Jobst Brandt, a highly respected mechanical engineer with many accomplishments under his belt, wrote the following back in 2003. It was part of a series of FAQ's hosted on Sheldon Brown's website but borrowed from Rec.Bicycles.Tech. Take a few minutes to read this and let me know how your mode of thinking differs or agrees.
The reason I bring this up is because one or two folks, who as usual withhold their identities, discussed with me the inadequacy of the chain drive and derailleur in a previous post on spokeless wheels and how a simple infinitely variable chain less drive is the definite solution to its ills. Will this truly revolutionize the bicycle?
"The Continuously Variable Transmission (CVT) is the holy grail of many inventors who are not convinced that it is an impossibility. That is to say, the positive engagement, continuously variable transmission, that does not rely on friction, electrical, or hydraulic ratios but uses mechanical gearing, is not possible. By definition, continuously variable is analog while gears and chains are digital.
The CVT does not exist, and I am convinced it will not. If it were possible, railway locomotives, trucks, buses, and cars would long ago have used them. Strangely, it is in bicycling that the strongest believers of the concept reside... as if there were more money to be made in bicycles. In fact, the bicycle, with its enormously adaptable human motor, doesn't need a CVT. In addition, its low input speed and extremely high torque, make the bicycle an especially difficult gearing challenge. For this reason high performance bicycles use derailleur chain drive that is found practically nowhere else.
Non-gear CVT's, currently used elsewhere, have poorer efficiency than both planetary gears and derailleur chains. More importantly though, the low-speed high torque of bicycling would require transmissions that would weigh more than the bicycle, which makes them impractical."
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Note: CVT is available on cars now (it wasn't at the time Jobst wrote that) Have a look here:
ReplyDeletehttp://www.drive.subaru.com/Sum09/Sum09_whatmakes.htm
However, as Jobst also points out, the power and torque curves differ very greatly between legs and an internal combustion engine. Although weight is sort of important on a car, it's critical on a bike - and that will mean that the "best" solution might be different on cars and bikes.
The idea of a bicycle CVT is interesting but not one I see as being necessary. Current drivetrains only exhibit problems under severe conditions. The problem isn't the interval between gears, it's the actual shifting performance.
ReplyDeleteSevere conditions such as snow, epic length off-road rides in bad conditions, damage to the rear derailleur from crashes/wayward rocks, and poor maintenance cause shifting problems (and how many of these problems are actually dirty cables) but with current 3x9 (3x10 now I suppose) I haven't found any issues with the size of the jumps between gears.
Something like the G-Boxx strikes me as a good compromise. You maintain the efficiency of a chain drive, the shifting is all protected from the elements and the length of chain that is exposed to the elements is an a straight run with no tensioner or derailleur to be damaged.
The straight run of chain means you can use a wider, longer-lasting, stronger chain.
One last thought:
-Call me OCD, but I know that a CVT on a bike would be really annoying for me, I'd constantly be fiddling with the shifter trying to find the perfect gear ratio.
I think CVT's are not needed. Jobst has a good point as the human body can adapt well to torque-power conditions. CVT chooses to replace the inefficient manual transmissions and helps shift staying on the optimum power band of the motor/engine. Human body is not like a traditional motor or engine. We don't have a torque or power curve.
ReplyDeleteThere are actually a wide variety of CVTs in cars, as well as in a number of other off-highway industries. A lot can change in 7 years. They're fairly well proven, and in automotive applications, notably more efficient than both manual transmissions and torque-converter automatics.
ReplyDeleteThat does not necessarily mean that it's a practical idea for bicycles, but Jobst is incorrect on that front, as well. The NuVinci CVPT is a continuously variable, and weighs only 4 kg - more than a traditional hub, cassette, and derailleur system, perhaps, but not really of much consequence on an urban bike that would actually be fitted with the unit.
I'm not sure if I think it's a brilliant idea - I get by just fine with derailleurs, and actually prefer them to planetary gears, but hey, if some people like them, more variety can't hurt...
