Jim’s Tech Talk
By Jim Langley
Last week’s Tech Talk answered a reader named Joe’s questions about his lower gearing plan. We asked for your help and you delivered with plenty of great ideas and advice – thank you!
This week, I want to quickly answer a couple of questions pulled from those comments and then spend a little more time on a third query which is the title of this Tech Talk. Again it would be helpful for any of you with experience on the subject to weigh in with what you’ve learned.
Reader Nat Haytcher asked the first question
“Let’s talk about cross chaining and wear some more. My bike has a single chainring (48-tooth). There are 11 options on the cassette. Most of the time I’m riding in the 2, 3rd or 4th smallest cogs (13T, 15T, 17T) cogs and that creates cross chaining. I’ve accepted the fact that the chain will just wear prematurely.”
Good news Nat! You can ride worry-free because there is actually no cross chaining on a bicycle with a single chainring (one of the advantages of 1X gearing). That’s because that one chainring should be right on the proper chainline for that bicycle. In other words, an imaginary straight line will bisect your cassette and your chainring.
With a double or triple chainring setup, there are two chainrings that are offset from the chainline. That’s why you get cross chaining.
Now, if you built the bike yourself or swapped out the crankset from a double or triple to a single (or just changed the crankset to a single chainring), there’s the possibility that the single chainring isn’t dead centered on the chainline. And if it isn’t then you could still experience cross chaining. Usually if that’s the case the chain will run rough or it might even come off the chainring, signs that it needs to be fixed and moved to the correct position. Or for some cranksets you might need a different 1X chainring.
Alex Pline has a Shimano Di2 electric gearing question
“Interestingly, with Di2 (at least my current setup – 11sp with a Frankenstein set of generations of pieces) the 11T and 12T cogs are not accessible in the small chainring. Not sure if this is programmable in the app or not?”
Your question is related to cross chaining too Alex (what Nat asked about above). It sounds like your Shimano Di2 derailleurs are set in Shimano’s Full Synchro mode (also available are Semi Synchro and Full Manual modes).
The mode you’re in puts the derailleur computer/programming in control of your shifting, which is pretty amazing. Basically, at certain points while shifting through the gears the front and rear derailleurs will shift simultaneously based on what Shimano’s programming believes is the best shift to make.
So you never end up in a gear that’s a little too easy or too hard because the derailleurs won’t allow it to happen. Or at least that’s the goal of Full Synchro. I don’t own a Di2 bike with this mode. But, I did put about 1,000 miles on one and did one race with it too. At first it’s strange to have both derailleurs shift at the same time, but you get used to it quickly and it’s nice to be in the right gear most of the time.
But one thing about Full Synchro is that it will refuse to let you cross chain. If you try to shift onto the smallest cogs when you’re on the small chainring, it won’t do it. And the same goes for shifting onto the large chainring and largest cog. It won’t do that either.
It’s been awhile since I had that bike so I’m not 100% sure, but I believe if you put your bike into Full Manual mode you will then be able to shift it without it overriding your wishes.
Here’s a helpful resource for learning about the different modes and how to change them:
NOW FOR THE MAIN QUESTION
RBR contributing editor bikefitnesscoaching posed this one:
“A thought Jim, please comment. Maybe gearing is only half the problem, the other half crankarm length?”
I’ll put some thoughts down here but it would be very helpful to hear what you experienced cyclists have to say about this too. Crankarm length is a much discussed and debated topic and opinions vary.
Personally, I have tried crankarms from 150mm up to 175mm. And when I say “tried,” I mean put them to the test in hard training and racing situations. When I tried the 150s, I had worked down in .5 cm increments all the way from 175 (which cost me a small fortune for all those different arms).
But I didn’t do it because I was trying to improve my gearing. I didn’t see it as related to the gearing at all. I did it to try to improve my efficiency pedaling the bike – a very specific bike too. I was trying to dial in my time trial bike in order to get my best result in the California Masters State Time Trial Championship and later in the year, in the USA Masters National Championship in Bend, Oregon.
On a time trial bike you’re in the most extreme position laying out on your forearms on the aero handlebars. The big issue is that in this position the knees can hit you in the chest at the top of the pedal stroke – at least if you’ve maxed out your aero position and are low.
Even if the knees hitting the chest isn’t a problem, it’s hard on the legs when you’re that far bent over. To fix these issues I went to shorter and shorter crankarms doing test after test. I discovered that 150s were too short, the same with 155s. It felt inefficient pedaling cranks that short.
But with only another 5mm, the 160s were the Goldilocks length and I knew I’d found the sweet spot. I didn’t feel any need to change the gearing on my bike as I messed around with all those different lengths. But what did change was my pedaling cadence and efficiency.
With the longer cranks my cadence was typically in the mid to high 80 rpms. With the 160s I was suddenly in the low 90s. This made a significant difference. Using this setup I managed 10th at the Nats and finally won the State Championship.
Still, I know that crankarms are the levers for pedaling bikes. That does have to do with gearing. A longer crankarm is a longer lever that should make it easier to push a gear. At least during the power phase of the pedaling circle.
When I first got into riding seriously in the late 1960s in New Hampshire there was an influencer named Jim Farnsworth. He set a new record up Mt. Washington with his secret weapon giant 223mm crankarms! He told anyone who would listen that super long cranks were the way to go.
Edmund R. Burke in his book Serious Cycling writes, “Biomechanics will be the first to tell you that crankarms influence your cadence and the leverage you can exert on the pedals. Longer crankarms are used for pushing large gears at a low cadence, and shorter arms allow for a higher cadence with smaller gears. You would use short crankarms for track sprints and criteriums, and longer crankarms for time trialing and climbing hills. On a mountain bike you would use longer crankarms for better leverage in climbing.”
I bought into the theory that longer crankarms were best for climbing and time trialing and used 175s for most of my early racing years. Partly I was influenced by Greg LeMond who recommended that length.
But back to bikefitnesscoaching’s question, “is gearing only half the problem and crankarm length the other half?” in my experience crankarm length is related to the gearing you use but I don’t think having too long or too short crankarms will be anywhere near as problematic as having the wrong gearing.
That’s my take on it. But, I’m very interested in hearing what you readers think. For anyone interested in experimenting with different length crankarms, you can buy some different lengths from the company that made the crank on your bike. That’s usually the easiest way to change lengths since nothing has to change except the crankarms.
If you’re looking for more length options than are offered by your crank maker, here are three resources:
https://www.lightningbikes.com/cranks/index.html (this is the carbon crank I run on my time trial bike)
Jim Langley is RBR’s Technical Editor. A pro mechanic & cycling writer for more than 40 years, he’s the author of Your Home Bicycle Workshop in the RBR eBookstore. Tune in to Jim’s popular YouTube channel for wheel building & bike repair how-to’s. Jim’s also known for his cycling streak that ended in February 2022 with a total of 10,269 consecutive daily rides (28 years, 1 month and 11 days of never missing a ride). Click to read Jim’s full bio.