Engineering Explained- wheel size effects on electric cars

[Mikedrives]

I have a gravel alley behind my house and asphalt out front. I usually floor it in the alley just to hear it roll about 5 pieces of gravel as it figures out the situation. Sounds about the same when I do it on asphalt- the wheels spin maybe an inch before it figures it out. I ride with the windows down a lot because I like to hear how quiet it all is.

 

[FelixtheCat]

It very much depends on the actual spoke design, and how smoothly the air flows past and around the spokes. You’d have to carry out proper aerodynamic testing in a wind tunnel to obtain a definitive answer.

Or even better, use a really good CFD simulation software package to capture turbulent flows around the wheels. You'd need to have the wheels spinning in the wind tunnel, and measuring flow details at that scale is very difficult. Here's a link about CFD software and Model S wheel aerodynamics - https://www.researchgate.net/public...la_Model_S_-_Part_2_Wheel_Design_Optimization

 

[Clivew]

Less weight is important! Road friction is constantly stopping your rotation. Hence 18s are superior.

Wheel weight will have an extremely minuscule affect on efficiency and range. The percentage difference of rotational inertia between a 8kg and 12kg wheel will be negligible when you consider the kinetic energy of a 1700-1900kg vehicle.

 

[dsnows]

Obviously there are many factors, maybe too many to easily account for: road surface, tire tread design, tire width, overall weight, spoke design, tire pressure... probably more that I don't know about. One thing that seems obvious to my simple brain is that you don't want air turbulence in the wheel. Why not have a solid wheel cover? is the air flow needed for brake cooling or any other reason? I realize many people will choose form over function. I would take optimal range over appearance every time.

 

[Clivew]

Why not have a solid wheel cover? is the air flow needed for brake cooling or any other reason?

Yes, airflow needed for brake cooling, this is why the aero covers are mainly a flat cover with the rounded slots to promote that smooth laminar airflow.

 

[Rudedog]

Are you 16 years old? Is that why you want to burn rubber? Give me a break!!

 

[Worldfishnski]

I reduced my rim size from a 20" to a 19" on my Model X because I read (somewhere) that I would gain 3%-5% range if I did that. I can't honestly say that I have noticed a difference.

There is consensus that range can be improved by using wheel covers that are designed for range efficiency.

Any ideas on how to find an aftermarket wheel cover that would improve range?

Would trial and error be the only way to know if a wheel cover could even work for my rims?

It also sounds like this open spoke design is the least efficient rim.

Have I screwed myself?

 

[ianc]

Badly flawed logic in the assumption that a larger wheel size is inherently less efficient. He talks about the coastdown coefficient in part three, but is the rolling resistance of the tires held constant for these numbers? Of course not, his conclusions are invalidated because the 20" wheels have sticky performance tires, while the 18's are optimized for efficiency. The ventilation drag theory sounds very plausible, but which will be more efficient: a 20" wheel with an aero cover, or an 18" spoke design? There's no data here to support the theory that a larger wheel is inherently less efficient than a smaller one to any significant degree. Less sidewall does offer superior handling however.

It's all about the tires...

 

[Clivew]

Badly flawed logic in the assumption that a larger wheel size is inherently less efficient.

A larger diameter wheel is less efficient aerodynamically compared to a smaller diameter wheel of the same design, this is a hard fact, based on science. It's exactly the same reason a wind turbine with a span of 18 meters produces less torque and energy compared to one with a span of 20 meters. Simple aerodynamics based on know physics. It's the same factual based physics that allows you to drive a car like the Tesla Model 3.

The ventilation drag theory sounds very plausible

Plausible? No, it's a hard fact based on scientific evidence.

but which will be more efficient: a 20" wheel with an aero cover, or an 18" spoke design?

No idea, it all depends on the design of each wheel. To make an accurate comparison about aerodynamic efficiency in regards to wheel diameter, both wheel designs must be the same.

There's no data here to support the theory that a larger wheel is inherently less efficient than a smaller one to any significant degree.

It's not a theory, it's a fact based on the physics model of this universe!

It's all about the tires...

No, it's a combination of wheels and tyres.

 

[jerry33]

The ventilation drag theory sounds very plausible, but which will be more efficient: a 20" wheel with an aero cover, or an 18" spoke design? There's no data here to support the theory that a larger wheel is inherently less efficient than a smaller one to any significant degree. Less sidewall does offer superior handling however.

1. The tires have far more effect than any wheel, which doesn't mean the wheels should be ignored--just don't expect miracles.
2. Less sidewall improves dry handling on smooth pavement, However, wet, snow, expansion joints, etc. suffer.
3. Less sidewall is far more likely to have pothole damage.
Rule of thumb is the shorter the sidewall, the less practical the tire. The main advantage of low profile tires is more profit to the tire and wheel manufacturers.

 

[ianc]

It's exactly the same reason a wind turbine with a span of 18 meters produces less torque and energy compared to one with a span of 20 meters

This is easy to understand. You mention that the air exciting the high pressure area in the wheel well causes ventilation drag when it exits.

Fine. Now I've got 18" and 20" aero wheels and both have the exit slots cut at a 16" radius, giving the same moment arm for the ventilation drag. Is the 18" still more efficient?

If the answer is yes, I'd be interested to hear the reasons why.

If the answer is no, then I guess the efficiency of the wheel doesn't actually depend on its size, but rather on its design, doesn't it? Which would kind of put paid to the fallacy that larger wheels are inherently less efficient...

I'd also be interested to know how significant these effects actually are in relation to those of the tire compound, which testing has shown to be in the neighborhood of 10%? Something tells me not very...

