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LastKraftWagen 03-18-14 08:43 PM

Frame design question
 
I tried to attach a photo, but I'm computer insufficient...

I had some pictures of frames explained to me, and I have a frame design question. In the photos, some of the frames have what I would call a more traditional attachment between the headtube, toptube, lateral, and downtube. (TT near the top of the HT, lateral in the middle, and DT on the lower part of the HT. In some of the pictures however, the lateral is at the top of the HT (the DT is still at the bottom), and the TT attaches about 6" or so down the lateral. WHY? All the frames described are supposed to be of the same model, and the person describing the photo didn't seem to be able to see a rational like frame size (both larger and smaller frames had both designs). Is this common, and my newly-to-tandems eyes just hasn't seen it yet? Could the second design (with the TT attached down the lateral) be stiffer? I'll contact Duratec in the morning, but they sometimes take a couple days to respond, and this may cause sleepless nights pondering...

Sorry, no pics, I haven't learned the secret handshake yet.
LKW

geronimo2000 03-18-14 08:58 PM

Not sure I'm totally following, but maybe this will help: Stiffener tubes and custom tandems

vrooom3440 03-18-14 11:08 PM

It is easier to build a frame with the fewest joints and with simple tube connections. Thus most tandems place the lateral into the middle of the head tube. That way there are only 3 tube connections and they are relatively simple fish mouthing shapes to make the tubes meet closely.

Running the lateral to the top of the head tube provides a better triangulation of the head tube for a stronger connection with less flex. However it does make a much more complicated joint between the top tube and the lateral. It also requires a larger area of thicker butting of both lateral and top tubes to accommodate the larger weld area.

Bezalel 03-19-14 12:49 AM

1 Attachment(s)
Are you asking about this design?

http://bikeforums.net/attachment.php...hmentid=369720

The designers want to keep both top tubes inline so they can't slope the top tube as much as the would on a compact single.

mstyer 03-19-14 06:33 AM

I think standover clearance plays a part as well. With a single bike it's simple; a frame with a shorter top tube has a shorter seat tube and thus the top tube is lower. Also, on a single bike the rider can lean the bike over if clearance is an issue. With a tandem the captain needs to be able to straddle the top tube comfortably with the bike fully upright while the stoker mounts. Also on a tandem, the captain's seat tube length can only be so short because the top tube must connect all the way back to the stoker seat tube.

Note on this Cannondale how much higher the top tube would be if it connected to the head tube:
http://www.francebikerentals.com/sit..._cannon_ms.jpg

Co-Motion chose to address this issue differently, by allowing the stoker seat tube to be exceptionally short and adding a periscoping seat post:

https://bikejournal.com/images/sweet...krP1030100.JPG

WheelsNT 03-19-14 07:33 AM

Quote:

Originally Posted by mstyer (Post 16591369)
I think standover clearance plays a part as well. ... With a tandem the captain needs to be able to straddle the top tube comfortably with the bike fully upright while the stoker mounts.

+1 on this -- more standover clearance is needed on a tandem than on a single.

LastKraftWagen 03-19-14 07:54 AM

Quote:

Originally Posted by geronimo2000 (Post 16590672)
Not sure I'm totally following, but maybe this will help: Stiffener tubes and custom tandems

I've read this one before...wrong end of the stiffener (what I referred t as the lateral tube), but thanks.

LastKraftWagen 03-19-14 08:00 AM

Quote:

Originally Posted by Bezalel (Post 16591079)
Are you asking about this design?

http://bikeforums.net/attachment.php...hmentid=369720

The designers want to keep both top tubes inline so they can't slope the top tube as much as the would on a compact single.

Sorry, blind, so I don't know if your image is what I am talking about, but in the design I am less familiar with, where the lateral [stiffener] attaches to the top of the head tube, the pilot's top tube is angled downward as compared to the stoker's top tube.

LastKraftWagen 03-19-14 08:05 AM

Quote:

Originally Posted by vrooom3440 (Post 16590951)
It is easier to build a frame with the fewest joints and with simple tube connections. Thus most tandems place the lateral into the middle of the head tube. That way there are only 3 tube connections and they are relatively simple fish mouthing shapes to make the tubes meet closely.

Running the lateral to the top of the head tube provides a better triangulation of the head tube for a stronger connection with less flex. However it does make a much more complicated joint between the top tube and the lateral. It also requires a larger area of thicker butting of both lateral and top tubes to accommodate the larger weld area.

