After nearly 30 years of mountain bikes being ﬁtted with three front chainrings as standard, 2010 saw the rise to prominence of single and double chainring setups. The old-school triple ring concept isn’t dead – it isn’t even broken – but riders, driven by the demands of pro racers, wanted to save a little weight and be able to ride and race making fewer front shifts (because these are always slower and more harmful to an even pedalling cadence than rear shifts).
This is achievable using nine-speed transmissions with a 32- or, more usefully, a 34-tooth large rear sprocket, but reducing the number of chainrings down from three really began to make sense with the arrival of the new 10-speed rear cassettes. With giant, climb-anything 36-tooth sprockets they make the concept of running a twin, or even a lonely single, up front a realistic proposition, even if you live in relatively hilly terrain.
There will always be times when the 22/24-tooth granny ring is useful, so before you launch into stripping back your triple, seriously consider whether it’s suitable for you. If you choose a single, this is the biggest leap of faith as you’ll occasionally ﬁnd yourself undergeared on the downhills and overgeared on the uphills. If you still want to proceed, read on to ﬁnd out how to go about it.
How to convert your bike to a single chainring setup
- Time: One hour
- Cost: From £10-£30 if you need to buy a chainring or chainguide
- Difficulty: Easy
- Essential tools: Chain tool; rag; Allen keys; Torx keys; bottom bracket tool; anti-seize compound; Shimano TL-FC20 bolt tool
1 Remove front derailleur and shifter
The ﬁrst thing you’ll have to do to affect the switchover to a single front chainring is remove the front mech and shifter. You can, at a push, leave it all in-situ and use the front mech as a sort of chainguide, but we don’t recommend it as it’s a bit hit-and-miss in terms of performance.
2 Remove the cranks
Assuming you’re using an external bearing bottom bracket and splined-axle crank (most newish £300+ bikes have these) you’ll need a long handled 8mm Allen key to undo the self-extracting cranks. Usually the left arm comes off, leaving the axle stub poking through the left side bottom bracket cup. Knock the end of the exposed axle to push the right side of the crank out of the frame.
3 Remove the rings
Give the crank arms and rings a quick run over with some degreaser and a rag – working on clean stuff is so much nicer. Using the appropriate tool you can then remove the rings. Most cranks use Allen bolts to attach the rings, though some newer ones use star T30 Torx bolts. You might need a Shimano TL-FC20 tool to grip the rear of the chainring bolts.
4 Choose the right size ring
Imagine yourself riding your most used trails and try to ﬁgure out which size ring to go for. If you ride everywhere in the standard 32-tooth middle ring mostly then go for that. If you think you’re a tough guy seek out a 34- or 36-tooth one. Most riders will settle on the 32 as they’re easy to ﬁnd (you already have one as your middle ring), and are easier on the climbs.
5 Use single chainring bolts
Before you can attach the single ring to the crank, you’ll need to track down some shallow single ring speciﬁc chainring bolts. The standard ones are longer for passing though a second chainring, and if used will bottom out before they tighten the chainring tabs. When you ﬁt the new single ring bolts use some anti-seize compound if you want to be able to get them off again after a winter of corrosion.
6 Fit to the inside of the tabs
To get the optimum chainline, you need to ﬁt the single ring to the middle ring position (on the inside edge of the chainring tabs). Make sure you clean these tabs thoroughly. Over time they can become pretty manky and any build up of dirt can affect the way the ring attaches and impede a good tight, even ﬁt.
7 Fit the chainguide
There are different types of chainguides. At one end of the scale are full-on downhill/four-cross racing-inspired enclosed upper guides like those from MRP and Superstar Components, with bottom bracket and/or seat tube mounts. Or you can take the minimalist route and use a simple sliding plate version like the Jump Stop from Ison Distribution. We’ve used both. For pure cross-country racing we like the Jump Stop, for rougher riding we like the enclosed type.
