# How is chainstay length defined for frames with horizontal dropouts?

Just reading our own definition of Chainstay Length in the terminology index. It is written currently as the distance between the centre of the chainring and the centre of the rear cog.

This is simple enough to follow for a bike with vertical dropouts at the rear, but what about frames with horizontal dropouts or track ends, the geometry of the frame is fixed, surely, even if the wheel can be moved F/A. So how do we define it in that case, where the cog can be moved?

From the diagrams I'm seeing online, the chainstay length by this definition is the same as the rear centre. Is this true, and is it affected by any of the above?

What I want to achieve is to understand and update the Terminology Index entry.

There is no universal standard for this. The most common way manufacturers do it is measure to the middle of the adjustment range, which is reasonable enough, except they don't also usually say what that range is, so it may not give you the exact information you're looking for, although in practice it's usually basically going to be +/-1cm or so for most dropouts, which isn't necessarily the whole length of the dropout but is about the working range where the rear derailleur (if applicable) won't wind up in a funky position and where the QR or axle nuts will have good engagement.

You could probably find some examples of manufacturers listing it based on the forward-most (slammed) position on a rear-opening horizontal dropout tri/TT frame, as on those bikes that's often the position it's most intended to be used in.

• The most forward position on a track bike would make it near impossible to remove the rear wheel without breaking the chain. Just thinking! Sep 28 '19 at 7:27

If I'm describing a complete bike, it's defined by wherever the center of cogs are at that time.

As we move into this bold world of full suspension mountain bikes with adjustable geometry, the ones with flip chips on the dropouts talk about the varying lengths of the chainstays.

Santa Cruz's description of their Megatower frame (emphasis mine)

In order to make the Megatower stride confidently across the globe it has an enormous amount of clever adjustability hidden in its bones. A tidy and concealed flip chip in the lower-link adjusts bottom bracket height and changes progressivity of the rear suspension. A second, in the rear dropouts, allows for a 10mm fore-aft adjustment in chainstay length to dial in the rider's rearward weight distribution - either set for play or for stability. The flip chips are a robust mechanism for riders looking to tune their ride according to their needs without compromising reliability and durability. No wackadoo mousetraps and frail proprietary shocks here.

So in the case of horizontal drop outs, I would provide the range of reasonable lengths, or measure to the center of the dropout and provide the available adjustment if I was describing only the frame.