Tandem Cycling - Carbon Stoker Stem

Had swapped notes some time ago with TandeemGeek on carbon stoker stems ex Zona via Precision Tandems. I think maybe design of same varies a little bit from stem to stem. Pics of ours attached. Clearcoat already coming off near extension clamping cutout. Hmmm.

FYI, stoker bars are inverted 46cm Kestrels that have been chopped off and capped. Stoker initially missed longer extensions of Profile stoker bar, but has since gotten used to these.

Got the first c/f stoker stem (prototype) on our Zona tandem with 20,000 miles on it. No issues.

I think maybe design of same varies a little bit from stem to stem.

Yes, there have been quite a few variations as these carbon stoker stems have gone through the design evolution process. Each new variant represents an improvement to an earlier version where an issue was identified, reported, and subsequently addressed with an incremental improvement or change in the design.

Clearcoat already coming off near extension clamping cutout.

Best bet would be to take a few very good digital photos of the blem and send them back to Mark at Precision Tandems so that he can pass them to the fabricator for evaluation. It could be purely cosmetic... as often times happens with painted carbon frames that get nicked, or it could be something else.

Above all else, the support we have received from Mark and his carbon stoker stem supplier has been outstanding. Although it's preferrable to never need that support, it's been very reassuring as we've worked out a few wrinkles with Debbie's stem and also underscores why these and other hand-made, nearly one-off carbon or machined parts cost as much as they do.

Got the first c/f stoker stem (prototype) on our Zona tandem with 20,000 miles on it. No issues.

I'm not sure Kay's weight-on-hands is fully representative of what many other stoker's will experience, particularly those who aren't as petite as Kay and who ride in a far more aggressive single-bike position. Debbie's not exactly a monster, but even her petite 5'2" frame and relatively light weight will generate some really high momentary loads in certain situations. Those types of situations exist on our local loops and have been enough to test the limits of a couple CF stoker stem designs (http://www.thetandemlink.com/Images/Calfee/calfee_details1.jpg). Our present CF stem (see photos below) appears to incorporate the necessary refinements (most of which are not visible) to yield a far more robust and durable final design; however, time will tell. So far, so good....

Again, customer & product support have been excellent so no regrets with the choice. However, I'm a long-time beta tester who has a lot of patience and enjoys the development process...

Those types of situations exist on our local loops and have been enough to test the limits of a couple CF stoker stem designs (http://www.thetandemlink.com/Images/Calfee/calfee_details1.jpg).

How many CF stoker stems have you had? Did failures lead to replacements, and if so, where/how did they fail?

How many CF stoker stems have you had? Did failures lead to replacements, and if so, where/how did they fail?

We haven't had any failures, per se.

Our first stem (http://www.thetandemlink.com/Images/Calfee/calfee_mar8.jpg) was a 1st Gen 31.6 mm model that had flat tabs for the two Ti bolts used to fasten the stem to the captain's seat post and used a 27.2 shim. Max torque spec + Tacx Dynamic Paste was required to achieve a secure grip on the seat post. By the end of March a fissure developed in the clear coat between the bolt holes on both tabs that did not appear to compromise the integrity of the tabs. We rode it for several weeks after the fissures were noticed with no ill-effect or indications of a weakened joint while a new stem was produced. Your stem appears to use a subsequent design that incorporates gussets on each side of the tab which addressed the durability issue we saw on our 1st Gen model.

Our second stem (http://www.thetandemlink.com/Images/Calfee/calfee_topo1.jpg) incorporated a very robust, proven 27.2 clamp design for the seat post that was bonded to a carbon boom. The joint between the carbon boom and seat post clamp was somewhat complex and included a sharp angle in an awkward orientation. A fissure developed above the sharp angle where the carbon weave boom part of the stem butted up against the filler that covered the junction between the seat post and clamp and boom.. The fissure migrated from the 12 O'Clock position out to the 3 and 9 O'Clock positions and eventually worked it's way all around the joint with the two dissimilar materials. I should note a fiberglass inner-sleeve was epoxied to and used to reinforce the junction of those dissimilar materials so there wasn't a lot of concern regarding the potential for a catastrophic failure; therefore, we rode with this stem for perhaps another 6 weeks before our current stem was delivered to include doing the Georgia & Tennessee Tandem Rallies without any concerns about the stem's integrity. Your stem doesn't appear to have the splice joints that our 2nd and even our 3rd stem have which also mitigates a potential durability issue.

