As you say - have a box each, - this contains clothes, sleeping bag and thermarest - everything - In my box I also carry tools and spares ! I would have a top box - either a Givi type or a Peli Case locked or secured to the bike - I have a Peli case attatched to rack permanantley- like the guys say for the daily essentials -food, water, stove, guide books etc for easy access.

Yes the Ortlieb dry bags are best for a tent, just bungee or strap to top of box or pannier.

For Security use a waterproof cover over the bike - this is a good detterent - a nice dark unobtrusive colour and people dont give it a second glance. Use a lock and covered chain to secure your helmets to bike.

All through CA there are kids who will watch your bike for a little money or a soda while you go off site seeing - use them - generally if you are out of town there is not much problem with theft - if you are staying in towns then find somewhere with a courtyard to ride the bike into or make sure you park in the Zocolo's - as public place as you can - dont park on quiet backroads.

If theres not enough room for all your gear leave some clothes behind, you shouldn't need much over the summer - you can always pick up some t -shirts on route !!!

Use very lightweight sleeping bags and Prolite Thermarests for where you are going - Though in the Central American highlands it can get pretty chilly at night - nearly freezing !!! especially Mexico & Guatamala - so have one warm item to wear !

Finally - If someonwe really wants to steal something they will - do your best, be careful with security and dont travel at night through CA - Its a great place to travel by bike, Don't Worry - Enjoy it !!
Cheers
Grif

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Ex RAF Regt, Ex Dragoman, LRE Instructor,
LR 90 300 Tdi Overlander
Suzuki DR650 Overlander
..and Bloody Nice Bloke!

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Cheers
Grif

'11 KTM 450 EXC
'09 Suzuki DR650
'00 Discovery Series 2 V8
'95 Defender 90 300 Tdi Overlander
http://gipperstravels.blogspot.ca