Ultimate guide to mountain bike gears

Why they're needed and how they work

Gears are the components that define the evolution of the mountain bike. Don’t believe us? Take a look back in history, to Marin County, California where it all began. The legendary Repack started as a downhill race for beach cruisers in 1976, and the riders used a truck to get them to the top. Later that year, over in Colorado, those same pioneers pushed their klunkers around the first Pearl Pass Tour to Aspen.

The breakthrough that turned klunkers into mountain bikes was bodging road bike derailleurs onto them, enabling the bikes to be pedalled up the hills too. In some ways, not a great deal has changed since - levers on the bars move some dangly bits on the frame. But right now, there’s a lot going on in the world of bike transmissions, with more options than ever before and even the odd challenge to the dominance of the derailleur. Join us as we sink out teeth into the world of gears...

Why use gears?

The ability of riders to put out maximum torque (crudely put: how hard you can push the pedals around) at low revs means that while multiple gear ratios are useful, they’re not essential. It’s not just modern-day singlespeed riders who demonstrate the truth of that, either. The Tour de France ran for 34 years before derailleurs were first used, although riders could turn their rear wheels around to use a different-sized sprocket on the other side.

However, if you’re a mere mortal, riding up big hills without gears means you either need strong legs and lungs, a high pain threshold, or a combination of the two. Once lots of gears became available, they were soon put into widespread use – no one’s winning races with one or two gears any more, unless it’s a race specifically for gear-deficient bikes. This is because gears allow you to make the most of your muscles.

You need a certain amount of power to move your bike – how much depends on how fast you want to go, what you and your bike weigh and the gradient you’re riding on. Power is a function of your torque and pedalling speed (or cadence) – to generate power you can either pedal slowly but push hard, or pedal quickly and push less hard. Thanks to the way your muscles work, you can maintain fast, easy pedalling for much longer than slow, hard pedalling.

A choice of gears enables you maintain a fairly constant cadence across a wide range of speeds and inclines. On a singlespeed, to go twice as fast, you need to pedal twice as fast, and it’s easy to run into muscle fatigue (or simple lack of strength) problems at one end of the speed range and the possibility of your legs flying off at the hips at the other.

Gear anatomy

Cassette: The stack of sprockets that acts as the bike’s gearbox. Small sprockets provide higher gears, and big ones result in lower gears. While 9-speed cassettes are the current standard, SRAM’s flagship XX delivers 10 and pretty much everything will be 10-speed for 2011.

Rear derailleur: Also known as a rear mech, this assemblage of links, pivots, jockey wheels and springs does two jobs. Firstly, it moves the chain between different cassette sprockets in response to the shifters. Secondly, it takes up slack chain as the chainring and sprocket sizes vary.

Front derailleur: This is attached to the seat tube with a wraparound clamp or bolted directly to a mount on the frame. Also known as a front mech, its cage shunts the chain across the chainrings.

Chainrings: Chainrings alter the range offered by the rear sprockets, a bit like the transfer box in a Land Rover. The middle ring’s for singletrack andmellow climbs, the inner (or granny) ring is for the grunty stuff, and the outer ring’s for going fast. Three is the current standard, but twin-ring set-ups are becoming popular.

Chain: The chain is pulled around by the chainrings and pulls the sprockets around to turn the wheel, thus converting your efforts into forward motion. It has to cope with high tension as well as being able to deflect sideways to change gear.

Shifters: Your point of contact with the transmission. The shifters pull cables to move the derailleurs. Twin-lever triggers (one for up, one for down) are the most popular choice, such as Shimano’s Rapidfire, but you’ll also find twist shifters, such as Grip Shift, and integrated brake lever/shifters.

How derailleurs work

Derailleurs, also called mechs, have been with us for a long time. Early models of rear mech, which have long arms pivoting backwards from the chainstay, are hardly recognisable as derailleurs now. But by the late 1930s, the familiar parallelogram design mounted near the rear axle had arrived. One end of the parallelogram linkage is attached to the frame, the other to a cage with two jockey wheels in it.

The cage is sprung to allow it to take up chain slack, which results from using the same length chain on different chainring/sprocket combinations. The linkage moves the cage in and out, pushing the chain onto adjacent sprockets. The shifter cable pulls the derailleur one way, acting against a spring that pulls it back when the tension is released.

This 1950’s Campagnolo ‘Gran Sport’ road bike rear mech shows how things have advanced!

Early derailleurs had the linkage moving horizontally, meaning that the distance between the upper jockey wheel and the sprocket got bigger as you shifted to smaller sprockets. That made for inconsistent shifting across the sprockets, but it took until 1964 for SunTour to realise that it would make more sense for the linkage to be tilted to allow the upper jockey to remain a more constant distance from the sprockets.

