Would you rock solid/flat/smooth wheels for more range?

[dhanson865]

Apparently you have missed my point, which is that it is the larger diameter brake rotors that eliminate the issue you noted above, about friction braking taking longer than you would prefer.

OK, this seems to be a reading comprehension issue because I never said a thing about friction braking taking longer than anything. What I said was the friction braking was quicker than it needed to be.

I'm still not sure how you get to saying that I said something I didn't say.

If you could somehow just say you disagree without putting words into my mouth I wouldn't take issue with an opposing viewpoint.

 

[SageBrush]

Due to the limitations of communicating with a keyboard on the internet, sometimes my quirky personal sense of humor is taken a mite bit more seriously than I'd like. Please, pretty please, with sugar on top, go back to my earlier posts and make an attempt to accept that they are posted in good fun, not with malicious intent at all. I do sincerely apologize for any misunderstanding in this regard, and I assure you I was just funnin' and stuff

As I hope you know, I find your writing to in general be delightful, and without any intent to pile on, I have to join the ranks of the impaired and admit that I also don't find humor in hyperbole. I just about always take it as misdirection or a straw-man, similar in intent to debating a point with ad-hominem.

Cheers!

 

[SageBrush]

There are situations where bigger is better. Apparently you have missed my point, which is that it is the larger diameter brake rotors that eliminate the issue you noted above, about friction braking taking longer than you would prefer. In those situations where Regen is not available, for whatever reason, I'd rather have the largest brake rotors that can fit under the car, no matter the weather.

What are the situations ? The only one I can think of outside of track racing are really long downhills where people ride the brakes leading to rotor overheating and so called brake fade. But that situation would presumably not occur in a car with regen and sufficient battery capacity.

I don't think inclement weather need be considered because the limiting factor in stopping distance is the tyre/road friction which decreases, so brakes can only do less in those situations.

 

[Red Sage]

What are the situations ? ... I don't think inclement weather need be considered because the limiting factor in stopping distance is the tyre/road friction which decreases, so brakes can only do less in those situations.

Bigger is better than smaller in certain situations. In perfect weather conditions, 12" rotors on a 3,500 lbs car would likely be sufficient to come to a halt, even in an emergency, if using tires that are only 185mm wide. But I would expect 235mm wide tires with 14" rotors to perform better even in slightly worse conditions. And I would further expect 275mm wide tires with 16" rotors to greatly exceed the capability of either even during inclement weather conditions. Of course, I could be wrong.

Cosmetically, to me, just about anything over an 18" wheel begins to look rather comedic on most cars, practically a caricature. I call them 'wagon wheels', especially the 24" or larger wheels on jacked up SUVs that never venture further 'offroad' than the back lawn. But again, the larger diameter wheels allow for larger diameter disc brakes in passenger cars. And since larger diameter wheels tend to also allow for wider tires to be applied, you have a greater contact patch on the tarmac. Combined those allow for better control and shorter stopping distances at all speeds. That should inspire both confidence behind the wheel, and a greater sense of safety for all occupants. Of course, I could be wrong.

 

[McRat]

Moon is still in Santa Fe Springs, California. My Bonneville Salt Flat wheels were 18" and they made discs for me.

Call them and see what they can do.

 

[SageBrush]

I would expect 235mm wide tires with 14" rotors to perform better even in slightly worse conditions. And I would further expect 275mm wide tires with 16" rotors to greatly exceed the capability of either even during inclement weather conditions.

I'm out of my league here, so apologies in advance for any nonsense to follow ...

It seems fair to distinguish between tyre traction and rotor diameter. Certainly a wider tyre will have more traction (all else being equal), but a larger diameter tyre is not a pre-requisite. And so long as the rotor is up to the task of maximizing the available tyre traction, I'm not sure that an even bigger rotor gets you anything.

 

[SageBrush]

Certainly a wider tyre will have more traction (all else being equal), but a larger diameter tyre is not a pre-requisite.

I know self-quotes are not PC; my hour to amend my post has passed.

Here is the list of available widths for 16" tyres at tire rack. They go up to 315 mm.

 

[SageBrush]

I agree that discs are not usually attractive, but if Tesla solar tiles have taught anything, it is that amazing imitation is possible with a good printer and a layered manufacturing process. I'm more interested in knowing how much discs impede brake ventilation.

