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.”
My reply
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?”
My reply
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:
https://bettershifting.com/di2-synchronized-shifting-the-complete-guide/
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?”
My reply
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.
Faster cadence
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.
Leverage
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.
Your turn
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.bigandtallbike.com/
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.
syborg says
Di2 in manual mode does in fact prevent shifting onto the smallest cogs when on the small chaining. It’s been a few years since I setup my bike, but I believe there is some adjustment as to the number of off limits small cogs. Is ready enough to check of your raft is interested by connecting their bike to the phone app.
Regarding crank length: Dr. Jim Martin has studied crank length. Use your favorite search engine to find his studies on the benefits of shorter cranks.
RickC says
The Ultegra Di2 R8050 on my previous bike prevented shifting into the smallest cogs while in the small chainring while in the manual mode and I appreciated that feature very much. I purchased a new bike last year with 12-speed Di2 Ultegra, and I was surprised and disappointed to find that feature missing, even through the phone app. If anyone knows how to set up my 12-speed Ultegra rear derailleur to do this, please post a note here.
Steve Esmacher says
I used e-Tube to remove the small-small restriction in Di2 manual mode (11-speed Ultegra) but after doing so, discovered that there was unmanageable slack in the chain so I reenabled the restriction. It may work with a long chain cage.
larry english says
re crank length – surprised you do not mention ‘gain ratio’ by sheldon brown
google that
it;s fascinating
includes crank length AND tire diameter into one common number
wle
David Le Fevre says
Ideally, a gear ratio would be expressed as a dimensionless number, such as pi, e, or Reynolds Number
In which case it’d be (chainring size / sprocket size) x (wheel radius / crank length), and would be the distance the bike moves divided by the distance the pedal moves
I’m guessing that that’s what Sheldon called a gain ratio
chris says
How could you omit mentioning Saint Sheldon Brown? One can input all sorts of variables and see the objective result: https://www.sheldonbrown.com/gear-calc.html
Jeff vdD says
Regarding Nat Haytcher’s point:
“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.”
While the situation he describes may not technically be cross chaining, it is riding where the chain isn’t perfectly straight. That will induce wear not just on the chain, but also on the cassette and perhaps also the chainring.
I’m just guessing here, but I suspect that in such a situation, it might make sense to have the chain ring not bisect the cassette (that is, be aligned with the 6th, or middle, cog). In his case, for instance, it might make sense to have the chainring align with with the 5th cog or perhaps even between the 4th and 5th cog.
Yes, the chain will be even more out of alignment when on the biggest (11th) cog, but if he’s not in that gear much, it should be better for the drivetrain.
I’m wonder what the experts say about this?
syborg says
On my old 1×11 MTB I lost teeth on my chaining and I always assumed it wore from rubbing when in the smaller cassette cogs. I destroyed multiple chainings on that bike.
The teeth that broke off were behind the right crank. I didn’t realize that until my second replacement. Luckily I kept the first chaining so I was able to compare and they both lost teeth in the same location.
I wouldn’t worry about chain wear as much as I would about chaining wear.
bike fitness coaching says
Hi,
Regarding crank arm length, I wont get into this too deep since I actually teach this in my bike fitting course. I’m surprised Mr. Burke discusses biomechanics and doesn’t mention mention the real issues, Range of Motion of the knee. The human body was built to walk and run not ride a bicycle. Walk around the block and/or run around the block and measure knee flexion. Now get on your 175-220mm cranks and measure knee flexion. Much more on the bicycle. Now, getting into anatomy, everyone knows about the knee joint (i.e., patella, patellofemoral head, acl, mcl, pcl, lcl, *patellar ligament/quadricep tendon, ), but not too many know how the knee actually works, i.e., what makes the lower leg extend on the downstroke? If you studied anatomy, which most fitters know nothing about, then you would tremble in sheer horror knowing how long crank arms are destroying ones knees. You mention the older riders talking about the benefits of longer cranks, then why did most world tour pros retire in their mid 20’s with blown out knees?
Power is basically Force x Velocity. How hard you push on the cranks x how fast you spin the cranks. I’m sure everyone remembers the Commutative Law in math? I think it was taught in 3rd grade.
