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Old 11-03-13, 07:40 PM   #1
kckulp
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Geometry & Efficiency

Look for opinions on which frame geometry is most efficient at transferring energy into the drivetrain.

Eliminate suspension. No road/trail conditions. No tire size. Not buying a bike. Not testing a bike.

I want to know what frame style (MTB, Road, Tri) most effectively transfers rider output into the system. Or places the rider in the optimum "power" position.

If you had two minutes on a trainer connected to a flywheel, and your goal was to get that flywheel to rotate as quickly as possible in those two minutes, what type of frame would you choose and why?
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Old 11-03-13, 07:48 PM   #2
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Recumbent, so long as no one is taking pictures
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Old 11-03-13, 07:57 PM   #3
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I say it has ZERO effect. It may as well be a slab of cement as a trainer.
Geo affects speed, weight and aero ONLY.
Now knee over pedal will make a power difference and slack angles affect steering.
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Old 11-03-13, 08:03 PM   #4
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There will be pictures...and I do have some pride.
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Old 11-03-13, 08:04 PM   #5
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The fastest TT bikes are regretably those fugly 20 inch wheel BSOs.
Would anybody watch them in the TdF ??
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Old 11-03-13, 08:07 PM   #6
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Aero is not an issue. Looking to get power into the system as efficiently as possible. So my position has no effect on transmission of energy?
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Old 11-03-13, 08:10 PM   #7
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Aero is not an issue. Looking to get power into the system as efficiently as possible. So my position has no effect on transmission of energy?
Position does, geometry (as long as it doesn't preclude the best position) doesn't.
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Old 11-03-13, 08:19 PM   #8
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Ok. So I'll rephrase. What type of frame will put me in the best position to maximize power output? BTW this is for a power generation experiment. Your thoughts are appreciated.
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Old 11-03-13, 08:25 PM   #9
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You want the angle between thighs and spine not to be too stretched out, e.g. the thighs at about a right angle to the spine. This lets the gluteous maximus muscles engage, and that is the big power.

There is a tricky issue here about the set-up of the trainer. Some stationary bikes allow a person to rock the bike back and forth, like a person would do when standing. This gets the arm muscles engaged, so should allow more power output. Other stationary bikes hold the cranks in a fixed position, so there is no rocking moving possible.
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Old 11-03-13, 08:26 PM   #10
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This may open a can of worms but geometry could affect stiffness which could impact either mechanical or bio-mechanical efficiency. Geometry of the frame also has something to do with the rider position I think.

I have no data but I strongly suspect that the most efficient position depends on the type of riding. Were I a betting man I'd wager that frames with racing bike geometry are more efficient for riding fast on smooth surfaces than mountain bike frames, and that for rugged downhill riding the suspension frames of mountain bikes are more efficient. Even though you said "eliminate suspension" the postulate implies that you will isolate the frame geometry from suspension when considering the "most efficient", which isn't necessarily valid.

Spinning a flywheel is not the same as cycling - is your focus actually on a trainer with a flywheel, or is the objective efficiency while on a bike?

edited, "best position to maximize power output" and you said two minutes earlier. Anecdotal, I have the most power when out of the saddle positioned forward. But for a more scientific answer I'd suggest looking at track cyclists and sprinting specialists.

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Old 11-03-13, 08:36 PM   #11
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Take a look at this. That's me on the machine. 660lb wheel. Using a Trek MTB and a cheap trainer. Drivetrain connected to flywheel via chain and gears - direct drive.

https://www.youtube.com/watch?v=eVD9...e_gdata_player
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Old 11-03-13, 08:51 PM   #12
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Wild party you have going there! Yeah you can see that even without rocking the bike but just getting up off the seat, one can get some power out of one's arms. Probably handlebars lower and more forward is going to allow getting up off the seat and using arms while still keeping the gluts well engaged.

You ought to be able to look at the spin biking crowd to see what folks like. They often have good power measurements and it's a popular work-out.

I wonder, though. Would a stationary rower be a way to get more power output?

Maybe cycling has the edge:
http://www.concept2.com/files/pdf/us...ErgArticle.pdf
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Old 11-03-13, 09:13 PM   #13
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I rowed in college. Very familiar with Concept 2 Ergometers. Torture devices.

We still think that a bike is the way to go. Thought about a spin bike because it provides a myriad of adjustments (I am not the only cyclist) but no gearing (although we could probably engineer a work-a-round). The gearing gives us the ability to break the wheel free from a standstill at the low end, and keep spinning it up at the high end.

