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Thread: What´s the reasoning behind dropped top tubes on a road bike?

  1. #101
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    Default Re: What´s the reasoning behind dropped top tubes on a road bike?

    Quote Originally Posted by sk_tle View Post
    I mentionned these because these are the bikes that sells. Outside of UCI sanctionned amateur racers you will see 10 roubaix or Domane on the roads for 1 tarmac or Madone.
    Not here. Here, for every Domane or Roubaix, you'll see 10 people who should be Domanes and Roubaixs on Madones and Tarmacs with 6cm of spacers.
    "I guess you're some weird relic of an obsolete age." - davids

  2. #102
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    Default Re: What´s the reasoning behind dropped top tubes on a road bike?

    Quote Originally Posted by zachateseverything View Post
    A longer tube is less stiff. The example you've given isn't good because changing your grip changes the how much force you can exert on a structure.

    Let's say you have an 8 foot 2x4 supported at either end by a pole and you hang from the middle of it. You see a certain amount of sag in the middle of it. If you move the post in from 8 feet to 4 feet and repeat the exercise you'll see less sag on the board.
    Your example isn't correct. You do not hang from the middle, with a headtube the forces come from the ends (and that will be true for most forces). Also the lever is in this case the fork and that doesn't change when you have a longer (effective) headtube (in this case the joints of the down and top tube). I just asked my colleague who is an avid cyclist and Physics teacher and he agreed that a longer headtube is stiffer.

    However he said it's a lot less clear for forces from the bracket and saddle. In those cases he assumes the larger plane of the two bigger triangles (rear and front) will be stiffer, but absolute deflection could be larger as in that case you do make the lever larger.

    The way to think about it is springs in parallel or springs in series. If you put two springs in parallel (by doing stuff like thickening the tube or board) the result is a stiffer equivalent spring. If you put two springs in series, you end up with a structure that's softer.
    That's not what happens if you have longer or shorter tubes?
    Support your local bike shop.

  3. #103
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    Default Re: What´s the reasoning behind dropped top tubes on a road bike?

    In my experience there are many reasons for the "need" to slope top tubes on road frames.

    The change from threaded steer tubes to threadless dramatically changed the height/length of headset stack and the steerer. Also a quill stem sits higher than most threadless stems. I found many riders preferred the look of a negative rise stem on their road bikes. That combined with carbon forks that have a maximum recommended extension of 20-30mm created a greater need for longer head tubes.

    With the increased head tube length you can extend the head tube above the top tube without increasing the slope or slope the top tube more to allow for a reasonable standover height. I will say that from my time racing in the early 80's we didn't really care about standover, I have short legs and a long torso so I always shrugged at standover. Over the years I saw customers felt standover became more of a priority and, as been stated, especially with smaller riders and frames. Speaking as a builder of titanium bikes the bowing that happens on the head tube where the top tube joins when the weld zone gets further away from the end of the head tube, creates challenges that are a pain in the ass to deal with. At Moots we only allowed a 2cm maximum increase before we increased the slope.

    Also I have found that more and more road riders have tended away from the grand tour fit on their bikes and are looking to be more comfortable and upright. I found that the customers companies like Moots were appealing to are older, less flexible and not interested in having a bike that is ridden in the pro peloton position. I find this refreshing, more comfortable fitting bikes, bigger lower pressure tires are better for most of us as we age.

    Another factor is that the newer generations of riders often come from BMX and MTB to road. Those of us in our advanced years went from road to MTB and we tried to mimic that road fit on our MTB's when that all started. Look at MTB's now and they are more upright than ever before. Also the aesthetic the newer generations are accustomed to are all about sloped top tubes.

    I think that when shopping for a stock sized bike, a bike that has a sloping top tube allows the customer look at stack and reach and they can ignore standover. For someone like myself, I like a longer top tube because of my torso length so I can select a size up potentially.

    IMO stiffness differences are very over rated. I think the vast majority of today's frames are more than plenty stiff. I know from testing forks for ISO standards, especially disc forks, that they are stiffer and stronger than ever before. I speak of carbon of course. I know forks that pass these tests need to be very strong and you will see they weigh a good 30% more than a rim brake fork.

    Traditional bikes with 180 mm long seat posts are/were a beautiful thing. Things develop and change and for me and my friends the shift to more standover, wider tires and disc brakes opened the door to a dirt roads, double track and singletrack rides away from autos with distracted drivers. Plus I ride with a 27.2 titanium seatpost that is a lot longer than 180 mm that makes a long ride more comfortable.
     

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    Default Re: What´s the reasoning behind dropped top tubes on a road bike?

    Quote Originally Posted by Loknor View Post
    Your example isn't correct. You do not hang from the middle, with a headtube the forces come from the ends (and that will be true for most forces). Also the lever is in this case the fork and that doesn't change when you have a longer (effective) headtube (in this case the joints of the down and top tube). I just asked my colleague who is an avid cyclist and Physics teacher and he agreed that a longer headtube is stiffer.

    However he said it's a lot less clear for forces from the bracket and saddle. In those cases he assumes the larger plane of the two bigger triangles (rear and front) will be stiffer, but absolute deflection could be larger as in that case you do make the lever larger.

    That's not what happens if you have longer or shorter tubes?
    Your friend might be a physics teacher but he isn't an engineer. I used the example I did because it's a similar loading setup to what you described except for using a force applied in the middle of the tube instead of a couple applied at the ends.

