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Old 2002-07-20, 07:13 AM   #1
wheelfreak
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dumb newbie question: uni with gears possible?

I've just started unicycling and have a 24" miyata which is really nice so far. I keep wondering if there is a way to make a multiple geared uni with like 2-3 different gears sort of like a bike. a chain could be put around the base of the cranks and then looped through a little gear under the seat perhaps...
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Old 2002-07-20, 07:30 AM   #2
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I recently read an article about wheelchair hubs that change gears automatically (mechanically) depending on speed, so normally they have a 1:1 ratio but at hight speeds they drop down a gear to 1:2. It made me thing that a geared hub uni is possible with similar technology. has anyone else heard anything about this? I'll try to find a web site about it to post it up with this sometime soon.

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Old 2002-07-20, 03:41 PM   #3
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Yes, people have built geared uni's. 3 of them I belive in total, world wide. Greg harper, whome I believe is away at the unicon, therefore will not be answering this himself, built the last one. The most successful solution seems to have been to use internal planatary gearing, so in other words, the gears are all within the hub, rather then using a chain. The problem is, nobody has ever put one into production for all to have, and nobody has ever successfully built a multible speed gearing system. For it to be a fully pratical, massproducable product, I feel it would have to be varieable speed and would have to be able to shift while in motion. You can learn all about Harpers uni by searching the rec.sport.unicycling archives for "uni.5". thats what he called it.
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Old 2002-07-20, 08:32 PM   #4
Mikefule
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The question was asked by a relative newbie, so all you experts probably know the following better than I do...

The big problem is that you can't have a freewheel on a uni (someone tell me they've ridden one, go on, say it...) so even with a chain-driven giraffe, a derailleur gear system or a standard Sturmey Archer 3 speed hub would be no good.

Most unis, of course, are not giraffes, and have a direct drive through the axle from the cranks.

Making a single speed 'step up' (or step down) gear using sun and planet gears is simple in the sense that similar arrangements were used in late Victorian times to make small wheeled versions of 'penny farthings'. The idea here is that the wheel is physically, say, 24 inches, but the single gear makes it turn faster than the cranks, so that it feels like, say, a 36 inch wheel. The basic idea is over 100 years old.

However, it is more complex than this because a unicyclist needs the hub to be light, strong, and smooth. When idling, or simply correcting your balance, you don't want the cranks to turn 5 or 10 degrees before they start to turn the hub. Transmission 'lash' which would be acceptable on a bicycle (which is normally moving forwards at a fairly steady speed) would be a pain in the bum on a unicycle.

Another problem is the very limited market. The proposed hub would need to be light, strong, and precision-engineered. Made on a one by one basis, they would be expensive to produce, but the demand is unlikely to be big enough to support the tooling required for mass production.

The thread started with a suggestion for a chain running up the outside of the forks to a 'gear box' and then a separate chain running down to the hub. This is a clever idea, but it would not eliminate the 'transmission lash' problem, and it would make the whole uni that bit wider, as well as introducing new problems like catching your leg or clothing on the outside chain.

In reality, the nead for gearing is questionable. A Coker 36 or a short cranked 28 or 29 can go rather faster than the rider can run. That means that if the uni stops from high speed, the rider is in difficulty.

Some control of 'gearing' can be achieved through selection of the wheel size, and by selection of the crank size. Very simply, short cranks = more rpm = more speed BUT at the expense of acceleration and hill climbing. Long cranks give more control on hills (both up and down!) but limit the rpm and therefore the max speed.

To my mind, the beauty of the unicycle is its simplicity, and although I'd like a go on a geared uni, I think half the fun comes from working within the limitations of the crank/wheel combination. It helps if you have several unis, though.
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Old 2002-07-20, 10:24 PM   #5
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ya, I agree with all that. Though, i personaly belive the problems with freewheels and shifting gears could be fixed by looking at car transmissions. They can't deal with backlash either. What they do is put in a clutch so that, you dissengage the driver from the driven, change the gear ratio, and reengage it. Why not do the same on a uni? (besides the unbeliveable difficulty in learning to shift).
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Old 2002-07-20, 11:08 PM   #6
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Re: dumb newbie question: uni with gears possible?

> The big problem is that you can't have a freewheel on a uni (someone
> tell me they've ridden one, go on, say it...)


Ok, I've ridden one. Back in about 1987 I built and attempted to ride a
coasting mini-giraffe uni. I was never very succesful so I ended up welding
the sprocket to the hub thus making it a non-coasting cycle. It had (has) a
12" wheel geared 2:1. The best distance I ever got when riding it as a
coaster was about 30 feet.

