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I've read John Allen's thoughts on tandem drivetrains at http://www.bikexprt.com/bicycle/tancrank.htm and find that his thoughts make sense.
I pick single-side-drive as the optimum choice for a tandem drivetrain because of the load canceling effect at the rear BB and weight savings.
Why don't manufacturers embrace single-side-drive? Who thinks it might be worthwile to upgrade the crossover system on the Trek T2000 to single-side-drive? Who thinks that 1000 pounds or so of twisting force on the rear BB is nothing to think twice about?
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1. Why don't manufacturers embrace single-side-drive? 2. Who thinks it might be worthwile to upgrade the crossover system on the Trek T2000 to single-side-drive? 3. Who thinks that 1000 pounds or so of twisting force on the rear BB is nothing to think twice about?
1. Because cross-over cranksets are now readily available, cost about the same as two equivalent single bike cranksets, are simple to set-up, work with kid-back cranksets, work with triplets/quads/quints, and -- given all of the foregoing -- have been adopted as the standard by the industry which, in turn, is why consumers are now comfortable with them.
2. If the cross-over system you have now works and you're not trashing bottom brackets, changing over to single side would be a solution looking for a problem.
3. Bottom brackets are incredibly durable & relatively inexpensive so long as you avoid models with Titanium axles which really aren't appropriate for a daily-rider tandem in the first place. On those infrequent occasions when one does start clicking or creaking from wear, replacement is relatively painless. Note: I tend to have more problems with my front bottom brackets creaking than my rear ones.
The logical follow-up question would be, "OK, so when WOULD I consider the use of single side drive?"
If I were building a tandem for racing or lived and rode almost exclusively in relatively flat to rolling terrain I would most likely use single side drive. However, given those conditions I wouldn't need more than a double crankset and would opt to still use a triple and use the inner, alpine/granny gear position to "host" the rear timing ring. This would permit the timing chain to run inboard as much as is possible which is also desireable as the further out you move the rings from the frame the more leverage they exert with respect to the loads the timing chain system places on the boom tube. Simply put, when you crank on a tandem's drive train, the front and rear bottom brackets are pulled toward each other which, as you would expect, causes lateral deflection of the boom tube.
Wow TandemGeek you are my kind of people.
as ussual I love the explanation..
you have a exelent perception of the factors involve and a K.I.S.S.(Keep It Simple Stupid) aproach that is very refresing..
exelent work keep it up.
Thousand and thousands of tandems out there a-twisting and a a-churning with cross-over drive, without any real problems.
If you want a 'weight saving' solution, single-side drive on a Trek 2000 ain't it . . . get a lighter frame/components.
After 5 tandems, 30+ years of riding and 200,000+ miles as a duo, we opt for cross-over drive.
However, that is just our opinion/experience.
Perhaps the reason (or a reason) manufacturers have not embraced single-side-drive is because it is not as superior as it once seemed.
With rear crossover drive the BBs are pulled toward eachother on the left side causing the boom-tube to want to bow to the right. The right side of the stokers BB and the wheel are being pulled toward eachother causing the chainstays to want to bow to the left. By causing the bow in 2 directions, the section bowing is shortened and could be expected be stiffer than if both sections wanted to bow the same direction.
With single-side-drive I see no force causing a bow to the right, and all forces want the bike to bow to the left thus making the bowed section longer and less resistant to the bowing force.
Thoughts?
FYI, here are some of my thoughts on the other advantages disadvantages all of which are speculative:
Advantages:
1. All the chains are on one side (which chain do you want to lay in the dirt while you're changing a flat? )
2. Slight weight savings from one less spider and chain ring, but the opportunity exists to select lighter cranks at the same time which is more significant.
3. Canceling much of the load at the stokers BB would cause the chainrings to run less when standing or otherwise going hard.
4. My stoker wants to spin slower than I do, so I'd jump at the opportunity to install longer cranks up front.
5. I could switch to outboard bearings at the same time which may stiffen the BB area.
Disadvantages:
1. Bye-bye granny gear. (Or kludge a chainring where it doesn't belong)
2. It would cost enough to consider a carbon fork instead (real weight savings)
Thousand and thousands of tandems out there a-twisting and a a-churning with cross-over drive, without any real problems.
If you want a 'weight saving' solution, single-side drive on a Trek 2000 ain't it . . . get a lighter frame/components.
After 5 tandems, 30+ years of riding and 200,000+ miles as a duo, we opt for cross-over drive.
However, that is just our opinion/experience.
Leave it to the guy riding a super-light CF tandem to tell me to save weight in my frame :) I picked his bike up at the Rally in the Valley last year, and I remember it feeling like picking up a single.
- Hi Rudy!
How's things in northern AZ?
We are hiding out (with our tandem) in northern Utah for a few months.
As young as U-2 are, weight-saving is not an issue!
Pedal on TWOgether!
Rudy and Kay/zonatandem
4. My stoker wants to spin slower than I do, so I'd jump at the opportunity to install longer cranks up front.
Isn't spinning defined by the RPM?
I think a change in crank length would not have the desired effect, but different size chainrings would. Then again, you would also change the crank positions relative to each other, since you would pedal at different RPM's.
:)
Isn't spinning defined by the RPM?
There are two aspects to "spinning". One is RPM which is a by-product of your final drive gearing at a given speed and the other another is the velocity of the pedal/foot which increases with the length of the crank at a given RPM. Think back to the childhood game of "crack the whip" or the Ice Capades figure skating pinwheel to get a graphic image of the concept.
I think a change in crank length would not have the desired effect, but different size chainrings would.
Timing ring size is addressed in this thread: http://www.bikeforums.net/showthread.php?t=125947
Then again, you would also change the crank positions relative to each other, since you would pedal at different RPM's.
No, being in-phase or out-of-phase with all other things being equal will not change the RPM or foot velocity.
No, being in-phase or out-of-phase with all other things being equal will not change the RPM or foot velocity.
I was referring to two different size chain rings for the timing chain on the same bicycle. That would result in different RPM for the two riders.
In the thread about chain rings you quoted, you mentioned that the captain would not know the pedal position for the stoker and you therefore would run into problems with leaning. True! But to scrape a pedal on the ground while leaning you actually have to lean pretty far, Probably much further than most stokers are comfortable with.
I was referring to two different size chain rings for the timing chain on the same bicycle. That would result in different RPM for the two riders. In the thread about chain rings you quoted, you mentioned that the captain would not know the pedal position for the stoker and you therefore would run into problems with leaning. True! But to scrape a pedal on the ground while leaning you actually have to lean pretty far, Probably much further than most stokers are comfortable with.
Feel free to put your theory into practice and then let us know how it goes. However, before doing so, consider testing your assumptions about how far you need to lean before you catch a pedal by riding a fixed gear bike on your regular routes with standard length cranks. It could be an eye opening experience or it may confirm that your riding style would be suited to your mixed chain ring configuration.
Appended below is a set of tables that should approximate the fixed differences in cadence that the captain and stoker would experience with various different timing ring combinations.
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