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
4WD and AWD have really got mucked up by marketing teams, but if we take the more traditional routes with AWD being always on with a center differential and 4WD being engagable with a Transfer Case in the middle then you can start to see problems
For an AWD system with a center differential there is also a front and a rear differential so the wheels can spin at different speeds if they need to incase one gets stuck or is just on the inside of a sharp corner.
The Transfer Case does not permit differences in speed between the front axle and rear axle, the average speed of the rear wheels needs to match the average speed of the front wheels, but when you go around a corner this isn’t usually the case. If you only use your 4WD in slippery conditions then this isn’t an issue because at least one of the wheels will slip on the mud/snow/ice, but if you run 4WD on dry grippy roads then every time you go around a corner there is a lot of twisting force being applied in the transfer case as the front and rear wheels attempt to travel at different speeds, in sharp corners you can hear the wheels skip as they lock up and have to break free from the road.
when you turn, your inside tire needs to spin at a different speed than your outside tire. 4WD doesn’t allow this on pavement because 4WD locks your differential. you scrub your tires and put stress on your drivetrain.
I wouldn’t say it’s particularly bad, but under certain conditions, less beneficial.
In a 2WD model, the engine only needs to power 2 wheels. In a 4WD model, the engine has to power 4. This can put more stress on the engine, and due to yhe increased weight of a 4WD drivetrain, it also increases fuel consumption.
Now, 4WD is good for offroading and icy or wet road conditions. All it really does is provide more traction where it’s due. Under normal conditions, it can help you accelerate faster than a 2WD vehicle, but more often than not, at an increased fuel consumption.
Traditional user-selectable 4WD systems found in trucks (typically) are designed to be engaged by the user – either via transfer case or by locking the center differential. When engaged, they require the front and rear axles to turn at the same rate. This is great off-road but taking turns on dry pavement results in drivetrain binding/tire skipping/scraping as the wheels try to turn at different speeds while the drivetrain fights it. Naturally this is a lot of stress that doesn’t need to happen and can result in anything between tire wear to something on the vehicle giving up. Same reason you should not drive around town with a differential locked. Great off-road when traction between the tires can vary, but horrible when they all have grip and you have to turn.
This isn’t necessarily applicable to ALL 4WD systems since some may be designed for permanent use and others may have an automatic mode.
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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