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Aero Wheels & Range - I'm confused

Aero Wheels & Range - I'm confused

With all of this talk about how range goes down at freeway speeds, how is it that they can get 320 miles with them? I'm guessing that the benefits of the aero wheels don't really help until you reach freeway speeds, at which point you wouldn't be getting the 300 mile range in order to add 20 to it...

Brian H | November 15, 2011

Thumped;
Yeah, I'm into puns too. And the ideal response is the fading scream of pain as the listener flees.
>:p

Volker.Berlin | November 16, 2011

Sudre++

sarge7359 | November 16, 2011

Aha, Brian H is a Callahan's fan. Or he picked up the pun reaction from same place Spider Robinson did.

And /agree with Denis -- I don't see why EVs are special in that mileage varies with how it is driven, etc. And that's the substance of the linked blog post. It could be that EVs vary more widely than ICE, but then again so do varies forms of ICE... which brings use right back to the numbers simply being a way of comparing 'ducks per liter per parsec' or whatever.

Brian H | November 16, 2011

MAJOR Callahan's fan. Best bar in the galaxy.

Brian H | November 16, 2011

Should stage a rush-hour road race; 'S' vs. "30 mpg" ICE car back and forth across town through traffic for 3 or 4 hours. Heh. Can you spell "sucking gas"?

;)

Larry Chanin | November 16, 2011

"Don't get the wrong impression, I do not disagree with any of the statements Siry's made, however, what I don't agree with their relevance;...What I really question here is the motivation of such an irrelevant article that makes reference to the obvious and does not take into account a level playing field."

Hi Denis,

The point is that the new 5-cycle testing that EPA is implementing is being applied to both I.C.E. and advanced technology vehicles to establish more reasonable expectations by the driving public. The new tests incorporate many of these common sense issues that Mr. Siry discusses.

Larry

Volker.Berlin | November 28, 2011

I wonder if the "aero wheels" will be "tall and skinny"...? In any case, the diameter of the Model S' wheels is on the upper bound of typical wheel sizes, so that may be a hint.
http://www.greencarreports.com/news/1066754_tall-skinny-tires-newest-gre...

I don't think that anybody has ever said that Elon Musk's statement regarding ultra aerodynamic wheels/320 miles range explicitly referred to the rims and to the rims only. IMO, there may be special rubber, too.

Brian H | November 28, 2011

Special one-way rubber, that grabs hard on the way down but releases easy on the way up ...
>:D
Ain't nanotech wonderful?

jbunn | November 29, 2011

I read something a few years ago about how geckos manage this. As I recall, the surface at the end of the toe pads divides finer and finer until it has a huge amount of surface area, clinging with van der Waals force, or perhaps capilary action.

If I have one way rubber, can I rotate my tires to the other side of the car when I'm ready to slow down?

EdG | November 29, 2011

I haven't seen the "Hot Wheels" feature listed yet. The one where you can spin the steering wheel 180 degrees. With that you've got a chance.

Volker.Berlin | November 30, 2011

I have a lot of fun reading your comments, however, I feel I should clear this up. When I said "special rubber" above I actually should have said "special tires". So far, the discussion in this thread has focused on aerodynamic rims, in particular the outside of the rims having an unusual shape such as plain flat closed or some kind of elaborate turbine blades.

That may have some aerodynamic impact, but as the article linked above points out, the shape of the tire as a whole makes a difference, too. It is trivial: Thinner wheels have less A in the CdA and therefore reduce aerodynamic drag. At the same time thinner wheels have a smaller contact patch with the road, which needs to be made up for by increasing the wheel diameter. Given that the Model S supports relatively large wheels, that may technically be an option. Question is if "skinny wheels" would be acceptable from a design perspective, assuming that safety and handling is the same, and assuming that they would significantly increase range.