    We also need to be clear if we're talking about the stiffness of a single tube or a structure. If we're talking about a single tube, a longer tube is softer both axially and torsionally than a shorter tube with the same cross section.

    In your example, if you apply the same couple to a simple structure at supports (ie. your hands) .2 m and then .4 m apart you will see more deformation in the .4m configuration. and more deformation for a given load means it's less stiff.

    Why? because springs in series sum together like electrical resistors in parallel.

    Let's say you have two massless springs of equivalent stiffness. If you attach one end of one of them to a shelf and the other end to a weight, you get a certain amount of deflection (D). If you hook the other spring to the same weight such that the two springs are operating in parallel you end up with half the deflection (.5 D) you had before. Makes sense right? But what if you had a series arrangement, a weight hanging from a spring that's also hanging from a spring? Well, the first spring doesn't care that the other end is attached to a spring and not a rigid shelf, it's going to give you that same deflection as the original case. and the second spring doesn't care that the force it's experiencing is coming from another spring and not the hanging weight, it's still going to deflect that same amount. So that mass is getting displaced 2*D. That configuration is half as stiff as the first.

    I'm using the example of springs because that's what each molecule along a structural member is. If a uniform tube is longer it means that each little bit of material has to deform less in order to allow for a larger total deformation.
     

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    Quote Originally Posted by zachateseverything View Post
    Your friend might be a physics teacher but he isn't an engineer. I used the example I did because it's a similar loading setup to what you described except for using a force applied in the middle of the tube instead of a couple applied at the ends.

    I'm using the example of springs because that's what each molecule along a structural member is. If a uniform tube is longer it means that each little bit of material has to deform less in order to allow for a larger total deformation.
    Tonight I had an EUREKA moment. I will explain why your premise is almost certainly flawed and why bigger frames are indeed stiffer,

    Let's start with sloping frames/lower toptubes.

    1. A sloping/lowered toptube leads to a more flexible frame. This is not just sound mechanics, in reality is demonstrated by how a mixed frame is less stiff.

    Also, a sloping toptube does not just lead to shorter seattubes, but also to a longer seat post. It's hard not to argue that's inherently more flexible. (Indeed unless you saw your seatpost, a seat post that's deeper in the seat tube will create extra stiffness^^).

    2. For a smaller frame, the mechanics are on the same line:

    - A lower frame leads to a longer post+stem.
    - A lower frame does NOT lead to appreciably shorter down tubes as the wheelbase does not or at best minimally change, so flex between the wheels is not positively influenced. Indeed, considering the triangles of the diamond frame become smaller, they actually are more susceptible to torsion. A framesize is generally reached by varying head and seat tube angle. Even if wheelbase grows, it's far, far from proportionally.

    The overall effect cannot but lead to a (slightly, ever so slightly) more flexible bicycle.

    So why does a small frame feel more stable?

    1. A smaller frame usually has a slacker headtube, which leads to more trail.
    2. A shorter toptube leads to a longer stem which also stabilizes steering.

    The result is that a smaller frame generally is less nervous than a bigger frame. This is an effect of the more or less fixed wheelbase and the fixed wheelsize. A smaller frame (Ceteris Paribus) is not stiffer, the mechanics are actually quite clear on that.

    Does stiffness matter? Unless the bike is a noodle to begin with, generally not. Sloping frames/lower top tubes are fine. But they do not lead to extra stiffness :D
    Support your local bike shop.

  6. #106
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    Default Re: What´s the reasoning behind dropped top tubes on a road bike?

    Quote Originally Posted by Loknor View Post
    Tonight I had an EUREKA moment. I will explain why your premise is almost certainly flawed and why bigger frames are indeed stiffer,

    Let's start with sloping frames/lower toptubes.

    1. A sloping/lowered toptube leads to a more flexible frame. This is not just sound mechanics, in reality is demonstrated by how a mixed frame is less stiff.

    Also, a sloping toptube does not just lead to shorter seattubes, but also to a longer seat post. It's hard not to argue that's inherently more flexible. (Indeed unless you saw your seatpost, a seat post that's deeper in the seat tube will create extra stiffness^^).

    2. For a smaller frame, the mechanics are on the same line:

    - A lower frame leads to a longer post+stem.
    - A lower frame does NOT lead to appreciably shorter down tubes as the wheelbase does not or at best minimally change, so flex between the wheels is not positively influenced. Indeed, considering the triangles of the diamond frame become smaller, they actually are more susceptible to torsion. A framesize is generally reached by varying head and seat tube angle. Even if wheelbase grows, it's far, far from proportionally.

    The overall effect cannot but lead to a (slightly, ever so slightly) more flexible bicycle.

    So why does a small frame feel more stable?

    1. A smaller frame usually has a slacker headtube, which leads to more trail.
    2. A shorter toptube leads to a longer stem which also stabilizes steering.

    The result is that a smaller frame generally is less nervous than a bigger frame. This is an effect of the more or less fixed wheelbase and the fixed wheelsize. A smaller frame (Ceteris Paribus) is not stiffer, the mechanics are actually quite clear on that.

    Does stiffness matter? Unless the bike is a noodle to begin with, generally not. Sloping frames/lower top tubes are fine. But they do not lead to extra stiffness :D
    I didn't say smaller frames were stiffer. Just tubes.
     

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