# Chainstay yokes - what thickness?



## cable_actuated (Jun 7, 2012)

I'm in the planning stages for my second [attempt at a] frame. I've got the front end all sorted out, but I'm still thinking about the chainstays. I've been studying the various chainstay and seat tube configurations posted on here to get short chainstays and clearance for fatter (2.3-2.5") tires. I'm attracted to chainstay yokes both aesthetically and because they seem to open up more design options for those of use without the expertise or tooling to bend tubing.

My question is what thickness of plate to use?

To clarify, I'm thinking something along the lines of...

this by Clockwork










or this by A_A










This second frame was a snow bike and A_A mentions that he used 1/8" but had to retroactively add a brace because it was too flexy.

I'm thinking that 1/8" would work for me because of my size (120 lbs) and because the yoke isn't going to nearly as wide as it is on a snow bike.

Any reason I would want to go thicker?


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## TrailMaker (Sep 16, 2007)

Hey;

If a part were cast, it could be thinner. Even .125" CroMo steel is just not going to have any rigidity at all by itself. There is the possibility add stiffness with gussets, webs, or boxing. I had been considering some designs in that realm. However, they were not necessarily intended to be a work-around for having no bending capability. I would think .1875" would be about as thin as I'd want to go, and CroMo for sure, at least. If you could source some quench-hardenable steel alloy, the stiffness would rise substantially.


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## dr.welby (Jan 6, 2004)

TrailMaker said:


> If you could source some quench-hardenable steel alloy, the stiffness would rise substantially.


Ummm, heat treatments do not change the stiffness of steel, and even between steel alloys the difference is only a couple of percent.


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## TrailMaker (Sep 16, 2007)

Ummmm....

Then I am to gather that this guy - and most of the the other links you will find on hardening & tempering - does not know what he is talking about?

How to Harden Steel With Motor Oil | eHow.com

I'm not a metallurgist, engineer, nor a current guest at Holiday Inn Express. I'm just curious.


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## dr.welby (Jan 6, 2004)

TrailMaker said:


> Ummmm....
> 
> Then I am to gather that this guy - and most of the the other links you will find on hardening & tempering - does not know what he is talking about?
> 
> ...


Though they seem very similar, hard does not mean stiff.

Hardness is the ability of a metal to resist permanently deformation, while stiffness is how much deformation you get under a given load. Say you had two identical springs, except one is hardened as hard as you could get it and the other annealed as soft as you could get it. If you put the same weight on the the two springs, both will compress the same amount. If the load is big enough, the hard one will spring back but the soft one won't.

So a hardened chainstay yoke may survive structurally, but you won't like your tire rubbing on the back of your seat tube!


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## TrailMaker (Sep 16, 2007)

Hey;

I do fully understand the difference between strength and toughness. Somewhere between the two, there must be a place where stiffness & ductility cross paths, and there has got to be an alloy out there that can be optimized for that. Maybe that place is plain old CroMo? A2 perhaps? Never tried to bend it. Or would simple hardening/tempering of mild steel be best?

With all the alloys out there, there has got to be a good/better/best choice.


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## dr.welby (Jan 6, 2004)

Well, no. 

Stiffness of a metal is primarily determined by the atomic bonds between the base metal atoms. 

Adding alloying compounds doesn't really change this much - you're still "stretching" the same bonds. They do help in strength though - you can think of them as getting in the way of the bonds breaking and reattaching.

Heat treating also doesn't change the bonds. It does rearrange geometry of these interlinked atoms, which can have an effect on how easily they can be displaced.


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## Clockwork Bikes (Jun 17, 2006)

The pipe and flat bar are 3/16" thick. I picked this out of thin air since I had never done anything like this. Even with the fairly thick material it was pretty flexy until the seatstays went on. It would be a much better solution to have something relieved or webbed.

-Joel


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## cable_actuated (Jun 7, 2012)

Joel - thanks for chiming in. After looking at that picture of your bike for awhile I figured it was probably made out of pipe. Then I found the build pictures on your Flickr stream. That looks like it was a lot of work! I don't have a lathe, so that's a no go.

Very interesting discussion on heat treatment. I was planning on using 4130 strip from Aircraft Spruce. It mentions in the product description that it responds to heat treatment.

4130 STEEL STRIPS from Aircraft Spruce

I'm thinking that I might actually aim for something like this from Lunar bikes instead of using a round (circular) yoke.










Simple is what I'm aiming for. Thanks for the input.


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## RCP FAB (Jun 15, 2011)

What wheel size and chain stay length are you shooting for?


