Technical FAQ: Wide Zipps, oval rings, and more

Lennard Zinn discusses the benefits of using oval chainrings, mixing Di2 components, and more

Tarmac SL4 frame with Zipp 303s

Dear Lennard,
I have a [Specialized] Tarmac SL4 and recently purchased a set of Zipp 303 tubulars; they fit but have very little clearance on the chainstays. Should I not attempt to use them? Is this going to be the case with the new tarmac frame?
— Fernando

Dear Fernando,
Here is the response from Specialized:

The Tarmac SL4 was originally designed before the firecrest 303 was released and thus didn’t have enough clearance (past a couple mm) in the chainstay. Once we caught this, there was a running change made to all SL4 frames, including Amira as well, that fixed this.

My guess is that this rider has one of the first year MY12 framesets and thus he might experience some run, depending upon his power, hub tightness, wheel stiffness, etc.

All frames following the MY12 Tarmac SL4’s has ample clearance for this wheelset, including the new Tarmac.
— Mark Cote

I would say that if the rim does not hit, you could ride the wheel some and see whether it touches the chainstays when it flexes during riding. If you see that the rim starts rubbing off the paint on your stays, that would be your warning. It’s not prudent to let it wear through layers of carbon. Also, you must avoid riding through mud puddles or other wet areas that could drag grit in alongside the rim, as that could rapidly wear away your carbon stays.
― Lennard

More on Di2 road mixed with XTR Di2

Dear Lennard,
Do you think the synchro mode with the Di2 XTR module would work with a compact double front road derailleur? It occurred to me that Shimano could easily have the module recognize what front derailleur was attached, be it road or mountain. In fact, it must to some extent, in order to be compatible with both the double and triple XTR options. The Shimano website does say the shifting is user programmable based on personal preferences, but I’m not totally sure what that means.
— Steve

Answer from Shimano:

The main issue for mixing road Di2 front and XTR Di2 rear systems is the massive 11-40T cassette used by XTR.

The 40T cassette is going to put the chain at an angle that the road FD is not designed for. There is a good chance it will rub on the top of the FD cage.

This incompatibility between the 11-speed road front drivetrain and the XTR Di2 11-speed rear drivetrain will keep this combination from working. Therefore, Shimano Synchronized Shift will also not function when this combination is used.

We don’t have a solution today for road riders as Shimano engineers prioritized Shimano Synchronized Shift for mountain biking, where it provides greater benefits and enhances the ride in riding environments where the terrain is challenging. However, we do feel that this is a function that will benefit many types riders. We will continue to study and should have more to say on this topic in the future.
— Dave Lawrence
Shimano Product Manager

More on oval chainrings

Dear Lennard,
Back in April last year I joined Verve Cycling; our first goal being to introduce a high-performance power meter, fully integrated into a crank. We took a ground-up engineering approach and designed the precision measurement device first and then a crank around that. The first customers are getting them delivered as I write. You can check out more of the details at

Anyway, that leads me onto your recent response in Technical FAQs to the “Using Biopace cranksets” query.

The “Oval ring pro/con” is a debate we are having internally right now, particularly related to power. Magnus is our lead affiliate in the UK and is a fan, (of some particularly extreme geometry). In your response, you say: “There is modern power data indicating that there is an advantage to be gained with these chainrings.” This is actually not our experience in development of InfoCrank. The way in which (crank-based) power meters collect torque and rotational speed data, and then subsequently calculate power do not accommodate the variation in angular velocity during the pedal stroke that an elliptical ring will generate. There is a good possibility that the way the averaging is done (the math assumes a circular trajectory and constant angular velocity for each measured segment) that the aggregated power number displayed on a head unit over the full stroke will be higher with an elliptical ring, but this is not necessarily the true case.

If there were definitive power measurements to prove or disprove the veracity of claims made for these and other “power enhancing devices,” then the battle would be won already.

Later this year, we will have some bespoke logging software to take advantage of the inherent data gathering capabilities we have built into InfoCrank, (capabilities the restrictions of the ANT+ protocols don’t allow us to utilize in commercial devices right now). Then we may be able to start getting some definitive data on the advantages or not of elliptical rings.

Right now it is still a subjective choice.
— Richard McAinsh
Chief Technical Officer
Verve Cycling

And from a former Paris-Roubaix winner:

Dear Lennard,
It was an interesting read about the oval rings. I agree that shifting is shocking with them.

I think we should probably go away from the statement that these rings will give you more power. Your legs will only be able to push so hard on the pedals regardless of what shape the rings are.

The one thing that I think they do offer is a micro recovery during the pedal stroke and with that, you may lower your heart rate and lactate a little bit and due to this, you may possibly be able to sustain a slightly higher power output (this is still subject to proper testing).

I do like riding them. And I certainly feel they give me something extra, but at the moment, until someone does a set that actually shifts properly, I think they are a bit of a gamble to ride.

This is my opinion.
— Magnus Backstedt
Lead Affiliate-United Kingdom
Verve Cycling