Ron,
ReplyDeleteI assume that I was one of the chain skeptics for which this post is aimed. My apologies for the anonymity; only because I don't have a goggle account did I "hide" my identity.
My name is Howard (moniker..JoeKing).
I agree that a true CVT is impractical in a bicycling application, but the device that I am working on is an... infinity (within a range) variable transmission.
I am not being crytic about this device..I have only built the first generation & the second isn't complete. When it is I will gladly share it with the readership for criticism.
Will it be the holy grail...who knows. I just think there is a need for a better system. I am using existing mechanical technologies so no "breakthroughs" are involved, just some lateral thinking.
JoeKing
Joe : Cat's out of the bag! :) Hey, I certainly posted this only for discussion's sake, and not to call you out or anything. It would be great to see some pictures and design ideas, but again, think where you would like the design to go. Will it be intended for sale? And what will you be trading off in the design?
ReplyDeleteb : I have to disagree. I'm pretty sure CVT's were around way before Jobst wrote that (2003). The Dutch car called DAF were equipped with "Variomatic" CVT's which were the first ever produced (1958). Have a look here : http://www.ritzsite.demon.nl/DAF/DAF_cars_p17.htm
I really have to ask Jobst to clarify what he meant by his statements. Will get back to you.
Cat out of the bag...hardly. I just wish I had something to show. The device is my summer project.
ReplyDeleteWhen roadworthy I'll post some pictures here. For sale? Way to early to think about that.
Trade offs....I have only thought of the upside; 57 advantages (& counting). I'll leave it up to others to figure out whats wrong with it.
What struck me about Jobst's comments, aside from the Inquistorial tone, was the line about as if more money was to be made in bicycles. Speaking for myself (& no doubt many other basement tinkerers)...money isn't what its about...its the process.
JoeKing
Long time reader, first time responder.
ReplyDeleteJobst Brandt reminds me of this highly respected professor that said in the 50's that a computer at every house would never happen, because they are too big, use too much power and, well.... people simply would not have any use for it! hmmm indeed
In other words if you want to remain 'highly respected' (also after your death) you'd better not use the words "it will never happen".
Consumers' priorities change, product use changes, technology changes; simply taking away drawbacks that were seen as insoluble. You cannot predict the future of certain technology, based on today's facts.
@JoeKing: I hope this will give you a positive boost after this overload of conservatism. Great companies did arise from basement fiddling, especially if it involves people that don't listen to the mainstream. Like Bill who didn't believe the highly respected professor and thought:
hey I am gonna make software so people can use computers at home.
Good luck!
a 'highly respected' bicycle designer
Another interesting development: http://video.google.com/videoplay?docid=-2667394369567824498#
ReplyDeleteAnon @ 3:03 AM : Jobst is respected because of the accomplishments he's had in his field. That and being able to predict future developments with some amount of talent are two unrelated things. Why should one have a bearing on the other? There's a difference between being grounded in reality and making a product of dreams alone.
ReplyDeleteAnon @ 4:23pm : Looks like this is the same beast that Jobst talked about before on R.B.T (Simkins?). Did it shows its face at the '96 Interbike by any chance? Jobst's observations were :
ReplyDelete"Its a pulsed multiple ratchet device that did not put out a constant speed. The reason these devices appear in bicycle applications is that they rely on the adaptability of the human to
make operation possible. No machine would work with it. There is a large market for such a device, if it existed, but it doesn't. It has multiple ratchets in what is essentially a single large freewheel, with the inside hub that carried the ratchets, its axis movable so that the more eccentric the lower the gear. It doesn't have a large enough range to be interesting for bicycles and it's pulsating nature makes it useless for machinery."
Anon @ 3:03AM :
Jobst also clarified what he meant in the quoted statements (subject of the post) long time ago, and I was stupid not to include that here. Perhaps this will clear your misunderstand as to what he meant, before you pull down on his "respected" status.
"There is no positive
engagement CVT in existence. All such drives use friction, hydraulic, or electric transmission. By positive engagement is meant GEARS.
People have been working on this since the beginning of mechanical
transmissions. Inventors make such devices that have multiple
differential gears. None of these devices have worked."
