[jcramer]

I bought my 2015 Volt Base last week for $15k. The previous owner had driven it 18,400 miles on under-inflated (36 psi) tires that were never rotated, with the result that both front tires are prematurely bald on both edges. Therefore I need new tires. I gather that the OEM Goodyear tires are good for range but terrible in snow and ice, so I'd like to try something else, perhaps Michelin Defenders. However, before spending big bucks on tires, I would like some quantitative data on the effect of various tires on range. There seems to be lot of anecdotal stories on this question out there, but no real data.

I'm a physics professor and I like data that is quantitative and definitive. The Energy panel on my Volt the gives distance traveled (in miles) on a given charge and the energy expended (in kilowatt-hours). Distance is always an integer, but if you take a reading of this display just as the miles counter changes, you can assume an additional significant figure of .0. So, for example, I find that when I go 23.0 miles in my Volt, we consume 8.35 kilowatt-hours of energy, therefore using 0.323 kilowatt-hours per mile. If you convert miles to feet and kilowatt-hours to foot-pounds (which involves multiplying by 502.88), this means that the thrust my Volt is providing is 182.5 pounds. That's how hard you would have to push from behind to get the same performance.

Now the curb weight of a 205 Volt is 3,786 pounds. With three 170 pound passengers (which I had at the time), the net weight is 4,296 pounds. That means that, treating my Volt like a block sliding on a frictional surface, the coefficient of friction is 182.5/4296 = 0.0425. That's the effective friction coefficient of my Volt on a typical trip involving some city and freeway driving on OEM Goodyear tires inflated to 42 pounds.

Before I replace my tires, I would very much like to see similar coefficients of friction from other Volts with other tires. Anyone willing to do the same calculation?

 

[Steverino]

My winter tire/wheels cost me 5 miles battery range. So that's likely worst case.

 

[bentbiker]

To get meaningful comparative data, wouldn't the 2nd vehicle have to:
Drive the same route you drove? Road surface is a big contributor.
Drive at the same speeds you drove and accelerate the same?
Make sure the internal friction of both vehicles was the same?
Drive at the same temperature and humidity?
Drive with the same climate control settings as you?
Drive with tires having the same wear pattern?
. . .

I think you see where I'm going with this. With typical reports of 5-15% reduction with most alternative tires, I think the variation between tested vehicles would overwhelm any similar calculation of a COF. The anecdotal reports based on months of experience would, I believe, be a lot more indicative of real world performance.

 

[saghost]

I bought my 2015 Volt Base last week for $15k. The previous owner had driven it 18,400 miles on under-inflated (36 psi) tires that were never rotated, with the result that both front tires are prematrely bald on both edges. Therefore I need new tires. I gather that the OEM Goodyear tires are good for range but terrible in snow and ice, so I'd like to try something else, perhaps Michelin Defenders. However, before spending big bucks on tires, I would like some quantitative data on the effect of various tires on range. There seems to be lot of anecdotal stories on this question out there, but no real data.

I'm a physics professor and I like data that is quantitative and definitive. The Evergy panel on my Volt the gives distance traveled (in miles) on a given charge and the energy expended (in kilowatt-hours). Distance is always an integer, but If you take a reading of this display just as the miles counter changes, you can assume an additional significant figure of .0. So, for example, I find that when I go 23.0 miles in my Volt, we consume 8.35 kilowatt-hours of energy, therefore using 0.323 kilowatt-hours per mile. If you convert miles to feet and kilowatt-hours to foot-pounds (which invives multiplying by 502.88), this means that the thrust my Volt is providing is 182.5 pounds. That's how hard you would have to push from behind to get the same performance.

Now the curb weight of a 205 Volt is 3,786 pounds. With three 170 pound passengers (which I had at the time), the net weight is 4,296 pounds. That means that, treating my Volt like a block sliding on a frictional surface, the coefficient of friction is 182.5/4296 = 0.0425. That's the effective friction coefficient of my Volt on a typical trip involving some city and freeway driving on OEM Goodyear tires inflated to 42 pounds.

Before I replace my tires, I would very much like to see similar coefficients of friction from other Volts with other tires. Anyone willing to do the same calculation?

In this approach, you're neglecting both the aero loads and the fixed loads.

The Volt idles at about 400W - the entire time the car is on it consumes this, powering the computers and drive inverters and contactors and such. If you use the HVAC, that adds to this number, potentially up to about 7 kW in extreme cases.

You didn't mention a speed that I saw - which is very important because the aerodynamic loads are a function of the air velocity squared. Despite all the effort to streamline modern cars, aero loads still dominate the equations at freeway speeds.

