Chainring/ chaing stay clearance

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jim g

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Oct 4, 2022, 3:46:26 PM10/4/22
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I'm planning out a frame to build for a friend who wants to use 29x2.35 tires, and I'm planning to use Nova CrMo Mtb S-Bend 25.4mm - 9/6 - 450mm long chain stays.  He also wants to run a triple crank.

Is there an easy way to determine crank & chainring clearance before I start cutting tubes?  I'm thinking of a PDF template that I can print out representing the cross section of the crank and chainrings, or something similar?  I feel like I've seen this before, but can't remember.

Thanks!
-Jim G

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jimg at yojimg dot net

Alex Wetmore

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Oct 4, 2022, 4:03:20 PM10/4/22
to jim g, Framebuilders
I'm thinking about a similar frame for myself (it's going to be a fully rigid MTB to ride on trails alongside my kids) and I used Fusion 360 to model it.  I'm happy to share my file.

This is the model of the chainstay area.  Note that this is a 1x bike, the second massive looking chainring shows the inside of the crank arm path (I measured a Deore crank that I had on hand).

Here are the types of parameters that I setup control it:

The whole frame is parameterized (roughly 50 parameters so far) so it's pretty easy to play with different geometry.

A limitation is that I can't figure out how to properly model ovalized and tapered tubes.  As a result my chainstays are modeled as round, and I just the smaller diameter of the oval as the size (since lateral width is harder to get right than height on chainstays).

Another option is to use Martin Manning's BG101 spreadsheet.  It handles double bend chainstays too, though it is more "road" than "MTB" oriented overall.  I found it on archive.org:
https://web.archive.org/web/20201112002802/http://www.os2.dhs.org/~john/Bicycle_Geometry/

Alex

From: frameb...@googlegroups.com <frameb...@googlegroups.com> on behalf of jim g <yoj...@gmail.com>
Sent: Tuesday, October 4, 2022 12:46 PM
To: Framebuilders <frameb...@googlegroups.com>
Subject: [Frame] Chainring/ chaing stay clearance
 
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david levy

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Oct 4, 2022, 5:29:22 PM10/4/22
to Alex Wetmore, jim g, Framebuilders
I would suggest not overthinking this, get a piece of cardboard and draw all the parts out, tire, crank arm, chain ring and then lay the stay you want to use on top and see what fits.  Kinda like making paper dolls...

Dave



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Dave Levy 
Ti Cycles Fabrication (Bicycle stuff) 
Cedar Ridge Fabrication (Car stuff) 
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503-621-9670 | http://ticycles.com
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Eric Keller

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Oct 4, 2022, 6:08:14 PM10/4/22
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I had a template like the one RH sells for their cranks laser cut out
of stainless. It's for 48/28 grx cranks though. Tbh, it's probably
more useful to see if you screwed up after the frame is built.
Although I'm trying to design a chainstay fixture that allows it to be
used prior to assembly. Have to hold a bb shell somehow.

Somewhere on flickr there is a picture of something similar someone
made that uses bolts, so it can be adjusted. Eric Estlund or Joel
Greenblatt maybe?
Eric Keller
Boalsburg Pennsylvania.

Alex Wetmore

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Oct 4, 2022, 6:44:43 PM10/4/22
to david levy, jim g, Framebuilders
Laying out stays on top works great if you have access to a variety of them to try and that is exactly what I'd do if I were hanging out down at Hahn Rossman's shop for example.  As a hobbyist I have limited tubing around (right now I have no bent chainstays on hand) and so I'm ordering what I hope to use on the final bike.  I also don't have a bender suitable for chainstays.

The first chainstays that I tried to make fit (Zona 29er S-bend) didn't have quite enough bend to clear the tire that I wanted, so I'm glad that I modeled it.

Drawing it on paper is great too, I'm just faster on CAD than doing it by hand.

Alex
From: david levy <titanium...@gmail.com>
Sent: Tuesday, October 4, 2022 2:29 PM
To: Alex Wetmore <al...@phred.org>
Cc: jim g <yoj...@gmail.com>; Framebuilders <frameb...@googlegroups.com>
Subject: Re: [Frame] Chainring/ chaing stay clearance
 

Hahn Rossman

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Oct 5, 2022, 10:04:13 AM10/5/22
to Alex Wetmore, david levy, jim g, Framebuilders
Jim-
You can find all the specs in the frame fit documentation from whomever is making the cranks. I made this gauge a while ago:
Because I wanted a final check I could use in the jig before brazing. It has also been handy for laying on top of full size drawings.
Hahn Rossman 

jim g

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Oct 5, 2022, 12:31:12 PM10/5/22
to Hahn Rossman, Alex Wetmore, david levy, Framebuilders
Hi Hahn,

That gauge you made looks perfect!  That's the sort of thing I was looking for -- I want something I can lay on a full-size drawing to see what my clearances are.

It looks like I can find the dimensions I need at https://www.peterverdone.com/wiki/index.php?title=Shimano_Framebuilder_Info#Chainwheel_Dimentions and then I can draw it up in Solidworks and water jet some material at work.

Thanks!
-Jim G

Alex Meade

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Oct 5, 2022, 5:07:53 PM10/5/22
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+1 for drawing them on heavy paper and cutting them out. Pro tip (not that I'm a pro):  label them and save 'em so you don't have to do it again unless you want to try a different stay.   I now have a nice collection and can try a bunch of shapes in just a few minutes. I've also done this in CAD, but I can draw them and cut them out a lot faster than I can model stays in CAD.

IMO, this is one of the few places BikeCAD could be improved.  A library of stays would be just super!

Alex

Chris Sikorski

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Oct 12, 2022, 12:09:36 PM10/12/22
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Anyone use this?  Looks like it would work

Eric Keller

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Oct 12, 2022, 2:46:16 PM10/12/22
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I couldn't understand how it worked, so I looked on their instagram.
https://www.instagram.com/p/B6TNBSYlaxE/

It's a good idea.
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