EV tyres — you’ve got a lot to learn

by Paul Murrell   WE made a presentation to a large group of people some months ago and were surprised to realise just how little most of th...

by Paul Murrell

 

WE made a presentation to a large group of people some months ago and were surprised to realise just how little most of them knew about electric vehicles. This is hardly unexpected since we have been exposed to a barrage of publicity about EVs, and almost all of it has been positive (as you’d expect from PR). The negatives (and there are a number of them) have been conveniently overlooked in the enthusiasm to convert us all to electric propulsion.

One matter that caused concern among the audience was the need for EVs to run on tyres designed for them, and inevitably, EV-specific tyres are more expensive than those for internal combustion engine (ICE) cars.

Most readers will know and understand that tyres have always been a compromise, a delicate balance of grip, longevity and noise. High performance tyres will provide higher levels of grip, but wear more quickly (thanks to the softer compound). Long life tyres use a harder compound but offer lower levels of grip, and often more road noise. EVs add a further element to the mix: range.

Tyre selection is crucial.

  Tyres for electric vehicles have different requirements than those for an ICE vehicle. For a start, EVs tend to be heavier, so the tyre needs to be able to handle the extra weight. EVs don’t have an engine to mask road noise, so a tyre for an EV needs to run more quietly than one fitted to an ICE vehicle. This is achieved by using specialised tread patterns, sound absorbing foam and different rubber compounds.

Tyres for an EV must be able to withstand strong initial acceleration and high output from the instant the driver hits the accelerator.

To cope with this, tyres suitable for EVs have specific stiff and wide centre rib patterns to reduce potential slippage and abnormal abrasions. Also, interlocking grooves in the tread pattern minimise hydroplaning and compensate for the higher torque. EV tyres must offer reduced rolling resistance to minimise energy loss. This is achieved using different techniques during the manufacturing process, such as a different rubber compound, and including rigid design patterns, tyre profiles and structures. When you purchase new tyres for your EV, you have to decide whether to forego efficiency, range, grip or long life. Quite simply, you can’t have it all. The best advice is usually to replace your tyres with the same tyres that were fitted when new.

Michelin ePrimacy

Tyre companies and car manufacturers spend countless hours and dollars ensuring OE (original equipment) tyres deliver the best combination of features. Choosing a tyre rated to wear less could affect the range by as much as six to nine percent. If your EV has a theoretical range of 600km, you could reduce your range by as much as 55km, which in turn could mean an additional charging stop. And that’s using new, unworn tyres. Replacing worn tyres with tyres optimised for long wear or maximum range could increase that difference to 15 percent or more. Even replacing worn tyres with identical new tyres can increase range by six percent. The difference is compounded if you choose a tyre with a high rolling resistance (an important factor in EV range). An A-rated tyre (the best rating for rolling resistance) can add up to 15 percent to your range over a D-rated tyre (offering poor rolling resistance performance).

It may seem logical that tyres that wear down more slowly would be less efficient because you’d assume a slow-wearing tyre is offering lower rolling resistance. In fact, it’s the opposite.

The whole range issue comes about because a tyre deforms and loses energy through the contact patch. This makes the ride more comfortable. If you want a tyre to last longer, the simplest solution is to add more rubber. But that becomes more rubber to deform, and more energy loss. Making the compound more wear-resistant is, unexpectedly, also bad for rolling resistance. It’s the flex of the tyre carcass at the contact point that determines how much energy the tyre loses. Because a softer rubber flexes more easily, it loses less energy and extending range.

Bridgestone Turanza Eco

All tyres generate noise. However, in a car with a petrol or diesel engine, the tyre noise is disguised by engine noise. Not only do EVs not have an engine masking the tyre noise, the problem is made worse because manufacturers use less sound deadening, because it’s not so important, and it helps reduce weight.

To reduce tyre noise (and with Australia’s coarse chip road surfaces, it’s even worse here), tyre companies are constantly researching ways to make their tyres quieter.

The size and shape of the blocks in the tread pattern have an effect on tyre noise. So does the compound. Careful attention to these factors allows the tyres to be acoustically tuned. Another trick is to add foam to the interior surface of the tyre, in much the same way that recording studios put foam on the walls. Increasingly, tyres on modern cars are not interchangeable. Of course, there are usually plenty of brands and types of tyres that will fit your car, but not all of them are suitable. The simplest advice is to replace your EV’s tyres with the same tyres it was fitted with from new, especially if you have been happy with their performance.

Hankook Kinergy

Often, EV owners will opt for ultra-high-performance tyres to increase the level of grip on offer. However, as mentioned earlier, rolling resistance is a critical factor in EV performance and efficiency. It’s even more important than in an ICE vehicle because EVs are so much more efficient than a petrol or diesel engine car. EVs lose considerably less energy to heat loss and drivetrain friction, making rolling resistance more of a factor.

Because EVs are typically about 10 to 20 percent heavier than an equivalent ICE vehicle, tyre wear can be as much as 20 percent higher.

But weight isn’t the only factor. EVs deliver much more torque, and this, plus regenerative braking, put much more stress on tyres. As described in the previous section, noise is also a consideration. Tyres designed for use with EVs have been designed to be quieter, because of the inherent lack of engine and drivetrain noise that mask tyre noise in conventional vehicles. When choosing new tyres for your EV, the load rating is very important. A new “high load” tyre has been specifically developed for EVs to cope with the additional weight of heavy batteries. Look for the HL marking on the tyre, even if the tyre is dimensionally the same size as the one you’re replacing. A tyre’s load rating is directly related to its safety, ensuring the tyre can withstand the strains and stresses if endures under braking, acceleration and cornering.

It's all about research.

Okay, we’ve established that tyres for an EV are even more expensive than tyres for conventional vehicles, so you’ll want to look after them, and ensure they give your EV the best possible range.

• Underinflated tyres

Tyres running at less than the recommended pressure, even by just a few PSI, will wear more quickly. However, for an EV owner, there’s another issue: underinflated tyres have a higher rolling resistance than tyres at the correct pressure. So your range will be adversely affected.

• Using tyres not intended for use on your car

Original equipment tyres have their noise profile, longevity, grip and rolling resistance optimised for the vehicle they are fitted to. The inevitable compromises have been assessed to please the majority of users. If you want something more or different, remember you’ll be making trade-offs: higher grip may improve performance and braking, but often with increased noise and reduced range; tyres that wear slower and last longer may result in reduced grip and/or longer braking distances.

• Using tyres not designed for EVs

Many will be tempted to save a few dollars (or, in this case, quite a few dollars). Don’t fall into this trap. Tyres that look outwardly similar can be quite different. EVs convert about 80 percent of their electricity into motion, unlike ICE vehicles that can only manage about 25 percent. That makes rolling resistance a much more important factor – about three times the effect it has in conventional vehicles. And tyres designed for use on ICE vehicles will wear more quickly, and not be as safe when used on an EV.

You could just fit the tyres that your car came with.

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