It's the interplay between the aero loads and the fixed loads that means the most efficient speed to drive an EV is typically in the 20 mph range, where the per mile cost of aero and fixed loads cross. The rolling resistance/friction loads don't play into this speed determination because they are the same for each mile you travel at most any speed.

 

[edk-austin]

As you can appreciate, there is always much more complexity involved. I suggest you google "rolling resistance", for more information. Some tires can be better on concrete, others better on asphalt. As rolling resistance decreases, so does traction and safety.

Obviously there are many other factors that determine EV range, such as air resistance, tire pressure, number of acceleration/deceleration cycles, drive train and regenerative braking efficiency.

 

[llninja]

I know you wanted quantitative and difinitive data... I don't have it. But I switched to 18" rims and Yokohama Avid Ascends. I lost 3-5 miles of ev range, but gained loads of stickiness around curves plus much longer life. My OEM goodyears lasted 36k miles through multiple rotations. My yokohamas have 42k miles on them with no rotations. The fronts have several mm before the wear bars, but the rears are like new. I'm sure I have at least 10k miles left in the front, then I'll buy two more, move the rear tires to the front, and rinse and repeat for hopefully 70k miles on the rear tires.

I'm sure my heavier tire/wheel package hurt range, as did my decision to stop driving like a grandpa for range and drive like jeff Gordon for enjoyment. I figure my lifetimempg dropped from about 90 mpg to 70 mpg as a result, but this still blows away any Prius.

 

[Dutch]

Too many variables (and my head explodes).

 

[llninja]

Too many variables (and my head explodes).

Speaking of head exploding... do you mean like this?

https://youtu.be/UI1Ovh5JnOE

If caught this movie while channel surfing at the hotel during a business trip. A complete mind frack.

 

[jcramer]

In this approach, you're neglecting both the aero loads and the fixed loads.

The Volt idles at about 400W - the entire time the car is on it consumes this, powering the computers and drive inverters and contactors and such. If you use the HVAC, that adds to this number, potentially up to about 7 kW in extreme cases.

You didn't mention a speed that I saw - which is very important because the aerodynamic loads are a function of the air velocity squared. Despite all the effort to streamline modern cars, aero loads still dominate the equations at freeway speeds.

It's the interplay between the aero loads and the fixed loads that means the most efficient speed to drive an EV is typically in the 20 mph range, where the per mile cost of aero and fixed loads cross. The rolling resistance/friction loads don't play into this speed determination because they are the same for each mile you travel at most any speed.

It's obvious that there are other load factors. However, the question is, how much does the coefficient of friction, as calculated as I described, vary from one trip to the next, and how much does the average change with tire changes. I've only had the 2015 Volt for a few days (and I'm away from it for a week at the moment), but I plan to accumulate more data and determine an average and variation of the friction coefficient. Then I'll do the same thing after getting new tires and see what changes.

The trip I described involved about 5 miles on city streets and 18 miles on I-5 at about 60 mph, so it represents an average over several conditions. I'd be interested in miles vs. kw-hrs data from other Volts, particularly those with non-OEM tires.

 

[caskConditionedAle]

In college, the physics professors always wanted exact equations. In engineering, we use simple equations to get us a number that was good enough.

Sticky tire, better traction, less range. Hard tire, poor traction, more range. Make your choice based on what you need.

 

[DonC]

I agree there are too many variables. However, I can tell you that, after about six years of running the OEM Goodyears and six months of running on the Michelin AS Premiers, the range hit is 3.5 miles +/- .25 miles. I wanted better performance and better handling on wet roads and was willing to take a hit on range.

 

[LANMAN]

I saved all the heavy thinking for work and just replaced the two front tires with OEM's (on the back) when I had a sidewall failure at about 30K. I also ended up with a nice spare for the garage. The original two tires that I moved to the front have made it to 54K, but are nearing retirement.

 

[jcramer]

In college, the physics professors always wanted exact equations. In engineering, we use simple equations to get us a number that was good enough.

Sticky tire, better traction, less range. Hard tire, poor traction, more range. Make your choice based on what you need.

The formulas used could hardly be simpler. They are: T = (E/D)*502.88 and mu_f = T/[W(car)+W(passengers)] where T is thrust in pounds, E is energy in kw-hrs, D is distance traveled in miles, mu_f is the coefficient of friction, and W is weight in pounds. The issue is whether mu_f is a meaningful number, or whether it varies too much with conditions to be useful. I intend to find out.