Our current stem (http://www.bikeforums.net/attachment.php?attachmentid=78172&d=1218060998) has a much more shallow angle and internal bracing at the junction of the seat post clamp and boom which addressed two potential weaknesses in the previous design. While there's no guarantee the satin clear coat won't develop a fissure, it seems far less likely given the latest enhancements and even if it does, the substructure is so robust that any life / durabilty issues are not something I'm concerned with.

So, in sum, what caught my attention were what appeared to be blemishes / cracks in the clear coat at highly stressed joints which could have been indicative of future durability issues. These were not something I saw as safety issues with the stems, noting in both cases we continued to use them instead of pulling a spare alloy stoker stem out of the parts bin pending receipt of a replacement.

Again, as I said, we really haven't had any failures... just some visual cues to suggest that we could experience reduced service life / durability issues down the road that I elected to address now rather than later. Unless someone has similar visual or audible evidence of potential durability issues I wouldn't be concerned about the stem's integrity or durability.

General Note: As mentioned in various other threads, carbon is tough stuff that seems to forewarn of most potential strength or durability issues so long as you are attentive to your equipment AND are very careful when it comes to applying torque to bolts that secure carbon parts: too loose and it creaks too tight and it cracks. Once you hear that "craaacck" it's too late and you can't go back.

Another way to skin the cat is a fixed length aluminum stoker stem.



We put a 140 mm stem flipped with minus 6 degree rise on in place of an adjustable stoker stem. With the negative rise, it ends up putting the bars in the same position (both height and extension) as the adjustable stoker with 175mm of extension.

If you can make the fit work with a fixed length stem, you'll end up dollars ahead,and I'd bet as light or lighter than an adjustable CF stem.

Our stem cost $7 on ebay, and is way lighter than the adjustable stem it replaced . If you want to really count grams Deda newtons are available from 140 mm down, and I'm certain are lighter than an adjustable CF stem.

The downside is you give up adjustability and the fit has to be dialed in before you purchase.

Yes these c/f stoker stems have evolved and are painstakingly built one-at-a-time . . . no production runs at all.
Adjustability in a stoker stem is important to us. When stoker asks for 1/16th of an inch more/less space (yes, she is that fussy) she gets it.
Three+ decades ago we used what was available (Pivo non-adjust stoker stem if I recall) on our Follis.
Had our first custom adjustable stoker stem built by Matt Assenmacher way back in 1976 and been using custom stoker stems ever since,
Our Zona c/f adjustable stoker stem also features a built-in water bottle holder (dead center) so stoker can drink, via plastic tube, from H2O bottle without removing it. Our last 3 stoker stems also included a mini braze-on (glue-on for c/f) to mount our mini garage door opener.
If your gonna go custom, get what you want . . .
Pedal on TWOgether!
Rudy and Kay/zonatandem

I really like this idea. I'm going to measure our stem out and see if it will work for us. A really cheap way to save some weight and very simple.



Another way to skin the cat is a fixed length aluminum stoker stem.



We put a 140 mm stem flipped with minus 6 degree rise on in place of an adjustable stoker stem. With the negative rise, it ends up putting the bars in the same position (both height and extension) as the adjustable stoker with 175mm of extension.

If you can make the fit work with a fixed length stem, you'll end up dollars ahead,and I'd bet as light or lighter than an adjustable CF stem.

Our stem cost $7 on ebay, and is way lighter than the adjustable stem it replaced . If you want to really count grams Deda newtons are available from 140 mm down, and I'm certain are lighter than an adjustable CF stem.

The downside is you give up adjustability and the fit has to be dialed in before you purchase.

Another way to skin the cat is a fixed length aluminum stoker stem.



