YACT: Question about Gearing Ratio

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ndee

Lifer
Jul 18, 2000
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OK, I always read something about 5:1, 6:1, 1:1, etc. What does that actually mean? If the motor goes 5000rpm and the ratio is 5:1, does that mean that the wheels are turning at 25000rpm?
 
S

Syringer

Lifer
Aug 2, 2001
19,333
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It means the opposite really..but you have to take into conderation axle/final drive ratio too.

So if you have a 5:1 ratio in first gear, and a final drive ratio of 2, at 5000 rpms the wheels are going at 250 rpms, or ~4 times a second.
 
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Eli

Super Moderator | Elite Member
Oct 9, 1999
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No. It means that the engine has to turn 5 times to turn the driven item once.

If you have a drive gear that has 10 teeth, and a driven gear that has 10 teeth, the ratio is 1:1. If the driven gear has 20 teeth, the ratio is 2:1.. etc.
 
N

ndee

Lifer
Jul 18, 2000
12,680
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Ah ok. So the higher the gear goes, the smaller the number gets? Like in the first gear it's 20:1, second gear 15:1, etc.?
 
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Eli

Super Moderator | Elite Member
Oct 9, 1999
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http://science.howstuffworks.com/gear-ratio.htm :)

Originally posted by: ndee
Ah ok. So the higher the gear goes, the smaller the number gets? Like in the first gear it's 20:1, second gear 15:1, etc.?
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Yep, exactly. Although the actual numbers are much lower than that.

Eventually, you get to overdrive.. which is less than 1:1, something like 0.81:1. That's why the engine only has to turn at slightly above idle, yet you're going highway speeds.
 
E

Eli

Super Moderator | Elite Member
Oct 9, 1999
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Originally posted by: ndee
Originally posted by: Eli
http://science.howstuffworks.com/gear-ratio.htm :)
Click to expand...

Thanks :D
Click to expand...

Just think of the gears on your bicycle.

When you're on the biggest gear on the front(where the pedals are), and the smallest gear on the back.. that's when it's hardest to pedal, but you can go the fastest.

When the gears are closer to the same size (when you're on the large sprocket on the rear wheel), it's much easier to pedal.. but you can't go very fast.
 
N

ndee

Lifer
Jul 18, 2000
12,680
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Originally posted by: Eli
Originally posted by: ndee
Originally posted by: Eli
http://science.howstuffworks.com/gear-ratio.htm :)
Click to expand...

Thanks :D
Click to expand...

Just think of the gears on your bicycle.

When you're on the biggest gear on the front(where the pedals are), and the smallest gear on the back.. that's when it's hardest to pedal, but you can go the fastest.

When the gears are closer to the same size (when you're on the large sprocket on the rear wheel), it's much easier to pedal.. but you can't go very fast.
Click to expand...

and it works kinda the same in a car?
 
S

Syringer

Lifer
Aug 2, 2001
19,333
3
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Yes. Gears in every aspect work the same way..

Another thing to make note of is that torquey cars like say a Trans Am have shorter gear ratios, whereas a high revving low torque car such as an S2000 has higher great ratios. This allows it to sort of make up for torque by allowing the car to run through the power band quicker. However in a torquey car high gear ratios would just be too much, and all you'd ever get is wheel spin.
 
N

ndee

Lifer
Jul 18, 2000
12,680
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