More than just a frame with two wheels attached, the bicycle is a collection of carefully considered parameters. Its design is dictated by component standards, intended riding style, weight considerations, and many more variables. We have full suspension mountain bikes that look a million miles away from the traditional touring bike, but we also have bikes that at first glance, look incredibly similar to one another and with almost no way of telling them apart.
But upon closer inspection of frame geometry numbers and component specs, we can see how this results in some very contrasting variations in bike design. It is the bicycle engineer’s job to define these differences in a tasteful and practical way, and deciding on frame components to realise the required spec of a frame is part of the journey. For example, we can look at things like drop-out choice – questions like ‘do we want to use the new UDH system?’, and making a decision about the type of brake mount we’ll use really has a huge effect on how the end product will turn out. Other parameters that almost seem invisible are also very carefully considered. Things such as the length and diameter of a frame’s tubing also need to be taken into account. This may dictate how the frame rides, and also tackle the issue of allowing adequate tyre clearance together with certain bottom bracket widths, not to mention chainline requirements and crankset specs. We touched on this subject in a previous article, where we discussed the thought process behind designing bikes for all-road use. But this time, we will focus more closely on one frame component in particular; the part that often serves as a bridge between the chainstay and bottom bracket, also referred to as the chainstay yoke.
As we shorten the chainstays of a bike frame, the closer we bring the rear axle to the bottom bracket. This reduces the amount of space behind the bottom bracket shell to fit the tyre. Fitting both chainstays, a tyre of sufficient volume, and enough clearance for the crank arms is a bit of a game of Tetris. It doesn’t leave very much room to mitre the chainstays to the bottom bracket, so designers have had to get creative, and think of new ways to make the junction work. In the late 1980s and early 90s, some frame designs adopted an ‘elevated chainstay’ configuration to allow more room for mud clearance in combination with shorter chainstays and a wider tyre. However, this was quite labour intensive in its manufacture and didn’t lend itself too well to mass production. Problems with frame reliability were sometimes encountered, and at times, the aesthetics of the end result were a little bit too ‘out there’ for some customer’s tastes. But as the years rolled on and CNC manufacture became more accessible, designers could explore new ways to build a reliable bottom bracket/chainstay junction without complicated workarounds. To be able to CNC manufacture a simple yoke that could be welded into the frame sped up the production line sevenfold, and it meant there was no need to depart from reliable and production-friendly manufacturing methods.
If you look closely at the titanium Bombtrack Hook EXT Ti frameset, down at the bottom bracket you will notice the employment of a chainstay yoke. As well as being quite tastefully decked out with the Bombtrack logo, it also serves to provide a solid connection point between the chainstay and bottom bracket, whilst allowing for adequate chainring clearance. The beauty of working with titanium is that we can build a frame with quite a sizeable, sturdy yoke, but at the same time keep the weight down. The shape of the yoke on the Hook EXT Ti is more traditionally shaped, but we can also use yokes to allow us to configure the chainstays in different ways. For example, if you look at the bottom bracket area of the Bombtrack Audax AL, you will see that the yoke used here facilitates a dropped chainstay, aiding the clearance for wider tyres and a double chainset. This is where the steel Bombtrack Audax and the Audax AL models differ in design – a steel frame will allow for slimmer tubing than aluminium due to the material’s metallurgical structure, so therefore a more conventional yoke shape can be used on the steel Audax (the Audax AL’s tubing is slightly thicker). The difference in yoke design also dictates a few differences in the component spec of the two bikes. The AL version has a few more millimeters of tyre clearance, but the steel version is compatible with larger chainring set-ups.
So despite the chainstay yoke initially appearing as being simply a chunk of metal that holds the chainstays together, we can now see that this is a bit more to this component. The yoke’s many purposes are sometimes obvious, sometimes more incognito. It is a component that helps to drive bicycle design, but at the same time maintains the bike in a form that we all recognise. Next time you are pondering about your bike’s tyre clearance, or wondering how you can squeeze in the chainrings you use, maybe take a quick look to see if there’s a yoke being used – the frame part that hides in the shadows but subtly influences our bike spec options.
SEIDO ComponentsSeptember 2025By Peter Skelton
