Tags: #ev-parts
Several factors are at play that makes EVs much harder on tyres than most conventional cars. For one thing, EVs are typically heavier than their ICE equivalents thanks to the weight of lithium-ion batteries. A regular Volvo XC40 starts at about 1600kg, but an XC40 Recharge EV adds at least 300kg to this figure.
Then there's the way electric motors deliver their power – in that all of the torque is available from zero RPM. Even in the torquiest of diesels they need to build a head of steam before maximum torque is delivered. Given that even EVs with modest power outputs can have prodigious torque figures, and it's easy to see why they spin up so readily on the wrong rubber.
According to a study by Goodyear in early 2018, EVs can chew through regular car tyres 30 per cent faster than a normal car.
It's not just Goodyear that has come to this realisation. Continental has offered tyres specifically for EV since 2012 when its Conti.eContact was first presented. The newest version of that tyre still comes on the Smart EQ ForFour as standard. Bridgestone, too, has its own EV-specific rubber in the form of the Enliten range which has 20 per cent less rolling resistance than “standard premium summer touring tyres”.
Perhaps the largest and best understood variable (among non-techy people) with tyres is rolling resistance. New tyres have had to display a rating (on an A, B, C etc. scale) for a long time, enabling people to understand how efficiently they roll. The worse the rating, the less efficient a car will be.
Whilst in a petrol car this might mean a couple less MPG, in an EV a 10 per cent difference is very noticeable. American car magazine, Road & Track, shod an e-Golf in sticky, performance-orientated Michelin Pilot Sport 4S tyres and found that its range plummeted from around 125 miles on its specified Bridgestone Ecopia EP422 tyres to below 100 miles on the Pilot Sports.
Harder compounds typically reduce rolling resistance as 90 per cent of energy loss is due to the constant changing of shape as a tyre rotates. However, hard compounds do have the negative effect of reducing available grip. Clever tread patterns come into play here, with the likes of Goodyear maximising the amount of rubber on the road by utilising smaller channels (for water dispersal) whilst not losing any wet weather performance. Bridgestone's Enliten tyres actually have shallower tread yet thanks to the compound don't wear out any quicker.
Tyre shape also helps lower rolling resistance, with EV-specific tyre favouring narrow, tall shapes rather than wide, low-profile sections.
Rotating mass isn't a big issue at consistent speeds, but getting heavy tyres up to speed requires a big slug of energy. This is not much good for efficiency. Continental's EcoContact 6 tyres keep weight to a minimum thanks to clever sidewall construction, whilst Bridgestone's Enliten rubber benefits from that thinner tread to keep the kilograms down to a minimum.
Whilst it might not do much for visual appeal, the best way of reducing rotating mass is specifying smaller wheels, however.
Here's a little test if you're really, really bored; the next time you're walking along a road with a speed limit of at least 40mph think about what constitutes the noisiest element of passing traffic.
As speeds climb above 35mph, tyre/surface noise becomes by far the dominant factor in noise. Below that, at neighbourhood and city speeds, EVs offer a massive advantage in terms of quietness, but above that there is a race to maintain the advantage and quieten EV tyres down. This isn't just for those on the outside either; thanks to the near silence of electric powertrains, tyre manufacturers are battling to keep noise down for the occupants of EVs.
Tyres have been rated for noise, like rolling resistance, for a while, but the technology in EV-specific tyres is advancing quickly. Continental's ContiSilent technology uses a foam inlay within the carcase of the tyre to quite literally absorb noise. Tread patterns will also have an effect on noise, so manufacturers are constantly honing them to keep EVs as quiet as possible.
It's important to remember that tyre noise is essentially wasted energy, so as a broad rule the quieter the tyre, the more efficient it is.
This is a contentious subject which hit the headlines back in March when Emissions Analytics released the results of a study suggesting that particulates from tyre wear were up to a 1000 times worse than those from vehicle exhausts.
Brand new and correctly inflated, it found around 5.8g/km of particulates were emitted compared to just 4.0 milligrams from a typical car's exhaust. These non-exhaust emissions are comparatively little-known, with studies into their effects only being called for with any significant weight within the last two years, with the UK government making a call for evidence as part of its 2018 Clean Air Strategy.
Back in 2018, then transport minister, Jesse Norman, said; “Particulate pollution from exhausts has been reduced substantially in recent years. But we must also take action to reduce the very serious pollution caused by the wear of tyres, brakes and roads. Tackling this issue is crucial for reducing air pollution.”
The need to understand the effects of NEEs is especially important for EVs if we're to truly be able to call them 'green'. Emissions Analytics' study found that vehicle weight and driving style were two big factors in their production. As we know, EVs are usually heavier than ICE cars, and their torque means it's easy to get carried away with the addictive acceleration.
Nick Molden, CEO of Emissions Analytics, told us: “They (NEEs) are mostly produced by thermal processes – i.e. the tyre getting hot during acceleration, deceleration, and especially in cornering.
“High speed cornering is by far the worst culprit and this is exacerbated with weight and quality of tyre. With heavier vehicles such as EVs, or indeed hybrids, fitting high quality tyres to counteract the mass is something that should be a major consideration for owners.”
The takeaway from our chat with Nick was that if you're going to buy an EV, it really pays to fit the best tyres you can afford.
You'll almost certainly have seen them on concept cars at motor shows, but 'airless' (aka non-pneumatic) tyres are being worked on by most of the major car brands. They offer a variety of potential advantages beyond simply never getting a puncture.
Michelin has developed what it calls Uptis for cars and small SUVs which wraps a wheel in composite materials, using 'spokes' rather than air for support. The process is akin to 3D printing, and the brand reckons that performance is similar to that of regular pneumatic tyres. What's more, when the tyre wears out, a new surface can simply be 'printed' back onto the Uptis carcase, making the idea far more sustainable. The French brand is looking to get the scheme on the road by 2024.
Bridgestone, too, is heralding airless as the way forward for greener tyres. It says it can better control the shape of tyres as they roll without air, helping to reduce rolling resistance significantly. Its non-pneumatic tyres are also 100 per cent recyclable – unlike regular tyres – enabling the brand to achieve what it calls its “cradle-to-cradle” strategy where tyres are continuously recycled.
Ultimately, though, we're going to have to wait a while before airless tyres become available at a price that most of us can afford. Until then, the best advice we can give is to always go for EV-specific tyres, and get the best you can afford. It pays off in the driving experience, and for the environment.
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