smatson

For lightness, it would be nice to use a 30T Wolf Tooth 104 BCD with a Lightning crankset. The crank arms would be 175mm for the captain and 172.5 for the stoker. Would this combination be a good one? I've read that as long as the two timing chainrings are the same size there would be no problem, except possibly faster wear on the chainring. However, one post mentioned a "mushy" feeling with a small timing chainring. I've only ridden a 42T timing chainring on a different tandem, so I don't have anything to compare it with. I'm not worried about faster wear, but I don't want "mushy" performance. Any thoughts?

Alcanbrad

I don't have answer to your question, but a couple of thoughts come to mind: I would expect that a smaller timing ring would yield a smoother performance just based on the experience of a smother pedal stroke one feels from the smaller chainring vs the large. Also, I would expect you would need to insure that the eccentric has sufficient range to take up any chain slack resulting from the geometries of the 30T rings and the center to center spacing of the cranks. I have never studied this but it might be an issue.

tandemdr

Smaller timing chainrings do result in a mushier feel (resultant of increased frame flex), greater chain tension, faster wear of everything, bottom brackets included, and loss of efficiency from greater friction. The one hour world record holder bikes seem to run large chainrings with a larger cog compared to a small chainring and tiny cog since one is more efficient than the other and the weight penalty of going big is pretty small. Here are some 58T chainrings to get rid of mush and bring back that direct drive feel to the captain!
https://www.precisiontandems.com/bra...ran_2015_3.jpg
Of course any chainring tattoos are bound to be more impressive.

reburns

smatson

Yes, I really wanted to use a Gates belt, but we needed to lengthen the stoker compartment and the belt does not fit the frame. Thanks for the replies! Any other opinions? Experiences? Recommendations?

Lightning recommends using Wolf Tooth components, and their 104 BCD ring comes in sizes 30T, 32, 34, 36, and 38. Like I said above, I was considering the lightest one, but based on the post by "Tandemdr" it sounds as though I should go with the 38T. The weight difference is 30 grams per ring for a total of 60 grams.

Alcanbrad

[QUOTE=smatson;20681406]Yes, I really wanted to use a Gates belt, but we needed to lengthen the stoker compartment and the belt does not fit the frame. Thanks for the replies! Any other opinions? Experiences? Recommendations?

If you are not already aware, Gates makes different size belts and chainrings. Perhaps a different combo would work. You might find this page helpful: https://www.precisiontandems.com/cat...belt_drive.htm

If the listed options do not not work, calculate the belt length you would need for the available chainrings and contact Gates, they may have something (you could also consider having custom chainrings machined for you.

smatson

Yes, I know there are a few variations possible to make the Gates belt system work. Calfee tried to accommodate these possibilities, but in the end it came down to getting the fit right for the stoker or getting the length right for the Gates belt. We went for the stoker fit.

OneIsAllYouNeed

For road use, I'd stick with 34t or larger chainring. With the excess chain wear, I think you'd be frequently adjusting the eccentric with smaller rings. 32t rings with 4-bolt pattern aren't recommended because it may make it impossible to get the crankarms to align.
With a Calfee frame and other relatively stiff frames, any "mushy" feeling will be driven by chain and chainring choice. On less stiff frames, the added chain tension could be significant enough to bend the boom tube.
You might also consider direct-mount S-works chainrings or the Wolftooth Camo system to appease the quest for lightness.

smatson

Thanks for the feedback, everyone. I think I'm going to get over my "weightweenie-ness" and go with the Wolf Tooth 38T 104 BCD chainrings for the timing side.

Doge

I would go larger on cross over (sync on left). I'd run 46 or something, but I'm big. Smaller rings = more chain tension. With cross over, 1 person power pulls forward and 2X backwards. If the rings are smaller there is even more torque on the BB spindle.
I like same side drive. I just bought a tandem and am playing with a 34 and decided on the 36 for less tension, although on the same side the tension pulling forward helps offset the two person tension from the chain pulling against the cassette. They are about equal chain tension now. That makes me happy.

bwebel

I'm not quite getting why smaller rings would lead to more torque on the BB spindle? Bigger rings would distribute the force over more chainring teeth, but it's the same amount of force being applied by the riders...why would the torque ultimately transmitted to the spindle be different? Why would the chain tension be different when it is between the timing rings? It's a 1:1 system regardless of whether it's a 34x34 or 54x54 and the same amount of power is going into each case. I understand the greater friction, but not the rest of the effects attributed to smaller rings. What am I missing?

Doge

-Chain tension is based on ring size - The distance from the front spindle pedal spindle is one lever arm, distance to the ring OD another. That ratio is multiplied by your foot/feet force/s on the pedal.

175mm crank / 67.5mm (~radius of 34T ring) = 2.59 multiplier to whatever your feet are doing. Tooling along smoothly, I doubt it is noticeable. Being I've always been a strong 200# plus guy, in a sprint, on a hill I can put over 200# of force just standing on the pedal (much more in a sprint pulling up and using bars). So 200# means ~500# front chain tension.


-The front cross over ring is pulling the rear sync ring forward so 500# forward.

For simplicity say we have no stoker and a 34T on the drive side - we are going up a hill.

We have 500# pulling the left side of the spindle forward, and 500# pulling the right side of the spindle backwards. The rings I removed were 215mm apart (more to the big ring) so a. There is 224kg/215mm torque (too lazy to do the torque units in ft/lbs) approximately around the seat tube - without the stoker. Move the sync chain to the drive side and the forward tension and rear tension cancel each other out in the same ring size. Add a stoker and you have torque again just like a single bike. When using a smaller sync ring than drive ring the greater chain tension pulling forward maks the torque less than on a single.


Anyway, the net effect is I notice it. I also found having the chains on one side made mounting on car racks easier as well as the times I needed to lie the tandem on the side.

Doge

After re-reading your post, a shorter explanation.

Torque on the BB, the twisting along near the seat tube axis.

NOT torque along the spindle axis - which is what you point to 34x34 and 54x54 being the same (and I agree).

Leisesturm

On the recumbent forum (not BF's) there is a similar anxiety expressed by several riders that they can flex the 2.5" diam. 'booms' of their bikes. Enough so to impair performance. For the record, I DOUBT that an ordinary tandem couple that do not extensively weight train for competition level output are in any way shape or form equipped to damage a Calfee Tandem frame simply by their choice of timing ring. A 46T timing ring is unnecessarily large (not to mention hard to find, actually a 48T would be far easier to source). A 30T is unnecessarily small. Our tandem came with 39T timing rings and so has every tandem we have ever owned, and we have owned several. Hmmm. I don't know. If only for sheer ... availability, I wouldn't be looking that hard to roll the rock uphill and look for something else. And FWIW the 5-arm crank is still the Gold Standard.

Doge

I road the 36T same side drive this morning. I'd go bigger if it would fit (using a standard DA double). I may move sync ring outside if the 48 will clear the chainstay. It may not.

MikeyMK

My Quatrefoil came with 38s. This is my first tandem so i have no other knowledge on this.

I can't see there being any notable difference between 34 and 44 in real world terms, especially if your stoker is a reasonably close match. I can imagine 22s being a little less refined, though. Incidentally, an eccentric adjuster will cater for any number of teeth, as the range is more than a complete pair of chain links.

I can't comment on my own experiences, as mine are now uneven (40/30), my rear crank turns faster than the front one (my stoker's crank arms are omitted, she has foot pegs instead).