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Thread: Stiffest frame?

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    Stiffest frame?

    I'm currently looking for a Cannondale RT frame (any year) but am finding it very hard to find anywhere that sells the frame only. Or I'd settle for an older model that uses the Shimano Octilink BBs and rim brakes but again, old models seem hard to come buy. I don't want to buy one of the newer complete C'dales becuse I've got most of the stuff I need to build the bike up and would be throwing away the disk brakes, crankset and wheels (although you can never have too many sets of wheels!). Obviously there's used C'dales coming up on cliassifieds all the time but I don't seem to be having much joy there either.

    I've ridden a few different tandems including Co-Motion and Trek but find the C'dale the stiffest out of all of them which is why I'm after a C'dale.

    I've contacted C'dale but haven't heard anything back from them yet. I suspect they probably get a lot of mail and shouldn't hold my breath.

    Faced with this problem, I contacted Craig at Calfee who were very helpful and I'm considering possibly getting one of his frames. And I notice they've dispensed with the lateral stiffener on some of their models which should be an indication of how confident they are with their stiffness.

    We race competitively, have a team weight of 374 lbs and so obviously the weight savings of using carbon are attractive. But we can deliver over 3000 Watts between us and so the frame has to be extremely stiff. It's a lot of money wasted if it isn't as stiff as the C'dale.

    We could obviously get a solid frame custom built out of steel that would be strong enough but we're would prefer to use a lighter material if posible.

    If anyone has ridden a Calfee and a C'dale and believes the Calfee is at least as stiff, or has any other suggestions I'd be very interested to here what you think. Obviously from the above you can tell I'm not interested in a Trek frame or Co-Mo but if you don't feel a Calfee is as stiff as either of these to frames it will tell me what I need to know anyway.

    Thanks in advance.

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    Flandrien
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    Cannondale sells their 2007 road tandem as frame only. I was given the price of 1199 USD by BackBayBcycles in Boston. Ask for Tim Libby. They do not sell their mountain (street) tandem as frameset only, even though the 2007 catalogue says they do..

    We have the 2006 mountain model and I can only agree that the Cannondale frames are plenty stiff.

    No idea about Calfee frames, but you can't go wrong with the Cannondale.

    Good luck with your decision and then enjoy,
    Eric

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    here too

    Cannondale does sell their road frameset for a reasonable price.

    I'm sure Calfee can build a frame to your stiffness needs,but it may be one with a lateral tube.
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    I have ridden with various stokers and my wife a 2005 C'dale, 2006 co-mo Robusta, 2006 Paketa and 2007 Calfee. Combined weight for my wife and I is ~330. The Paketa and Calfee subjectively felt the most stiff. While we did not do structural testing on the frames, based on our riding times we concluded the Calfee was stiffer and more comfortable than the others with the Paketa a very close second.

    Our primary outcome varible was the time to climb a certain hill in our area. I won't bore you with the details of the testing, our times were faster on the Calfee, then Paketa, co-mo followed by the cannondale and trek t-2000.

    I am curious as to how you know you can produce 3000 watts as a team? Is this the summed max 6 or 12 second effort of both cyclists while on a single bike? What is your sustained watts over the course of a typical race and how are you calculating these data?

    We really like the Calfee and Craig and his group can tailor the frame to your weight and power capabilities.

    Jay

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    Thanks for getting back to us. Nice to know that it should be possible to get the 2007 C'dale frame on it's own. But there's nothing conclusive on the frame choice though. To clear up the detail, Jay you are correct in assuming that the figure of 3000 W is the sum of both my and the pilots peak power over 6 seconds measured on a turbo trainer. Not wonderfully accurate but I thought it would just help to vie people an idea of the forces that need to be handled. We mainly ride the track over kilo and pursuit distances so we will hit that mark off the line the kilo and be fairly close on the pursuit. Not easy to get going tandems! Another reason why shedding a few pounds using a carbon frame whould help. Although I suspect I could probably still she a couple of pounds myself at the moment as well!

    My concern is that less powerful pairings (although not necessarily slower pairings!) would feel that a frame was stiff, where as a heavy stronger pairing would not.

