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Sunday, January 4, 2015

Rolling Resistance



As you drive down the road, there are two primary forces that slow your car: aerodynamic drag & rolling resistance (assuming flat ground and you are not breaking).

What is rolling resistance?

Rolling resistance is the energy lost to tire deformation as you drive. Reducing rolling resistance is one of the reasons that trains have steel wheels. Depending on the weight of the vehicle, up to 25% of the vehicle's power is used to push the tires down the road.

Rolling resistance applied to EVs?

The rolling resistance can have a big impact on the range of an EV. The wrong tires (or under inflated tires) can significantly reduce the range. This can then impact how deeply the battery pack is cycled. Excessively cycling causes wear on the batteries and this too will (eventually) reduce the range and battery lifespan. So you see, that tires can really matter.

How do you measure rolling resistance?

I recently needed new tires for my car. Since I drive an EV, I wanted low rolling resistance tires. I started shopping for them, and all I could find was imprecise marketing terms. They had names like fuel-saver, or eco tire and they said things like 20% better than a standard tire. These vague terms gave me no way to compare one low rolling resistance (LRR) tire to another.

I called my local tire store and asked if they had rolling resistance data for the tires they sold or knew where I could find it. They said that they didn't have it or know of anywhere that it was available to the public.

A little more digging and I found that there are standard metrics for rolling resistance. It is defined by the Society of Automotive Engineers (SAE).


Let's take a look at the above formula (F=CW/r). The weight of my car (W) is fixed and so is the radius of the tires (r) that will fit it. So that leave only C, the coefficient of rolling resistance.

The coefficient of rolling resistance  (Crr) for most new passenger tires ranges from 7 kg/ton to 14 kg/ton.


I Want Low Rolling Resistance Tires! 

OK, so now we know there is an actual unit of measure that we can compare and what range of values to expect, but where can I find the data?

Googling for Low Rolling Resistance Tire Lists, I found a list of tires from 2003 that were LRR with their Crr values. But that list is nearly 12 years old and LRR tires have made a lot of improvements since then.

Checking Consumer Reports was no help. They recommend that rolling resistance only be used as a tiebreaker and that other performance characteristics like handling and braking distance should be the first things that you consider. That is probably good advice, but this just for my "around town" EV, so I am going to ignore it. For me, this is not just about saving 3% on my fuel bill, it directly impacts how far I can drive and how useful my EV is for my transport needs.

This left me with nothing but the marketing terms and sifting through anecdotal data on various car forums. I could just buy the same model of tire that came on the car when I bought it, but they wore out after less than 30,000 miles.


Tesla Knows This Matters

Tesla recently announced the Roadster 3.0. This updated Roadster is expected to have over 400 miles of range. In this upgrade they improved the batteries, the aerodynamics, and the tires. The new tires have a rolling resistance coefficient (Crr) of 8.9 kg/ton. This is a 20% improvement from the older Roadster tires that had a Crr value of 11.0 kg/ton.

So Tesla can get this information. I am sure it is not too hard to find if you are in the auto industry and know where to look. If so maybe you can help the EV driving world out.

If you know of a source where I can find Crr data for tires on the market today, please let me know in the comments below.