So i was reading a bit about 4wd vs awd but i couldn’t understand why using 4wd under normal circumstances is bad. Most articles were pretty vague and basically just said dont do it. Why specifically is it bad for a vehicle?
In: Engineering
Vehicles with an all time all wheel drive system (not to be confused with most AWD systems, which are 2WD until they detect slippage) use special differentials which allow the wheels to spin at different speeds without damaging the differential. Below is a link to one such technology. You will hear a lot of people talk up the ‘torsen diff’, it is an amazing technology.
[https://www.youtube.com/watch?v=JEiSTzK-A2A](https://www.youtube.com/watch?v=JEiSTzK-A2A)
If you ‘lock’ the diff, as is the case in many selectable 4wd systems, both wheels will turn at the same speed regardless of the traction or the g-forces. This is fine, even preferred, if you are at low speed and need to get out of mud or snow or something. However, if you are going around a corner at 60 MPH+ this will cause a lot of force to be exerted on one wheel, which will cause the tire to wear out very quickly. In those cases, you want what is called an ‘open diff’ which will allow the wheels to turn at different speeds as you go around a corner;
[https://www.eaton.com/us/en-us/products/differentials-traction-control/open-differential.html](https://www.eaton.com/us/en-us/products/differentials-traction-control/open-differential.html)
So, often, if you hear a 4wd offroading nerd talk about their awesome new Land Rover, they will mention that you can ‘lock the diff’, and other nerds will nod in agreement. In this application, locking the diff can be the difference between getting up a hill and not.
If you hear a german car nerd talk about having a ‘torsen diff’ system (as opposed to the Borg Warner Haldex system) and other nerds nod in agreement, they are talking about an AWD system that will help you get around a corner.
If you don’t care about cornering like a mad-man or giving people the ‘full Audi experience’ (look it up) then the typical AWD system is on-demand. It will remain off until a computer detects slippage at the front wheels and it will engage the rear wheels. I used to have a 2012 VW Tiguan which had this type of system and by the time you consciously realized that you were in slipping conditions the car would work itself out, provided you have good tires then you really don’t *need* anything different than that. In fact, most Audi Quattro systems in passenger cars are now Quattro ‘ultra’, which is this kind of system. Mercedes AWD is very similar in this regard. I know a bunch of enthusiasts (and the guys from Throttle House) will whine about this incessantly, along with how far over the front axle the engine is mounted – but I challenge anyone to drive at street legal speeds to honestly tell the difference.
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