Question: Do you have any tips for installing oval chainrings and fine-tuning the front derailleur for their use? From: Rhett
Answer: As more and more riders make the switch to oval chainrings, this question is coming up a lot these days. Before we dive in it should be said that front shifting with oval chainrings isn’t quite on par with round chainrings, so even with these tips, which will help immensely, you should temper your expectations.
Nearly every oval chainring front derailleur setup can benefit from a shim placed between the frame braze-on and the front derailleur. This shim angles the rear of the derailleur down and moves it back to help clear the taller rings while putting the cage in a better position to push the chain down to the small ring.
Both SRAM and Rotor make these types of shims. The type and size of shim that you need will depend on the bike’s seat tube angle and the position of the front derailleur braze-on. In some cases they are not required at all, however Rotor makes both a large shim and a small shim to accommodate any bike. Simply choose the shim and its orientation that places the outer arch of the front derailleur’s cage 1-1.5mm from the tallest portion of the chainring as shown in this pdf. They are easy to install because the tuning process is essentially the same as a round chainring setup.
Start by shifting the front derailleur into the small chainring and then loosen the cable anchor bolt. Put the derailleur into a position that places the outer plate on the derailleur as close to the tallest tooth on the ring as possible without it touching while making sure the outer cage is parallel to the big chainring, unless it is a SRAM Yaw front derailleur (for instructions on how to align a SRAM Yaw front derailleur click on the highlighted link). Then use a torque wrench to torque the derailleur fixing bolt down to spec.
Set your low limit screw so that when the rear derailleur is shifted into the large cog, the inside of the derailleur cage is as close as possible to the chain without it touching. Then pull the cable tight and torque the cable fixing bolt down to spec.
Shift the rear derailleur into the middle of the cassette and then shift the front derailleur into the big ring. If the front derailleur is struggling to push the chain up to the big ring, then you’ll need more cable tension. Use your barrel adjuster to do this. If you don’t have a barrel adjuster then you’ll have to tighten the cable at the cable anchor.
Shift the rear derailleur to the smallest cog and check to see if the chain is rubbing on the cage of the derailleur. If it is, back out the outer limit screw. You want the outer part of the derailleur cage as close to the chain as possible without rubbing. If the derailleur is shifting into the big ring just fine, check to make sure that your limit screw is tightened down until it bottoms out on the derailleur. This will prevent it from over shifting and dropping the chain off the outside of the big ring. With the high limit set, back it off just a quarter of a turn to allow the derailleur to barely overshift and push the chain to the big ring just a little quicker.
Perform some front derailleur test shifts while the rear derailleur is shifted into a variety of gears in the back. You may want to fine tune your cable tension and limit screws after performing the test shifts to make sure that your shifting is as good as it can be.
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