 

[holmgang]

"copious amounts of rubber left at every light" not detected. and i suspect will never be found.

 

[holmgang]

T
If the answer is no, then I guess the efficiency of the wheel doesn't actually depend on its size, but rather on its design, doesn't it? Which would kind of put paid to the fallacy that larger wheels are inherently less efficient...

he did say it depends on the design.
the claim of larger wheels being less efficient is only fallacious in a pedantic sense. wheel designs scale with size. a 15" wheel offered in a 19" size doesnt gain all its 4" just in the lip. so for all practical purposes, the statement holds true.

 

[holmgang]

Or even better, use a really good CFD simulation software package to capture turbulent flows around the wheels. You'd need to have the wheels spinning in the wind tunnel, and measuring flow details at that scale is very difficult. Here's a link about CFD software and Model S wheel aerodynamics - https://www.researchgate.net/public...la_Model_S_-_Part_2_Wheel_Design_Optimization

not better. faster and easiest to do quick experiments with, but in the end, needs physical testing to determine the actual truth.

a simulation is only as good as the model used. in my field, we make all sorts of thermal and fluid simulations using $100k/license software + compute cost to run it on hpc..... then we spend a few $M on a physical process to gain an extra data point and re-run the models again.

it would be great if that multi-million dollar piece of the workflow was magically eliminated from the picture.

 

[Clivew]

This is easy to understand. You mention that the air exciting the high pressure area in the wheel well causes ventilation drag when it exits.

Yes, ventilation drag adds to the aerodynamic drag of the rotating wheel spokes.

Now I've got 18" and 20" aero wheels and both have the exit slots cut at a 16" radius, giving the same moment arm for the ventilation drag. Is the 18" still more efficient?

This, again, is not an accurate comparison as the design of both wheels is different.

If the answer is no, then I guess the efficiency of the wheel doesn't actually depend on its size, but rather on its design, doesn't it? Which would kind of put paid to the fallacy that larger wheels are inherently less efficient...

You seem to be ignoring the phrase 'same design' which I have previously posted three times (shown below). In engineering and science, when testing something like this, you have constants (in this case the wheel design), to make sure the results are as accurate as possible, otherwise the comparison is absolutely pointless.

A larger diameter wheel produces more ventilation drag and thus reduces range compared to the same design wheel of a smaller diameter.

A larger diameter wheel is less efficient aerodynamically compared to a smaller diameter wheel of the same design, this is a hard fact, based on science.

To make an accurate comparison about aerodynamic efficiency in regards to wheel diameter, both wheel designs must be the same design.

Holmgang weighs it up perfectly in his post #53

I'd also be interested to know how significant these effects actually are in relation to those of the tire compound, which testing has shown to be in the neighborhood of 10%? Something tells me not very...

An average of 3.4% IAW this test;

The Tesla Model 3's Aero Wheel Covers Improve Efficiency

 

[freeAgent]

Larger wheels of the same design weigh more. So, all else being equal, larger wheels are less efficient. More weight = more power used to overcome inertia both in acceleration and braking. That's Newton's first law of motion.

 

[dfwatt]

Larger wheels of the same design weigh more. So, all else being equal, larger wheels are less efficient. More weight = more power used to overcome inertia both in acceleration and braking. That's Newton's first law of motion.

Efficiency losses associated with larger wheels are minimal at least in terms of their weight if not their aerodynamics (where our esteemed colleague Clive has made it clear that their contribution is non-trivial). And most of that extra energy used to accelerate is recouped in regen braking in any case.

Where heavier wheels really do have an impact is on acceleration times where you can potentially gain two-tenths of a second going from the OEM 20 inch wheels to a lightweight 18in forged wheel. Even that's rather modest when you think about it. Efficiency losses associated with drag on the other hand are pretty significant. So if you lower your car and you put on a front air dam, you can gain significant efficiency we've chopped about 15-18 watt hours per mile off our consumption by lowering both cars with MPP coilover kits.

 

[freeAgent]

Efficiency losses associated with larger wheels are minimal at least in terms of their weight if not their aerodynamics (where our esteemed colleague Clive has made it clear that their contribution is non-trivial). And most of that extra energy used to accelerate is recouped in regen braking in any case.

Where heavier wheels really do have an impact is on acceleration times where you can potentially gain two-tenths of a second going from the OEM 20 inch wheels to a lightweight 18in forged wheel. Even that's rather modest when you think about it. Efficiency losses associated with drag on the other hand are pretty significant. So if you lower your car and you put on a front air dam, you can gain significant efficiency we've chopped about 15-18 watt hours per mile off our consumption by lowering both cars with MPP coilover kits.

It sounds like we agree. I was not saying that the difference due to weight is large, but it is there.

 

[dfwatt]

It sounds like we agree. I was not saying that the difference due to weight is large, but it is there.

Well I'm not disagreeing with Newtonian mechanics. But I think we might not agree on where wheel weight sits in a sequence of quantifiable efficiency losses. For sure it comes after general body aerodynamic losses, after wheel aerodynamic losses and also after rolling resistance losses. I'd describe it as minimal in that sense and almost academic. Would you agree?

 

[Gasaraki]

He discussed the tire/contact patch, in his first example of a 205mm tire vs a 305mm. Given his numbers, the difference was minor. Point 2 discussed the rolling resistance, which had a greater impact than the first, but not massive. The wheel diameter definitely had a greater effect than both, due to the amount of open area. A flatter wheel, with less room for air to pass through spokes, is ideal.

Understand but he can't say that a larger wheel will cause this. You can put a 19" eco wheel designed for EVs vs an 18" wheel, for example the Tesla Model 3 wheel without hub caps and the 19" wheel will be more efficient.