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Range penalty for 21" wheels vs 19"
- Thread starter ReturnZero
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- Driving Dynamics Model S
Hmm. It should be more of a difference. When I weighed the wheel + tire on the 19s and 21s, the wheel + tire was 8 lbs more than on the 21 than the 19. The 21" tire weighs less than the 19" tire so the difference in weight of just the wheel should be more than 8 lbs.
Good point...and that weight should all increase the affects of this....
To comment on someone above who said the measurement from the ground to the center of the hub is the same - that is correct! No matter what wheel size you go with, you MUST maintain the same Circumference of the tire or else your speedometer would be wrong. Since Pi * Diameter = Circumference, and 2 * Radius is the Diameter, you were reading the Radius thus it must be the same for all wheel/tire combos.
Below is an article on wheel size and how it affects many different aspects of automobiles. While not specifically related to Tesla, it does relate to automobiles, weight, friction, etc. between different tire and wheel combos.
Effects of Upsized Wheels and Tires Tested - Tech Dept.
Below is an article on wheel size and how it affects many different aspects of automobiles. While not specifically related to Tesla, it does relate to automobiles, weight, friction, etc. between different tire and wheel combos.
Effects of Upsized Wheels and Tires Tested - Tech Dept.
FlatSix911
Porsche 918 Hybrid
Keep in mind that most 21" tires weigh more than a 19" tire. Additional 5 lbs for the wheel and 3 lbs for the tire.
More information about rotating mass. Listed below is the part that pertains to wheel size.
Rotating Mass, Available Horsepower, and Acceleration
What does a rotating mass actually do?
A rotating mass does not really consume or dissipate energy. A rotating mass stores energy. The rotating mass eventually either returns energy to the system in a useful way, or something converts the stored energy to some other form of unwanted energy. The conversion might be with a friction, converting to heat. The energy stored might be helpful, like the smoothing of cylinder pulses in an engine flywheel. The energy stored also might not do anything at all, or the stored energy can even be harmful, reducing acceleration or braking.
Accelerating an unnecessary rotating mass requires energy, and the acceleration process saps some of the horsepower we have available to accelerate our vehicles. Reducing available horsepower affects acceleration in a very predictable manner, and the horsepower amount needed to spin something up gives us some feel for how important a part change might be.
Four things determine the effect of rotating mass. Every one of these things is important:
Wheel Changes
Let's assume, just as an example, all of a wheel's weight is at the outer edge and remains at the outer edge. If we reduce a wheel's diameter but keep the overall weight the same, the wheel is a spinning ring with smaller diameter. The smaller diameter increases the wheel's RPM at the same vehicle speed. The smaller diameter also moves the spinning weight closer to the center.
Let's say we cut diameter in half. Now think about how fast the wheel spins. RPM will be twice what it was at the same speed. The half size diameter reduction spins the wheel twice as fast, and that would increase stored energy to four times the original amount if the weight was the same distance out. But the weight isn't the same distance out. The spinning weight is now half size. This 1/2 size reduction decreases stored energy by four times!
If we did not change the weight or weight distribution, and we reduced a wheel and tire diameter by half but drove the same speed, nothing would change. It would be a major change that just broke even. Moving the weight closer to the rotation center reduced stored energy, but the increased RPM to maintain the same speed increased stored energy the same amount. One cancelled the other, and stored energy did not change!
If we change tire and wheel diameter without changing weight distribution and weight in the tire and wheel, we don't change a thing. In this example, we gained nothing from a significant physical change. We also lost nothing.
Lightening the tire or wheel some distance out from the hub reduces stored energy. This is especially true if the weight reduction is far out from the center. If we change the weight one-pound fourteen inches out, it is like changing weight four-pounds seven inches out. Which brings up an important point we almost never hear mentioned, a lower weight part might not be lighter at the outside edge. It might be lighter in the center, where the weight reduction doesn't mean much.
It is more important to make something as light as possible on the OUTER edge, rather than near the (wheel) center. Spending money on smaller or lighter rotors to save rotating weight should be down the list, because the rotating weight is closer to the wheel hub. Unless the rotors are huge and we take weight out of the rotor's outer areas, things will not change much. (A light rotor and wheel is good for reducing un-sprung weight, and that helps keep our tires in contact with the road. It also reduces vehicle weight. But this is a different problem. Here we are talking about rotation, not the bounce inertia or "dead weight".)
