Hopefully, we all have a drivetrain of one type or another on our bikes. They transfer our sweat, blood and tears into forward motion and sadly I find the amount of forward motion and the tears are directly related. Fundamentally very simple, drivetrains have become increasingly complicated with a bewildering array of ‘standards’, options, manufacturer compatibilities and terms, so we thought they were an excellent candidate for our latest product focus! Split into two parts, in part one we’re going to look more at components and compatibility and in part two we’ll look more at manufacturers, technologies and a few maintenance tips. As always, there is a balance to be struck between giving a good overall guide and focusing on some important points, so we’ve not been able to cover everything, but we hope you’ll find it informative and helpful.
Drivetrains broken down.
Your drivetrain comprises what most people probably call the gears and for the purposes of this article, the drivetrain is comprised of; shifters, chainset (including bottom bracket), front mech (derailleur), chain, cassette, rear mech (derailleur) and gear cables. When buying a groupset the brakes are often included, so keep an eye out for this too.
Much like the other componentry we have talked about in this series of product focus blogs, generally the more you pay the lighter the components get. At the top end, groupsets like Campagnolo Super Record, SRAM Red and Shimano Dura-Ace utilise materials like carbon, titanium and advanced manufacturing processes to create products which are light without sacrificing strength or durability. High end groupsets often feature the brands latest technologies as well giving smoother, more precise shifting. Ultimately, the goal for any drivetrain is to provide a clean, precise and quick gear shift at the lowest possible weight. Efficiency as we shall see, is the name of the game.
As much as I may want to leave this until later, lets get bottom brackets out of the way first. Bottom brackets stand guilty of having a bewildering array of options and standards. For this reason we’re going to take a look at the most popular types and options, but it would be impossible (and really boring) to look at them all. If having read this you have further queries relating to your frame then please don’t hesitate to get in touch with our technical department who will be more than happy to help.
Bottom brackets are subject to the design of the bicycle frame and as frame design has evolved, a key area has been the bottom bracket. The bottom bracket is where frame stiffness and rigidity can provide significant performance advantages due to its key role in transferring power (or in my case at the moment weak and feeble leg spinning!).
If you have a traditional bottom bracket, whether its an internal cartridge style or a modern external design, then the bottom bracket screws in to the frame. The bottom bracket shell (this is the part of the frame that holds the bottom bracket) is threaded to accept the bottom bracket – a bit like how a nut is threaded to accept a bolt. There are two types of thread; Italian and British (sometimes called English), this refers to the direction of the threads. Both standards are still used today so you need to double check what thread your frame has before you buy your bottom bracket.
External BB: This is where the bearings sit in cups outside of the frame (see the left hand image below). These cups screw in to the frame according to the thread type (Italian or British). This external bearing design has been used by Shimano (they call it Hollowtech 2), SRAM (they call it GXP) and Campagnolo (used in both the Power Torque and Ultra Torque systems). The bottom bracket needs to be compatible with the chainset because of spacing and design as well as being suitable for your frame. Shimano and SRAM use different spacer arrangements, so you need to use the correct spacers and the correct bottom bracket for the correct chainset. As a starting point, Shimano Hollowtech 2, FSA Mega Exo and Race Face X-Type bottom brackets are all compatible with the chainsets from each brand. SRAM GXP, Truvativ GXP and Bontrager GXP bottom brackets are all compatible with the chainsets from each brand. Compatible replacements are also available as after market products from the likes of Chris King, Ultimate Ceramic Bearings (SRAM compatible and Shimano compatible) and Enduro.
Internal cartridge style Bottom Brackets: These are what bottom brackets always used to look like (see the right hand picture above). The axle runs on internal bearings which sit inside of the bottom bracket shell. This is not as stiff an arrangement as an external bottom bracket, because the bearings are closer together, but they can be more durable than external style bearings because the bearing races are protected inside of the cartridge style bottom bracket. These BB’s are still used on the track, in some budget groupsets and on the lower end Campag chainsets. Compatibility wise, you need to ensure they are the correct thread for your frame and you have the right shell width, axle width and axle type. The axle type refers to the shape of the ends of the axle which the chainset and crank arm fix onto. Axle width, type and shell width are fairly easy to work out, just look at your existing BB when you take it out (or check with the frame specs).
Integrated BB’s: The latest advance in BB and frame design has been the integrated BB. This is where the cartridge style BB bearings sit directly in the frame. This design has come around thanks to the advances in carbon fibre moulding, meaning a frame can be beefed up around the BB area creating a stiffer bike with less flex for more efficient power transfer in a lightweight package. Thanks to the larger bearing diameter, a larger axle can also be used providing more rigidity. There are three sizes of integrated BB used – BB30, BB86 and BB90.
BB30 is the most common where the bearings are pressed directly in to the frame, a job best done by your local bike shop unless you are experienced and have the correct tooling. Designed by SRAM, there are fewer compatibility issues between brands that use BB30, the Ultimate Ceramic Bearings BB30 upgrade kit fit all BB30 systems and make a fantastic upgrade. Ceramic bearings (like we highlighted in the Wheel Product Focus last week) make a fantastic upgrade which will work more efficiently and last longer than steel bearings.
BB86 and BB90 are slightly less common. A Shimano system that uses press fit style bearing. They offer the same advantage as the BB30 system but with a wider axle diameter and instead of pressing the bearings directly in to the frame, the bearing assembly is press fit in to the frame.
