Hi all, I’m currently trying to work out how you determine if / when to change front crank size. Currently I’ve got dura-ace 50/34 - 30/11 (front - rear). That’s OK for most of the time, but there are times when I’m spinning out (basically you can’t effectively pedal > 55Kph). I also do a lot of hill climbing, where I use 34-30 on a couple of the steepest pinches.
It doesn’t happen a lot, but it’s happening more often as I’m slowly getting stronger. I’ve had a look at the new cranks on offer, and the next steps up would be 52/36, 53/39.
What I’d really like, is a 52-34 or 54-34 (i.e. retain the smaller ring), but I assume there’s a good physics based reason why they don’t do that (as opposed to a marketing-segment-based-reason).
52 has 4% more teeth than 50… but does that translate in a linear fashion to the max speed? I imagine not, because you’re looking at the circumference, so wondering what real world experience people have making this type of change?
Other stuff in case it’s relevant…
I only use this bike on the road - not on a trainer
Typical road ride is 150Km-180Km 2000-4000m climbing
This isn’t urgent, but it’s on the wish list of “upgrades I’d like but don’t really need”
I would have thought that your crank-set, if you look, allows you to remove the rings separately so you could simply replace the larger ring.
That said, while I don’t want to doubt you, are you sure you’re “spinning out”?
A quick calculation, assuming you’re running 700c 23-28mm tyres would suggest that at 35mph (56kmh) you only need a cadence of 97 rpm in 50/11, which many would only just consider to be starting to “spin”.
A cadence of 111 gives you 40mph (65kmh). I’d, personally, consider the “unsustainable” spin to be somewhere around 130 rpm, which in 50/11 would give me 47.5mph (76kph).
So, I fully suspect you can replace just the large ring on your cranks, which answers your original query, but I would also suggest maybe considering the cadence drills on offer in Sufferfest, which will approach the problem from the other side and help you get more comfortable with spinning fast.
For contrast, at 35mph (56kph) your current 50/11 would require a cadence of 97 rpm, while 52/11 would be 93 rpm and 53/11 would be 91 rpm.
Finally, just a quick question. Is this happening on the flat, or downhill mostly? If downhill, are you racing? The effort required to add significant speed over a good tuck freewheel downhill typically isn’t worth it, use it as an opportunity to let your legs get ready for the next push on the flat/up.
You can replace just the rings. If you’re genuinely spinning out, you want to change the chain rings not the cranks. Crank length is about bike fit.
I also second Jon’s comments. Unless your av cadence is over 90 and actually more like 120, then you might just need to work on your cadence. If your cadence is low and you switch to bigger rings, you may really put some pressure on your knees. That said, I know quite a few cyclists who run a bigger chain set and it sounds like you’re super fit. If you’re more comfortable at a lower cadence (which is my preference) then it may be worth going up from a full compact to a semi compact or an even bigger size!
I run a compact chainset and only spin out doing 70km/h down hills. I’ve an 11-32 on the rear. My preference is to ride at a lower cadence but I never spin out.
If you really want a harder gear setup, you can also consider switching your cassette for a 10-25 rather than the standard 11- 28/32
I have compact 50-34 on my 2014 TCR with 11-25 (mainly on trainer now) and on my 2019 TCR I have mid-compact 52-36 with 11-30 (recent change from 11-28). My cycling terrain: I live in a valley with climbs all around and indeed at the base of an HC climb (which Richie Porte still holds the Annual TT record for).
I agree with the comments regarding cadence of 130 so it’s not spinning out, but holding it for long on fast descents (e.g. chases) is not comfortable so I battled with the 50-11 combination. Just like the change from 11-28 to 11-30 has really helped in enabling me to keep a more comfortable higher cadence on steep climbs, the 52-11 combination has really helped in keeping a more comfortable fast cadence on the long “gunning it” descents. The teeth numbers don’t seem much but the cycling effect (for me) is significant.
I’m not sure that you’d miss the 34 by going to a 36 (with 11-30 on your casette and you could always head to 32).
One of the things you might notice if you go to a 52/34 is that your FD might start banging on the chain. Most FDs are NOT designed for this big of a gap. The second is that if you go small/small, your chain will wrap around the RD and jam. Not that this would be an issue, but if you are using Electronic shifting and the batteries go flat, you will end up in this situation. Suggestion: Try to see if you can fit an 11-32 on the rear and go to a 36 small chainring on the front.
Thanks all for all your feedback. And… I have fixed the subject to talk about the actual ring size and not the crank (thankyou @DameLisa ).
