On a scale of 1-10 how serious is small hair line crack in the rear wheel?
The cracks are less than 1 mm in length and start where the spoke enters the rim. The cracks are so smooth when you run a razor blade over them you can’t feel them. The wheel is still true, but occasionally creeks (not sure if the cracks are causing the creeks). Should the wheel be replaced ASAP or should I just keep an eye the cracks and wait for them to get worse?

Cracked rims happen. If you feel comfortable enough to ride around on a cracked rim...well....then any advice you get here probably won't help. Sounds like you've made up your mind.

Would I do it? No.

The rim is toast. How much longer you ride on it should be proportional to your inconvenience when the wheel leaves you stranded.

You may be able to simply replace the rim and reuse some of the wheel. However, I think most wheelbuilders would recommend a completely new wheel.

The spokes probably won't pull out catastrophically.

But the nipple will pull through the rim where its cracked and detension the spoke, causing the wheel to become untrue.

It's time for a new rim. If the hub is good you can get your LBS to lace a new rim to that hub.

Depending on the wheel, you might be able to easily swap out the rim & reuse the spokes. I've done it often with standard build wheels. Odd ball spoke patterns and drillings might make it harder to find a similar rim.

Cracked is BAD...very BAD....
Good luck on your decision!

+1. I'm sure there are many people out riding with cracked rims and don't even know it. I'd replace the rim asap but you can still ride it for some time.

Are they by chance Bontrager wheels? If so and they are less then 5 years old and you are the original owner go to you bike shop with them. Possably a warrany issue.

The rim is still functional till you kiss the asphalt

That's like asking if the car that doesn't stop is a Toyota.

So you can get another pair that will crack. That's crack-tastic.

My crappy Neuvation rims cracked.

A friend of mine recently had a crack in his Mavic ksyrium SL. Went from nearly nothing noticeable to barely ridable in about 20 miles. He managed to limp home on the wheel, but it wasn't something I'd ride for 15 ft further than I had to.

What is all the Bontrager Hate from. A 5 year warrantee is good as gold if you ride lots of miles and the wheels take allot of pounding. I have had my Bontragers for 3 years now, and had a hairline crack by one of the spokes, LBS gave me a loaner and I had the brand new wheel a week later. Now I have a new rear wheel and the warrantee starts new on that wheel. Whats bad about that? They always stay true and strong.

That being said, I would not ride on a cracked rim, all it would take is a wheel coming apart on a down hill to make that one enjoyable ride.

Tommy: Let's think about this for a sec, Ted. Why would somebody put a guarantee on a box? Hmmm, very interesting.
Ted Nelson, Customer: Go on, I'm listening.
Tommy: Here's the way I see it, Ted. Guy puts a fancy guarantee on a box 'cause he wants you to feel all warm and toasty inside.
Ted Nelson, Customer: Yeah, makes a man feel good.
Tommy: 'Course it does. Why shouldn't it? Ya figure you put that little box under your pillow at night, the Guarantee Fairy might come by and leave a quarter, am I right, Ted?
[chuckles until he sees that Ted is not laughing]
Ted Nelson, Customer: [impatiently] What's your point?
Tommy: The point is, how do you know the fairy isn't a crazy glue sniffer? "Building model airplanes" says the little fairy; well, we're not buying it. He sneaks into your house once, that's all it takes. The next thing you know, there's money missing off the dresser, and your daughter's knocked up. I seen it a hundred times.
Ted Nelson, Customer: But why do they put a guarantee on the box?
Tommy: Because they know all they sold ya was a guaranteed piece of Poo. That's all it is, isn't it? Hey, if you want me to take a dump in a box and mark it guaranteed, I will. I got spare time. But for now, for your customer's sake, for your daughter's sake, ya might wanna think about buying a quality product from me.

I went thru three rear Bontrager wheels due to nipple hole cracks - it was nice they kept replacing them about every year or so, but a waste of my time.

Just get a new wheel or rebuild it. You are not saving money waiting for it to get worse. Maybe put a hundred or so more on it while waiting for the right time window to rebuild.

If it's not under warranty, get the rim replaced.

And if it is under warranty, just ride it until your stranded on a mountain.

Thank you for all the advice! I had the wheel checked out at the LBS. They agreed it was starting to fail and needed to be replaced. Since the LBS is a Trek shop you would think the owner would recommend a set of Bontrager wheels, however he didn't. Earlier in the winter, when I was browsing in the shop, the owner was explaining how Bontrager had gone away from the paired spokes and the new wheels are much better. When I stopped by today he recommended Mavic or Easton. In the last month he has had a number of the newer Bontrager wheels in for warranty work. I doubt he will be ordering any more Bontrager wheels for shop stock.

Looks like I am in the market for a new wheel set! FYI the failure occurred on a set Of Neuvation SL2. I am the second owner, so I am not sure how many miles are on them. Not enough, that is for sure!

Tommy Boy!!!