My current base has a rule against riding bicycles on the main road and will write a $140 ticket if you even cross the road on a bicycle.

Does a base commander have the authority to levy fines like this?  I was under the impression they can make rules and enforce them via UCMJ but they cannot enforce fines for breaking a rule that has no law to back it up.

Think of the Base Commander as GOD they see themselves in that role because they can assign you duty that will get you killed. Don't fuck with the Base Commander's bullshit rule - its His base and you will loose no matter what the law is. Stay positive think of this as a learning experience regarding authority, it's certainly not about bicycles or traffic safety.  Remember that many years ago He also had to put up with bullshit rules made by Base Commanders and having successfully survived that and learned a lesson about authority, he is now teaching you that what ever he says in how it is, without question. In combat that becomes important.  Have an opinion when you are a civilian until then suck it up and "Yes Sir" is the only wording you are looking for.

I guess I always figured that his two stars would ensure that he would win any pissing contest between the two of us, and it never even occurred to me to ask if he "had the authority" to do whatever the hell he did.

I absolutely agree with both of you. There is no way in hell I'm going to challenge this. I was just trying to figure out how it worked?  If it could really work?  What would happen if somebody challenged it?  I didn't get to where I am by challenging these rules.

Also even if you win in court and didn't have to pay the fine he could still smack you with a gomar for violating a policy rule

The "way it works" is that he says "We are going to do this, and we are going to do it this way."

Then everyone says "Yes, sir." and everybody does that, and does it that way.

Why not ask IG?

Look at the alternative:  You manage to beat the $140 ticket.  So the CO makes it a standing order that no Soldier shall ride their bike on the street, nor permit a dependent to do so.  Now instead of a ticket, you can have UCMJ for violating a lawful order.  Winning?

Army Regulation 190–5
OPNAV 11200.5D
AFI 31-218(I)
MCO 5110.1D
DLAR 5720.1

Look up the applicable regulation and you should find where the installation Commander has the authority to create local restrictions against certain activities.  My base had a hard-on for people that didn't carry their vehicle insurance cards and instituted a bunch of hoops they had to jump through to avoid having their on-base driving privileges suspended.

There may also be a local supplement to the above regulations that applied specifically to your installation.

You know... I don't know if they can write fines like that.  

Would you prefer they sent the offending people to NJP instead?  IIRC the max punishment for disobeying a lawful order can be 45 days restriction & extra duty, plus demotion in rank, and half-pay for two months.  (And for civilian offenders they can be banned from the base - I've seen it done too.)

Maybe it's me but $140 is a hell of a lot cheaper.

ETA: Oooh yes, base driving privileges.  Yep I've seen that revoked too.  Sometimes you take the lesser of two evils.