Also, about the SIVAT video, I watched it and he goes "its infinite because it can go to zero." Now that's a bucket full of hot air. Infinite means there's infinite number of variables between any two numbers, not because it can go to zero. Also, it would be nice if Simkins explained how zero speed is applicable in a bicycle? Its all too sketchy at this point.
ReplyDeleteThere's a difference between making something for a need and making something that is TOTALLY unnecessary. You would think over 100 years of bicycle design would teach a good history lesson to some folks. What's next? A CVT for a rower?
ReplyDelete"Bucket of hot air"!
ReplyDeleteFrom Wikipedia
"The IVT is a specific type of CVT that has an infinite range of input/output ratios in addition to its infinite number of possible ratios; this qualification for the IVT implies that its range of ratios includes a zero output/input ratio that can be continuously approached from a defined 'higher' ratio. A zero output implies an infinite input, which can be continuously approached from a given finite input value with an IVT."
The Milkman "You would think over 100 years of bicycle design would teach a good history lesson to some folks."
ReplyDeleteIt obviously hasn't to you.....
If you know your numbers, there's a whole set of numbers before zero on the number line. Like I said again, by definition, infinite means a lot of numbers between two points as a range. An IVT accounts for that by even having negative reverse ratios to make it "Infinitely variable" (forward and reverse), and it is not infinite just because output can go to zero like the SIMKINS video says. Just by virtue of including the zero does not include "everything" , as the video suggests so its a little misleading.
ReplyDeleteAgain, what is the need for an IVT on a bicycle? Does it need to have a "park" mode while someone is climbing a hill with it? The IVT can find so much better use elsewhere in other machinery. By doing something for bicycles, it's possibly trying to answer a question no one asked or wants or makes any sense. Feel free to correct me.
"Again, what is the need for an IVT on a bicycle?"
ReplyDeleteClearly, there isn't a need for zero speed but it means that there is no limit to the lowest ratio, which means that you can have whatever range you want.
Ron
ReplyDeleteI think you & Jobst are focusing to narrowly on semantics. Simkins only speaks of zero mph in terms of the device's capabilities, not that it has an advantage for bicycles. In an automotive application this capability has the obvious advantage of eliminating the clutch or torque converter.
My device will also be able to travel at zero mph & transfer power pedaling in both directions...usefull unlikely, just an unintendid consequence of the design.
I think you both are missing the larger point...sure there is a multi-billion$ market for a practical CVT in industry, but realisticaly the odds of making dime 1 are infinitesimal; the reason people like Simkins (& myself) are persuiung the bicycle CVT (IVT) is our passion for bicycles...power generation doesn't turn me on.
JoeKing
Joe : Passion is one thing and if you like doing what you're doing, go ahead who's stopping you. I see it this way - Time = money.
ReplyDeleteThe SIVAT was invented for most transmission applications involving adjustable ratios in several venues. It is a fact that CVT's and IVT's do exist. The value of an IVT on a bicycle is not in having zero speed capability, but in having enormous range instead. An added benefit of the SIVAT is that ultra-low ratios which can produce extremely high output torque are prevented from reaching destructive levels of output. The first application of the SIVAT will be on bicycles because that is an excellent proving ground with its high torque input; there is also evidence that some MTB users will like such a transmission because it will allow more attention to MTB technique without the annoying events associated with shifting gears. So, yes, this is an endeavor of passion, and is also intent on providing a useful product that people will buy. A real SIVAT-equipped MTB will be tested in a few months. It's in the re-design & engineering stage now for a special MTB. Test user input will help determine whether this will become a bike product.
