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space savers - TUSTs

introduction

In May 2003, RoyalAuto highlighted how the trend towards the use of temporary use spare tyres (TUSTs) found in many new cars was compromising vehicle performance and creating inconvenience for motorists. The practice of fitting the small, skinny and yellow space savers in sports cars has now grown to the point where there is a wide array of temporary tyres, fitted to a wide range of vehicles including 4WDs and family cars. Unfortunately a large proportion of motorists still learn about the presence of a TUST upon first changing the tyre.

RACV position

All motorists should check their vehicle for the presence of a TUST and familiarise themselves with the handling limitations of driving with a TUST. This applies in particular to:

  • vehicles not equipped with ABS or stability control,
  • 4WD vehicles,
  • use in rural and remote areas, where distances are greater,
  • towing caravans or trailers,
  • off-road operation, and/or
  • use in ice or snow.

When purchasing a new vehicle, car buyers should:

  • seek information from the dealer as to whether the vehicle is fitted with a any form of temporary use spare tyre: the sales person will probably not raise the issue, and
  • consider insisting on a full size spare as a condition of sale if the vehicle is capable of taking a full-size spare.

In the event where a TUST is required to be fitted to a car, motorists should only drive it for the minimum distance required to repair/replace the damage to the full-size wheel, and drive at all times within the conditions specified in owner's manual

background

Temporary Use Spare Tyres (TUSTs) - spare wheels that are not the same size as a vehicle's on-road wheels - have grown from being a viable alternative in space-starved sports cars to standard issue for anything up to roomy four wheel drives.

The most widespread TUST is the traditional space saver tyre. These tyres are highly prominent due to their reduced sectional width, reduced rolling radius, and brightly painted rim. While it is more common to find one of these tyres on a luxury car or sports car there is an increasing trend to fit these on many popular small cars too. Refer to figure 1 for an illustration of the types of TUST.

The term small wheel spare tyre applies to a smaller wheel sourced from a less luxurious version of the same model. These tyres may look deceptively like full size tyres, however an 80 km/h speed rating usually applies because of the compromise to the vehicles handling when it is fitted.

Other types of TUSTs may even be supplied deflated and folded up into the boot to save more space.

compromise to vehicle handling

While TUSTs come in many different forms, it is the traditional space saver that will most significantly compromise both motorist's convenience and handling of their vehicles. The problem starts with the space saver tyre having a smaller rolling radius and sectional width than a standard spare tyre. This results in a dynamic deficiency during emergency braking, emergency swerving and cornering.

RACV conducted three tests on vehicles from four different market segments to assess vehicle handling when a TUST is fitted. Three involved traditional space saver spare tyres, and one was a smaller wheel spare tyre.

The worst result was attributed to a vehicle running on a space saver tyre and not equipped with ABS. The space saver tyre increased braking distance by 15.4 metres or three and half car lengths when fitted to the front axle.

The cornering traction of the tyres was assessed by cornering the vehicles on a 50 metre circle at a proving ground. Cornering traction suffered appreciably when a traditional space saver was fitted; particularly on heavier vehicles such as large 4WD where the deterioration in grip levels was 13.5 percent.

The International Standards Test for Emergency Lane Change (the "Moose test") indicated that fitting a traditional space saver significantly increased the difficulty of emergency swerving without the vehicle becoming unstable.

Keeping control of the vehicle under emergency braking and swerving was also increasingly difficult. The tyre continually locked up when attempting to stop quickly causing the vehicle to pull to one side of the road, and leaving significant "flat spots" on the tyre.

In comparison, small wheel spare tyres perform almost equivalently with regular road wheels.

inconveniences to motorists

One mitigating argument for TUSTs is that housing a full size tyre absorbs boot space that might otherwise be used for cargo. What remains puzzling though is that in some cases these cars have room for a full size tyre. Alternatively the space saved by the tyre is packed out with foam to keep the boot floor flat.

While perhaps suitable for use on European or Japanese roads the life of some of these tyres is questionable given Australia's expansive road network. RACV testing has revealed that a common type of space saver tyre is only capable of around 450 km of road driving before it becomes un-roadworthy. Motorists may easily cover this distance in country driving because many rural tyre centres do not stock a full range of replacements

Traditional space saver spare tyres are difficult to replace as our investigations have revealed that they are not stocked at most tyre centres or distributors, are sometimes not even stocked in the country, and can cost between $135 and $270 to replace. By not having replacement space saver spare tyres readily available, many motorists may be forced to retain the spare tyre even when it is unroadworthy.

roadworthy requirements

Fitment and driving on a TUST is acceptable under Victorian roadworthy regulations as long as it is used within the specifications provided by the vehicle manufacturer in the vehicle's handbook. However, two TUSTs cannot be fitted to any one vehicle and the vehicle cannot be presented at a roadworthy inspection with a TUST fitted to one of its axles.

insurance

Insurance claims involving TUSTs are treated like any other insurance claim whereby the contributing factors are determined individually for each case. While TUSTs do not automatically exclude claims for approval, the insurer may determine whether the tyre was used within the specifications (e.g. speed requirements, fitment to a specific axle) of the vehicle manufacturer when assessing the claim.

 

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