That simple development allowed SunTour to dominate bike transmission production for 20 years until its slant parallelogram patent ran out, at which point everyone else started using it, including previous also-rans Shimano.

Shifter ergonomics

Early shifters were simple levers with friction devices to keep them in place. Then indexing came along and the levers got distinct clicks, providing helpful feedback. Next, the simple lever evolved into a pair of thumb levers, or a twisting barrel on the bar. Then one of the levers became a finger trigger, until double thumb levers were reintroduced. Finally, the finger trigger could also be pressed with a thumb. Or the shifter was married to the brake lever and you moved it up and down to change gear.

Dual lever gear shifters like this Shimano SLX unit are popular today

These days, the field of shifters has settled down somewhat. The underbar trigger is the dominant design, although integrated Dual Control brake levers and twist shifters still have niche appeal. Having settled on a shifter design, you’re left with some set-up options. Dual Control’s double-duty brake levers let you choose how far along the bar they’re set and at what angle, but that’s about it.

Twist shift users can opt for different length fixed grips, setting their hands closer together or further away from each other. Triggers can be set at different positions or angles relative to the brakes, and some models can be fitted inboard or outboard of the levers too. Test and see which set-up works best for you.

Twist shifters used to be big, but they’re a niche product now

Compatibility

Most bikes come with a mish-mash of components from different groupsets, but not all parts will work together. There’s no problem with chains or cassettes – all 9-speed cassettes have the same sprocket spacing, so you can drop an SRAM one into an otherwise Shimano set-up (or vice versa). You can also use 9-speed derailleurs with 8- or 7-speed cassettes as long as you’re using a shifter with the correct indexing set-up. The only real no-no is mixing SRAM and Shimano shifters and rear derailleurs. Shimano uses a 2:1 actuation ratio (two units of cable pull give one unit of derailleur movement), while SRAM uses 1:1, so they won’t work together. SRAM does sell Shimano-compatible versions of its trigger and twist shifters, though.

Planetary gears explained (Truvativ HammerSchmidt and hub gears)

Planetary gear systems are a way of packing gears into a small space, and so are rather handy for bicycles. A planetary gear comprises a ‘sun’ gear in the middle, a ‘ring’ gear around the outside and a number of equally sized ‘planet’ gears connecting the two. Depending which component is held still and which turns, the parts will move at different speeds.

In a Truvativ Hammerschmidt crank, the crank arm is attached to the smaller planet gears via what look like chainring bolts. The actual chainring is driven from the ring gear, while the central sun gear is attached to the frame and can’t rotate. In the low gear, the whole lot is locked up so the chainring and crank spin together. Unlocked, the chainring is driven round 1.6 times every time the crank rotates, providing the ‘overdrive’ gear.

Truvativ's HammerSchmidt is an enclosed planetary system that provides two modes – normal and the 1:1.6 overdrive for trail mashing

You can see the central ‘sun’ gear, the ‘ring’ and the four tiny ‘planets’

In a hub gear, the sprocket is attached to the sun gear and the ring gear is on the inside of the hub shell. To get more gear ratios, a whole bunch of different sun/planet combinations are mounted side by side, with spring-loaded clutches to engage different ones. The Rohloff Speedhub’s 14 gears are achieved with a couple of planetary gears giving seven ratio options, plus another planetary gear that multiplies up those seven to yield the full 14.

Rohloff's hub gear uses a planetary system to provide 14 distinct, evenly spaced gears

The rather confusing innards of a Speedhub

Range, steps and options

Your gear preference all depends on your strength, favoured cadence and how fast or steep you plan to ride. If you’re scaling mountain passes then descending the other side as fast as possible, you need as wide a range of gears as you can muster. But as there are only so many different gears you can have and the wider the range you want, the bigger the jump between one gear and the next will be.

Racers like close ratios so they can find just the right gear for maximum efficiency, and they’re willing to sacrifice range for it. But as bikes have sprouted ever larger numbers of gears, this tradeoff becomes less acute; if you’ve got more gears to cover the range, they’ll sit closer together. On a typical mountain bike, one gear is about 10 percent higher or lower than the next, and given the variability of terrain there’s not an awful lot of justification for going closer than that.

If your gears are too closely spaced, you end up having to shift several at a time in response to changes of incline in the trail. The conventional triple chainring setup leads to three overlapping ranges of gears and you’ll be able to find a couple of different chainring/sprocket combinations that deliver the same overall gear ratio.

The bigger the range of gears you want, the larger the difference between each gear becomes

So while you may have 30 gears on paper, depending on your exact choice of chainrings and cassette, that might mean you only have 18 or so unique ratios. That’s the thinking behind Rohloff’s 14-speed hub – every gear is different, and they’re all equally spaced. The jumps between them are a little bigger than between adjacent cassette sprockets, though.