    Craig said that they use 55mm diameter tubing but I'm just concerned that even with the lateral stiffner, this wouldn't be enough. The C'dale uses 60mm alluminium. This provides a greater degree of structural stiffness, even if the tubing itself isn't a stiff as the carbon but I'm not even sure of that.

    Any more feedback welcome.

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    Too complex

    You can't only consider the outer wall diameter as a measure of stiffness. Wall thickness also plays a role.

    Also,I'm pretty sure Calfee can add a stiffening sheet inside a tube to further stiffen it or hand wrap joints etc.... Just looking at diameter does not tell the whole story.

    I have no idea if the Cannondale would be more or less stiff,but I suspect either would be a great choice.
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    [QUOTE=Craig said that they use 55mm diameter tubing but I'm just concerned that even with the lateral stiffner, this wouldn't be enough. The C'dale uses 60mm alluminium. This provides a greater degree of structural stiffness, even if the tubing itself isn't a stiff as the carbon but I'm not even sure of that.[/QUOTE]

    C'dale frames aren't particularly light. If you're comfortable with the cost of a Calfee frame, just do it. The guy knows his stuff and should be able to build what you want.

    FWIW, compact, "lateral-less" tandem frames can be extremely stiff. To understand why, think of them as "toptube-less" instead. My stoker and I recently rode a steel Co-Motion Periscope tandem; I do believe that it is the laterally stiffest tandem frame that we have ever tried.

    Good luck & have fun,

    Dave

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    Absolutely agree with everyone's saying. And I know that wall thickness is also important. But I don't know how thick the carbon tubes are, nor the C'dales alluminium 60mm diameter tubing. All I know is that pound for pound, if constructed correctly, c/f tubing is stiffer/stronger.

    I guess I was hoping that maybe somebody had already 'gone for it' or at least tried both frames and could tell me about their experience. If I knew catagorically that the Calfee would be at least as stiff, it's a no brainer. But I suspect I'm not going to know unless I do 'go for it' myself.

    It's encouraging to hear you all talking so favorably about the c/f frames and in particular those made by Calfee. I just read one review a while back now that suggest the Calfee frame wasn't that stiff compared to an alluminium Co-Mo and having ridden a Co-Mo that concerned me greatly.

    Thanks again.

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    Quote Originally Posted by IanS View Post
    I'm currently looking for a Cannondale RT frame (any year) but am finding it very hard to find anywhere that sells the frame only.
    Here ya go, listed at the bottom of the page:

    http://tandemseast.com/frames/cannondale.html

    -Greg

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    SDS
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    Seems to me that with wall thickness you ought to be able to calculate the stiffness of a Cannondale bottom tube and ask Calfee if he wants to match it. Cannondale never answered my e-mail about a part, but a phone call got that part in the next day's mail. A phone call might get you the current wall thicknesses.

    I doubt that anybody has ever considered bothering to build a tandem as stiff as a pre-'98 Cannondale, because the durn thing would beat you to death. It's noticeably unyielding over Texas chipseal. I double-wrapped the handlebars with handlebar tape to cut back the buzz. It was also the best-sprinting tandem I ever used because all the power went straight to the back wheel instead of winding the frame up. When we switched to a long-wheelbase Easton 7000 tandem, on our first ride, we were headed for a bump, I called "bump" and....nothing. The relatively springy tubeset ate the bump and passed almost nothing on.

    A run out to the garage with dial calipers, finds that the bottom tube diameter was around 60mm in the early '90's as well, though Cannondale may well have since reduced the wall thickness.

    Back to back comparisons might be difficult, but it occurs to me that if the Cannondale frameset is much less, it might be better to order that first, and then only switch to the Calfee if you are not satisfied.

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    FWIW: power output does not dictate stiffness needs. It would impact handling some, and comfort some, but power and frame stiffness are independent measures. Stiffness is more of a comfort concern -- you're not losing any power to frame flex.

    Nothing wrong with wanting a stiffer frame if it makes you more comfortable, but you said you need it because you and your stoker can put "x" wattage.

