It is amazing to think that just 30 years ago the mountain bike drivetrain didn’t even exist. It wasn’t until 1982 that Shimano launched the iconic six-speed XT component group, and even this high-end equipment didn’t feature the basic stuff that we take for granted today, such as indexed shifting and slide on cassette sprockets.
Gears is an area that has seen rapid development over the years: the fundamentals may be very similar, but with manufacturers pushing their new technologies and releasing 10-speed parts in a host of configurations, the modern mountain bike transmission is a world away from its forefather. Confused? Read on as the twisted chain of cogs, mechs, shifters and crank arms is slowly unravelled.
The need for speed
Why do we even need gears, and why so many? The simple answer is that not all of us do. People have been riding singlespeed since the bike was invented and continue to do so for many reasons such as mechanical simplicity, low weight or just easy terrain that doesn’t demand so many cogs. However, ultimately we are all limited by one thing – the human powerplant.
The power we output is simply a function of torque (how hard we pedal) and cadence (how fast we pedal). Our muscles are most efficient at sustaining a cadence of somewhere between 60 and 120 crank revolutions-per-minute. Changing gear allows us to stay in that optimum range and tailor our output for the variety of speeds and terrain we will encounter.
A motorbike engine for example can supply its torque over a much larger rev range, and for that reason doesn’t need quite so many gears. Also, as we’re powered by cakes rather than petrol, the supply of energy to fuel our engine isn’t always as smooth as we would like. Efficiency is key, and over pushing a gear that is too hard, or over spinning too easy a gear can lead to fatigue or muscle strain very quickly.
Gear systems and ranges have evolved over the years to reflect how we ride and the changes in the types of bikes on the market. Mountain biking is no longer just the cross-country orientated sport it was in the 1980s. With the advent of downhill at the end of the last century, and the introduction of all-mountain go-anywhere machines at the beginning of this one, manufacturers and riders soon realised a change in direction was needed.
The large touring-inspired triple chainsets that were standard on nearly all bikes were replaced with smaller, compact versions that provided greater clearance. Wider ranging cassettes with more sprockets were introduced, not only to provide higher and lower gears, but to reduce the gap to the adjacent gear for more precise cadence control. Harder hitting bikes often lost the front shifting altogether or used a double ring crank and guard for powering down hills with rock clearing confidence.
The biggest leap forward in mountain bike drivetrain technology since Shimano launched the Mega Nine equipment in 1999 is 10-speed. Both Shimano and SRAM have released 10-speed groups at a variety of pricepoints, but rather than push the benefits of that single extra cog they've looked more in depth at how it integrates with the front shifting. By understanding the basics of how transmission works and the pros and cons of different setups even the novice rider can make an informed decision on what is best for them. Whether that be seven, eight, nine, 10, or just the one gear!
Derailleurs and drivetrains uncaged
The bicycle derailleur, or mechanism (mech), was envisaged as long ago as the late 1890s, but it wasn’t until the 1930s that the familiar parallelogram shape was born. Suntour revised the design for rear mechs in 1964, and it's this layout that survives today, albeit with a few tweaks. That dangly piece of metal that hangs off the back of your bike has quite a few tasks to perform.
As well as moving the chain up and down the cassette in precise increments, it must also compensate for the change in chain length as we shift, and keep the chain under sufficient tension. It does this by using a sprung parallelogram off which hangs a cage. This cage contains the top guide pulley and the bottom tension pulley, and moves in and out of the frame, taking the chain with it. A second spring pulls the cage to tension the change, while the whole mechanism is connected to the frame via a rotating link.
The front derailleur works on a similar principle, with a linkage to shunt the chain back and forth, albeit this time without any tension duties to perform. Both mechs have two screws to limit the overall movement (preventing the chain derailing completely).
None of this can happen without the shifters. Housed in the familiar ‘pods’ that bolt onto the bars, a ratchet and lever system pulls the chain in set increments that result in a gear change. It hasn’t always been this way though – indexed shifting (one click per gear change) wasn’t fully realised until 1984. Before that we relied on a simple friction system to adjust the derailleur action. Setting the correct cable tension is the key to precise shifting, with the barrel adjusters on the shifter body allowing for fine tuning.
So, how does the chain actually jump to the next cog or sprocket? If you look closely at your cassette and chainrings you will see a series of ramps and pins. These are located in specific places to allow the chain to climb and drop to the next gear smoothly and quickly. Some systems, such as the Dynasys equipment from Shimano even have directional chains that take this concept a step further.
