Doctor Diesel

A conflict of opinions?

BMWHi Doc,
So what’s all this about wider tyres? (Variable Resistance – Diesel Car 339). You claim, much to your surprise and mine, that wider tyres consume no more energy than narrower ones, due to having differently shaped road contact areas: the narrower tyre having a long and narrow one, the fatter tyre a shorter but wider one. Apparently, you say, the end result is the same, or even possibly favouring the wider tyre for lower rolling resistance. Well maybe you should read the next page where Victor Harman’s “The Extra Mile” claims that BMW’s i3, with its skinny 19-inch wheels, could be the “beginnings of a healthy trend away from fat alloys” and that using larger diameter, but narrow wheels and tyres reduces both rolling resistance and aerodynamic drag thus saving fuel. Hmmmmmmmm! Regards,
Malcolm Archer

Well, of course in the case of the BMW i3 we’re not just looking at a normal narrow and higher profile tyre, like maybe a 195/65/15 or 205/55/16 compared with say a 225/45/17, or even a 235/40/18 which might be the sort of extremes that one might encounter, dependent on trim level or optional wheel and tyre combinations selected by a buyer. So the i3 is not just a case where it might have a lower rolling resistance compared with a smaller sized wheel/tyre combination. With the i3 and its 155/70/19 tyres, you have a wheel/tyre combination where the overall rolling circumference is significantly greater than normal. So the gains come from a much larger tyre diameter, but with a narrow section, which means that there are gains in rolling resistance from that tyre – in the same way (in the extreme) that a small wheeled bicycle has greater rolling resistance than a 26-inch wheeled cycle – there is less tyre distortion because the approach angle is smaller. Along with this, the aerodynamic resistance is significantly lower and, I imagine, the i3’s wheel itself has been designed for minimum aerodynamic resistance, and maybe that’s where the biggest gains are derived: 

Press quote:

“BMW commissioned Bridgestone to make special tyres just for the i3. Even though they carry the (existing) “Ecopia” name, they are quite different from any other tyre that Bridgestone makes. Franco Annunziato, CEO and President of Bridgestone Europe said: “The BMW i3 is very much a car for the future. Developing a unique tyre for this unique vehicle was therefore an enormously challenging, but also rewarding experience. Energy efficiency is an important development criteria for all our tyres at Bridgestone. However, it becomes an even more critical factor in an electric car. Which is why we have put all our know how, skill and passion into developing this unique tyre. One that we are confident delivers the premium performance, safety and economy towards consumers who have come to expect it from both brands.” As for performance, since BMW wanted to use a narrow tyre for efficiency, but not sacrifice on performance, they needed to use a much taller tyre than most would expect on a car of this size. By increasing the height, they were able to increase the contact patch so it is similar to that of a MINI Cooper, which is well known for its great handling. Talking about the i3’s tyres and their grip on the road, Ulrich Krantz, BMW’s product manager for BMW i said: “It’s not rocket science. All that matters is the size of the contact patch. The 19-inch tyres may be skinny, but their tall height generates the same contact patch as a low-section 16-inch MINI tyre”.

So hopefully that bridges the credibility gap between what I wrote and what Victor Harman said, and I also see it as a healthy departure from the norm, whereby those owners who want huge wheels and tyres that fill the wheel arches can now have the best of both worlds. From what I have read from road tests (I haven’t had the opportunity to drive one yet), the end results in handling and steering feedback terms are quite successful. I hope that clears things up, but I must say that it’s really rewarding to have readers picking us up on things like this. It shows that some Diesel Car readers (as I’ve always suspected) have a far deeper insight and more questioning attitude than most car owners, or for that matter most motoring magazine readers! Long may it continue that way, and I thank you again sincerely for your contribution! Best regards,
Doc Diesel

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