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  1. #1
    Have bike, will travel Barrettscv's Avatar
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    Achieving chainstay stiffness, does length matter?

    I have two steel framed bikes and a titanium bike. Iíll be selling the titanium bike and replacing it with a steel bike. Iím trying to select a non-custom steel bike based on geometry, but Iím baffled about chain-stay length and its effect on stiffness.

    I have a Soma Double Cross in Tange Prestige and this is my favorite bike. I also have a Lynskey Ti Bike. This bike climbs and sprints better than the Cyclocross bike, but its not comfortable enough for 200k and longer rides. The primary fit issue is handlebar drop, the road bike put too much weight on my hands, which start to tingle and can tingle for days. Nerve damage is not a risk Iíll fool with.

    I would just use the Soma, but itís really too flexy for climbing and staying with faster groups. I donít need tire clearance for anything more than 700x25 tires.

    Iím looking at two bike frames: the Gunner Crosshair Cyclocross bike and the Soma Smoothie road bike. The Crosshair has 430mm chain-stays and room for 700x35 tires. The Smoothie has 415mm chain-stays and room for 700x28 tires.

    Will a shorter chain-stay on a road bike frame be stiffer than the longer chain-stay on the Cyclocross bike?

  2. #2
    Senior Member bobbycorno's Avatar
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    Quote Originally Posted by Barrettscv View Post
    I have two steel framed bikes and a titanium bike. I’ll be selling the titanium bike and replacing it with a steel bike. I’m trying to select a non-custom steel bike based on geometry, but I’m baffled about chain-stay length and its effect on stiffness.

    I have a Soma Double Cross in Tange Prestige and this is my favorite bike. I also have a Lynskey Ti Bike. This bike climbs and sprints better than the Cyclocross bike, but its not comfortable enough for 200k and longer rides. The primary fit issue is handlebar drop, the road bike put too much weight on my hands, which start to tingle and can tingle for days. Nerve damage is not a risk I’ll fool with.

    I would just use the Soma, but it’s really too flexy for climbing and staying with faster groups. I don’t need tire clearance for anything more than 700x25 tires.

    I’m looking at two bike frames: the Gunner Crosshair Cyclocross bike and the Soma Smoothie road bike. The Crosshair has 430mm chain-stays and room for 700x35 tires. The Smoothie has 415mm chain-stays and room for 700x28 tires.

    Will a shorter chain-stay on a road bike frame be stiffer than the longer chain-stay on the Cyclocross bike?
    Not necessarily. AFAIK, stay length would effect rigidity linearly (ie, 2x as long = 1/2 as stiff), wall thickness also linear, and cross section exponentially (2x cross-section = 4x rigidity). That said, I've owned and ridden both a Crosshairs and a Roadie (415mm stays) and there is no perceptible difference in rear-end rigidity.

    BTW, what size frame are you looking for? My 62cm (2007?) Crosshairs frameset is available.

    SP
    Bend, OR

  3. #3
    Have bike, will travel Barrettscv's Avatar
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    Quote Originally Posted by bobbycorno View Post
    Not necessarily. AFAIK, stay length would effect rigidity linearly (ie, 2x as long = 1/2 as stiff), wall thickness also linear, and cross section exponentially (2x cross-section = 4x rigidity). That said, I've owned and ridden both a Crosshairs and a Roadie (415mm stays) and there is no perceptible difference in rear-end rigidity.

    BTW, what size frame are you looking for? My 62cm (2007?) Crosshairs frameset is available.

    SP
    Bend, OR
    PM sent

  4. #4
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    2x Section = 8x stiffer and 4 times stronger, however that applies to it only as an elelment, the stay itself is part of a triangulated structure relative to the global loads.

  5. #5
    Have bike, will travel Barrettscv's Avatar
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    So, 15mm of chainstay length should have a 7% change in stiffness.

    How about the ovalizing of the chainstay to fit the tires. CX bikes take tires up to 700x38, while road bikes fit much smaller tires. Will the shape of the stay, to fit larger tires, going to make a CX bike more flexy?

  6. #6
    Randomhead
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    Quote Originally Posted by Barrettscv View Post
    So, 15mm of chainstay length should have a 7% change in stiffness.
    I don't believe that it's that much. It's not really an easy problem to approximate.

    As far as your other question, the crimps have to make for at least a small increase in flex. But again, it would be hard to quantify. There is a load path through the rear axle, so a cantilever isn't quite the right model.

    When I was a teenager, it was the style to build really short stays, steep angles, and short fork rake for racing bikes. As it turns out this is basically pointless. The frame I built for myself has 420 stays to fit a 28mm with fenders. I think it's a good length, and it has the significant advantage of keeping the front wheel on the ground on very steep hills. I can't imagine an extra centimeter having a significant effect on stiffness, and I may well go to 430 for my next frame.

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