Recumbent - speed as a by-product of efficiency

does more efficiency always add up to more speed? for example, on an FWD bike with pivoting BB (aka swivel nose) the drive train is even smaller than that of an uprwrong. does this automatically mean speed? (assuming the bike is pretty light for a bent) are there drawbacks that make FWD swivel nose, while more efficient, less fast? (such as traction issues, etc.) From my understanding, to be efficient means to use power only on the task at hand and use as little of it as necessary- no more, no less. Is that close? In that case, wouldn't more efficiency neccesitate faster speeds for the same power input? What would the "most efficient 'bent" look like? (in your opinion) I think it would be an aluminum (or lighter material) FWD lowracer with a carbon seat. (no frame-based suspension). let me know what you think.

All things being equal, yes. In terms of bicycle layouts, I'm not really sure, but from what I've gathered, drive train and RR losses are minimal compared to aerodynamic losses. The most efficient "bent" is something along the lines of a Varna Diablo, and allows the rider to sprint up to ~80mph on flat, smooth ground with no wind. Obviously, this isn't a pragmatic design, suitable for daily use. In this case, the best deisgns I've seen pull something like ~25mph@150w in the same conditions. If a rider wants to extend the useful range of faired efficient bent, a 1-2hp two stroke gasoline engine is suitable because it only adds ~20lbs but increases power by a factor of 5-10. Ack.. I'm getting side tracked, fire away if you have any questions.

that one that you said will do 25 mph at 150 watts, is that a streamliner you're talking about? that's interesting about the engine, but I really am a purist (I'm sure others will agree). Plus, if I did put the aforementioned engine on my bike, I'd have to register it as a motor vehicle and deal with like a billion pounds of red tape just like cagers do, so where's the freedom in that? I know the Varna is impractical but FWD is not in itself impractical. I prefer the "swivel nose" design to the other option (not sure what it's called) because a.) it leaves the drivetrain looking a WHOLE lot better (as elegent as a DF drivetrain or better!) and b.)since the nose swivels, the bike can be ridden "hands off" once one masters the steering principles. the only disadvantage i see to FWD pivoting BB (from now on abbreviated as FWDPB)is the bigger turning circle and genral inability to turn on a dime like I'm used to on my DF (yes, i am a DF rider currently). Would FWDPB be straining on the leg muscles when you turn your legs with the front wheel? would this present a health risk? I ask because to me, it looks REALLY weird seeing that guy make sharp turns with his python (he actually turns pretty sharply). the guy i am refering is the guy in this video http://kotisivu.mtv3.fi/jviirret/python.mpg
also, what about putting hubs with built in suspension on both front and rear instead of opting for frame based suspension?

Yes it is, supposedly this (http://www.adventuresofgreg.com/HPVlog/03-09-06.html) does ~25mph@150w faired, and ~19mph@150w unfaired, so a relatively comfortable fairing seems to reduce the CdA by ~80%. That site has gobs of useful information btw. I also think the lean stear trike (check out Nov 14th 2004) may be able to turn on a dime while still being FWDPB if I'm interperting your anacronyms correctly. That red tape is odd. In CA bicycles with engines that have less than or equal to two brake horsepower are considered to be motorized bicycles aka mopeds, and only require a one time fee of ~$15(?) for a license plate. No more, ride it until the wheels fall off, even licensing isn't strictly adhered to, I see plenty of road or mtbs with a small gas two stroke and no plates. The only downside is it can't be ridden on freeways, if you like to live dangerously, with certain exceptions. If two regions only have a freeway connecting them, no other streets, then I think mopeds are permited on freeways until side streets allow a path to where they're going. In any event, long distance commutes (50mile) just aren't practical at ~20-25mph for me, I can't spend four hours riding to and from work each day, YMMV. ;)

what different effects do trails have on handling? I know the python has a negative trail, which is theoretically unstable but the weight of the rider self centers it. correct me if i'm wrong, but it is this feature that limits coasting speeds to something around 50-55 kph according to the python site. (couldn't you just keep pedaling and go faster?) Are there any FWD's with a positive trail that you know of and if there was such a bike, wouldn't that mean that the coasting speed is not limited? It seems strange that the python "lowracer" as it is called, has a limited coasting speed. I'll bet even lowracer pilots coast from time to time. everything else about the bike looks wicked fast. one guy even built a python with a seat height of about 7 inches! the aerodynamics must be crazy. the drive wheel is adequately big to get good speed on the flats as well. i don't see any reason why this bike couldn't compete with the baron or M5 except for weight.

