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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 10-27-09, 08:51 AM   #1
oldacura
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Frame design

I was going to append this to the Paketa thread because it had morphed into frame materials & construction but I thought it better to start a new one. This is not really tandem specific but a tandem could benefit more from advances because of the larger frame.

This goes back 30 years when I had just graduated from college and had interviews with 2 aerospace companies that supplied rockets to the government: MacDonnell-Douglas and General Dynamics. They made Atlas and Titan rockets (don't recall which was which). They showed me the process to make the rocket casings.

One took very large, curved aluminum panels (partial cylinders) about 3/4" thick (guess). Using a giant CNC mill they removed most of the material from the inside leaving a thin aluminum skin with a spiderweb of ribs. This was welded to other similar panels into a giant cylinder that would support its own weight. The smooth skin was on the outside.

The other process was to make a giant soda can - but out of thin stainless steel. The can was unable to support itself unless it was pressurized - just like standing on an unopened soda can -vs- an opened one.

Would it be possible to make a bike frame like the latter method? My understanding is that the reason they cannot make thinner wall tubing is because it would crush under light pressure. If it were possible to make some (maybe not all) of the frame members (e.g. top tubes, down tube, boom tube & part of the seat tubes) out of very thin wall metal (steel, titanium, aluminum, magnesium) and pressurize it so the skin was always under tension, could they not make the walls significantly thinner?

I realize that depressurization could result in a catastrophic frame collapse but if they can make it work for rockets, why not bikes? They could include a pressure gauge and possibly a valve to pump up the frame (just like tires).

If this is not practical for consumer bikes, how about a controlled fleet of racing bikes?

I doubt that I'm the first one to consider this. If this has been considered before and discarded, why?
Do great minds think alike (or not)?
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Old 10-27-09, 09:11 AM   #2
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My bet is the weight, and cost of making the system able to stay pressurized exceeds the theoretical advantage.
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Old 10-27-09, 11:46 AM   #3
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I doubt that I'm the first one to consider this.
Hmmmm. I'm going to go out on a limb here and suggest that you may, in fact, be the first.

That is, unless somone considered building a one-time use, gas-charged frame that could be used to launch a cyclist into orbit.

By the way, that would have been the Atlas-Centaur developed by Convair / General Dynamics. I believe it was an insulated beer can, i.e., two thin layers of stainless steel with a fiberglass core.
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Old 10-27-09, 01:01 PM   #4
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An inflateable frame? Would I have to pump it up at the same time I pump up my tires?
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Old 10-27-09, 01:08 PM   #5
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Originally Posted by oldacura View Post
I was going to append this to the Paketa thread because it had morphed into frame materials & construction but I thought it better to start a new one. This is not really tandem specific but a tandem could benefit more from advances because of the larger frame.

This goes back 30 years when I had just graduated from college and had interviews with 2 aerospace companies that supplied rockets to the government: MacDonnell-Douglas and General Dynamics. They made Atlas and Titan rockets (don't recall which was which). They showed me the process to make the rocket casings.

One took very large, curved aluminum panels (partial cylinders) about 3/4" thick (guess). Using a giant CNC mill they removed most of the material from the inside leaving a thin aluminum skin with a spiderweb of ribs. This was welded to other similar panels into a giant cylinder that would support its own weight. The smooth skin was on the outside.

The other process was to make a giant soda can - but out of thin stainless steel. The can was unable to support itself unless it was pressurized - just like standing on an unopened soda can -vs- an opened one.

Would it be possible to make a bike frame like the latter method? My understanding is that the reason they cannot make thinner wall tubing is because it would crush under light pressure. If it were possible to make some (maybe not all) of the frame members (e.g. top tubes, down tube, boom tube & part of the seat tubes) out of very thin wall metal (steel, titanium, aluminum, magnesium) and pressurize it so the skin was always under tension, could they not make the walls significantly thinner?

I realize that depressurization could result in a catastrophic frame collapse but if they can make it work for rockets, why not bikes? They could include a pressure gauge and possibly a valve to pump up the frame (just like tires).

If this is not practical for consumer bikes, how about a controlled fleet of racing bikes?

I doubt that I'm the first one to consider this. If this has been considered before and discarded, why?
Do great minds think alike (or not)?



What is a controlled fleet of racing bikes? A pro peloton? UCI provides equipment rules for bikes including weight and the shape of tubing. I would hate to race in a peloton of gas filled pressurized bikes maintained by bike racers. In fact, not even maintained by NASA.

One approach is to work with a shop such as Calfee and optimize everything with cost not being a limitation and go for the lightest most aero frameset, components and wheels for your team's characteristics.

Last edited by Hermes; 10-28-09 at 07:52 AM.
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Old 10-27-09, 01:24 PM   #6
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We build gas lasers where I work. These are basically an extruded aluminum tube with ends welded on. They need to maintain a certain level of vacuum. The welds maintain this. With some careful design, one may never have to re-pressurize the bike frame. I think the way a thin-walled tubing frame is likely to fail is by buckling. If the internal pressure could ensure that the skin is always in tension, this could work. I don't know how thin a wall could be achieved but likely quite a bit thinner than a non-pressurized frame.

