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Old 11th July 2010, 21:42   #1
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Default Understanding gear ratios

Guys,Could somebody explain in a simple layman term as to what is meant by gear ratio?
I searched the web for answers but was not able to find any simple answer.
Is it something like as to what ratio of speed to RPM should a gear shift happen?

P.S:I had searched the forum for an answer but was not able to find a thread dealing with this question.
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Old 11th July 2010, 21:57   #2
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In the simplest words, Gear Ratio is just the ratio of the diameters of two gears which are meshed together. Gear ratio can also be defined as ratio of angular velocities (or RPMs) of the two gears, but this will be the inverse of the first definition.

For example, let us assume Gear 1 and Gear 2 are meshed together. Gear 1 has a diameter of D1, and Gear 2 has diameter of D2. Then, Gear Ratio is D1/D2. If the two gears have RPM of R1 and R2 respectively, then note that D1/D2 = R2/R1. Therefore, R2/R1 can also be called as gear ratio.

In automobile context, Gear Ratios are the ratios of driving gears with respect to the crank shaft gear. In other words, these are the ratios of engine RPM to the RPM of the wheel. For example, if first gear ratio if 4.0, then the engine RPM will be 4 times the RPM of the wheel or axle, when you are in first gear. And the same goes for other gears. Typically, gears 5 and sometimes even 4 have ratios less than 1, which means that engine RPM will be less than wheel RPM in those gears. For example, gear ratio for 5th gear can typically be 0.9. Such gears (with gear ratios of less than 1) are called overdrive gears.

Hope this helps.
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Old 11th July 2010, 22:05   #3
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Quote:
Originally Posted by Dr.AD View Post
In the simplest words, Gear Ratio is just the ratio of the diameters of two gears which are meshed together. Gear ratio can also be defined as ratio of angular velocities (or RPMs) of the two gears, but this will be the inverse of the first definition.

For example, let us assume Gear 1 and Gear 2 are meshed together. Gear 1 has a diameter of D1, and Gear 2 has diameter of D2. Then, Gear Ratio is D1/D2. If the two gears have RPM of R1 and R2 respectively, then note that D1/D2 = R2/R1. Therefore, R2/R1 can also be called as gear ratio.

In automobile context, Gear Ratios are the ratios of driving gears with respect to the crank shaft gear. In other words, these are the ratios of engine RPM to the RPM of the wheel. For example, if first gear ratio if 4.0, then the engine RPM will be 4 times the RPM of the wheel or axle, when you are in first gear. And the same goes for other gears. Typically, gears 5 and sometimes even 4 have ratios less than 1, which means that engine RPM will be less than wheel RPM in those gears. For example, gear ratio for 5th gear can typically be 0.9. Such gears (with gear ratios of less than 1) are called overdrive gears.

Hope this helps.
Thanks Dr.AD. I think I am beginning to get a feel of it. What is the significance/importance of the gear ratio? Is there something like good gear ratio or bad gear ratio?
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Old 11th July 2010, 22:15   #4
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Quote:
Originally Posted by roadracer View Post
Thanks Dr.AD. I think I am beginning to get a feel of it. What is the significance/importance of the gear ratio? Is there something like good gear ratio or bad gear ratio?
Actually gear ratio is a very critical parameter and it has huge significance in terms of drivability as well as fuel economy (just to name the most obvious things). Gear ratios will determine if the engine will run at high RPM (which may deliver more torque if the RPM is in the right band but may consume more fuel) or low RPM (which means the torque is less but fuel consumption is also less) for a given wheel speed. Overly high RPM can cause engine noise as well as loss of FE, whereas overly low RPM can cause engine lugging and loss of engine power. The gear ratios need to be very carefully designed considering the vehicle weight, engine torque and power curves, intended drivability and performance parameters and of course intended fuel economy.

It is difficult to say what is a bad ratio or good ratio because bad ratio for performance may be good ratio for fuel economy. However, if the ratio is really bad and engine runs at the wrong RPM band, then one can lose on both fronts, which means less power as well as less fuel economy. In such cases, the gear ratio can certainly be called as bad.
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Old 11th July 2010, 22:22   #5
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Did you try Wikipedia?
Gear ratio - Wikipedia, the free encyclopedia
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Old 11th July 2010, 22:39   #6
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Thanks for starting the topic roadracer. I was confused about the same topic.
Thanks for the explanation Dr.AD.
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Old 11th July 2010, 22:45   #7
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Quote:
Originally Posted by Dr.AD View Post
Actually gear ratio is a very critical parameter and it has huge significance in terms of drivability as well as fuel economy (just to name the most obvious things). Gear ratios will determine if the engine will run at high RPM (which may deliver more torque if the RPM is in the right band but may consume more fuel) or low RPM (which means the torque is less but fuel consumption is also less) for a given wheel speed. Overly high RPM can cause engine noise as well as loss of FE, whereas overly low RPM can cause engine lugging and loss of engine power. The gear ratios need to be very carefully designed considering the vehicle weight, engine torque and power curves, intended drivability and performance parameters and of course intended fuel economy.

