So as many of you know, I have a Cannondale 2012 RT2 which features a "direct lateral" tube. Further, I know many of you with custom frames have gone with an open frame design. Now I understand that some of that decision may be based around weight savings, but what other reasons would you go with an open frame? I also wonder how do open frame designs do in the context of touring?
Last edited by CaptainHaddock; 08-30-12 at 11:29 PM.
Reason: go vs. gone
Open tandem frames have been around as long as tandem frames that have used diagonally aligned internal tubes and/or external bracing stays to yield sufficient lateral stiffness to provide good handling for the intended loads the frame will carry while still yielding enough vertical compliance for a comfortable ride.
An open frame simply uses larger diameter tubing often with more compact geometry to mitigate the need for adding an internal bracing tube or external stays. Eliminating the diagonal bracing reduces the amount of material & time needed to build and finish a frame, and therefore cost and complexity.
In many cases, eliminating the diagonal bracing tube also yields some weight savings, again bearing mind that the horizontal and vertical tubes typically need to be increased in diameter and/or thickness to off-set the the stability normally provided by horizontal bracing tubes or stays. In some cases, and for lightweight teams, production frames using diagonal bracing are sufficiently overbuilt to allow for the removal of diagonal bracing without doing anything else which does yield pure weight reduction: however, this the exception and not the rule.
As to whether an open frame would be appropriate for a given team will always come down to the size and weight of the riders, and in the case of touring, how they will carry luggage. These are details that must be provided to the frame builder who will then have to consider if he can practically achieve sufficient torsional resistant to frame flex without using some type of bracing. After all, at some point the horizontal and vertical tube dimensions needed to provide the needed frame stiffness will become too large or produce a frame that would be heavier and less comfortable than one using smaller tubing and diagonal bracing.
For production tandems, again... the frame's designer would need to be consulted and provided with the same information on rider size, weight and luggage loading to determine if the stock frame would still deliver the riding qualities that the builder intended.
As for consumer desire for wanting an open versus a more conventional frames with diagonal bracing, it's typically a combination of the aforementioned economies of construction (the potential for cost & weight savings) coupled with vanity and desire to have something that's visibly different and cutting edge. The latter tends to mitigate the potential cost savings at the high-end of the open-frame offerings, more so on entry-level open frame models where the cost-savings are used to preserve the lowest price point possible for value-conscious buyers.
650B tandem converted from Santana Arriva, Santana Noventa, Boulder Bicycle 700C, Gunnar Sport, Trek TX700,
I am a special case in that I enjoy a somewhat flexible frame. I have discovered this over a period of years methodically trying different frames with known steel tubes and therefore known stiffness.
I weigh 150 lbs and feel production single and tandems are designed for larger riders. It is better for a manufacturer to have too stiff a frame for the small riders than to have larger riders breaking frames. This also holds true for tandems. Our team weighs 280-290 and production tandems need to handle teams that are much heaver and stronger and sometimes carrying touring loads.
Our current tandem started life as a Santana Arriva and I had a local frame builder remove the lateral resulting in an open tube frame. This was not to save weight but rather to make the frame more flexible. As a side benefit it did save about two pounds. We love our bike but it does take a team that is in sync to enjoy it. We like the ride characteristics so much that I would not have a lighter carbon frame unless we could be assured that it would have the liveliness of our current bike.
I believe by attempting to save weight builders often design a bike that rides better not because it is lighter but because it is slightly more flexible. Many disagree with me on this and feel a stiffer frame is more efficient. That is fine with me. I know what I like to ride.