dreaming because he wants this for His bike. Wink
Either way, we don't really care.This is BT and some of us eat this up. Big fun!

Brother. There is no 'overkill' in any application design. It's MFG tolerances that eat us alive. How will our Mass Produced, Stamped from Sheetmetal brain-farts perform in the real world? You are on the right path. Measure twice. Cut once. Sound familiar?
Wheelies don't pop themselves. (from a QBP fortune cookie)
A formal introduction is long overdue, so here it is. My name's Daniel, 25, student and machinist. I graduated 2005 and study at the local community college as an Apprentice Toolmaker. I work in a small custom machine shop that specializes in jigs and fixtures for automation, inspection, and other applications. We also do parts & tooling and structure & framing 80/20 designs. Primarily small runs from prototype to build-only.

The only formal AutoCAD and drafting experience I have was in high school. Most of the experience and knowledge I've gained is from being in-industry reading prints for machining and assembly, and understanding the application for certain designs and tolerancing (different stack-ups and open vs tight tolerance).

About the lateral position: I measured the bottom bracket and based on the figures I calculated (see 0.251 step) this will theoretically match the chainline. Thinking about it, I may increase the step so I can add spacers to bring it medial if I need to. I'll dig up my notes and post them.

About the chainring depth: I redrew the chainring, I should add it is a 3/32 chain using the same tutorial as I did before (http://www.gizmology.net/sprockets.htm). The basic drawing system they provide comes up with sharp teeth, so there is a formula for the outside diameter (the toothform compares to existing chainrings). This creates a tooth depth of 0.2964, and the roller is 0.3125. What I understand the mesh between the chain and sprocket is that a minimum 120 degrees of the sprocket must be engaged.

I've been playing with the idea of switching into engineering. Any thoughts? A broad question, but bike stuff is where I want to apply my skills.
"I've been playing with the idea of switching into engineering. Any thoughts? A broad question, but bike stuff is where I want to apply my skills."

Engineering is all about math. If you can tough it out through Calculus and Differential Equations, the rest of Engineering is comparatively easy.

As far as focusing on bikes - the market is limited, and the income is not very good, even if you own the company. But income is not everything - passionate people make a difference.
Thanks for the advice about Engineering. Our designer said he wasn't up for four years of Calc so he opted for the less mathematical route.

Here are the notes I base my designs from:

Spindle Details:
-- Taper angle: 2 deg
-- Minor square: 0.4960 (12.6)
-- Major square: 0.5433 (13.8)
-- Taper length: 0.6575 (16.7)
-- BB Shell to end of spindle: 0.9843 (25)
-- Frame center BB Shell end: 1.3779 (35)
-- Chainline: 1.7717 (45)
Note: The major square and taper length dimensions are approximate based on where the radius blends with the flat.
Have you also considered that most cranks are curved out from the bottom bracket, so they clear the chain stays. Without a curve, you've have to use a very long bottom bracket and it will look a bit odd, also you may find that you clip your ankles on the bottom bracket end of the crank arms.

Here's what happens if you have a BB axle that's too short:

[Image: 003-3.jpg]
Thanks for reminding me to look at that. With my straight shank design it would not clear the chainstays. A rough ballpark estimate indicates a 3-7 deg relief or offset to achieve clearance. A clearance angle is easier to produce than an offset to the shank but also removes material. Then again, we're not building watches here.
Calculating all the angles and stuff to achieve the clearance I needed, I opted for a minimal cutout at the pedal end of the crankarms. This should in theory provide the necessary clearance between the crankarm and chainstays. The step is 1" from the end.

Put a generous radius at each end of the 17.4° ramp.

You can taper the arms a great deal more, and maintain the strength you need. Think about the torque in the crank arm - which is constant the length of the arm, meaning the shear force in the arm is linearly increasing from the pedal to the center of the crank. And you also need to think about the bending force on the crank (side to side on the bike).

If you look at really high end cranks, Hollowtech for example, you see that the pedal end is just big enough for the pedal threads, then in gets wider and deeper as you go towards the crank centerline.

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