If I convert a bike to a single speed/fixed and decide to leave the double ring on the front, is there any reason I should use the bigger or smaller ring? Should I just find whatever gear feels good and go with that one?

I might convert and old LeTour this weekend. It has 2 rings in the front and 5 in the back. The gear I found I liked was small ring in front - 2nd smallest in back. I don't know what sizes they are.

I figured I could leave the front double on just to go as cheap as possible (not a lot of extra cash right now). Other than weight is there any reason to not leave the double?

They give differing chainlines if you leave both of them on.

I have a really unique front crankset so I just took my big ring off and theres essentially a chain guard on there. Im running my small ring with a specific flip flop hub and my chainline is PERFECT!

Its a 40x16 or 14 setup.

If you have a standard BCD (like 130mm) you could easily find bigger or smaller rings to go in either position.

Its just about what gear setup you want. Count ur teeth (or read it off the rings) and calculate ur gearing on sheldons website. Muy bueno.

The inside of a Shimano double, and many others, will give you a good chain line.

this is info from sheldon's website for fixed gear conversions:

"Big or Small Chainring?"

Once you have decided on your gear ratio, then there's the question of which of several different equivalent sprocket/chainring combinations to use.
For instance, 36/12, 39/13, 42/14 45/15 48/16 all give the same 3:1 ratio. Which to choose?

Bigger Chainring Good:
- Slightly less friction
- Longer chain/sprocket life
- less chain tension
Bigger Bad:
- Slightly heavier
- Chainstay clearance may be a problem on some frames

Smaller Chainring Good:
- Slightly lighter
- More log-jumping clearance
- More chainstay clearance
Smaller Bad:
- Rapid chain/sprocket wear
- Greater chain tension (increased liklihood of the axle slipping in the frame)

These differences are mostly pretty minor. Most riders will be best served by a chainring somehere in the 30s for technical off-road use, 40s for road or bike path use, low 50s for track use.
Since 42 tooth rings are very commonly available on road cranksets, this size is particularly popular for conversions.

If you use a flip/flop hub, running smaller sizes gives you a bigger gearing difference for each tooth difference on the flip flop.

read the whole article: https://sheldonbrown.com/fixed.html

i run a larger chainring and a larger cog (52x18) because i find it rides much smoother than a smaller set up. plus less wear and longer life is appealing to me...

I've done a couple on-the-cheap conversions with dual chain rings, and I will usually choose whichever has the best chainline. Gear ratio can be defined by your selection of cog on the rear wheel.

There are lots of chainring/spider variations out there on older road bikes, so your options will depend on what you have. take apart the chainring/spider set up and see what your options are. sometimes you can put chainrings on different sides of the spider, sometimes you cannot. sometimes you can install a single chainring, sometimes one is connected to the other. You just have to take it apart and see. choose the combination that has the best chainline and put everything back together. (if you find out you can move the large ring in towards the frame, make sure it is a few millimeters away from the stay if you decide to go that route.)

once you've done that, get yourself a cog that will give you a gear ratio you like. Most conversions like this will get you to within a couple mm's of perfect chainline.

have fun!!!