Lateral Stiffness Testing: Steel vs Carbon Spokes
Most existing carbon-spoked wheels use adhesive bonding or resin to attach the spokes (essentially making them integral to the structure of the rim - not ideal for serviceability). However, the TaperLock construction method has enabled us to develop wheels with carbon spokes that are as usable as steel ones. At the hub end of the of the spoke an aluminium mandrel is placed around the spoke during the curing process, and is pulled tight against the (tapered) head of the spoke. The rim end of the spoke is fitted in the same way with a threaded steel mandrel, and can be trued using a spoke key on a nut from within the rim bed (accessible through the internal spoke holes). The steel mandrel has a standard square outer shape allowing the spoke to be held straight whilst any adjustments are made to the internal nipple.
Lateral Stiffness Testing
An increase in stiffness is very beneficial for a wheel. It helps with manoeuvrability and efficient power transfer, and is therefore a performance gain.
As a material, carbon is inherently 2x stiffer than steel, and as one-directional fibre can be up to 5-10 times stiffer in that direction (for the same weight). However, it can achieve this whilst being just 1/5th of the density – hence its popularity in application for frames, forks, components, and wheels (including rims).
Knowing the material properties and therefore the potential in using their specially developed carbon fibre spokes opposed to steel spokes, HUNT wished to quantify the lateral stiffness performance gains. Further with increased stiffness and reduced weight of carbon fibre there is potential to optimize other characteristics of the wheel such as weight, spoke count and therefore its ride performance thanks to carbon fibre.
Introducing the new 44UD carbon spoked rims, we took two of these identical (in profile) rims and prepared one with our carbon spokes (20h) and one with traditional steel spokes (24h) and used HUNT’s in-house stiffness test rig to conduct the test.
The test wheel is securely bolted horizontal into the test equipment ensuring it is level. The DTI clock (which measures the deflection) is used to determine the surface difference and then to zero a reference point (always between spokes for this test case).
A measured weight is hung off the rim at 180o from the DTI clock and the deflection is measured, the weight is removed and the total deformation is measured for analysis. The test is repeated three times to remove any anomalies.
|Wheel||Deflection (mm)||Percentage Difference|
|44 (24 steel spokes)||0.25|
|44UD (20 carbon spokes)||0.235||- 6%|
Even though the carbon spoked wheel has 4 less spokes it is 6% stiffer than a steel spoked wheel, it shows that we can achieve greater lateral stiffness whilst also reducing spoke count and weight with carbon spokes.