I’m on 170mm now, my bike fit said 170-172.5 is my ideal crank length, so haven’t worried too much about it. Was interesting seeing taller cyclists like Froome not using longer cranks. One day I might try just to see how it feels though.
I can spin ~100, but I can’t hold 110 - 130 for extended periods of time.
My max cadence is > 200 but that’s only for a couple of seconds, and on the trainer which is far more stable than my road bike. I find on the road bike > 110 is too jerky, so I’ve been practising high cadence for this specifically, whilst doing easy laps around a local park.
If I’m trying to hold a really hard effort for a long time then my preference is a lower cadence ~80. If I’m climbing I’ll happily go on v low cadence (50-60) out of the saddle for extended periods of time (30 - 60 minutes), or mix it up with low cadence / high cadence.
If it’s a sharp descent I’d probably just tuck once I hit those kinds of speeds, but if you’re on a gentle 1-2% decline you can’t tuck, but I find myself stuck not being able to pedal effectively.
Thanks @jmckenzieKOS for the notes on the FD, no point in trying if it’s just going to make it unreliable. Unreliable is far worse than occasionally running out of gears…
In which case… what are people’s thoughts on a different cassette (10-30) instead of different front ring (50->52)? Instinctively I’m thinking that the 10 ← → 11 change should have more impact on maximum speed, because the size of the lever is changing more as a % (vis a vis changing the front ring). Or is that wrong?
And @Sir_Brian_M I obviously have more work to do, to be worthy!
There’s a limit of 16 teeth difference between big and inner ring for a Dura-Ace front derailleur according to the spec sheet. FD-R9100-B (shimano.com)
There’s a limit of 16 teeth difference between big and inner ring for a Dura-Ace rear derailleur according to the spec sheet. RD-R9100-SS (shimano.com)
Total capacity for the rear derailleur is 35, so if you max out the 16T difference on your chainrings, you can only have a 19T difference on your cassette i.e 11-30. minimum sprockets size is 11t so you can’t even put on a SRAM cassette with a 10t.
You shouldn’t do 52-34 (18t difference) or 54-34 (20t tooth difference) It might work but the shifting will be poor, the chain will be more likely to drop and jam.
If you switch your whole drivetrain to SRAM you can get a wider range. SRAM red has a rear derraileur with a 36t capacity so you can get the 10t sprocket overdrive gear. Downside is the 10t sprocket is about 2.5% less efficient than an 11t or 6w at 250w, its why no pro uses a a cassette with a 10t sprocket.
I changed to a semi compact but with the 32 tooth on cassette, and a long hanger cage. this gives a bit more oomph when defending and has an easier enough gear for the toughest climbs
This will work just fine, you should do this. You don’t need to change your whole setup.
Just change both front chain rings, not one, the typical setups are below:
full compact - 50-34 (what you currently have)
Semi compact - 52-36
Standard - 53-39
No need to change your front derailleur, as well as indexing, you may just have to shift it up the seat post a couple of mm to get the clearance for the bigger ring, but the big-small chain ring ratios above are absolutely typical and won’t give you shifting issues. If you’re using Di2, it’s literally a change in the settings you can do using your phone and the app. Triple check your indexing.
If you still need a granny gear for climbs, a bigger cassette is the way to go.
I have three road bikes all running a full compact up front (50-34). And a range of rear cassettes, 11-25 (10 speed); 11-28 (10 speed); 11-32 (11-speed). The larger cassette runs on a long rear derailleur cage. The others are both on medium cages.
Just bear in mind if you go for a standard front setup and an 11-32 rear, you’re going to have q very long chain, and that comes with efficiency issues. My feeling is, if you’re strong enough to run a semi or standard up front, you shouldn’t need a 32 granny gear in the back. A 25 or a 28 will be fine.
P.S. for an epic hilly insane ride here, we’ve also been known to go the other way and reverse mullet the front with a 46 big ring, and something crazy like a 32t small chain ring. It’s the bomb when all the climbs range from 9-26% and there’s 60km of them. Ran this crazy setup with an 11-28 rear. No issues at all. Of course it’s the reverse of what you’re after, more of a Cyclocross setup I guess
I was previously running a 34t compact/28 cassette. looking at the ratios with a semi compact, I reckoned the 11/32 would cassette would give me both a higher top and lower bottom than my old set up. I upgraded the BB while I was at it, the front changer was raised slightly and the di2 coped as if it was born for the configuration. I was going to ride a lejog , 1000 mile plus ride with some very steep stuff on a loaded bike , otherwise I think I would have chosen a cassette with closer ratios.
Ran this crazy setup with an 11-28 rear. No issues at all. Of course it’s the reverse of what you’re after, more of a Cyclocross setup I guess