Design notes: Ratio Chainrings
Chainrings have long been a staple part of our offerings here at Ratio, designed and manufactured by us in the Lake District. They may seem like a fairly simple part of the drivetrain − they’re a circle with teeth that spins, right? − but there’s a lot more going on in our Ratio Chainrings than first meets the eye. We thought it was high time we gave the humble chainring its moment in the spotlight and explained what sets our chainrings apart.
Spline Pattern
Firstly, let’s look at how the chainring fits on the bike. The spline pattern of the drive-side crank dictates which chainring you can use. We’ve designed our chainrings to fit on a variety of different spline patterns to suit most modern bikes: direct mount SRAM 8 bolt and 3 bolt, 5 bolt 110 BCD and, most recently, 4 bolt 110 BCD patterns to fit on Shimano GRX cranks. You can see more about our newest Rings for Shimano GRX which come in 38t, 36t and even 34t sizes (spin it to win it, and all that!) in this recent post on The Radavist.
Tooth Profile
The tooth profile is where the really interesting engineering is hidden.
The main function of the chainring is to transfer power from your legs to the chain so the wheel turns and you go forwards. So it’s a pretty ineffective system if the chain drops off − not to mention grubby as you have to put it back on! The chain is much more likely to bounce off as you ride over rough and uneven ground, with all the undulations of the trail transferred to the chain. Now, clutched derailleurs have helped reduce the impact of the terrain on the chain, keeping the chain under tension to avoid chainslap for the most part. But the chainring can also play a role here too. We’ve designed our chainrings to reduce the “wave” behaviour that can cause the chain to whip off the bottom of the chainring, and the secret is asymmetrical teeth.
On the top side of the chainring, the roller of the chain needs to engage smoothly with the chainring. This is why the leading flank of the tooth (highlighted in yellow) leans back slightly, so the chain engages smoothly and the load is shared across multiple rollers. We’ve used a mathematical model developed over the course of two Masters projects at Cambridge University to optimise the angle of this flank.
Additionally, the force (or load) on the roller should be low to reduce wear on the tooth. Again, this is where the angle of the leading flank of the tooth is key, so the load is shared widely to reduce wear, but not so widely that the chain slips at very high pedalling forces.
You might also notice that the top of the tooth isn’t square. This is designed to conform to the bottom edge of the chain link, so that if it’s deflected to the side − for example, when you’re in the largest sprocket of the cassette − the link engages evenly across the top of the tooth. On conventional chainrings with square-topped teeth, this contact is concentrated at the leading top corner of the tooth which is bad news for longevity.
On the bottom side of the chainring, there is much less tension on the chain (this is why it’s often called the “slack side” of the chain). It’s on this side that the “wave” behaviour of the chain can cause a bump to transfer the energy around the chain and throw it off. The trailing flank of the tooth (highlighted in red) is aligned with the radius of the chainring; in other words, it points straight out from the middle. This means all the load from the bottom span of chain is concentrated on one roller, which keeps the chain locked in and prevents the energy of the “wave” from being transferred all the way around the chainring.
On top of this, there are reliefs to make sure mud doesn’t clog up the chainring and to make sure our chainrings are compatible with a wide range of 11, 12 and 13 speed chains.
It turns out chainrings are a lot more complicated than they first seem… We think chainrings deserve more love!
To get your Ratio Chainring, head to the link below.