If we spent money on the same weight reduction in the wheel, reducing weight out a little further away from the center, we would do much better. We would be removing weight further out from the center, where it does the most good.
If we spent our money on a lighter tire we would be getting the very most return for the weight change. The tire's weight change is mostly outside between the rim edge and the tread area. We get maximum effect from the weight change!
Think about this carefully. If we buy a lighter tire, we know for sure the weight comes off the most critical area. If we buy a lighter rotor, it is close to the center and, for the same weight change, the return is much less.
The wheels also speed up and slow down gradually. With an 11-second car, we have 11-seconds to speed the wheel up. Most of the horsepower pushed into the wheel and stored is pushed in near the end, when acceleration is least. Since we have more time to push the bigger amount of energy into the wheel, it takes less horsepower than we might expect.
Rotating Mass, Available Horsepower, and Acceleration
What does a rotating mass actually do?
A rotating mass does not really consume or dissipate energy. A rotating mass stores energy. The rotating mass eventually either returns energy to the system in a useful way, or something converts the stored energy to some other form of unwanted energy. The conversion might be with a friction, converting to heat. The energy stored might be helpful, like the smoothing of cylinder pulses in an engine flywheel. The energy stored also might not do anything at all, or the stored energy can even be harmful, reducing acceleration or braking.
Accelerating an unnecessary rotating mass requires energy, and the acceleration process saps some of the horsepower we have available to accelerate our vehicles. Reducing available horsepower affects acceleration in a very predictable manner, and the horsepower amount needed to spin something up gives us some feel for how important a part change might be.
Four things determine the effect of rotating mass. Every one of these things is important:
- How quickly and often a rotating mass speeds up or slows down. Every time it is forced to speed up or slow down, it takes or releases energy
- How heavy the rotating mass is. More weight (with no other changes) stores or releases more energy
- The rotating weight's distance outwards from the centerline. The further out, the more energy pushed in and out of a given weight
- How fast the weight spins, or the speed the weight travels in a given circle diameter. The higher the RPM, the more energy stored
- If we push energy into the rotating mass and pull energy out several times, we move more power around than if we make a slow, smooth, change in speed. It takes much more effort to repeatedly speed and slow something in a short period of time than to gradually speed it or slow it
- The amount of weight is the least important thing! If we double the weight (with no other changes) we only double the stored energy
- Weight distance from the center line is very important, because it determines the weight's circular velocity (speed)! Stored energy goes up by the SQUARE of the radius change. If we replace a 4-inch diameter hollow driveshaft with an 8-inch diameter tube of exactly the same weight, it is not just double. It is twice the size squared, or four times the stored energy when it weighs the same!
- The faster we spin the weight, the more energy it stores. If we double RPM, we multiply stored energy four times. Again it is a square of the change, just like weight distance from centerline is a square.
- If we reduce mass from twenty pounds to ten pounds, keeping the same distance out and same peak RPM, we reduce stored energy to half the original amount. Reducing weight is a one-for-one change.
- If we cut diameter in half while keeping the same weight and RPM, stored energy will be 1/4 the original stored energy. This change is a square. Twice is a "four times" effect. 2*2=4. Four times is a sixteen time effect on stored energy. 4*4=16
- If we cut RPM in half, we would reduce stored energy to 1/4 the original amount. Once again this is a squared change. Change RPM three times, and the stored energy changes nine times. 3*3=9
Wheel Changes
Let's assume, just as an example, all of a wheel's weight is at the outer edge and remains at the outer edge. If we reduce a wheel's diameter but keep the overall weight the same, the wheel is a spinning ring with smaller diameter. The smaller diameter increases the wheel's RPM at the same vehicle speed. The smaller diameter also moves the spinning weight closer to the center.
Let's say we cut diameter in half. Now think about how fast the wheel spins. RPM will be twice what it was at the same speed. The half size diameter reduction spins the wheel twice as fast, and that would increase stored energy to four times the original amount if the weight was the same distance out. But the weight isn't the same distance out. The spinning weight is now half size. This 1/2 size reduction decreases stored energy by four times!