You may want to deliberately limit your gear options for various reasons. Twin-ring setups are popular with all-mountain riders doing without an outer chainring for added ground clearance and less chain flapping around. At the other other end of the spectrum, cross-country racers may opt to do without an inner ring for more reliable front shifting.

Trail riders are starting to experiment with 1x9 setups, using a middle-size chainring and wide-range cassette for a simple solution that offers most of the gears you’ll ever need. Or there’s the even simpler singlespeed option, which can be a simple, reliable and (on the right trails) rewarding setup.

Some riders choose to go singlespeed for the ultimate in simple reliability

Setup comparisons

There are plenty of gearing options available off the shelf. The chart you’ll find below shows the gear ranges offered by five common setups. A gear ratio of one means one revolution of the wheel for every revolution of the cranks or direct drive. Anything below that is a very low gear. The left-hand end of each bar is the lowest gear, the right-hand end represents the highest.

All these ranges can be tweaked by choosing different parts. For the sake of comparison, we’ll say the 27-speed setup is the common 22/32/42T chainset and 11-32T cassette. Twin and bash is the all-mountain/freeride-friendly 22/36T double chainset with a bashguard instead of a big chainring and a 11-34T cassette. SRAM XX is a race-oriented 28/42T chainset and has a 11-36T 10-speed cassette. Finally, the Rohloff and Alfine hub gears can be used with a wide variety of chainrings and sprockets, and different ones will shift the bars left or right. The illustration here assumes a 40/18T for the Rohloff and a 32/18T for the Alfine.

Mechanical efficiency

Measurements have been made of bicycle drivetrain efficiency and the results are quite impressive. In a clean lab, a bike transmission can be almost 99 percent efficient – just one percent of the power put in at the pedals fails to make it to the back wheel. That’s amazing for a mechanism that’s remained fundamentally unchanged for about 130 years.

You’re not likely to be getting 99 percent efficiency out in the real world, though, and some of the factors that affect efficiency aren’t immediately obvious. The main one turns out to be sprocket and chainring size, with bigger ones being significantly more efficient. For a given chainring size, you might see 99 percent efficiency with a 21-tooth sprocket but only 95 percent with an 11-tooth sprocket.

That’s quite a difference, suggesting that if you can achieve the same gear ratio using a bigger sprocket (42/21T instead of 32/16T, for example) then you’ll be pedalling more efficiently. Surprisingly, lab tests show there’s hardly any difference in measured efficiency if the chainring and sprocket are offset sideways, contradicting the common dictum that extreme crossover gears like big chainring/big sprocket should be avoided.

Lubricating the chain doesn’t seem to make much difference in the lab either. But there’s a world of difference between a clean lab with a simple run of chain between two sprockets and the grubby outdoors with derailleurs and worn parts. Extreme crossovers tend to be noisy, which is always a bad sign, and tend to stretch the chain. And as soon as there’s dirt, grit and water around, lubrication is clearly a good idea.

You’re not likely to be getting 99 percent efficiency out in the real world

What does the future hold?

Ten-speed cassettes are no longer the preserve of the rich or the sponsored. SRAM's 2/3x10 X7 groupset is already available and the new 3x10 Shimano Deore is set to be released in June. We’re expecting 2011 to be something of a watershed year for hub gears, too. Shimano’s Alfine is getting a bump to 11-speed with a gear range equivalent to that of an 11-45 cassette. It can’t quite match Rohloff’s derailleur-rivalling range, but it’s likely to be far cheaper.

Then there are systems that have been bubbling away for years. Centrally mounted gearboxes have weight distribution and suspension benefits but add complexity. Toothed belt drives are low maintenance, clean and quiet but require high tension hub gears (or a single gear) and frames with gaps for fitting them. It seems likely that chain drives and derailleurs will dominate the mountain bike market for some time to come.

Gearbox bikes have been in development for several years now

However, the Achilles’ heel of current gears is that they use a mechanism that dangles off the bike waiting to be mashed by rocks Looking further ahead, a replacement must surely be the next big advance. Conventional hub gears are too heavy and inefficient for use outside of specialist applications, but continuously variable transmission (CVT) might be the answer.

CVT systems promise to free us from the constraints of choosing gears by giving us as many as we like across a range, with instant shifting. Current CVT systems are too heavy for performance use but there are some determined engineers out there working to fix that. Finally, Shimano’s electronic transmission for road bikes (Dura-Ace Di2) has proven hardy enough to stand the  almost off-road conditions of the Paris-Roubaix race. Will we see it on mountain bikes one day? We already have.

Shimano's Dura-Ace Di2 road groupset offers electronic shifting

Related Articles

Comments

Back to top