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o n , frequently use a 4 x 4 vehicle for
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towing a heavy lorry, following a breakdown. Four-wheel-drive is normally engaged to prevent overloading the rear axle. I have been advised that running on metalled roads with both axles engaged causes rapid wear of the transmission and tyres. What is your view?
AWhen both axles of a four-wheel-drive
vehicle are engaged and the vehicle is run on a metalled road for any distance. "wind-up" of the transmission is incurred because of the high coefficient of friction of the tyre tread on the hard surface. A slight difference in overall tyre diameters varies the effective transmission ratios and this creates higher-than-normal stresses in the transmission components that accelerate wear and tear.
When the vehicle is negotiating a curve, wind-up is produced because of the different radii of the paths on which the front and rear wheels travel. Eventually the transmission "unwinds" and this causes one or more of the tyres to slip. Wind-up and unwinding are repeated at regular intervals and thus increase tyre wear. If a vehicle is travelling at a relatively high speed, wheel bounce will tend to reduce wind-up. An inter-axle differential would obviate wind-up.