Pedal Symmetry is Not a 'Given'
As I wrote in Make Sure Pedals are Symmetric!, it’s wise to verify that pedal offsets are symmetric.
Just today I installed my DuraAce SRM crankset onto my Moots Vamoots RSL, swapping out the Cannondale Hollowgram SRM crankset (FOR SALE). With the latter, I had been running the wide-spindle DuraAce pedal on the left side (+4mm longer pedal spindle) along with a 0.75mm washer to obtain near symmetry.
And so I thought that the symmetry DuraAce crankset would result in symmetric pedal offsets. To my surprise and chagrin, the DuraAce crankset installed using the new Chris King bottom bracket for PressFit 30 is NOT symmetric.
Depending on where I measured, I saw somewhere around 2mm of asymmetry ±0.5mm. I measured pedal offsets versus outer side of head tube, seat tube and also from the center of the water bottle bolt. This is harder to do that one might think, but I measured repeatedly and at multiple places. Nor can one assume perfect rotational offset through entire pedal stroke, especially with a modified crankset (e.g., SRM).
In the end, I added 1.5mm of washers to the left pedal. Measuring from the center of the water bottle bolt, I found that I now had perfect symmetry ±0.5mm. But 1.5mm of spacer (washers) is not a great idea as some threading is lost (which causes more stress on the crank arm). So I will instead move the cleat position slightly to make the compensation, then remove the washers.
At that point, I need to measure to the cleats to verify symmetry, which is better since that is the ultimate goal (actual offset of the foot).
With a near perfect symmetry, I now hope that the hip twist that still lingers will gradually let go. And in my case, the wider Q factor seems to be more natural, so I hope this will help also*. The trick is getting cleat positions precisely-set; moreover my feet are not exactly the same size, so there is more slop in the right shoe. So all of this is on the order of “get it as close as one can determine”.
UPDATE: I measured from the center water bottle bolt to the center of the bolt on the cleats. I measured before and after removing the washers. It turns out to be a happy happenstance: without the washers, I measure the same distance to the left and right shoe cleat bolts, so problem solved.
The cleats must have been mis-centered by ~2mm just by chance (or prior intent for the prior crank), but happily in just the right asymmetry to perfectly compensate for this new crank.
* Mild right knee discomfort has been nagging me, and some ache in right quad. Raising the seat might help the right quad, and I have done so now.
Shown below, I am measuring from the outside of the seat tube to some point on the pedal.
Measuring is trickier than it seems:
- Being at 90° to a tube
is not obvious; it is easy to make a 1mm variance by a slight angular change.
- Some bicycles have asymmetric seat tubes (my LOOK 595 Ultra does).
- Parallax from eye to ruler.
- Various bike parts tend to get in the way.
In the end I found it best to measure from the center water bottle bolt (marked with a pencil on the tube) to the outer chromed edge of the pedal. That longer spread makes it easier to see that the ruler is aligned at 90° to the frame.
This measurement gives a pedal offset. The cleats on the shoes must be positioned precisely also (and are the shoes perfectly symmetric?!). Then those of us with feet 1/2 size different might find the foot fits differently in left or right shoe! It is a more complex thing in total than one might assume.
Measuring pedal offset