Wheels and range reduction...

[ellisina4]

Hello,
I am in the process of looking to lease another MX for my wife. Her current P90D lease is ending soon.
Current car has 22" OEM wheels. The range reduction due to them is fairly high... close to 15-20% as documented everywhere.
I started looking into purchasing lighter than OEM truly forged 22" for a new car but here is what I do not understand:
The OEM 20" rear wheel and tire weigh about 33.2lb for the wheel and 33lb for the tire = 66.2lb
The OEM 22" rear wheel and tire weigh about 40.4lb for the wheel and 36lb for the tire = 76.4lb
See attached screenshots for weights.
What I do not understand is how 10lb (per corner) of unsprung weight can cause such a huge decrease in range... plus the rolling diameter of both 20 and 22” tires is identical.
It is equivalent of only about 160lb of dry weight on a 5000lb+ car
Please let me know if I am missing something as buying lighter 22" forged wheels should not have that much effect on range.
Thank you for your input.

 

[SubSolar]

I thought the consensus was that the width of the tire matters a lot more in range reduction than weight of wheel.

 

[ellisina4]

I thought the consensus was that the width of the tire matters a lot more in range reduction than weight of wheel.

The OEM 20” wheels come with 265/275mm wide tires... 22” tires are 265/285...
That can’t be it.

 

[Troy]

I found EPA documents that show the following:

21" Model S wheels have 89.5% range compared to 19" wheels.
22" Model X wheels have 81.6% range compared to 20" wheels.
20" Model 3 wheels have 90.5% range compared to 18" wheels with aero covers on.

Looking at the numbers, larger wheels seem consistently drop range by ~10% but the 10mm extra width of the Model X rear wheels cause another 8% loss.

 

[Kentegra]

Hello,
I am in the process of looking to lease another MX for my wife. Her current P90D lease is ending soon.
Current car has 22" OEM wheels. The range reduction due to them is fairly high... close to 15-20% as documented everywhere.
I started looking into purchasing lighter than OEM truly forged 22" for a new car but here is what I do not understand:
The OEM 20" rear wheel and tire weigh about 33.2lb for the wheel and 33lb for the tire = 66.2lb
The OEM 22" rear wheel and tire weigh about 40.4lb for the wheel and 36lb for the tire = 76.4lb
See attached screenshots for weights.
What I do not understand is how 10lb (per corner) of unsprung weight can cause such a huge decrease in range... plus the rolling diameter of both 20 and 22” tires is identical.
It is equivalent of only about 160lb of dry weight on a 5000lb+ car
Please let me know if I am missing something as buying lighter 22" forged wheels should not have that much effect on range.
Thank you for your input.

I am just as curious. I purchased a set of 22'' rotary forged wheels and should be as heavy as my 20'' OEM or slightly lighter. I'll keep you updated on range difference.

 

[MP3Mike]

Please let me know if I am missing something as buying lighter 22" forged wheels should not have that much effect on range.

I don't think it is the weight that impacts the range, but the aerodynamics. So a different 22" wheel that weighs the same could impact your range differently based on how it impacts the air flow.

The larger the wheel the more space for the spokes to impact the aerodynamics.

 

[StealthP3D]

There are three components to the effect of different tire/wheel setups on range:

1) Aerodynamic - This affects highway range primarily and can be a quite an impact. As pointed out by @MP3Mike , a larger diameter rim is more disruptive to airflow. Thats why the Model 3 Aero wheels have spoke covers to make the outside of the wheel/tire as flush as possible. Narrow tires/wheels have less drag also. The offset of the wheels can have a significant impact on range. I have to assume the factory offset is the one used when Tesla was optimizing the aerodynamics.

2) Rolling resistance - Sportier rubber compounds (grippier) cause more rolling resistance. Some tires are specially designed to minimize rolling resistance and are sold as "low rolling resistance" tires. Low rolling resistance tires have sidewall construction designed to minimize flexing and heat build-up. A LRR tire will have less pressure rise after a drive on the freeway because they build less heat. More air pressure can greatly reduce rolling resistance in any tire. The factory PSI recommendation is more for comfort.