Bottom brackets do wear as they take a lot of abuse and strain. There is not a whole lot you can really do to prevent this wear either other than looking after your bike generally. Check that there is no water in the frame – if there is it will tend to sit in the BB shell and corrode. When you clean your bike and if you use a degreaser, thoroughly rinse the area and make sure no degreaser is left on the seals or shell. Also when washing your bike, never point a concentrated or high power stream of water at components like hubs, BB’s and headsets because the water can get in to the bearings and strip the grease out, severely cutting the lifespan of your components.
Chainsets – compact or standard?
There we go, I told you bottom brackets were a bit complicated! As already mentioned, chainsets need to be compatible with the bottom bracket (which in turn needs to be compatible with the frame). Once you’ve got the right chainset for your bottom bracket, the main choice you’ll be faced by is what ratio to choose. This is a tricky one, everyone has their own opinions and your riding style will have an impact on what is right for you. The main choice is between standard ratio and compact ratio. Triple road cranks are available, but they are generally less popular since the advent of ten speed which gives an impressive and realistic range of gear ratios with just two chainrings up front.
Compact chainsets are usually 34/50, this is where the smaller chainring has 34 teeth and the larger chainring has 50 teeth. This gives you a lower gear for those big climbs, without sacrificing too much top end speed depending on your cassette choice (we’ll get to that). A standard double chainset will usually be a 39/53 combination, which will give you a higher overall set of ratios. Which is right for you? A difficult question, it depends on how fit you are (can you push the big gears?), do you like to spin or grind away in a big gear (if you like spinning the lower gears of a compact might be more suitable) and how hilly is the region you live in (unless you are pretty fit those 30% climbs are going to sting on a standard)? There is no right answer – choose whichever you think compliments your riding the best and if necessary be prepared to change. Check out the Ultegra groupsets available in both options and the Campagnolo Chorus chainset in compact, a great compromise between cost and performance.
Cassette choice can also play a big part, cassette ratios range from the 11-21 high speed, close ratio cassette to the 12-27 option which gives you a nice low gear for climbing (the 27) but in a widely spaced ratio. Also, remember when choosing your cassette that the cassette needs to fit the freehub design (check last weeks Wheel Product Focus for more info). A 10 speed rear cassette is now the standard on mid to high end drivetrains from Shimano and SRAM, Campagnolo use 11 speed cassettes which offer a wider ratio but use a thinner chain.
So ever so neatly, we move on to chains. The critical thing about a chain is choosing the right speed. 8, 9, 10 and 11 speed cassettes use specific chains. 8 and 9 speed chains are wider than 10 speed and 11 speed is narrower still. This is because the sprockets in the cassette are closer together to fit 11 sprockets in the same space as 10. As long as chains are the right speed, they should be interchangeable between brands.
However, top end Shimano 10 speed chains are now chamfered and directional (check out the 7901) for optimal shifting, so if you use a Shimano drivetrain it may be advisable to use the corresponding Shimano chain to ensure you get the best shifting experience. Companies like KMC produce after market chains which are compatible with all manufacturers as long as they are designed for the right speed (8, 9, 10 or 11). The more you spend the lighter chains get and features like hollowpins (found on SRAM 1091 for example) and outer plates with material removed to save weight where it is not needed but cost more to produce. Chain coatings sometimes change as well, harder wearing options like the titanium nitride coating used on the gold KMC chains maintain their appearance and last longer, in theory.
Derailleurs, Shifters and Cables.
The final piece of the jigsaw; the derailleurs, shifters and cables. Together, they enable you to move the chain from one sprocket or chain ring to another. The aim is for this to be as smooth, precise and quick as possible, so you get the gear when you really need it.
An often overlooked and neglected component are the cables – these are critical. Old, corroded and neglected cables will have a massive impact on shifting. Poorly maintained cables will not change properly because the cable will not move freely, which in turn will not move the derailleur properly, stopping the bike from changing gear properly and frustrating you. My top tip is to fit cables like the Gore Ride-On system. Available for gear and brake cables, these fully sealed units are resistant to the elements and stay smooth and trouble free for a very, very long time. The set on my mountain bike (which get a lot of mud, water and abuse) are currently 18 months old and still going strong. On a road bike these cables are probably as close as you can get to fit and forget.
We’ll consider shifters and derailleurs together: Different manufacturers use different cable pull systems and ratios, so you cannot pair a SRAM shifter with a Shimano mech or a Campagnolo shifter with a SRAM derailleur. We’ll talk more about this in Part 2, but SRAM and Shimano use different pull ratios – the distance the cable moves to change gear is different – so they will not work with each other. The shifters also need to be for the same speed as the derailleur, a ten speed shifter should be used for a ten speed derailleur and a double front chainset will require a double front mech and shifter. A triple chainset with a triple front derailleur will not work with a double shifter.
As long as the shifter and derailleur are the same speed and the same manufacturer, you can mix and match within that – an ultegra shifter will work with a Dura-Ace derailleur for example. If you are building a bike this might be a good way to save money letting you upgrade the shifters at your convenience in the future.
So there we go, that’s part one of the Product Focus on Drivetrains! In part two we’re going to look at the main brands and the technologies, designs and ranges available. We’ll also take a look at basic drivetrain maintenance.
Have you got a favourite chain you swear by? Is there a certain bottom bracket or cable system you’ll only use? Let us know what works for you!