Aero bladed spokes, round spokes, straight pull spokes, and J-bend spokes – what for whom and why?

Straight pull vs. J-bend spokes

Straight pull (SP) spokes are no longer an exotic curiosity and are now widely offered by most major wheel manufacturers. The idea behind their introduction is simple: by eliminating the weakest point of a spoke, which is its bend, they should be less prone to cracking, resulting in better reliability. The combination of SP spokes with a finely milled hub flange, adapted to them, is a technically superior construction than well-known spokes with a bend (J-bend) laced through a traditional flange. However, every coin has two sides – changing the way spokes are attached to hub flanges has a number of consequences that affect the entire wheel design.

Straight pull spokes – advantages and disadvantages

• The most important advantage of SP spokes is their theoretical higher durability due to the elimination of their bend. In practice, spokes most often crack at the other end, on the first coils of the thread, and the straight pull design cannot help with that. Moreover, cracking spokes are a problem that is affected by a number of factors, the most important of which depend on the competence of the wheel builder. In carefully built wheels with skillfully matched components, cracking spokes are rare;
• lower mass of hubs with flanges adapted to them. This saving is small – a few to fifteen percent, and it occurs in a place that does not affect how the wheels behave during driving (the mass of the hubs is accumulated close to the axis of rotation of the wheel, and therefore does not significantly affect the moment of inertia of the wheels), but it is a fact;
• the last issue concerns aesthetics. There is no arguing about taste, but most people would agree that SP hubs have a more attractive appearance than classic J-bends.

That would be it for the advantages of this solution. Let’s move on to the disadvantages of straight pull spokes.

• SP hub flanges have a complex, difficult-to-produce design. The large amount of work done by CNC machines significantly affects the final cost of hubs;
• lower lateral stiffness of SP wheels – the design of SP flanges requires them to be moved closer to the center of the hub, which results in a decrease in lateral stiffness. In addition, SP hubs require the use of spokes that are a few to fifteen millimeters longer, which also negatively affects stiffness;
• the last problem is the lower availability of SP spokes. If an SP spoke cracks on the way, on a trip, far from home and a friendly wheel builder, you may have a problem finding a replacement for replacement. Ordinary J-bend spokes are available in practically every bike shop in the world.

Straight pull spokes have taken over the market probably mainly due to their modern and attractive appearance, but that does not mean that they are technically better than J-bends. Some hub manufacturers, such as Chris King, Onyx, and White Industries, have not succumbed to the trend of straight pull spokes and are sticking to classic flanges. And straight pull is not such a modern idea after all, as evidenced by the photo of a hub from the late 19th century.

Strength of Spokes

It happens that customers ask us to build their wheels on a specific spoke model, justifying it with revelations heard from other riders or based on the parameters given on the manufacturer’s website. The first argument obviously closes any discussions, in the second case, however, the differences in strength are due to the different types of steel used to produce the individual models. Sapim gives the force at which the spoke breaks – for example, for the middle section of the Sapim Laser spoke, the manufacturer gives a strength of 1500 N/mm2, and for the Sapim Strong spoke 1250 N/mm2. At first glance, it might seem that the latter is weaker, but the values are given in mm2 and can only be compared after calculating the cross-sectional area. This inaccuracy is precisely the belief that CX-Ray is “the strongest spoke in the world” – for the material in the middle of its section, the highest value of 1600 N/mm2 is given among all Sapim models – of course in terms of a square millimeter. We asked Marcin, who is the representative of the Sapim brand for Eastern Europe, a few questions:

– Why is the strength of the middle section given for an area of 1 mm2 instead of a value that determines the strength of a given spoke? We give the strength of the middle section because this is where the tapered spokes are thinnest. This is important when calculating the cross-sectional area of the spoke – in the case of tapered spokes, it changes depending on the section. It is true that since we give the strength of the middle section, we could give it in terms of the cross-sectional area, but this is a commonly accepted way of presenting the strength of spokes by most manufacturers.

– What is the cross-sectional area of the middle section of CX-Ray and CX-Sprint? (It is easier to calculate the area of a circle than the area of an oval) – The cross-sectional area of the middle section of the CX-Ray spoke is the same as the Laser, and CX-Sprint is the same as Sprint. The semi-finished product used in producing aero spokes are tapered spokes, which we flatten. The surface area does not change.

– Apart from aerodynamic properties, are flat spokes better than round counterparts providing comparable stiffness? – The lateral stiffness is smaller, but the radial stiffness is higher. This gives us better power transmission. In addition, the spoke flattening process increases their strength.

– Does Sapim have plans to introduce new models, and if so, in what direction will it develop its offer? – New designs are under development, but I cannot provide any information.

– What is your favourite spoke? – Limiting myself to aftermarket spokes, definitely CX-Ray due to its high strength and low weight. Wheel building is also relatively easy thanks to the ability to hold the spoke, so it is easy to prevent it from twisting. Next in line is D-light, which gives greater lateral stiffness of the wheel and is less susceptible to mechanical damage. But I also like the Strong spoke, ideal for heavier riders and bikes for “special tasks” 😉

Below we present the Sapim spoke models with the calculated cross-sectional area multiplied by the value that determines the strength of the middle section of each model.

Round spokes: Sapim Strong 3,14 mm2 x 1250 N/mm2 = 3925 N Sapim Force 2,54 mm2 x 1400 N/mm2 = 3556 N Sapim Leader 3,14 mm2 x 1130 N/mm2 = 3548 N Sapim Race 2,54 mm2 x 1300 N/mm2 = 3302 N Sapim Sprint 2,26 mm2 x 1300 N/mm2 = 2938 N Sapim D-Light 2,10 mm2 x 1370 N/mm2 = 2877 N

And flat spokes: Sapim CX-Sprint 2.26 mm2 x 1300 N/mm2 = 2938 N Sapim CX-Ray 1.76 mm2 x 1500 N/mm2 = 2640 N

As you can easily see from the above list, the flat CX-Sprint spoke has the same cross-sectional area as its round counterpart – Sapim Sprint, identically in the case of CX-Ray and Laser. This is because flat spokes are simply rolled-out round spokes, with the cross-sectional area remaining the same, only the shape changing.