3) Rotational inertia - This is often referred to simply as "weight" because it's so difficult to accurately measure rotational inertia which is determined by how much the tire/wheel assembly weighs AND how far that weight is from the axle. As the weight is moved farther from the axle, rotational inertia increases. That is why larger rims have higher rotational inertia - the heaviest part of the wheel, the barrel, is moved farther from the axle. This negatively impacts city efficiency and acceleration performance. Race cars generally do not have large rims and low profile tires because that increases demands on the brakes and slows acceleration. Modern tires have carcasses are constructed using sophisticated lay-ups of stiff fibers and steel to prevent sidewall tuck-in under hard cornering forces. Coupled with adequate tire pressure, there is not a lot of reason to go to large rims and low profile tires from a performance perspective, in fact, the larger amounts of rotational inertia will actually cause slower acceleration and less efficiency in any kind of driving where the speed is variable from moment to moment.

There are a lot of practical reasons why most cars don't come from the factory with low profile tires and not a lot of practical reasons to to downgrade to larger rims and low profile tires. It's done primarily by people who think it looks better. But looks are subjective and I think things that work well, look better so I am not a fan of the trend to lower and lower profile tires. When I see them I think of the guy on the side of the road with a flat tire. That's the guy I don't want to be.

 

[natethegreat]

I'm still trying to find someone who has the 19" on the model x and can attest to the range difference. Problem is, most folks are running them in the winter, so it's not a fair comparison.

 

[ellisina4]

Thank you for for taking time to explain... all of your points make a lot of sense. It is still hard to believe that these 3 items (ones that have to do with wheels and tires only) will bring the range down by 20% but it is what it is. At the same time, I have always been driving ICE cars with 500HP+ V8 engines and never really pay attention to fuel economy. Now that my wife has been driving MX, I really only started paying attention to the range when we get 170 miles out of 250 charge.

Aerodynamics... will be probably hit or miss based on wheels purchased.
Rolling resistance can be addressed by since I really like Continental DWS 06 tires for all season driving in Chicago, I may stick with these.
The last point, wheel weight, this is something I can definitely work with. I will try to get down to 30lb for each wheel... HRE and Vossen may be a good choice. These are my favorite as of now:
TESLA MODEL X - VOSSEN FORGED - PRECISION SERIES: VPS-305T - Vossen Wheels

Anyways, thank you again. Feedback is much appreciated.

There are three components to the effect of different tire/wheel setups on range:

1) Aerodynamic - This affects highway range primarily and can be a quite an impact. As pointed out by @MP3Mike , a larger diameter rim is more disruptive to airflow. Thats why the Model 3 Aero wheels have spoke covers to make the outside of the wheel/tire as flush as possible. Narrow tires/wheels have less drag also. The offset of the wheels can have a significant impact on range. I have to assume the factory offset is the one used when Tesla was optimizing the aerodynamics.

2) Rolling resistance - Sportier rubber compounds (grippier) cause more rolling resistance. Some tires are specially designed to minimize rolling resistance and are sold as "low rolling resistance" tires. Low rolling resistance tires have sidewall construction designed to minimize flexing and heat build-up. A LRR tire will have less pressure rise after a drive on the freeway because they build less heat. More air pressure can greatly reduce rolling resistance in any tire. The factory PSI recommendation is more for comfort.

3) Rotational inertia - This is often referred to simply as "weight" because it's so difficult to accurately measure rotational inertia which is determined by how much the tire/wheel assembly weighs AND how far that weight is from the axle. As the weight is moved farther from the axle, rotational inertia increases. That is why larger rims have higher rotational inertia - the heaviest part of the wheel, the barrel, is moved farther from the axle. This negatively impacts city efficiency and acceleration performance. Race cars generally do not have large rims and low profile tires because that increases demands on the brakes and slows acceleration. Modern tires have carcasses are constructed using sophisticated lay-ups of stiff fibers and steel to prevent sidewall tuck-in under hard cornering forces. Coupled with adequate tire pressure, there is not a lot of reason to go to large rims and low profile tires from a performance perspective, in fact, the larger amounts of rotational inertia will actually cause slower acceleration and less efficiency in any kind of driving where the speed is variable from moment to moment.

There are a lot of practical reasons why most cars don't come from the factory with low profile tires and not a lot of practical reasons to to downgrade to larger rims and low profile tires. It's done primarily by people who think it looks better. But looks are subjective and I think things that work well, look better so I am not a fan of the trend to lower and lower profile tires. When I see them I think of the guy on the side of the road with a flat tire. That's the guy I don't want to be.

