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  1. #1
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    basic frame question.

    when the seat stays are shortened, meeting the seat tube lower, you lose the traditional road frame geometry. Is there a term for this (like a mixte is a mixte, a stepthrough is considered "women's", etc)?
    just curious. And if anyone can shed light on reasons for this frame style or history on it..

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    Senior Member masi61's Avatar
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    What you're describing is a "compact". I think they became popular about 10 years ago. Some will claim it was a manufacturers tactic to make fewer standard frames work with a larger number of riders. Giant has alway been big into this, but many other companies are too. The current Serottas use compact geometry nicely. I like compacts, my next roadbike hopefully will be a compact. I believe they have the potential to climb better and be stiffer in the rear triangle while still keeping a halfway normal headtube so that your back doesn't complain.

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    Senior Member Cactus's Avatar
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    There isn't a single name for a lowered seat-stay junction. It's only a compact if the rear end of the to-tube is also lowered.

    Sometimes, the lower seat-stay junction is accompanied by extension of the seat-stays to the top tube. This style also has no one name.

    Why do these things? It depends on the builder. Some of it (perhaps most) is about acheiving a visually distinctive style. Sometimes, a builder may want to limit heat near the seat lug - especially with a fileted or tigged joint - to avoid distorting the seat tube. The seat tube can, and often is reamed to round - but this can be problematic for thin walled tubing.

    Sometimes it is claimed that rigidity is enhanced when the stay joins the seat-tube low and then joins the top-tube several inches ahead of the seat=tube. Sometimes builders figure that they're saving a few grams by shortening the seat-stays.

    Some people just like the style, especially with monostays.

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    Question part two: Why is it that many sprint/TT/pursuit/tri etc bikes tend to have a lowered/shortened seat stay?
    when you talk about compact and the top tube, do you mean that the top tube is sloping and not parallel to the ground?

    I was checking out a used frame that was as you described: "Sometimes it is claimed that rigidity is enhanced when the stay joins the seat-tube low and then joins the top-tube several inches ahead of the seat=tube". Out of pure boredom with the typical road frame geometry, I was thinking about building up a frame like this.
    Last edited by Ready to Ruck; 06-11-06 at 04:21 PM.

  5. #5
    works for truffles pigmode's Avatar
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    Quote Originally Posted by Ready to Ruck
    when the seat stays are shortened, meeting the seat tube lower, you lose the traditional road frame geometry. Is there a term for this (like a mixte is a mixte, a stepthrough is considered "women's", etc)?
    just curious. And if anyone can shed light on reasons for this frame style or history on it..


    Question part two: Why is it that many sprint/TT/pursuit/tri etc bikes tend to have a lowered/shortened seat stay?
    when you talk about compact and the top tube, do you mean that the top tube is sloping and not parallel to the ground?

    Tom Kellogg mentioned that the only advantage to the compact configuration is it is more responsive when standing on the pedals and moving the frame from side to side. Its an Nth degree type of thing.

    Also, frame type or configuration has nothing to do with frame geometry.
    Attached Images Attached Images

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    Senior Member Cactus's Avatar
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    It's not clear to me that there is a stiffness impact on the location of the stays or from a sloping tube. While I don't know that there is no impact, here are the issues that cause me concern with accepting these assertions.

    1) I have yet to see a test proclaiming to prove this thesis where the weights/forces are described as being applied to the frames equally. That is, with the shorter seat tube, if the weight/force is placed at the top of the seat collar, then on the longer seat tube (the frame with a horizontal top tube) the weight/force needs to be applied somewhat below the seat collar (the same distance from the BB). Otherwise, the leverage of the longer seat tube will cause greater deflection. That might at first seem reasonable, but the rider doesn't shrink and doesn't sit closer to the bottom bracket when on a compact frame. So, from what I've seen, the comparisons are apples and oranges.

    2) A tube is a more efficient stiffening-agent than a bar. Bend a bar, and there is some material in the middle that is neither stretched or compressed - that material doesn't resist the bending with much effect. In a tube, that middle material is missing. On an equalized weight basis, the tube should better resist bending because all of it material is located where the stretching or compression is maximized - and thus encounters the maximal resistence of the material. In the vertical plane, a bike frame is like a giant tube with the down-tube being stretched and the top-tube being compressed. By effectively bringing them closer together with a compact frame, one is reducing their leverage to resist flexing.