We put a 140 mm stem flipped with minus 6 degree rise on in place of an adjustable stoker stem. With the negative rise, it ends up putting the bars in the same position (both height and extension) as the adjustable stoker with 175mm of extension.

If you can make the fit work with a fixed length stem, you'll end up dollars ahead,and I'd bet as light or lighter than an adjustable CF stem.

Our stem cost $7 on ebay, and is way lighter than the adjustable stem it replaced . If you want to really count grams Deda newtons are available from 140 mm down, and I'm certain are lighter than an adjustable CF stem.

The downside is you give up adjustability and the fit has to be dialed in before you purchase.

Personally - I wouldn't want my stem up on the highest part of the seatpost - especially a CF one - due to leverage and torquing by the stoker. You could achieve the same bar position by flipping it and putting it in the middle of the seat post.

Personally - I wouldn't want my stem up on the highest part of the seatpost - especially a CF one - due to leverage and torquing by the stoker. You could achieve the same bar position by flipping it and putting it in the middle of the seat post.

I thought about that. However, you can't get as much extension with the stem flipped the opposite way. (remember the seatpost is running at a 17 degree angle from vertical, so as you move up the post, you're also moving the handlebars backward.)

If you look at the way Co-Motion has their bikes set up in their photos the stoker stem is reasonably high up the seat post.

And my stoker is fairly light, so she's not putting tht much torque on the bars.

Finally, with the stem further down, my thigh rubs on the bolts.

While we are in our mid-70s now and do not ride as far/long/aggressively as we used to just 5 years ago, Kay can put stress on her stoker stem; she still pulls on the bars when climbing.
A few times she pulled so hard that she twisted the stem in the insert. She is a little Mity-Mouse!
But age, and some health issues like cancer for both of us, has helped us to ride and smell the flowers and have a coffee break, rather than continually riding with nose to the wheel. Been there, done that and loved it!
Still gettin' in 100 miles a week TWOgether!
Rudy and Kay/zonatandem

....can put stress on her stoker stem; she still pulls on the bars when climbing. A few times she pulled so hard that she twisted the stem in the insert.

Those aren't the loads I'm referring to.. It's her riding posture (very upright, low weight on hands) combined with light weight that put less demand on the stem than do the taller and heavier stokers who ride more stretched out with a greater amount of their static weight sitting on the handlebars.

The part of a stem that takes the most abuse is the clamp and the joint between the boom and the steerer tube / captain's seat post clamp. Moreover, the longer the stem the more tensile strength that joint must have to deal with those big hits where the front wheel nails a bump while the tandem is descending at speed. It's at the moment of impact that a stoker's weight-on-hands can be magnified x2, x3, x4. Moreover, the stoker's boom is essentially a lever in that scenario and as that boom/lever get's longer, those loads are further amplified.

In fact, below are some photos TandemRacer took of a 25 lb drop test that Thomson subjects its own stems to as well as their competitors to evaluate the tensile strength of that critical joint.

Twisting forces and long-term fatigue are also the source of some stem failures, but IMHO those just don't routinely generate the kind of forces that a front wheel hit does on a stoker's stem.... particularly those very long adjustable stoker stems that are needed for an extra-long stoker compartment.

Kay, at 4'10 3/4", has an extremely short stoker compartment and no need for a long stem.
If stoker is in the drops in descent/big hits as you describe then stresses will be multiplied.

Kay, at 4'10 3/4", has an extremely short stoker compartment and no need for a long stem.

Exactly, which was my point earlier on in the thread with regard to why you have not had any issues with your stem whereas Debbie -- who at 5'2" uses 190mm of boom extension while still maintaining her single racing bike riding position -- has put much greater loads into the clamp and base area of our stem(s).


...it ends up putting the bars in the same position as the adjustable stoker with 175mm of extension.

I concur with the earlier post regarding double-loading the top-end of your seat post; that's something you'll want to keep an eye on.

However, to your point, it is very true that most tandems have stoker compartments that are short enough to allow for the use of regular threadless stems, so long as the stoker is either much shorter than the captain or will use a very aggressive racing position that already has them "spooning" the captain's backside where captain hip clearance doesn't pose an interference issue with the stoker's bars or arms.