 

[ellisina4]

Just a word of advice from someone who learned the hard way the difference between truly forged wheels and pseudo forged ones (rotary or flow forged). These terms are purely marketing and they did fool me until my car started shaking on the expressway at high speeds due to 3 wheels being bent only after 6 months of driving. Low profile tires like 30 or 35 (% of width) do not help either in absorbing impact from pothole and etc.

I am just as curious. I purchased a set of 22'' rotary forged wheels and should be as heavy as my 20'' OEM or slightly lighter. I'll keep you updated on range difference.

 

[ajdelange]

3) Rotational inertia - This is often referred to simply as "weight" because it's so difficult to accurately measure rotational inertia ...

It is quite easy to determine the inertia. Roll the tire down an inclined plane and measure the time it takes to go a measured distance with a stop watch. The angular acceleration is the distance divided by two pi times the radius times two divided by the time squared. The rotational moment is the radius times the mass of the tire times 9.8 m/sec^2 times the sine of the inclination angle divided by the angular acceleration.

A more refined procedure would spin the tire in the horizontal plane using a motor whose torque can be controlled and measuring the angular acceleration produced by a given torque. Just a Force = mass*linear_acceleration in the linear domain so torque = rotational_inertia* angular_acceleration in the rotational domain.

...which is determined by how much the tire/wheel assembly weighs AND how far that weight is from the axle.

The square of the distance. It is the second moment of the radial mass distribution.

As the weight is moved farther from the axle, rotational inertia increases. That is why larger rims have higher rotational inertia - the heaviest part of the wheel, the barrel, is moved farther from the axle.

And there is more of it. Thus it takes 22/20 = 1.1 times as much steel to make a circle of 22 inches as it does to make one of 20 inches. And as the radius is 1.1 times larger that factor alone increases J by a factor of 1.21. Combining extra material and greater radius J for a 22" time would be bigger than J for a 20 inch tire by a factor of about 1.1^3 = 1.33 i.e. bigger by a third.

But just as the kinetic energy stored in the vehicle is one half its mass times its velocity so in the kinetic energy stored in a rotating tire equal to one half its moment times the square of its rotational speed and just as slowing the vehicle with regen operable recovers the translational energy (or a good part of it) so does it recover much of the rotational energy in the wheels. Besides that, note that for a given highway speed the angular velocity of a 22" wheel is going to be 1.1 times lower than that of a 20" wheel and so, for a given linear speed, the rotational energy of the larger wheel is going to be approximately 1.1^3 / 1.1^2 = 1.1 times greater than the smaller.

These are all approximate figures of course (not considering that the tread is wider by a bit on the 22" wheel but they suggest that rotational moment isn't going to be that big a factor. That leaves energy absorption by the sidewalls and aerodynamics. I would be hard pressed to explain how they can increase energy consumption by as much as 20% but apparently they must.

 

[StealthP3D]

Thanks for the additional color. I was trying to keep it simple, in general terms but you have done a good job with the math (with one huge mistake in the application of that math which I'll point out below).

It is quite easy to determine the inertia. Roll the tire down an inclined plane and measure the time it takes to go a measured distance with a stop watch.

Yeah, but a lot of wheel resellers are too lazy to put their wheels on a shipping scale and list individual weights. I doubt you're going to get many of them rolling them down a ramp and timing their acceleration. That's what I meant when I said it's "difficult" to measure rotational inertia.

But just as the kinetic energy stored in the vehicle is one half its mass times its velocity so in the kinetic energy stored in a rotating tire equal to one half its moment times the square of its rotational speed and just as slowing the vehicle with regen operable recovers the translational energy (or a good part of it) so does it recover much of the rotational energy in the wheels.

"A good part of it"? You do so well on the math, then leave the regen losses to such a general term? Of course, we don't know exactly what the overall efficiency is but it's a good guess that regen only returns around 50% of the original energy. To explain: The energy contained in a spinning wheel already experienced motor losses and motor controller losses and battery/wiring losses to get it spinning in the first place. So the extra energy contained in a heavier spinning wheel (vs. a lighter one) has already consumed additional energy beyond what is embodied in its rotational inertia. That is a loss right there. When regen braking that wheel, some of the embodied energy is returned to the battery (but none of the energy wasted to get it spinning in the first place). This effectively (roughly) doubles the losses. It's a round trip, first to get it spinning and then to recover some of the energy that became embodied as rotational inertia.

