zzyzx_xyzzy

I'm working on a homebuilt tandem frame using two donor frames for the front and rear triangles. I'd like to have some ideas about how long to make the stoker compartment. It will be more of a touring kind of bike as opposed to a go-fast bike and the stoker will be around 5'6". I see a lot of complaints that the stoker compartments on commercial tandems are too short; what would you build if you were going from scratch?

Homeyba

I would think that you would be the best one to answer this question. Since you are "making" this bike I think you should measure your stoker and and make the bike a length that will fit him/her properly. You could even take measurements from their single bike (assuming they are fitted properly). As far as I'm concerned their fit on the tandem should be exactly the same as their single bike so the measurements should be easy to get.

zzyzx_xyzzy

Sure, the seat/crank/bar relationship should be the same as on single bikes, but that leaves the question of how much space to give between the stoker's bars and the captain's seat.

bobthib

It seems to me that longer is better than shorter. You can always get a long or adjustable stoker stem. If it's too short, oh well....

I suppose the problems with longer has to do with frame flex, weight, and increased aero drag. Since your not worried about racing, the aero drag issue is not in play. I doubt if the weight issue it a big factor. It's only frame flex/strength to consider.

Homeyba's suggestion is a good start.

I guess I would also suggest looking at the geo. specs on some commercial bikes. If there some commercial bike that are too small, you can use that as a gage as to what not to do.

Good luck. Post Pix.

2frmMI

My wife loves our Erickson because Glenn designed the entire bike around her cockpit, and her cockpit was designed for her efficient riding. Top tube is probably a bit longer than her single bike and then the stoker stem extends her bars back and up to the position she would want on her single. Wheelbase is way longer than our prior, production, tandem, but we are way faster/stronger/happier because she is in an efficient position.

Homeyba

That would depend on where you mount the stokers stem and how long the stem is. The stem length will determine the length of the stokers area beyond the handlebar/seat location as they should be fixed by the fit geometry. If you are making a custom stem then you have some leeway to make it a little more roomy.

PMK

Here's the direction I'd aim, pull up the published specs from the quality tandem builders. Most of them will show dimensions for various sizes of the frames they offer. Use that as a guideline. You may also want to pay close attention to the headtube angles and wheelbase parameters.

Saying that, 28.5" would be a minimum for my stoker of 5'6" ish.

PK

osurxbiker

My wife (and stoker) is 5'5" and our stoker compartment is 32 inches long. The reason for the length is that I am 5'4.5" tall and her handlebars need to completely clear the back of my saddle. To have the handlebar height where she wants it (about even with her seat), the handlebars need to be an inch or two higher than my saddle. She will not ride the old tandem anymore it is just too short in the back and she can't get comfortable. You can always put a longer stoker stem on the bike, it is much more difficult to make it longer once you are finished.

swc7916

You're not really going from scratch; you're starting with a couple of pre-built bicycle frames. Tandems that are actually built from scratch use tandem-specific tubing along with design experience in order to create that long stoker compartment without sacrificing rigidity and handling. Old tandems were built with short stoker compartments because the designers couldn't get the rigidity they needed with the single-bike tubing that was available. If you're using single-bike frames as the basis of your tandem, make the stoker compartment as short as you can get away with.

WebsterBikeMan

The original poster said he was using donor frames for front and rear triangles. Given the nature of the question, I would assume he's also adding additional tubing - for the boom tube, and possibly a lateral, and presumably the rear top tube.

To answer the original question, you may be best off looking up adjustable stoker stems and seeing the range they offer. Now, given that that is a distance from the seatpost, you want a single-bike like fit from the saddle to where the bars will be, then add the length of the stem. The further back the bars can be relative to the captain's saddle, the less the captain's legs will interfere with the stoker's bars/hands. Although that partly depends on whether the stoker uses drop bars or cowhorns (more of an issue with drops). Also think about where the stoker's head/helmet is when in the tucked position, if that will ever happen - and whether it is running into the captain's back.

Retro Grouch 

That's what I think too.

When I had my own shop I used to keep a Santana, a Burley and an entry level Univega or KHS in stock. It was interesting to ride one right after the other. The frame flex was quite disconcerting to ride the entry level tandem right after getting off of the Santana.

GingerSpice

Another consideration is what type of handlebars the stoker plans on using. I stoke (and occasionally pilot) our tandem, and have found that although the length of the cockpit is similar to my single, I can't have drop bars in the rear. Reason? I'd like the riding position but my face would end up in the pilot's butt crack.

Anywho, it's something to keep in mind.

zzyzx_xyzzy

Yes, I'm starting with overbuilt (.9mm wall thickness, straight gauge, oversize) mountain bike frames and adding 4 more tubes to make a direct-lateral.

Thanks for the input, all.

Retro Grouch 

Sometimes bib shorts are a really, really good thing.

lhbernhardt

I've always thought the stoker compartment on modern North American tandems was too long, more a reflection of North America's love for bigger cars (remember the original '55 T-Bird, and how it morphed into the ugly 60's T-Bird that was as big as a Continental?). But looking more closely, I think it's much more practical than that. Tandems started off shorter not only because they used single-bike tubing and needed the stiffness, but because they were built for racing, with two guys of about equal height. The stoker thus had plenty of room to put his arms close to the pilot's hips and his head over the pilot's back. In order to put a larger male and smaller female on the same bike, the rear compartment has to be stretched out in order for the stoker to have somewhere to put her head. So to get back to the original question, I think the size of the rear compartment depends on where the stoker's head is going to be when in the "down" position.

Luis

zzyzx_xyzzy

That's a point I hadn't thought about. I have huge legs and my seat is going to be pretty far up there.