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What is up with this chainstay?

Old 11-24-13, 01:44 PM
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tdsherman325
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What is up with this chainstay?

So I went to take a photo of my bike and noticed that the right chainstay appears to be severely bent:



At first I though it might be an odd angle but I looked and sure enough, it has a bend in it. However, had it gotten bent at some point, that would surely throw off the geometry of the rear wheel, right? The rear wheel sits perfectly straight in the dropouts and they're nice and even. Plus the seatstays aren't deformed and I would figure that anything that would bend a chainstay like that would deform the corresponding seatstay. Also, it's aluminum, and it's my understanding is that aluminum does not bend like that in the event of an impact. I could be wrong, though.

This bike's a bit of an oddball - I've never been able to locate any information about it online. The frame resembles the construction of a Schwinn Homegrown, however, and I've found one photo of a homegrown with a similarly bent chainstay:



So is this normal? Just some weird quirk of how schwinn aluminum frames were built in the 90's? Or did my bike somehow get damaged without affecting its geometry?
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Old 11-24-13, 01:49 PM
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curved to reduce chain slap on top of the tube.
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Old 11-24-13, 01:53 PM
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Originally Posted by fietsbob View Post
curved to reduce chain slap on top of the tube.
Agreed. It's an early-mid-'90's bike, when there was a lot of experimentation going on with regard to chain slap and clearance. It's a feature, not a bug.
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Old 11-24-13, 02:15 PM
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You can see it described as an "Asymmetrical G-Force chainstay" on the catalog page here: http://bikecatalogs.org/SCHWINN/1992...992_Pmt_14.jpg
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Old 11-24-13, 02:41 PM
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+1 to all the above.

Chainstays haven't been symmetrical for decades. The right has been bend, curved, crimped, ovalized and just about anything you could think of to address tire, chainring & crank clearance, chain slap and other issues. This is just more obvious than some of the other tweaks.
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Old 11-24-13, 05:41 PM
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I could be wrong, but that looks remarkably like a crankset that was recalled by Shimano many years ago. These cranks were known to have defects that allowed them to crack easily, leading to the crank-arm sheering off under force.
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Old 11-24-13, 05:43 PM
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Nope it's the underground version of homegrowns. Go to bonus tomato.com.
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Old 11-24-13, 07:43 PM
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Thanks, everyone, that explains it. I've never owned a bike from that era before, so it looked a bit odd to me.

Originally Posted by cannondale125 View Post
Nope it's the underground version of homegrowns. Go to bonus tomato.com.
I looked at those, but I don't think that's it. Several details, including the bottom bracket and the rear dropouts are completely different, and the Project Underground bike appear to have been labelled "Project Underground" on the top tube, whereas mine is labelled "High Timber Aluminum". Plus, mine's all aluminum, and from what I gather the Project Underground bikes were some kind of odd thermoplastic or carbon fiber composite.

The frame looks an awful lot like a 95 homegrown frame. This one I found online is identical, AFAICT:



Hmmm... I think I'll start a thread over in the C&V (Is '95 old enough to count as classic? I see folks posting about 90's stuff in there) or MTB forum to try and get more info. If anyone here knows anything, let me know.

Thanks again!
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Old 11-24-13, 10:02 PM
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Originally Posted by FBinNY View Post
The right has been bend, curved, crimped, ovalized and just about anything you could think of to address tire, chainring & crank clearance, chain slap and other issues. This is just more obvious than some of the other tweaks.
I've never seen a chainstay bent like this to reduce chainslap before; it's pretty neat. I'd also say uncommon.

And typically there's only an extra dimple on the right chainstay for chainring clearance; the asymmetry generally stops there, cable stop and derailer hanger aside... right?

Or am I missing part of the picture on this side of the pond?
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Old 11-24-13, 10:06 PM
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Originally Posted by nhluhr View Post
I could be wrong, but that looks remarkably like a crankset that was recalled by Shimano many years ago. These cranks were known to have defects that allowed them to crack easily, leading to the crank-arm sheering off under force.
Originally Posted by Bixid View Post
The cranks have been installed on more than 200 models of bicycles manufactured since 1994 and sold under at least 49 brand names. Most bicycle manufacturers have sold bicycles that are involved in this recall. The cranks also may have been installed as an upgrade as part of the Altus, Acera and Alivio groups of components. The cranks are numbered FC-CT90, FC-M290 and FC-MC12. These numbers are located on the back or inner side of the right crank arm.

Consumers should check their bicycles' crank assemblies. If they are involved in this recall, consumers should stop using their bicycles immediately and call Shimano at 800-353-4719 to arrange for the free replacement and installation of a new crank assembly at a nearby authorized repair shop. Consumers can also arrange for the free replacement with their bicycle dealers.
Originally Posted by Feldman View Post
Though ten years on [this post is dated 2007 - Kimmo], Shimano is still honoring this recall--they really, really want those cranks off the road. It may be fear of liability, but to this bike business lifer it's an example of a company that really does give several dozen damns about its customers.
Wonder if you can still get a free replacement...
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Old 11-24-13, 10:09 PM
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Originally Posted by Kimmo View Post
I've never seen a chainstay bent like this to reduce chainslap before; it's pretty neat. I'd also say uncommon.

And typically there's only an extra dimple on the right chainstay for chainring clearance; the asymmetry generally stops there, cable stop and derailer hanger aside... right?

Or am I missing part of the picture on this side of the pond?
There are chainstays bent way down up front so they don't pass directly between the chainring and tire at the narrowest place. One builder even split the right chainstay so there was a gap where the chainring and tire were closest.

On my hardtail mtn frame the right and chainstays aren't matched, so they look very different if seen from top or bottom. This improves chainring clearance allowing low Q-factor fro the triple, but the wheel looks like it isn't centered in the stays. Not an issue since it has hvertical dropouts, but a few people have asked if my frame was bent.

Over the years there have been a number of creative ideas relating to right chainstays, but many don't survive despite being quite functional simply because they look "wrong".
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Old 11-24-13, 10:46 PM
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I looked on the right crank arm - no number like that is stamped, just "175 TG SHIMANO JAPAN VIA H", although it is an Alivio crankset.
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