Note: every company has a policy slighlty different to the next and even different depending on distance traveled or times of day. You should find the actual policies of who you’re considering traveling with
Sometimes with a layover you have to pick up your checked luggage, and you might be asked to pay (again) to check your bike.
Train/Tram/Metro/Ferry
I say “train” but it applies to all
In a lot of cases you have to pay extra, but you can avoid that.
In some trains you can be asked to pack your bike, without extra charge for the bike.
In some trains you have to book your bike in a special carriage, and you can get entry to the train refused if not.
It can be avoided by “hackpacking”.
In some trains, it’s free, with dedicated hooks, and no limits to the amount of bike.
Hackpacking:
Because you likely don’t need an actual bike bag in trains, I have “hackpacked” my bike in my sleeping bag or a blanket cover by removing the 2 wheels and “fiting” the bike and wheels in.
I would recommend this if you are at least an ok mechanic and you travel with some tools.
This can be a hassle as sometimes you “check in” 200-300m away from your door through stairs and you have to carry your janky hackpack and it can be exhausting!
Hydraulic disc brakes users need to fit something in between the brake pads when removing the front wheel, hydraulic brakes have a self centering and adjusting function that get the pads at the same distance to your rotor as they both wear out. It is like that so that the distance in-between them is always the same.
Bus
Similar to trains,
For connection buses (to get to the airport) or city buses, you have to “hackpack” (see above) the bike as a minimum I’d say.
For Intercity or intercountry buses, sometimes you can load your bike as is, sometimes you are not allowed a bike at all. You might have to pay extra if you “check in” your luggage, and maybe extra if you have a bike. You can always lie about the content, but I recommend not to hackpack but rather stealthpack (see below).
Flying
First: please don’t fly when you can avoid it. Or consider playing closer to your home for fun tourneys!
Avoid booking with third party websites because you might not be able to book your bike in, and also you might not be able to choose your dietary preference off the menu and it will be hell to rectify after.
To carry your clothes/ipad: large rolltop backpack or classic cabin suitcase is great to put everything in.
Most times the sizes and weight of carry on luggages is usually not checked.
You are most of the time allowed an additional “handbag”, it can be a small backpack: use it to put your ipad/laptop, small blanket (planes get cold!), neck pillow and noise cancelling headphone in there, another advantage for this is that you usually have to take your laptops and tablets out during a custom’s check, so you don’t have to empty your neatly packed backpack/suitcase.
Remember you’re not allowed tools, razor, nail clippers, lighters, knife or non certified battery packs in the plane.
Pro tip: forgot your charger at home? Buy an external battery (powerbank) with the compatible cable attached to it, it will be cheaper and more reliable than the cables sold in airports. Sometimes they can double as wall charger too.
Budget airlines are awful, you might have to physically print your ticket in advance, check in online up to 24h prior, or pay a fee on the spot.
They will charge any and everything they can (overweight, oversize, overly dressed, overly happy…).
They always have a trick up their sleeves, read the small prints. Don’t let a 50£€$ cheaper price fool you…
Polo mallets and bike helmets can be taken with you in the plane as carry on just by asking or you can tell it is a walking stick with the head removed and it should do the trick. Fit them in your bike bag preferably though.
Stealthpack and lie to save some £€$:
People have had luck not paying a bike fee in flights where it applies saying that they carry an art project.
It’s a fairly common lie that people get away with, usually work when you have a bike bag that’s not printed with bike pics all over!
Ground Effect has a great bike bag that’s smaller than the standard size (as opposed to oversize luggage), some companies will assume oversize when it doesn’t fit on the belt, but you can ask them to proove that it is oversize (ie. Mesure compared to the definition of oversize).
A cheaper alternative is to disassemble your bike entirely and wrap it in Cling Wrap/Cellophane (that plastic roll you pack your sandwiches in), until everything has a thick layer of plastic around it, use plenty of zip ties, and your polo protective gear to protect critical parts (head tube, dropouts, seat tube), and that’s a lightweight and pretty hacky way of doing it.
Consider using old hubs to protect your dropouts (frame and fork).
As you disassemble your bike DO NOT TOUCH your rotor with bare hands, your body oil and your bike grime will contaminate the pads and worsen your braking experience! It can be reversed with dish soap to a varying degree of success…
Saying the truth, Normal bike bag:
Tell the truth, no special advice, just pay in advance, as it is cheaper.
Sometimes companies will ask random stuff as: did you remove your pedals, deflate your tires, turn your handlebar, just say yes. In practice it useless to do so.
Sometimes they also do not allow for clothing to be in the bag, just tell them you brought tools and spares that make up to the maximum allowed weight… If you chose to say it is an art project/installation you won’t have the hefty fee, odd questions and sometimes you can go oversize and overweight at no extra cost… just sayin’…
NEVER LIE TO CUSTOMS!
In some countries customs might not allow for stuff that have been in contact with soil if not properly washed (to protect their biodiversity), if it is a polo bike tell them it is a city bike, but don’t lie, customs have nothing to do with your airline bike policy.
Those points only apply to bike polo bikes, as they are most likely tough as hell and can take a beating, although there is a non zero chance to receive your bike with some damages, if you have damages to your packing notify the helpdesk of your own plane company immediately.
Also there’s a non zero chance that your bike gets “lost” in a transfer flight, avoid transfers when possible. You can usually get it sent home within 10days for free, assuming it just went the wrong way. Because it is not a normal suitcase you usually can’t apply for compensation.
For your other bikes, gravel, road, touring, use a proper bike bag, with sufficient pading, axle mounts, and derailleur protection.
Telling the truth should be your best option for safe handling.
Hope this helps. 
Full disclosure:
Do this at your own risk, i am not your parent, I am just a stranger on the internet after all.
I have had to pay more than I anticipated using these tricks. But I’d say that overall I saved more money doing all this than if I had to pay full advertised prices.