 

[Red Sage]

I'm out of my league here, so apologies in advance for any nonsense to follow ...

It seems fair to distinguish between tyre traction and rotor diameter. Certainly a wider tyre will have more traction (all else being equal), but a larger diameter tyre is not a pre-requisite. And so long as the rotor is up to the task of maximizing the available tyre traction, I'm not sure that an even bigger rotor gets you anything.

Yeah. My presumption was that the tires themselves would be the same approximate diameter, plus/minus 1" or so. That's why in earlier posts I spoke of wheel diameter, as you probably can't fit an 18" rotor inside a 16" wheel. I believe the other individual was arguing against pretty much any wheel diameter over 17" or 18" being completely unnecessary except for cosmetics. I was just pointing out that if having a 20" wheel diameter allows for larger diameter brake rotors that potential improvement in safety is, or may be, an acceptable non-cosmetic basis for the design decision by Tesla, or any other automobile manufacturer.

I apologize for perhaps being hyperbolic in my earlier replies. I am just really passionate about Safety. That is being able to drive quickly and fast and still being safe is important to me. I consider Safety to be a fundamental component of Performance. I have held this position even more firmly since three things happened in automobile racing: 1) Ayrton Senna died; 2) Alex Zanardi lost his legs; 3) Dale Earnhardt died. In my opinion the accidents in F1 and CART happened because the governing bodies outlawed technologies that made the cars safer at higher speeds. The fatality in NASCAR happened because there was a known technology that could prevent it, but the governing body didn't mandate it until after there was a high profile fatality. In each case, the technologies were eliminated, disallowed, prevented from being used, or simply ignored for the sake of 'preserving the formula', under the belief that there was some sort of 'unfair advantage' for those teams that employed them before (making it so everyone else was racing for second, third, or fourth place), or that using such things made racing more about the machine than about the driver, or that there was too much 'Pansy-ass Nanny State' meddling in the affairs of Racing Teams. Thus, the idea was that a 'Real Man' would be able to 'tame the machine' and still perform at their utmost, so that the 'skill of the Driver would shine through' no matter the driving conditions or vehicle operated. Essentially, their position was that race car driving was SUPPOSED TO BE UNSAFE -- by design, and that the Drivers 'knew what they were getting into' that's why they got paid the big bucks...

If an electric car can be built so that it stops from 60 MPH in the distance that an ICE does from 40 MPH, that is a GOOD thing that should be DONE. Anything that contributes to that possibility is a GOOD thing. Everything that makes electric cars ordinary, simple, the same as everything else because, "Meh. That's good enough. No one will notice anyway.", makes the adoption of sustainable transportation that much less compelling. I'd rather electric cars be seen as extraordinary, advanced, better than everything else. IMHO.

 

[SageBrush]

I was just pointing out that if having a 20" wheel diameter allows for larger diameter brake rotors that potential improvement in safety is, or may be, an acceptable non-cosmetic basis for the design decision by Tesla, or any other automobile manufacturer.

I presume that DHanson is saying that rotors larger than what can be put on a 17" wheel is over-spec'd for any passenger car tyre. If he is right then I agree with the sentiment to avoid the larger sizes only out of a bow to bling.

Wikipedia had a lot of interesting stuff to say about the components of tyre design that affect traction. New to me, it turns out depending on tyre materials diameter can increase, decrease, or negligibly effect traction. Pneumatic tyres are placed in the last category.

Cheers, and thanks for the interesting discussion.

 

[David99]

I'm more interested in knowing how much discs impede brake ventilation.

I have done some testing regarding temperature while I had them on and the difference is insignificant. I have measured temps with and without the disks and the difference is small, but more important, the brakes never get to temperatures that would be concerning. Unless you are on a race track, there really isn't a situation where your brakes would get anywhere near overheating in an EV.

 

[SageBrush]

I have done some testing regarding temperature while I had them on and the difference is insignificant. I have measured temps with and without the disks and the difference is small, but more important, the brakes never get to temperatures that would be concerning. Unless you are on a race track, there really isn't a situation where your brakes would get anywhere near overheating in an EV.

Thanks for the data

How much friction braking was in your testing ? Any emergency braking, or extended friction braking ?

 

[brianman]

IIRC, Tesla tried to sell Model Ss a few years ago with mostly flat wheels and nobody bought them so they discountinued them. I wouldn't buy them either.

