One of the imagined appeals of owning a Brodozer MonstroCity FX4 SUV or Truck, is the idea that if you wanted you could drive off into the bush and go camping, and engage in hand to hand combat with kangaroos or something. As I am a 47 year old man who likes watching motor racing, then the algorithms assume that I would also want to fork out an entire year's salary on one of these things.
As a result, the adverts that I get include adverts for the Isuzu D-Max, Ford Ranger, Mitsubishi Triton, Nissan Navara, et cetera et cetera et cetera. What these advertising companies fail to realise, is that I do not have 120kg children and do not need a Brodozer MonstroCity FX4 to do the school run. What I actually want would be a Gemini, Fiesta, Lancer, or Pulsar. The ironic thing is that my Mazda 2 is actually a more capable off-road vehicle than the D-Max, Ranger, Triton, Navara, et cetera et cetera et cetera, and it all comes down to the black art of tyres.
My Mazda 2 sits on 185/65 R15 tyres. Quite frankly I think that 15 inch tyres on a car this small is absurd, and that the 165/65 R13 on the Ford Ka that I used to have, were entirely adequate.
However, as I walk to the bus stop in the morning, I pass a Brodozer MonstroCity FX4 truck which sits on 255/25 R21 "all terrain" tyres, and the really really dumb thing is that I know that there is no way in Hades, Sheol, or Abaddon, that this monstrosity would ever leave the black top, let alone drive on whatever surface "all terrain" imagines. This is for the simple reason that it would be deeply unpleasant.
A motor vehicle is suspended upon tyres, springs and shock absorbers. Spring and shock absorber return and damper rates are determined by woo-woo, goblins, numbers, and imagination. However, the tyres and the wheels themselves are pretty easy to understand.
In a tyre specification:
XXX/YY R ZZ
XXX is the width of the tyre in millimetres.
YY is the aspect ratio; that is the height of the sidewalls expressed as a percentage, relative to the width.
ZZ is the size of the wheel, expressed as a 'wheel size' which just happens to correlate 1:1 to inches (what a coincidence).
The important number here when it comes to the ability of a tyre to drive across rough surfaces, is the aspect ratio YY. The bigger the number, the more space that there is for the tyre to flex and deform in the vertical dimension; which is what you want when the car is suspended above. If we set aside the tyre pressure for a bit (because higher tyre pressures will give you a rougher ride), then the aspect ratio actually becomes the most critical measure when it comes to the ability of a tyre to go over rough surfaces nicely. This is why buses and trucks and actual proper Four Wheel Drive vehicles have nice lovely chunky sidewalls.
Back in the days that we call 'the past', when Auto makers weren't in a race to create the biggest and chunkiest vehicle possible, cars (do you remember those?) used to have tyres with accept ratios of 65, 70, and 75. You could and might think about driving a Falcon to Cameron Corner. A car/truck/SUV thing which sits on 25 aspect ratio tyres, would likely miss Cameron Corner by about 400km which is where the black top runs out.
The Brodozer MonstroCity FX4 truck thing that I walk past, sits on 255/25 R21 tyres; which means that instead of the tyres being asked to do the work of suspending the vehicle, the springs and shock absorbers are instead. The fun thing about tyres is that as they are filled with air, they are a lot squishier that the gas inside shock absorbers, or the compressibility rates of the metal in a spring. Whereas my Mazda 2 can, has done, and will continue to drive over rough surfaces for fun, this Brodozer MonstroCity FX4 truck thing is highly likely to give you an absolute bone shaker of a ride.
If you like turning your latte into a milkshake through the simple act of driving down Parramatta Road, then 25 aspect ratio tyres are for you. However, don't pretend that this thing or these tyres are "all terrain" because I suspect that even driving over a grassy field is likely to be an unpleasant experience.
What makes this all the more ironic is that Isuzu in selling their D-Max and MU-X uses the tag line from the Fleetwood Mac song "Go Your Own Way". Selling their D-Max and MU-X on 25 aspect ratio tyres, disqualifies their own tag line. You can not go your own way if you can not leave the pavement.
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