In short. A wheel spacer is a plate that goes between the hub and the wheel and moves the wheel outwards by its own thickness. It does what a lower offset does: a 10 mm spacer under an ET45 wheel puts it where an ET35 wheel of the same width would sit, and it widens the track by 20 mm across the axle. It does not change the bolt pattern; an adaptor does that. Whether it is safe depends on four things that sales pages rarely spell out. The wheel must still be centred on the hub. With a slip-on spacer, the bolts or studs must be longer by its thickness, so that they engage the hub as far as the manufacturer requires. The fixings must be torqued with a wrench and checked again after a short run. And the wheel must still clear the arch, the brakes and the suspension. A spacer also lengthens the lever through which the wheel loads the hub: engineers expect that to shorten the life of the wheel bearing, but we found no published limit on thickness and no test that puts a number on it. In Great Britain the MOT manual has no item for spacers; it does fail a wheel and its fixings that are not compatible, a loose or missing wheel nut or stud, and a tyre fouling the car. Tell your insurer.
What a wheel spacer is
The hub does not move, and the wheel sits against its mounting face. A spacer is a disc or plate, usually in aluminium or steel, placed between the two, so the wheel sits further out by the thickness of the plate. The words are used loosely, so here is how we use them.
- Slip-on spacer. A plate that goes over the hub and under the wheel, with the car's own studs or bolts passing through it. Because the plate takes up some of the length of the fixing, the studs or bolts have to be longer.
- Bolt-on spacer or adaptor. A thicker plate that is fixed to the hub with its own nuts or bolts and carries its own studs for the wheel. An adaptor is drilled differently on its two faces, so it can also change the bolt pattern (PCD): it is how a wheel with another pattern is fitted without re-drilling it, and it adds a second joint.
- Hub-centric. The plate has a bore that fits over the hub and a raised ring that fits the wheel's centre bore, so it is centred by the hub and not by the fixings alone. Eibach's instruction sheet asks the fitter to check that the hub centre and the bolt pattern of the spacer suit the vehicle and the wheel, and that the spacer sits on the hub without play. Our guide to PCD, bolt pattern and centre bore explains why a wheel that is not centred runs out of true.
What a spacer changes
A spacer of thickness t behaves like a wheel with its offset lowered by t. Offset is the distance between the wheel's mounting face and the centre line of its rim, and each millimetre taken off it moves the wheel a millimetre outwards; our guide to wheel offset sets out the arithmetic. With one wheel on each side, the track changes by twice the thickness.
| Spacer thickness | Wheel moves out by | Track, across the axle | Same as an offset lowered by |
|---|---|---|---|
| 5 mm | 5 mm | wider by 10 mm | 5 mm |
| 10 mm | 10 mm | wider by 20 mm | 10 mm |
| 15 mm | 15 mm | wider by 30 mm | 15 mm |
| 20 mm | 20 mm | wider by 40 mm | 20 mm |
| 25 mm | 25 mm | wider by 50 mm | 25 mm |
To see where a wheel will sit, enter it in the wheel offset calculator twice: once as it is, and once with the same width and its ET reduced by the thickness of the spacer. It compares; it does not tell you whether the wheel will clear your car.
Two things do not change. The bolt pattern stays the same, so a spacer cannot make a wheel with the wrong PCD fit. And the wheel itself is unchanged: its stamped width and ET are what they were; what moves is where it sits.
Why the hub notices: the lever
The wheel is carried by the hub, and the load of the car acts along the centre line of the tyre. The farther that line is from the hub's mounting face, the larger the bending moment the load puts on the hub and its bearing. A spacer adds its thickness to that distance.
For an ET45 wheel the distance from the mounting face to the tyre's centre line is 45 mm; fitting a spacer of thickness t makes it 45 mm plus t. Take a corner of the car carrying 4 kN (about 408 kg, which is a car of roughly 1.6 tonnes on four equal corners). This is our arithmetic, in statics and with a made-up corner load, not a measurement:
| Spacer | Distance, mounting face to tyre centre line | Increase | Moment of the vertical load about the mounting face |
|---|---|---|---|
| None | 45 mm | none | 180 N·m |
| 10 mm | 55 mm | 22 per cent | 220 N·m |
| 20 mm | 65 mm | 44 per cent | 260 N·m |
| 30 mm | 75 mm | 67 per cent | 300 N·m |
On a wheel with a low offset the same spacer is a larger fraction: 20 mm on an ET20 wheel doubles the lever. The bearing sits a little behind the mounting face, which adds the same amount to every row, so the real percentages are a little smaller; the direction does not change. Braking, cornering and bumps add to this static load.
What the engineers say
The Autopian's suspension engineer, Huibert Mees, looks at what a spacer does to three dimensions of the front suspension, using a 50 mm spacer as his example (The Autopian).
