Sullalto

Like the title says. In reviews and marketing stuff, you hear about how one bike is a great road bike for unpaved roads, while another is nearly an old-school rigid MTB with some road manners.

But when you look at the geometry, it's nearly identical. Maybe .25, .5 degree difference in head tube angle, maybe a maximum of 10mm difference in bottom bracket drop, stack or reach to bikes described differently. Fork rake usually the same, chainstays are all over the place.

But a lot of that is variable with your setup, barring bottom bracket drop.

So what's the big difference here? Is it down to tire clearance Because the geometry and marketing gets pretty blurry across brands.

Abu Mahendra

Consider the geometry of, for example, the Salsa Fargo.

HTupolev

That's a good chunk of it.

A lot of what's left is probably the shape of the front end. Bikes aiming to feel more like road bikes tend to have steeper head angles and/or more offset, and handlebars that are fairly narrow and road-like; bikes that aim to feel more like mountain bikes tend to have more steering trail, and might use wider handlebars with more flare to offer wide hand positions with more flat-bar-like wrist angles.

chas58

gravel and mountain bikes tend to be longer, lower, slacker.

+10mm chain stays, - 10mm BB, -1.0 degree head tube angle (all highly variable).

you are right, there isn't much difference.

For me, the difference is rider position and tire size. But all new designs take 40mm tires. Bikes intended to go over 20mph are going to have less stack and lean me over more (usually with a shorter head tube).

I'm not sure the small differences in geometry make much difference, as the human body is highly adaptable. Although I do avoid a low BB because I hate pedal strike and like powering out of a corner (or jumping logs).

Marcus_Ti

The lines get blurry. Simple test IMHO YMMV NTYTAWWT:

1) What is the designed front-fork axle-crown measurement? Road-based bikes will be ~400mm +/-10mm or so. MTB will be more in the realm of 490mm realm, with a shorter head tube.



You might also consider BB standard basis (68mm road versus 73mm MTB)...but which is chosen tends to follow the axle-crown used. But only "tends to", as always "gravel" bikes are a very loosely defined area best summarized as "run what ya brung".

Kapusta

Head angle is one big difference.

Look at the Salsa Fargo. That is something I would consider a true drop bar mountain bike. It has a 69deg HA on most sizes.

Compare that to my bike, a Soma Fog Cutter which is in my view a fat-tire road bike (as opposed to a gravel bike). HA on those are mostly 73deg.

That is a huge difference.

I imagine bikes marketed a “gravel” bike are going to span a wide area between these two extremes.

In my mind “fat-tire road bike” means a bike with pretty standard “road” geomentry (though maybe not racing geo), but with bigger tires. “Gravel” to me indicates that the geometry is more geared toward stability on loose conditions.

However, there are no set standards, so bike companies will market bikes however they see fit.

It is similar to some mountain bike descriptors such as “XC” “Trail”, “All Mountain” and “Enduro”. There is a lot of overlap and nobody defines them quite the same. Yet the basics are generally understood.

Spoonrobot 

It's tire clearance and all the design compromises to fit tires wider than about 50mm. Usually to fit tires that wide the bb will be 73mm and/or the chainstay must be designed specifically to accommodate road size chainrings. Most often mountain cranks or 1x will be used. Also a front derailleur designed to clear fatter tires will be used. Tubing type, diameter and butting profile are some other factors. A bike designed with MTB theme will ride significantly different than a bike with road/dirt road theme.

The main thing IMO is intent. Categories may be blurry and be hard to pin down on the consumer end, but the person designing the bike absolutely has specs for which they will apply to anything considered a road bike, a dirt-road bike or a mountain bike. Mainly because the CPSC mandates it as so, there is a ASTM International specification for the terrain bikes are meant to handle. Here is a link with the explanation. And here is a link to the CPSC webpage.

Fat tire road bikes/All road bikes/Gravel Bikes are mostly going to be designed with lighter tubing that is more flexible, as well as a lighter fork.

thehammerdog

I just love riding my 1994 mongoose with drops seg up is comfy feels stable fast bombproof on the road setupvery near to road bike. Riding 2.1 will drop ti 1.75 next fun fun fun bike.

antimonysarah

I'd add that if you're on the short end and looking at the geometry of your size (rather than the middle of the range) you may be seeing the compromises made to make those sizes work with wheel size+tires driving the geometry towards being all the same. (Especially if they're sticking with 700c in the small sizes.)

Sullalto

I'm on the other edge of the spectrum, but you're right-that's something I overlooked, and happens at both ends of the size spectrum.