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    Senior Member zonatandem's Avatar
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    With today's quality tandems with oversized tubes, with or without a lateral, all should be quite stiff.
    The stiffest? Depends on your perception of stiff, team weight, load, riding style etc.
    Have ridden over 30 brands/models of tandems. In the early days (70s - 80s) some were scary just going around a corner!
    Have ridden Calfee and C'dales . . . personally we prefer Calfee. Carbon fiber's plenty stiff for us and more comfortable than alu.
    If you are so concerned about stiffness/racing, design your own tandem and have someone cutom build for you. That's what we did.
    Pedal on TWOgether!
    Rudy and Kay/zonatandem

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    Cannondale's frames are about as stiff as any you'll find. Only Craig can speak to what he'd recommend for your team. I know that he mentioned building an open frame Calfee Tetra Tandem for a couple in Europe with a combined team weight of 380 lbs which suggests that he has a lot of confidence in his designs. Moreover, he's also mentioned that any of his carbon tandems (or single bikes) can be "beefed-up" if needed after the fact. For example, I seem to recall that many years ago just after he'd delivered the first few Tetra Tetra models (of which there were only a few before they were renamed the Tetra Tandem) there was one very strong team who were overpowering the frame enough to derail the timing chain during hard sprints. Again, going from memory, Craig reworked the frame by adding more material to the joints to generate the needed stiffness for this team.

    So, I think you have just a couple paths here:

    1. Take a chance on the Cannondale which would be the least costly / lowest risk solution since it's a standard production frame that could be acquired used or new as a frame only without a huge outlay of cash. If it works, great. If it doesn't, sell it and move on to option #2 or #3.

    2. Get back on the phone with Craig and have a very specific discussion that focuses on your team's weight and power output with regard to what he'd recommend in the way of a frame design and talk at length about how he'd go about making sure it was stiff enough and/or, if not, what the process for increasing the stiffness would be.... to include responsibility for cost. In other words, will he guarantee the frame's stiffness and back it up by covering any shipping and rework if it turns out that the frame needs to be beefed up?

    3. Have a conversation with the folks at Co-Motion and Bushnell with regard to what they could provide in the way of a super-stiff aluminum tandem frame. From a cost standpoint, I would expect a custom aluminum frame to be a bit less expensive than a Calfee, although not as inexspensive as a C'dale frame only.

    Good hunting.

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    Quote Originally Posted by TandemGeek View Post
    .... I know that he mentioned building an open frame Calfee Tetra Tandem for a couple in Europe with a combined team weight of 380 lbs which suggests that he has a lot of confidence in his designs. Moreover.....
    That would be Steve and Kirsten Dunne. We met them at the Co-mo race in Oregon. Their bike is an open frame design with a couple of "rods" along the the boom tube. They had it set up as a double with disk brakes front and back. Inspite of being a tall and big team they were climbing steep grades very fast. They were very happy with their custom Calfee tandem.

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    The beauty of carbon fiber is that more carbon can be wrapped in areas of stress.
    Know of one of the early Tetra Tetras that had some BB flex issues with a couple state TT champs . . . Craig reworked it, and all was fine.

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    Very interesting and wise words again thanks. Not exactly sure why someone suggested that frame flex wouldn't affect power to the rear wheel though? Surely if power is being dissapated through frame flex, less power will be available to drive the back wheel? Am I missing something? And that apart from the handling issues and link chain derailing problems mentioned by someone else. We've ridden supposedly stiff frames on a track and believe me, the handling is more of an issue than you may think if the frame isn't solid when getting off the line and accelerating through the first/second bends. My pilot tells me it's like trying to steer a rubber eal! Which in turn, compromises power delivery because my pilot is concentrating on just keeping the thing on the track and I'm just trying to keep the bike upright!

    I do agree that the stiffer the frame to more unconfortable the ride but I can live with that just on the track and time trialing. I've got a couple of training bikes that aren't as stiff and a little more confortable.

    The other reason I'm trying to get as much info as possible now is that we are considering changing our frame dimensions so a custom build would obviously be better. But as is usually the case, we have limited resources and there's no guarantee, chaning frame dimensions to get us in a better position would improve performance. Better aero usually leads to not being able to deliver the same power and takes a long time for your body to adapt to. There are just too many variables and not enough cash! Same old I guess.

