Re: [Frame] Chainstay Dimples

[jon norstog]

None whatever, Joe.

jn

"Thursday"

On Mon, Mar 18, 2013 at 1:04 PM, Joe Graham <jgra...@gmail.com> wrote:

Hey All,

I've been wondering what the story is with chainstay dimples. I get what they do in terms of tire clearance and crank arm clearance, but are there other benefits? If a chainstay doesn't need the dimple to fit within the constraints of the tire and crank arms, is there any reason to add it?

Thanks,

Joe Graham

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[Tom W]

in connection with that, what is the best tool/method to mae such a dimple. I don't suppose you could buy a dimpler. Seems simple but I struggle with how to get it done wiht regular tools.

[Richard Sachs]

...buy a dimpler.

atmo -
http://www.flickr.com/photos/9866331@N08/sets/72157600706475164/detail/


e-RICHIE
http://www.richardsachs.com/site/category/atmo-bis/

[Andrew R Stewart]

Tom- The way I've done the handful of true dimples is this- I have a hand
carved/filed set of ovalizing blocks, one becomes the backing block for the
side of the stay I wish to keep as is. Any form that roughly fits the stay's
profile will do. This gets placed in the bench vice against one jaw. I
carved/ground/filed,sanded a dimpling form out of a 3/8" piece of steel rod.
It's kind of cucumber shaped with a flat filed along the long axis. This
form is placed in the vice on the second jaw, double sided tape holds it in
place where it is centered in the backing block's form. The stay is placed
in the backing block and care is taken to align the three pieces so the
dimpling form is straight with the stay, the stay is centered within the
backing block and the drop out is vertical. Then the vice is turned down.
More care is taken during this step to insure all is still lined up. If done
well there is no change of the stay's contour on the undimpled side and the
dimple is centered and straight at the right spot along the stay's length.

I have also tried to hammer a dimple into a stay with less controlled
results. I usually try to use my ovalizing blocks first to get clearance as
I like the look of round/oval/round stays better then dimpled ones. Andy.

----- Original Message -----
From: "Tom W" <twec...@yahoo.com>
To: <frameb...@googlegroups.com>
Sent: Monday, March 18, 2013 10:18 PM
Subject: Re: [Frame] Chainstay Dimples

> in connection with that, what is the best tool/method to mae such a
> dimple. I don't suppose you could buy a dimpler. Seems simple but I
> struggle with how to get it done wiht regular tools.
>

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Andrew R Stewart
Rochester, NY

[omar-...@cox.net]

This is how I do chain stay dimples.
Omar Khiel

--------------------------------------------------
From: "Tom W" <twec...@yahoo.com>
Sent: Monday, March 18, 2013 7:18 PM
To: <frameb...@googlegroups.com>

Subject: Re: [Frame] Chainstay Dimples

> in connection with that, what is the best tool/method to mae such a
> dimple. I don't suppose you could buy a dimpler. Seems simple but I
> struggle with how to get it done wiht regular tools.
>

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> http://search.bikelist.org (older content).
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[William Chitham]

I can see why and I understand how but what is the downside? Do dimples weaken the stay, are they ever a cause of cracking?
William.

[Mark Bulgier]

William Chitham wrote:

I can see why and I understand how but what is the downside? Do dimples weaken the stay, are they ever a cause of cracking?

 

Yes they weaken the stay, but that only means they yield a bit easier, which almost never has any practical impact other than when you spread the stays later for a wider hub.  Then you need to remember that the right side stay will bend easier, because it (usually) has indents inside and out, where the left stay is dented inside only.  You know frame tubes never yield in use, other than crashing, right?  So strength per se is pretty irrelevant, except to the extent that it correlates with fatigue endurance.  If anyone disagrees that strength in a chainstay is almost irrelevant, please speak up so we can converge on that point.

 

You’re also taking some fatigue endurance out of it, but if done smoothly, this is seldom a place for fatigue cracks. One exception I can think of is Reynolds 531 Superlight from the mid-late ‘70s, which came with chainstays that were flattened on the inside for the tire clearance.  At least that was the most common option I saw, for a few years there, though they probably came in other CS shape options also if you ordered enough sets.  The flattened ones frequently cracked at the edge of the flattened area – it was a bad design.

 

So I think the classic indent shape, like ‘70s Columbus SL/SP and most of the shapes linked to in this tread, is correct somehow.  I can’t describe it in words but it just feels right to me.  And the excellent record, of mega-miles under strong riders without cracking, is all the evidence I need.

