Ask RIDE… Can you please explain the Deakin graph?

I love the RIDE magazine and was intrigued by your article about testing done with Deakin University (page 211 of issue #52) where accelerometers are used to measure the ride quality damping. In the second graph (Pinarello FP6 carbon-fibre frame) the peak measured at the seat comes in a fraction of a second before the peak in the G-force at the chainstay. I might be wrong, but I would have thought that the peak measured at the seat would come slightly after the chainstay peak, because a bump picked up by the rear wheel travels up towards the seat which should take a fraction of a second – hence the peak at the seat should come later. Did you flip the graph around? If not, can the person from Deakin please explain this graph to me?

Also, how would the Eddy Merckx bike in the retro review (or similar bike from that era) measure up against the modern bikes in terms of stiffness, weight, maneuverability and all the other parameters you present with the other bikes in the bike review? I’ve been out of cycling scene for many years, but back in it now and it would be interesting to see how far bikes have progressed.

Regards, Derk

figure 1 - incidence of G-force

figure 1 - incidence of G-force

Hi Derk,

Thanks for the feedback! Your assumption of how the test is run is correct. When you have two accelerometers the seat peak G-forces should occur directly after those on the chainstay, and this happens on graphs when G-force is compared against time. If you read RIDE #53 we have those graphs included, but also I’ve included a small time section for you as figure 2 (below). As you can see over 1 second there is a slight delay in peaks especially at 124.2 seconds, but also realise there is a lot going on at the seat which can influence the accelerometer as well – i.e. you bouncing up and down and pedaling, this all relates to the feeling of cycling and feedback from the bicycle.

The graph you are referring to is different. This one (figure 1) is a normal distribution of incidence and G-force – so time is not directly considered. Incidence is the amount of times we see a G-force over a given period of time (in this case 30 minutes), and G-force axis shows the magnitude of force that is being felt. In the 30min period we measure G-forces at 10 times per second and collate all that data into two curves (one for seat and the other is chainstay) which is what you see as a normal distribution. The peak of each distribution curve is considered the mean. So the mean G-force magnitude being seen by the seat is lower than the mean G-force magnitude at the chain stay for the Pinarello FP6, this is why it occurs earlier on the plot. From this we consider the movement in the average G-force the rate of dynamic stiffness and damping and express it as a percentage.

As for the circa 1980 Eddy Merckx bike, that is something we have not tested yet – how older style thin steel tube bike compares against today’s evolutions. In terms of static stiffness, the modern day bikes would be stiffer. The reason most bikes have gone to larger diameter tubing is that you achieve stiffer tube geometry for the same given weight compared to smaller diameter thicker wall tubing. As for dynamic stiffness and damping I honestly don’t know the answer, but my suspicions would be that modern day bikes have better ride characteristics due to a greater understanding of material parameters.

Enjoy getting back into cycling!

Best regards,

Dr Paul Collins

figure 2 - incidence over time

figure 2 - incidence over time

Author: design@ride

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