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).





Quick maths shows that's an equal proportion of energy wasted per minute when braking compared with the energy used pedaling forward [n.b. this varies a lot, but let's stick with that figure for now].


So, considering an equal amount of energy can be wasted by braking, it is probably pretty important to focus on pedalling hard and maximizing efficiency with braking. In doing this, a rider can pedal at the same workload (with their limited engine), but maintain a higher average speed due to wasting less energy on braking.


This makes sense even for XC, but especially so for DH and enduro where there are only marginal gains for even very high power outputs. This is due to the already high speeds. On downhills the speeds are so high that a massive effort won't actually gain that much more speed.


Yes, sometimes coasting can actually be faster!




OK, SO PACING IS MORE IMPORTANT THAN WEIGHT?


So is it the case that weight isn't important at all?


No, weight is important! On climbs.


A set amount of weight takes a set amount of work to be moved. In physics, we think of this as force*distance, where force is equal to mass*gravity. Gravity, mass and distance are all equally important in this equation, and gravity is for sure one thing we cannot change. If you think about picking up a rock, the work required to pick this up will be,


Work = mass of rock*distance you're lifting the rock*9.81


Thus, lifting a 1 kg rock 1 m high will require 9.81 J of energy. Pick up this rock very slowly over 1 second will require 9.81 W (maths: power = work divided by time, so 9.81 J/1 second).



It's easy to figure out how much energy we use when picking up a rock. Gravity is one of the major factors affecting the work needed. Uphill riders take note!


Since we are thinking about riding bikes up a hill, this hill will usually have a set distance which we cannot change. We also cannot change gravity.


At the same time, we also can probably not change our fitness very much until we spend many hours over many months of training.





In this case, we can actually ‘buy’ ourselves some time up the hills by saving a bit of weight. But how much time can we save?

SCIENTIFIC STUDY ON WEIGHT IN MTB


I've never seen a good report on weight, so I dug deep into the archives for a small publication in a NZ MTB magazine by my amazing PhD supervisor, Dr Paul Macdermid. Paul pioneered research into MTB vibrations and 29 v 26 wheel sizes, but was also pretty interested in how weight affected performance. Paul travelled the World Cup XC circuit for years with his wife Fiona, where she even earned a few times in the top 10.

This research on weight was never widely published, but Paul sent me a pdf copy of the article. I can't find it anywhere online, but I really do think it's important to share with everyone.


In the study, riders rode a 1.125 km climb that gained 125 m of elevation. Power was steady at 2.8 W/kg. This climb took about 11ish minutes. Riders did a number of tests, and in a random order the same bike was stacked with water bottles full of sand. Total bike + sand weights were 10.7, 13.0, 14.4 and 19.0 kg.


Read the study here:

MTB Weight Research Study | Dr Paul Macdermid | NZMTBR

weight110113 (2)
.pdf
Download PDF • 971KB

If TL;DR:

The study found that adding even just 2.3 kg to a lightweight XC bike resulted in a significant time penalty. However, the study also noted that 2.3 kg was a much greater time penalty for a 28 kg rider (46 seconds) versus a 95 kg rider (22 seconds).

To me, this says that lighter weight riders have more to lose with heavy bikes and components, whereas heavier riders have less to lose, which is an interesting –albeit not surprising—finding.


For very big riders, maybe it doesn’t even pay at all to have lighter things that may just break in the end?


The study went even further to use the results to calculate the time lost when adding a dropper post versus straight post up the hill. Dr Paul calculated that a 70kg rider would lose about 4 seconds up an 11 minute climb by carrying and extra 335 grams between the seatposts.


Roughly, a 70 kg rider would lose 2 seconds on a 5.5 minute climb, and 1 second on a 2:45 minute climb by riding a dropper seatpost.

Digging a bit deeper, this extra 335 grams for the dropper post would have almost no penalty for a 100kg rider.


This isn’t a huge margin at all--even for a lightweight whippet, and it’s possible that at least some of these seconds could be saved on the next descent—because remember, MTB has varied terrain and you also have to go down the next hill!


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WHAT ABOUT ACCELERATING AND ROTATING WEIGHT?


Objects in motion will stay in motion unless an outside force acts on it. Object not in motion will tend to not move without an outside force. This is as true for a rock on the ground as it is true for a bike posing for an Instagram shoot.


This is inertia.


Unlike most things we calculate, inertia is actually easier to understand when we consider rotation. Think of this as your rotating wheels.


Rotational inertia can be calculated as,


I = m * r^2