Combinations and limitations
The drivetrain you end up with will depend on a few factors, not least your personal preference. The terrain, preferred cadence, your bike and strength/weight will all influence what you choose and you will often find that you have two different setups on two different bikes.
If you want to climb every mountain and nail every descent then a wide ranging gear set with triple chainset could be the perfect compromise. If you're a cross-country racing snake with tree trunk legs you may want to drop the granny ring altogether and favour a large double chainset with close ratio cassette. Alternatively simplicity may be your thing so a single ring could be the ideal solution, but if you value a bit of clearance on your all-mountain mosher, a smaller twin front set-up with bash guard will tick all the boxes.
More sprockets can increase your range and the step between each gear. On the flipside, too less a step may result in you making more changes. Twin chainrings can provide a more usable range of gears but limit the absolute top and bottom end, though smaller rings can provide great log clearing capabilities.
Even before 10-speed was thought of, there has existed a large variety of chainset and cassette combos at the seven-, eight- and nine-speed level. Cogs with 34 teeth started to appear on cassettes at the turn of the century, and companies such as Ritchey and Middleburn were experimenting with proprietary double chainset systems even before then. Triples have been tweaked a few times with some interesting and odd combinations.
A typical gear system may have three chainrings in a 44/32/22 format and eight or nine sprockets on the cassette, usually 11-32. The smallest rear sprocket equates to the hardest gear, while at the front this is reversed. The gear you're in is a simple ratio of the number of teeth on the chainring to the number of teeth on the sprocket in use. For example, with a 32-tooth ring and 16-tooth cog your ratio is 2:1 (32/16).
However, the size of the wheel has to be taken into account to determine how hard that gear actually is. Multiplying the gear ratio by the wheel diameter gives us a number called the gear inch. This allows us to compare gears across a wide range of wheel sizes, for example comparing an average 29er bike to an average 26in-wheeled bike, with exactly the same gearset, the 29er will feel about 10-11 percent harder to pedal. The total range of your gears is simply a comparison of the lowest ratio to the highest and is often expressed as a percentage.
A quick look at our typical triple chainring will reveal something that at first glance seems rather odd: a lot of the gear ratios are duplicated! The 32/16 combo will pedal pretty much the same as the 44/22 and 22/11. In total there may only be 13 or 14 unique ratios across the whole gearing range. The reason for this is the physical limitations imposed by the gearing.
Front mechs can only handle jumps of around 12-14 teeth in one gear change, and because of the angle the mech moves at, a minimum number is usually quoted too. This isn’t always a bad thing. If the mech could jump say 20 teeth in one go you might find that the gear change is simply too much and you have to make numerous ‘recovery’ shifts at the rear to get the next gear you wanted. Smaller steps in the chainring sizes may cause more overlap, but can provide a more economical gear change, and using certain sprocket/ring combos can cause the chain to lie at an angle, increasing wear and tear.
The same questions can be asked of the cassette. Why limit our choices to 11-32 or even 11-36 as found on some newer drivetrains? Why not use a range like 6 to 42 teeth for a huge sequential range? Again it comes down to technicalities involving the drivetrain equipment. The modern sprocket cluster slides onto a unit called a freehub body, which houses a freewheel mechanism and the hub bearings. The physical size of this has limited the size of the smallest cog to 11 teeth.
Derailleur specifications can't handle a sprocket larger than 36 teeth, which is why currently the widest ranging cassettes we see are the 11-36. Also the smaller the rear sprocket gets the less efficient the drive will become. By weighing up all the options, studying the numbers and even testing a few groupsets out, you can quickly familiarise yourself with the choices and make the best decision.
Shift to the future
Will we stop at 10-speed? With Campagnolo already releasing 11-speed on their road groups it seems logical that mountain bikes will eventually follow suit with the ‘more is better’ mantra. There have been some interesting developments with cassettes, with Shimano redesigning the freehub body to allow a nine-speed sprocket to be fitted, and Hope achieving a very similar result with a proprietary system.
It seems that wider ranges are at the forefront of developers’ minds with pushes towards systems that may do away with front shifting altogether. Some hub gears have already realised this, and no doubt we will see more planetary drives and gear boxes popping up over the next few years. We may even see instantly shifting Continuously Various Transmission (CVT) appearing on bikes in the future with a few people working on this concept.
Will the derailleur itself disappear at some point in the future? When it works, it works great, but show it a stick or any other trail gremlin and it is rather prone to self-destruction. One thing’s for sure, we will have more choice, even if the parts are not necessarily better. And more choice allows us to tailor our bike ever more towards that holy grail of riding perfection...
