02GF74
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| posted on 6/12/10 at 01:47 PM |
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mechanics type question
Let's say you have a multi-gear bicycle, with 2 chainrings at the crank and more than 2 sprockets on the rear.
Let's say you put two chains on.
Chain A goes on the bigger of the two chainrings and onto a sprocket at the back.
Chain B goes on the smaller of the two chainrings and onto a sprocket at the back, the sprocket being larger than for chain B. (stand gear layint on a
bike; chains are not crossing each other). There is no front or rear derailleur.
Now what would happen if you got on the bike and tried to pedal?
Answers are to not include any mathematics but a simple description.
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matt_claydon
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| posted on 6/12/10 at 01:51 PM |
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You wont be able to turn the pedals. If is has derailleurs then you'll turn it until all the slack has been taken out then it'll lock up.
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02GF74
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| posted on 6/12/10 at 01:52 PM |
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yes - but why?
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Sellsie
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| posted on 6/12/10 at 02:17 PM |
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Its all to do with gear ratios.
By using two sets of gears each differing in size you are in effect trying to turn the wheel at two speeds at the same time.
Clearly this will not happen so the two rotation forces fight each other and the wheel locks up.
For this reason small ball bearings and plungers are used in the gear selectors of car gearboxes to prevent two gears being selected at the same
time.
I hope this explanation helps.
Sellsie
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BazzaMonty
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| posted on 6/12/10 at 02:34 PM |
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Move forward........maybe!!!!!!!!!!!!!!!!!!!!!!!
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liam.mccaffrey
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| posted on 6/12/10 at 02:39 PM |
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the back wheel will want to turn at 2 different speed corresponding to the 2 different gear ratios.
Imagine having 2 seperate bikes one with each ratio and assuming a constant front sprocket RPM on both bikes, the difference in this speed is the
important thing. Imagine one bikes back wheel was going at 10 RPM and one was going at say 30. The difference is 20 RPM.
So imagine then if both ratios were on a common gearset the 30 RPM chain will immediatly tension the bottom of 10 RPM chain and bind up.
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grub
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| posted on 6/12/10 at 02:45 PM |
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fall over if your using SPDs
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MikeR
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| posted on 6/12/10 at 03:17 PM |
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all of the above unless ...........
if the two front gears are 30 teeth and 15 teeth + the two rear ratios are 10 teeth and 5 teeth. The Reality is both are turning at the same 3 to 1
ratio and therefore the bike will work.
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marcjagman
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| posted on 6/12/10 at 04:00 PM |
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Why would you want to know that?????
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Liam
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| posted on 6/12/10 at 04:52 PM |
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quote:
Originally posted by MikeR
all of the above unless ...........
if the two front gears are 30 teeth and 15 teeth + the two rear ratios are 10 teeth and 5 teeth. The Reality is both are turning at the same 3 to 1
ratio and therefore the bike will work.
Ah but no - he said the chain on the smaller front sprocket goes on the bigger rear sprocket, so there's no way the gear ratios can be the same
for both chains.
It will lock up - one chain in tension accross the top and the other in tension accross the bottom.
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loggyboy
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| posted on 7/12/10 at 01:14 AM |
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If its on a typical pedal bike with a 'ratchet' style sproket which ever is the higher gear ratio will be the one governing the
speed/output. the lower ratio will just turn on the rachet as if you were rolling.
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slingshot2000
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| posted on 7/12/10 at 01:17 AM |
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I think a re-post is in order;
''Philosophers Type Question''
Any decent mechanic/engineer/locoster would not have given this the time of day.
If you need to know badly enough, go build it and see what happens!
P.S. Take a first aid kit for the scratched/bruised chin !
P.S.S. if you need someone to build you a life-size working model, my numbers are
available......................................................................................................................
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slingshot2000
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| posted on 7/12/10 at 01:20 AM |
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quote:
Originally posted by loggyboy
If its on a typical pedal bike with a 'ratchet' style sproket which ever is the higher gear ratio will be the one governing the
speed/output. the lower ratio will just turn on the rachet as if you were rolling.
Hah, well spotted! Are both systems running ''to'' or ''fro''. in the same direction or opposing???
Hmmmmm, some more I think !
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Liam
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| posted on 7/12/10 at 09:47 AM |
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quote:
Originally posted by loggyboy
If its on a typical pedal bike with a 'ratchet' style sproket which ever is the higher gear ratio will be the one governing the
speed/output. the lower ratio will just turn on the rachet as if you were rolling.
No - wont work because on a normal rear cassette all the sprockets are joined together - they can't turn independantly of each other, so one
can't turn faster than the other.
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MikeRJ
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| posted on 7/12/10 at 09:50 AM |
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Try removing the interlocks from your gearbox and engaging two gears at once and see how far you get!
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02GF74
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| posted on 7/12/10 at 01:42 PM |
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the question really was to see if you could vvisualise what would happen and provide a simple explanation.
Way I see it is that if the front chain rings turn, the larger one, A, would pull more chain.
If the rear sprockets turn , the larger one, B, is feeding more chain.
Since the lagrer chainring A cannot be supplied with enough chain, the bike stops moving.
Chain A is under tension at the top, slack below and chain B is slack at the top and under tension below.
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