At university I majored in “Energy Conversion” (a blend of ME & EE at that time) and am an active supporter of electric vehicles (EVS)… at both the casual and technical levels. For us it’s not just all “talk”; been driving a BEV (Battery Electric Vehicle, not a hybrid) for four years. Since April, no longer even own an ICE (Internal Combustion Engine), just two BEVS.
“Charged Electic Vehicles Magazine” is a good source of both consumer and technical info… and back issues of the magazine can be read online (free). An article on EV Energy Consumption from the January-February 2018 issue is attached:
“Conversely, overinflating the tires to reduce rolling resistance isn’t really worth the reduced tire life and increased risk of a blowout.”
Some recent reading I’ve done on bicycle tires says that the pressure/rolling resistance curve is sort of V-shaped, with hysteresis from tire deflection decreasing with increased pressure, but losses related to pavement roughness increasing with pressure.
I’ve been wondering whether this applies to auto tires, or if the stiffer, belted tire carcass changes things. Can anyone comment?
@Zed I don’t have knowledge of the analogy between bicycle and automobile tires. I do know the Electric Vehicles (EVs) have requirements with different priority than Internal Combustion Engine (ICE) tires.
For EVs, energy efficiency is most important. Tire manufacturers have developed tires with lower rolling resistance by using certain rubber compounds. My 2017 Chevrolet Bolt has the factory-installed “Michelin Energy Saver A/S” tires. Michelin claims they give up to 8% higher efficiency than typical all-weather tires.
For their size, EVs are much heavier than similar ICE (primarily because of the large battery). This is taken into consideration for the tires used.
Torque characteristics for an electric motor are vastly different than an ICE. An electric gives maximum torque at zero RPMs. You can take off from a stop light leaving ICE behind, if you want to. Of course you consume excess energy doing this. There is a problem with high torque, see below. It’s not just more powerful electrics that can do this; even my wife’s modestly powered 2015 Nissan Leaf can do it. BTW, concerning tires, you don’t have to use energy efficient tires, her Leaf has a typical all-weather set and does just fine… except for some range reduction.
Tires designed to be quiet are also desirable. At highway speed about all you hear are wind and tire noise… which can be surprisingly loud, but still quieter than an ICE.
One energy efficient tire compromise made is traction, even thought tire manufacturers deny it. With the high torque, you can “spin tires” anytime if you choose to.
Yes, there is an optimum psi for a given tire for rolling resistance. If you overinflate the tire it drives high frequency inputs into the shock absorber which then turns them into heat.
If you underinflate the tire there is excessive deformation in the contact patch rubber.
@GregLocock How does the optimum tire pressure for rolling resistance compare with the auto manufacturer’s recommendation for tire pressure?
I’ve “heard” for years that the manufacturer’s recommendation is for comfortable ride, but a slightly higher pressure is better for fuel economy and tire life. Is that true?
For my Bolt, Chevrolet recommends 38 psi in all tires, but I try to keep 40 psi. Will change if you say so.
Tire pressure is set by a rather subjective tradeoff between attributes. Formally we use a Kano chart, but like any of these games, the weightings are set by he who wishes to control the outcome.
There is a move, partly driven by the Firestone/Explorer experience, to have the tire manufacturer decide what the tire pressure should be in a given application. I don’t think that’s going to fly.
Yes for a typical car the drivers for ride, noise, impact harshness and dry grip are all downwards, so i suspect the average car has lower psi than optimum for wet braking and aquaplaning and fuel consumption.
Unfortunately the literature is all chasing the politically correct line about low tire pressures, and not exploring the negative consequences of higher psi.
FWIW I usually overinflate my tires by 2-3 psi, more for reasons of laziness than anything else. Also aquaplaning terrifies me.