The short answer: a concave wheel's spokes dip from the outer lip towards the hub, and how deep they dip depends on the size. Wider rims and lower offsets leave more room between the hub face and the lip, so one design can be flat in a narrow size and concave or deep concave in a wide one. Sparco's FF4 in 18" is flat at 8 to 8.5J, concave at 9 to 9.5J and deep concave at 10 to 11J.
What “concave” means
Concave describes the shape of a wheel's face, not its colour or finish. On a flat-faced wheel the spokes run more or less straight from the lip to the centre. On a concave wheel they sweep inwards, so the centre sits deeper than the rim edge and the face looks like a shallow bowl.
Makers that label it tend to use steps. OZ Group's stock list uses four: flat, semi concave, concave and deep concave. Those labels belong to an exact size, not to a design as a whole. That's the part most concave guides miss.
Why width and offset change the dish
The hub mounting face sits somewhere inside the barrel. How far it sits from the outer edge depends on two numbers:
- Width (J): a wider rim puts the outer lip further from the middle of the wheel.
- Offset (ET): a lower offset moves the mounting face further inboard, away from the outer lip.
As a rough guide, the room between the mounting face and the outer bead seat is half the rim width in millimetres, minus the offset:
| Size | Rough room, mounting face to outer bead seat |
|---|---|
| 8J ET45 | about 57 mm (101.6 minus 45) |
| 9.5J ET20 | about 101 mm (120.7 minus 20) |
| 11J ET20 | about 120 mm (139.7 minus 20) |
More room lets the maker sweep the spokes deeper before they reach the hub. That's why the same design gets more dish as it gets wider or as its offset drops.
It's a rule of thumb, not a formula. Each size has its own casting and machining, and the maker decides how much of the room to use. The MSW 51 in 22" shows it: the 10.5J at ET45 has a little less room (about 88 mm) than the 10J at ET36 (about 91 mm), yet MSW label the 10.5J concave and the 10J flat. The only sure answer is the maker's label for the exact size.
The makers' own labels, size by size
These come straight from OZ Group's stock list, which covers OZ Racing, MSW and Sparco. Only four designs carry face-profile labels in it, and the list only covers the sizes in stock when it was issued. A size missing from these tables isn't necessarily flat. It just isn't labelled.
Sparco FF4, 18"
| Width | Offsets labelled | Face |
|---|---|---|
| 8J | ET25 to 45 | Flat |
| 8.5J | ET30 to 47 | Flat |
| 9J | ET30 to 48 | Concave |
| 9.5J | ET20 to 45 | Concave |
| 10J | ET17 to 25 | Deep concave |
| 10.5J | ET7 to 35 | Deep concave |
| 11J | ET20 to 44 | Deep concave |
Here width is what moves the label: every 9J and 9.5J is concave, and every 10J and wider is deep concave, whatever the offset.
Sparco FF3, 18" to 20"
| Size | Offsets labelled | Face |
|---|---|---|
| 18 x 8J | ET25 to 45 | Flat |
| 18 x 8.5J | ET30 to 47 | Flat |
| 18 x 9J | ET30 to 48 | Semi concave |
| 18 x 9.5J | ET20 to 45 | Semi concave |
| 19 x 8J | ET40 to 45 | Flat |
| 19 x 8.5J | ET40 to 50 | Flat |
| 19 x 8.5J | ET25 | Concave |
| 19 x 9J | ET20 to 24 | Concave |
| 19 x 9.5J | ET20 to 38 | Concave |
| 20 x 10.5J | ET20 | Deep concave |
The 19 x 8.5J shows offset at work: at ET40 to 50 it's flat, and at ET25 the same width is concave.
MSW 51, 22"
| Width | Offsets labelled | Face |
|---|---|---|
| 9J, 9.5J | ET29 to 49 | No profile label |
| 10J | ET36 to 56 | Flat |
| 10J | ET18 to 28 | Concave |
| 10.5J | ET18 to 45 | Concave |
| 11J | ET15 to 50 | Concave |
| 11.5J | ET14 to 61 | Concave |
The list carries no profile label for the MSW 51 in 20" or 21".
MSW 53
Labelled concave in 10.5 x 21" and 10.5 x 22" at ET19. Its 8J and 9J sizes in 19" carry no profile label.
Concave on a staggered car
Staggered cars run a wider wheel on the back (see staggered vs square). Because the face depth follows the width, the back wheels can come out with more dish than the fronts on the same design. Take a Sparco FF4 set in 18 x 8.5J front and 18 x 9.5J rear: by Sparco's own labels, the fronts are flat-faced and the rears are concave.
Some people want exactly that look. Others want the fronts and rears to match. If it matters, check the label for both sizes before ordering.
What concave changes about fitment
Fitment still starts with PCD, centre bore, width and the offset range the car accepts at each axle. Check a setup with the wheel fitment calculator, and see where the face will sit against the arch with the camber and spacer calculator. But a concave face changes two things.
Concave wheels have less brake clearance
A concave spoke dips in towards the hub before it sweeps out to the lip. Where it passes the caliper, the back of the spoke sits further in than a flat-faced spoke would on the same size and offset, so there is less room between spoke and caliper. The deeper the concave, the less room.
With a big brake kit, or a car with large factory calipers, check the exact wheel and size against the caliper before buying. See big brake kit wheel clearance.
A deeper dish usually means a lower offset
A lower offset moves the whole wheel and tyre out towards the arch. On most cars the arch is the real limit, not the dish.
Questions
What is a concave alloy wheel?
Why is the same wheel concave in one size and flat in another?
Do concave wheels need spacers?
Do concave wheels have less brake clearance?
Which wheels do the makers label concave?
Sources
- OZ Group stock list (OZ Racing, MSW, Sparco), April 2026: face profile labelled per size (flat, semi concave, concave, deep concave).
- Room figures: half the rim width in millimetres (width in inches x 25.4 / 2) minus the offset, measured to the bead seat and ignoring the outer flange.