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Tandem Cycling A bicycle built for two. Want to find out more about this wonderful world of tandems? Check out this forum to talk with other tandem enthusiasts. Captains and stokers welcome!

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Old 06-24-10, 08:16 AM   #1
jim_pridx
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Question about tandem frame rigidity.

Hey all,

As some of you might be aware, my wife and I finally purchased a second tandem earlier this year after having ridden our first one for about 25 years. Needless to say, we've been riding tandem for quite some time, but we have very little experience riding other tandems than the two we've owned.

Our first tandem was literally a tank and very heavy, but it was designed as a road racer by a guy who just dabbled in building bikes. Although, they were really built well. This particular bike was entirely constructed of straight-gauge aircraft tubing with two small laterals extending from the front of the bike to the rear. In a word, the bike was very "rigid," and we could feel every bump in the road with little or no flex in the frame.

Our new bike, however, rides like a cadillac; it's very smooth and handles very well. However, I notice much more flexibility in the frame, especially when climbing hills or simply just wiggling the bike while on a ride. This certainly doesn't bother me, for the added flexibiltiy just seems to make the bike a much more comfortable ride for both of us.

So, my question(s) is this: how much flexibility is inherent in your frame? and how much flexibility do you find acceptable? Of course, I'm not looking for a fix.....just curious. Thanks!

Jim
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Old 06-24-10, 09:18 AM   #2
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Our 89' Burley would toss the timing chain when we were beginners, we were not smooth back then, boy that thing would flex, rode it for years. When you hit a bump, it would bounce 13 times. Moved up to a Cannondale, much stiffer everywhere, your behind really noticed. It was good for a gear everywhere, and when you hit a bump, it bounced a couple of times. This bike was very stiff, almost over the top, a mid 90's bike. We moved back to steel in 07', bought a Co-Motion Supremo, and there are no more bumps in the road. Comfort first for us. Tubing materials today are simply amazing. Combined weigh under 300lbs
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Old 06-24-10, 11:39 AM   #3
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My entry level Pacific Dualie has a thick heavy steel frame (in fact having a wide oval cross-section bottom bar connecting the eccentrics) and exhibits surprisingly little frame flex as compared with the older gas-pipe diameter framed rental tandems. It is comfortable both on and off-road on knobbies at low neighborhood cruising speeds.

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Old 06-24-10, 01:25 PM   #4
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One advantage to a hand built frame built by an experienced frame builder is that the tubing is selected for the riders' weights. The builder can then select tubing that will hit the "sweet spot" between comfort and efficiency. That is why the frame builder will often ask for the captain and stoker weights.
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Old 06-24-10, 01:39 PM   #5
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Frame flex of any sort is undesirable not only because it dissipates pedalling energy, but also because it causes fatigue in the alloy which leads to breakage. Compliance should come from components that are designed to give, like tires and suspension. Of course, these are blanket statements and I know that there's a happy medium for most people.

That being said, my stoker says that our Mongoose Wanderer exhibits no difference in comfort from our Cannondale RT1000 with a telescoping suspension seatpost and of course, the Cannondale is ridiculously faster than the Mongoose.
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Old 06-24-10, 02:51 PM   #6
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Interesting thoughts indeed! Thanks!

Well, here's what I notice between the two bikes. First, here's a photo of the bike frame we rode for 25 years:



This is the bike that was super rigid with steel straight-gauge tubing. Even minor bumps in the road were quite jarring with little or no give. I don't think my wife would've enjoyed riding that bike for very long if we hadn't installed a shock-post for her. It really didn't bother me too much, for it felt very much like my old Italian racing bikes in many respects. Of course, the stoker will always take the bumps a little harder. And yeah......we know.....haha.....it's kind of an ugly frame, but it went well beyond the call of duty.

Now, here's the new bike that was built for us by a custom builder by the name of Bob Brown in St. Paul, MN:



Again, it's a steel-framed bike, but it's constructed of Reynolds (I forget the exact name) double-butted tubing. It's about 10-12 pounds lighter, but the most noticeable difference is that the wheelbase is considerably longer, thus giving my wife much more room as a stoker. The very first thing I noticed upon riding this bike is that it felt much more like riding my single touring bike ('83 Trek 720). The ride is considerably softer, but not necessarily slower. Upon hitting bumps in the road, it doesn't bounce as mkane described of his Burley. The bike seems to absorb the bumps very well while recovering very quickly. While I'm perhaps describing this bike as having a good share more of flex than it really has, when compared to our old bike, there's certainly a very noticeable difference, for the old bike was an extremely harsh ride in comparison. Also, I'm not really convinced that a little bit of flex is such a bad thing, for it surely does feel as though it gives the bike a more comfortable ride. BTW, our combined weight is currently at about 350 lbs., so the frame has a little more work cut out for it as opposed to much lighter teams.

Thanks again, and please feel free to keep the thoughts coming if you like, especially about flex within your frames if any.