I can see the design with the lateral [stiffener] attached to the top of the headtube being stiffer on the vertical axis, but wouldn't two attachment points (lateral and down tube) be less stiff rotationally?

waynesulak 03-19-14 08:06 AM

Quote:

Originally Posted by WheelsNT (Post 16591537)
+1 on this -- more standover clearance is needed on a tandem than on a single.

Tandems do need good standover for the captain and that is one reason to use this design. The other reason is to allow for fewer sizes to fit more riders. Make a short standover and the frame fits a wide range of customers on a very limited number of frame sizes to be manufactured by standardized production facilities. Each additional size manufactured in means a whole lot of new slightly different sized tubes and angle cuts on tubes at the joints.

This is a very important cost factor where the total volume of tandems sold is fairly small compared to singles. In addition to the obvious cosmetic difference, the trade offs are in things like water bottles not fitting in traditional locations without side loading cages or maybe not fitting at all and cramped cable runs that add friction to brake and derailleur cables.

LastKraftWagen 03-19-14 08:10 AM

Quote:

Originally Posted by mstyer (Post 16591369)
I think standover clearance plays a part as well. With a single bike it's simple; a frame with a shorter top tube has a shorter seat tube and thus the top tube is lower. Also, on a single bike the rider can lean the bike over if clearance is an issue. With a tandem the captain needs to be able to straddle the top tube comfortably with the bike fully upright while the stoker mounts. Also on a tandem, the captain's seat tube length can only be so short because the top tube must connect all the way back to the stoker seat tube.

Note on this Cannondale how much higher the top tube would be if it connected to the head tube:
http://www.francebikerentals.com/sit..._cannon_ms.jpg

Co-Motion chose to address this issue differently, by allowing the stoker seat tube to be exceptionally short and adding a periscoping seat post:

https://bikejournal.com/images/sweet...krP1030100.JPG

Maybe, but the way the photos were described to me, both designs were found on both larger and smaller frames. If standover were the primary issue, then why not build the large frame for a pilot who needs a little more standover a little smaller?

LastKraftWagen 03-19-14 08:17 AM

Quote:

Originally Posted by WheelsNT (Post 16591537)
+1 on this -- more standover clearance is needed on a tandem than on a single.

Seems like if this were the primary reason, it could be more easily achieved by attaching the top tubes lower down on the seat tubes as opposed to recalculating and readjusting all the jigs to move the lateral [stiffener] up the headtube and attach the toptube somewhere down the length of the lateral.

WheelsNT 03-19-14 08:19 AM

Quote:

Originally Posted by LastKraftWagen (Post 16591679)
If standover were the primary issue, then why not build the large frame for a pilot who needs a little more standover a little smaller?

Making the frame smaller also lowers the bars.

qspencer 03-19-14 08:26 AM

I think more frame builders are in fact doing that. You can't discount aesthetics and tradition in these kind of design decisions. Mountain bikes were the first to depart from the orthodoxy of horizontal top tubes, and road bikes eventually followed. Now we're seeing the same thing gradually happen in tandems. Historically, bikes were sized by their seat tube, but the cycling community has come to appreciate that the top tube length is probably the most important contributor to a good fit. On modern bikes, stems are easy to swap out, and long seat posts are available, so why not make a frame a little stiffer, lighter, and easier to stand over by shortening the seat tube? Joining the top and lateral tubes before the head tube I think is a way to avoid a very complex joint at the head tube on shorter head tubes, and it probably saves a little weight as well.

vrooom3440 03-19-14 09:30 AM

Quote:

Originally Posted by LastKraftWagen (Post 16591654)
I can see the design with the lateral [stiffener] attached to the top of the headtube being stiffer on the vertical axis, but wouldn't two attachment points (lateral and down tube) be less stiff rotationally?

Perhaps minimally since the lateral on a conventional setup attaches near the center of the head tube (and thus at the rotational center so to speak) it provides less strength. One has to consider the loads on the bike frame and the weight carrying is the much larger load here than twisting loads. The structure of the entire frame is rather handicapped dealing with twisting loads being close to a flat plane.

LastKraftWagen 03-21-14 06:31 AM

Update: The official word from the builder is based on the difference in height between riders... With a pilot much taller than a stoker, the toptube drops into the lateral and when they are closer in height the toptube meets the steer tube.


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