Lemme see, Google says... Negative trail implies ease when turning but an "unstable" feeling bike at higher speeds, positive implies a very "stable" bike at high speed that's harder to turn. If the center of gravity is ahead the steering axis the bike will tend steer in the direction of the lean. The python has negative trail, and if the weight and center of gravity are behind the steering, it won't go steer where it leans. There are also other concerns like the force required to turn the headset (needle versus ball bearings) and

More subtle effects, such as the fore-to-aft position of the center of gravity and the elevation of the center of gravity also contribute to the dynamic behavior of a bike.
In the case of that bent I mentioned earlier, negative trail isn't as much of a problem since it's always balanced and the steering is regulated by steel cable (it seems the rider has to push/lean the bike into turns, and with no force on the steering, the bike would return to nuetral), so it seems like a best of both worlds deal, lots of negative trail for turning, but with needle bearings and the cable system, it can be stable at high speeds too. With the python, not so much, and because of that there may be trouble steering at higher speeds. In fact, after googling the python it seems as if trail changes with seat height, but even with different builds, it seems to always have negative trail (http://www.python-lowracer.de/projects.html). The python is nice and quick because by lowering the bike as much as possible, reference area is minimized, but it won't be as fast as a faired version because the drag on any unfaired bicycle is huge, for example the CdA of your average upright bike is about the same as the CdA of a Honda Insight (car!). By cutting the A roughly in half, the python has roughly 1.5 times the top speed of a typical bike at some power level, and faired with the Cd in half as well, probably twice. Anyway, the python is evidently very nimble and can easily turn in a tight radius at low speeds, but at high speeds is probably a bit on the twitchy side, which means it would be harder to match a M5 or baron at higher speeds, not that it couldn't do it, just that it'd take more luck and skill. Otoh, the baron and M5 would probably not be as agile at low speeds. It's a trade off. YMMV

A moving-bb bike like the Python loses efficiency because some of the rider's output must be used in counteracting the pedal steer. IOW, you can't put full power into propulsion. I suspect the negative trail has something to do with trying to minimize that effect. Although not the case with the Python, most FWD bikes I've seen are fixed-BB and have at least a power idler near the top of the head tube, which means a large deflection of the chain's power side. The rider's arms in the video certainly aren't very aero, although again that wouldn't necessarily be true for other FWDs.

OTOH, a RWD bent has a long chain, which means a long lever which must resist bending; and bending force goes up as the square of the length. So a RWD will have to be much stiffer to resist frame deflection. Plus it requires an extra 600g of chain. Maneuverability is compromised if the return chain is dropped alongside the front wheel, otherwise my Baron can turn every bit as sharply as the Python. Running the chain 'up' results in some increased chain drag, which it would be more accurate to class as a small fixed amount rather than a % of the rider's output. Running a 'dropped' chain is cool, but I really haven't noticed much difference in speed either way.

Which is better/faster? I'm not even going to try to guess; but if I were getting a FWD, my personal preference would be fixed BB due to the pedal stroke issues. YMMV.

When you say that on the fixed BB FWD's the idler means "a large deflection of the chain's power side", do you mean that less power goes into the chain? As far as I know, the "praying hamster" position is the most aero, correct? if so, couldn't handlebars w/ shifters and brakes be rigged to put the rider's arms in that position, regardless of whether or not they steered with them? I'm guessing the amount of chain drag is negligible, your experiences in consideration. My question about fixed BB FWD's is how turning is achieved. I know that the chain must be able to flex a bit (not something i'm really comfortable about). does this mean you can't do really sharp turns? if so, how sharply can they turn? ( a u-turn?) I know John Tetz has a carbon fiber fixed BB FWD dual 20 which looks pretty cool to me, until i look at his drivetrain, which weirds me out. but i understand his bike is considerably fast.

I hear that FWDs are considerably better at climbing than other bents. Is this true? I pretty much believe it, but i'm not sure why it climbs better.

On a Fixed-BB arrangement, the power side of the chain runs from the cassette, straight up along the fork, then deflects 90 degrees or more at/near the top of the head tube, and from there along the boom to the chainring. More deflection is generally associated with more power loss (inefficiency,) although having a large diameter pulley lessens the effect. Fixed-BB are sometimes referred to as "twist chain" because the chain twists as the front wheel is turned. A chain can accomodate a surprising amount of twist, but there are definitely limitations to how tightly you can turn. In most riding conditions, the front wheel is turned a very little amount, so the limitation usually goes unnoticed. It would not turn as sharply as the Python in the clip. Maybe not too much worse, though. I'm pretty sure you could U-turn in a 2-lane road. Here's an example: http://tinyurl.com/r56q3


At really ridiculous grades, I suppose that weight transfer to the rear would leave the front wheel too unweighted for traction; but up to that point, gearing and frame stiffness would be more of a factor than which wheel is doing the driving.