Brompton for Triathlon?
 

You should look at other bikes that can also fold as small, maybe bike Friday or something, others should be able to come up with some. You need much lighter weight, drop bars and good gearing, Bromtpon cant provide any of those.

Something like this would smash the brompton for speed:
http://d.hatena.ne.jp/roy-t/20170117

Really?
https://www.cyclechat.net/media/fran...-go.10628/full
Source: https://www.cyclechat.net/threads/le...n-ever.238033/

If the OP's requirement "bike must fit in an overhead compartment" stands true there is not much choice apart from a Brompton (if any at all). So if this is the core requirement one has to start from there and see what's possible. It is obvious that a Brompton is not the perfect (or even a competitive) Triathlon bike but the OP seems to be aware of that. And I know personally people who have used a Brompton for Triathlon (no official ones but an inofficial fun event with much smaller distances than normal) and came out well.
This on the other hand makes me wonder if the OP is aware of the technical limitations of a stock brompton. Clearly it can be done but needs a lot of fiddeling and expensive aftermarket parts (like i.e. a Vostok rear triangle) and will at the same time influence weight and folded size as well to a degree - IMHO w/o delivering additional value accordingly. So maybe the OP should either rethink the overhead-compartment part (which at least in Europe would possibly not work reliably anyway) and this way open up the choice of possible bikes or start with a S2Ex model and stick relatively close to the factory configuration/shape, adding maybe things like a third cog, a bigger chainwheel, a lighter seat-post, replacing steel with ti-parts, and add (removable) triathlon bar add-ons. Stuff like that. Will probably work but needs time and effort.

Maybe it would, maybe it wouldn't - but it would not fit into an overhead compartment I'd assume and thus not in the OP's requirements as well.

While I am not into racing or even fast riding, every once in a while I end up being faster on Brompton than people on road bikes in sporty clothes.

While this may seem minor, if life makes you ride in a variety not particular familiar places, invest time in the manner in which you might want to mount phone on the handlebars, including in the rain, as the latter can surprise you away from your lodging that you do not know how to get back to. Also collect a variety of navigating apps. Google Maps may work fine for bike at home, but not in places you may get to. When the internet barely works at a hotel, downloading a 200MB map may turn into a nightmarish challenge. I keep 12GB of maps for the whole world on my phone, so I can pretty much recover no matter what.

I log only 10 days/month away from home. Having a bike along for riding makes a world of difference for a variety of reasons and training for me is not even on horizon :D

Ori (née Mezzo)
 

The Ori is a tad thicker than the Brommie, but has more off the shelf components and is more amenable to less costly modifications. That being said, I love IGHs for commuting (the benefits outweigh the weight disadvantage). I don’t see the gain for the triathlon scenario, but that is strictly me.

For what’s it worth, my wife is a captain and she tried the Brommie overhead route, She stopped the experiment after 3 weeks. There were too many variables involved from no overhead space (and no closet space) left to getting stink eye when non-reving on a sister airline, as well as others.

Since you already own a Brompton I would try modding that one first; or consider a Rodriguez 6 pack. I have a Brompton M2LX with 58t chainring and 12t cog. I can can get up to 40kmh before my gearing starts to run out. I think 12t is the smallest rear and 60t is the largest front with the ability to fold. You could go bigger in front but then the fork/swingarm hook might be blocked by the chainring so you would have to use a strap or something to hold the bike together. Not sure if it will feel stable, but I would get a cheap flat mtb bar and clip on aerobars for less than $50 off Aliexpress so you can keep the stock brake levers. Road bars might be nice but doubtful it will fold small and requires new brake levers. If you spent a whole lot of money you could get your bike under 8kg.

Lots of cheapish things to do...
kojaks or primo comets
Flat bar with bar ends (lower than an S bar with an M handle post)

Chameleon with the fork popped off and frame in a soft bag may do the trick. Depends on if you could store the 24 inch wheels in a separate bag overhead. I think overhead is 22 inches deep so maybe on an angle?

