Condensed answer: The gears on downhill bikes increase the bike’s efficiency and acceleration abilities. The extra gearing makes the bike more versatile and adaptive to the terrain ahead.
The Benefits of Extra Gears
On the surface, a downhill bike doesn’t need many gears because the rider is descending. This is true, but only to a certain extent.
Below is a list of reasons why gears are beneficial to downhill bikes:
Gears allow the rider to spin the pedals at a more optimal cadence. The result is smoother acceleration.
Flat + Uphill Segments
Some downhill tracks include flat and uphill parts too. During those sections, the extra gearing results in easier and faster pedaling.
Downhill Bikes Are Heavy
Downhill bikes have dual crown suspension forks, a rear shock and a massive frame designed to sustain a lot of abuse.
The wheels are overbuilt too. Otherwise, they would bend and get out of true very quickly.
The final product is a heavy bike that would be quite difficult to pedal as a single-speed.
The table below contains the weight of popular downhill machines:
|Model||Weight (kg)||Weight (lbs)|
|Commencal Supreme DH 27||15.1kg||33.22lbs|
|Canyon Sender CF 9.0||15.83 kg||34.8lbs|
|Santa Cruz V10||16.59kg||36.5lbs|
|Saracen Myst TEAM 2019||15.83||34.8lbs|
|Nukeproof Dissent 290 Comp||17.49kg||38.4lbs|
|Saracen Myst AL||18.5kg||40.7lbs|
|Trek Session 9.9||15.65kg||34.43lbs|
In comparison, XC bikes weigh a lot less as shown in the next table:
|Model||Weight (kg)||Weight (lbs)|
|BMC Fourstroke 01 One||10.7kg||23.54lbs|
|Scott Spark RC 900||10.45kg||23lbs|
|Specialized Epic Expert EVO||10.22kg||22.5lbs|
|Kona Hei Hei||14.21kg||31.26lbs|
|Specialized S-Works Epic||9.46kg||20.812lbs|
|Yeti SB100 GX||13.1kg||28.82lbs|
|Trek Top Fuel 9.8||12.2kg||26.84lbs|
|Giant Trance 29 Adv Pro||12.4kg||27.28lbs|
|Santa Cruz Blur Carbon CC XX1 AXS||10.27kg||22.594lbs|
With an average weight of 11.43kg, XC bikes are notably lighter than downhill models.
Downhill Bikes Aren’t Aero
Downhill bikes have a slack head tube angle and put the rider in a very upright position. This stance increases stability during descents but at a cost – the drag rises too.
As a result, it takes even more energy to get the bike up to speed. Gears facilitate this task.
High Rolling Resistance
Downhill bikes have wide tires with aggressive patterns. This is helpful for downhill tracks, but on flat sections without many obstacles, the end result is greater rolling resistance making it difficult to accelerate and maintain speed. An extra set of gears minimize this effect.
Some people like to ride their downhill bikes on trails too. In that scenario, gears are indispensable.
What Gears Do Downhill Bikes Use?
The drivetrains of downhill MTBs reflect the main function of the bikes, namely quick off-road descending, and have the following properties:
1. One chainring
Downhill bikes have a single chainring at the front protected by a bash guard.
The reasons for relying on a single chainring instead of multiple ones are:
a. The use of a front derailleur during quick descending increases the chances of dropping the chain and essentially losing your ability to pedal.
By sticking to 1x drivetrains with highly-efficient chain guards and tensioners, this scenario is prevented.
b. Downhill bikes don’t need a wide range of gears.
2. High Gearing
Since downhill bikes are rarely used for “brutal climbing”, they only need top and mid gears.
Consequently, most downhill models come with limited gearing in comparison to trail and XC bikes.
The table below contains the number of chainring teeth and the cassette range of popular high-end downhill bikes:
|Commencal Supreme DH 27||34Т||7||9-21Т|
|Canyon Sender CF 9.0||36Т||7||10-24Т|
|Santa Cruz V10||36Т||7||11-25Т|
|Saracen MYST AL||36Т||10||11-25T|
|Nukeproof Dissent 290 Comp||34Т||7||11-25T|
|Trek Session 9.9||36T||10||11-25T|
|CUBE TWO15 Race||34T||7||11-25T|
|Specialized DEMO RACE||34T||7||11-24T|
- Most downhill bikes have 7 speeds.
- The lowest gear/cog has about 25 teeth.
- The front chainring is of medium size.
Note: Downhill bikes are starting to use super small cogs (e.g., 9T) as a way to increase the highest gear while keeping the front chainring small for extra clearance.
To understand this principle, one has to be familiar with the term gear ratio.
Тhe gear ratio describes the “relationship” between the chainring and the rear cogs.
You can find the gear ratio of a combination by dividing the number of teeth on the chainring by the number of teeth on the rear cog.
The gear ratio indicates how many times the rear cog and consequently the wheel turns per 1 rotation of the cranks.
Combining a 34T chainring and a 9-teeth cog gives us a 34/9=3.77 gear ratio.
This means that the rear wheel turns 3.77 times per pedal stroke at that gear.
Meanwhile, the more common combo of 34T + 11-teeth cogs produces a gear ratio of 34/11=3.09. Or in other words, the rear wheel turns 3.09 times per one crank revolution.
A larger gear ratio results in higher top speed because the wheel is turning more times per one pedal spin and thus the bike is moving the furthest.
The only way to match the gear ratio of a 34T/9T combo with an 11-cog would be to get a bigger chainring.
You can find the number of teeth that the chainring would need by solving a simple equation.
y/11=3.77 (y=the number of teeth on the chainring)
y= 3.77×11= 41.47
This tells us that to match the gear ratio/speed that a 34T/9T combo offers, one would have to run a front chainring with 41-42 teeth.
The downside of having such a big chainring would be reduced clearance and loss of low-end gears.
Chainless Downhill Competitions
One can find proof that gears aren’t fundamental to downhill tracks without flat or climbing sections in the chainless downhill competitions which were more popular in the past.
As the name suggests, chainless competitions are races in which the competitors do not have a chain. Obviously, similar events can work only on downhill terrain.
Interesting fact: In 2015, the American downhill biker Aaron Gwin won a race at the Leogang UCI MTB World Cup despite breaking his chain at the beginning of the run.
Frequently Asked Questions
Why Aren’t “Pure” Downhill Bikes Single-speed?
Downhill bikes gain close to nothing from being single-speed for the following reasons:
The rear triangle of full-suspension bikes moves. Therefore, the chain should be free to dynamically expand and shorten without losing tension.
The only way for this to happen is to have a derailleur or a dedicated chain tensioner. Otherwise, the chain would either drop because it’s too loose or break because it’s too tight.
The fact that you still need a tensioner reduces the reliability and lightness that single-speed offers and begs the question of whether it’s worth it to lose your gears.
The only way to avoid using a tensioner is to find a full suspension downhill frame built for single speed use. To my knowledge, such a frame is nowhere to be seen.
Derailleurs Are Tougher Than People Think
Contrary to popular belief, rear derailleurs do no break easily. Modern models are low profile and tuck away nicely.
Sure, they can get hit, but it doesn’t happen frequently enough to remove them completely and lose the functionality of gears.
Loss of Versatility
A single-speed downhill bike doesn’t have the versatility of a geared one. Therefore, the number of potential riders that would want such a product is low.