    It's been extremely helpful reading your feedback though and please feel free to add anything else if you feel it may be of interest.

    Thanks again.

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    Making a kilometer blurry waterrockets's Avatar
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    Quote Originally Posted by IanS View Post
    Very interesting and wise words again thanks. Not exactly sure why someone suggested that frame flex wouldn't affect power to the rear wheel though? Surely if power is being dissapated through frame flex, less power will be available to drive the back wheel? Am I missing something?
    Yeah, most cyclists are missing that this is a myth (stiffness and efficiency correlation). I guess the easiest path is to read this thread. The strongest arguments start at the end of page 4:
    Are stiffer frames actually faster? Discuss.

    The cycling industry has been pushing the efficiency thing for a long time as it gives them an upgrade path, and one that is expensive because it's not easy to make a frame stiffer without increasing, or even while decreasing the weight. It makes 3-year-old frames appear obsolete and plays to the gearhead mentality.

    The drivetrain and handling issues are real, and if you prefer a stiffer frame, that's real too. There's just not any power loss. I'm sure a c'dale is stiff enough for any strong tandem team to keep the thing on the road under power without jumping the drivetrain.

    I even had an email conversation with Cannondale trying to get test results from their extensive testing facility regarding stiffness and efficiency. After a lot of hollow arguments, the conversation was escalated to engineering, and their only argument was that "it's intuitive and all the elite racers prefer stiff frames." Riiight. This was an engineer's response? Yeah, he's driven by marketing.

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    I am no expert and I do not want to read four pages worth of posts either. I do know that lateral stifness is crtical for tandems: large boom tubes, double boom tubes, oval boom tubes, reinforcing rods on boom tubes. Vertical stifness is another matter.

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    Seems that when considering spending an additional $3000 (?) on a mildly stiffer custom frame, you might want to take the time to read a couple of pages of arguments on the subject. Handling on a tandem will certainly benefit from some stiffness in some areas, and may well warrant that custom frame, but frame stiffness does not affect power efficiency.

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    I'm affraid I haven't had time to read your reference yet but will do later. Just thought I'd propose another argument that may help. And again, I'm no engineer and can't explain why anyone who knows any basic physics couldn't provide this argument but...

    If you take it to the extreme and had an extremely flexible bike, and ignore the andling and other issues, most of the energy that you would be putting into the system would be 'absorbed' in the frame as it flexes rather than being transmitted to the rear wheel. Ergo, less power to the rear wheel means you don't go as fast. The energy stored in the frame is released when the frame flexes back, but can't be transmitted to the rear wheel to turn it and get back the power because the frame is only connect to the wheel via the spindle. So the energy would be lost through friction between the tire and road as the frame tries to 'whip' the rear wheel. OK, I've exaggerated the actual movement that occurs but this is just basic physics.

    As you say, if it feels better for us then that's real to us which is the most important thing. After all, we have to ride the bike so it has to feel right.

    I would however be happy to hear why the argument above could be flawed though.

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    Yeah, that's the way it would seem to work, and is the basis for the "it's intuitive" argument.

    The energy isn't actually "absorbed" -- it's stored. Just like with any other spring. If the spring is flexed well inside its normal operating range, it returns all of its stored energy as the opposing force retreats. There's actually more of an argument that CF is less efficient since it is often used as a dampener for high-frequency vibration -- and you'd never use metal in such an application (think about bells and guitar strings).

    This article is referenced in the above thread, and explains clearly how the stored energy is returned to the crank and results in forward motion: http://www.bikethink.com/Frameflex.htm

    The best quote is this one:
    Quote Originally Posted by Bike Think
    Try this experiment. It will work best if you mount your bike to a trainer and disengage the resistance roller.
    1. Put the cranks in the horizontal position.
    2. Place a rigid block or stool under the forward pedal so that there is a small gap under the pedal.
    3. While holding the rear brake firmly, stand on the pedal so that it is pushed down to the stool.
    4. Keep holding the pedal down and release the brake.
    When you pushed the pedal down, the chain did not move since the brake locked the rear wheel. Since the chain did not move, no work energy was delivered through the chain. The crank moved down with the pedal as the frame was strained. When you released the brake, the frame was able to move the center of the crank back up to relieve the strain energy. But the pedal remained in it's lower position, so the crank had to rotate around the pedal as the bottom bracket went up. There was a reaction force in the chain and the chain moved as the crank rotated around the stationary pedal. The strain energy of the frame was converted to rotation kinetic energy in the wheel.

