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TT gearing advice
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Need some gearing advice from the forum experts.

My normal TT set up is a 54/42 on the front and an 11-23 cassette on the rear.

I plan to race a 40km TT course in a few weeks that is a long steady 20km uphill on the outbound (average 2-3%), with a turnaround then a long fast 20km descent to the finish.

The caveat is that there is a steep 10% kicker for approx 750m just before the turnaround. After 20km hard work on the outbound, I need to be really sure I have suitable gearing for the kicker as by this stage the legs will be very tired!

I really want to stick with a big chainring (ideally the 54 tooth, but I do have a 52 that I could use) to give me plenty of legs on the long, fast descent. To get me up the kicker at the turnaround I will switch to an 11-28 cassette on the rear, but I will still need a smaller gear on the front, but prefer to keep the 54 tooth big ring for the fast descent - is it possible to go with a 34 tooth small ring on the front???? If 54/34 with 11/28 will not work, what small ring options do I realistically have to pair with the 54 big ring??

Advice appreciated......
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Re: TT gearing advice [CG99] [ In reply to ]
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What (realsitic) average speed and power are you expecting to keep on the course? What average cadence do you expect to keep?

54/34 will be a risky combination when it comes to shifting. You might drop the chain when you shift down to the small ring, and/or struggle to move it up from the small ring to the big ring. I would stick to the recommended combinations. 52/36, 55/42, 54/42, 50/34 etc. Will your RD allow you to use a bigger cassette, like an 11-30? I would go that path if you're worried to run out of gears going downhill, but still want to have a lighter gear going uphill.
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Re: TT gearing advice [CN] [ In reply to ]
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16 tooth front difference is normally the limit of the front derailleur. You might be limited to 38 tooth inner depending on you crank sets BCD. But you'd check both things depending on the specifics of what you have.

If I had a race in 2 weeks I'd take it to a shop now. I could do the work but I don't like it if it takes longer than expected and I start getting stressy if the shifts aren't great. I'd assume the shift will be a bit clunkier if you are at the limit of the derailleur anyway.

Chain rings work in pairs so get the same make model of what you already have so that the tooth ramps coincide properly.
Last edited by: OddSlug: Feb 4, 19 5:02
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Re: TT gearing advice [CN] [ In reply to ]
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average cadence of 95-98.

Based on data from people who rode this course last year, I expect an overal average speed of 41km/h with 33-36km/h average on the outbound and an average of 50-55km/h on the downhill return leg - but topping out at approx 70km/h.

Thought about using an 11-30 cassette, but I have the short cage di2 derailleur and Shimano don’t recommend going beyond 11-28 - although many claim it is possible to go 11-30 it seems very marginal. Don’t especially want to invest in a medium cage di2 derailleur for just one race.
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Re: TT gearing advice [CG99] [ In reply to ]
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What kind of crank do you have?

It will either be a 130BCD or 110BCD. This governs the size of rings you can add

I’m guessing yours are 130, whereas the 34 is a 110 (meaning incompatible)

Could you do an 11/32 in the back?

See here for what cadence you’ll need for the gears you have available, to hit your pace goals at slope

https://www.cyclingabout.com/...shold-steepest-hill/
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Re: TT gearing advice [CN] [ In reply to ]
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I currently have 50, 52 and 54 big rings.

...and 34, 36 and 42 small rings.

All are Rotor brand - NoQ.

I get that 54/34 and 54/36 are too big a jump for the derailleur to cope with - so if I bought a Rotor 38 or 39 tooth inner ring, whilst this is not a “standard combo”, would this work ok with my 54 outer ring??

Before I invest, is 38 or 39 front and a 28 on the rear likely to get me up a 10% incline??
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Re: TT gearing advice [mvenneta] [ In reply to ]
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Rotor NoQ 110BCD.

.....have thought about using and 11-30 or 11-32 cassette - but would need the medium cage di2 derailleur for those - reluctant to invest in a new di2 derailleur for one race.
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Re: TT gearing advice [CG99] [ In reply to ]
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CG99 wrote:
Rotor NoQ 110BCD.

.....have thought about using and 11-30 or 11-32 cassette - but would need the medium cage di2 derailleur for those - reluctant to invest in a new di2 derailleur for one race.