ReplyDeleteRebuttals to some statements in this blog are in order. First: IVT's must be distinguished from CVT's by fundamental differences between them, so 'infinitely' is used to differentiate IVT's because their 'infinite' input speed ratio is possible. Both IVT's and CVT's have infinite ratio choice, but only IVT's have infinite range. It's merely a naming convention, widely accepted. Furthermore, reverse gearing has absolutely nothing to do with it! Second: the SIVAT does indeed pulse, but its pulsing magnitude is a tiny fraction of that in nearly all prime movers; engines, for example, have torque pulsing that literally reverses direction on every other cycle (i.e., 100% F.S.), electric motors have sinusoidal torque pulsing. Torque pulsing translates to speed pulsing if not for inertial effects of cranks, rotors, legs, wheels and flywheels. So, the SIVAT is very useful for all types of driven devices. Third: bicyclists do have a power/torque curve, just like motors & engines do; that's why cadence is so important. In fact, propulsive efficiency drops off from a peak at about 85 rpm of pedaling speed for most of us, radically so at speeds below 50 rpm & above 120 rpm. Ask why Lance Armstrong modulates his cadence within a very tight range. All prime movers have a sweet spot. Fourth: CVT's & IVT's are not intrinsically linked to a need to constantly tweak the shifter to get the right ratio; I intend to prove that bicyclists will be able to forget about their geartrains and concentrate on just the good stuff while riding through hills & valleys! Having designed, built & tested an earlier, automatic bicycle CVT, I know that forgetting the shifter is possible & desirable. Fifth: The SIVAT is definitely a positive-engagement device with gears and levers; it's real and it works. The challenge to make it work very well in the 'real world' will be met soon.
ReplyDeleteSixth: the implication that fascination with a 'useless' technology as motivation ignores the fact that this inventor (for one) always was very annoyed with derailleurs as were his fellow cyclists, so this invention is a solution to a lack of choice between range and gear ratio precision, and the maddening chain suck & other frailties of derailleur systems. In projecting this annoyance with other cyclists, it became obvious that others also complained about bike transmissions. The goal is to produce a fairly light, small, trouble-free MTB transmission that is capable & fun. If successful, cyclists will wonder why anyone bothered to invent the other stuff.
"You would think over 100 years of bicycle design would teach a good history lesson to some folks."
ReplyDeleteAre you riding a recumbent yet? If not, you've learned nothing from the last 100 years of bicycle design when it comes to efficiency.
Is it an eRecumbent? Does it have 3 wheels? These are all improvements to the standard "safety" bicycle. The UCI killed bicycle innovation back in 1934 when they banned the recumbent from bicycle racing.
I for one am looking forward to Simkins device. Forgetting about the shifter (as well as forgetting about the need to balance, or getting soaked, or keeping up with traffic) is the key to wider sustainable vehicle adoption by the masses for everyday uses such as eBikes/Trikes/Twikes.
There is a reason why derailleurs root word is derail, as it typically is the first thing to go out of wack with your bicycle and derail your ride. While they are highly efficient and lightweight for racing (when well-maintained), they are poor for mass market reliability and dependability, and difficult for a novice (ie, the general public) to maintain. They are a very large reason behind the movement to fixed-gear bicycles. Good riddance = good riding.
I would love to test this out on my MTB or Twike (www.twike.com)... imagine pedaling to one's heart content, at whatever cadence, without needing to worry about shifting gears.
Never say never. Technology is ever evolving.
ReplyDeleteYears ago it was said that aluminum is not usable as a bicycle frame material, now its practically the only frame material used.
Doubters will spend there energy on negativity, inventors will spend there energy on looking for solutions to problems. Its a growth process to make something that doesn't work, in order to figure out how to make it work.
"I didn't fail, I just figured out a 1000 ways on how not to do it"
Thomas Edison
"The difference between Genius and Stupidity is that Genius has its limitations"
Albert Einstein
So... its been 15 years since the SIVAT patent issued. Where's the product? Has it been renamed, because I don't see it for sale on the net.
ReplyDeleteI am the one who posted to r.b.t about riding the prototype in the parking lot outside Interbike in 1996. It was a Simkins transmission. Pics are at http://minortriad.com/cvt.html and it is described in US patent 5,516,132.
ReplyDeleteI can report that it did function but I have doubts that it could be made practical (efficient, reliable, not overly heavy).
Yes belt CVT's have problems for bicycle.
ReplyDeleteBut take a look to mine:
http://yves.maguer.free.fr/home/CVT_fort_couple.html