Jim

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Old 06-24-10, 04:21 PM   #7
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Originally Posted by jim_pridx View Post
While I'm perhaps describing this bike as having a good share more of flex than it really has, when compared to our old bike, there's certainly a very noticeable difference, for the old bike was an extremely harsh ride in comparison. Also, I'm not really convinced that a little bit of flex is such a bad thing, for it surely does feel as though it gives the bike a more comfortable ride.

Santana's Bill McCready is conducting studies of tandem flex, as discussed on this post, and at more length on TG's blog linked down thread.
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Old 06-24-10, 04:32 PM   #8
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Our daVinci's steel frame is much more comfortable over large bumps than my single, but the torsional rigidity seems to be outstanding. I had my 6', 195 lb. cyclist son on the back with a long seat post and I could not tell whether he was in phase. To me this is the sign of a good design: provides some flex in the vertical, but torsional rigidity for cornering and standing. I'm sure other modern designs have similar attributes.
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Old 06-24-10, 10:22 PM   #9
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Santana's Bill McCready is conducting studies of tandem flex.
Or, more specifically, bottom-end, axle-to-axle deflection from pedal-induced loads. Sadly, what's missing as it pertains to this thread is top-end resistance to torsion which is induced by the rider's bottom-ends (pun intended) and arms which comprise the other 2/3's of rider touch points with a tandem's frame. Again, it will be interesting to see how the results are presented and what types of conclusions will be drawn from the data collection exercise.

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Originally Posted by jim_pridx
I notice much more flexibility in the frame, especially when climbing hills or simply just wiggling the bike while on a ride. This certainly doesn't bother me, for the added flexibiltiy just seems to make the bike a much more comfortable ride for both of us.
Do you happen to know how much steering trail your old tandem had, or what the fork rake & head tube angles were? How did that compare to what Bob used for your new tandem? The only reason I ask is, adding steering trail to a tandem will make the bike feel a bit more 'twitchy'. Or, put another way... it becomes far more sensitive to both unintended and intended steering inputs. For some teams, it takes a little getting used to and one of the places where it's most obvious is on steeper climbs where the typical in-phase, side-to-side swaying action of the bike under the riders tweaks the handlebars with each pedal stroke... increasing the captain's task load. Just riding along, having your stoker lean down unannounced to grab a water bottle can also throw the bike out of kilter, as can a stoker who rocks their hips or who moves their upper body around a lot. None of this is frame flex, per se... it's unintentional, stoker-induced steering inputs.

A whippy frame, on the other hand, tends to have a lot of frame deflection under torsion loads like the ones caused by sprinting, while climbing or when cornering... where a whippy frame is noticably twisting under those torsion loads. In addition to just not handling well or inspiring confidence, you will often times find that flexy frames will ghost-shift under load, have intermittent brake pad rub when standing and sprinting or climbing out of the saddle, or trailing riders will notice that your front and rear wheels don't appear to be tracking in the same plane under hard efforts.

Now, if you really want to make your head spin don't forget to factor in the effect that wheels, tires and forks have on how a bike feels and handles. Your Bob Brown looks pretty solid in all those departments with a 40h conventional wheelset, steel fork, and 28mm or perhaps even 32mm tires. However, for other readers don't underestimate just how much stability you sometimes give up when using certain carbon forks (the True Temper Alpha Q X2 that we use comes immediately to mind) or low-spoke count wheelsets.

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Originally Posted by jim_pridx
So, my question(s) is this: how much flexibility is inherent in your frame? and how much flexibility do you find acceptable? Of course, I'm not looking for a fix.....just curious. Thanks!
As for our frames, our Ericksons and the Calfee are very small frames which give them very good resistance to the kind of torsion induced frame deflection along the top or bottom end that you really don't want. We're also a small team: I'm 5'8" and Debbie's 5'2" with a combined weight in the 275# to 285# range depending on the time of year, and that also gives us an edge since we're not putting as much of a physical load into the frame as a much taller and heavier team would. The aforementioned Alpha Q X2 fork does tend to deflect a bit on hard cornering efforts and that can be a bit disconcerting if I have to change my line in a fast downhill corner or make any other type of abrupt steering input, but is otherwise just fine the rest of the time. Note: having recently revisited a steel fork on our Erickson I can without hesitation state that the trade off on a little loss of stability is far outweighed by the comfort factor that you get with a carbon fork. We did have a bout with some very flexy '08 Rolfs and replace them with less flexy '07 Rolf. However, even then when going through a compression at the bottom of steep descent with an off-camber turn even the less-flexy '07 Rolfs scared the bejeesus out of me given how unpredictable the handling became, i.e., I couldn't keep our bike in our own lane which we'd never experienced before using conventional 36h wheelsets in 6 previous visits to that very technical corner on three different tandems.

