What are gear ratios and what does final drive mean?

 

If you think of it in the terms of a ten speed bike, it may be easy to understand. If you look at the gears on the rear wheel, you'll notice that there are several sizes( 5 sizes on a ten speed bike) and then there are 2 larger gears on the crank by the pedals (hence 5x2 = 10 speeds)

Imagine that your rear gears are the gearbox on your car. First gear will be the largest gear. When you put into the largest gear, you'll notice it'll be easy to pedal, so you can accelerate faster. Now, as you shift down into smaller gears you'll notice that it gets more inceasinglly difficult to pedal, but you gain more speed. If there is a big size difference in the gears, you can regard this tall gearing. So every time you switch gears, its hard to build that speed back up. But by the time you get to the smallest gear, you'll be really flying (providing you have strong enough legs to cut through the resistance of the tall gears)

If the sizes in the gear get smaller more gradually, the gear ratio is shorter, and you accelerate quickly. But considering that you only have five gears to go through, your last gear will be relatively big. This is good for getting through the gears quickly and accelerating quickly, but unless you've got fast legs (a high reving engine) you'll reach your terminal velocity quicker with shorter gearing, hence a lower top speed.

Now you can contol a lot of the gearing with the final drive, or the gear a the pedals. If you have a final drive gear close to the size of the first gear...you will accelerate quickly, but top off...and vise-versa... If only we can have a shiftable final drives like ten speeds...hmmmm




[Modified by MiraiZ, 2:52 PM 9/30/2001]


[Modified by MiraiZ, 2:52 PM 9/30/2001]

 

Oh I see.. So the closer your gear ratios are, the faster you accelerate, but then you will have a lower top speed than a car with taller gears? Where does the number from the Final drive come from? Is there some sort of formula?

 

Final drive ratio is the number of teeth on the ring gear divided by the number of teeth on the pinion gear.

-T

 

Newbie, the closer your gear ratios are to EACH OTHER, the faster you will accelerate, provided your car does not produce constant HP over the rev range (it can be said that every gasoline car does NOT). The closer gear ratios mean that you can stay in the best part of the powerband and thus accelerate faster.

There's both short gearing, and tight gearing. Short gearing helps to accelerate, tight gearing helps even more by keeping you in the powerband. The shorter and tigher your gearing, the lower your top speed, but the faster your acceleration.

There are exceptions...a car can have long, tight gearing or short, spaced gearing. You'll find spaced gearing in 3 and 4 speed trannies, and tighter gearing in 6 speeds. A good example of a car with short gearing would be an ITR...long gearing would be a big diesel truck that spins <1000RPM @70mph.

 

Thanks for the response Stephen, it helps alot.. I am kinda getting it now but can't visualize the actual GEARS though.

 

If I want to be at lower rpms at highway speeds with my b16a2 (em1), what kind of final drive would this require? or is there a way to only change the 5th gear ratio?

 

LS or an LS 5th gear. Expect the fuel mileage to be worse, it will cut down the noise a bit though.