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If I my 2007 Forester transmission into its low range, which has a ratio of 1.6 I think, it will accelerate faster (eg. From 0 to 100 km / h). Why is this? below is a picture.

enter image description here

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    Welcome to the site. Could you explain what a "reducer" is? Also, not really sure what your question is here, especially considering I have no idea what a reducer is. – Pᴀᴜʟsᴛᴇʀ2 Nov 20 '16 at 18:04
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    sorry for a bad name , I have found good using wikipedia – Adam Nov 20 '16 at 18:46
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    @Adam by the way your english is pretty good! – Cc Dd Nov 20 '16 at 20:20
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    People should quit downvoting because of bad english! He is talking about low range. he is asking why the car has a faster 0-60 time when it is engaged. People should get used to using the edit button rather than immediate downvote button. – Cc Dd Nov 20 '16 at 20:31
  • @CcDd, yes, it's way better than my Polish :-) – dlu Nov 20 '16 at 23:16
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I added that picture for you. I am not sure if this is the same subaru AWD dual range transaxle your Forester has but it will do for this discussion.

The image below shows the direction engine power takes when the collar selects the forward gear set or the rear gear.

enter image description here

when the collar is moved towards the front of the trans, where the clutch should be, it engages the low range gear set. you can see the power will go through gear one which is smaller through gear 2 which is permanently attached to a short idler shaft with gear 3. gear 3 is again smaller than gear 4 which gives another reduction and off into the rest of the transmission portion of the transaxle.

When the collar is moved backward it simply connects the input shaft to the output shaft and lets the other gears spin freely.

Gear 1 is actually floating on needle bearings and does not engage the input shaft at all. Instead it is that collar that slides back and forth on splines which are locked into that shaft. The collar has teeth to engage the small teeth you can see in the image on the collar sides of the gears. here is an excellent explanation of how these slide back and forth to engage and disengage the different gears.


So that is all great but why does it make the car get up to speed faster? This has an excellent explanation of what torque actually is:

Torque is a twisting force- (it doesn't do any 'work' itself- it is simple an application of energy).

Work (or 'stuff') happens, when torque is applied and movement occurs. "Torque is a force that tends to rotate or turn things. You generate a torque any time you apply a force using a wrench. Tightening the lug nuts on your wheels is a good example. When you use a wrench, you apply a force to the handle. This force creates a torque on the lug nut, which tends to turn the lug nut.

English units of torque are pound-inches or pound-feet; the SI unit is the Newton-meter. Notice that the torque units contain a distance and a force. To calculate the torque, you just multiply the force by the distance from the center. In the case of lug nuts, if the wrench is a foot long, and you put 200 pounds of force on it, you are generating 200 pound-feet of torque. If you use a two-foot wrench, you only need to put 100 pounds of force on it to generate the same torque."

In summary:
Torque equals Force multiplied by Distance

How does gear ratio affect Torque?
Simply put, torque at work (such as at a wheel) is your motor's torque times your gear ratio.
Motor Torque x gear ratio = torque at the wheel
Let's say we have a 10rmps motor that is capable of 5 oz Torque (we know this from our motor spec.)

Let's say we have 2 gears. Our input gear (attached to our motor) has 10 teeth Our output gear has 50 teeth

Our Gear ratio is 5:1

Motor Torque x gear ratio = torque at the wheel

5oz x 5:1 = 25 oz

What if our gear ratio were 1:3 ?

5oz x 1:3 = 1.6oz


great so why does the car start up faster though? I mean the engine is still putting 200 ft lbs to the ground right? not really after going through your 1.6:1 ratio you are actually increasing the effective torque the engine is giving to 320 ft lbs before it even gets to the transmission. This also means your engines RPMs will be 1.6 times higher to reach the same speed.

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The short answer is that in low range the gear ratios are reduced and the engine has to do less work on each revolution – as a result it can accelerate the car faster.

  • thank you dlu for reducing my short novel to one sentence ;) – Cc Dd Nov 20 '16 at 22:22
  • @CcDd, I thought a Cliff Notes version might inspire people to read the novel. – dlu Nov 20 '16 at 22:28
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    you know they have code golf on one stack? we could compete here to see who can use the least word count to accurately answer the questions. >:} – Cc Dd Nov 20 '16 at 22:54
  • @CcDd, I like that idea! It would be fun, and for some readers it might make the site/answers more accessible. We could still have long answers too :-) But how would we count photos? – dlu Nov 20 '16 at 23:11
  • @CcDd, I think we would need to ban the use of '42' as an answer :-) – dlu Nov 20 '16 at 23:13

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