Shift Ramps and Short Teeth: How Geared Chainrings Work

When you’re riding fixed and your chain is your brake, you want that sucker to stay put. But what about when you go geared? How does a flick of the shifter get the chain to move up or down the chainring? It turns out there’s a lot of engineering that goes into making a geared chainring work. In today’s post we’ll look at the little idiosyncrasies that make everything shift smoothly – let’s dive in!

Derailleur

First things first, what actually happens when you shift? When it comes to the chainrings (left shifter), the lever pulls the cable that moves the front derailleur towards or away from the frame and pushes/pulls the chain up or down on the rings.

And, once you’re in gear, there shouldn’t be any rub as the chain can pass cleanly through the derailleur again. But it takes more than a push to get the chain to jump from one ring to the other. Things like…

Short, Squared Teeth

On a single chainring, all the teeth will be uniform and look like those pointy peaks we’re so used to seeing. Those peaks are designed to slip perfectly between the plates of the chain and hold everything in place while driving you forward.

But, on a geared chainring, when you don’t want the chain stuck in place permanently, you have to do something a little different. That’s why you’ll notice interspersed stouter, flatter teeth that don’t penetrate the chain plates as deeply. That gives the chain a chance to come free when guided by the derailleur and find its new place after you’ve shifted.

Moving from the big to the small chainring is easy then. The derailleur bumps the chain towards the frame, the short teeth let the chain jump the cog, and gravity drops it down on the inner chainring, right where you want it! But how about moving back up to the big ring? The derailleur can still push, the short teeth can still let the chain get free, but gravity isn’t going to raise the chain back up – we’re going to need some ramps.

Shift Ramps

Shift ramps are small grooves or indentations on the inside of the big ring that help grab the chain plates and drag them up and over and back onto the big ring as the cranks come around. There are lots of different designs out there, but we’ve found the most efficient design is the chain-plate sized indentations you see below.

They’re perfectly sized to scoop the chain, carry it back up over the top of the ring, and let it drop right back where you want it – on the big ring and ready to put the power down.

TaDa!

And that’s it! Those tiny tweaks are what makes chainring shifting work smoothly, and why it takes more than a slap on the chain to get into the gear you’re looking for.

What you do with those gears is still up to you, though. Happy riding.

We'll see you out there.

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