 

I’m not saying indented is better than round-oval round.  It might be, in some really minor way, but R-O-R is plenty good enough.  So I doubt we’ll ever prove either indented or R-O-R is definitively better than the other – I think they’re too close to identical, in strength, stiffness and fatigue endurance.  Providing, that is, that the indents are done smoothly with tapered transitions.  R-O-R is always going to be good in fatigue because it always has smooth transitions, whereas indents are sometimes done crudely, so that’s a point in the R-O-R camp, compared to average indents.  I don’t think R-O-R is better in fatigue than excellent indents however.  And indents can theoretically be a little better than R-O-R in terms of stiffness.

 

What any tire/chainwheel-clearance scheme does, compared to leaving the stays full-diameter round, is reduce stiffness.  Assuming we want the stiffness (and that is debatable), the clearance scheme should preserve as much stiffness as possible compared to the full-diameter round stay, which is assumed to be close to ideal.  But you can’t just leave them round, unless the stays are very long, without clearance problems. (Yes, “Rapid-Taper” stays, like Reynolds made in the 60s and 70s, technically remain round.  But they go to such a small diameter at the clearance point that they might be the worst design of all, from a stiffness point of view.)

 

In theory, an indent can be sized/positioned to give just the right amount of clearance, right where you need it, leaving as much as possible of the remainder of the stay round.  (In practice, many indents are too long, or positioned wrong, but let’s assume a properly-sized/placed dent for the moment). 

 

An oval generally makes more of the stay less stiff.  For example on the left stay, it is bringing the inside of the stay outward for tire clearance (good), but it’s also bringing the outside of the stay inward, for no reason at all.  Less stiffness with no benefit.  On the right stay, oval makes more sense, because of the need for chainwheel clearance.  But the oval doesn’t make the clearance only where needed, like an indent can.  The inner indent for the tire and the outer indent for the c-ring are often ideally in different places, so an oval needs to be longer to cover both areas.  More of the stay is left in the less-stiff shape.

 

Now, I am actually a fan of having some flex in a bike frame, but my gut reaction is chainstays should be optimized for “light and stiff”, within the constraints of tire and c-ring clearance, and of course without making the chainline wider than it needs to be.  That is, make them as light as you can without them becoming too flexible.  A minimum stiffness then becomes the limiting factor in how light you can go; not strength, and not fatigue endurance (assuming care is taken to avoid stress-risers, or weak heat-affected-zones from welding/brazing).  If you can afford to throw away stiffness with suboptimal design of your tire/c-ring clearances, then you’re building it heavier than you could/should.

 

Mark Bulgier

Seattle

 

[Mathias Scherer]

I’d say no. With roadbikes you usually can get away without dimples, on MTB frames (29er with shortish stays in particular) that often is not an option.

I’d love to see what people use for making these dimples. With steel it’s not that big a deal, I’d say, I find it a lot more challenging with Ti, even if it is annealed stuff.

Cheers,

M.

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Mathias Scherer
Ludwig & Scherer GmbH
Wintringerstrasse 19
66271 Kleinblittersdorf

Geschäftsführer Mathias Scherer

[John Caletti]

If using a thinner/harder steel alloy or Titanium - the dimple can be a bit … harsh a shape and I've cracked some ti chainstays doing it. I will ovalize it with a couple of slightly rounded forms and the arbor press (or vise), to get the needed tire/ring clearance.
Many ways to skin this cat.

John Caletti

Caletti Cycles
www.caletticycles.com

[Tom W]

Thanks for the dimpling advice.  I can make simple tool based on what you guys have posted.  Thanks!

[starmichaelbowman]

Hi, this is the tool i just made and it worked great. Harbor Freight arbor press with a big old second hand bolt brazed to the bottom, then i ground and filed the bolt down to a nice tear drop shape.
http://www.flickr.com/photos/starmichael-bowman/8514189406/in/set-72157632685818716

StarMichael Bowman

Portland OR

starmichael-bowman.tumblr.com

On Monday, March 18, 2013 12:04:36 PM UTC-7, Joe Graham wrote:

Hey All,

I've been wondering what the story is with chainstay dimples. I get what they do in terms of tire clearance and crank arm clearance, but are there other benefits? If a chainstay doesn't need the dimple to fit within the constraints of the tire and crank arms, is there any reason to add it?

Thanks,

Joe Graham

[Mathias Scherer]

Looks nice, but pretty much impossible with Ti, I’d say. Even with CP. Is that ok with all types of steel?


Am 20.03.2013 7:27 Uhr schrieb "starmichaelbowman" unter <starmich...@yahoo.com>:

http://www.flickr.com/photos/starmichael-bowman/8514189406/in/set-72157632685818716

[Joe Graham]

Thanks for all the good information everyone! Much appreciated.

Cheers,

Joe

[John Thompson]

On 03/19/2013 05:48 AM, William Chitham wrote:

> I can see why and I understand how but what is the downside? Do dimples
> weaken the stay, are they ever a cause of cracking?

Yes:




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-John Thompson (jo...@os2.dhs.org)
Appleton WI USA