Cycling and bicycle discussion forums. 
   Click here to join our community Log in to access your Control Panel  


Go Back   > >

Tandem Cycling A bicycle built for two. Want to find out more about this wonderful world of tandems? Check out this forum to talk with other tandem enthusiasts. Captains and stokers welcome!

User Tag List

Reply
 
Thread Tools Search this Thread
Old 03-18-14, 08:43 PM   #1
LastKraftWagen
Senior Member
Thread Starter
 
Join Date: Feb 2014
Location: Wind Tunnels of Cheyenne
Bikes: Burley Duet [of some unknown year] (the guinea pig); 2001 Ventana ECDM (the project); And always one less than I think I really need.
Posts: 361
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
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
LastKraftWagen is offline   Reply With Quote
Old 03-18-14, 08:58 PM   #2
geronimo2000
Senior Member
 
Join Date: Dec 2013
Bikes: Rodriguez, Ibis
Posts: 91
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Not sure I'm totally following, but maybe this will help: Stiffener tubes and custom tandems
geronimo2000 is offline   Reply With Quote
Old 03-18-14, 11:08 PM   #3
vrooom3440
Senior Member
 
Join Date: Mar 2013
Location: Roseville/Folsom
Bikes: Co-Motion Primera Co-Pilot, Trek Madone 3.1, Trek 7300, Electra Townie 21D
Posts: 135
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
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.
vrooom3440 is offline   Reply With Quote
Old 03-19-14, 12:49 AM   #4
Bezalel
Senior Member
 
Join Date: Mar 2010
Location: City of Brotherly Love
Bikes: Raleigh Companion, Nashbar Touring, Novara DiVano, Trek FX 7.1, Giant Upland
Posts: 1,505
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Are you asking about this design?



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.
Attached Images
File Type: jpg 81daKdZu7JS._SL1500_-300x137.jpg (13.5 KB, 12 views)
Bezalel is offline   Reply With Quote
Old 03-19-14, 06:33 AM   #5
mstyer
Senior Member
 
Join Date: Jun 2013
Bikes:
Posts: 94
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
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:


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

mstyer is offline   Reply With Quote
Old 03-19-14, 07:33 AM   #6
WheelsNT
Senior Member
 
Join Date: Nov 2011
Bikes:
Posts: 128
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Quote:
Originally Posted by mstyer View Post
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.
WheelsNT is offline   Reply With Quote
Old 03-19-14, 07:54 AM   #7
LastKraftWagen
Senior Member
Thread Starter
 
Join Date: Feb 2014
Location: Wind Tunnels of Cheyenne
Bikes: Burley Duet [of some unknown year] (the guinea pig); 2001 Ventana ECDM (the project); And always one less than I think I really need.
Posts: 361
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Quote:
Originally Posted by geronimo2000 View Post
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 is offline   Reply With Quote
Old 03-19-14, 08:00 AM   #8
LastKraftWagen
Senior Member
Thread Starter
 
Join Date: Feb 2014
Location: Wind Tunnels of Cheyenne
Bikes: Burley Duet [of some unknown year] (the guinea pig); 2001 Ventana ECDM (the project); And always one less than I think I really need.
Posts: 361
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Quote:
Originally Posted by Bezalel View Post
Are you asking about this design?



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 is offline   Reply With Quote
Old 03-19-14, 08:05 AM   #9
LastKraftWagen
Senior Member
Thread Starter
 
Join Date: Feb 2014
Location: Wind Tunnels of Cheyenne
Bikes: Burley Duet [of some unknown year] (the guinea pig); 2001 Ventana ECDM (the project); And always one less than I think I really need.
Posts: 361
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Quote:
Originally Posted by vrooom3440 View Post
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?
LastKraftWagen is offline   Reply With Quote
Old 03-19-14, 08:06 AM   #10
waynesulak
Senior Member
 
waynesulak's Avatar
 
Join Date: Jan 2008
Location: Ft Worth, TX
Bikes: 650B tandem converted from Santana Arriva, Santana Noventa, Boulder Bicycle 700C, Gunnar Sport, Trek TX700,
Posts: 1,909
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Quote:
Originally Posted by WheelsNT View Post
+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.
waynesulak is offline   Reply With Quote
Old 03-19-14, 08:10 AM   #11
LastKraftWagen
Senior Member
Thread Starter
 
Join Date: Feb 2014
Location: Wind Tunnels of Cheyenne
Bikes: Burley Duet [of some unknown year] (the guinea pig); 2001 Ventana ECDM (the project); And always one less than I think I really need.
Posts: 361
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Quote:
Originally Posted by mstyer View Post
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:


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

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 is offline   Reply With Quote
Old 03-19-14, 08:17 AM   #12
LastKraftWagen
Senior Member
Thread Starter
 
Join Date: Feb 2014
Location: Wind Tunnels of Cheyenne
Bikes: Burley Duet [of some unknown year] (the guinea pig); 2001 Ventana ECDM (the project); And always one less than I think I really need.
Posts: 361
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Quote:
Originally Posted by WheelsNT View Post
+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.
LastKraftWagen is offline   Reply With Quote
Old 03-19-14, 08:19 AM   #13
WheelsNT
Senior Member
 
Join Date: Nov 2011
Bikes:
Posts: 128
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Quote:
Originally Posted by LastKraftWagen View Post
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.
WheelsNT is offline   Reply With Quote
Old 03-19-14, 08:26 AM   #14
qspencer
Senior Member
 
Join Date: Dec 2013
Location: Lubbock, Texas
Bikes: VeloBuild VB-R-027 (road), Miracle Bikes MC-286 (cyclocross), 1986 Raleigh Olympian, X-Peria 5200 (tandem)
Posts: 81
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
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.
qspencer is offline   Reply With Quote
Old 03-19-14, 09:30 AM   #15
vrooom3440
Senior Member
 
Join Date: Mar 2013
Location: Roseville/Folsom
Bikes: Co-Motion Primera Co-Pilot, Trek Madone 3.1, Trek 7300, Electra Townie 21D
Posts: 135
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Quote:
Originally Posted by LastKraftWagen View Post
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.
vrooom3440 is offline   Reply With Quote
Old 03-21-14, 06:31 AM   #16
LastKraftWagen
Senior Member
Thread Starter
 
Join Date: Feb 2014
Location: Wind Tunnels of Cheyenne
Bikes: Burley Duet [of some unknown year] (the guinea pig); 2001 Ventana ECDM (the project); And always one less than I think I really need.
Posts: 361
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
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.
LastKraftWagen is offline   Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off



All times are GMT -6. The time now is 05:03 AM.