# Why do car wheels have holes?

If you have a look at the car's wheels, you'll notice that they have holes which can be of different forms (mostly circular or rectangular).

Why do they have such holes? Doesn't that reduce the stiffness of the wheels?

• I don't know the real answer but common sense tells me that the weight is not a real factor (insignificant % of reduction that is likely outweighed (pun intended) by tire wear or size). Heat reduction is also unlikely as there is already a lot of air movement and less metal means less heat exchange - you would get more surface area by closing those holes. Aerodynamics and aesthetics are the only viable things I can come up with. I suppose I could have googled it for you though :) – user1675 May 19 '15 at 18:10
• Weight is a much greater factor than you give it credit for. Some car enthusiasts spend thousands to get wheels that are made of a lighter alloy. It gives incredible performance and handling gains to minimize the unsprung weight of the car – M.M May 20 '15 at 5:05
• @Mike: Common sense depends on who that commoner is. As someone who builds model airplanes I can tell you for certain that common sense tells me that tiny holes are for weight reduction. You see that one wheel and you think that's a small amount of material removed. But if you strip a car down to it's chassis you'll see lots more tiny holes everywhere in the body work. All those holes add up. There's an adage among plane builders (model and full scale): it's a lot easier to reduce 1g of weight in a thousand places than it is to reduce a kilo of weight from a single place. – slebetman May 20 '15 at 6:02
• It allows you to inspect the brakes without removing the wheel – user1695 May 20 '15 at 7:48
• When considering the % of weight saved by creating the holes, it's not the % of the weight of the car, but the saving against the original weight of the wheel itself. That % can be significant. See my answer regarding the importance of reducing unsprung weight. – Steve Ives May 21 '15 at 8:08

Car wheels have holes mostly due to weight and cost considerations. Each hole is a chunk of material that you aren't wasting and weighing down the wheel with.

As another bonus, the holes help with cooling the brakes by allowing airflow between the inside and outside.

The shape and size of the holes are calculated to have a minimal impact on the structural integrity of the wheel.

• Exactly what I would have said! Additionaly: look at the rims of these holes, they have a little fillet to help against buckling and crack propagation – Knigge46 May 19 '15 at 14:42
• While weight and cost are certainly factors, I would expect that brakes would be more prone to overheating without holes in the wheels for ventilation. – supercat May 19 '15 at 16:38
• Also, reducing the mass of the wheel has the added benefit of being easier to turn, which reduces the amount of energy used to make the car move. So not only is the car lighter but wheels spin without so much loss in power. Correct? – Jasper May 19 '15 at 20:03
• I'm just spit-balling, I don't actually know - do the hole-y rims have a better strength to weight ratio than a solid rim? – coburne May 20 '15 at 15:20
• although these are some decent/good points, the most benefit, especially from an engineering point of view, comes from reducing unsprung weight. this is not an engineering answer, its more of a everyman's answer or an answer from the view of a technician. – agent provocateur Mar 18 '16 at 22:38

Many of the answers so far have mentioned that part of the purpose of the holes is weight reduction, but most of them don't express why weight reduction in the wheels is important. There are two major reasons; the first (also mentioned by Steve Ives) is that the suspension systems in vehicles operate better if the 'unsprung' mass is kept as low as possible, and the second (not mentioned so far) is that shaving weight from the wheels contributes more significantly to performance than shaving weight from the rest of the vehicle.

