# tandem frame size



## Kama (Sep 2, 2018)

Hi folks,

We are currently riding a Cannondale MT4000 MTB tandem and many components are getting too old and the frame size is not ideal. That´s why we are thinking of selling it and buying a new one.

But when looking at what´s on the market as standard frame sizes in Europe (Poison, Drössiger, Lapierre, Duratec) or at Cannondale, all have a back top tube that seems short to me.

Sure, you don´t want to have your nose too close to the front person  and maybe some riders want to see something when they are riding at the back (but come on, it´s more fun not to know when the bike is going to turn. Some people pay for a roller coaster!). But I currently have 72cm top tube length (along tube) for a height of 46cm. That makes both of us sitting at the back vertically as on a city bike. We calculated we would need a length of the top tube of about 88cm to sit at the back as we would on a normal MTB.

Can you folk explain us why the top tube is always so short? Would it be an error to request a custom frame with a longer top tube at the back?

Hope you folks can help! Thanks!

Greetings from France.
Karo and Manuel


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## Darth Lefty (Sep 29, 2014)

The length of the boom tube is pretty standard 28.5 or 29 inches, road or mountain, does not vary with frame size. Stoker position is set by the stem and bar, the rear size is purely about seat tube length and stand-over.

I guess I'd like to hear the case that "modern" trail bike geometry works for a tandem, since it has a lot to do with your fore/aft weight distribution which shouldn't make sense when there's two of you. Something fun to ask in the frame builder forum, maybe.

If you look at older road racing tandems from the 70's (Motobecane, Paramount, Peugeot etc.) they are a lot shorter than anything today!


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## mactweek (Oct 3, 2011)

Many teams opt to have a longer rear top tube. I don't know why this is not a standard option. Ventana will do this as a custom frame, and other makers will do this also. Our Curtlo was built with a 31.5" stoker top tube back in 2003 and my wife fits in her normal position. At that time it was necessary for him to use a straight guage rear top and boob tube because there were no butted tubes available at that time in the lengths required. That is still probably the case, and may be one reason for the lack of options.


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## Kama (Sep 2, 2018)

Thanks a lot, it helps to know there are folks out there riding such tandems! I have some more questions for you: Did you notice a pretty low stiffness of the frame (I am worried this might also be one reason for it not to be standard)? Is the distance towards the back of the captain sufficient in your case? Did you opt for the same sitting angle as on a MTB or did you do something in between MTB position and the position you would have on a commercially available frame? 

I think I will talk about this topic with a tandem frame manufacturer and tell you what their opinion on the topic is.


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## Kama (Sep 2, 2018)

Hi folks,

I spoke with the manufacturer of the frame. He confirmed he sees no issue in making the frame longer at the back. He also explained the non-availability of longer frames on common commercial tandems by the effort to use the same pieces for all bikes to reduce costs. So they use tubes with a fix wall thickness. As for static you would have to adapt the wall thickness to the length of the tubes and to the weight of the biker and they have to make it work for all weights, they have nearly no other choice than to search for a trade-off between weight (using heavily over-dimensioned tubes for the S size) and tube length (making the L sizes too short).


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## Kama (Sep 2, 2018)

Hi again guys,

It's a bit of a monologue here between me and me, but I thought I document the result. I got the bike now and made some small trips, and frankly, I recommend a longer frame at the back to anybody. Finally a tandem where you don't get thrown on the pilot each time he hits the brake. It's great comfort, finally a tandem where you are both riding a MTB.

Hope it helps somebody!


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## unikid28 (Oct 11, 2017)

Additionally I would expect a longer wheelbase.

More difficult on turns and switchbacks?

Not to mention longer timing chains, cable runs, etc.


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## Kama (Sep 2, 2018)

Hi unikid28,

On the turns, for our use it's not an issue. I fact, maybe because of the wheel rigidity, it even feels more maneuverable. But yeah, I'm not a downhiller, I do mainly off-road trekking over hiking trails with 40kg luggage, so I'm not really doing switchbacks with it. Regarding the cable runs, it's really not an issue: we are speaking of 10 more cm, the same parts can be used as on my old tandem.


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## mactweek (Oct 3, 2011)

Longer wheelbase, of course, but better balance more than makes up for the 2" of extra length. I live in an area where switchbacks are everywhere, and I don't find the extra length a hinderance over a "normal " tandem


unikid28 said:


> Additionally I would expect a longer wheelbase.
> 
> More difficult on turns and switchbacks?
> 
> Not to mention longer timing chains, cable runs, etc.


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## unikid28 (Oct 11, 2017)

Perhaps the manufacturers are trying to save money on materials then. Or don't understand stoker ergonomics.

On the other hand, my stoker who is tiny, and doesn't care about bar position, DOES complain that she can't see anything from the back.

I wonder how much having the longer stoker setup improves visibility? That might be something I might pursue to make the spouse happier about joining me on rides more often


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## bakerjw (Oct 8, 2014)

One thing that I can say about a bit of a stretched tandem boom tube is that it adds room for more frame bag space. Our tandem was a Curtlo custom and I never checked the length of the boom tube. 

LOL... My stoker, who does not ride a bike solo due to eye problems, doesn't care at all about seat, handlebar, saddle, or any other position. I constantly ask how she feels. Should the stoker bar come up or down? go forward or back? rotate one way or the other? Seat angled ok?

I ride a Jones H-Bar in the front and have a swept angle bar for her in the back. My time riding the Tour Divide has shown me that a swept bar like the Jones adjusted to put even pressure across the palms helps with numbness and allows me to ride 10+ hours with no issues.


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## TandemBear (Aug 18, 2013)

I understood that short top tubes were simply the result of trying to reduce frame flex. Traditionally, tandems utilized somewhat normal steel tubes. There wasn't a wide selection of oversized tubes available until around the mid 1980's. The 1 1/4" head tube, for example, wasn't a standard production item until Gary Fisher, Klein and Cannondale incorporated them into their designs. Then 1 1/8" became the standard. And now 1 1/2 to 1 1/8" taper is now the norm. And I remember mountain bikes of the late 80's bragging about having "Oversize tubing" or "OS tubes" in their frames to increase rigidity and "efficiency." It was thought that the added stresses placed on mountain bike frames necessitated using oversized tubes. The industry followed suit and myriad different diameter tubes became available.

So up until this time, tandems with traditional tubes suffered from a lot of frame flex. Two ways to reduce tube flexibility is to reduce length or increase diameter. Thus, frame length was reduced. This resulted in short stoker top tubes, especially taking into account the stoker stem pointed BACKWARDS, effectively shortening the top tube even more.

Then along came Cannondale with their oversized aluminum tubes. They could produce a tandem with a much longer stoker top tube and still keep frame flex to a bare minimum. This was an advantage of early Cannondale tandem frames.

Today, the really wide variety of tubes available has solved this issue. Add to that carbon fiber and you eliminate frame flex issues pretty much altogether, also accommodating whatever stoker top tube length is desired. (Within limits of course.) 

At least this is how I understand it to be in a very brief summary.


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## Kama (Sep 2, 2018)

unikid28 said:


> On the other hand, my stoker who is tiny, and doesn't care about bar position, DOES complain that she can't see anything from the back.
> 
> I wonder how much having the longer stoker setup improves visibility? That might be something I might pursue to make the spouse happier about joining me on rides more often


I am not sure about that... If you just increase the top tube length, she will also have a more leaned forward position... I don't know if the distance towards your back or the height is more important in the end? But you could get a pretty oldschool design : http://www.juchem-bike.de/images/Bild10_Tandem_kl.png  That could help!


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