Sliding Dropouts
When assessing a bicycle’s versatility, we can look at a number of different areas of the frame and consider the scope for adjustment and customisation. Some bikes allow for more tyre clearance with more space between the chainstays and seatstays, others feature fork geometry that allow for the interchanging between rigid forks and suspension forks. Provisions for fitting mudguards and luggage racks are very simple elements that can dictate the bike’s character and purpose, but some frame designs take the spirit of versatility a step further. They allow the rider to make significant changes to drivetrain and/or handling characteristics of the bike – some full-suspension mountain bikes make use of ‘flip chips’ to adjust the geometry for different disciplines and ride-feel, but that is a subject for another time. This time we are looking more towards the back end of the bike to bring the dropout area into focus. On most bikes, this area is quite binary, the rear wheel is fitted into the frame with the axle in a fixed position at the end of the chainstay (the exception being single speed frames with a horizontal dropout, and older or more classically designed non-disc brake frames). This fixed ‘effective chainstay length’ limits the rider to running only a geared drivetrain setup, or at least makes the conversion to a singlespeed set-up a much more complicated process. The fixed positioning of the rear axle also means that the frame has only one useable effective chainstay length, therefore not allowing the rider to fine-tune any of the characteristics concerning responsiveness or stability. Enter the sliding dropout…
The sliding dropout is a modular system that incorporates the use of an aluminium insert that is bolted into a horizontal-type dropout. The dropout does not look a million miles away from those that can be found on track or single speed frames, but a closed end stops the insert from sliding too far. Having this system employed on a frame allows the rider to alter their setup by effectively changing the length of the chainstay. The dropout can be moved backwards to therefore lengthen the chainstay – this not only frees up space for a larger tyre diameter, but a long chainstay length can make for more stable ride characteristics. This is one of the reasons why we often see a longer chainstay on a touring bike, making things more comfortable and stable when loaded up with luggage. A longer chainstay also leaves more clearance for rear pannier bags, therefore reducing the risk of heel-strike.
When we slide the dropout further forward, we can achieve the same effect as having a shorter chainstay, be that only by a few millimetres. A shorter chainstay makes for a more responsive ride, in that it changes the weight distribution on the bike by shortening its wheelbase. And as the rider’s weight is moved closer to the rear axle through this effect chainstay shortening, we may also see an improvement on how the bike climbs. One thing to bear in mind, however, is that when we adjust the dropout insert to shorten the chainstay, the space for a larger tyre may be reduced – the wheel will be pushed into the tighter space between the chainstays as they triangulate towards the bottom bracket.
But the option to specify a bike frame with sliding dropouts isn’t just considered from a geometric point of view. The modular nature of the sliding dropout insert means that a whole world of different axle types and brake mount compatibility is opened up. For example, the rider can choose to fit a dropout with an insert made to work with a 12mm thru axle and a post-type brake mount, or even a quick-release type axle with a flat-mount brake interface. And, when it comes to drivetrain options, there are dropout inserts available to the end user that facilitate not just the use of conventional derailleur hanger configurations, but also the use of the newer SRAM T-type interface. The rider can also choose to run their bike single-speed with any of these dropout types, and if their frame facilitates it, they can also be made to work with belt drive and hub gear systems. This is because the chain or belt can be tensioned in the same way as any single-speed dropout, by pulling the insert back. But if the single-speed rider is looking for a cleaner aesthetic without the presence of a redundant derailleur hanger, there are also dedicated single speed dropout inserts on the market. These dropout inserts are available without, the previously mentioned brake mount types.
So there you have it, the sliding dropout – a very simple but effective piece of engineering, that aims to widen compatibility and breath even more versatility into the design of the bicycle frame. Maybe the perfect choice for the cyclist who is on the lookout for maximum reliability, for when a broken derailleur means that single speed is the only way home. Or maybe this is the perfect answer for the rider who simply loves to conjure with the spec of their bike every so often.
SEIDO Components
December 2025
By Peter Skelton