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The Science of Weight in MTB: Does it Matter?

MAIN TAKEAWAYS

· We ride fast by having a high speed. We can pedal more or brake less to go fast. Pacing is important since our tiny little human motors are not that great.


· A study by Dr Paul Macdermid showed that heavier bikes were slower up hills—no surprise! A bike 21% heavier was 3.3% slower for a 95kg rider. This same study indicated that a dropper post will lose you only 1 second up a steep climb.


· Rotating weight is important, and get even more important as the diameter gets bigger.


· Once a bike is up to speed, it wants to stay at speed. Heavier bikes will want to stay at speed more, but they will be harder to get going again if you slow down.


· You can ‘buy’ time, but getting the right mix of pedalling, braking and pacing is better than buying expensive, fragile parts.


· Don’t worry about weight for DH or Enduro.




If you prefer this article as a free podcast episode, we talked all about bike weight here on the Performance Advantage Podcast:



WEIGHT A MINUTE


To mountain bikers of all types, weight seems to be one of the most important factors they consider of any bike or component. It’s all anyone ever seems to talk about.


*Online Magazine: “Here’s a new bike. This is what it weighs!”


*To your mate who just got a new thing: “Wow, that’s light!”


*You about a power meter: “Nah, too heavy.”




These 11 grams are guaranteed to make you faster.



I've always found this to be a bit strange. I mean, for racers going uphill it always made sense to stress about weight, but for a 100kg weekend warrior? For enduro racers aiming to go down hills all day? Really??


When you are racing XC and it matters how fast you get to the top of the hill, it's for sure important to maximize your speed at your given effort. We can do this lots of ways. For example, one can train properly to gain fitness and the ability to hold power (that is, propulsive power, measured in watts (W) with your power meter). Or on the other hand, one can pace properly using their power meter to maximize efficiency over the length of a race. And similarly, one can minimize their body weight through good old clean eating, which means that their weight is down and they have a greater (W/kg) at a given effort. So yeah, for XC where going uphill is the most important thing, you definitely need the watts up and the kg down. Counting grams could be helpful.



Check this scale out if you still want to count grams after you've read this.


But then at the same time, I've been told by World Cup downhillers that they won't run component X because it is too heavy. This is when things started to sound a bit weird for me. Add in anecdotes from weekend warriors purchasing the absolute lightest parts (and breaking them), and this really does need further investigation.

HOW DO WE RIDE FAST?


To ride fastest in an enduro, XC or DH race, you want to cover the set distance of the trail in the shortest amount of time. By doing this, you need to have the highest average speed. Have the highest average speed and you win the race. High speed on uphills and downhill in XC will win. High speed on downhills in enduro and DH will win. Perfect.

In any form of MTB, there are all sorts of variable terrain. There are bumps, turns, various gradients, some chances to pedal, and plenty of opportunity to maximize your efficiencies in braking, propulsion, bump absorption, focus, etc.


To get to these high average speeds, we will need to use our relatively tiny human engine; the human engine is incredibly limited. For example, even the best riders in the world can maintain 2,000 W for about 10 seconds. A top XC pro can average about 400 W over ~2 hours. A normal weekend warrior of about 75 kg can maintain ~200 W for an hour going full tilt. That about equal to 6 bananas going full gas (720 kJ); after this it's light out.


But this is just half of the story.


On a 3 minute downhill, riders can brake over 30 times for almost 45 seconds, burning 33% of a banana (35kJ).