-mg


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Old 2002-07-21, 06:57 AM   #7
harper
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Quote:
Originally posted by thisistrev
Though, i personaly belive the problems with freewheels and shifting gears could be fixed by looking at car transmissions. They can't deal with backlash either. What they do is put in a clutch so that, you dissengage the driver from the driven, change the gear ratio, and reengage it.
Car transmissions don't care about backlash. They only run in one direction at a time when in gear. They could have tremendous backlash and no one would ever know or care.

Bicycles similarly use some form of clutch in that a ratchet or freewheel is a form of clutch which disengages the drive from the drivetrain. As Trevor and Michael Grant both noted, the difficulty in having a freewheeling unicycle is obvious. I saw John Foss ride a 12" wheeled freewheeling unicycle with coaster brake today. Sarah, the daughter of the inventor, could alledgedly ride it uphill. I was more taken by her two baby oppossums than the unicycle, however.

Planetary gear systems have some inherent backlash. To produce a shiftable planetary geared hub for a unicycle a splined shaft for sliding one of the gear assemblies must be used which will necessarily increase the overall backlash of the drivetrain.
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Old 2002-07-21, 09:37 AM   #8
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Mr. H says that backlash is present in a car, but seldom a problem. On motorcycles with shaft drive (pretty similar to a car in mechanical terms) backlash can be a problem - mainly because of the funny things which happen to the weight distribution of the bike as it changes from acceleration mode to deceleration mode. Even with a chain drive, there can be a bit of snatch which can upset the handling as you enter a bend. (Just out of interest and not really relevant to unicycles!)

As for gearing systems for unicycles... Twist and go mopeds have continuously variable transmission. What happens is that when the load on the transmission becomes heavier, the gearing automatically drops. As the load becomes lighter, it automatically rises. It's not like an automatic car where it shifts gear from time to time. It's a smooth progression of ratios, rather than a number of steps.

I vaguely understand it's done with two spring loaded cones which have a drive band running between them. The cones compress under pressure, and expand when the pressure is reduced.

It seems to me to be not beyond the wit of human ingenuity to introduce some element of control into such a system - maybe a cable control to alter the tension on one or both of the sets of springs.

I wonder whether a similar idea could be used on a giraffe uni?
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Old 2002-07-21, 04:23 PM   #9
Lowell Terry
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Re: dumb newbie question: uni with gears possible?

I think you have some very good ideas here. Maybe someone, maybe
you, can develope them into a workable model.

Lowell

----- Original Message -----
From: Mikefule <Mikefule.84x5a@timelimit.unicyclist.com>
Newsgroups: rec.sport.unicycling
To: <rsu@unicycling.org>
Sent: Sunday, July 21, 2002 4:37 AM
Subject: Re: dumb newbie question: uni with gears possible?


>
> Mr. H says that backlash is present in a car, but seldom a problem. On
> motorcycles with shaft drive (pretty similar to a car in mechanical
> terms) backlash can be a problem - mainly because of the funny things
> which happen to the weight distribution of the bike as it changes from
> acceleration mode to deceleration mode. Even with a chain drive, there
> can be a bit of snatch which can upset the handling as you enter a bend.
> (Just out of interest and not really relevant to unicycles!)
>
> As for gearing systems for unicycles... Twist and go mopeds have
> continuously variable transmission. What happens is that when the load
> on the transmission becomes heavier, the gearing automatically drops.
> As the load becomes lighter, it automatically rises. It's not like an
> automatic car where it shifts gear from time to time. It's a smooth
> progression of ratios, rather than a number of steps.
>
> I vaguely understand it's done with two spring loaded cones which have a
> drive band running between them. The cones compress under pressure, and
> expand when the pressure is reduced.
>
> It seems to me to be not beyond the wit of human ingenuity to introduce
> some element of control into such a system - maybe a cable control to
> alter the tension on one or both of the sets of springs.
>
> I wonder whether a similar idea could be used on a giraffe uni?
>
>
> --
> Mikefule - Roland Hope School of Unicycling
> ------------------------------------------------------------------------
> Mikefule's Profile: http://www.unicyclist.com/profile/879
> View this thread: http://www.unicyclist.com/thread/19435
>
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Old 2002-07-21, 05:28 PM   #10
Mark Newbold
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Re: dumb newbie question: uni with gears possible?

How about a chain-drive unicycle with two chains, one on each side of the
crank? They would be geared differently and only one would be engaged at a
time.

Problems:
1. During shifting there would be a brief moment when neither was engaged.
To prevent UPD, the shift mechanism would need to lock the axle momentarily
during the shift. You would have to remember not to shift while moving.

2. Adjusting chain tension would be tricky. Adjust one correctly and the
other one is too loose or too tight. Maybe it would need some kind of idler
sprocket halfway up (at least on one side) to tension the chain.

--Mark


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Old 2002-07-21, 05:34 PM   #11
Mikefule
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Re: Re: dumb newbie question: uni with gears possible?