As previously noted and as alluded to above, our Ericksons and the Calfee use fairly long stoker compartments @ 31.5" and 30" respectively, even though Debbie is only 5'2". This was an Erickson concept that we have embraced because it gives Debbie a lot more breathing room while still allowing her to replicate her single bike riding position. However, as you'd suspect, for us an extra-long adjustable stoker stem is a necessity that can't be substituted with a stock-sized stem given it would need to be 195mm long. Therefore, either an adjustable stoker stem or a custom-length fixed stem would be required and, like zonatandem, a fixed stem is just a non-starter for Debbie as we've both reached the age when subtle adjustments seem to be the norm these days.

I thought about that. However, you can't get as much extension with the stem flipped the opposite way. (remember the seatpost is running at a 17 degree angle from vertical, so as you move up the post, you're also moving the handlebars backward.)

If you look at the way Co-Motion has their bikes set up in their photos the stoker stem is reasonably high up the seat post.

And my stoker is fairly light, so she's not putting tht much torque on the bars.

Finally, with the stem further down, my thigh rubs on the bolts.


It's all up to you - but it certainly won't feel that good when you go to sit down on shards of CF :)

Originally Posted by merlinextraligh
...it ends up putting the bars in the same position as the adjustable stoker with 175mm of extension.

I concur with the earlier post regarding double-loading the top-end of your seat post; that's something you'll want to keep an eye on.

I disagree that attaching the stem at the top of the post is a bad idea. Why?
1) It makes no difference to the stress on the seat post where it is clamped by the frame, since for a given bar position, the moment arm from any force on the handlebars onto the seat post, where it is clamped by the frame, must be the same.
2) For a given force on the handlebars the longer seat post load path will allow a greater deflection, thus theoretically soaking up the bumps better (or robbing you of sprinting energy depending on your view point)
3) The top of the CF seat post is actually the strongest place to clamp anything. At the top the CF cylinder of the post is supported internally by the aluminium head which is normally glued into the inside end of the post.

I disagree that attaching the stem at the top of the post is a bad idea.

I'm not a smart guy, but I'm pretty sure that as you make a lever longer and/or apply more force at the distant end of a lever, mechanical advantage increases at the axis... pick your analogy, cranks, linear pull brakes, a torque wrench or breaker bar.

How is it that the placement of the stoker stem at the top of a seatpost as compared to a point closer to the seat post clamp seems to be exempt from this aspect of the moment of force?

I thought about that. However, you can't get as much extension with the stem flipped the opposite way. (remember the seatpost is running at a 17 degree angle from vertical, so as you move up the post, you're also moving the handlebars backward.)

Well, no. The point at which the bars attach to the stem follows a line parallel to the seat post. Actually a line segment, the endpoints of which are determined from the highest and lowest points at which the stem can be attached to the seatpost. If you flip the stem over, this point continues to lie along the same line, which is (approximately, depending on the bike) 17-18 degrees from vertical. But the endpoints of the segment change. Depending on the length of the stem, and the length of the exposed portion of the seat post, these two segments may or may not overlap. But they are colinear. This would be easier with a picture, but I don't know how to do that...

Imagine a carpenter's square on the seatpost so it hits the centreline of the bars, now measure the distance up to the stem attachment. If you flip the stem, and move it down to as far below the square point as it was above, the bars should be in exactly the same place.


Finally, with the stem further down, my thigh rubs on the bolts.

I'll grant you that.

^ I don't think the math works out that way in practice. It might if the stem rise was 17 degrees, (correlating to the seatube's 73 degree. )But the stem is 6 degrees +/-.

Because I have a compulsive need to be right, I went and measured it, dropping a plumbline to the top tube. In both cases I set the top of the bars 5" above the top tube. This meant approximately 2" difference in the clamping point on the seatpost (which unfortunately I didn't record) from the stem flipped negative or positive.

