The 4 Things Slowing You Down



There are 4 things slowing you down when you head out for any kind of ride. These often unseen ‘forces’ can be felt by all of us, and are oftentimes annoying. These are the 4 Trail Nemeses.


MAIN TAKEAWAYS
  • There are 4 main trail nemeses slowing you down. Some you can fix but others are a waste of time.

  • Gravity is the main force slowing you down up hill. Saving weight is important, but don't be a weight weenie.

  • Aerodynamics is important in MTB, but they will never be 0 watts. Ride in control and in a smart aero position.

  • Rolling resistance is more important than aerodynamics (especially on MTB), but it's a trade-off between rolling speed and braking traction.

  • Braking is the most important factor slowing you down. Brake smarter for free speed.



We can think of the 4 Trail Nemeses as forces slowing us down as we complete an entire circuit - some we see more on uphills, some more on flats, and other more on descents. As you leave the parking lot, these forces start slowing you down immediately. The 4 Trail Nemeses continue slowing you down until you return back to the car. Some are working against you all the time while others are concentrated to particular parts of the trail. Some of them keep us up at night, but others we never even thought of. Some of these you can remedy by throwing money at the problem. Some of these you can improve with smart choices and training. Others still are a trade-off, whereby solving it will inflate another issue.


It’s important to note that the 4 trail nemeses are not all equal. The point of this article is to help you decide how you might be able to spend your own time and energy to improve your times and expend your limited amount of human power to get around a circuit.


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Gravity


Gravity is a force acting on us all of the time. It’s the acceleration of a body towards the center of the Earth. We can measure gravity in meters per second squared. It’s what gives your mass weight. An object (or person on a bike) that has more mass will have to fight gravity more.


Gravity is a pretty obvious force acting on us. You’ve faced this trail nemesis the moment your bike pointed up a hill. You had to use your muscles to do work to overcome gravity. You might have measured this one with your power meter. In an instant you could go from riding at 100W to riding at 200W when you’re faced with a slight incline. You have two choices: slow down or push harder.


Weight weenies are well-versed with gravity. Weight weenies know that they can buy titanium bolts and flimsy tires to reduce the mass of their bikes. This makes the person able to travel faster up the same hill at any given effort since gravity has a lesser effect.


WHY BIKE WEIGHT DOESN'T MATTER


Gravity is a huge force acting on us when we travel up a hill. It’s probably the biggest force and our greatest foe.





You can think about the how much gravity is working against you by think about how much work (total energy) it takes to pick up a bike. Let’s say you’re picking up your bike from the ground and lifting it straight into the air 1 meter. Obviously you’re posing for a photo for Instagram to show off your whip!





Bike: 10kg

Distance lifted: 1 meter

Gravity: 9.81 m/s2


Work = Force*Distance

So let’s get force;

Force = mass* gravity

Force = 10*9.81

Force = 98.1N


Then of course the distance is 1 meter, so,

Work = 98.1N*1m

Work = 98.1 Joules


If you want to think of lifting this bike in terms of watts, just imagine that you lifted this bike up over 1 second. You do work/time and it was a bout 100W to lift your bike 1 meter in 1 second.




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You can see then with this example that the greater the mass of you or your bike, or the greater the hill, the more work you will need to exert to get your bike up any given hill.





You can imagine this work increasing as you eat junk food and add fat mass to your body. It’s hard to resist that Twinkie, but it’s also hard to carry more Twinkies up a hill!


This is where weight weenies come in.


They will buy the lightest parts to save a bit of weight, because, duh, I can spend money to get faster without putting in more effort and continuing to eat Twinkies!


And they’re right. But they’re not as right as they might think.


I wrote an article on Why Bike Weight Doesn’t Matter based on results from a study by my PhD supervisor, Dr Paul Macdermid. They put bottles of sand on riders' bikes and sent them up hills. I messed with the numbers a little bit, and it turns out that saving an entire 335 grams will save you 1 second up a 150 second climb.


This amount of weight saving is what you would get by running a rigid seatpost versus a dropper post, and 1 second up a 2.5 minute climb is not much. And you’d possibly be able to ride faster down the hill by using that same dropper seatpost.


For comparison, going down to the tiniest brake rotors would save you about 150 grams. Running only 3 rotor bolts would save you about 20 grams. Switching from full suspension back to a hardtail would save you about 1000 grams and it would be a bumpy ride. You can see that these marginal gains might not really be worth it, especially considering you want to get back to the car as quickly as possible. Some of these - like running fewer rotor bolts - are downright risky for almost no time gain. In the same way though, you could also eat cleaner to reduce how much energy you must expend to fight gravity.


It's important to note here that unless you are racing up hills all the time, gravity might not be that big of a worry for you.


Nevertheless, when considering gravity, it is a huge force acting against us uphill, but can be our friend when riding downhill.




Aerodynamic Drag


This one is new to a lot of MTB riders, and something you might not have ever considered. Air resistance is something that any moving object must overcome, since the air has mass. As we increase our speed, we have to fight air resistance even more. You have to fight it all the time - any time you’re moving. Think of the air as a wall that you need to continuously break through as you ride.






Back in the earlier days of downhill, riders knew all about aerodynamics. This is why they rode in lycra (which was eventually banned). After that - in the early to late 2000s - downhillers had some super baggy kit, going along with the skater-style of the day. Now downhillers ride with tighter fitting baggy clothes, and tuck in their shirts for races. Without all that extra clothing flapping in the wind, air resistance is reduced and the riders are more streamlined - they can coast at a faster speed without doing anything different. Essentially the snug clothing helps to reduce their frontal area (cdA).


