"variable sprocket" anybody got any experience?

[bobc]

This is an idea for our greenpower race cars; I just wondered if anyone had actually tried to use anything similar.
2 of the 4 lobes will have their teeth removed, and this will allow the ratio to change. There will be light spring action trying to spread the sprocket outwards, most of this force will come from the motor torque. Ratios will be altered by moving the motor closer or further away.
Effective sprocket size goes in 2 tooth jumps from 48 to 62

[breakwellmz]

That looks very much like the principle behind Archibald Sharp`s variable hub gear system patented in 1900!
Excuse poor illustration-

[bobc]

There were quite a few wacky expanding chainrings around throughout last century - I was just wondering if anyone had used one? my scribblings above are not for a bike, but the experience might well be transferable...

[mrjemm]

Would the non round/smoothly changing (as per Q/Rotor/Biopace) radius possibly cause pulsing or vibration (depending on revs), perhaps leading to damage?

Just wondering.

Edit... Also only the arm not in contact with the chain would be able to move outwards I think.

[jb]

Not wanting to throw cold water on it but I'm guessing as the motor moved away the sprocket tries to contract but chain trapped between two segments will try to buckle, the motor won't move. Conversely it can't expand. The principle might work with a rubber band drive.

[[XAP]Bob]

it just takes a rev to expand, one arm at a time.

Seems like an interesting idea, but I have to suspect that it's use is limited, as it seems like the kind of thing the victorians would have gone for...

[bobc]

I'll make one next week to try out (it's just a few laser cut parts after all). I couple of respondents hadn't read the bit where I said 2 of the lobes had their teeth removed....
It really looks as though is addresses all the gearing issues for greenpower - I'll follow up with a couple of pix of the real thing
Shame no- one seems to have tried any of the many 20th century patents of things like this..... !!

[[XAP]Bob]

I'll make one next week to try out (it's just a few laser cut parts after all). I couple of respondents hadn't read the bit where I said 2 of the lobes had their teeth removed....
It really looks as though is addresses all the gearing issues for greenpower - I'll follow up with a couple of pix of the real thing
Shame no- one seems to have tried any of the many 20th century patents of things like this..... !!

Erm, you'd have very few teeth in use at various times - do you need to strip the teeth at all, jut move the arms in/out in sequence...

[LWaB]

I've ridden a Cambiogear many years ago but there have been a number of expanding chainring (or sprocket) systems.
http://www.zoxed.eu/photos/bikes_drives.html

[bobc]

That is great LWaB - interesting to see the 'deal drive' also has just 2 small toothed sections & the other 4 lobes are toothless - same idea as mine.
You say you had a go on the cambio - can you remember anything about it, I'm interested in how much rattling went on & how smooth it felt.
Here's the design which will be lasered next week. BTW it sits on (is clamped to) a 16 tooth freewheel.

It's not quite as complex as it looks, there are just 5 different pieces, the links, the freewheel clamps, the 2 sorts of lobe and the outer sheets (which link the lobes together and act as chain guides)
I'll take a picture of one when it's done

[[XAP]Bob]

"Ratios will be altered by moving the motor closer or further away."

Am I being thick?

[bobc]

Bob - I don't necessarily expect this to work particularly well, esp straight out of the box. Physically moving the motor is a well established way of controlling continuously variable V belt drives with a sprung pulley, this is just drawing on that, if you pull the motor away it must force the sprocket to a smaller diameter. Hooking up the control was always going to be the trickiest bit of this & moving the motor looks like the simplest solution.
TBH I love this kind of stuff & if it works as I hope it will really give us an edge in the competition. I'll be picking the parts up from the laser cutters on friday (designs dropped off today) & will stick a test rig together at the weekend.
Only then will we start to have a better idea about practicality. Trying it is only going to cost £15 or so & it's a bit of an engineering adventure for the girls in the greenpower team.
Still hoping for feedback from someone who tried something similar in real life....???

[sreten]

Hi,

I don't understand "moving the motor". As it stands it looks like
with the correct springing it would react to applied torque,
or the lack of, to change down and up.

Of course "moving the motor" would affect the maximum gearing,
and very stiff springing would make that the only gear possible.

By the looks of it re-engagement of the teeth with the chain
is going to be very messy between the "natural" gear torques.

Some sort of ridging to the sprung positions that the arms
would only jump between, corresponding to the right chain
gaps seems a good idea to me.

Though TBH I'm looking at it from a front chainring perspective,
with a derailleur rear. I'm just guessing here, but not much,
I'm pretty sure the ICU has banned such things for racing.

rgds, sreten.

[[XAP]Bob]

Can't see why they would have banned this?!

Can't see what action moving the motor away is providing either. Can't even see what *direction* the motor would move

[RickH]

Can't see what action moving the motor away is providing either. Can't even see what *direction* the motor would move

If I am understanding it right, the motor is at the other end of the loop of chain & the springs in the above mechanism act as a chain tensioner. If you move the motor end closer the segments spring out to take up the extra chain length & in doing so form an effectively larger sprocket.

As a side note, I think with this design there will be problems with slippage as the teeth do not cover 180 degrees, so there will be times when no teeth are in contact with the chain unless you plan to use some sort of jockey wheel arrangement to increase the chain wrap.

Rick.