    A bit of a corallary to this is that the top tube is the most critical tube in the main triangle regarding it's straightness. Under stretching, a tube naturally straightens out - and then as more force is applied, it suffers not from any initial lack of straightness. This is the case of the down-tube. Under compression (the top-tube), any deviation from straightness causes forces to vector out and further bend the tube. Consequently, any initial bend in the tube will be excacerbated by compression and limit the stiffness of the frame. In most cases, this should only be an issue when a frame is highly stressed, that is at its maximal lightness for its assigned task. But, when you see a frame with a curved top-tube, you should assume that the design has not be optimized for the purposes of a bicycle frame.

    Most frame tube produced today are pretty darn straight. Still, a good hand-builder checks his tubes before building to ensure that the frame performs as intended.

  7. #7
    mousse de chocolat Moose's Avatar
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    Quote Originally Posted by Cactus
    It's not clear to me that there is a stiffness impact on the location of the stays or from a sloping tube. While I don't know that there is no impact, here are the issues that cause me concern with accepting these assertions.

    1) I have yet to see a test proclaiming to prove this thesis where the weights/forces are described as being applied to the frames equally. That is, with the shorter seat tube, if the weight/force is placed at the top of the seat collar, then on the longer seat tube (the frame with a horizontal top tube) the weight/force needs to be applied somewhat below the seat collar (the same distance from the BB). Otherwise, the leverage of the longer seat tube will cause greater deflection. That might at first seem reasonable, but the rider doesn't shrink and doesn't sit closer to the bottom bracket when on a compact frame. So, from what I've seen, the comparisons are apples and oranges.

    2) A tube is a more efficient stiffening-agent than a bar. Bend a bar, and there is some material in the middle that is neither stretched or compressed - that material doesn't resist the bending with much effect. In a tube, that middle material is missing. On an equalized weight basis, the tube should better resist bending because all of it material is located where the stretching or compression is maximized - and thus encounters the maximal resistence of the material. In the vertical plane, a bike frame is like a giant tube with the down-tube being stretched and the top-tube being compressed. By effectively bringing them closer together with a compact frame, one is reducing their leverage to resist flexing.

    A bit of a corallary to this is that the top tube is the most critical tube in the main triangle regarding it's straightness. Under stretching, a tube naturally straightens out - and then as more force is applied, it suffers not from any initial lack of straightness. This is the case of the down-tube. Under compression (the top-tube), any deviation from straightness causes forces to vector out and further bend the tube. Consequently, any initial bend in the tube will be excacerbated by compression and limit the stiffness of the frame. In most cases, this should only be an issue when a frame is highly stressed, that is at its maximal lightness for its assigned task. But, when you see a frame with a curved top-tube, you should assume that the design has not be optimized for the purposes of a bicycle frame.

    Most frame tube produced today are pretty darn straight. Still, a good hand-builder checks his tubes before building to ensure that the frame performs as intended.
    The stiffness referred to in compact frames is primarily in the rear triangle. I submit also that bringing the top and down tube closer will reduce leverage that creates flexing. The seat tube is the "lever" and since it is shorter in this design, it has less leverage.

  8. #8
    Senior Member Cactus's Avatar
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    Moose,

    Help me out.

    The forces in the frame won't change due to the a sloping top tube. How they are absorbed may change. As to the reduced leverage, it would appear that you are referring to a relative twisting (torque) motion between the head-tube and bottom-bracket. No?

    The reduction in leverage, in this case would cause the down-tube to provide more of the resistance to this torque because the top-tube was providing less resistence. With all else being equal - this might appear to make the frame more flexible, rather than less.

    More likely, in my estimation is that the top-tube would absorb more bending and torsion, and the seat-tube would see less bending from this reduced leverage scenario.

    How any of this translates to rear-triangle stiffness is very unclear to me. Especially, because there are many ways that the rear-triangle can flex, and its not clear which of them you're referring to.

    In vertical compliance, making the seat and chain stays more nearly parallel will have the effect of softening the rear triangle.

    If you're thinking of the rear triangle bending sideways, there may be some stiffening to the lowered stays, but it doesn't seem likely to be much - probably adjusting the diameter of the stays would have a larger impact. Also, if you look at www.sheldonbrown.com/rinard/fea.htm, you will see that the majority of strain-energy is absorbed by the down-tube, not rear triangle.