That’s great the rotor crank is all 110 - for my SRAM red I need 130BCD for standard and 110BCD for compact cranks

You have two gear shifts in the race - as you approach the big hill at turnaround and once right after the turnaround

I’d get the smallest ring you can in the front and try it out. While it might not be perfect quality - not the end of the world!
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Re: TT gearing advice [CG99] [ In reply to ]
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Like OddSlug mentioned, chainrings usually work in pairs, so you won't get the best shifting performance if you match a 54T (designed to shift well with a 42T inner), with a 39 inner. Not to mention that the jump might be a little to big for the FD to handle well.

Given the average speed and the fact that you ride with a pretty high cadence, I wouldn't be too worried with running the 52 instead of the 54 chainring. It would be a different story if you preferred a low cadence like 80 though.

Dura-Ace 9100 and Ultegra R8000 RDs are designed to work with up to 30T sprockets, the older 6800 and 9000 RDs are in theory limited to 28T. They might work okay with a 30T though; I'd just avoid running the big-big / 54-30 combo though.
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Re: TT gearing advice [CG99] [ In reply to ]
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CG99 wrote:
I currently have 50, 52 and 54 big rings.

...and 34, 36 and 42 small rings.

All are Rotor brand - NoQ.

I get that 54/34 and 54/36 are too big a jump for the derailleur to cope with - so if I bought a Rotor 38 or 39 tooth inner ring, whilst this is not a “standard combo”, would this work ok with my 54 outer ring??

Before I invest, is 38 or 39 front and a 28 on the rear likely to get me up a 10% incline??

With a 39 front with a 32 rear you will be going ~8.6 mph at 90 rpm or 6.7 mph at 70 rpm.

When I climb 10% grades I think I'm nearer 5.5 mph which would put me near 60 rpm for me with that gear ratio.

That is on the very slow side of pedaling for me to be climbing over threshold power. My threshold power is normally 3.5-3.7 W/Kg

So the question is, how strong of a rider are you? If you are over 4 W/Kg I would think a 39 front and 32 rear would be enough and a 28 would be stretching it. If the section of road is 750 meters at 10% I might go with the 39/28 if I was over 4 W/Kg.
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Re: TT gearing advice [jaretj] [ In reply to ]
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4,2 watts per kg for a 30-40 min TT effort
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Re: TT gearing advice [CG99] [ In reply to ]
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With those choices I'd run a 52/36 front and an 11/32 rear.

With a 52-11 you'd be going 37 mph at 100 rpm which wouldn't be hard to spin going downhill. I doubt you'd be doing anywhere near threshold watts on that portion of the course and will probably be tucked rolling near 45ish so you wouldn't be pedaling anyway.

You'd gain 2 mph using the 55 over the 52 but the gearing probably won't work without a long cage or running an extra pair of chain links to hack the system and that creates other problems.
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Re: TT gearing advice [CG99] [ In reply to ]
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I'd keep your 11-28 cassette; in my experience with two bikes neither short cage di2 worked well with the 30t cog, much less a 32.
I'd go with 52/36 chainrings. If you can do 4.2 w/kg for 40 minutes you can do 4.8 for the 10% section and get up with a cadence in the 90's.
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Re: TT gearing advice [CG99] [ In reply to ]
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CG99 wrote:
Shimano don’t recommend going beyond 11-28 - although many claim it is possible to go 11-30 it seems very marginal.
I'd got the 11-30. As you said, many people have done it.

Shimano are very conservative in their recommendations. Unless you ran it regularly/hard in the big-big combo, it is unlikely to be a problem.
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Re: TT gearing advice [MattyK] [ In reply to ]
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MattyK wrote:
CG99 wrote:
Shimano don’t recommend going beyond 11-28 - although many claim it is possible to go 11-30 it seems very marginal.

I'd got the 11-30. As you said, many people have done it.

Shimano are very conservative in their recommendations. Unless you ran it regularly/hard in the big-big combo, it is unlikely to be a problem.

I've used an 11x32 on a Ultegra Di2 rear derailleur from a few years ago with no problems. As has been mentioned, shimano is conservative. You just want to be careful about a full cross over. FYI, I did not add any chain and it was just fine. Again, don't put it in the big - big combo.

Another option is to get a Wolf Tooth adapter

Kevin

http://kevinmetcalfe.dreamhosters.com
My Strava
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Re: TT gearing advice [CG99] [ In reply to ]
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Some more thoughts...

