If you are chasing drivetrain efficiency, I suspect it could have hidden benefits when used part way up the cassette i.e. Same overall gear, but with less losses.

WD :-)

I'm always thinking the same thing but I'm not sure if my Trek Speed Concept will accept a 58T. Have had one in the past for a long 40km TT off a pass (PR!!!) and it was nice to keep pedaling, felt more stable at high speed. I don't have data to back it up but lots of us used them and I took about 1 1/2 min off my previous yrs time (but was in better shape).

I know Sam Gyde runs a 56 on a flat course but that is more for drivetrain efficiency much like the pro peloton do in TT's as he pushes a big gear and runs a low cadence. Scroll down he answers my question.

https://forum.slowtwitch.com/forum/Slowtwitch_Forums_C1/Triathlon_Forum_F1/Sam_Gyde_on_IM_Texas%2C_drafting%2C_doping_...._P6624052-3/

Unless its a point to point race, any long descents or tailwinds will be matched by a climb or headwind. So keep your small gears in mind or else you'll just be saving a little time and paying back much more.

True, and I'd certainly be in a 34/28 in most races a lot more than an 11/58 but it just seems like there's some time to be saved there IF you could run a wider variety of gears.

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http://www.fitspeek.com the Fraser Valley's fitness, wellness, and endurance sports podcast

It all depends on how strong you are. If you're strong enough to make the BMC pro team then the 58 is the way to go in some courses. If you're the average age grouper, probably not a good idea.

I'm pretty sure you can't run a 58-34 double as the jump is too big. That is why you can have a 53-39, 52-36 or 50-34 as the jump can't be more than 16 (maybe 17).
That means you would need to have a 58-42 double, unless you want a triple. That means you'd need to have a 34 cassette to get your equivilent 34/28 gear as a 42/34.

Seems like a faff. I think the pros sometimes use the bigger cranks so that they have a better chain line, but apparently that is unimportant nowadays and 1x drive systems are a great idea!

I don't see any need for 58T unless you're well in excess of 70km/h or you really don't like a high cadence.
You can hit 65km/h with 53/11 at about 102-103rpm. I wouldn't think that's excessive even for a prolonged descent. I've pedalled to 80km/h with a 50/12 combo although I wouldn't hold that cadence for long.

58/11 would allow you hit 65km/h with a cadence in the mid 90s but I can't see it being worth the inevitable compromise on the climbs. Any course with descents that might tempt you would surely also have climbs. If you stick to the normal max tooth difference at the chainrings you'll have a 42T small chainring. I'm sure most riders on most courses will lose far more time due to this being excessively big for the climbs than they could gain by pedalling more comfortably on descents.

Two things.

One: Say you've got a course that goes up and down a hill. You spend an hour going up it at 20 kmh, then 20 minutes descending at 60 kmh. Going 65 on the downhill saves you 1.5 minutes. You would save 1.5 minutes by going ~0.5 kmh faster on the uphill instead. So better to spend your energy there, and recover on the downhill.

Two: powering down hill doesn't gain you very much speed anyway compared to spinning out and coasting. You're probably facing 1500W (total guess) of headwind drag. Adding another 200-300W will only make you a small amount faster, thanks to the cubic relationship between speed and aero drag. And because the descent takes not much time in the first place, your time saving is minimal.

If you're using a compact 34 and a 28 to start with, the biggest ring you could run is about a 50 and sounds like you wouldn't have the strength to push a 58... Only place I have considered a bigger ring than the 54 I run is Kona and that depends on the tailwind return can be huge but it changes each day almost of if it would be a real advantage or not.

I run a 52/36 (TT bike) and a 52/34 (road bike)...both 6870 Di2. When the road bike is dialed in, it only shifts marginally worse from the little to big ring.
Yeah, 58/34 is a no-go.

So it sounds like the reality is that a 58/11 & 34/28 will not work. I wonder if any ST have any success using a triple in this case then? Smoothness of shifting is nice but not a deal breaker as long as it can get into the desired gear without too much persuasion. Ideas/experiences?

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http://www.fitspeek.com the Fraser Valley's fitness, wellness, and endurance sports podcast

Triples are great for getting a nice big spread of gears but I still don't think it'll be worth it on the vast majority of TT/Tri courses. A much cheaper, simpler, lighter, more aerodynamic and tunable setup would be to simply swap in a medium or long derailleur and put on a wide range road or MTB cassette, to compensate for the big chainrings. For example, I have Ultegra 6800 on my road bike and purchased a GS (medium length) derailleur for my road bike last year to allow me use an 11-32 cassette, just in case I needed it doing the Marmotte des Alpes. Much easier than switching to a triple although you may have slightly larger gaps between some gears. Not really a problem in the mountains with 11 speed in my opinion.

If a compact or semi compact double with a wide cassette is about the best option for most doing a really, really mountainous road ride like say the Marmotte or Maratona, which I think it is; then I don't think there's going to be many cases where you need a wider gear range for a tri bike.