Mountain bike drivetrain components explained
Cassette: The cassette is basically a pile of different sized sprockets that make up one half of your gear system. They mount onto the freehub body of the wheel, and are locked down with a specific ring. If you have an alloy freehub body look for alloy spiders on the cassette as this will prevent scarring and damage
Chainset: The chainset forms the second half of your gearing, usually comprising of one, two or three rings joined to the crank arms via a spider. Different lengths are available to suit different lengths of leg, as well as a variety of bottom bracket fittings, just check what your frame takes.
Rear derailleur: The rear derailleur shunts the chain up and down the cassette as well as taking up chain tension differences between the ratios. Longer cages can handle a greater range of gears, but shorter lengths can mean less rattling and a snappier shift.
Front derailleur: The front derailleur handles shifting duties across the chainset and acts as a retainer for the chain. Different mounts and cable entries are available depending on the needs of your frame. Look out for specific versions to tailor shifting ie for double chainsets or very large chainrings.
Shifters: The shifters perform the magic, hauling a cable through the outers, eventually operating the derailleurs. Trigger shifters with paddles are most common, but twist type and brake levers that double up as shift levers are available. The number of shifts must match the number of sprockets on your cassette.
Chain: The chain combines with the shifting ramps and pins on the sprockets and rings to enable it to smoothly change between the gears. Always ensure it is the correct length, and fit direction chains appropriately – wrap the chain around the largest cog and largest sprocket (bypassing the derailleurs), and add two links where they overlap.
Devil in the detail
Pay attention to the individual specifications of the items in question. Front mechs are available in a number of clamp types, cable entries and cage heights so ensure it fits your frame and double check it can cope with the range of gears and size of rings you are using. Similarly for the rear mech; most bolt on the same way but there are always exceptions.
The cage length depends on the capacity of your system (the difference in number of teeth on your very highest and very lowest of gears). Most rear mechs will state this, as well as the actual largest sprocket it can handle. In many cases you can actually get away with going over the prescribed specs a little, but with so many combinations out there, always err on the side of caution and always ensure the chain is the correct length. Bottom brackets have many standards from the simple cartridge type to press-fit external bearings. Your frame will dictate much of what fits and what doesn’t.
With seven-, eight- and nine-speed formats having been on the market for over a decade, well established rules have emerged on how different kit integrates together. While it is pretty obvious that your indexed shifters must match your cassette for speeds, it's less well known that the ‘speed’ of the derailleur doesn’t have to match shifters or cassette as the cable pull ratios remain the same.
Well, at least for the same brand. The brand of shifter has to match the rear derailleur. With SRAM shifters pulling twice as much cable per click than Shimano, you can’t use Shimano shifters with SRAM derailleurs and vice versa. Front mechs are fully interchangeable though, except for a little rubbing with the narrower cages of the higher speed stuff on the lower equipment. Similarly almost all chains, regardless of designated speed, seem to work perfectly well on eight or nine-speed cassettes and chainsets despite slightly different dimensions.
The introduction of 10-speed has thrown a big oily spanner in the spokes, with a lot of this compatibility not being carried forward. Shimano and SRAM have tweaked the cable pulls, so that 10-speed shifters must match their own 10-speed derailleurs. Interestingly, front derailleurs seem to work fine with any brand and any designated speed, but there is no doubt that matching for speeds across the parts results in less rubbing.
If you must mix, try and mate the derailleur speed to that of the crankset. Similarly chains work best when in their own group, but using a higher speed chain on lower named cranksets works fine – though keep the chain matched to the sprockets for best results. Also remember that all cassettes bar seven-speed will fit on a modern freehub body as the sprocket spacing gets progressively narrower, and if you do use a seven-speed cassette, a 2.5mm spacer must be fitted first.
SRAM first released a 10-speed groupset a couple of years ago, aimed squarely at racing with their flagship XX kit. The technology has now trickled down to the mid-range X7 level. Shimano have hit back with 10-speed groups from SLX level all the way up to the top of their tree, XTR, for the 2011 season. A 10-speed cassette with a 36-tooth sprocket is where the similarities end, as each company has pushed different concepts revolving around the chainset.
Shimano champion the triple configuration. By reducing the largest ring to 42 teeth and increasing the smallest ring to 24, Shimano propose riders are more likely to stay in the middle ring with that dinner plate sized 36-toother at the rear. When you do shift down into the granny ring or up into the large ring, the smaller jump will equate to less recovery shifting and a more efficient ride. Shimano also claim that full-suspension systems will suffer from less bob due to the optimised chainline.