Stick with the 54 as you'll want that on the way home.

Maybe consider a 36 inner ring. Yeah, it's outside spec, but it's going to be two shifts, one down and one up. Be careful and you'll be fine. Perhaps put on a chain catcher to make sure you don't drop it inside. I've run a 34x52 before and that is the same tooth difference. It was fine, I shifted up and down all day long with no problems (Ultegra Di2).

I was worried about the 11x30 or 32 due to the jumps in the middle. I'm always concerned about a missing 16 tooth cog when it's relatively flat. However, considering your outbound speed on the grade it looks like you'd mostly be in the 54x17 or 19 so the missing 16 probably won't be noticed.

Another thought, and keep in mind that I am a BIG believer is using small gears when warranted. Maybe don't sweat the low gear so much. It's only 750 meters, probably just a bit over a minute. You'll be fine bumping it up to VO2 max level power or so as you'll get some recovery coming back down and even really the whole rest of the way back with that slight grade. It would be a totally different story if after the top of that grade you had to get back up to speed on a flat road or worse a slight grade. But this is one time when you can dig yourself a fairly deep hole and recover from it. That might be worth it if you get something in return for it. Something like a tighter freewheel with better gear combo's for the trip home.

Kevin

http://kevinmetcalfe.dreamhosters.com
My Strava
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Re: TT gearing advice [CG99] [ In reply to ]
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If you take some time to do the math, cyclingpowerlab.com and a gear calculator website can figure all of this out for you as far as what type of gearing would need.

For example, if I know that my overall target wattage for a 40k is 300w, total weight (bike, rider, gear) is 85 kg, preferred cadence is 97, and that my cda is .25, then I can run multiple scenarios of power required for both the 10% uphill and the 3% downhill and you can then use a gearing calculator to see if such power/cadence will match the resulting speed.. Which I assume are the gearing factors you're most concerned about. How easy of a gear do you need for the steepest section, and how big of a gear do you need for the extended downhill where you will be pedaling.

So lets take an example from cycling power lab.

At all the assumptions above with a 10% grade (adjusted cda to .35 since I assume you will be on the horns...not that it matters much)
300w = 11.81 km/h
310w = 12.19 km/h
320w = 12.56 km/h
330w = 12.93 km/h
340w = 13.30 km/h

Now take those speeds and use a gear calculator (my favorite is mike sherman's site) with various gear combinations and cadences. Also note that a 750m section at 12.5 km/h will take you 3.6 minutes...not just over 1 minute for us mere mortals. I believe Kevin is on a different level than all of us... ;)

So if you assume 97 rpms, even a gear of 36/32 only gets you 13.8 km/h. So if you want to maintain that cadence, you'll need more than 340w for 3.6 minutes. But lets say you're willing to drop down to 80 rpms for that period of time. Then it becomes clear that even a 36/28 gearing allows you to maintain 13 km/h, so you'd need to do maybe 332w for 3.3 minutes or so...or roughly 110%. Not that bad considering you should have an immediate recovery going down the 10% grade. Would be tough if the 10% grade was at the 10 km mark and you still had to climb a 2-3% grade for another 9 km right after the steep section with no recovery.

Ok...so now for the downhill...assuming -3%. Here the spread of wattages don't make that much of a difference because at high speeds, additional watts don't matter as much compared to going uphill. But the calculation basically says that anywhere from 275-310 watts is 57.4 to 58.7 km/h. So at 97 rpms, what type of gear do you need?

The gear calculator says that with a 52/11, you can hit 58.4 km/h. With that same gear at 102 rpms, you'll hit 61.5 km/h. If you solve for 54/11, then it's 60.7 and 63.7 km/h respectively.

So just by sheer math...with all of the assumptions I made, I would think that 52/36 with 11/28 should be fine. But you need to plug in your own numbers and figure it out yourself. The bottom line is that there are useful tools that if your information is correct, you don't really have to guess.
Last edited by: Jason N: Feb 5, 19 10:25
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Re: TT gearing advice [CG99] [ In reply to ]
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Biggest gears that you can push! 11-23 cassette with a 1x AeroCoach 58T ARC ring!!
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Re: TT gearing advice [nslckevin] [ In reply to ]
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I had 53/39 and 11-32 on short cage original DA Di2
Loved the gearing, could handle anything.
Big/big was a stretch but it worked.

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