Whiplash and concertina effect in cycling and the importance of bunch positioning!

14 November 2016

We’d like to present to you an inadvertent case study of two riders who recently competed in a local criterium to highlight in raw numbers, the importance of bunch positioning in any bike race and explain the concepts behind these results.

In this case study we look at the race file of 2 riders (Rider A and Rider B). Rider A is a local club level cyclist who has been consistently training and racing for 5+ years. Rider B is a former A-grade cyclist with 2+ years out of the sport who decided to jump in the deep-end in order to try and regain a little fitness and have a bit of fun. Both cyclists began the race together in the same grade. Rider A positioned himself towards the front of the group and rolled steady turns around the 2 km criterium circuit while Rider B who was lacking fitness decided to sit towards the rear of the group and simply aim to hang on as long as possible.

Right now you may be thinking one of two possible things:

  1. Rider A got it right by sitting towards the front of the pack to stay out of trouble and minimise the effects of surges.
  2. Rider B was the smart one by simply sitting on the back of the group – he’ll surely get the easy ride around the course.

The correct answer is Rider A had a considerably easier ride due to the effects of a phenomenon known as ‘whiplash and concertina’. We will cover this theory below but first we’d like to present a few numbers from this event. To begin with, Rider B only managed to complete just under 10min of the race prior to being dropped so we’ll focus our analysis on that first 10min period.

[table width =”100%” style =”” responsive =”false”]
[th_column]Rider A[/th_column]
[th_column]Rider B[/th_column]
[row_column]Average Power[/row_column]
[row_column]212 W[/row_column]
[row_column]264 W[/row_column]
[row_column]Adjusted Power[/row_column]
[row_column]229 W[/row_column]
[row_column]279 W[/row_column]
[row_column]Peak Power[/row_column]
[row_column]739 W[/row_column]
[row_column]902 W[/row_column]

You can see from the above table, Rider B was forced to expend far greater energy just to remain with the bunch and this provides a great explanation for why he was dropped so early in the race. In addition to that, see below a screenshot from the Ride graph with both athletes 10 min laps compared to each other. You can see that Rider B is having to produce much greater power with every corner and every surge simply to maintain contact with the group.

In addition to this – Rider B was required to spike above 200% of his threshold power an incredible 33 times, that adds up to once every 17 seconds. In contrast, Rider A was only required to spike above 200% of his threshold 3 times in that same period or once every 3m:10s.

Note the larger spikes in power from Athlete B throughout the file

So if you answered that you thought Rider B would have had the easier ride and are now scratching your head you won’t be alone. There is a very simple answer- the ‘whiplash and concertina’ effect. It’s really easy to view this phenomenon in practice when watching any bike race on the road. You see as the leaders head into a sharp corner the bunch will quickly group together, then as the front of the group exit the corner it quickly spreads out into a long snaking line of riders.

As the leaders of a group approach a corner it forces the riders at the front to slow their speed to safely navigate the turn which allows the riders at the back to roll up to the group and this forms that bunching effect, however as the leaders accelerate out of the corner, their speed is increasing rapidly and the people at the back of the bunch are still braking into the corner. If those back-markers in the group wish to remain with the bunch they need to produce a much larger acceleration in order to maintain contact and ensure they don’t get dropped.

This effect is not restricted to tight criterium races. You can witness it as riders approach and then crest a climb, in cyclocross races approaching obstacles or MTB races when entering into narrow sections of trail. So if you’re someone who was thinking that Rider B would have had an easy ride by sitting on the back of the group, think again. Next time you’re in a race try to move yourself closer to the front of the race and don’t be afraid to get involved with the pace setting. Overall you’re going to be saving yourself a lot of energy for later in the race and if you’re feeling strong, don’t be afraid to put in a little extra effort as you accelerate out of a corner, the people hanging on the back are going to feeling that pressure a lot more!

Happy Riding!