Aero spokes? - Fuel Economy, Hypermiling, EcoModding News and Forum

serialk11r · 08-11-2014, 07:59 PM

So moon disks and pizza pans are great and all but some people do care about brake cooling, and many people (not here) think covered wheels look weird. So I was thinking, there are aero spoke bicycle wheels, why not cars? Would the improvements be too small?

Some cars already come with "aero spokes", in that the spokes get a lot wider as they hit the edge of the wheel where the air would be moving the fastest.

Painted black, spoke fairings could look pretty good IMO. Kind of like if you took these concept wheels and made the black bits bigger:

Grant-53 · 08-11-2014, 08:11 PM

Anyone seen data on mag wheel spoke configuration? What would be preferable for brake cooling, inboard to outboard? Wait! Ecomodders don't usually use their brakes enough to generate much heat. That's a road racer thing.

Smokeduv · 08-11-2014, 10:52 PM

I was thinking about brake cooling, as I've had some minor episodes where the brakes fade to the point that the pedal goes all the way and there are no brakes, so you have to release the pedal an apply the brakes again (it was an old Nissan Sunny '87 or so). It's pretty scary if you don't know what to do, so I'm a bit skeptical about the pizza pans. I think that the best solution could be a brake duct cooling, but I don't know if it would be worse, aerodynamically speaking.

serialk11r · 08-11-2014, 10:59 PM

I am not trying to imply that brake cooling is necessary during normal driving, I just list it as one of the several reasons people are opposed to wheel covers.

oldtamiyaphile · 08-12-2014, 07:34 AM

Quote:

Originally Posted by Smokeduv

I was thinking about brake cooling, as I've had some minor episodes where the brakes fade to the point that the pedal goes all the way and there are no brakes, so you have to release the pedal an apply the brakes again (it was an old Nissan Sunny '87 or so). It's pretty scary if you don't know what to do, so I'm a bit skeptical about the pizza pans. I think that the best solution could be a brake duct cooling, but I don't know if it would be worse, aerodynamically speaking.

This is easily solved by keeping your car in good condition and not driving like an r'tard.

My 2012 Fiat has solid (non vented) disks front and drums rear, and it will still go around a race track without significant fade. Anyone serious about performance driving will have a spare set of wheels and track tyres, so the problem of brake cooling on the street is all but imaginary.

It would be nice if we could get CD data on wheels, even if that data didn't mean much as it would be dependent on the way the wheel's aero interacts with the car body.

Smokeduv · 08-12-2014, 08:46 AM

Quote:

Originally Posted by oldtamiyaphile

This is easily solved by keeping your car in good condition and not driving like an r'tard.

My 2012 Fiat has solid (non vented) disks front and drums rear, and it will still go around a race track without significant fade. Anyone serious about performance driving will have a spare set of wheels and track tyres, so the problem of brake cooling on the street is all but imaginary.

It would be nice if we could get CD data on wheels, even if that data didn't mean much as it would be dependent on the way the wheel's aero interacts with the car body.

Yeah, I know. My Fiat and now the Fiesta ST have excellent brakes and I don't even use them or slam my foot on them, they last more than 50k miles and I know that they are not going to fade easily, but the old Sunny didn't have good brakes and I was working in a place with lots of hills where it is silly to the point that when you go down, there is a speed bump on the lowest part, so you have to brake to almost zero when you are going downhill and then accelerate again to go uphill again, so the old Sunny had brake fading a lot and very easily, and it was serious because you are going downhill, so, that was my main concern, as it really happened and all without driving and using the brakes as if there were no tomorrow. As you say, performance-wise, you are going to have a different set of wheels for track use.

Smokeduv · 08-12-2014, 08:51 AM

Anyway, many cars nowadays are not going to have brake cooling issues (we have better brakes and better brake fluids now), and as seen on many cars here, they are not going to have any problem with pizza pans and those things, I was just stating that there is a possibility, although minimal, but there is. I'm already planning on putting some wheel covers on my Fiesta.

RedDevil · 08-12-2014, 09:08 AM

Wheels are open at the back where the brakes are. I doubt closing the front makes a big difference.
You cannot really lock up the air like that. It would start spinning round with the wheel and blow away through centrifugal force, being sucked in around the axle and pushed out at the rim edge. This may even aid rather than restict brake cooling.

As for spokes, it is the big rim bucket that causes most of the drag. The skinnier the spokes the bigger the gaps between them and the worse the air resistance would be.

ChazInMT · 08-12-2014, 10:18 AM

It is easy to think of a wheel spinning under a car as a fan, like when it is on the wheel balancing machine. How would the air be effected in this situation is pretty easy to see, it would act pretty much like a fan.

But this is NOT the environment a tire rolling under a car is in, the air is stationary and the car is moving through it. The contact surface of the tire is coming to a compete stop, and the top surface of the tire is moving through the air at twice the speed of the vehicle. The air at the hub is moving along at the same speed as the car. In light of this, you start to realize that the wheel and tire system rolling underneath the car is a Very complex aerodynamic environment.

When the wheel & tire are rolling along under the car and Through the air (which is stationary), to the air, the tire comes to a dead stop when it is on the ground and the tire surface is going twice as fast as the car when the tire is at the top of its rotation as the wheel spins on its axle. So it is a tremendously dynamic system that exists here.

This is known as a cycloid if you want to Google it. there is a lot of history behind it and a ton of math.

Looking at this, you can sort of see that the “blades” on a wheel are doing nothing when they are at the bottom, but, on top the blades are moving quite fast, so it would seem that they are moving most of the air when at the top half of the wheel and not so much in the bottom half.

Kind of a cool thought puzzle when you really consider everything going on. Way more to it than you would consider at first glance.

It is also another reminder that we need to be mindful of the fact that our cars are moving through the still air, the air is not “Blowing” around our cars. With a boat it is pretty intuitive that the water just sits there and the boat move through it, but for cars, we all seem to think that the car is just sitting there and the air is blowing over it, like in a wind tunnel. But it doesn't and a rolling tire/wheel system really illustrates the fact that it's much more complicated than you'd think.

RedDevil · 08-12-2014, 10:37 AM

There can be something else at play too.
The air trapped in the wheel well moves with the car, more or less. The air outside of the wheel is pretty much standing still.
This may cause a pressure difference, but I cannot work out whether this would suck the air through the wheel, and if in which direction that would be.

With solid wheels or moon disks it would not happen, that's for sure.

I wonder whether the wheel can help dissipate brake heat. The brakes are fitted to the hub, not the wheel itself; otherwise changing tires would be cumbersome.
Nonetheless, if the wheel can double as a heat sink then (semi) solids may be more effective at cooling the brakes than spoked wheels...