EdG | November 30, 2011

I can't see how handling could possibly be the same. You're removing contact area on the ground. Small bumps, rocks, ice patches, rain, etc. will have a magnified impact when reducing the tire width. More weight per square centimeter, but can the rubber increase its static friction linearly as you do that? In other words, if you halve the width, half the rubber needs to use twice the weight to perform as before. I'm guessing that's not going to happen.

Increasing the height of the tire will, in general, make for less side-to-side stiffness, making turning a little softer, too.

Volker.Berlin | November 30, 2011

You're removing contact area on the ground. (EdG)

Which should be compensated for by a larger diameter and possibly an improved tire pattern, so that we end up at about the same contact area, albeit a different shape. I agree that a significantly reduced contact area would be an unacceptable disadvantage. For the sake of the argument, let's assume that it's possible to solve this.

Increasing the height of the tire will, in general, make for less side-to-side stiffness, making turning a little softer, too. (EdG)

Increased diameter does not necessarily require a higher tire wall. The height of the tire could stay unaffected when using larger rims.

EdG | November 30, 2011

Correct. Nix the comment on the height of the tire. I don't know why you'd want a higher tire anyway, unless you want to save tread life (bigger circumference means longer tread life).

If they could improve the tread pattern, I don't know why they wouldn't do it with any width. Why wait for thinner tires?

How can thinner tires "end up at about the same contact area" without softening the tires (which might allow more contact area front to back)?

Volker.Berlin | November 30, 2011

I don't know why you'd want a higher tire anyway

Precisely for this reason:

How can thinner tires "end up at about the same contact area"

Larger diameter -> larger contact area.

If you are interested, you can read the article (and the comments). It's all explained there:
http://www.greencarreports.com/news/1066754_tall-skinny-tires-newest-gre...

EdG | November 30, 2011

Personally, I'll leave it to the pros to decide these things.

But higher, thinner tires to change the contact pattern? It increases its front to back depth only because of lower curvature (not much help for, say, 20% higher tire) and increased flex (due to the same weight on less width).

For city cars (as in the article you reference) with limited battery range, this might be a good idea. I don't understand why a performance car would go this route.

Volker.Berlin | November 30, 2011

I don't understand why a performance car would go this route.

The only reason I can think of would be: "To achieve 320 miles range." That's the topic of this thread... ;-)

EdG | November 30, 2011

I don't want an extra 5 miles of range at the expense of good traction and cornering. I'd like to get there without undue worry about whether the thin tires will hold the turn. It's a matter of how much better "tall and thin" would be, range-wise, and how much worse (if at all) handling.

When all those wide, short-walled performance tires were being designed, I doubt they were thinking "Let's put in a mediocre tread pattern and poorer handling characteristics and make a tire we can easily beat by making it tall and thin later." On the other hand, they probably were not thinking about the front facing area that would induce higher wind resistance.

Larry Chanin | November 30, 2011

I gather if the tire were mounted outside the body of the car that increasing the diameter while decreasing width would have offsetting effects with regard to cross-sectional area. However, as long as the tire is surrounded by the car body wouldn't the increase in the tire's cross-sectional area due to greater diameter (height) be somewhat mitigated by being enclosed by the body? (I imagine the proximity of the tire to the body or fenders would effect the degree of mitigation.) Wouldn't the portion of the tire exposed below the body of the car contribute the greatest effect on aerodynamic drag? Isn't this the principal behind lowering the ground clearance of the car particularly at high speeds?

Larry

EdG | November 30, 2011

@Larry: I think what you're saying is "when you look at the tires from the ground in front of the car you'll only see a small portion of the tire" due to most of it being surrounded by the body and the car as low as possible to the ground (especially with the optional air suspension lowering the car at higher speeds).

If that's what you're saying, I agree. But we're talking about a car where side door handles are being tucked away, too. I have no idea how much difference a two centimeters of wheel width change would make.

Larry Chanin | November 30, 2011

Hi Ed,

Yes, that's what I'm saying. So if the tire is made taller the additional height doesn't contribute to the aerodynamic cross-section as much because it is tucked within the body of the car.