To see why this is true, consider the energy that the engine must put into the vehicle to get it moving at a speed $v$: $$E=\frac12 m_tv^2+\frac12 I\omega^2$$ For the wheels we can express the angular velocity in terms of the linear velocity as $\omega=\frac{v}{r}$ where $r$ is the radius of the wheel. The moment of inertia of the wheel can be expressed as $I=\eta\ m_dr^2$ where $m_d$ is the mass of the wheel. Here $\eta$ is a number between $\frac12$ and 1, but is closer to 1 since a wheel is closer to a thin hoop than a solid disk. Sticking all of this back in gives \begin{align} E&=\frac12 m_tv^2+\frac12\eta\ m_dr^2\frac{v^2}{r^2}\\ &=\frac12 (m_s+m_d)v^2+\frac12\eta\ m_dv^2\\ &=\frac12[m_s+(1+\eta)m_d]v^2, \end{align} where I have used $m_s$ as the non-rotating mass of the vehicle. So, you can see that shaving mass from the wheels is equivalent to shaving a factor of $1+\eta\simeq 2$ as much mass from the non-rotating parts of the car.

There is an additional, relatively minor, effect due to angular momentum for which it is advantageous to reduce the weight of the wheels. Due to conservation of angular momentum, the body of the car will tend to roll toward the outside of a turn when the wheels are rotated to initiate the turn. Reducing the moment of inertia of the wheels reduces their angular momentum and thereby reduces the amount of body roll upon steering.

• This is actually the most accurate answer. To put it in layman's terms, for every pound of weight taken off of the wheels is like 10 pounds off the sprung weight. This is the reason Centerline designed the Convo-Pro wheels, and why Ford has gone to Carbon Fiber wheels for the Mustang GT350. Great answer. – Pᴀᴜʟsᴛᴇʀ2 Mar 1 '16 at 15:35
• This is the only engineering answer. Unsprung weight is one of the biggest factors for vehicle suspension design and ride quality. – agent provocateur Mar 18 '16 at 16:58
• Perfectly said answer in the perspective of engineering principles. :) Great! – Jem Eripol Sep 9 '17 at 12:51

Mainly to reduce weight. A car's handling characteristics are improved by keeping the 'unsprung weight' (the weight of the car not isolated from the ground by springs i.e. the wheels, axles, hubs, brake disks, calipers, etc.) as low as possible. Holes in the wheels reduce this weight.

The lower weight helps the unsprung portions of the car to follow the bumps and dips of the road more closely.

The holes in wheels serve a few purposes. They reduce the weight of the wheel itself, although not by much. The holes in those particular wheels actually appear to be adding rigidity and strength to the wheel. The extra folds in the steel make it stronger than if it was just flat. The holes may also help prevent the build up of brake dust. I believe Ratchet Freak is correct about airflow.

It's for airflow to allow for extra cooling. In most situations, that extra airflow isn't going to help. But when you're doing heavy braking coming off of a mountain, it can make the difference between your being able to brake and your brakes failing from over-heating.

Yes I agree, the "moment of inertia" is a factor in making "spoked" wheels, the holes in pressed wheels will reduce weight, and allow circulation.

The truth is, for this kind of wheel, it is largely cosmetic. It also makes them easier to manually handle (finger holes).

For this type of wheel it would not make a lot of difference if they were not there. But even if they were discs milled from billet magnesium alloy they can be a lot thinner between the hub and the rims, in the same way an I-beam profile is thinner than a square beam.

• Hi mckenzm, welcome to Engineering SE. Can you clarify "this kind of wheel" for readers who don't intuit your meaning? I assume you're referring to stock wheels on typical passenger cars and light trucks, as opposed to high-performance sport and luxury cars. Or perhaps you simply meant spoked vs. solid. Also - not clear who you're agreeing with unless you provide an explicit link. :) – Air May 21 '15 at 15:09
• This is a pressed metal rim, closely matching OEM rims found on many cars since the 1980's. It is not one piece, and there will be subtle seams. The stressing of the deformation in manufacture adds strength. It is not a "split rim", nor wire-spoke, nor machined, nor cast. Without the holes the concave inner surface may develop a partial vacuum at speed (Bernoulli effect). But turbulence is generally considerable. They are built to a spec, and a price. Typically accompanied by a scissor jack that is good for 3 uses, that stores inside this "cup" in the spare wheel well. – mckenzm May 22 '15 at 0:51