You would have noticed air resistance when you were riding with your mates. Even on a calm day, you can notice that you’ll travel faster when you jump into the slipstream of the rider in front of you. As the wind speed increases, this effect is greater, meaning you can gain more speed or save more energy by jumping in the slipstream.


When we head up hills, air resistance is a relatively small force acting against us. The speed is so low that it’s almost a non-issue. Of the 4 trail nemeses we are fighting, air resistance is the least of our concerns when going slow up hills (assuming no wind).


The effect of aerodynamic drag can be calculated by knowing our effective frontal area. This gets a lot of attention in the triathlon world, where they call frontal area cdA. Bigger riders have a greater cdA, as do riders with flappy jackets and bigger heads, or riders sitting straight up or riders with XXXL wide bars.






We’ve estimated cdA on the MTB before when we needed to validate our brake power meter. Every joule of lost energy needed to be accounted for if we were to show that BrakeAce was able to measure the rest. To do this we calculate the total energy lost to aerodynamic drag (Ed) with the addition of air density, distance traveled and velocity. The equation for energy lost to drag is below. You can find the paper here, along with access to the primary reference of a paper by William Bertucci (2013).



Ed=1/2cdAρv2d


From this, you can see that the faster we go, the more energy we lose to drag. This is because v (velocity) is squared. Plus considering cdA, the ‘bigger’ (wider, more flappy, taller, etc.) we are, the more air resistance we’ll be met with.


To give you an idea of how much of an effect this has when you’re riding, let’s assume a small example like we did with gravity. Travelling at about 14kph (~8.6mph) we are losing about 17W to drag.


You're losing about 17W at 14kph from air resistance. This will never be 0W.




Now this actually sounds like a lot - and it is! This is because at any time you’re riding at this speed, you’ll be losing this many watts.


So I know where this automatically gets you thinking… Let’s save these watts!


It’s important to note though that you’ll NEVER get aerodynamic drag down to zero, even with the best lycra and the goofiest aero helmet. Or with your hands down on your fork crowns! (Don't do this. It's unsafe and won't help.) However you can reduce the effects of drag in number of ways, including those above, or even milder methods like normal fitting clothing, tucking at key moments, or sliding into a slipstream.


Extreme sketch. via: http://www.treadmtb.co.za/specialized-introduces-another-hand-postion-on-s-works-models/


One of the things I always tell my athletes is that at higher speeds, you have less to gain by pedaling harder. This is because the power required to overcome air resistance exponentially increases the faster you go. Sometimes it’s better to just maintain speed - or even coast when you’re going fast.



Rolling Resistance


This trail nemesis is a trade off. Rolling resistance comes from the friction and fracture of tires in contact with the ground. It’s where the interaction between the knobs and the surface is converted to heat and noise as we move along. Little tiny bits of the tire breaks off as it passes over the ground. Bigger, knobbies tires make more noise (also energy).





Rolling resistance (RR) is a big one, but at the same time messing around with rolling resistance is a trade-off with traction. We need traction for stopping, cornering and for when the terrain gets slippery (gravel, ice [?], wet roots, etc.). Sure, you can roll fast with those slick, but can you stop on a dusty trail?





We can calculate the energy lost to RR with,


Err = mgμrrd


Where, m is mass, g is gravity and d is distance travelled. μrr is the rolling resistance coefficient, which can be estimated. When we did this we used a method from Bertucci et al (2003). We used 0.0218 for a Schwalbe Nobby Nic tire in 27.5 x 2.25. There are a number of online calculators for μ.


To get the power lost to drag, you need to know how long it took you to travel this distance. You’d divide the energy by time to get watts.


Interestingly, rolling resistance is not dependent on speed. That is, it is a constant based on this equation. You’ll lose x watts with this tire versus that tire, no matter how fast or slow you’re going.


Using the same data set we explained above, RR with a knobby MTB tire was about 4x the amount of watts lost versus air resistance: 68.1 joules every second or so (68W). About 40W every meter. (Again, I’m extrapolating from averages here, but you get the idea.)


If you think about what it feels like to pedal at 68W, consider that it is almost nothing (compared to 0). Of course as you go faster and closer to your limit, adding another 68W eventually becomes impossible.


We can feel the effects of reduced RR immediately. They only way to do this is to change tire tread.





I often ride with Maxxis Ardent Race tires, and even did a trail comparison with those versus a set of downhill tires. The test consisted of a controlled climb followed by a fast descent, and I measured my braking the whole way down.


On the way up the hill, I saved a whole minute by using low-profile XC tires. The tires were lighter, but the time saving was mostly due to the reduced RR.





The trade-off was on the descent: I didn’t have enough traction with the low RR tires, so I had to start braking earlier to ensure that I slowed down enough and didn’t lose control. The result was more time braking and a higher FlowScore.


You might be thinking that this extra braking meant I was a LOT slower down the hill with the XC tires, but actually I only lost 3 seconds compared to the DH tires. I believe this was another trade-off: the low RR tires picked up speed faster, but slowed down slower.In a way it was a wash between the two sets of tires.


You can watch the video here: https://youtu.be/IQXZqKsKlDY