Black Inc Rim White Paper
Black Inc Wheels White Paper
There’s a lot of technology packed into each and every Black Inc wheel, so much so that it is worth explaining in greater detail. The following diagrams and text will fully explain the science behind the choices we made when designing our wheelsets.
Aerodynamics, stability and strength were the goals, and this is how we achieved it.
The Aerodynamic System & Aerodynamic
Benefit of the Dual Direction Cross Section Profile
Tire and wheel together form a coherent wheel system. There’s no point considering one without the other.
It is well known nowadays that wheels built with deep and aerodynamc-sectioned rims are significantly faster than shallow box rims.
Deep section aero rims (i.e. greater than 30mm depth) work quite well when apparent airflow over the wheel is close to a headon direction (i.e.
yaw* is close to 0°). The deep section of these types of rims helps with the airflow coming around the tire.
(Yaw refers to the apparent angle of the wind in relation to direction of the wheel). Since high-performance tires were most usually produced in the range of 20mm to 22mm in width, it followed that high-performance rims were designed to work optimally with the tires that were most likely to be mounted on them.
The result was that the optimal rims were similar in width (usually around 20mm to 21mm width).
Not surprisingly, rim and tire together form an aerodynamic system and ideally must be considered together in order to fully optimize the aerodynamic performance of a wheel system.
Recently, new trends have developed that have resulted in a reformulation of deep section rim widths.
Firstly, wider and wider high-performance racing tires have become more common, particularly in widths of 23~25mm.
Secondly, deep section rim construction in carbon has improved and matured to the point that wider rims are feasible without a significant weight increase
It’s all about that leading edge… and what’s
Generally speaking, the round leading edge of the tire limits how the tire/rim system can be optimized aerodynamically. When the air flow hits a round tire, the airflow is disrupted. A deep section rim helps clean up the disrupted air flow.
(Figure 1 (a))
When the tire size increases relative to the rim width, the deep section aero rims still helps somewhat. However, the wider tire begins to dominate the aerodynamics of the system because the rim now has a diminished role in
the system (Figure 1 (b)).
If we consider the tire/rim system in a vacuum then we would quickly realize that the system should work well when the deep-section rim and tire are at least as wide as each other.
For example, if we consider a 23mm wide tire to be the current standard and reference for tire width, then for a 23mm tire a 23mm wide rim would be better than a traditionally narrower rim. the airflow around the tire has less time to
stay disturbed before it is picked up by the rim Also, a rim at exactly the same width as the tire adds no extra frontal area to the system.
Why wider rims? Surely they will be slower?
You would think so, wouldn’t you?
However, there are further gains to be had by refining the shape of deep section rims such as to selectively increase the width of the rim even wider than the intended tire.
This does seem counter-intuitive, because widening the rim can also increase the frontal area significantly. However, frontal area is only one part of wheel aerodynamics; widening the rim with a carefully considered aerodynamic shape will actually allow improved airflow across the tire/rim system.
By making the rim/tire cross section widest at a point further down its cross section, we then create an optimal cross section where the tire and rim start to approximate an ellipse. Even though the rim may be wider, the resulting
elliptical shape is a further improvement over the basic aero oblong cross section (Figure 2(a)).
We also start to gain some very important side benefits.
The first, is that when conditions require an even fatter (wider) tire for either comfort or control or flat protection, then we still realize an aerodynamic benefit. For example, if our extrawide rim which is optimized for a 23mm tire then this wider rim will still be quite good even with a 25mm or 27mm tire (Figure 2(b)).
The greatest of these is that the tire/rim system is not only more aerodynamic but also more stable at a wider range of yaw angles (meaning cross wind conditions) advantages: in aerodynamics that is significant over traditional width rims, and also a tangible advantage in stability, and therefore in
handling, in particular when cornering, descending and riding in cross winds.
A traditional-width deep-section rim with a typical flattish body starts to lose its aerodynamic benefits as the yaw angle of the crosswind increases.
The air flow around the rim starts to separate or break-up early on the opposite side of the system and creates a large wake. This produces both increased drag and instability that affects handling (Figure 3 (a)).
An extra-wide, deep-section rim with a more elliptical body, such as those that characterize our Black Inc wheels, helps to keep the flow on the opposite side of the system close to the rim, reducing drag and promoting stability (Figure 3 (b)).
Combined Aerodynamic Benefits & Increased Lateral Stiffness
Wider, more aerodynamic and stiffer. Is that possible?
In a word, absolutely.
Black Inc rims, with their increased width and optimal elliptical shape, are also stiffer laterally. This follows the same principle as when you compare an oversize tube to a regular tube is stiffer, for example with a regular head tube and an oversized head tube.
The result is that the wheel with the wider rim has a higher lateral stiffness than a wheel built around a narrower rim.
Durabiliy & A Minimized Tire Bed
‘Aero’ does not have to mean ‘weak’.
Many companies are so narrowly focused on ultimate results in the wind tunnel for their aerodynamic wheels that they forget that in real life, the roads that their wheels must travel over are far from perfect.
Many of these modern ‘aero’ rim profiles wrap so high and tight around the tire that they reduce the distance between the tire and the road significantly.
(Figure 4 (a)).
This means that under a given impact (for example, a hard impact with the edge of a pothole) there is a greater chance of the tire damaging the rim. (Figure 4 (b)).
An aerodynamic rim should never result in sacrificed durability. A damaged rim and subsequent loss of time for a wheel and/or tire change negates any aerodynamic advantage you may have been enjoying until then.
Rear Wheel Differential Spoke Gauges
Black Inc rear wheels feature differential spoke gauges. Modern rear wheels must be highly dished to accommodate the cogs for 10 and 11 speed drive-trains.
This dish creates a large lateral asymmetry between the drive side and it’s opposite side, with the latter being inherently stiffer and stronger.
Therefore, Black Inc wheels counteract this phenomenon by utilizing asymmetric spoke gauges. On the drive side we choose stouter spokes, having an optimally maximized cross section, and then pair them with optimally minimalist spokes on the non-drive side side.
Compared to using the same spoke gauges on both sides, Black Inc’s differential drive side spokes are 55%+ stiffer, but only add a nearly negligible 1% to the weight of the wheelset.
The result is a highly optimized spoke system and a wheel that delivers the most balanced combination of aerodynamics, lateral stiffness, low weight and maximized durability.