Is there a definite line that separates the two?

Is it the width of the trail? The amount/size of obstacles?

Can a jeep trail be a mountain bike trail?

I'm not a mountain biker. I've started riding around on some of the un-maintained dirt roads near me and it got me wondering.

Any time a trail is gnarly enough to necessitate an mtb is the short answer.

I've done plenty of road rides and even a few road races with off road sections, like fire roads and gravel.
Noting with drops tho obv.

I've never read any definition, but I regard any cycling on dirt as off road. Is riding a dirt or gravel MUP mountain biking? Not in my book. On all my posts I've listed road bikes, mountain bikes and MUP riders. I define mountain biking as riding on rough terrain, too rough to seriously ride a road bike (although I've seen some nut cases try it.). Also, there are the obsticles such as logs, jumps, rock gardens etc where a road bike is just not suitable.
I think of MUPs such as Rails to Trails, including MUPs that are not paved with plenty of bumps and uneven surfaces as roads, smooth enough for road bikes with a wider tire to ride on. So just because it's off road, doesn't make it rough enough to call it mountain biking in my book.
Just my opinion.

Meh, I've seen some really skilled cx guys on some of my mountain bike rides. Heil Valley Ranch into Picture Rock in Boulder, for example, is non-technical enough that I've seen cx guys on it, but it's a far cry from a road ride.

There's no clean line. You could try to separate it using the surface you're riding on, but then I'll give you the example of a guy on a road bike on a fire road versus a guy on a mountain bike riding urban assault. Don't worry about trying to label it, just do what you enjoy and don't overthink it.

Oh no... I'm just out having fun and was curious as to what you folks thought on the matter.

if it's paved, it's road riding. if it's dirt, it's mountain biking. the type of bike, size of path, etc, is irrelevant.

road biking stops and mountain biking starts when the pavement turns to dirt.

of course, you can get a bit more specific or fine tune it, such as:

lycra and snobbishnes = road riding, baggies and getting high = mountain biking.

What about dirt roads?

don't ask me; i smoke tobacco from a pipe and wear lycra under my baggies while i'm riding my cyclocross bike.

when you're having fun, you're mountain biking.

when they hit pave in paris roubaix, they are mountain biking....

this is probably one of the best images ever. i shall go to sleep with this image comforting me and making me smile.

glad i could oblige.

i assume you are smoking tobacco from a pipe while you ride your cx bike. at least, that is the image i have that makes me so happy.

Why am I picturing the monopoly guy riding one of those trikes with the GIANT front wheel, smoking a pipe...

i dunno. i can't grow facial hair to save my life.

I can see CX riders hitting PR, but Heil?
Dang!!

Note to self: Don't ask Darth.

The great thing about cycling is you can do anything you want on a bike you own. Rentals, not so much.

Pete, if you are in the Phoenix area, you know about the Black Canyon and South Mt. and the Sonoran Loop. I can see taking a CX on the Sonoran Loop and any bike you want on the Sonoran Bike trail. BCT and dirt trails on South Mountain definitely MTB. Baseline, Dobbins and the pavement climbing to Dobbins lookout, outstanding skinny tire stuff.

BTW, Harley Davidson University used South MT Park to film their new promo commercial. It ends at the burned out shell of Scorpion Gulch. Kinda cool. I drank my first Mt Dew there in the 60's. A couple different beverages in the 70's.

If you are on a road bike you are road riding. If you are on a mountain bike you mountain biking. What you ride on or how you ride is all personal preferance.

What if I'm on a comfort bike?

And am riding on a washed out jeep trail?

Daspydyr, I'm in Sedona.

SEDONA! While it gets a tad chilly there from time to time, you live in a true biking mecca. With that slick rock you have around town you can ride skinnier tires where ever. But for a washed out jeep road, I would consider that Mountain Biking and prefer fatter tires for the potholes and gripping the washouts. But a comfort bike (single speed or 3speed?) should be fine. Post some pictures, love that area.

SEDONA?!?!?!?

SWEET!

Pete, while you are riding your bike on the dirt roads next to the pink jeeps filled with tourists, you are mountain biking.

Single speed? In Sedona??

Um... no.

I have a "21" speed Raleigh Venture 3.0 (Don't laugh, it works for me at the moment) with 2.1 inch wide tires. The bike shop has a Scott 29" rental bike that they are willing to sell me. I'm thinking about it.

And Pablo... I'm usually standing next to the road while the large motor vehicle passes by. If I tried to ride next to them, I would probably end up under them. I'm not that good yet, I still need a lot of space.

So far, it is a heck of a lot more fun then riding on the road. I'm not sure about those 20' jumps, but I sure am going to get out in the back country more.

if you need info on where to ride, stop by ote in sedona, right on hwy 89, where the old mountain bike heaven used to be. jason and mike are great guys and will help you out.

edit: only now did i see you replied to my "going to sedona" thread, you clearly know about ote, please disregard

As kids, we used to ride our ten-speeds and cruisers down the steep, tree-littered hills of Sowinski Park in Cleveland, Ohio. We encountered rocks, stumps, crevices, and sometimes trees.

How 'bout that?... MTN biking with english racers and cruisers!

Who'd ever imagine such a crazy idea?