Why is a diff geared
Discussion
For efficiency and the packaging of drivetrain rotating components, you want to keep the rotational speed as high as practically possible to the last minute. This means you can transfer the most Power with the smallest Torque. This enables you to use smaller shafts/bearings/gears etc!
On certain WRC cars, the engine rpm is actually increased before it enters the transimission to help reduce gear sizes etc
The rear diff of a RWD car is pretty much the last availible location to "downspeed" the transmission without going to outboard hub reduction gears (and all the unwanted unsprung weight and complexity that brings) (although certain off-road and comercial vehicles do this)
On certain WRC cars, the engine rpm is actually increased before it enters the transimission to help reduce gear sizes etc
The rear diff of a RWD car is pretty much the last availible location to "downspeed" the transmission without going to outboard hub reduction gears (and all the unwanted unsprung weight and complexity that brings) (although certain off-road and comercial vehicles do this)
Actually far from being a stupid question I think this is one of the best questions I've seen on here for a while. It made me stop and think about something I hadn't really considered before anyway. You do indeed just sort of take it for granted that diffs are normally in the 3:1 to 4:1 range.
There's probably a bit more to it than has already been said. Traditionally in rwd gearboxes which of course was all gearboxes for many years, top, or 4th in a 5 speed box, is a straight through gear where the drive is direct from input shaft to output shaft so saving a couple of percent transmission losses and increasing power to the wheels. Once you mandate that ratio the rest of the gearing has to follow from engine rpm, top speed, and tyre radius and it all ends up with the diff needing to be in the normal range.
Secondly, as already said, if all the gearing was done inside the gearbox you'd need one of the gears in the lowest ratio to have a very large number of teeth which means a physically big gear and a consequently big gearbox which causes packaging problems. For instance my Focus has a 4.062 diff and a 3.67 1st to achieve the desired 30mph or so at peak rpm in 1st gear to make sure the car can get off the line easily without bogging down. If the diff was 1:1 then 1st gear would need to be 14.89:1 which would be impossible to package. One of the two meshed gears in 1st would need to be a foot or more in diameter. That then mandates the shaft spacing and means all the other gears have to be huge. You'd never fit anything like that into the car. The gearbox would be bigger than the engine. The inertia of gears that size would also absorb a massive amount of the engine's horsepower during acceleration.
Finally big heavy gears like that spinning round at high speed would increase the load on the synchromesh mechanism which has to slow the gears down to match speeds for smooth gearchanges. Better to have a good part of the total ratio in a constantly meshed diff and a smaller, easier to design box.
The final answer comes out as being the best compromise is to have the total 1st gear ratio somewhat evenly split between the diff ratio and the 1st gear gearbox ratio which is why in most cars they are similar numbers. Swing the equation too much in either direction and you end up with either a huge diff or a huge gearbox.
There's probably a bit more to it than has already been said. Traditionally in rwd gearboxes which of course was all gearboxes for many years, top, or 4th in a 5 speed box, is a straight through gear where the drive is direct from input shaft to output shaft so saving a couple of percent transmission losses and increasing power to the wheels. Once you mandate that ratio the rest of the gearing has to follow from engine rpm, top speed, and tyre radius and it all ends up with the diff needing to be in the normal range.
Secondly, as already said, if all the gearing was done inside the gearbox you'd need one of the gears in the lowest ratio to have a very large number of teeth which means a physically big gear and a consequently big gearbox which causes packaging problems. For instance my Focus has a 4.062 diff and a 3.67 1st to achieve the desired 30mph or so at peak rpm in 1st gear to make sure the car can get off the line easily without bogging down. If the diff was 1:1 then 1st gear would need to be 14.89:1 which would be impossible to package. One of the two meshed gears in 1st would need to be a foot or more in diameter. That then mandates the shaft spacing and means all the other gears have to be huge. You'd never fit anything like that into the car. The gearbox would be bigger than the engine. The inertia of gears that size would also absorb a massive amount of the engine's horsepower during acceleration.
Finally big heavy gears like that spinning round at high speed would increase the load on the synchromesh mechanism which has to slow the gears down to match speeds for smooth gearchanges. Better to have a good part of the total ratio in a constantly meshed diff and a smaller, easier to design box.
The final answer comes out as being the best compromise is to have the total 1st gear ratio somewhat evenly split between the diff ratio and the 1st gear gearbox ratio which is why in most cars they are similar numbers. Swing the equation too much in either direction and you end up with either a huge diff or a huge gearbox.
Max_Torque said:
For efficiency and the packaging of drivetrain rotating components, you want to keep the rotational speed as high as practically possible to the last minute. This means you can transfer the most Power with the smallest Torque. This enables you to use smaller shafts/bearings/gears etc!
On certain WRC cars, the engine rpm is actually increased before it enters the transimission to help reduce gear sizes etc
The rear diff of a RWD car is pretty much the last availible location to "downspeed" the transmission without going to outboard hub reduction gears (and all the unwanted unsprung weight and complexity that brings) (although certain off-road and comercial vehicles do this)
in the case of off-roaders the hub reduction is usually a 'portal' hub though for ground clearance purposes ...On certain WRC cars, the engine rpm is actually increased before it enters the transimission to help reduce gear sizes etc
The rear diff of a RWD car is pretty much the last availible location to "downspeed" the transmission without going to outboard hub reduction gears (and all the unwanted unsprung weight and complexity that brings) (although certain off-road and comercial vehicles do this)
stabbed rat said:
Max_torque, What happened to your Ibiza R-SR thread? are you still building it or is it on hold?
Still here:http://www.seatcupra.net/forums/showthread.php?t=1...
recent update as to progress to date ;-)
Max_Torque said:
stabbed rat said:
Max_torque, What happened to your Ibiza R-SR thread? are you still building it or is it on hold?
Still here:http://www.seatcupra.net/forums/showthread.php?t=1...
recent update as to progress to date ;-)
Thanks 
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