So, what is a steering wheel boss?
A steering wheel boss (often referred to as a hub or adapter) is the critical, structural link between a vehicle’s steering column and a steering wheel. Modern wheels usually incorporate a boss within the structure of the wheel but for many classic cars, the boss was supplied in addition to the wheel and is almost always specific to the model of car.
A correctly designed boss provides precise, secure, and safe steering control and is essential to ensuring your new wheel fits correctly.
Generally, bosses are a fixed single unit, made from aluminium, but quick-release versions are available for sports and security applications, to allow removal of the wheel from the vehicle.
Here is the anatomy of a typical steering wheel boss:
The main body:
The body of a boss / hub is generally cylindrical in form and is typically crafted from material such as aluminium, which offers high-strength, low weight and an ability to be engineered to an accurate, high quality finish.
Moto-Lita bosses are CNC milled from raw billets of aluminium, polished and / or painted to suit each individual customer’s requirements. Other manufacturers might pour molten aluminium into moulds and in fact, that’s how we used to do it; whilst the results can be good, it’s more difficult to obtain consistent build quality and a really great cosmetic finish.
Wheel mounting face:
The driver-facing surface of the boss incorporates pre-drilled holes for safe attachment of the steering wheel. This is usually achieved by bolting through the steering wheel (and often a cosmetic top ring) into either pre-tapped holes in the boss itself or through plain holes into loose nuts, which would be tightened behind the boss.
The Moto-Lita mounting hole standard for wheels and bosses is 9-holes, with a PCD (Pitch Circle Diameter) of 101.6mm.
Other manufacturers may use different numbers of holes - 6 holes with 70mm PCD is quite common, for example.
Did you know…
Moto-Lita can make wheels to order that will fit other manufacturers’ bosses?
Helpful if you already have an aftermarket boss and simply want to
upgrade your wheel to a genuine Moto-Lita!
Splined mounting hole:
A boss is designed to slide over and mate closely with the vehicle’s splined steering column via a corresponding centre hole with splines that mirror those of the steering column and which provide rotational grip. To create the boss splines, we ‘broach’ the boss using a press and a hardened steel broaching tool, which accurately carves out the spline shape from the softer aluminium. An alternative technique is to use spark erosion, a technique which is a requirement for more complex designs.
Taper and keyway:
Some vehicle manufacturers choose to add a taper to the steering column and some add a locating keyway for further security. Bosses for older model vehicles may also incorporate pinch bolts to secure the boss to the steering column although the vast majority relies on a top nut positioned inside the upper end of the boss to secure it to the column.
It’s important to recognise that each boss is suited to a specific car or range of cars i.e. sized and shaped to be a perfect fit on a specific car manufacturer’s model. For example, the boss for an Alfa Romeo Alfetta to 1981 is different from the boss for say, an MGB BT 1975, and so on.
In addition to the number of splines, the diameter across them and the column taper, each boss is designed individually to meet the needs of the specific vehicle. This includes the overall shape of the boss so that it ideally looks like an original fitment or in some cases covers over any aperture left in the steering column shroud once the OEM wheel has been removed.
Horn operation:
Where a steering wheel-operated horn button(s) exist, there is likely to be an inner channel or contact point in the boss that allows for the wiring of the horn button to pass through and remain functional.
A slip ring or wiring connector on the back of the hub is another option that keeps the horn circuit active throughout a wheel’s rotation.
For vehicles with stalk or floor-mounted horn activations controls, centre caps are available in engravable billet aluminium or plastic. In both cases recessed centre options exist in the Moto-Lita range for badging to be added.
Quick release functionality:
Some hubs are designed to enable quick-release, which means that the steering wheel is removable for the purposes of ingress / egress, driver safety or legislative compliance, particularly in track racing situations.
In most cases, the lower half of the boss remains attached to the steering column whilst the upper half and wheel detach, with separation activated by a pair of spring-loaded buttons.
Key Specifications:
The depth of a boss has a surprisingly important part to play in overall driver comfort, as it’s one of the things that determines how far away the wheel sits from the driver. Total depth can range from shallow (50mm) to extended (80mm+) to bring the wheel closer to the driver.
Spline count:
Spline count is vehicle-dependent, such as 36 or 40 splines for specific models.
Material:
Aluminium is the standard material most manufacturers use in order to achieve light weight as well as high-strength. Some performance models are available in high-strength alloys.
Finishing touches:
Where an original steering wheel incorporates a boss, removing it can sometimes leave an unsightly hole. Moto-Lita bosses are designed wherever possible to not only provide the physical connection between the steering wheel and the steering column, but also to provide a finished cockpit, ready to enjoy.
If you have any questions at all, ring us on +44 (0)1264 772811 and speak directly to someone who knows Moto-Lita wheels and bosses inside out.
Explore our full range of boss kits, with expert advice always included!