To cut to the chase, I believe that experiencing the allure of the GR Yaris’s four-wheel drive is more enjoyable on surfaces with uneven friction coefficients.
By the way, “μ” is a term used in mechanics to denote the coefficient of static friction. Generally, a higher μ means less slipperiness.
On Dunlop’s website, the coefficient of friction for dry asphalt is around 0.8, for wet surfaces it’s between 0.6 and 0.4, for snowy roads it’s between 0.5 and 0.2, and for icy roads it’s between 0.2 and 0.1.
It’s worth noting that μ is not only determined by road conditions but also by tire conditions (such as material and tread).
So, winter tires are designed with materials and tread patterns that increase the coefficient of friction on snowy roads.
Regarding icy roads, it’s not that the tires slip because of a low μ, but rather because the weight of the vehicle melts the ice, turning it into water, which creates a barrier between the tires and the road surface, preventing proper traction.
Water acts as a lubricant and can lead to hydroplaning. It seems like hydroplaning could occur.
Returning to the main topic, the GR Yaris is not designed for extreme off-road conditions like the Jimny; instead, it’s meant for flat roads with gravel, small puddles, uneven surfaces, and manholes scattered about (in other words, uneven μ) that it can traverse at high speeds.
It’s on roads with uneven μ that the GR Yaris RZHP truly shines, as I realized during my previous drive in rough weather.
However, we often tend to think that μ on roads is uniform, which is probably incorrect in everyday reality. In reality, there are various μ values scattered randomly.
On circuits, for example, the course is built on a homogeneous high-μ surface, so on a clear day, most parts of the track would likely have similar μ values.
Therefore, on circuits, whether it’s 4WD or 2WD doesn’t matter. With high-grip tires, even 2WD can handle steering well and provide solid traction, making it faster due to its lighter weight.
On rally courses or regular roads we drive on, there are countless factors like puddles, rain, residual snow, gravel, small stones, paint, manholes, rainwater flowing from the center divider to the shoulder, etc., making the μ of the four contact patches of the wheels uneven.
The μ values differ between the right front, left front, right rear, and left rear tires. And as the tires rotate and the car moves, the contact patches change, causing each wheel’s μ to probably change rapidly and independently.
Furthermore, real-world roads have irregularities within the acceptable range (as roads), further complicating matters. If the tire doesn’t make contact with the ground due to a depression, traction won’t be effective, regardless of how high the ground’s μ is. (This is related to suspension. I plan to explain it separately someday.)
In such situations, with a 2WD car with an open differential, if one driving wheel slips and loses traction, the torque is transferred to the wheel with less traction, resulting in a situation where the driven wheel doesn’t receive power, despite having the desired load on it.
Hence, the necessity of LSD arises from such situations.
Therefore, in a 2WD car with an open differential, if one driving wheel slips, it effectively becomes 0WD.
It might look like it’s running, but in reality, it’s just moving due to inertia.
However, with a LSD-equipped 2WD, it can continue to run with one driving wheel even in similar situations.
However, that’s limited to a maximum of two wheels. Well, that’s obvious. It’s 2WD after all.
Even if a non-driving wheel is in contact with the ground, traction won’t be applied.
How about in the GR Yaris RZ?
With features like a Torsen differential or an electronically controlled multi-plate clutch connecting the front and rear, the GR Yaris RZHP can apply traction to the ground with more driving wheels than a 2WD car, even on surfaces with uneven μ and irregularities.
Unless it’s in a situation like a jump where all four wheels are off the ground, it won’t become a 0WD situation.
If any tire is in contact, there’s a 100% chance that it’s a driving wheel.
And under ideal conditions, traction can be applied to all four wheels.
Unless there are very specific conditions, 4WD has twice the chance of meeting driving wheels and high-μ (tractable) ground compared to 2WD.
This is why on roads with uneven μ, 4WD has an advantage over 2WD.
In ideal places like circuits where μ is extremely homogeneous and irregularities are artificially minimized, 4WD doesn’t have an advantage over 2WD.
Oh, let’s summarize a bit. On roads with μ of zero, any drivetrain is meaningless. And also, if μ is high and uniform everywhere on the road, the difference in drivetrain doesn’t matter much.
The decisive difference between 4WD and 2WD occurs in situations where the road or course is formed as a collection of surfaces with uneven μ.
In such roads or courses, the GR Yaris RZHP truly demonstrates its strengths.
So, time trials on circuits frequently seen in magazines or YouTube, or drag races on runways, may not be very meaningful as performance evaluation tests for the GR Yaris RZHP.
Because those places lack irregularities and are made of homogeneous high-μ materials.
Tire grip and engine power (and driver skill if it’s manual) are what matter. The drivetrain doesn’t matter much.
So “Mr. Theory” plans to deploy the GR Yaris during rough daytime weather in the future. Nights are dangerous, you know.
That’s it for this time.
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