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    SDS
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    I read some of the thread mentioned above. I wish to note that tandem bikes are different from single bikes, and that some of the argument for single bikes seems to me to be weak. Specifically:

    1. Windup of the timing chain on tandems does occur (substantial slack accumulating on the bottom timing chain run), indicating bowing of the bottom tube toward the timing chain. This can be alleviated somewhat with bigger timing rings (52T is better than 40T) and minimal offset (use the smallest bottom brackets spindles possible, i.e., 118mm is better than 127mm) of the timing chain plane from the frameset centerline.

    2. The energy stored in the bowing of the frame is mostly not recovered, because pedaling torque is not constant. The energy stored during peak torque is released in resisting the legs where they are weak at the top and bottom of the pedal stroke, and is not well transmitted to the rear wheel. Stiffer tandems without windup are faster tandems because they transmit the power peaks better. It seems to me that frame stiffness does affect power transmission on tandems.

    3. Keeping the head tube in the plane of the frame of a single bike or a tandem keeps the bike going straight. I used to ride a Reynolds 531 single bike, and it seemed to me that the bottom bracket was going an inch to the left and an inch to the right during sprints. That had to steer the rear wheel away from the direction I was trying to go. It appears that that frame is now cracking around the spigot for the seat tube. Seems it wasn't strong enough for a 180 lb guy.

    4. At least one of the posters has not mentioned their flyweight size. There are some issues that larger riders may have that they cannot comment about from experience, and that includes frame stiffness and ride quality--frames seem less vertically stiff with more weight on them, and laterally less stiff with more power.

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    Quote Originally Posted by SDS View Post
    2. The energy stored in the bowing of the frame is mostly not recovered, because pedaling torque is not constant. The energy stored during peak torque is released in resisting the legs where they are weak at the top and bottom of the pedal stroke, and is not well transmitted to the rear wheel. Stiffer tandems without windup are faster tandems because they transmit the power peaks better. It seems to me that frame stiffness does affect power transmission on tandems.
    Yeah, energy return to forward motion is the biggest hangup people have with this idea. The energy is not returned at the top/bottom of the stroke. That's where the return finishes. The energy is returned continuously following the peak pedal pressure in the stroke, probably around 4 o'clock. The result is that that portion of the pedal stroke goes ever so slightly faster (crank rotation) as the energy is returned. Even the infinitesimal amount of energy returned at TDC is going to get to the wheel, as it raises the rider, who gets to put that potential energy into the other pedal immediately.

    Frame-twist and timing chains is a complication specific to tandems, but the twist will be in the same direction for both BBs on an in-phase configuration.

    I'm not condoning noodles here that would allow massive amounts of drivetrain flex. My assertion is that there's no way to get more power to the wheel than is allowed by a frame as stiff as a Cannondale.

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    [QUOTE=waterrockets;5084701]- you're not losing any power to frame flex.

    Frame flex creates heat,and bicycle frames have no means to convert heat to power the rear wheel.

    Differences may be small,but they must be there.
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    Making a kilometer blurry waterrockets's Avatar
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    [QUOTE=dfcas;5092673]
    Quote Originally Posted by waterrockets View Post
    - you're not losing any power to frame flex.

    Frame flex creates heat,and bicycle frames have no means to convert heat to power the rear wheel.

    Differences may be small,but they must be there.
    At that level, it's completely immeasurable, and by no means a criterion for frame selection. Aerodynamics of head tube badges vs. paint would make more difference.

    Also, if you look at the practical options for going stiffer than the c'dale, you have to go with carbon. Carbon is an inefficient spring and will lose more of this immeasurable power. So a slightly more flexible AL frame may be microscopically more efficient than a stiffer CF frame.

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