Handlebar Stiffness Testing

 

Have you ever considered the effect handlebar width has on stiffness? Manufacturers frequently make claims about the stiffness of new products in the market, either due to new materials or construction methods.

Using our Stiffness Rig here at Silverstone Sports Engineering Hub, we recently tested a range of handlebars enabling investigation. The stiffness rig is designed to test and quantify different systems and components found on a bicycle. This is done by measuring the amount of displacement for a given force applied to the component using sensors. The data logging system measures force in Newtons (N) and the displacement in millimetres (mm). Stiffness is therefore calculated and reported as Newtons per millimetre (N/mm). A linear force is applied with an adjustable loading actuator that contains two data logging sensors: a single axis load cell and a linear displacement potentiometer. The actuator uses a turnbuckle thread that can be wound to increase or decrease the length of the actuator. Extending the actuator will push the component applying a compressive load, shortening the actuator will pull the component and apply a tensile load.

In the case of handlebars, our stiffness rig here at SSEH provides two methods of investigating handlebar stiffness. In the first, the handlebars are mounted on the bike as normal, and the vertical stiffness is determined by applying a vertical tensile load to both drops. The handlebars can also be tested in isolation using the handlebar torsion test, by constraining the handlebars in a clamp block and then applying the tensile test load to one of the drops. A handlebar torsion test is used to replicate the force of a rider pushing down on one side of the handlebar (i.e. when sprinting), the sole variable is the handlebars, meaning for the purpose of this investigation, it is the most informative test.

Method

The handlebar torsion test was applied to a range of different handlebars, with a range of widths from 36-44cm. All of the handlebars used were the same brand and model of alloy road handlebars, the only difference was their width.

Once each of the bars were set up in the rig, the load actuator adjusted to give no load. Three settling runs were then carried out on each setup prior to testing, to ensure keep the same force preload prior to testing. The test load of -280N was then applied whilst the displacement was recorded. The same test load was applied to all of the handlebars. The gradient of the load-displacement graph was then calculated in our Cycling Test Rigs (CTR) app, giving the stiffness of each handlebar.

Results

For the majority of the bars, an increase in stiffness with decreasing bar width is observed. This is true for all of the handlebar widths, other than from 44-42cm. The differences in stiffness from 44-40cm are very small, however for 38 and 36cm the differences are very large.

Discussion

It is known that stiffness is an inversely proportional to width. Excluding 44-42cm, the results support this, with a clear increase in stiffness visible with decreasing bar width. The relationship between the stiffness and width should be linear, however the relationship shown in the results graph is far from this. As previously mentioned, the same brand and model of bars were used, the only difference between the samples was their width. However, the non-linear relationship between stiffness and width suggests there were more geometric changes occurring between the bars than simply just the width. The bars tested were not of constant cross section. The clamping region of the bar was larger in diameter, the bar would then taper gradually into a constant diameter for the rest of the bar. From inspecting the bars, this put some of them at a succinct advantage. For the wider bars, this difference lead to relatively small changes in stiffness with the narrower region getting smaller being decreased in size to make the bars narrower. However, for the narrower two sets of bars, the narrower diameter tapered region disappeared completely to narrow the bars. This meant that the total portion of the bar in bending was of greater cross-sectional area, enhancing stiffness. It can therefore be said that stiffness is a function of width, wall thickness and diameter.

One geometric feature of the bar that would likely have an impact on stiffness would be wall thickness. Without cutting sections of the bars, there was no way of quantifying and investigating this.

Conclusions

Overall, bar stiffness was found to be a function of bar width, wall thickness and diameter. However, the results demonstrate that the most important variable was the width.

Although, simply put narrower bars are stiffer, these results show there is a lot more to consider. Depending on where a rider’s bar width sits in the spectrum, the magnitude of stiffness gains available from choosing a narrower bar varies massively.

The following test, as well as many others can be performed on our stiffness rig here at SSEH.

If you would be interested in arranging a session on the Stiffness Rig to optimise your own equipment choices, please refer to the Cycling Test Lab section of our website for further information – https://silverstonesportshub.co.uk/cyclingtestlab/