When the stem was attached with negative rise, the bars moved 4mm toward the stoker's seat measured by a plumbline dropped to the top tube from the bars.

I'm not a smart guy, but I'm pretty sure that as you make a lever longer and/or apply more force at the distant end of a lever, mechanical advantage increases at the axis... pick your analogy, cranks, linear pull brakes, a torque wrench or breaker bar.



I'm pretty certain you're right. Moving the stem 2" up the seatpost gives a 2" longer lever, and will increas the total torque put on the seatpost.

I'm also pretty comfortable that we're well within the anticpated forces the bike was designed for.

The seatpost is 350mm. I could raise it another 8" and not be near the minimum insertion.

My stoker weighs 130 lbs.

Presumably you could flip the stem negative, but raise the Captain's seatpost 2" to accomodate a taller team. You'd have just as much of a lever. And I bet you could do that with 2 male 185lb Cat 2 racers,and still not break the seatpost.

I'm also pretty comfortable that we're well within the anticpated forces the bike was designed for.

Recalling it is a Alpha Q Pro (220g) and not the Pro Light (190g), I suspect you are correct. Moreover, Co-Motion would not have spec'd it unless they had a lot of confidence in its durability.

I was actually quite surprised at how robust the FSA SL-K seat posts were compared to our Easton EC-70's and in looking at the Alpha Q, they are very much like the FSA SL-K.

Regardless, it appears to be a good set-up and lightweight to boot.

^ I don't think the math works out that way in practice. It might if the stem rise was 17 degrees, (correlating to the seat tube's 73 degree. )But the stem is 6 degrees +/-.

Because I have a compulsive need to be right, I went and measured it, dropping a plumbline to the top tube. In both cases I set the top of the bars 5" above the top tube. This meant approximately 2" difference in the clamping point on the seatpost (which unfortunately I didn't record) from the stem flipped negative or positive.

When the stem was attached with negative rise, the bars moved 4mm toward the stoker's seat measured by a plumbline dropped to the top tube from the bars.

Geometry doesn't lie, but it does assume rigid bodies. It may be that the stem and/or its attachment have just a little flex, and since this is always toward the ground, there's an assymetry that may explain it. 6 vs 17 degrees doesn't change the isoceles triangle you would start with. But a degree plus or minus in the angle of the stem at the point where the bars attach would.

Captain's seat posts do break. We broke a Control Tech aluminum seat post several years ago. It snapped at the frame and and we removed the broken off piece and inserted the "stub" to ride back to the van. The seat post was only 4 or 5 years old. Fortunately, we were going slow up a hill and got stopped without hitting the pavement.
Sheldon

Geometry doesn't lie.

:thumb: A picture's worth a thousand words...

A bit off topic and I'm sure its been addressed elsewhere, but aside from issues like double butted frame tubing, loadpoints, etc. has anyone used a `standard stem like' approach on the top tube of the stoker cockpit rather than the captain's seatpost? It might be particularly useful with small children or shorter partners as a way to get the bars back from the captain's seat without huge long stems (and lever arms) hanging off the seat post.
I ride with my 8 year old and fabricated a longer non adjustable stem via two cheap stems and a welder, but had I had an amenable diameter top tube I could have clamped something directly to that.

your thoughts and opinions welcomed...???

cheers
mike

:thumb: A picture's worth a thousand words...

Someone has way too much time on their hands.:)

On a practical note, I found a 140 Deda Netwon with 4 degrees rise plus/minus. It will be close to 100 grams lighter, and with only 4 degrees of rise, the whole issue of the stress on the seat post, and the a mount of extension, from flipping it positive or negative will be essentially be moot.

Someone has way too much time on their hands.

No, I'm just REALLY proficient when it comes to doodling with PowerPoint, PageMill and other low-tech applications...

But, even you must admit, it WAS a rather compelling image :thumb:

well played

Yes, by several builders for several reasons/purposes.
Oedal on TWOgether!
Rudy and Kay/zonatandem

I have a 150mm stem made by System 2 on my mtb that's held up great over the years. 216g.

Just letting you know to keep your eyes open if you're interested in long stems.