Are you bent out of shape?
06-16-15 | 07:27 PM
#1
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Joined: Sep 2014
Posts: 126
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From: Pasadena, Ca
Bikes: 1972 Gitane Super Corsa Frankenbike, 1972 Motobecane Le Champion, Motobecane Grand Jubile, 1980 Bianchi Campione di Italia, 1984 Paramount, Trek 620, Trek 720, Cannondale 3.0, Kestrel 200sci, Kestrel 200EMS, Bob Jackson Tandem
Are you bent out of shape?
I have taken on the hopelessly romantic task of rehabbing a 1974 Motobecane Grand Jubilee and need some advice on frame alignment.
I built a nice (and cheap) copy of the Park Tool frame gauge. Follow this YouTube link for your DYI'ers out there:
https://www.youtube.com/watch?v=z9bPiAi_KQc)
Sure enough I found that the rear stays are not symmetrical. I also measured using the old tried and true string test. Both methods yielded the same result… the drive side dropout sticks out about 3/8" further out than the non drive side. Spacing at the rear dropout is currently at 121mm.
I have no problems getting out the ol' 2x4 and leveraging things back, but which stay to bend? Is the drive side too far out (therefore making the non drive too close to keep the 120 dropout distance) OR is the non drive side too close, so I need to bend it out, but then shove the drive side in a bit to keep the 120mm spacing correct?
Seems to me one has to establish a centerline off the head tube/seat tube line then symmetrically space each drop out from that reference line, all the while maintaining your 120mm drop out distance. I'm I missing something here? It really has me bent out of shape!
I built a nice (and cheap) copy of the Park Tool frame gauge. Follow this YouTube link for your DYI'ers out there:
https://www.youtube.com/watch?v=z9bPiAi_KQc)
Sure enough I found that the rear stays are not symmetrical. I also measured using the old tried and true string test. Both methods yielded the same result… the drive side dropout sticks out about 3/8" further out than the non drive side. Spacing at the rear dropout is currently at 121mm.
I have no problems getting out the ol' 2x4 and leveraging things back, but which stay to bend? Is the drive side too far out (therefore making the non drive too close to keep the 120 dropout distance) OR is the non drive side too close, so I need to bend it out, but then shove the drive side in a bit to keep the 120mm spacing correct?
Seems to me one has to establish a centerline off the head tube/seat tube line then symmetrically space each drop out from that reference line, all the while maintaining your 120mm drop out distance. I'm I missing something here? It really has me bent out of shape!
06-16-15 | 07:53 PM
#2
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Joined: Apr 2009
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From: New Rochelle, NY
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
If the dropout width (separation) is 120mm or so, which is the intended width, then BOTH stays are bent to the right. You'd want to move both to the left by half the right/left difference to establish symmetry.
BTW - it's common for both stays to bend together like this pair did. Figure a bike hit by a car with the wheel bolted in. The rear triangle is bent sideways, but the width is constrained by the wheel's axle leading to what you see now.
You have an opportunity here. Consider bending only the left stay to the left by 9mm. That would give you 130mm axle width for a more modern wheel, and close symmetry. Of course it would be serendipity if it worked out exactly, so a bit of fine tuning, or calculating (or both) will be called for.
BTW - it's common for both stays to bend together like this pair did. Figure a bike hit by a car with the wheel bolted in. The rear triangle is bent sideways, but the width is constrained by the wheel's axle leading to what you see now.
You have an opportunity here. Consider bending only the left stay to the left by 9mm. That would give you 130mm axle width for a more modern wheel, and close symmetry. Of course it would be serendipity if it worked out exactly, so a bit of fine tuning, or calculating (or both) will be called for.
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Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
06-16-15 | 08:52 PM
#3
Thread Starter
Senior Member
Joined: Sep 2014
Posts: 126
Likes: 5
From: Pasadena, Ca
Bikes: 1972 Gitane Super Corsa Frankenbike, 1972 Motobecane Le Champion, Motobecane Grand Jubile, 1980 Bianchi Campione di Italia, 1984 Paramount, Trek 620, Trek 720, Cannondale 3.0, Kestrel 200sci, Kestrel 200EMS, Bob Jackson Tandem
If the dropout width (separation) is 120mm or so, which is the intended width, then BOTH stays are bent to the right. You'd want to move both to the left by half the right/left difference to establish symmetry.
BTW - it's common for both stays to bend together like this pair did. Figure a bike hit by a car with the wheel bolted in. The rear triangle is bent sideways, but the width is constrained by the wheel's axle leading to what you see now.
You have an opportunity here. Consider bending only the left stay to the left by 9mm. That would give you 130mm axle width for a more modern wheel, and close symmetry. Of course it would be serendipity if it worked out exactly, so a bit of fine tuning, or calculating (or both) will be called for.
BTW - it's common for both stays to bend together like this pair did. Figure a bike hit by a car with the wheel bolted in. The rear triangle is bent sideways, but the width is constrained by the wheel's axle leading to what you see now.
You have an opportunity here. Consider bending only the left stay to the left by 9mm. That would give you 130mm axle width for a more modern wheel, and close symmetry. Of course it would be serendipity if it worked out exactly, so a bit of fine tuning, or calculating (or both) will be called for.
Ah hah! The light bulb just went off...thanks for your insight. It all makes sense now. I'm debating to go for an all out retro rebuild whch will call for 120 spacing..though I might be persuaded to 126 as a compromise.
Anyhow, thanks to your comments, I now know which stays to bend and in which direction.
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Hellhunde
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06-18-15 12:38 AM