Quote:
Originally posted by Lowell Terry
I think you have some very good ideas here. <snip> maybe
you, can develop them into a workable model.

"Not I," said the cabin boy! I'm more your ideas man than your artisan. Bulsh a speciality.
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Old 2002-07-21, 06:08 PM   #12
Nick Grey
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Re: dumb newbie question: uni with gears possible?

wheelfreak wrote:

> I've just started unicycling and have a 24" miyata which is really
> nice so far. I keep wondering if there is a way to make a multiple
> geared uni with like 2-3 different gears sort of like a bike. a chain
> could be put around the base of the cranks and then looped through a
> little gear under the seat perhaps...


Okay, here is myt silly idea for adding gears to a uni: variable length
cranks.

Well, actually that's a bit too hard. What you really want is special
cranks in which the pedals can slide up and down. Obviously you go
closer to the hub to move up gears and closer to the rim to move down
gears.

The bit I haven't worked out is how do you actually control sliding of
the pedals?
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Old 2002-07-21, 06:35 PM   #13
Mikefule
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Re: Re: dumb newbie question: uni with gears possible?

Quote:
Originally posted by Nick Grey
Obviously you go
closer to the hub to move up gears and closer to the rim to move down gears.

The bit I haven't worked out is how do you actually control sliding of the pedals?

2 things:
FIRSTLY: Altering the length of the cranks is not exactly the same as altering the gearing. It alters the leverage ratio, which means that with longer cranks, you get more torque. This is similar to having lower gearing when going up or down hill or across difficult ground, but it is not identical - your feet have to move further per revolution of the wheel, and the wheel still turns once for every turn of the cranks.
Following on from this, shortening the cranks does not change the number of revolutions of the wheel per revolution of the cranks. It simply makes the circle described by your feet smaller, which makes it easier to pedal with a fast cadence.
Altering the gearing on a bicycle, car, motorcycle etc., or on Uni.5 or any hypothetical geared unicycle would mean altering the number of revolutions of the wheel per revolution of the pedals. As on a bicycle, the ideal would be to keep pedaling at a comfortable cadence, and change the speed of the wheel according to the terrain.

SECONDLY: Having said the above, there would be a lot to be said for variable length cranks. I find 150 mm cranks ideal for most applications, because they fit my leg size best, but there are times when I would appreciate the extra torque of a pair of 170s, or the extra speed of a pair of 125s.
Are you suggesting cranks you could adjust 'on the move'? This would be hard to achieve. Cranks that could be adjusted quickly at the side of the trail would be a simple proposition. The simplest answer would be cranks with 2 or more threaded sockets for the pedals. These have been discussed eslewhere on this forum.
'Telescopic' cranks would give an infinite degree of adjustment within a limited range. As a simple engineering problem, they would be easy to make. The mechanism to adjust them would probably be similar to the threaded adjuster used to tighten the chain on a motorcycle, or the similar device used on giraffe unicycles. As with geared hubs, the biggest problem would be the cost, arising from the low demand.
As for cranks which could adjust in length 'on the move', I suppose that would need servo motors and stuff. To all intents and purposes, impossible. Unless of course...
Instead of cranks, use discs cut into sections like the iris of a camera, or those doors in 1970s science fiction films. The discs would expand to produce effectively a 'longer crank', or contract to provide a 'shorter crank'. I dare say that a spring mechanism could be introduced which would be set to allow the cranks to lengthen when a certain level of torque is exceeded. The cost would be astronomical, the device would be heavy, it would be unreliable and so on, but, hey, it could work. ;0)

Which returns me to my earlier view that half the fun is getting the most from the combination of cranks and wheel that you have. It would be a pity if unicycling were to become complicated.
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Old 2002-07-21, 08:43 PM   #14
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Quote:
originally posted by Mikefule: It would be a pity if unicycling were to become complicated.
Yeah, nobody here would want to see that happen!

Right?

Don't all yell out at once, now...

........................................................................ ..... Hello?
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Old 2002-07-21, 08:57 PM   #15
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Re: dnq:uwgp?

Quote:
Originally posted by sendhair


Yeah, nobody here would want to see that happen!

Right?

Don't all yell out at once, now...

........................................................................ ..... Hello?
You Knoooooooooooow what I meant!

Unicycling is such a simple sport, and it would be a pitty to overcomplicate it.

Why, all you need to enjoy the sport to the full is a lightweight frame, decent hub, 48 spoke rim and 48 spokes, high volume tyre, kevlar tube, a selection of cranks, sealed bearing pedals with spare pins, a two bolt seat clamp, seat post, Miyata air seat conversion, digital trip counter, a handle, padded shorts, shin pads, elbow pads, wrist guards, gloves, helmet, eye protection, selection of tools, first aid kit, Camelbak or drinks pouch, energy drinks, GPS... Why add the unnecessary complexity of a geared hub? ;0)
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