Please don't take your aggressive dogs out where other people and dogs are at risk. Instead, seek out trails at non-peak times or go with a group of friends who know your dog's limitations and are willing to work with them.
It may be the case your dog is biting because he feels unsafe. Yelling, scolding, smacking, or other negative forms of discipline will only make this problem worse. Your dog will build more negative associations when around other dogs, and will react more and more aggressively. Talk to other dog owners and your veterinarian to find the name of a behaviourist (not a dog trainer) in your area.
Under the guidance of a certified behaviourist, with time and patience you can manage your dogs fears and reactions to other dogs.
One reason your dog has become aggressive, or reactive to other dogs may be due to fear. Your dog may have suffered an attack from another dog, and is not seeking to protect himself.
The problem with this, is that this behaviour can easily spread. Max bites Rascal, Rascal bites Woody, Woody bites Happy. Now Happy is no longer happy, and we have a bunch of dogs ready to lash out.
If you find yourself on the other end, with dogs who are not reactive, and are charged by another dog(s) here's what you can do.
* Pull your team in when passing or being passed. If you know the team to be friendly, hold your team, and ski over to the side. If the team is passing from behind, a well trained team will respond to "On-by" and will keep moving ahead. If the teams are head on passing, give your dogs a "Gee" command and then "ON-by" Again, a well trained team will move over and go on down the trail.
* If you are working a new team, or don't know the team you are passing or being passed, put your dogs on your right side, and walking down the right hand side of the trail, move the team past the other team.
* Train a "Watch me" command. This works best with high value treats, something your dog does not get often. Watch me just means, you reward your dog for looking at your eyes. If they are looking at you, you have more control over your dog, and they pose less of a "threat" to another team or other dogs you might meet on the trail.
* Do not allow contact between your dog and other dogs while in harness. A harness should be a clear sign to your dog that this is working time. When your dog gets "suited up" he should be focused on the task at hand, and not looking to sniff butts or engage in social (or anti-social) behaviour. Don't confuse work and play time.