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The Science of Downhill Mountain Biking
So when talking science by the way I am not talking bike components or clothing although these are worthy conversations of other posts. The science discussion point here is how gravity affects a bike at different gradient of slopes and accordingly what this means for our body positioning and requirement to react quickly.


So lets break it down into the main parts. Firstly gravity is the force that is taking our bike down a descent. I'm not going to get into Newton's theory or the law of relativity as I couldn't pretend to really understand them.. Here mind is just one interesting fact in relation to the speed of gravity. In 2002 two pretty darn smart scientists ("Kopeikin" and "Fomalont"  concluded that the speed of gravity is between 0.8 and 1.2 times the speed of light. So clearly powerful stuff this gravity thing and needs treating with respect whatever it means. Moving on...


The two points touching the earth as we hurtle down a descent are our wheels. No sh&*$.t Sherlock!! I hear you say. Well if we then draw two parallel vertical lines through the centre of the wheels of a bike then the part of the wheel that is then touching the ground has the most gravitational force pushing it down. These lines or more arrows pointing downwards represent the force of gravity. If we then draw a line horizontally between these parallel lines and mark a cross in the centre then this here is where the centre of gravity of a bike is. Right you still with me?


This centre of gravity is a mountain bike riders sweet spot. Looking at the gentle gradient below in the first pic we can see that the centre of gravity is pretty much just above the saddle a little behind the mid point of the bike. The riding rules are simple ... keep off the saddle, knees bent , body loose yet alert, one finger on the brake (assuming powerful disc brakes otherwise two but no more)  and keep your body within this center point of gravity


Gentle gradient slopes.



Ok..So what happens when the gradient gets a little steeper. It gets a lot more fun that is for sure but.....let's see below on a moderate gradient slope what this means. We can see below that the distance between the vertical lines of gravity and it's force pushing down on the bike have got closer together. As a result we can see that the centre of gravity is going back further down the length of the bike. This is why as we descend it is key to move your butt further back.


Moderate gradient slopes



When riding gets to extreme slopes it gets really good fun....Looking at gravity we can see below how much closer the vertical gravity lines have got together and subsequently how much further back behind the bike is the new centre of gravity. Get your butt seriously right behind the saddle and low.


The other thing to take important account of here is as these vertical gravity lines get closer and closer together the margin for error gets smaller and smaller. If you leave the centre of gravity point you may become a cropper. Hence your movement path is severely restricted with less area to play and move in ..


Extreme gradient slopes



So many of us look at downhill bikers with a smug type of thought that it is their bikes that are keeping them on and as a complete novice riding my Big Hit of a sofa I would accept a level of truth to this but it is also their ability to keep a controlled body position within a very limited area whilst subtly moving back and around as the terrain and gradient demands. Envisaging this small area to play in and then adding the complexity of a) technical terrain with tight corners, rocks and drops, b) physical stamina to keep focused and in control, c) mental agility for decisive line choice and d) doing all this at stupid speeds... then you need to have just a wee bit of tiny respect for these folks. Well I think it is cool anways


Wear more body armour than you can carry in both arms. Makes sure you have a full face helmet, goggles, gloves, knee and shin guards the rest. Ride with experienced riders. Eat Weetabix, porridge with salt and three shredded wheat before setting off.
I dislike the use of the word "smart" at the top if they can only conclude its somewhere between 0.8 and 1.2 times the speed of light. That is known and you can surely work out the speed of fall and therefore gravity whilst taking into account other factors such as wind resistance etc which is also easy to work out.

Anyway....related to what this is on about is centre of gravity in corners. You must lean the bike far more than yourself. I dont and because im so tall this means my effective centre of gravity is way off. Dropping a vertical line down would show it to be quite a way inside of the bikes turning circle and contact points on the ground, this leads to no grip and both wheels potentially sliding. The few times when I get on it and ride it properly and chuck the bike around and lean it considerably more than my body, not only is the centre of gravity far more over the tyres contact points but also as the wheels are lent over more I am using the tread on the edge of the tyre to dig in. For shorter riders this isnt such a big issue as they are lower to the ground but leaning the bike more helps regardless.
You need to move your hips and knees into the corner and not your bike (feels unnatural) but once you hook it up it feels fantastic.

Like you said if you lean with your bike the Centre of gravity will push the tyres out from under you. (unlike on a motorbike because of the speeds involved) now if there is a berm or lip of some sort you can angle it in much faster and get it to grip without fear of washing out.

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