- Scrub radius is the distance, seen from the front, between the centre of the tyre's contact patch and the point where the steering axis meets the ground. With the spacer it grows by the spacer's thickness: from 15 mm to 65 mm in his example. If it was negative, it can change sign, so that the force of braking tries to steer the wheel the other way from the one the suspension was designed for. Wikipedia's article on the subject says the scrub radius "is changed whenever there is a change in wheel offset", and that the greater it is, in either direction, the greater the steering effort (Wikipedia).
- Kingpin offset, the distance between the centre of the wheel and the steering axis, grows in the same way: from 50 mm to 100 mm in the example.
- The bearing load line is the vertical line through the centre of the wheel and tyre. For the longest bearing life, he writes, it should pass between the two rows of balls in the bearing; with the spacer it moves outside them, and the bearing then carries a moment it was not built for.
Each of these moves by the thickness of the spacer: a 10 mm spacer changes them by a fifth of what a 50 mm one does. The article's editor adds that how much this affects the way a car drives or how long its parts last cannot be answered from his analysis, and that a wheel with a different offset is, physically, the same as an original wheel with a spacer. We found no published test of bearing life against spacer thickness, and no text that sets a maximum thickness. The sellers' guides we read quote "typical" thicknesses (from a few millimetres to around 30 mm) without a source, so we do not repeat them.
Fixings: the length that matters most
A wheel stud or bolt has to engage the hub by enough thread to carry the load. A slip-on spacer takes up part of its length, so the stud or bolt must be longer by the thickness of the spacer. Two documents are explicit.
- UN Regulation No. 124, which covers replacement wheels for passenger cars, asks that the length of thread engagement of wheel bolts, nuts and studs be the same as that of the vehicle manufacturer's own wheel and fixings, and that the material be at least equivalent (Annex 10, § 2.3; Regulation No. 124). It is a criterion for assessing a wheel, not an instruction to a fitter, but it states the principle: whatever you put between the hub and the wheel, the engagement must not fall.
- Eibach's instruction sheet for its Pro-Spacer System 6 kit is the only maker's document we found that prints figures. It tells the fitter to check that the shaft of its special studs is longer than the original stud by the thickness of the spacer, and sets a minimum number of turns of thread engagement (Eibach, D2356):
| Thread | Minimum turns | Support length, about |
|---|---|---|
| M12 x 1.25 | 8.0 | 10 mm |
| M12 x 1.5 | 6.5 | 10 mm |
| M12 x 1.75 | 6.5 | 12 mm |
| M14 x 1.5 | 7.5 | 11 mm |
The sheet adds that, if its information conflicts with the vehicle manufacturer's workshop manual, the manual takes priority. These figures are Eibach's, for its kit, not a universal rule: the right length for your car is the one in your car's documents.
The same sheet says that a combination of two or more spacer rings "is not safe and should never be done", that a torque wrench must be used for the final tightening (the figure comes from the workshop manual or the wheel maker), and that all fasteners and torques must be checked again after about 25 miles and again at 500 miles. UN Regulation No. 124 asks the maker of a replacement wheel to tell the buyer to re-tighten after 50 km (Annex 10, § 1.4).
After fitting: what to check
Eibach's sheet lists what to inspect after installation:
- Wheel alignment (camber, castor and toe).
- Clearance between tyre and wing or arch, in the whole range of the suspension and steering.
- Brake lines and their fixings.
- ABS and anti-skid sensors.
- That the wheel and hub turn freely, with one full turn of the wheel by hand.
Then balance the wheel and tyre and, as above, check the torque again after a short run.
The law and your insurer in Great Britain
No text we read bans spacers, and none sets a limit on thickness. Several apply to the result.
| Text | What it says | Why it matters for a spacer |
|---|---|---|
| Construction and Use Regulations, regulation 100(1) | A motor vehicle and "all parts and accessories" must at all times be in such a condition that no danger is caused or likely to be caused to any person in or on the vehicle or on a road | A spacer is an accessory: short fixings or a loose wheel are danger |
| Construction and Use Regulations, regulation 63(2) | A car must have wings or similar fittings to catch, so far as practicable, mud or water thrown up by the rotation of its wheels | A wheel moved out can leave the arch behind |
| MOT manual, 5.2.1 | A wheel with a loose or missing wheel nut, bolt or stud: major; more than one: dangerous | Under-length or untightened fixings |
| MOT manual, 5.2.2 | A wheel and its fixings not compatible: major | Wrong seat, wrong length, wrong centring |
| MOT manual, 5.2.3 | A tyre fouling a part of the vehicle: major, but only if it is fouling at the time of the test; contact caused by flexing or suspension movement is not a defect | A wheel moved out can touch the arch or the inner lining |
The first two rows quote regulation 100 and regulation 63 of the 1986 Regulations, which apply in England, Wales and Scotland; the last three rows quote the MOT inspection manual, section 5. The manual has no item that names spacers or adaptors, and none that measures how far a wheel sits out of the arch, so what a tester decides depends on what the car shows at the test: our article on new forged wheels and the MOT sets out what is checked.
