I know…….this post’s super late but life is what life is.
The final round of the downhill world cup saw Rachel Atherton further cement her name in the record books and saw Danny Hart take his third consecutive world cup win though he couldn’t stop Aaron Gwin taking the overall title.
Centre of Gravity
The Vallnord track was recognised as being the steepest track this season with an average gradient of 23% and the steepest section being 75%. To be dominant on this track the riders needed to demonstrate their mastery of maintaining their centre of gravity within the lines of stability available to them.
So what are centre of gravity & lines of stability and why do they matter?
Centre of gravity refers to the point inside an object where all its weight seems to be concentrated. Calculating centre of gravity becomes very complicated very quickly when considering mountain bikes as gradient, rider weight, rider position, suspension setup, wheel diameter, tyre choice etc etc all play a factor in determining it so for the purpose of this post I’ve ultra simplified things.
On the flat (when you take into consideration the bike plus the rider) both wheels will tend to be loaded equally therefore the centre of gravity sits roughly in the centre of the bike horizontally. If you then adjust the centre of gravity vertically (imagining there’s a rider sat on the bike in the diagram) the centre of gravity will roughly lie just above the height of the seat. In the diagram this point is marked by the intersection of the green and blue lines. The other thing we need to consider are the lines of stability (the red lines in the diagram). If we want to stay on our bikes we need to keep our centre of gravity within these lines of stability.
If we now consider what happens when we start to roll down a hill what we find is the centre of gravity shifts forwards as the front wheel is loaded more than the back wheel. As riders, this makes us feel unstable and we naturally move our weight backwards. If we look at the diagram, by moving our weight backwards we move our centre of gravity back and this keeps us between the lines of stability.
The problem is that as the gradient increases the lines of stability get closer together meaning you have less room to manoeuvre within (between the two diagrams there’s a reduction of around 15%) in order to stay balanced on your bike. If you look at the picture of Wade Simmons below you’ll see he’s virtually resting his chest on the seat in order to keep his centre of gravity within the lines of stability.
If you consider that, as gradient increases gravity plays a bigger part and so speed increases too, the level of skill and balance you need to stay on your bike as the gradient increases ramps up pretty quickly.
One of the big factors that allowed Rachel and Danny to take their respective wins at Vallnord was their ability to subtly adjust their centre of gravity within the stability limits they had available to them. If you’re aiming to copy these guys then practicing on the flat and shallow gradients won’t get you there. Practicing manuals, jumping etc on steeper gradients is key to progressing.
If you’re going to take the time to practice, practice like the elite