Besides that, note that for a given highway speed the angular velocity of a 22" wheel is going to be 1.1 times lower than that of a 20" wheel and so, for a given linear speed, the rotational energy of the larger wheel is going to be approximately 1.1^3 / 1.1^2 = 1.1 times greater than the smaller.

You are forgetting that wheels must be mounted to tires. When speaking of 18" vs 20" wheels, it is generally assumed a tire will be mounted that keeps the rolling diameter the same. Therefore the angular velocity of an 18" wheel will be identical to the angular velocity of a 20" wheel (at the same vehicle speed). Strangely enough, the tires to achieve the same rolling diameter generally weigh approximately the same (or very similar) regardless of whether you go with 18" or 20" rims. One would think the 18" tire would weigh more with it's taller sidewall but sidewalls are a very minor part of the weight of a tire with most of that being in the tread.

These are all approximate figures of course (not considering that the tread is wider by a bit on the 22" wheel but they suggest that rotational moment isn't going to be that big a factor. That leaves energy absorption by the sidewalls and aerodynamics. I would be hard pressed to explain how they can increase energy consumption by as much as 20% but apparently they must.

I believe the comparison involves tires with different rubber compounds and treads. So rolling resistance can vary quite a bit right there. But I do think you are underestimating the inefficiencies due to rotational inertia. Mostly due to the factors I've mentioned above but also because not all braking is regen braking (only works down to 5 mph on a Model 3). All of these factors add up to the fact that rotational inertia of the wheels has significant effects on efficiency. Admittedly, only when changing speeds. But changing speeds is a fact of driving on public roads and while this is most apparent in stop and go city traffic, it also has significant effects on most freeway traffic.

 

[ajdelange]

"A good part of it"? You do so well on the math, then leave the regen losses to such a general term? Of course, we don't know exactly what the overall efficiency is but it's a good guess that regen only returns around 50% of the original energy. To explain: The energy contained in a spinning wheel already experienced motor losses and motor controller losses and battery/wiring losses to get it spinning in the first place. So the extra energy contained in a heavier spinning wheel (vs. a lighter one) has already consumed additional energy beyond what is embodied in its rotational inertia. That is a loss right there. When regen braking that wheel, some of the embodied energy is returned to the battery (but none of the energy wasted to get it spinning in the first place). This effectively (roughly) doubles the losses. It's a round trip, first to get it spinning and then to recover some of the energy that became embodied as rotational inertia.

I hear you but would like to point out that all these remarks apply equally well to the translational kinetic energy. It is revealing to roughly calculate the translational kinetic energy in a 5500 pound car going 100 km/hr (approx 1 Mjoule) and compare to the rotational kinetic energy in its 4 tires assuming they are all 22" and weigh 76.4 lbs with all the mass at the rim (the case where the moment and, consequently the stored energy would be the greatest). This works out to about 50 kjoule or 5% of the translational. Thus, if going to the 20" tire saved you half the rotational energy and you couldn't recover any of it you would change you rotational loss from about 5% of the total to about 2.5% of the total. That's why, I believe, in a typical Sankey diagram the translational kinnetic load is shown but, while roll resistance (which includes sidewall deformation) and wheel slip are shown, rotational kinetic load is not.

Based on this example diagram perhaps we should be looking at wheel slip!

]

 

[StealthP3D]

I hear you but would like to point out that all these remarks apply equally well to the translational kinetic energy. It is revealing to roughly calculate the translational kinetic energy in a 5500 pound car going 100 km/hr (approx 1 Mjoule) and compare to the rotational kinetic energy in its 4 tires assuming they are all 22" and weigh 76.4 lbs with all the mass at the rim (the case where the moment and, consequently the stored energy would be the greatest). This works out to about 50 kjoule or 5% of the translational. Thus, if going to the 20" tire saved you half the rotational energy and you couldn't recover any of it you would change you rotational loss from about 5% of the total to about 2.5% of the total.

Your math is wonky.