Larry

EdG | November 30, 2011

It's not the height of the tire that does that: it's the area that's exposed as seen from the front. Only the width of the tire tread and the height of the bottom of the car is important.

Larry Chanin | November 30, 2011

"It's not the height of the tire that does that:"

Hi Ed,

I believe that's what I said. :-)

"So if the tire is made taller the additional height doesn't contribute to the aerodynamic cross-section as much because it is tucked within the body of the car."

Larry

EdG | December 1, 2011

Sorry. Mis-read what you were saying. With some rest, it's hard to see what I was thinking you said!

Brian H | December 3, 2011

jbunn;
Yeah, I considered that. But they release by gently pulling loose from the leading edge. Not workable for rotating surfaces!

Brian H | December 3, 2011

Counting on TC to offset the loss of surface contact? Narrower wheels also reduce rolling resistance, of course.

Volker.Berlin | December 4, 2011

Counting on TC to offset the loss of surface contact? (Brian H)

Certainly not. TC can compensate for a lot of things (like incompetent or inattentive driver, at least to some degree), but it cannot magically create traction that plain isn't there physically.

Brian H | December 4, 2011

No, but it can prevent catastrophic loss of contact and hence traction, hence maximal use of all available traction. That's no help in sideways skidding, of course; just accel and braking.

Volker.Berlin | December 5, 2011

[...] hence maximal use of all available traction.

Oh sure. You were mentioning "loss of surface contact" which translates immediately into less available traction. Was there anything wrong with what I said?

brianman | December 20, 2011
David M. | December 20, 2011

Bad photo? Or just goofy looking?

Volker.Berlin | December 21, 2011

brianman, thanks for the link, I noticed that as well. It is the first picture ever of these suspicious aerodynamic wheels.

David M., I'd say it's photoshopped, suggesting that these wheels do not exist -- yet (still). I'll get them in any case, and if they look something like in the picture on the "Options & Pricing" page, I'll be happy. They do look unusual, as expected, but for me the additional efficiency/range outweighs a look that admittedly needs some getting used to.

brianman | December 21, 2011

Has anybody seen anything official on the expected impact of the 19" Aerodynamic wheels on the 40kWh flavor?

More specifically, the options page doesn't have the 300/320mi. indicated for the 85kWh nor does it mention a % impact (6%?).

Volker.Berlin | December 21, 2011

Aerodynamic 19” wheels are designed to reduce wind resistance. Disc-like in shape, they channel air along the sides of the vehicle and can add up to 5% range during highway driving.
http://www.teslamotors.com/models/facts

mwu | December 29, 2011

Apologies for reviving an older thread, but I've been thinking about the aero wheels and they get their performance by basically diverting air around the wheel. This means that not as much fresh air reaches the brakes.

If you plan to do performance oriented driving for extended periods, you probably want to avoid the aero wheels since they would reduce how quickly the brakes cool and lead to brake failure faster. For most people this won't be a concern, though.

The only person I know myself who has ever had brake heat issues on normal roads with a well maintained road vehicle was my brother in his VW R32 while running the Tail of the Dragon (http://tailofthedragon.com/dragon.html -- US 129 in eastern Tennessee which has something like 318 curves in its 11 mile length) when he was reaching upwards of 90Mph between the curves. When he finished the run his brakes and calipers were smoking.

Mycroft | December 29, 2011

With regen braking and normal driving, I don't think brake overheating will be a problem.

mwu | December 29, 2011

Agreed -- I wouldn't be concerned unless I wanted to visit a track or on anything like the dragon. Autocross shouldn't be an issue since it's short bursts and lower speeds the brakes should have sufficient time to cool between runs.

Brian H | December 29, 2011

Heh. Ask Roadster owners about brake wear and heating. I think all you'll hear back is a few snickers.

ggr | December 30, 2011

Snicker.

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