How are Subarus from the 2000s able to pull trucks and buses out of snow?
In: Engineering
a modern Subaru Ascent has a towing capacity of 2,000-5,000 pounds (for comparison a F150 is 5,000-11,300). Subaru are universally all wheel drive (excluding the sporty BRZ) and have better traction than some 2wd trucks depending on the situation.
Fun bonus fact Sedans in the 70-90’s could tow 7,000 pounds or more.
Mostly the same way that Porche Tractors from the 1930s were able to do it.
A limited slip differential.
Most cars are two wheel drive. A rear wheel drive will have a “drive train” which is a long bar running under the car chassis that reciprocates the spinning of the engine, which then gets transferred to spinning the back wheels.
A front wheel drive will recipient the spinning of the engine directly to the front wheels.
(There’s also all wheel drive cars, which aren’t quite four wheel drive cars, but let’s skip over that)
In reality a lot of cars only had one wheel actually driving the car. Some still do.
In any case, you have to transfer the spinning motion of the engine to drive the wheels via the gear box (gear ratios are something that I barely understand, and aren’t really relevant here).
If you think of a 90° gear pushing this (✓) motion to this (-) motion.
This (/-) motion is lost without complicated gearing.
Complicated gearing will transfer power to both wheels, and is used in most cars today. So let’s think about moving both wheels together.
If you put even power into both wheels and one wheel got bogged, the car would try to put even power into both wheels, and go nowhere slowly. One weel would spin wildly (the not bogged one), and one wheel would not move.
If the wheel that was bogged allowed the differential to “slip” a little bit, then the other would take up the slack.
There’s still the same amount of (✓) motion, but because some of the (-) motion is lost to the gears turning without the wheel turning, the (/-) motion is more.
A Subi Forrester has power going to all four wheels. It is a front wheel drive and a rear wheel drive both at the same time. You will lose a little speed, but gain power.
Both differentials will have a limited slip diff, which means that all four wheels will pull mostly evenly, even in slippery surfaces.
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