I did triathlon on a Brompton... by mistake!
I wanted to try out a folding bike. I wanted to see how easy it would be to take it on a plane. I wanted to try out "wild camping" in a campervan in the Canaries. This is how it all came together...
I joined the excellent Brompton Hire scheme and booked a return flight to Tenerife with hold luggage. I hired the Brompton and parcelled it up with protective material and a few bits of luggage as an odd-shaped bundle suitable for transport in the hold. I arranged to hire a compact motorhome in tenerife and flew out without a hitch. I found a nice beach at Las Teresitas, just north of Santa Cruz de Tenerife, near the TF-11 "motorway". As well as the official car park there was an unofficial, but clearly tolerated, overnight parking area for campervans and motorhomes. After a good night's sleep and breakfast on the beach I unfolded the Brompton and set off in my beach shorts and tee shirt... and cycle helmet. The town of San Andres had been taken over by the Santa Cruz triathlon (oceanlavatenerife) for the cycling component. The only road along the coast, theTF-11, had its southbound carriageway closed to traffic, with 2-way traffic on the northbound side. Of the 2 southbound lanes, one was dedicated to the tri cyclists and the other for pedestrians. I was directed to avoid the motorway traffic and the pedestrians but carefully share the cycling lane. I attracted wide-mouthed stares, laughter and cheers. After a while I decided that a 3-gear commuting bike wasn't going to cut it, especially when I found a turning off into the hills!
It also convinced me that I should try a folding bike with more gears and bigger wheels. I've just bought a used Tern Verge P18 which is light, fast and has a great range of gears. Can't wait to fly to tenerife with it. Maybe I should learn to swim?

Pocket Rocket bike fridays go in a suitcase, checked luggage, not carry on,
you can spec any race-road components you like, on those...
451 high pressure tires won't hold you back..


And , now there is the newer 349 wheel PakIt, which eliminated a few folding features to save weight ,

so it, like their 20" wheel travel bikes is a mix of rear fold and knock down with quicker reassembly ..

they too can take Alfine's 11 speed hubs, and gates belt , so you wont have a grimy chain to deal with as you handle it stowing in the OHC...








...

M6R w/ it's rack and fenders is designed to be a good commuter/grocery getter. Lots of things one can do to modify that bike to make it faster.
No cost mods: take off rack, fenders, reflectors(+ brackets). Folding Kojak tires are pretty low cost replacements I'd also recommend. Titanium
and/or lighter pedals(clipless or not), aero/lighter wheelset(Aeroheat, SMC, etc.) starts to get expensive. But I would start with those mods; these
are rotating mass weight savings. More important than static weight savings; seatpost, saddle, bolts. Although if you can do/afford all of these mods;
that would really help.
My 2013 S6L-X has done 100 mile charity rides, long weekend trips, etc. No problems keeping up with roadbikes:

I say go for it. The base Brompton frame/fork/wheels is probably as aerodynamic as most of the 700c wheeled bikes you'll be riding against. The most important improvements over a stock Brompton:
1) Aero position. Ideally use aerobars with bar end shifters. Profile ZBS TT aerobars would probably pack the flatest on the Brompton straight bars. It's really, really important that you're comfortable in your aero position. This might require swapping the stem for one with much more reach.
2) Gearing. You'll need high enough gears to be competitive. I'd pick their 6-speed configuration for sure. I think you'll want to switch to a larger chainring for races.
3) Tires. Fastest tires that fit the bike... Maybe Schwalbe Marathon Racer or Primo Comet

What are you smoking? How can you possibly justify saying that?

Totaly agreed.
A heavy modified bromptom with dropbars And roadparts would be aero enoght.

Wonder if you made an aerodynamic front bag for it , would it be legit?

A pointy tail fanny pack for good measure...

I might go against the fold and recommend getting a custom bike with s and s couplers.

There was a framebuilder that used maybe 4-8 couplers and had a mini velo that fit in a briefcase. I can't find the website, but he used it for touring for years.

Frankly, if I was ever interested in triathlons (instead of being an oversedentary fatty), I would consider making my own bike...pr probably following the Patek framebuilder manual...or just hire someone to build it for me.

Come to think of it...I need to get back on my bike.

A little for April Fool's jokes... If you ride a Brommie in a triathlon, not only will you be last, you'll still be struggling on your ride while the person in front of you is crossing the finish line.

My daughter rode a 2sp automarix hub Java 14" foldie for a kids triathlon.
Beat a fair number of kids on fancy racers even...
But for the adult field (esp if its OD), I'd say that you need at least a 406 wheeled bike.
My wife did the sprint distance and had folks asking her if her Pocket Rocket was motorized.


I think the best approach is to come up with the dimension that is allowed in the overhead bin.
Once you establish that, you can look around at folding bike specs for their folded size.
You might still end up with a Brompton, but then you'd know that its the only option for the compartment size.

Better times on the other 2 ? cycling can be just not losing that much time in the middle...