In short. Unsprung weight is the mass that the springs do not carry: the wheels, the tyres, the hubs and bearings, the brakes mounted at the wheel, and part of the suspension. A lighter wheel helps in two ways. The suspension has less mass to control, so the tyre follows the road better. And the wheel spins, so the engine and the brakes have less to speed up and slow down. A rotating part counts for more than its weight in acceleration, up to twice as much for mass at the edge of the tyre, which is why a wheel is worth more than the same mass somewhere else in the car. It is not worth ten times as much: the rule that one pound off a wheel equals ten pounds off the body is repeated on many pages, and we found no source or arithmetic that supports it. A published test with timed runs put figures on it. A BMW M3 on wheels of 20, 38 and 52 lb took 5.76, 5.93 and 6.07 seconds from 31 to 87 mph, and 99.13, 99.92 and 100.74 seconds round a circuit. Our arithmetic reproduces that spread to within 0.03 s. Scaled down, a saving of 2 kg per wheel is worth about 0.05 s over a run of six seconds: small on the stopwatch, more in the feel and on bad roads. Ask the seller for the weight of one wheel at your size, in kilograms.
What unsprung weight is
The unsprung mass of a vehicle is the mass of the suspension, the wheels and the parts directly connected to them. Wikipedia lists the wheel axles, bearings, hubs and tyres, and a portion of the weight of driveshafts, springs, shock absorbers and suspension links; brakes mounted inboard, on the driveshaft and not at the wheel, count as sprung mass (Wikipedia). The sprung mass is what the springs carry: the body and what is attached to it.
So a wheel is only part of it. The tyre on the wheel counts, and so does the brake at the wheel. The wheel is the part you can change most easily, which is why it gets the attention.
Two effects, not one
The pages that rank for this subject tend to mix two things that are different.
- Unsprung mass matters for the suspension. It sets how quickly the wheel can follow a bump and settle, and so how much grip the tyre keeps on a poor surface.
- Rotating mass matters for acceleration and braking. A wheel has to be spun up when the car accelerates and slowed when it brakes; that costs energy over and above the energy of moving its mass in a straight line.
A wheel is both, and a lighter wheel helps with both. Machine Design's short explainer notes that lighter wheels and tyres need less energy to spin up and stop, and treats that as a separate matter from sprung and unsprung weight; it gives no figures (Machine Design). They are separate, and they can be put in numbers separately.
Rotating mass: the arithmetic
A wheel that rolls without slipping, at speed v, has two kinds of kinetic energy: ½ m v² for its movement along the road, and ½ I ω² for its rotation, where I is its moment of inertia and ω = v ÷ r is its rotation rate, r being the rolling radius. If the moment of inertia is written I = m k², with k the radius of gyration (the radius at which the mass would have to sit to give the same inertia), the sum is:
½ m v² × (1 + k² ÷ r²)
So for acceleration a rotating mass counts as m × (1 + k² ÷ r²), whatever its size. The factor depends only on where the mass is:
| Where the mass is (k as a share of r) | Factor |
|---|---|
| At the hub (k close to 0) | 1.0 |
| Half way out (0.5) | 1.25 |
| 0.7 of the radius | 1.5 |
| At the edge of the tyre (1.0) | 2.0 |
A real wheel and tyre have their mass spread from the hub to the tread, with much of it towards the outside, so a factor of about 1.5 is a fair working figure; the factor cannot exceed 2 for mass on a wheel that rolls. A factor of 10 would need mass spinning three times further out than the tyre. That is why the rule of "one pound off a wheel is ten off the car" is not an acceleration figure. It might describe something about the suspension; we found no source that says what. Pages that give it do not cite one.
For a car of mass M, a saving of s kilograms on each wheel takes about 4 × s × 1.5 kg off the mass the engine has to accelerate. With constant power and no drag, the time to go from one speed to another is proportional to that mass. This is a simplification: it ignores aerodynamic drag, gear changes and the limit of grip, so it is good for proportions, not for the second decimal.
A measured test
Jonathan Benson, who runs the Tyre Reviews channel, ran a BMW M3 (F80) on three sets of wheels of 20, 38 and 52 lb each, with the same tyres, a sticky Bridgestone, on all three. Jalopnik reported the results on 24 December 2025 (Jalopnik). The article does not say whether the wheel weights include the tyre; 20 lb is 9.1 kg, 38 lb is 17.2 kg and 52 lb is 23.6 kg.
| Wheels, each | 31 to 87 mph | Lap time |
|---|---|---|
| 20 lb (9.1 kg) | 5.76 s | 99.13 s |
| 38 lb (17.2 kg) | 5.93 s | 99.92 s |
| 52 lb (23.6 kg) | 6.07 s | 100.74 s |
| 20 lb, with 130 lb (59 kg) of ballast in the car | 5.88 s | not run |
Three observations follow.
