dfcas

Looking at the Co-Motion sizing charts,some questions came to mind.

i need the 23 front to get enough top tube,but choosing the rear size is confusing.They offer both a 23x20 and a 23x 18.I'm trying to figure out why.

The stoker top tube is the same in both,so I assume the wheelbase is also. What benefit does the taller rear end bring?less seatpost equals less stress on the seat tube junction,but I can't imagine that being an issue on a tandem. The stokers weight would be loaded lower or higher into the frame,maybe changing handling? Is the frame stronger with more triangulation with the taller seat tube?Aesthetics? A tall rider may need the taller seat tube,but I would think at that point the rear tt would be too short.

Enquiring minds want to know..

SDS

It may be fairly argued that for all stokers over 5'2", all production tandem rear top tubes are too short. I suppose the taller rear seat tube for the larger size is intended to allow a taller saddle height, but when you can get a 430 mm seatpost, the utility of that also falls into somewhat questionable territory.

If you are in the position of considering the purchase of a new Co-Motion, take note of the small premium charged for a custom frame. Then consider the various positions you adopt on a single bike, i.e., low and forward when seated and going hard, and moving forward when you stand up, and you may conclude that in addition to the horizontal spacing between the center of the seatpost and the center of the handlebars, an additional 8-10 inches of bottom bracket spacing may provide just enough space. So if you have a stoker of 5'2" with a horizontal bike fit as described above of 22", you may then add 9" of space in front of the stoker to get 31" of horizontal bottom bracket spacing. Aerobars usually require the most space of all, so if you have room for them, then you have room for all other position requirements.

This additional spacing allows the stoker to use the back of the bike more like a single bike, to move into many different positions instead of being limited to a few that quickly become uncomfortable, and to have enough space to work efficiently.

TandemGeek

The 23x20 is Co-Motion's Large size and uses conventional frame geometry with a slight slope to the top tube, proportional to Co-Motion's Medium (22x19) and Small Size (21x18) frames.

The 23x18 is also called the Large/Small: Simple deductive reasoning would suggest the primary logic behind the Large/Small is offering a shorter frame to accommodate "small" stokers who aren't comfortable on a tandem that they can't straddle.

A by-product of the shorter stoker's seatpost is a more steeply sloping top tube that also happens to afford the captain a little more standover height which, I would suspect, adds a fourth size for captains with just a little more standover height than the Large size frame.

FWIW, Cannondale offers similar sizing options with it's Med/Small, Lrg/Med, Lrg/Small, XLrg/Small, XLrg/Med.

As for the benefits of a "compact" tandem frame, that one's been hashed about quite a bit with regard to the lateral-less (lat-less) or my preferred moniker the top-tube-less (topless) frames. Just search on Paketa or Bruni to find those threads.

dfcas

I believe both sizes have lateral tubes.is this correct?So,in this case,with a 5'6" stoker,either size seems to work.The conventional 23x20 says it will fit a stoker up to 6'0",which is 2" more than the 23x18.A 6' stoker on that bike would have my sympathy...

It seems to come down to the visual preference.

Not that it matters,but Cannondale does not offer a L/M road frame.I think they should,or maybe just increase the stoker TT 1" across the line.They certainly have the stiffness to support it.

zonatandem

Visual preference has nothing to do with it . . . proper fit does.
As Mark says, it has to do with stoker standover room. Can your 5'6" stoker stand with both feet *flat* on the ground on a 20 inch frame . . . or would that be a close call for her?
If it's a close call, then we suggest the 18" frame that'll give her 2" more standover space.
We have never bought into the idea that stoker doesn't have to be able to get both feet flat on the ground . . that it's the captain fault if, in a panic stop, she hits her crotch hard onto that too high toptube. No way to ruin a weekend in our opinion!
And yes, both sizes have the internal lateral tube.
Pedal on TWOgether!
Rudy and Kay/zonatandem

TandemGeek

If that makes you happy, then go with it....

They do, but they don't call it L/M except in the off-road frames. But, yes, I just checked and I did screw up and recalled the off-road frame nomenclature instead of the road but it's all relative. What Cannondale calls a Large is on par with Co-Motion's Medium and so on. C'dales front seat tubes "seem" short because all but the Jumbo have the stepped top tube which is where the extra standover height also comes from. I've included a comparison table with Co-Motion and Cannondale's frame sizing tables below.

dfcas

The Cannondale stoker compartments get longer with size changes-S=27.1,M=28.1,L=29.1. this makes a decision distinctly different from Co-Mo,where the top tubes hold at 28.5 across all sizes.

I would expect that the 18" rear on the Co-Mo ,due to the different angle of the seatstays and more exposed seatpost for a given rider,would be softer riding than the 20" rear.This may be better or worse,depending on preferences and team weight.Obviously tubing changes could offset this.

Since a 5'10" stoker sould standover either one,it still seems muddy to me how to choose sizing.(I have a rapidly growing daughter.)

bikeriderdave

A 5'6" Stoker should easily fit either size, so visual preference really could be the deciding factor for some. IMO, the 23x18 will be more versatile. It is also more likely to have room for the shock-absorbing seatpost of choice -- if that matters. My 5'6"-ish stoker is quite comfortable on our small Trek, which has a 44 cm (17.3") stoker seat tube. If the OP can't ride both sizes before making a purchase decision, the 23x18 is clearly the safer choice.