Help - Bike rolls while in gear

[MetalFaceMays]

Hello CC forums!
So I'm a new rider, I just recently bought a '99 XLH 883 with 10k miles on it and have gotten fairly comfortable riding around town. Bike seemed in decent condition (for a bike as old as me) when I bought it.
I tried taking off from a stop and to my surprise I went nowhere. Looked down and my belt had snapped, I'm thinking when I was downshifting into first which I found out apparently is a big no-no. Got a tow home, all good.
Anyway, my bike rolls when in gear and the front pulley will not spin with it. I ordered a new belt and plan on changing it myself, but I'm worried that the belt is not the problem, so while I'm in there I want to see if I can find out what the problem is, and if I ought to take it to a professional.
What could have caused the belt to snap? Upon closer inspection it was pretty worn down on one side of the belt. Also, how fucked is my transmission? I bought the bike so I could have something to wrench on and learn from, but if I can't fix it myself then I'll swallow that pill and take it to a shop. I want to wrench but I want to ride even more.
Cheers!

EDIT - I'm a dumbass. Of course the bike is going to roll freely without the belt. I checked the front pulley by spinning it with my fingers and shifting through the gears. Spins freely in neutral, does not spin in gear with or without the clutch engaged. Shift lever will get stuck and refuse to gear up sometimes but I'm going to assume that it's normal when the bike is totally off. I'm going to do a full regular service on the bike while I bother replacing the belt, make the proper adjustments etc. Leaving this post up for posterity and also shits and giggles.

[Dragstews]

[docmel]

Ok You rode it and the belt broke: Outside of the expense, not a big deal. No, your pulley will not rotate easily with just your fingertips when in gear even with the clutch engaged

I had rear belt break on a high mileage BT when I started it when in gear (Clutch lever pulled in). I couldnt get it N, so I pulled the clutch in and BAM

You prob nailed the prob: Worn belt while downshifting caused it? Maybe. I doubt anything else is wrong But why/how was the belt worn in the first place? PICS PICS PICS! Get why the belt was worn, fix it and ride. Just be sure you adjust the rear belt correctly

[JBinNC]

Many folks use the opportunity to do a chain drive conversion. 'Bout the same price. More durable, or at least predictable, but more maintenance.

Jim

[7and7is]

When you get it back together make sure to check your belt tension, PO may have not known how to set it and just pulled as tight as he could, and if it was not evenly adjusted could have very well been rubbing one side of the belt, I would also check the throw out bearing and do a clutch adjustment, and shifting into first while moving (or not moving) with the clutch pulled in is not a "no no" I had an 03 sportster with 80+ thousand miles on the original belt and probably could have gotten another 20,000 out of it (Until some little lady with a cell phone took me out), these bikes are not made out of beer cans they can take some abuse.

[Tater66]

shifting into first while moving (or not moving) with the clutch pulled in is not a "no no"

Just for clarification...are you saying that , on a sportster, you should only shift into 1st gear from a dead stop ?

[7and7is]

Just for clarification...are you saying that , on a sportster, you should only shift into 1st gear from a dead stop ?

Negative, you can shift into 1st moving or not moving, you don't want to slam it into 1st at some stupid high speed, just normal down shifting, like when you are coming to a stop, or slowing in traffic.

[docmel]

Couldnt agree more with the proper belt tension: I cant count how many times a primary or secondary belt I have seen was WAY TOO TIGHT. IMHO, thats what happened to the dual belt FXB in the 80s. Shadtrees tightening the primary belt on thier own terms

A rear belt should last almost the life of the engine to its first rebuild, if properly adjusted/not damaged