If each 22" wheel/tire combo weighs 76.4 lbs. then that's 305.6 lbs. (four wheels) which is over 5% of the total vehicle weight of 5500 lbs. right there! That's before you consider their rotational inertia. When you move to lighter wheels you reduce the amount of energy required to accelerate the car by both the reduction in translational kinetic energy of the entire mass of the car in addition to the reduction in rotational kinetic energy.

 

[ajdelange]

305/5500 = 5.5%. Now according to OP the 22" wheels weigh 10 lbs more than the 20's so presumably the 20's comprise (305 - 40)/5500 = 4.8%, a change of 0.7%. I don't think we can explain the 10 - 20% mileage reduction on an 0.7% change in the kinetic translation demand or by the less than 5% change in the rotational kinetic demand (assumes 0 energy recovery). Assuming we recover 50% of that energy we are now talking 0.35% and less than 2.5%. Again I direct readers attention to the Sankey diagram. I have no idea which vehicle that typifies nor what the grade was when this diagram was drawn. Again I note that rotational energy was not considered significant enough to include on this diagram which, as it is an example from a text book, should be at least reasonably representative of a typical EV. And again I point out that slip, which has been up to this point completely ignored here was deemed important enough to include and is, apparently, the largest load after inertial load.

I have been, up to this point, totally puzzled as to how changing the tire size could cause such a large change in range. I think I have the answer now. The slip properties of the two tyre types must be appreciably different. At the same time I recognize that rolling resistance and aerodynamics do make some difference as does rotational moment of inertia but that appears to be settling into 4th place.

 

[SSedan]

On the tires and rolling resistance which I believe are the biggest factor and feel is it truly sad it took till post 7 to address I would go a step further and say that a given tire model does not guarantee "the same tire" across sizes.

The fixation on rim diameter is wrong.

Aerodynamics certainly are a factor including width but the tire itself is HUGE. On my wife's ICE vehicles we have seen 10% swing from tire changes just going from OEM LRR to a "all season" that is well rated for snow and that is on the same rims. I also do not believe there is a standard for LRR I believe it is a relative term much like the wear ratings, those are NOT standardized but some folks will use that as an indication of how "soft" a performance tire is.

Short and simple is low profile tires are usually meant to improve cornering via less sidewall to flex, as such "stickier" compounds and tread designs are usually part of that package too.

 

[mxnym]

Thank you for for taking time to explain... all of your points make a lot of sense. It is still hard to believe that these 3 items (ones that have to do with wheels and tires only) will bring the range down by 20% but it is what it is. At the same time, I have always been driving ICE cars with 500HP+ V8 engines and never really pay attention to fuel economy. Now that my wife has been driving MX, I really only started paying attention to the range when we get 170 miles out of 250 charge.

Aerodynamics... will be probably hit or miss based on wheels purchased.
Rolling resistance can be addressed by since I really like Continental DWS 06 tires for all season driving in Chicago, I may stick with these.
The last point, wheel weight, this is something I can definitely work with. I will try to get down to 30lb for each wheel... HRE and Vossen may be a good choice. These are my favorite as of now:
TESLA MODEL X - VOSSEN FORGED - PRECISION SERIES: VPS-305T - Vossen Wheels

Anyways, thank you again. Feedback is much appreciated.

I was just commenting on exactly this in another thread. Assuming the cars you're talking about are rated 18 MPG highway, which is presumably a high estimate, you'd probably not bat an eye at the 20% reduction in mileage (down to 14.4 MPG), and the real reason you notice the effect so much more on the Tesla is likely one of the following:
1) They put it in your face on the MCU, and it's making you think about what you're consuming for the first time
2) The 100kWH "tank" simply isn't big enough for the vehicle (this hypothetical 18MPG rated V8 beast would have a "range" of something like 360 miles while the Model X P100D has a "range" of 280 miles, is it?)

 

[onlinespending]

With all the discussion of this thread, it would be nice if we knew of a set of lightweight 20" (or even 19") wheels and tires that we knew provided a material benefit in range over the OEM 20" setup. I really wish Tesla offered wheels caps for the S and X (much like the 3) to improve aerodynamics. They could easily sell a set for $500, which would be a mostly pure profit item for them. I've considered buying the Model S Aero wheels that Tesla no longer offers, but am concerned they wouldn't be able to support the weight of the X.