SRAM, on the other hand, push the double chainset, or as they put it, the 2x10. By pairing the chainrings in a 3:2 ratio (42/28 and 39/26) with perfectly timed ramps and pins, SRAM have increased the front shifting speed and fluidity. A variety of cassettes are offered, not all with the large 36-tooth option though. SRAM claim that the advantages of such a system are simplicity, lighter weight and a perfectly suitable range of gears that are fast changing and easy to use for all riders.
The hub gear is an age-old solution that comprises of an enclosed planetary cog assembly inside the rear hub. A standard looking shifter and cable is used to shift the cogs about while a normal chainring and sprocket is utilised to drive the system. The enclosed system resists the elements and requires little maintenance. However, the design can add weight and mechanical efficiency isn’t always as high. The range can be also low, but companies such as Rohloff and Shimano with their Alfine products have upped the ante, reproducing useful gear ranges and lowering weight to near that of an equivalent priced derailleur set-up.
Planetary drive gears aren’t just limited to the rear hub. Truvativ made big waves with their twin-geared HammerSchmidt front chainset. Chains aren't even needed in some cases – a small number of bikes have appeared using belts with their planetary drive gear sets. Even electronics have found their way onto bikes. Shimano Di2 has been on road bikes for some time now with rumours abounding that we'll see it on mountain bikes at some point in the future.
Problems and cures: Make your shifting sweet again
You’ve probably noticed that bicycle transmissions are complex things. Hub gears hide their parts inside their shells and have maintenance needs more akin to a car than a bike. You change the oil every so often and if anything goes wrong enough to need further intervention it’s time to seek out a specialist.
Derailleur systems wear their moving parts on their sleeves. Having everything out in the open is something of an anachronism in wheeled vehicles, but with exposure comes accessibility, and that’s one of the reasons that derailleurs still dominate bike transmissions. Cleaning goes a long way to keeping things sweet, but sometimes something will go badly wrong and need mending.
The most common problem is slow or hesitant shifting at the rear. Reluctant downshifts (to larger sprockets) are often the result of cable stretch and can quickly be dialled out by turning the barrel adjuster on the mech or at the shifter. Wind the adjuster out (anti-clockwise) a quarter-turn at a time until everything’s happy again. If you go too far, upshifts (to smaller sprockets) will become slow, so turn the adjuster back in again, being careful not to overshoot the other way.
If you can’t get snappy shifting in both directions, something else is wrong. The usual cause is drag in the cable – when you upshift, the derailleur spring has to pull the slack cable through, and any stickiness in the cable run will impede that. Stick the bike on a workstand, shift into the biggest sprocket and then click the shifter all the way back the other way, but without turning the pedals. This will give you enough slack to pop the cable housings out of their stops, allowing you to slide them along the cable and give it a degrease and wipe clean. A bit of light oil sprayed down the ends of the housings should see everything sweet once more.
While the cable’s exposed, check for kinks or frayed strands. Inspect the housing too. Outer cable rubbing against the frame can lead to the wire strands bursting out, and sometimes the ferrules at the end of the cable can split or crease. If the cable checks out, turn your attention to the rear derailleur. Possible issues include a build-up of dirt, a broken spring stopping it springing back, a bent cage, worn jockey wheels or sloppiness in the parallelogram linkages. Eventually, mechs get sufficiently worn that most of the cable pull just drags the slop out of the links rather than moving the cage sideways. And, of course, a heavily worn chain or cassette won’t run smoothly either.
The final possibility is a bent derailleur hanger on the frame itself. That’s easy to spot – it should hang straight down. If it’s bent, don’t panic. The hanger normally bolts on and is easily replaced. Even if it doesn’t, it can usually be levered straight. You can do this with two spanners, a good eye and nerves of steel, but it’s best to take it to a competent bike shop and let them use the proper tools.
Tip 1: Start with the barrel adjuster to dial out any cable stretch
Tip 2: Light oil at the end of the housings will help ease cable drag
Tip 3: Check the cable for damage or fraying and replace if needed
Tip 4: The rear derailleur is the next place to look for grime, wear and tear – adjust where necessary
Tip 5: If you still can’t get a smooth shift, re-tension the cable and run through these tips again
Latest mountain bike drivetrains reviewed
- Shimano Deore XT, £515
- SRAM X7, £482
This article was originally published in What Mountain Bike magazine.