I'm not sure "sticky" is the whole story with range vs. traction, it should be possible to have hard tires with treads that grip well against slippage (imagine gear teeth). Also, the Goodyear OEM tires weigh only 20 pounds each, while most other tires weigh 4-6 pounds more.

 

[llninja]

The formulas used could hardly be simpler. They are: T = (E/D)*502.88 and mu_f = T/[W(car)+W(passengers)] where T is thrust in pounds, E is energy in kw-hrs, D is distance traveled in miles, mu_f is the coefficient of friction, and W is weight in pounds. The issue is whether mu_f is a meaningful number, or whether it varies too much with conditions to be useful. I intend to find out.

I'm not sure "sticky" is the whole story with range vs. traction, it should be possible to have hard tires with treads that grip well against slippage (imagine gear teeth). Also, the Goodyear OEM tires weigh only 20 pounds each, while most other tires weigh 4-6 pounds more.

I
I actually went much further than range vs. traction. Bought all season Yokohama stickies for most of the year, then got stickier Yokohama snow/ice tires for winter. A lot of good that did, in two years the snow and ice have been on the car a total of 2 weeks. So they've done a great job of preventing snow.

 

[saghost]

It's obvious that there are other load factors. However, the question is, how much does the coefficient of friction, as calculated as I described, vary from one trip to the next, and how much does the average change with tire changes. I've only had the 2015 Volt for a few days (and I'm away from it for a week at the moment), but I plan to accumulate more data and determine an average and variation of the friction coefficient. Then I'll do the same thing after getting new tires and see what changes.

The trip I described involved about 5 miles on city streets and 18 miles on I-5 at about 60 mph, so it represents an average over several conditions. I'd be interested in miles vs. kw-hrs data from other Volts, particularly those with non-OEM tires.

That's not a coefficient of friction, and I really wish you'd stop mis-using the term. Your number can't be compared correctly to numbers other people generate without controlling those other variables - speed and starts/stops alone can make a huge difference on the exact same route with the same car.

 

[srmarti]

It's rolling resistance that's more important here. If the tire has a low coefficient of friction it slides all over the place instead rolling and steering.


You really need much more controlled conditions to get reliable numbers. Something like coast down tests come to mind.

 

[Fyodor]

The formulas used could hardly be simpler. They are: T = (E/D)*502.88 and mu_f = T/[W(car)+W(passengers)] where T is thrust in pounds, E is energy in kw-hrs, D is distance traveled in miles, mu_f is the coefficient of friction, and W is weight in pounds. The issue is whether mu_f is a meaningful number, or whether it varies too much with conditions to be useful. I intend to find out.

Ha. I guess I'm just a humble microbiologist, and while I can see that the maths is not complicated, I can imagine simpler equations.

It does seem to me that maybe this is being over thought just a bit, but what does someone without an iron ring know?

 

[jcramer]

That's not a coefficient of friction, and I really wish you'd stop mis-using the term. Your number can't be compared correctly to numbers other people generate without controlling those other variables - speed and starts/stops alone can make a huge difference on the exact same route with the same car.

OK, call it a coefficient of energy dissipation. Do you have evidence that your "huge difference" obscures differences in the average coefficient for driving with different tires? That's what I'd like to establish, one way or the other.

 

[edk-austin]

I think people are getting caught up in the range issue. I agree that a coefficient of rolling resistance exists that a tire manufacturer could use to advert their tires. Tires Are being advertised as LRR, I too would like to see a standardized metric to compare (how LRR are they!?). There are so many other variables affecting force, that we need to take energy, range, speed and surface conditions out of the discussion. With a well defined scenario, tire manufacturers could advertise a LRR score, but I haven't seen one yet. The LRR score would apply to all vehicles, maybe just like the letter codes for treadwear and traction.

 

[saghost]

OK, call it a coefficient of energy dissipation. Do you have evidence that your "huge difference" obscures differences in the average coefficient for driving with different tires? That's what I'd like to establish, one way or the other.

Data on how much difference different sets of tires make is pretty hard to come by - especially data where all the variables are adequately controlled.

Most of what we get on the forum is from owners who just changed to new tires talking about how their brand new tires compare to their old worn tires in similar conditions. Of course, brand new tires always behave differently than worn tires...

That makes it hard to draw definitive conclusions, but the largest variation I've read from old to new was about 20% range loss.

On the other hand, it's quite easy to see from available data that big speed changes can double or halve consumption, and extreme climate can do the same, especially on short trips (where the massive energy bill to heat/cool the cabin is spread over only a few miles of driving.)

So quoting exact numbers is going to be difficult, but I'm quite confident that the uncontrolled variables greatly exceed the variation that you're searching for.