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## smdubovsky (Apr 27, 2007)

4130 on something that thick is completely unnecessary. Mild steel like 1018 will be the same stiffness at a fraction of the cost.


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## cable_actuated (Jun 7, 2012)

My goal isn't really that ambitious. I'm thinking 16.0" (~400mm) stays for 26er with good room for 2.35" tires. I'm replicating the geometry from my favorite frame - the Zion 853EBB, but fixing it's one flaw - that it only has room for 2.1" tires. I'm also shortening the rear end a bit (only 0.25").

The reason I'm thinking about a yoke, and strongly leaning towards something similar to the Lunar design is that I'm planning on using an EBB (mostly since I've invested a bunch of dough in the Bushnell Featherweight), so I'm dealing with 66mm wide shell (TANDEM SHELL CRMO FOR BUSHNELL ECC :: TANDEM & SINGLE SPEED SHELLS :: BOTTOM BRACKETS :: Nova Cycles Supply Inc.). That doesn't give you a lot of room to spread out the attachment points for the stays.


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## DEFCON4130 (Dec 19, 2009)

I made this one out of 1020 1in thick plate. Hasnt found its way to a frame yet but.. soon. The two long pieces that get mitered will be cut down to more than half what they are now.


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## GrayJay (May 16, 2011)

cable_actuated said:


> My goal isn't really that ambitious. I'm thinking 16.0" (~400mm) stays for 26er with good room for 2.35" tires. I'm replicating the geometry from my favorite frame - the Zion 853EBB, but fixing it's one flaw - that it only has room for 2.1" tires. I'm also shortening the rear end a bit (only 0.25").
> 
> The reason I'm thinking about a yoke, and strongly leaning towards something similar to the Lunar design is that I'm planning on using an EBB (mostly since I've invested a bunch of dough in the Bushnell Featherweight), so I'm dealing with 66mm wide shell (TANDEM SHELL CRMO FOR BUSHNELL ECC :: TANDEM & SINGLE SPEED SHELLS :: BOTTOM BRACKETS :: Nova Cycles Supply Inc.). That doesn't give you a lot of room to spread out the attachment points for the stays.


Here is a shot of the chainstays I added to a 68mm BB shell that have clearance for a 3.7" fatbike tire. i just bent the CS myself on a simple forkblade bending jig. Obtaining clearance for a 2.3" tire on the 66mm BB should be no problem by comparison even without a yoke.


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## Feldybikes (Feb 17, 2004)

cable_actuated said:


> My goal isn't really that ambitious. I'm thinking 16.0" (~400mm) stays for 26er with good room for 2.35" tires. I'm replicating the geometry from my favorite frame - the Zion 853EBB, but fixing it's one flaw - that it only has room for 2.1" tires. I'm also shortening the rear end a bit (only 0.25").


Another option: you might get the clearance you're looking for with Deda chainstays. Give Joe Bringheli a call to get 'em. They might also be available from Nova, but I'm not sure.

I recently used these stays with a 73mm bottom bracket (wider than what you're using, I know) and have clearance for 2.3" tires (well, Schwalbe Tabletops, so not particularly knobby but they are reasonably wide) at only 15.7" chainstays. I could imagine with slightly less room at the BB, you could achieve 16" chainstays without too much problem. Another factor is that I used Paragon sliders, so the dropouts are quite a bit wider than normal, so thay might've helped my clearance a little bit, Clarence.


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## TacoMan (Apr 18, 2007)

The stiffness of all steels is basically the same, it is the measure of Modulus of elasticity which is 29.1*10^6. Titanium is about 1/2 as stiff at 14.9-14.5 and alum 1/3 as stiff at 10.5. Heat-treating increases yield stength, higher quality steels can be heat-treated to a higher yield before becoming brittle, 4130 is not considered a very high quality steel since you can only really go up to 130ksi, but 4340 and 300M is and you can go up to 280ksi.

Low carbon mild steel can only be surface hardened, so no increase in yield. 

FYI, our yokes are cut from .187 4130 plate and then milled to a taper of .060 at the end.


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## DavyRay (Apr 13, 2012)

Simply put, the amount of deflection under load of any steel structure depends on the geometry of that structure, not the strength of the steel. A thick steel structure will deflect less under load than a thin one. The strength of the steel determines whether the structure returns to its original shape, or deforms permanently. All steel deforms about the same amount under stress. 

Weak steel will deform permanently. Stronger steel will return to its original shape.


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## teatreetim (Nov 14, 2011)

Not sure you realise but the bikes you are showing are 29ers where getting the wheel closer to the seat tube is a bigger deal. I'm sure the pros could elaborate.


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