I believe the position of the MTB is not aggressive enough. Would a TT/Tri frame put us in a position to pump more watts into the system?
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Old 11-03-13, 10:21 PM   #14
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I believe the position of the MTB is not aggressive enough. Would a TT/Tri frame put us in a position to pump more watts into the system?
Here again, the position is what helps efficiency/power output -- the rider, being bent forward, can put more weight on the pedals than in the upright position of a mountain bike. For the purposes of a stationary bike that does not need to be aerodynamic, you could make the thing out of a solid chunk of metal with attachment points for a crank, seatpost, and handlebars. What would the "geometry" be then?

(One thing to keep in mind with tri bikes is that the positioning is designed to keep some muscles fresher for the run -- for your project, you'll want to choose a position that doesn't make that compromise.)
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Old 11-04-13, 03:48 AM   #15
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I'm going to assert that the most efficient geometry is whatever closest matches what your volunteer powerplant rides to class/work on.
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Old 11-04-13, 03:59 AM   #16
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Bikes are always a compromise between optimum power generation, comfort, aerodynamics and balance.
The actual power transfer efficiency of the bike from pedal to wheel is independent of style and more to do with structural stiffness.
The position for max power generation will vary from rider to rider, depending on flexibility, muscle mass and type.

Peak power is only significant in sprints. For most riding you need sustainable power which includes a degree of comfort.
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Old 11-04-13, 08:54 AM   #17
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Comfort is not necessarily an issue, but would help.

The official rule is one human, two minutes of effort. I have to get the machine from a dead stop to max RPM's in that time frame - watch the youtube video link a few posts up. We're not trying to sustain power, we want the tanks to be empty at 1:59. So in effect, it's a sprint.

The designer of the machine found a used MTB bike and it became the bike. He had a few friends/family spinning it before he found actual cyclists to spin it for him.

The frame we have is too small for both volunteer cyclists. So now we (the cyclists) want to re-engineer the power plant. We're don't want to bodge together a completely new "riding aparatus" (ie. chunk of metal w pedals), but rather drop a frame in place that suits our needs and maximizes our efforts. So we're at a point where we can start (almost) from scratch and build-to-suit. So where to start? Stiffness is a very good consideration.
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Old 11-04-13, 11:23 AM   #18
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Look for opinions on which frame geometry is most efficient at transferring energy into the drivetrain.
the one that fits your body best. and individual's bodies differ .. (duh)

Mass produced stuff / vs made just for you , is a world of difference..

Roller races? small light wheels will have less inertia. to spin up ..

a 451 20" wheel is the small diameter equivalent to a road-race-bike wheel . 25mm folding bead tires at 100+ psi.

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Old 11-04-13, 12:53 PM   #19
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Go find a bike shop with one of those guru fitbikes - they can move the seat/pedal/bar positions around relative to each other and measure your power output. That might help answer your geometry question, although the answer may only be relevant for you personally.

Are you limited to just one rider? I reckon a tandem would do even better.
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Old 11-04-13, 01:01 PM   #20
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What kind of drivetrain are you using? Do you have clipless pedals?

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Old 11-04-13, 01:24 PM   #21
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The fit bike would be an interesting option. Loads of adjustability. Only one rider allowed. We could easily mark our personal measurements for each rider. Gearing still an issue.
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Old 11-04-13, 01:33 PM   #22
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What kind of drivetrain are you using? Do you have clipless pedals?
All using Look Keo's.

Rear cluster is bolted to a carbon steel axle. Axle spins large sprocket ring mounted directly adjacent to the bike/trainer. Chain drive up to the top of machine/wheel. We're firing again this weekend. I can take some pictures of the drive system.
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Old 11-04-13, 01:38 PM   #23
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the one that fits your body best. and individual's bodies differ .. (duh)

Mass produced stuff / vs made just for you , is a world of difference..

Roller races? small light wheels will have less inertia. to spin up ..

a 451 20" wheel is the small diameter equivalent to a road-race-bike wheel . 25mm folding bead tires at 100+ psi.
Both cyclists ride custom fit bikes recreationally. We were wondering if there would be a way to drop our bikes onto the machine, but are concerned that we'd beat the s#*t out of them.

Rollers may slip too much because of the mass of the throwing wheel? Maybe a Wahoo or LeMond Revolution trainer?
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Old 11-04-13, 01:46 PM   #24
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easier to ask than answer questions, are common here ...
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Old 11-04-13, 07:25 PM   #25
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in the vid
your saddle is too low
get that fixed before you worry about changing style of bike

and fwiw
performance oriented upright bikes
have pretty similar geometry
and i would bet what works best is different for every rider
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