I realize this is a basic and naive question so I won't be offended by short responses, if any. What is the fundamental advantage of using a single speed frame over a frame from a geared bike?

I'm imagining using one cog and some spacers (using a converted frame), and I could swap out wheels with my road bike in a pinch which I'd count as a plus. Is it just the extra weight? Or some other reason to prefer the narrower dropout?

Is there an advantage having to do with the angle of the chain on the ss frame as opposed to using the inside ring on a geared bike frame? What else is there?

I'll just monitor the answers. Thanks.

I'm pretty sure the most important aspect is the dropouts. SS/FG frames have horizontal dropouts which allow adjusting chain tension.

Ah! So you wouldn't need one of those tensioning arms or DR hanging there. Just shorter chain and pull the wheel back, which I hadn't thought about. Thanks!

IMHO, geometry is the major difference. Especially bottom bracket drop, but that doesn't matter all that much if you're running singlespeed.

Older road frames and some modern steel frames have horizontal dropouts, which will work just as well for SS/FG as a dedicated SS/FG frame. There are no major advantages of track ends over horizontal dropouts, but traditional track frame looks a lot cleaner without all the brazed-on cable guides, deraileur hanger (etc.) which are unnecessary for SS/FG.

In theory, you could pull the axle out of the horizontal dropouts of a road frame if your axle nuts aren't proberly tightened, but in practice, this is not a major concern.

Its easier to remove a fendered wheel with horizontal dropouts, but I've never had a problem using fenders with track ends, so, again, not a significant difference.

Track frames also have higher BBs, although these days, many dedicated SS/FG frames have geometry similar to road frames and most people seem to be able to manage riding fixed with the lower BB just fine.

Most modern road frames have vertical dropouts which will require a chain tensioner for SS. You cannot run FG with vertical dropouts unless you are lucky enough to find a magic gear ratio that will work with your frame.

The wider (130-135mm) spacing of modern road frames is to accomodate a cluster of gears. Older road frames had narrower spacing, and as the number of gears have increased, the frame spacing has continued to widen. The 120 mm FG/SS is a much older standard for which there has been no reason to change because the number of gears (1!) hasn't changed. These days it is easy to find a SS/FG hub with whatever spacing you need, as long as your hub is spaced correctly, it really doesn't matter.

Looks

Good to know, thanks. "Track frames also have higher BBs" so that the pedals don't hit the ground? It's a good thing I asked ... it seems like higher BB's would also let you put longer crank arms on the single speed with a freehub, and the extra leverage might be handy on hills? Or is the difference not significant enough to be a factor?

The higher BB on a track frame is to prevent pedal/ground contact on the outside pedal when going around a steeply banked track, which ironically, allows you to pedal around the track more slowly. Running longer crank arms on a FG is a bad idea, even with a higher BB. With one gear an no ability to coast shorter cranks allow you to spin faster more comfortably.

this is the ss/fg forum, not the track forum. theres no reason to not run longer crank arms.

You can run whatever crank arms you want, but there are advantages to shorter arms on and off the track.

well yeah, but not nearly the same reasons.

I dunno. Pedal strike can happen on the road as easily as on the track.

Get off the drugs.

I was thinking, perhaps simplistically, that using a freehub the SS could coast around corners and not have pedal strike. I think everyone has answered the question though, that enabling a longer crank is not a particular feature for a single speed frame. Thank you for the insights.

What about durability of modern whells - comparing hubs with 120 mm and 135 mm hubs? Will it be better using a track frame with 120 mm or a road frame with 135 mm for a LD wintertrainer?

at what point do you draw the line? 2 foot off the ground?

With an SS, pedal strike is no more or less of a problem than with a geared bike, unlike with a FG where there is no option to coast around corners, so there is less reason to use shorter cranks. Personally, I still prefer shorter cranks on a SS for road riding. I normally use 172.5-175 mm cranks on a road bike, and 167.5-170 mm cranks on singlespeeds and fixies. Your cadence is far less constant on a one-geared bike compared to a multi geared bike and having shorter cranks increases the upper limit you can hold a comfortable cadence.
I have never heard of anyone using longer cranks for extra leverage on a road SS. Some SS MTBers use longer cranks, however, I've done a lot of SS and fixed riding on and off road and I've never felt like I needed longer cranks in comparison to a geared bike. I always feel like the same length or shorter is better, again because of the higher cadences. Generally, rather than increasing crank length, you gear the bike appropriately to climb the steepest hills.

With 120mm, you could run cranks with a narrower Q, and the axle would be more resistant to bending, but in practice, it doesn't matter...you aren't going to bend your axle regardless of your frame/hub spacing.

What about the strenght in the building of the spokes? Isn't the 120 mm a weaker construction?

You are confusing the over locknut dimension (O.L.D.) with the spacing of the hub flanges. The spacing of the hub flanges effects the lateral stiffness of the wheel...wider is better. The O.L.D. has nothing to do with it. However you will find that track wheels are stronger and stiffer than typical road hubs because, having no gear cluster to accomodate, the drive side flange is positioned further out and the wheel is bilaterally symmetrical (or very close to it).

Oh I see,

a big thing people aren't thinking of is toe overlap

We are thinking about Pompinos but have toe overlap at my stokers frame size - the 48 cm... We might have to find another brand.

Another factor of toe overlap has to do with the type of cages, too.
I run Large plastic wellgo pedals with nylon straps, and I can just barely touch the wheel with my feet if my wheels are turning more than 45 degrees... which realistically doesn't happen unless I'm stopped.

Make sure that your feet (with largest type of shoes you'll be commonly riding with. Like runners) don't touch the wheel when cornering or turning at a reasonable speed. This made me go from 175mm to 165mm and right now i'm golden.