What are the disadvantages of buying a 2 or 3 ring crankset and converting to a track crankset ?

you can't say you have track cranks.

Some "track cranks" are really just road doubles that come with a single ring, anyhow.

maybe getting the chainline right, but that shouldn't take much work

There really aren't any. Unless it's octalink, which I've heard is liable to strip out in fixed street riding.

If your frame is a conversion, you'll most likely want to put the ring as close as possible to the chainstay - having double/triple crankset, the spider might interfere with the chainstay before the chainring.

^^I have an old set of Campy Record road cranks that I changed into faux-pistas. I took a file to the inner chainring shoulders so it would clear the chainstay. No worries and more money in my pocket.

If you're running a 1/8 cog then you'll have to run a 1/8 chain on the 3/32 chain ring which will have some play.

More flex in a 3/32 chainring.

If the whole drive train is 3/32 you will run a wimpy chain and be a wimp.. jk

Well there is less material in the 3/32 chainring so it will wear faster.

But these aren't really big disadvantages.

I doubt this is true. I know of octalink dura-ace track bottom brackets. I was thinking about running one with an Ultegra 6500 double with one chainring instead of getting a track crankset.

There are no disadvantages, only advantages.

Not really. With the ring in the middle spot on a triple/the inner spot on a double, it should line up reasonably well with almost all fixed rear hubs..

Track cranks in name only, no more "track" than an IRO is a track frame.

chainline is the only real problem, buts its easily corrected by flipping your spindle

That's more a matter of using a road double or triple spindle rather than a road crank.

Ha ha, ****ing brilliant!!!!!!!!!!!!!!!

Some really cheap stuff tends not to have as round of a spider as you might want for FG.

Cranks are stiffer when they dont have those inner chainring tabs.

that is all there is and all you need is a good ruller a peice of string and a set of vernier calipers
to measure the chainline and then calculate bb spindle length or you could spend a grip on five or ten bbs to get it straight

its more difficult to find road cranks in 165mm
and you have to deal with a non-track standard
bcd which limits your choice of chainrings
...unless you find an older set with a 144bcd.

1. They look horrible if you have to put the ring on the inside to get the chainline right.
2. They can be difficult at times to find in 165 if that's what your tires need. I ran 28mm tires on the only conversion I had, so I felt comfortable with 170, but if I'd wanted to use 23s, I would have worried about pedalstrike.
3. (minor) It's probably easier to buy a track crank and matching BB than it is to find the BB that will give your non-track crank the chainline with the ring on the outside (see #1).

???

Never mind the question marks, people- I think I figured out the poster's motivation.

Advantages?

You'll win the favor of the one or two asstroll busybodies on every cycling forum who won't simply ride their own bikes and be nice about everyone else's- they have to assume the role of Utility Police, making sure that every component on your ride is being used as the factory intended.

I use my running shoes to walk as often as I use them to run; hope that isn't some sort of violation of the Shoe Usage Code.

The only + to a track crankset is that the c-ring shoulders on the crank spider are matched for a 1/8" chainrings. The only advantage I can see of 1/8" rings for your particular situation (since you are asking about road cranks I am assuming you are not running on a track) is that they tend to have a wider selection of tooth counts than 3/32" road rings. I can not for the life of me find a 47t 3/32" ring in a 135 BCD (Campy), but can easily find a 130bcd 1/8" 47t track ring. Same goes for a 49t ring....

103mm bottom bracket +/- 1 spacer ring seems to be the ticket for the outside position to line up nicely with a 120mm hub. sugino makes a nice square taper one.

Also, a 1/8" 130 bcd track ring fits on a road crankset just fine.

Hard to find a 1/8" 110 bcd ring, though.

cool points deduction for running the ring on the inner spider...

dude I personally like fmf bmx cranks they are 20 bucks and they work fine 110mm bcd
I have converted an old road double on my conversion it is an old sugino gs 144bcd I used a file to shave off the double tabs and a shorter bb to acheive the 42 mm line I also had to releive the chaistay slightly to make everthing clear I have run it like this for about 9 months with no problems

also 130 bcd road cranks are easier to find rings in mulitiple sizes