For gearing, why do you think what you have is wrong? Did you spin out on the 12 tooth that you'd need a 11?

On the second question: Practice running on tired legs. Brick (Bike-Run) workouts help. I do a lot of duathlons and I do quite a few bricks, even during the season. What is your current long run distance?

Congrats on your first Duathlon!!

What was the issue you had with gearing? 20km bike in 35 min is about 34km/h? that cassette and crank should be fine for that since a 50t x 12t gearing @ 90 RPM would be 47km/h. Only thing you may be able to optimize with a different cassette is smaller gapping through the middle of the cassette but that is marginal gains. Biggest gains will be practicing Brick workouts! (running right after a bike workout).

FWIW, I run 50T chainring with 11-28 cassette and ride about 36-40 km/h for 20km bike leg depending on the course. I definitely don't run out of top gearing especially if it is flat.

Unless your average speed for this (your first ever) duathlon on a flat course was in excess of about 32 mph, I highly doubt the change from a 12-27 to an 11-25 would make any difference at all. Now if you tell me you were spinning at a 120 cadence in the 50-12 on level ground, then I'd say maybe an 11 tooth cog or a bigger chainrings would be in order.

While I do believe you can train yourself to get used to the cement leg feeling by doing brick workouts, those workouts can only do so much. My guess is that you did what a lot of us do...rode way too hard and killed your run legs. For that matter, you probably also did the first run too fast.

For those distances, it wouldn't be that difficult to do some testing at different intensities/power/paces to see what is optimal for you. Set asside an hour and see if you can finish the whole course faster if you dialed back the bike intensity just a bit.

What was your average cadence? Were you spinning out using the 50/12 combo?

Sorry, not sure how much of this you can assign under Mistake #2, most of this sounds like it's a mistake #1 problem. What was your average cadence? If there are any gear ratio issues it's probably due to spacing and not top end/bottom end.

This has nothing to do with gearing and everything to do with inexperience and training. Get out there and train more. Do a few brick sessions to practice what it feels like to run on tired legs. I personally don't believe that bricks are a critical type of workout, but everyone should do them on occasion, especially newer multi-sport athletes.

Given your speed, the available gearing is an unlikely limiting factor of your bike time.

Sounds like you got tired. At your current fitness you probably should have started at a gentler pace.

As far as gearing goes, what about the experience made you feel as though your gearing was inappropriate?

One more chime in on the gearing. Curious why you think your gearing was an issue. Was there a lot of elevation gain/loss? If not, 50/36 12-27 seems perfectly reasonable to me. I run that exact cassette with a 50/34 crankset on my road bike.

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"They're made of latex, not nitroglycerin"

How close was your first 5k time at this race to what you would usually do for a straight-up 5k running race? (In other words, compared to your typical time in a 5k running race, how much slower was your opening 5k in the duathlon?).
Pacing of your race probably had more impact than your gearing.

I suspect that you would have felt better with an 11/25 or some other, tighter cassette ratio. I can't say you would have raced better

The problem on a flat course isn't "running out of gears". It's not a matter of not having the right gear in the middle when you need it. You find yourself frequently searching for but not finding the "right" gear. One gear is too easy, the next is too hard. You can get by on either but it's never quite comfortable.

A tighter cassette eliminates this problem. As long as the course doesn't get too hilly in either inclination, there is no downside.

I do a flat 70.3 race in my area every year and I always make sure to change myself over to 12/25

11/25 cassettes aren't any tighter than 12-27.

Whoops, my bad. I said 11-25 in one sentence and then 12-25 in the next. Seriously short term memory

Hi,

Thank you for your response. I do not my exact average cadence since I cycled without my garmin... and YES! I was spinning using the 50/12 combo.
Maybe a lack of experience, and weak cadence made me preform weaker than my fellow competitors. Is there any specific tactic/strategy on how to pedal? Should I go straight on the highest gear and just pedal my a** off?

Hi,

Thanks for the response. My personal best on 5k is 21:30. Maybe I've started pretty fast and therefore I've burned my legs on the second run. I mean, I barely moved my legs from all the pressure from the bike. What kind of tactics do you have on sprint duathlon? Do you pace yourself strategically in Both run/bike?

You should probably be looking for a pro contract.

Thank you so much for your response. Well, now I see that I've made 100 mistakes at the race day. I was pedaling the bike in 50/12 combo non stop. But my cadence was not suitable obviously. What's your riding style? Do you go instantly on the highest gear when you are racing? Cause I definately felt that I was running out of gears.
Can you explain please. Thanks

you're not running out of gears, you're running out of fitness.

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Eric Reid AeroFit | Instagram Portfolio
Aerodynamic Retul Bike Fitting

“You are experiencing the criminal coverup of a foreign backed fascist hostile takeover of a mafia shakedown of an authoritarian religious slow motion coup. Persuade people to vote for Democracy.”

There is your problem. Unless you are putting out 500+ watts like grand tour riders, grinding the 50/12 combo at 50-60 rpm will kill your legs before the run. You are basically doing heavy leg presses for the entire bike leg. I suggest getting a cadence sensor.

this is why I ride an 80t front chainring and a 9t rear cog... it's all about the gear ratio. I expect to win Kona and TT Worlds next year...

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Eric Reid AeroFit | Instagram Portfolio
Aerodynamic Retul Bike Fitting

“You are experiencing the criminal coverup of a foreign backed fascist hostile takeover of a mafia shakedown of an authoritarian religious slow motion coup. Persuade people to vote for Democracy.”

Time out.

I finally stopped laughing long enough to type this response.

This has to be trolling.

If it's not - my sincere apologies - but yes, I instantly go into 56-9 and breeze through traffic at 45 mph.

If you have a 90 cadence, then you will ride about about 30 mph in your 50:12 gear.
You averaged about 21 mph, so would have been in your 50:17 or 50:16 spinning at 90rpm.

As lots of people have said, the cassette ratio was fine. If what you say is right, it would be your 65rpm cadence in the 50:12 that would be a problem.

Yes, you cadence would be very low (lower than 70 RPM?) in that gear at that avg speed! I don't specifically monitor my cadence but I usually ride in a gear that allows me to spin around 85-90 RPM which for riding along on a flat road might be 50/15 gear. I really only use the 50/11 on descents.

Try different cadences in a workout from 70 RPM to 110 RPM. This will help you get a feel for where you are comfortable. "normal" range for a lot of cyclists is 85-95 RPM but everyone is a bit different.

PS-I made as many or more mistakes in my first race (and second and third...). Half the fun is pursuit of the perfect race!

Holy cow, people. OP is obviously new to this and doesn't get the terminology yet. Sounds like he ran a bit slower than his 5k pr (about right if you are well trained), over biked in the wrong gear, and just ran out of gas on the run.

OP: you went out too hard for your fitness. Your gearing is fine. You need to learn to ride a bike at proper cadence, and build up your fitness for this type of event.

OP: no, people don't wind their way up to their hardest gear. They wind their way up to the gear that lets them go the hardest at their desired cadence (typically around 90rpm). You change gears based on terrain, wind, fitness, etc.

OP: this is the only response you need. Ignore the haters. Hope you had fun racing, now go figure out the training and cadence parts!