8 Refit the cranks
With the cranks stripped, cleaned and reﬁtted with the single chainring of your choice in the middle chainring position, you can remount them to the frame. This will be the reverse of what you did to get them off. Make sure you line up the splines accurately. Tighten them down to the manufacturer’s recommended settings.
9 Check chainline
Sighting from the rear of the bike the chainring should line up exactly with the middle sprocket on the rear cassette for equal access to both ends of the cassette. With some frame/crank/bottom bracket combinations it can be useful to add a millimetre wide washer between the ring and the tab to space the ring inboard if you need to improve the chainline to the bigger sprockets. We err on the side of a better hill climbing chainline than the speed sprockets.
10 Position the chainguide
With the chainguide ﬁtted you’ll need to put the bike through its gears to make sure everything is running smoothly and quietly. Chainguides have a habit of running noisily and can take a bit of detailed ﬁ ddling to stop chain rub or chatter at the extremes of the cassette, so adjust and ﬁne-tune the guide’s position as necessary.
11 Shorten the chain
With no big chainring to wrap itself around, you’ll have a few extra links in your chain. You don’t want an over-long chain as when the bike encounters a bump the derailleur arm swings forward, creating a loop of untensioned chain which can unseat itself or get chain sucked. To work out how many links to remove, shift into the biggest sprocket and look what chain length you need when the rear derailleur is at about 45 degrees.
12 Shorter cage rear derailleur
Along with ensuring that you’re not using an over long chain, it’s worth making sure that you’re using a mid-length, rather than a long (standard) arm rear derailleur. This will require less chain, and help keep the chain under control. It’s not a deal breaker though if you haven’t got one or got the funds for a new one.
How to convert your bike to a double chainring setup
1 Choose new chainrings
Because crank manufacturers haven’t envisaged too many home brew double ring conversions, there aren’t the perfect 28/39T 104mm bolt circle diameter (BCD) rings about, so you’ll have to compromise. A combination of Truvativ (28T) and TA Chinook (40T) rings can be easily sourced. Control Tech do a dedicated 29/40 set for 94mm BCD cranks. We’ve plumped for a simple-to-achieve 34/24 combo from Race Face.
2 Fit rings in the right position
The most common BCDs on aftermarket triple cranksets are 104mm and then 94mm; on a 104mm BCD crank, you’ll need to ﬁt the outer chainring on the middle ring tabs, with the inner chainring on the inner ring tabs. For 94mm BCD cranks, you’ll need to ﬁt the outer chainring on the outer (big) ring tabs, with the smaller chainring on the middle ring tabs.
3 Adjust front mech stops
As you’re only running two chainrings you’ll need to adjust your front derailleur accordingly, as it’s optimised to shift across three, not two, rings. Use the limit screws to make the necessary adjustments. The Control Tech rings will use the middle and outer shift stops, so lock out the inner position. The TA/Truvativ option will use the inner and middle, so lock out the outer (former big ring) position.
4 Check shifting
When you’ve made the necessary front mech adjustments on the workstand, go for a quick spin and try the shifting. Usually the action under load is a bit different and can require a second tweak for perfection. It takes a few minutes to get used to the feeling of the new shift, especially when you accidentally try to overshift against the newly imposed shift stops, but you’ll quickly get the hang of it.
- Double banger: If you happen to see one going cheap on your travels, you could pick up a speciﬁc front derailleur that's been optimised for use with twin compact rings. It’s especially useful on full-suspension bikes where occasionally it's possible to have clearance issues using standard front mechs that have been lowered for use with compact rings.
- Spread ’em: Sometimes it’s hard to get the chain to run neatly (and quietly) into the chainguide. You can use a few thin washers to gently spread the rear entrance to the guide to allow easier access to it when the chain is coming from the extreme ends of the cassette.
- Carbon care: If the edges of your chainring tabs are pressing directly onto your carbon crank arm tabs, gently radius the ring’s bolt tabs where they touch the carbon to prevent them damaging the crank.