Search

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.

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!


Get ANY MTB Training Plan

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


where, m is mass and r is the radius of the wheel.


Anything with inertia (which is anything with mass), will take some sort of energy to get going or change speed in any way. Heavier objects normally have greater inertia, which means that more energy will be needed to change the speed of it.


You may have heard it said before that rotational weight is the most important thing to consider in your bike's weight. This is why we see riders choosing lighter tires and rims, and lighter weight pedals—they are rotating.



You might remember this argument from one of the bike companies pushing 27.5 in 2015. This really only told half the story.


Considering inertia, it will most definitely be more difficult to accelerate a heavier tire. This heavier tire has more inertia, and thus will want to remain motionless as we use our limited pedal power to get up to speed.


At the same time, however, these same tires will want to stay at speed once they get rolling. This is great for downhills!


BUT. But consider the other part of the equation: the radius. When calculating rotational inertia, radius is squared. This means that actually the radius of the wheel affects the inertia of a wheel much more than the weight.


In practice, your 29er will be harder to get up to speed than your 27.5. HOWEVER, the 29er will carry speed better — due to greater inertia. Add to it the fact that, by being bigger, your 29er is heavier, and you have an altogether better ability to maintain speed. But also harder time accelerating…

SO DO WE KEEP SPEED OR ACCELERATE?


There is no better way to win a race than by maintaining the highest average speed. Actually, having the highest average speed is the only way to win a race! However, there is no faster way to lose a race than by accelerating too many times.


Remember how limited our tiny human engine is!


To have that higher average speed in XC, we could either maintain better speed or accelerate many times…or both accelerate and maintain speed!





A heavier bike going slow will really want to stay going slow uphill. This means we will need to spend a lot of energy to accelerate the bike up to the speed we want. However, a heavier bike going fast will really really want to keep going fast.


As riders on any bike, it takes a lot of effort for us to get our bikes to go faster, but on the other hand, it doesn’t take much for us to keep them going fast—especially when gravity is on our side and actually giving us more energy.


[See here for coasting versus pedalling article]





When a hill is giving us energy or we have a heavier bike, it actually makes the most sense for us to brake really efficiently to keep going fast. Sure, we can pedal, but this has a high cost and there is only so much we can do before we get tired.


It is for this exact reason that we invented BrakeAce. Now riders can understand the efficiency of their braking and work on improving it to conserve energy.


The bottom line for slowing down and speeding back up again is that eventually the rubber band will snap, and we will be totally smoked and unable to get back up to reasonable speed again.

SO IS WEIGHT IMPORTANT FOR DOWNHILL?


I'd argue that having a lightweight bike for downhill is not at all important.





To understand why, let’s go back to energy, pacing, and braking.


The energy that hill can give you is equal to mass * gravity * height, or, in the case of a rider:


Gravitational potential energy = (Weight of rider + bike) * height of hill in meters * 9.81


If you work this out, there is an incredible amount of energy the hill can give us. You’ll understand this if you’ve ever coasted down a hill—you can get lots of energy and go fast very quickly! Based on the equation, there is actually more gravitational potential energy with a heavier rider or bike, and once we are up to speed, we are more likely to stay at speed due to the greater inertia.


Tentative riders may end up taking away a lot of this energy once they are moving [when we are moving it is called kinetic energy] through the use of the brakes. If these riders brake too much and take away too much speed, they will need to pedal hard to get back up to speed. For the rider slowing down and accelerating back up many times, a lighter weight bike may actually make more sense for as this will save energy.


[NOTE: this is why we need bigger rotors]


Pros on the other hand might be able to get around more parts of the same downhill without braking or by braking more efficiently overall. If they can do this, it is actually true that a heavier bike could maintain an even higher speed for them thanks to our old friend gravity.


Go back and read the article on how hills give us free energy and speed.


Aaron Gwin showed us that by braking more efficiently, he could win a downhill World Cup – even with no chain. The only time the chain was really important for him was at the start when he accelerated from 0 kph to X kph. After this point he was able to ride smooth by using the energy the hill gave him. Had Aaron used a heavier bike, it would have been more difficult to get going off of the start. While this might not have had any effect on whether he broke his chain or not, had he used a lighter bike, it may not have carried as much speed on the rest of the descent. What Aaron did show to us was that it’s not all about getting your bike back up to speed by pedalling really hard – sometimes going fast is all about maintaining speed.


Check out his winning chainless run below. Luckily his bike wasn't any lighter!




SO DOES BIKE WEIGHT MATTER FOR ENDURO?


Enduro races are big days. The timed descents are hard and fast and the untimed climbs are hard and long.


Enduro ends up being all about pacing properly. An astute rider will know exactly the power she will hold on all the climbs, will fuel properly, and ride the descents as fast and smooth as possible. One mistake or mechanical and the race can be over. Therefore, the bike’s components need to be robust.


Due to the long climbs, riders often weigh up the importance of low weight and durable equipment. Sometimes riders will even swap to lighter tires if a day has ‘more climbing’.

However, we know that we need a super robust bike for the race stages so we can go fast when it counts. Knowing what we know and pacing like we do, we can easily work out how much slower a heavier bike will be.


I really like the study by Macdermid we wrote about above. Since it is paramount to pace properly on liaisons, we can use the same W/kg model on a long climb that the study used as an example.



I love a good DHR tyre. If you're trying to go fastest down tricky hills, go with full sticky, full DH setup and ride with confidence.


Adding 2.3 kg to a bike is the difference between a set of super lightweight trail tire and a set of full DH tire.


Big difference, right?


Well, this massive difference in tires is also arguably one of the most robust and control-inspiring changes you could make. You simply need beefy, sticky tires to ride in control and change direction and speed easily.


For a 70kg rider this additional 2.3 kg will make them 27 seconds slower per 11 minutes climbing at a very steady 2.8 W/kg.


Note that the proper enduro tires will not make them have to work harder--these tires simply mean that for the workload that they normally pedal at, they will simply need to go slower!


The DH tires are a no-brainer.

FINAL POINTS


So as we can see, weight is important when fighting gravity up hills or when we are trying to accelerate, but if we ride smooth and smart, heavier bikes can actually help us go faster. Mountain bikers spend way too much time trying to have lightweight stuff and trying to "buy" time, when in reality much bigger gains can be had by riding smoother, smarter and training right.


Hopefully this makes your next ride more fun!

Check out BrakeAce.com to stay up on the automatic coaching software


Check out all my TrainingPeaks plans with smart trainer integration


5 Ways to Use the BrakeAce For Self-Coaching


Why do you have to be fit for gravity racing?


Don’t forget about our other web articles!


LATEST SCIENCE: Validity of a device designed to measure braking power in bicycle disc brakes


5 Science Hacks to Help You Race Mountain Bikes Faster


Anthem Advanced 27.5 1 Review


Picking the right power meter for mountain bike racing


How to avoid blowing up in MTB racing

© 2020 MTB PHD LIMITED