It is difficult to say what is a bad ratio or good ratio because bad ratio for performance may be good ratio for fuel economy. However, if the ratio is really bad and engine runs at the wrong RPM band, then one can lose on both fronts, which means less power as well as less fuel economy. In such cases, the gear ratio can certainly be called as bad.
Thanks Dr.AD.Say if the gear ratio for the first 2 gears of a bike is as below. How this needs to be interpreted?

First Gear - 3.000
Second Gear - 1.750
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Old 12th July 2010, 14:08   #8
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Quote:
Originally Posted by roadracer View Post
Thanks Dr.AD.Say if the gear ratio for the first 2 gears of a bike is as below. How this needs to be interpreted?

First Gear - 3.000
Second Gear - 1.750
For simplicity's sake, let us assume that the engine is connected directly to gearbox, and the gearbox is then connected directly to the wheels.

Now, the above gear ratios can be interpreted as follows :

First Gear - 3.000 : 1
In the first gear, for every 3.000 revolutions that the engine makes, the wheel will turn once.

Second Gear - 1.750 : 1
In the second gear, for every 1.750 revolutions that the engine makes, the wheel will turn once.


So, for the same engine RPM, the wheel will make more rotations (vehicle will move faster) in higher gears.
This also explains why the engine sounds louder (it is rotating more) in the lower gears , than in the higher gears.
Hope this helps .

Last edited by Blue Thunder : 12th July 2010 at 14:15.
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Old 12th July 2010, 14:19   #9
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Quote:
Originally Posted by roadracer View Post
Guys,Could somebody explain in a simple layman term as to what is meant by gear ratio?
Let me try to put it in simple form.

Forget the cars, cause you don't really see the gears rotating. Check out a bicycle which has its gear clearly visible. For example say an old Atlas cycle.

Say its crank(the tooth wheel attached to the pedals) has 44 teeth and suppose its freewheel has 11 teeth. Now when the pedal goes one round, the wheel will go 4 rounds - 44/11 = 4. Now depending on the radius of the wheel, you can calculate how much distance you would cover in one pedal circle.

All this and much more goes inside the gearbox of an automobile.

Hope it helps.
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Old 12th July 2010, 16:55   #10
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Quote:
Originally Posted by Blue Thunder View Post
For simplicity's sake, let us assume that the engine is connected directly to gearbox, and the gearbox is then connected directly to the wheels.

Now, the above gear ratios can be interpreted as follows :

First Gear - 3.000 : 1
In the first gear, for every 3.000 revolutions that the engine makes, the wheel will turn once.

Second Gear - 1.750 : 1
In the second gear, for every 1.750 revolutions that the engine makes, the wheel will turn once.


So, for the same engine RPM, the wheel will make more rotations (vehicle will move faster) in higher gears.
This also explains why the engine sounds louder (it is rotating more) in the lower gears , than in the higher gears.
Hope this helps .
Thanks Blue Thunder. From the gear ratio,can I deduce at what RPM the gear shift should happen among the individual gears?
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Old 12th July 2010, 17:37   #11
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Originally Posted by roadracer View Post
From the gear ratio,can I deduce at what RPM the gear shift should happen among the individual gears?
I doubt. I believe it would depend upon engine torque & power curves also. Might be a few other parameters.

Query -> When a review says that the 3nd gear is 'tall', my interpretation is, the speed-'range' that the 3rd gear would support is wide. My question is how is the tallness/shortness of the gear associated with the gear-ratio (if at all)?
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Old 12th July 2010, 17:47   #12
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@BlueThunder - Very simple explanation to help me understand the point. Taking a motorcycle as an example, isn't it the ratio between the front & rear sprocket wheel? What's that calculation called?
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Old 12th July 2010, 18:46   #13
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Quote:
Originally Posted by roadracer View Post
...From the gear ratio,can I deduce at what RPM the gear shift should happen among the individual gears?
gear ratio, along with other parameters like engine power/torque curves.
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Old 12th July 2010, 19:22   #14
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Quote:
Originally Posted by roadracer View Post
Thanks Dr.AD. I think I am beginning to get a feel of it. What is the significance/importance of the gear ratio? Is there something like good gear ratio or bad gear ratio?
Very simple to experience the difference in gear ratios.

Drive a good Suzuki - SS80 for over 20 kms and get the proper feel of it.

Later

Drive the Maruti - SB308 for over 20 kms and get the proper feel of it.

You will know the difference on how a closed ratio gearbox works against the simple ordinary gearbox.

Do tell us the difference.
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Old 13th July 2010, 14:56   #15
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All, Many thanks for the inputs. I have a fair understanding of gear ratio now.
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