If we did not change the weight or weight distribution, and we reduced a wheel and tire diameter by half but drove the same speed, nothing would change. It would be a major change that just broke even. Moving the weight closer to the rotation center reduced stored energy, but the increased RPM to maintain the same speed increased stored energy the same amount. One cancelled the other, and stored energy did not change!
If we change tire and wheel diameter without changing weight distribution and weight in the tire and wheel, we don't change a thing. In this example, we gained nothing from a significant physical change. We also lost nothing.
Lightening the tire or wheel some distance out from the hub reduces stored energy. This is especially true if the weight reduction is far out from the center. If we change the weight one-pound fourteen inches out, it is like changing weight four-pounds seven inches out. Which brings up an important point we almost never hear mentioned, a lower weight part might not be lighter at the outside edge. It might be lighter in the center, where the weight reduction doesn't mean much.
It is more important to make something as light as possible on the OUTER edge, rather than near the (wheel) center. Spending money on smaller or lighter rotors to save rotating weight should be down the list, because the rotating weight is closer to the wheel hub. Unless the rotors are huge and we take weight out of the rotor's outer areas, things will not change much. (A light rotor and wheel is good for reducing un-sprung weight, and that helps keep our tires in contact with the road. It also reduces vehicle weight. But this is a different problem. Here we are talking about rotation, not the bounce inertia or "dead weight".)
If we spent money on the same weight reduction in the wheel, reducing weight out a little further away from the center, we would do much better. We would be removing weight further out from the center, where it does the most good.
If we spent our money on a lighter tire we would be getting the very most return for the weight change. The tire's weight change is mostly outside between the rim edge and the tread area. We get maximum effect from the weight change!
Think about this carefully. If we buy a lighter tire, we know for sure the weight comes off the most critical area. If we buy a lighter rotor, it is close to the center and, for the same weight change, the return is much less.
The wheels also speed up and slow down gradually. With an 11-second car, we have 11-seconds to speed the wheel up. Most of the horsepower pushed into the wheel and stored is pushed in near the end, when acceleration is least. Since we have more time to push the bigger amount of energy into the wheel, it takes less horsepower than we might expect.
The 21" Extreme ContactDW weight = 26 lbs. The 19" Primacies weight = 27 lbs. This is straight off the tire rack.
FlatSix911
Porsche 918 Hybrid
Apples and Oranges when you compare two different brands.
Check out the Michelin performance tires on Tire Rack ... 3 lb. difference
Also remember that 8 lbs for the tire and wheel is a huge difference and will significantly impact performance as stated by Tesla.
Apples and Oranges when you compare two different brands.
Check out the Michelin performance tires on Tire Rack ... 3 lb. difference.
Um no. I'm comparing *THE* tires that come on these wheels from Tesla. OE tires, not other brands or models.
And agreed that 32 lbs of extra rotating mass will make a huge difference as evidenced by my vbox 0-60 runs with both sets of tires at the same SOCs, battery temps, and locations with no wind.
busaman
Member
i have both 19" and 21" for my MS the range has not altered at all although the 19" rolls easier and is quieter it does not handle as well as the 21" next time i need tyres im buying 21" they look that much better if there is a range difference its so small not worth the bother
busaman
Member
reading the comments above i cant believe the small weight difference of the wheel /tyre combination can make that much difference well actually it doesent because ive not noticed any difference in range.
As someone stated on the first page Tires are not the same. The fixation on rim diameter is misplaced. Lower profile tires are generally focused at better cornering, higher performance tread pattern and compound.
Even the focus on weight is a bit shortsighted, it can matter in city driving but on the highway it wouldn't matter as you are not really accelerating much.
If we want to focus on one detail and attribute everythinge to that 21" use a higher pressure and get less range so therefore we must conclude higher tire pressure is bad for range right.
Even the focus on weight is a bit shortsighted, it can matter in city driving but on the highway it wouldn't matter as you are not really accelerating much.
If we want to focus on one detail and attribute everythinge to that 21" use a higher pressure and get less range so therefore we must conclude higher tire pressure is bad for range right.
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