The original OEM 19" wheels were cheaper than the Aero option. If they were the same price (and still available), I would have opted for them on my P85D.

 

[JoeS]

My plea goes for not only seriously-aero wheels, but clip-on rear fender skirts. In my book, function trumps currently-in-vogue aesthetics (sorry for the use of the "t" word).

 

[David99]

Thanks for the data

How much friction braking was in your testing ? Any emergency braking, or extended friction braking ?

I measured the temps a few times after normal city driving. I did this with and without the disks on and the difference was small. I don't remember the exact numbers. Maybe 10-20 degree difference. Variations in weather are much greater. On road trips I drive mostly on freeways where I don't touch the brakes at all except in emergencies. Even slamming on your brakes at 150 mph and coming to a stop won't get the brakes near their critical point. Extended friction braking is really hard to do in a Model S. Maybe if you live on top of a large mountain and you charge to 100% and drive down just on friction brakes. LOL
Thermal issues are at the very bottom of the list of possible problems.

 

[SageBrush]

Even slamming on your brakes at 150 mph and coming to a stop won't get the brakes near their critical point.

Any data to support your statement ?

 

[Frank99]

Well, let's see. A 2000 KG car (roughly 4400 pounds) at 250 kph (155 mph) (about 70 m/sec) will have roughly KE = 1/2 m v^2 = 4.9 megajoules of energy that need to be converted to heat to brake to a stop. Assuming that all of that heat energy goes into the disks and none is converted into heat at the tire/road interface and none is dissipated to the air because of the short period of time, and that the disks weigh about 10 kg (22 pounds) apiece and are made out of iron (about 460 J/Kg-K), we end up with
Temp rise = 4900000 Joules / 460 J/Kg-K / 40 Kg = 260 degrees K = 260 degrees C.
So if your disks start out at 40 degrees C (about 104 degrees F), and you do this emergency stop, the disk temperature will end up at about 300 C (572 F). This is about where "hot brake smell" starts becoming noticeable, and well before there's a risk of brake problems (Marathon Brake Systems - Know Your Brake Temperatures)

So I'm buying David99's statement.

 

[30seconds]

You can also get custom print vinyl for the disks if you want a different look. Bascially anything you want

 

[SageBrush]

Well, let's see. ...

Looks like good math to me -- thanks.

So here we have it -- a sensible explanation for a massive rotor ;-)

 

[dhanson865]

Looks like good math to me -- thanks.

So here we have it -- a sensible explanation for a massive rotor ;-)

Note the rotor has to have mass not diameter. It can be thick, it can be dense, it does NOT have to be a large diameter.

And it's for heat fade avoidance not actual normal stopping power.

Mechanical Engineering: What limits the size of brake rotors in a vehicle? Do bigger rotors mean more braking? - Quora

The single best advantage from larger rotors isn't better braking through mechanical advantage (though that does theoretically exist, the limiting factor is almost always the grip on the road, so it doesn't help braking).

The single biggest advantage is improved resistance to heat. For two reasons. One, the larger rotor will have more area to cool, so it's more resistant to fade. Two, the larger diameter will allow for a greater mechanical advantage after fade sets in, so it performs better under fade.

Vented, slotted, and drilled will improve cooling. Vented the single best for that. Slotted and drilled will improve resistance to fade. Fade is usually accompanied by gas pressure from the pads heating up making a lubricating bufffer between the pad and the rotor. The slots and drill holes give the gas a place to go, so for the same temperature, the fade will be less with slotted and drilled rotors.

...

Race cars almost always have vented slotted and drilled.

Really once your brakes have enough stopping power to keep the car from running away when some idiot puts both pedals to the floor at the same time you've got big enough rotors. And it doesn't take 19" wheels to do that.

It takes more braking power to stop the pull power of the motor at stop than it does to stop the car from highways speeds.

So unless Unintended Acceleration is your primary concern then very large wheels are about style, not safety.

You can argue about where between 15" and 19" true safety max is for the Model 3 but I'm saying it's probably around 16", maybe 17".

And I'm saying I'm willing to use 16" wheels if max safety is not outside the range of 15" to 17" and I'm willing to use 17" wheels if max safety is not outside of the range of 16" to 18". And if by some chance 15" is in the range of max safety I'm willing to use those.

I just don't want to have to use 18" wheels if max safety occurs at 16" just because the style police don't like the look of 16" wheels.