So, how much flex is acceptable? Anything that's not unacceptable. Seriously, if a bike or tandem's handling causes you to lose confidence under routine or demanding conditions then something needs to be changed. Every tandem will have its nuances and most of those just take a little getting used to. However, if the captain or stoker gets spooked by unpredictable handling at least once a ride or feels like they're having to work a lot harder than they should (e.g., as evidenced by sore arms or shoulders after a long ride and fighting with the tandem or are just fatigued), once again something needs to be changed. It could be as simple as adjusting tire pressure, using a different wheelset, or a more significant change might be called for, such as more focus on becoming "clean riders" if either the captain or stoker have any bad on-bike riding habits.

More than you wanted to hear...

Last edited by TandemGeek; 06-26-10 at 05:30 AM. Reason: Grammar
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Old 06-25-10, 03:31 PM   #10
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More than you wanted to hear...
No, that's great! It's really nice to get a feel for what other riders go through. It just tells me I'm not the only one that feels these little "nuances" from time to time. Thanks, everyone for taking the time! It's really appreciated! And TandemGeek, if I find time later to reply, I will, but it's time to ride with sailing on schedule for the remainder of the weekend! Thanks again!

Jim
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Old 06-27-10, 02:23 PM   #11
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Frame flex of any sort is undesirable not only because it dissipates pedalling energy, but also because it causes fatigue in the alloy which leads to breakage. Compliance should come from components that are designed to give, like tires and suspension. Of course, these are blanket statements and I know that there's a happy medium for most people.
This is just simply wrong.
First, every frame, tandem or single flexes. Just take any high end racing frame and press down on a pedal with your full weight and see how much the BB deflects, or put your CDale on a trainer, and watch the BB go back and forth as you pedal..

Second, frame flex does not waste energy. To the extent a frame flexes when you pedal it, you get the energy back (minus a tiny amount of energy lost as heat) when it rebounds. There are long threads on this in the ROad forum.

Third, if you could design a frame with no flex, you would not want to ride it. A bike frame with no flex, under normal pedaling loads, would not only have an unacceptable weight, it would ride like a tank. Take for example the early CDale road bikes which were way overbuilt to address issues with the potential for aluminum to fatigue. Those bikes while they still flexed, were very stiff, and were a teeth jarring ride. A completely flex free frame would raise the idscomfort of the original CDales by an order of magnitude.

By contrast the "magic carpet ride" of Titanium bikes, and the "Steel is Real" bit, are the result of the fact that most Ti and Steel frames have a fair amount of flex.

The issue in designing a frame is how to manage flex, i.e. how much it flexes, in what direction, and under what loads. Thus, the oft quoted "horizontally rigid,and vertically compliant. This is one reason that Carbon fiber, whih gives a designer the opportunity to lay up fibers in different directions, different thicknesses, and different modulus, in order to dial in the flex charateristics of the bike, is such a good material to build a frame out of.
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Old 06-28-10, 11:17 AM   #12
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Merlinextralight, I agree with you on all accounts. What's your opinion on this... say we have some imaginary material and frame design that indeed make a completely flex-free frame at a reasonable weight. It would certainly ride like a tank and be horribly uncomfortable, but would that be desirable for people going for all-out speed? Would the riders be able to tolerate the totally-rigid frame if there were other provisions for comfort, like beach cruiser saddles and thudbuster seatposts?
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Old 06-28-10, 11:41 AM   #13
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^ If you start with the premise that you get any energy taken up by frame flex returned to you when the frame rebounds, you're not going to be any faster by making the frame more rigid.

As long as the bike is rigid enough carve the line you set it on, and to feel like it's responding crisplly when you accelerate, there isn't much, if any performance advantage to making it stiffer.

Making it stiffer past a certain point is going to beat up the rider, and fatigue them, decreasing performance.

Modern Carbon fiber racing bikes could be a lot stiffer than they are. Take for example the Cervelo R3SL. While the front end and the bottom bracket are designed to be quite stiff, the seat stays are as thin as a pencil, and are designed to flex. Riders have won Spring Classics as well as the Tour de France on that bike. If there was a substantial advantage to being absolutely rigid, Cervelo would beef up the seat stays and eliminate the flex.

So I think its still a matter of determing how much flex, under what loads, and in what direction, is optimal for the perfomance criteria you're trying to meet, rather than just trying to make a bike that is stiff as possible.
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Old 06-28-10, 12:59 PM   #14
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Third, if you could design a frame with no flex, you would not want to ride it. A bike frame with no flex, under normal pedaling loads, would not only have an unacceptable weight, it would ride like a tank. Take for example the early CDale road bikes which were way overbuilt to address issues with the potential for aluminum to fatigue. Those bikes while they still flexed, were very stiff, and were a teeth jarring ride. A completely flex free frame would raise the idscomfort of the original CDales by an order of magnitude.
This is myth, IMO and IME.
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Old 06-28-10, 01:00 PM   #15
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MWould the riders be able to tolerate the totally-rigid frame if there were other provisions for comfort, like beach cruiser saddles and thudbuster seatposts?
Or how about such provisions as pneumatic tires? Or pencil-thin seat rails?
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