    Torsion, or twisting of the rear triangle shouldn't be much of an issue. There is nothing holding the dropouts in place except the lateral resistence offered by the rear tire. This isn't a level of force that should cause concern.

    Perhaps more important is the question of: Why is additional stiffness necessary or desireable?

  9. #9
    mousse de chocolat Moose's Avatar
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    You are definitely over thinking this...

    The stiffness that compact frame designers are shooting for is linear, from the bottom bracket to the dropouts. The benifits a smaller rear triangle has for acheiving this are obvious to me. This is desireable for performance oriented riders who want to accellerate quickly.

    Are you a frame designer/builder?
    Last edited by Moose; 06-18-06 at 03:37 AM.

  10. #10
    Senior Member Cactus's Avatar
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    Ok, Moose, help me out. What do you mean by linear? Is this compression of the chainstays along their axis? Horizontal bending of the chainstays in the plain of their axis? Something else?

    You might want to look at www.bikethink.com/Frameflex.htm for an alternative view on the benefits of stiffness to performance oriented riders. The current issue of Vintage Bicycle Quarterly (vintagebicyclepress.com) has a couple of good pieces on the same topic. Fred Parr (one of the originators of oversize tubing) also shares some of these views, especially the need to adapt stiffness to rider weight and power.

    Yes I design and build.

    Stiffness is a poorly understood phenomena, but one that is heavily marketed. It doesn't appear, however, that empirical race results do anything to substantiate the importance of stiffness.

    Unfortunately, the popular press doesn't question what the marketeers have to say. Instead they repeat the same old saws until they become common knowledge. So, at the risk of sounding like the ultimate retro-grouch, I don't see lowered stays as stiffening the rear triangle, nor do I see the clear benefits of stiffening the rear triangle.

  11. #11
    mousse de chocolat Moose's Avatar
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    A stiffer frame feels more responsive to the rider, the article you linked to corroborates this. In your rather condescending tone you suggest that compact frame designers are simply forcing these frames down our throats at the behest of the marketing departments of big bike manufacturers. (do I hear conspiracy?)
    I think they are responding to what riders are asking for.

  12. #12
    Senior Member Cactus's Avatar
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    Moose,

    I'm sorry if my tone sounded condescending - not the intent. Guess they call me Cactus for a reason. Why don’t you help me understand what I should have phrased differently.

    Do I detect a conspiracy? No. However, they have figured out a pitch that sells (stiffness) - and they push the dickens out of it. Do you ever see objective tests of the stiffness of bikes? Do you ever see objective claims as to how stiff a given frame is, in what direction? No. Only vague nebulous claims like 10% stiffer with greater vertical compliance. What does that mean?

    It's interesting to note that all the big MFGs are trying to increase stiffness generally, except they want to increase vertical compliance. Look at a diamond frame. It's built like a bridge truss - that is to be stiffest in reaction to vertical forces. Heck, we could leave the top tube off if the goal was vertical compliance.

    So, the manufactures are generating consumer needs, as they should to make money, by creating advantages – even specious ones. Caveat Emptor. You can buy into their claims or not.

    One of the things that Jan Heine has pointed out is this: Back when the top pro racers could and regularly did choose their own frame-builder (always hidden by the sponsor’s paint job), the inevitably choose the lightest frame possible. This was due to experience with what worked. Jan’s theory is that frame flex can work sympathetically with varying pressure applied to the pedal during its rotation, allowing one to stay in a higher gear longer when climbing.

    I hope this hasn’t sounded condescending. I’m just trying to share the benefit of the experience and thinking of many folks who are smarter than me. Its your prerogative to disagree.

  13. #13
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    The vertical stiffness of the rear triangle is greater in the traditional frame, but racers are concerned with sideways flex under pedaling loads. Making the rear triangle with a shorter distance from the rear drop-outs to the seat tubes, increases the sideways stiffness

  14. #14
    mousse de chocolat Moose's Avatar
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    I don't really care about stiffness personally, I prefer a good old flexy steel frame. For the large majority of the bicycle-buying public, stiffness not dreadfully important.

    Perhaps Cactus is right, that the advantage of stiffness is very hard to quantify, but you'll have a hard time convincing racers that they have as good a chance at winning a race on a Trek 520 as they would on a Giant TCR Advanced Composite. This has nothing to do with Big-Bicycle Marketing tactics and everthing to do with rider experience.

    Take the argument that stiffness is meaningless to the Road Forum and see what they have to say.

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