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Spiderman
12-11-2004, 12:15 AM
Just how fast for any given speed is the chain moving. I think its either a product of the rpm & final ratio & chain length. Or road speed & rear wheel diameter & chain length etc. but I just can’t come up with a formula for either.

A Yerbury
12-11-2004, 12:22 AM
I dont know about a formula but it would be easy to work out, factoring in chain stretch would make things a bit either way though. nice question though, next week "whats with gravity" and "can you jump start a helicopter"

NBs
12-11-2004, 01:23 AM
.

I hearby admit that this post was full of crap!
Thanks to Zimbo for politely pointing it out!!

n

Zimbo
12-11-2004, 08:00 AM
Nice question indeed!
The actual speed of the chain on the bottom run in relation to the road wouldn't be 0mph, the theory is correct but it assumes the chain is looped round the outside of the rear wheel. As it's looped round the sprocket, which has a smaller diameter to the wheel, it will always have a positive speed in relation to the road.
So, in relation to the bike:

Road speed in kph can be converted to rear wheel rpm by dividing (pi x diameter of rear wheel in metres) into it, and dividing the result by 60 to convert to rpm rather than rph, then multiplying by 1000 to convert from kilometres to metres.
This will also be the speed in rpm of the rear sprocket, since the two are fixed together.

Measure the effective diameter of the rear sprocket in m (diameter to inside of teeth?) and multiply by pi (3.142) to get circumference of rear spocket in m. Multiply by rpm to get actual speed of chain in metres / min in relation to a fixed point on the bike, then divide by 60 to convert to metres / second, or multiply by 60 and divide by 1000 to get kph.

The actual road speed of the chain will be (bike speed plus chain speed) for the top run, and (bike speed minus chain speed) for the bottom run.

Any advance???

NBs
12-11-2004, 09:50 AM
zimbo, in the sober hour of day I agree...

relative to road speed:
The moment the chain passes along the bottom of the rear sprocket, i.e. the moment it's travelling parallel to the road, then its velocity would be
Cv=V-(r1/r2)V
At the top of the sprocket it would be
Cv=V+(r1/r2)V
where
Cv = chain velocity
V = velocity of bike
r1 = diameter of sprocket
r2 = diameter of tyre

Along any other point, where the chain is not travelling parallel to the road, then the angle of the direction of travel relative to vertical would need to be factored in, modifying the equation to:
Cv=V±((r1/r2)sinA)V
where A is the angle of attack

How does that sound?

Northman1
12-11-2004, 10:33 AM
My head hurts !

nik_the_brief
12-11-2004, 10:45 AM
Bloody hell that's a complex concept for this time of the morning.

More pertinently - why would you want to even know? :D

NBs
12-11-2004, 10:58 AM
yeah, why?

I'm just waiting for a maths type bod to pop up cos i'm still not sure my equation is right???

Spiderman
12-11-2004, 11:15 AM
What I wanted was the actual speed of a chain in m/s. After a sleepless night, I came up with this;
Road speed (m/s) / rear wheel circumference (m) * rear teeth * tooth pitch (m)
It looks about right. Any comments anyone.
Try out the little spreadsheet attached and see what you come up with.
And thanks for those wacky replies.

Ps. you can jump start a helicopter. If the engine stalls, reverse the pitch of the blades, the falling mass (that’s you attached to 1500kg of metal) then acts to turn the rotor. When the engine speed indicator is back in the yellow, reverse the pitch to give lift, hit the start button and pray. The hardest part of this manoeuvre is cleaning the cockpit after. Oh and don’t try it below 2000ft.
:eek:

NBs
12-11-2004, 11:38 AM
from your spreadsheet it looks like you're after the constant rotating speed of the chain rather than the forward velocity relative to the world.
Then using the bike's linear velocity translated through the dimensions of the tyre to give rpm, then factored in the size difference between tyre and sprocket.

Haven't checked the formula, but the final number looks about right.

Now try'n work out how many G the chain experiences as it changes direction!

Pedro
12-11-2004, 01:14 PM
Do you guys find your anorak zips scratch your paintwork?


:D

A Yerbury
12-11-2004, 01:54 PM
not when you factor in tank pad abbrasion divided by zip coating to the velocity squared...now pay attention next time! cheers for the tip spiderpants Ill bear that one in mind. carbon fibre wingspan for monster take off any one?

Duncan
12-11-2004, 08:21 PM
As Yerbs will testify, the speed of the chain on most bikes here is 0 mph, rising to about 2 mph when they get rolled out of the garage to get the mower out then zero again.

Garry Hogan
13-11-2004, 10:26 AM
What's Patrick Mower doing in your garage? :burnout: