19's vs. 21's vs. AfterMarket. What affects efficiency?

[rick325]

Hi all.

I understand (I think) that the 19 inch wheels are more efficient than the 21's. How much more efficient and why? This is a short conversation if they really aren't more energy efficient.

Why I am asking: I've been lurking on the Aero wheel thread, 19" cyclone wheel threads, and have some theories. Some of this is motivated by my not liking either wheel option, and considering After Market rims (I use the term "wheel" and "rim" interchangeably, knowing that is incorrect). I don't want to shoot myself in the foot range wise though.

My thoughts:

• Seems that the weight and rotational mass of the wheel+tire matters. Less mass is more efficient than more mass. Hence, the 19's are more efficient.
• The 19's have less rubber on the road than the 21's. Hmmm.. they are both 245/xx/yy, so should have the same amount of rubber on the road. Probably a wash on those two. However, if you use 265 (like Elon), do you end up with more friction, or does it not matter since the weight of the car is distributed over a wider area.
• How much does the spoke pattern matter? The stock 19s are very open, the stock 21s are far more 'closed'. I would have figured this is irrelevant, but The Aero thread indicates a potential for 6-8% using true aero wheels. However, my sense is that this is binary---either you are Aero and you get Cd benefits, or you are not and you don't. Two wheels in the "not" category would be similar, so that doesn't affect the 19 vs. 21 efficiency debate.

Help!

(P16302)

 

[Owner]

I too spent a lot of time analyzing wheel choices. I ended up with the 21".

I hadn't spent too much time thinking about range.

Here are some more potential factors to consider but may just be wild ideas:

Range on curvy roads, where the 21" would stick and perhaps be more efficient? Or am I trying to justify my 21" decision!

Regen factor - Is there any relationship between regen and more rubber on the road?

How much will the choice depend upon road / speed - freeway vs. stop and go etc..?

 

[ElSupreme]

I think most of the range loss is due to tire choice, not necessarily rim size. Sure the taller sidewalls will be a little better resistance wise but I am sure 90% of the 'range loss' with the 21" wheels is due to performance tires.

I think rim size boils down to three things. Aesthetics. Tire cost. Tires selection. I personally think the 19" rims win on all three accounts. I would find it hard for anyone to argue about the 21" rims winning tire cost or tire selection.

There will probably some slight 'performance' gains with the 21" rims (assuming same model tires). But I bet that very few people in the world would, or could, actually use that performance gain.

 

[rick325]

Thanks. Any sense for how much "range loss" there is between the 19's and 21's?

 

[ElSupreme]

Thanks. Any sense for how much "range loss" there is between the 19's and 21's?

Your Questions Answered | Tesla Motors

You have to scroll down a bit but there is a range calculator. It depends on lots of stuff. There is a small 19" button near the rear wheel. It toggles wheel choice. It also has a tool tip

Range decreases slightly with larger wheels. 21" wheels have more contact with the road and are made of stickier rubber. Both characteristics increase friction.
GAS CAR
Gas cars are equally less efficient.

 

[gtimbers]

The 21" rims have more rotational inertia than the 19" rims due to more mass being farther from the center of the wheel/tire combination. The bigger rim will be less "free" to accelerate and decelerate thus using more energy during any transient condition. This is well known in the Automotive community and it is the main reason companies like Audi, Mercedes and BMW try to stay with 18" wheels. Of course, market pressure from us, the buyers, who think big wheels look cool have forced them to increase their wheel size somewhat. With that said, it has been my experience that when oversizing wheels, the major effect is in the feel of the car, not so much fuel economy. The bigger wheel is more sluggish in cornering and has greater unsprung weight which makes bump absorption sloppier. There is the advantage of the lower aspect ratio which actually improves turning response and steering bite. I would assume that the Tesla was designed from the outset with the 21" wheels in mind and I'm sure the engineers took all of this into consideration and tweaked the suspension accordingly. I doubt that the 19" vs. 21" size actually accounts for more than 1% or 2% range difference, at most. HOWEVER, the difference in tire design is significant. I am a major tire guy and I usually dump OEM tires immediately in favor of much higher performance/safety tires. I can say with good personal experience that going from a high performance summer tire to a high performance All season tire can often account for a 5% difference in range (MPG). Of course this varies from brand to brand but overall, the super sticky rubber will wear faster and consume more HP to roll than the all season rubber. I would expect the 19" rubber to offer something like 5% more range if driven in an equivalent manner to the 21". Both wheels have the same contact patch as the tires are both 245mm so it really just comes down to rubber and tread design.

 

[Bearman]

According to the interactive range calculator on Teslas website the difference is 301 vs 283 miles just changing between 19" and 21" rims/tires, thats a 6% difference ceteris paribus.
Their explanation is: "Range decreases slightly with larger wheels. 21" wheels have more contact with the road and are made of stickier rubber. Both characteristics increase friction."
http://www.teslamotors.com/goelectric#range
(The interactive thing is 3 pictures down)

 

[rcc]

On performance: I would expect slightly quicker turn-in with the 21" vs. 19" and slightly more precise handling all other things being equal (same tires on both). The 21" tires will also have less sidewall roll which probably results in slightly better lateral grip.

Overall though I think the dominant factor in the stock S handling is the weight of the car. I think few people will feel the performance difference in 21" vs. 19" tires assuming the same tires on both wheels.

What should make a bigger difference is switching from 19" all-seasons to 19" summer max performance. That increases the overall grip significantly. I just doubt we're going to get real-world data on that anytime soon. (Anyone feel like buying a new set of max performance 19" tires and pitching your almost new 19" all seasons? Or lending your car to Tire Rack for a day?)

So I expect reasonable numbers of people to feel a difference between the stock 19" and 21" tires but that's more because the 19" tires are all-seasons, not because of the 2" difference in tire diameter. I think folks with 19" wheels can get most of that performance/handling gain by switching to 19" summer max performance tires. And they'll get hit with most of the range penalty too.

Note that Elon's car is a different story: the wider rubber on his 21" wheels could make a real difference when paired with matching suspension changes.

On range: I have to agree that majority of the 21" range loss is due to stickier tires which result in higher rolling friction losses. As for better range in curvy roads, dream on. On curvy roads, if you actually care what tires you're using, you're probably burning so much power keeping your speed up that range loss due to tires is noise. Nice rationalization though .

 

[Owner]

As for better range in curvy roads, dream on. On curvy roads, if you actually care what tires you're using, you're probably burning so much power keeping your speed up that range loss due to tires is noise. Nice rationalization though .

I tried at least!

 

[rick325]

Thanks all. I wonder what sort of range these wheels give you. 22"'s, probably wide and super duper sticky...

Model S With 22 Tires * World's First*

 

[jiaotong]

I don't want to shoot myself in the foot range wise though.

I was thinking Tesla had solved range anxiety... = )

There is more bang for the buck in being efficient elsewhere. I woudl imagine driving style and speed by far outweigh tire size.

Tire size should choice should probably be as personal as exterior color. As another post mentioned, if you are tryign to get the most out of the car through tire size, 19" is already too big.

My .02
jim

 

[gtimbers]

The same continental tire as supplied on the 21" rims is available in our 19" size. I am a little surprised that this wasn't the tire of choice by Tesla. Of course, you will now get essentially the same range as with the 21", slightly less precision in turning maneuvers and a slightly smoother ride. There are also max performance all seasons that will most likely outperform the Goodyears. That tire is not particularly a great performer.

 

[rick325]

I don't want to shoot myself in the foot range wise though.

I was thinking Tesla had solved range anxiety... = )

I can give you $100,000 reasons (er, 100,000 reasons) to prove they've solved it. But going from 300-250 just because you want eye-candy-ghetto-rim bling (not that I'd get those rims) would be a bummer.

There is more bang for the buck in being efficient elsewhere. I woudl imagine driving style and speed by far outweigh tire size.

Sure. But all else being equal, how much are you giving up?

Tire size should choice should probably be as personal as exterior color. As another post mentioned, if you are tryign to get the most out of the car through tire size, 19" is already too big.

What, size matters?! Lol.

I'f I had my car already, I'd stop being overly obsessed about random hypotheticals. But I don't, so I am.

 

[jerry33]

Seems that the weight and rotational mass of the wheel+tire matters. Less mass is more efficient than more mass. Hence, the 19's are more efficient.

This depends on where the mass is. Closer to the centre of rotation has less effect than when the mass if further out. 21" wheels will have a greater effect in the city where there are many stops and starts than on long trips. On rough roads the larger mass of the 21" tires and wheels will also use more energy.

The 19's have less rubber on the road than the 21's. Hmmm.. they are both 245/xx/yy, so should have the same amount of rubber on the road.

The contact patch is dependent upon load and tire pressure, not tire size. If you have a 1000 lbs of weight on the wheel and have 40 psi, the contact patch will be 25 sq. in. (minus a small amount for casing factor--even if you installed 235/80R15 tires, assuming you could). The 21" tire has a slightly wider tread width because the lower aspect ratio doesn't have has much "bend" as the 19" tire. (Check the tire manufacturers' sites for the actual tread width.)

However, if you use 265 (like Elon), do you end up with more friction, or does it not matter since the weight of the car is distributed over a wider area.

You can develop larger cornering forces with the 265. Total sheer force is based on the contact patch size, belt construction, and tread compound.

How much does the spoke pattern matter?

The turbine wheels act like a fan to move air. This may--big may here until someone measures this--use more energy than the 19" wheels. However, the tread compound difference between the tires plus the weight of the driver's foot are going to trump every other factor.

 

[7racer]

here is a nice article that is apropos

Effects of Upsized Wheels and Tires Tested - Tech Dept. - Car and Driver