Anyway, something I've pondered while hunched over the handlebars pedaling along long straight stretches of road.
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Old 10-27-09, 03:07 PM   #7
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You could possibly do major parts of major tubes, but ... seat tubes would have to allow for both seatposts and bottom brackets, down and boom tubes involve bottom brackets, etc. It would be akin to double-butted tubes, except the butted part would have to have a bulkhead of sorts to keep the innards pressurized.
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Old 10-27-09, 03:44 PM   #8
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And if the tubes were oversized and pressurized with Helium...........
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Old 10-27-09, 03:46 PM   #9
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While the support may be achieved from the internal pressure. Would the pressurized gas vs non pressurized gas weight be worth the savings in empty weight.

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Old 10-27-09, 04:07 PM   #10
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Quote:
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One approach is to work with a shop such as Calfee and optimize everything with cost not being a limitation and go for the lightest more aero frameset, components and wheels for your team's characteristics.
Yup, that'd be my approach.

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I participated in a hill climb race on Saturday...
I did that hill climb too, and thereby found the best approach. Don't worry about frames, get a stoker like what Hermes has in Velodiva. I just edged her, and she finished near the very top among 20 very strong female riders. Stoker watts/kg like that make any frame enhancements trivial in comparison.
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Old 10-28-09, 07:49 AM   #11
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I've been trying to upgrade the motor for many years but it seems to be sliding the other way. Once again - It's not about the bike.
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Old 10-28-09, 09:10 AM   #12
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Yup, that'd be my approach.



I did that hill climb too, and thereby found the best approach. Don't worry about frames,
get a stoker like what Hermes has in Velodiva. I just edged her, and she finished near the very top among 20 very strong female riders. Stoker watts/kg like that make any frame enhancements trivial in comparison.
And she is not available........to others
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Old 10-28-09, 09:46 AM   #13
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With respect to frame design, it strikes me that tandem frame builders have done little to nothing with frame aerodynamics. Round tubes of different sizes dominate the frame sets.

Compare and contrast that to single road and track bikes which have become much more aerodynamic. They use carbon fiber to create shapes that require less wattage to propel the bicycle. And the aerodynamic frames offer benefit under more cycling terrain conditions including flat, downhill and even uphill at speeds above a certain point (assuming the aero shape is heavier than it traditional counterpart).

The removal of the lateral from the tandem frame by some builders for cost and weight savings is an interesting point of discussion. The lateral is removed and the size of the remaining tubing increased thus potentially increasing the aero drag on the frame. One could argue that the removed lateral added some aero drag to the frame and I am sure that is true. The question is what is the correct tradeoff of ride comfort, weight, cost, stiffness and aerodynamics to produce a superior frame that reduces the wattage needed to propel the tandem under the most possible terrain conditions. It strikes me that carbon fiber or a mixture of carbon fiber and other frame materials contains the solution. This is doable today and I suppose that the potential number of tandem frames sold would not payback the investment in engineering and manufacturing setup costs. However, this is what is done in the single road, track and time trial bikes.

IMHO, tandems, frames and components are marketed by price and weight. Cyclists assume if it is lighter it is better and take into account the $$$/gm to make buying decisions along with short test rides.

Finally, the characteristics of the team dominate tandem performance which includes weight, power and aerodynamics and of course team work. But for racing a tandem where winning and losing by seconds may determine a championship, a better conceived frame set would add some value.
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Old 10-28-09, 10:55 AM   #14
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But for racing a tandem where winning and losing by seconds may determine a championship, a better conceived frame set would add some value.
How about $32k in then-year dollars for the one-off EDS Corima track tandem?

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Old 10-28-09, 11:06 AM   #15
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As with weight, I'm sure that the riders (not the bike frame) dominate aerodynamic drag.

Also, if aerodynamic drag of the frame members is a concern, tubing can be extruded in many shapes (eliptical, tear-drop, rectangular, etc).
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Old 10-28-09, 11:14 AM   #16
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How about $32k in then-year dollars for the one-off EDS Corima track tandem?

Nice...as an interesting side note, I met a bike manufacturers rep and he was telling me that right now it is possible to get lower volume CF frame orders into overseas factories. The context was that the lower sales volume for a CF track frame (single not tandem), that they plan to offer, would match the factory minimum order. Under normal conditions, they could not offer a CF track frame.
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Old 10-28-09, 11:31 AM   #17
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As with weight, I'm sure that the riders (not the bike frame) dominate aerodynamic drag.

Also, if aerodynamic drag of the frame members is a concern, tubing can be extruded in many shapes (eliptical, tear-drop, rectangular, etc).
IMO, one must separate the impact of the riders from the frame and components. Rider power, endurance and aerodynamics as well as mental focus always dominate performance. However, with respect to frame design and subsequent performance, aerodynamics must be part of the equation or else one could pay more for a lighter frame and receive lower performance.

And because of the dominance of the team's metrics and the difficulty of getting repeatable test results, it may be difficult for buyers to know if they spent more money for ultimately less performance.

The solution to this is wind tunnel testing of the frame by itself as well as with a tandem team. Once again, the cost of such engineering, testing and optimization would have to be amortized over the cost of the yearly sales volumes.

Here is a pic of a BT Stealth track bike. It is a great sprint bike. I suspect it would be difficult to create this frame out of tubing.

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