BMW i3 takes advantage of taller, narrower tyres
Credit: BMW/The Telegraph
Recently, a few trends in cars have been:
• wider tyres
• larger diameter wheels
• improved fuel efficiency
All other things being equal, & within reasonable limits, for each centimetre increase in tyre diameter, rolling resistance reduces by 1%. This happens because the tyre suffers less deformity in the contact patch (the part of the tyre in contact with the road) & sidewalls, as it rolls.
Increased tyre diameter is consistent with the other current trend toward improved fuel efficiency.
Odd man out in this equation is wider tyres, which increase rolling resistance & aerodynamic drag. Taken together, these two factors reduce fuel efficiency of a vehicle.
Wet weather performance of a tyre is substantially determined by it’s ability to displace water. A tyre which displaces more water, effectively drives on a drier road. As the tread rolls into contact with the road, it begins to displace water sideways, to be expelled from the sides of the contact patch.
A narrow tyre has a relatively long, narrow, contact patch. On a wet road, water has a shorter distance to travel to be expelled at the sides of the tyre. A long contact patch has more patch-length in which to displace water. The rear edge of a long, narrow contact patch has more contact patch ahead of it to dry the road, & so has more grip.
Wide tyres have the reverse situation: a short, wide contact patch, which displaces less water.
Taller, narrower tyres offer a range of advantages, but raise a question about grip in dry conditions. At least in part, this will be compensated for by an increase in the length of the contact patch in the larger diameter tyre.
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