For example, If you want to produce 10 units of power, it can be 5×2 (high force x low cadence) or 2×5 (low force x high cadence). Believe me and every orthopaedic surgeon and physical therapist, it is much safer and healthier on the knee joints to use shorter cranks and spin them faster.
In closing, I am 6′ tall and use 170mm cranks on all my bicycles except my track bike with 165mm. No more knee pain!
* Everyone knows the difference between a tendon and a ligament. What I see often is that the medical industry combines the patellar ligament/quadricep tendon calling this the patellar tendon.
Larry K says
This comment about strain on the knees is right on target and reflects my experience, as a 66 year old cyclist living in SW Utah surrounded by mountains. During my bike fit I couldn’t believe how much of a difference it made switching from 175 to 170 cranks. My knees felt better and my power went up. On steep climbs I spin up the hills with a cadence in the mid 90s and it is so much kinder to my knees than grinding up the hills on longer cranks at a lower cadence. The other thing that really helps my knees is having an oval chainring on all of my bikes, making it easier to push across the top at 6/12 o’clock. All of my bikes are 1x setups (mountain, gravel, and road) which enables this.
Kerry Irons says
Quoting a Fred Matheny Summary: There have been studies of crankarm length, but the results aren’t consistent. Some show that longer cranks provide greater leverage for turning big gears. Some show that shorter cranks foster greater speed via a faster cadence. And some show that crank length is completely individual.
So, longer crankarms aren’t a panacea for time trialing. In fact, there are dangers associated with them. The added length makes your knees bend more at the top of pedal strokes and extend more at the bottom — both of which can lead to biomechanical injuries if you jump from 170 mm to, say, 180 mm.
Also, longer cranks reduce cadence — and a brisk cadence is the key to good time trialing.
Big Ring Bob says
“is gearing only half the problem and crankarm length the other half? Many production road bikes today come with 172.5 mm cranks. If you reduce this to 165 mm, you are shortening the vertical stroke by 15 mm, a little over 1/2 inch. This means your knee is going to move a little less than 1/4 in lower at the top and higher at the bottom of the stroke. This is the change in vertical motion of the knee. Work done to move the pedal through one revolution is changed by (172.5- 165)/165 which is about 4%. Shifting between a 16 tooth and a 17 tooth gear gives you a change of approximately 6%. Hopefully this demostates the relationship between crankarm liength and gearing.
Gary Keene says
First, let’s stipulate that most of us are worrying about bouncing our knees off our chest in a deep TT tuck, so, good on you Jim! Second, I am surprised the issue of fit has not yet been addressed, specifically, that–within the dynamic relationships of the body’s contact points, the length of the crank is relative to the length of the femur = there’s a range, and there’s adjustments at the saddle, and there are preferences, but it is a vital factor. Finally, all of us fall on a range between say Contador and Ullrich: spinners and stompers. The former puts more of the load on the lungs, the latter on the legs. While gears can be changed in the moment, crank arms cannot, so wise personal choices are the order of the day. Bottom line: we have options, not rules.
Filippo Moneti says
Jim, How tall are you? What’s your inseam.
Jim Langley says
Hi Filippo,
Height: 180cm
Inseam: 86.5cm
Thanks for commenting,
Jim
Russ Marx says
Change the chainring to 50 tooth so you are using the middle of the cassette. The limit of 1x for the average cyclist is the limit on speed range, can’t have climbing & road speed.
Higher RPM = more power. Shorter crankarms allow more RPM, change the gearing to place your cadence in the speed range or power range you are looking for. Its more than just crank length
alan lott says
With repeated suggestions from Rick S to go with shorter cranks for my knees I finally ended up with 160’s and haven’t looked back. Noticeable reduced knee pain, increased comfort, more relaxed pedaling and wish I had done it sooner! I ended up with Rotor ALDHU 3D DM for my 11-spd yet also looked at an 11 speed 105 160mm which are no longer in production(I got one on ebay from Japan for my second bike.) Shimano now makes 12-spd 105, ultegra and dura-ace in 160 and 165 and grx 12 2x a 165 coming soon.