- The spread is 0.31 s in acceleration and 1.61 s over a lap. The author of the article calls the first imperceptible from the seat of the pants, and the second large in racing terms.
- The ballast run shows that the same extra weight, carried in the car, costs less than when it is in the wheels: 130 lb of lead added 0.12 s, the heavier wheels added 0.31 s for about the same extra mass.
- Braking was not published. Benson called his braking runs less scientific because of the road surface, said the distances were very close, and worked out a theoretical difference of 2 feet between the 52 lb wheels and the 20 lb ones. He also described a feel difference: on the light wheels, he said, the car had more feedback through the steering.
Our arithmetic against the test
Here is the factor of 1.5 applied to the test, for a car of 1,600 to 1,800 kg with its driver (an assumption: the article does not give the mass). The estimates are ours.
| Run, against the 20 lb wheels | Mass added | Our estimate | Measured |
|---|---|---|---|
| 38 lb wheels | 32.7 kg in the wheels (4 × 8.2 kg) | 0.16 to 0.18 s | 0.17 s |
| 52 lb wheels | 58.1 kg in the wheels (4 × 14.5 kg) | 0.28 to 0.31 s | 0.31 s |
| 130 lb of ballast | 59 kg, not rotating | 0.19 to 0.21 s | 0.12 s |
The model reproduces the two wheel runs to within 0.03 s and overestimates the ballast run by about 0.08 s. We read that as a reminder of the model's limits, not as a test of it: one car, one driver, one day. It is enough to show the scale, and to show that the rotating part of a wheel's mass is the one that matters.
What a lighter wheel is worth
With the same factor, for a car of 1,600 kg with its driver and a run of 6.0 seconds, at constant power and without drag:
| Saving per wheel | Mass removed from the car's acceleration (four wheels) | Share of the car's mass | Time saved over 6.0 s |
|---|---|---|---|
| 0.5 kg | 3 kg | 0.2 per cent | 0.01 s |
| 1 kg | 6 kg | 0.4 per cent | 0.02 s |
| 2 kg | 12 kg | 0.75 per cent | 0.05 s |
| 3 kg | 18 kg | 1.1 per cent | 0.07 s |
| 5 kg | 30 kg | 1.9 per cent | 0.11 s |
Two lessons come out of it. A saving of one or two kilograms per wheel is a few hundredths of a second over six seconds: real, and not what makes a car quick. And the benefits that matter to most drivers are elsewhere, in the suspension and in the feel.
The suspension: what less unsprung mass does
The quarter-car model, the standard simple picture of one corner of a car, has two masses: the body's share and the wheel's. The wheel mass sits on a spring of its own (the tyre) and under the suspension spring (pymycar documentation). If the body is held still, the equation of the wheel's motion in that model is m_u × z̈_u + (k₁ + k_w1) × z_u = 0, where k₁ is the suspension stiffness and k_w1 the tyre's. The wheel then bounces at a frequency of (1 ÷ 2π) × √((k₁ + k_w1) ÷ m_u). Whatever the stiffnesses, the frequency varies as one over the square root of the unsprung mass:
| Unsprung mass reduced by | Wheel-hop frequency rises by |
|---|---|
| 5 per cent | 2.6 per cent |
| 10 per cent | 5.4 per cent |
| 15 per cent | 8.5 per cent |
On a corner whose unsprung mass is 45 kg (an illustrative figure, not a measurement), 2 kg off the wheel is 4.4 per cent. A lighter wheel therefore reacts faster to a bump. Wikipedia describes the consequence: a lighter wheel "which readily rebounds from road bumps will have more grip and more constant grip" on an imperfect road, which is why lighter wheels are sought for high-performance use. It also says the effect cuts both ways: the lighter wheel soaks up less vibration, so road irregularities reach the cabin more and ride quality and road noise are worse, and high unsprung mass makes wheel hop worse under hard acceleration or braking where the wheel is poorly located (Wikipedia). It describes a trade-off, not a free gain. We found no source that puts a number on any of it for a given wheel.
