The BoB Yak trailer, probably the most popular bike trailer around, has a single wheel. The Burley Nomad, also popular with tourists, has 2 wheels.

The Nomad does have a higher load rating than the Yak, but it is a moot point for most users as few ever reach the lower load limit for the Yak.

There are more reports of handling issues with the Yak than the Nomad, however these are infrequent and uncommon.

Pulling the Nomad can occasionally result in the right tire dropping off the pavement, however the tow arm is fashioned so that the towed trailer position is biased to the your left, to help prevent right wheel drop, as well as offering a small safety advantage of forcing overtaking motorists to steer farther to the left.

More comments on Yak vs Nomad from earlier post:

http://www.bikeforums.net/showthread...1#post13478523

The Yak Sak is as a tough as any roll-top bag, and is waterproof when closed adequately. You need to roll the top at least twice to seal it well, and each additional roll diminishes the volume of the bag significantly. Bob claims a Yak Sak volume of 5600 cubic inches/92 liters (they evidently can't do unit conversions accurately), but every way I've ever measured it I get ~4500 ci with the top rolled only once (not a secure closure), and close to 4000ci with a proper double roll closure.

The Yak Sak weighs about 4.5 lbs (I've weighed it). BOB has never mentioned this in their specs all these years. They DO say the trailer weighs 13.5#, which is true, but when you add the bag (which most people use) it totals 18 lbs. This is a little trick they do to make their trailer sound comparable to their main competitor, the Burley Nomad. The Burley Nomad claims 105 liters space under the hood, with total weight of 14.8 lbs - it's lighter and holds more stuff. You can stack stuff on top of the Yak Sak for more capacity, but you can do the same with the Nomad if you add the accessory rack. Bob carries 70 lbs, Nomad carries 100 lbs (according to specs). Also worth noting Nomad isn't 100% waterproof under hood - you need to store electronics in another WP bag, but you'd probably do this also with the BOB Yak.

As an alternative to the Yak Sak, Ortlieb makes a bag that's larger, weighs less and and fits the BOB Yak very well - available in several colors, even the same yellow BOB Yak color. The Ortlieb Rack Pack XL weighs nearly 2 lbs less than the Yak Sak but costs $20 more, which may be a fair deal to many people. Plus it could color match your Ortlieb panniers. It's been discussed here before.

http://www.ortliebusa.com/CartGenie/prod-62.htm

http://www.ortlieb.com/_pdf_en/rackpack.pdf

http://www.bikebagshop.com/ortlieb-r...xl-p-1039.html

http://www.burley.com/home/bur/page_454/nomad.html

http://www.bobgear.com/yak

http://www.bobgear.com/trailers/drysak

Even more comments:

http://www.bikeforums.net/showthread...1#post14876836

http://www.bikeforums.net/showthread...=1#post8312265

http://www.bikeforums.net/showthread...1#post13696674