TYRES: Bus mass limits

By: Steve Skinner

From extra wide bus wheels to simply keeping the wheels on

TYRES: Bus mass limits
Super single truck wheels and tyres, being advocated for bus tag axles

It’s one of the modern dilemmas of coach operators in particular: increasing body weight running up against bus mass limits.

For Michael Musgrave, however, there is a simple solution: extra wide wheels on the tag axles, with a small corresponding increase in mass limits.

Pre-sales Technical Services Manager with Scania Australia, Musgrave says manufacturers have been butting heads with road authorities on the issue for many years, but they’re not giving up.

"It’s been going on for a long time," Musgrave says.

"We probably resurrect it with the authorities roughly every two years … but we’re still not getting very far with it."

Musgrave believes the case is compelling to allow 11.75-inch wheels on the tag axle -  the lazy axle behind the drive – rather than the standard 8.25-inch bus wheels.

It should be noted that Musgrave as an engineer would usually speak in metric, but imperial measure of inches is the worldwide language of all things wheels.

He says operators in Europe and New Zealand are allowed to run 11.75-inch tag axle wheels with higher axle mass limits than in Australia.

"If we were allowed to fit the wide wheels on the tag axle it would give us a better weight capability, they’re sort of on the limits now," Musgrave says.

"With all the extra weight that’s in coach bodies these days, as far as Australian Design Rules (ADR) issues like rollover, seats, and toilets are concerned – and then of course there’s increasing comfort levels – to try and control that weight is getting very difficult for the body builder, and being allowed to carry a little bit more weight over that axle group with 11.75-inch wheels would be a huge advantage.

"Being a wider rim it’s allowing a wider tyre to spread that load and potentially cause less damage to the road pavement … because you’re spreading a similar weight over a broader footprint."

Musgrave points to another reason he believes the axle group and total mass limits should be increased by a tonne or two: to legitimise what’s already happening in reality now.

He says the authorities in Europe and New Zealand assume an average passenger weight of 80 kilograms, whereas in Australia it’s a much slimmer 65 kilos.

"By the time you put real people on there with their luggage, I wouldn’t be surprised if in most cases the coach is borderline overweight or actually overweight," Musgrave says.

Meanwhile wouldn’t there be a big problem for an operator if a wide tag tyre blows?

"The availability of the 11.75-inch wheel is quite good because a lot of trucks are using them on their steers," answers Musgrave. These wheels hold the "super single" tyres that have been around in trucks for many years; "15-inch" in the old language or "385/65" in the new.


Meanwhile another European innovation involving tag axles is allowed in Australia, and is increasingly being used.

Steerable tag axles for city buses and coaches are an option on Scanias, Volvos and Mercedes-Benz, and have been used in Europe for many years.

The tag steer wheels turn at the same time as the front steers, but at different angles according to the situation.

"It’s really just the advantage of manoeuvrability, particularly for around town to get out of tight places, but even out on the road, and particularly with winding roads, the steering and movement of the vehicle is better controlled," Musgrave says.

There’s also less scrubbing of not only the tag tyres, but the drives as well.

However, drivers have to be extra-careful in pulling away from kerbs, because the rear ‘kickout’ is bigger than normal, but only by a few hundred millimetres.

Nevertheless this is enough to highlight discrepancies in state overhang allowances – for example New South Wales is more generous than Victoria.


Almost all the wheels that come to Australia on Scania coach chassis are Alcoa forged aluminium, whereas their city bus cousins generally have steel wheels.

A big advantage of aluminium wheels is obviously their lighter weight.

The Bus Industry Confederation (BIC) says they can save 100kg per bus.

They also allow better dissipation of heat from the brakes and tyres.

"So tyres run cooler, which prolongs the tyre operating life," Musgrave says.

However, alloy wheels are more expensive and Musgrave says that "early in the piece" there was concern about keeping aluminium wheels looking clean.

"But of course there are now wheels available which have special surface finishes which means you just wash them with good quality soap and rinse it off."

A more significant issue is the conventional wisdom which Musgrave agrees with, that it’s easier to damage aluminium wheels compared with steel.

"Operators are worried about drivers kerbing the wheels, running the wheels into the gutter and damaging them," Musgrave says.

Nevertheless Technical Manager for the BIC, Luke Hardy, says the impact resistance of alloy wheels has improved greatly, and there are many operators running them in city service.


What concerns Hardy regardless of the type of wheel, is the small risk of wheels actually flying off.

"From time to time it happens," Hardy says, ominously.

So it’s understandable that Hardy warns about nuts coming loose.

"This is an issue with the ISO type wheel nuts and yellow indicators starting to be widely used," Hardy says.

He says these ISO nuts have been around for about a decade on disc wheels, and are a flat nut with an encaged flat washer.

The yellow indicators clip over nuts, and when the pointy ends are not facing each other, you know you have a problem.

Another type of simple gismo called a ‘link retainer’ is spring-loaded and clips over two nuts, helping prevent them coming loose.

Of course these precautions shouldn’t be needed if nuts are tensioned correctly.

"You have to get the torque settings right," Hardy says.

To this end he points out that a lot of chassis manufacturers recommend re-tensioning after a certain period, say four hours after a wheel has been changed.

Hardy advises operators or their tyre service providers to check with the manual and use a tension wrench.

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