What a wheel weighs
Weights depend on diameter, width, design and construction, and many makers do not publish them. Enkei does, for its RPF1, a cast wheel whose barrel is spun by its MAT process: an "estimated weight" for each size, with no unit printed (the page's text speaks of pounds, so we read pounds) (Enkei).
| Size and offset | Estimated weight, lb | In kg |
|---|---|---|
| 14x7 ET28 | 8.40 | 3.8 |
| 16x7 ET43 | 13.70 | 6.2 |
| 17x8 ET45 | 16.05 | 7.3 |
| 18x8 ET45 | 17.95 | 8.1 |
| 18x10 ET38 | 18.45 | 8.4 |
Weight rises with diameter and width, and with a lower offset: our guide to deep dish and concave wheels gives the pairs. These are one maker's estimates for one design. A forged wheel is made differently, and the guide to forged, cast and flow-formed wheels says why no single percentage of saving is true for all wheels.
Makers' claims vary in how they are worded. Manthey, which makes a forged 20/21-inch wheel set for the Porsche 911 GT3 (992.2), says its set reduces the weight of the unsprung and rotating masses by six kilograms against the standard wheels (Manthey); the page does not say whether that is per wheel or for the set. The two readings differ by a factor of four, and on the table above, by a factor of four in what it is worth. It is the question to ask.
When it is worth it
On the stopwatch, lighter wheels matter most to someone chasing lap times: 1.61 s over a lap in the test above is a lot. On the road, the Jalopnik article's own reading is that they will not make much difference to performance by themselves, and that a change you can feel through the steering wheel, the seat and the pedals may be worth having for that alone. That fits the arithmetic: the saving in time is small, the effect on feel is a matter of taste, and neither is a reason to give up strength. Whatever the weight, a wheel is judged by the same trials, and its load rating is its limit.
What to ask a seller
- The weight of one wheel at your diameter, width and offset, in kilograms, per wheel and not per set.
- What the weight includes: with or without the centre cap, the bolts and the valve.
- The load rating, stamped on the wheel or in its specification.
- The moment of inertia, if the maker publishes it; most do not.
- The construction and the alloy, for the centre and for the barrel.
- The weight of the tyre you will fit, which is part of the same unsprung and rotating mass: in the test above, the tyres were the same on every set.
How Onyx Forged does it
On our own pages, the monoblock is the construction that weighs least of the three, because it has no separate barrel and no assembly bolts: see the monoblock range. We do not quote a percentage: the weight of a wheel depends on its diameter, width, offset and design, and the figure that counts is the one for your wheel. You tell us the car and how you want the wheels to sit; the Onyx Forged design office works out the wheel for the car as you described it, and checks it with you before anything goes into production. A different tyre or ride height can change the result, so say so.
Frequently asked questions
What is unsprung weight?
The mass the springs do not carry: the wheels, tyres, hubs, bearings and wheel-mounted brakes, and a share of the suspension arms, springs and dampers.
Do lighter wheels make a car faster?
A little. In a published test, wheels of 20 lb against 52 lb changed the time from 31 to 87 mph in a BMW M3 by 0.31 s, and a lap by 1.61 s. A saving of 2 kg per wheel is worth about 0.05 s over six seconds in our arithmetic.
Is one pound off a wheel worth ten pounds off the car?
Not in acceleration: a rotating mass counts as at most twice its weight, and about one and a half times for a typical wheel. We found no source for a factor of ten.
How much does an alloy wheel weigh?
From about 4 kg for a small 14-inch wheel to more than 8 kg for an 18-inch one, on the one maker's published estimates above. Forged wheels of the same size are usually lighter, but by how much depends on the two wheels you compare.
Do lighter wheels improve braking?
Slightly: the wheels and tyres have less inertia to stop, and the tyre keeps its grip better on a bumpy surface. In the test above the measured stopping distances were not published; the author's theoretical figure is 2 feet.
Do lighter wheels ride better or worse?
Both effects exist: better contact on an imperfect road, but more vibration reaching the cabin. Wikipedia describes it as a trade-off.
Are lightweight wheels weaker?
Not by definition. Wheels are run through the same trials, whatever they weigh, and the load rating is the limit. Ask for it.
Does a bigger wheel weigh more?
Usually, yes: on the published table the weight rises with diameter and width. The tyre on it counts too.
Where to go next
For how forged, cast and flow-formed wheels differ in weight and strength, read forged, cast and flow-formed wheels; for which sizes your car lists, what size wheels fit my car; for the tyre that goes on the wheel, how to read a tyre size. Onyx Forged is a French brand of made-to-order forged wheels, designed to JWL/VIA standards: see Onyx Forged and the catalogue.
Sources
- Wikipedia, Unsprung mass; Machine Design, sprung and unsprung weight (17 May 2016); pymycar, Quarter car model.
- Jalopnik, This Test Shows The Limits Of Lightweight Wheels (24 December 2025).
- Enkei, RPF1 specification (estimated weights); Manthey, lightweight wheel set for the 911 GT3 (992.2).
All pages were read on 15 October 2026.
The Onyx Forged design office.