Insurance. The Financial Ombudsman Service lists changing alloy wheels as an example of a modification and advises telling the insurer as soon as possible (FOS). It does not mention spacers; ask your insurer whether it wants them declared, and keep the answer in writing.
Outside Great Britain the rules differ, and we have not set them out. Look up the ones that apply where you drive.
Spacers or a wheel with the right offset
People fit spacers for three reasons: to move the wheel out for the look, to clear a brake caliper or a suspension part, or to make a wheel with the wrong offset fit. In the first and third cases, the same position can be reached with a wheel that has the right offset in the first place. The Autopian's editor is clear that the load on the bearing is the same either way: what a wheel made for the car removes is the extra part, with its second joint, its longer fixings and its centring ring. It does not remove the effect of moving the wheel.
That is why Onyx Forged wheels are designed to be fitted without spacers. You are not asked for the offset (ET), the PCD (bolt pattern) or the width: they follow from your car. The Onyx Forged design office works them out for the car as you described it and checks them with you before anything goes into production. We do not suggest an offset in conversation, because a figure only means something for one car, with its brakes, ride height and tyres.
Common mistakes
- Reusing the original bolts under a slip-on spacer: they are too short by the thickness of the spacer.
- Stacking two spacers to reach a thickness: Eibach says that is never safe.
- A spacer that does not fit the hub, so that the wheel is centred by the fixings alone.
- Tightening by feel, or finishing with an impact gun instead of a torque wrench, and never checking again.
- Forgetting the brake lines and the ABS sensors, whose clearance Eibach asks you to check after fitting.
- Treating "legal" as a thickness. No text gives one: the test is whether the car is safe, the wheel is covered and nothing touches.
Frequently asked questions
Are wheel spacers legal in the UK?
No text we read bans them. A car must still be in a condition that causes no danger (regulation 100(1)) and have wings that catch what the wheels throw up, so far as practicable (regulation 63(2)); a wheel that is not covered, or fixings that are short or loose, are the usual problems.
Are wheel spacers safe?
They can be, if the spacer is centred on the hub, the fixings are longer by its thickness, the torque is right and checked again, and nothing touches. They also change the geometry and lengthen the lever on the hub, and no published test says how much a given thickness costs in bearing life.
Will wheel spacers fail the MOT?
The manual has no spacer item. It fails loose or missing nuts, studs and bolts, a wheel and its fixings that are not compatible, and a tyre fouling the vehicle at the time of the test.
What is the difference between a spacer and an adaptor?
A spacer keeps the bolt pattern and moves the wheel out. An adaptor has a different drilling on each face, so it also changes the PCD; it adds a second joint.
Do I need longer bolts or studs?
With a slip-on spacer, yes: longer by its thickness, so that the thread engagement is the same as without it. Use the figure in your car's documents.
Do spacers change the offset?
Yes. A spacer of 10 mm acts like an offset 10 mm lower on the same wheel, and widens the track by 20 mm over the axle.
How thick can a spacer be?
There is no figure in the law or in the documents we read. It depends on the car, the wheel and the fixings; ask the maker of the spacer for the limit for your car, in writing, and note that Eibach's sheet allows only a single ring.
Do spacers wear out the wheel bearings?
Engineers expect a longer lever to load the bearing more, and the effect grows with thickness. We found no measurement that gives a figure.
Where to go next
For the arithmetic of where a wheel sits, read wheel offset; for the pattern that fixes the wheel to the hub, PCD, bolt pattern and centre bore; for how a wide wheel with a low offset looks and what it costs in clearance, deep dish and concave wheels; and for finding out which sizes your car lists, what size wheels fit my car. Onyx Forged is a French brand of made-to-order forged wheels, designed to JWL/VIA standards: see Onyx Forged and the catalogue.
Sources
- Eibach, Pro-Spacer System 6 instructions D2356.
- UN Regulation No. 124, replacement wheels: Annex 10 §§ 1.4 and 2.3.
- The Autopian, Huibert Mees, wheel spacers and offsets (21 February 2023); Wikipedia, Scrub radius.
- GOV.UK, MOT inspection manual, section 5, sections 5.2.1 to 5.2.3.
- The Road Vehicles (Construction and Use) Regulations 1986, regulation 100 and regulation 63.
- Financial Ombudsman Service, car modifications and insurance (1 October 2024).
All pages were read on 15 October 2026.
The Onyx Forged design office.