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The Extra Mile

If you’re anything of an enthusiast of driving for economy, and a keen fan of this column, you’ll know that the energy in the fuel we use is dispersed as heat, in one form or another. Overcoming air resistance ends up heating the air, the energy dispersed in braking to slow down is turned into heat at the brake friction surfaces, and the energy used at the tyres mostly ends up heating them. We know that low rolling resistance tyres can help save fuel, by reducing the heat generated by physical distortion of the tyre carcass, and tyre/road friction, and the generally accepted picture is that between five and ten per cent fuel savings can be achieved when these tyres are fitted. 

What has come to light recently are some interesting facts about the energy losses at the tyres, and their relationship to front wheel geometry, or tracking, particularly in relation to cars with driven front wheels. Manufacturers specify tracking geometry figures, in terms of millimetres or degrees of toe-in or toe-out when measured in clearly specified conditions, although in reality, the figures actively change on the road, with the dynamic changes of suspension systems. The same goes for the rear wheels, although they are somewhat less critical, unless they are the driven wheels, and are less likely to be disturbed from the correct settings. For minimum tyre scrub though, and therefore tyre wear, and fuel consumption, zero degrees of toe-in/toe-out at the front wheels is an optimum figure, and most front-wheel-drive cars usually have specified settings close to zero toe-in – for the reason of avoiding any of the undesirable torque steer when serious power is applied. Similarly, many manufacturers may specify small amounts of toe-in on rear-wheel-drive cars.

What has been revealed though is that, with the steering geometry as little as a quarter of a degree out (around 2mm with an 18-inch wheel), the tracking error can increase tyre rolling resistance by five per cent or more. Now large errors in tracking should become fairly quickly evident in the form of uneven tyre wear; but, if your fuel economy drops noticeably for no particular reason, it may be that your steering geometry has recently been disturbed, and is costing you serious money in excessive tyre wear and high fuel consumption. Free geometry checks are offered at many garages and tyre depots, although you’re at risk of unscrupulous ones that may particularly exploit, dare we say it, those of the fairer sex, so it is best to use a garage that you know, and can trust.

But we’ve been thinking mainly about the straight ahead steering position, yet the tyres absorb even more energy when we are taking bends, and making steering corrections for all sorts of other road situations. In these conditions, the rolling resistance increases more significantly, due to the significant lateral forces involved, as the tyre scrub angle increases, and it will increase even more so if the steering geometry is incorrect. So those whose motoring mix is based mainly on lower speeds – such as urban commuting, and on 40mph and 50mph speed limit roads – will see the biggest effects, when rolling resistance is a significant energy consumer, and there’s constant steering activity. They need to be most aware of the potential problems of steering misalignment.

Finally, we learn that low rolling resistance tyres are most beneficial when they are transmitting torque, either to the road, of from the road to the brakes, and/or any energy recovery system. So they have proportionately less rolling resistance when accelerating or decelerating, and their benefits are greater than in steady speed driving. With that prospect in mind, the benefits of fitting fuel-saving tyres to the non-driven axles are thus reduced, and it may well be that the optimum cost/benefit ratio comes with low rolling resistance tyres fitted only to the driven wheels. But those whose motoring is stop/start, and mostly at lower speeds, not those who travel fast on open roads at steady speeds, will benefit the most from fitting low rolling resistance tyres, but they should keep an eye out for any signs of uneven wear, and maybe have their tracking checked regularly. 

Victor Harman 

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