XC MTB stem/reach length vs CX?

[Lewis Moon]

So... After about 25 years I'm going to buy a mountain bike. Presently, I'm riding cross bikes on the road, gravel and trail and both are set up in a kind of soft road positionin; seat a bit lower, reach to thr hoods a bit shorter and shallower.
I always rode in more of a road position on all of the mountain bikes I have owned, because sitting too upright felt just weird and awkward.
My question is: should I base my MTB stem length on, say, the distance to the bar tops on my CX bike? With the 30 cm wider bars that will put me somewhere near my CX position, only with a bit less drop to the bars due to a lower saddle?
Im a neo rookie so be gentle...

[KonaSS]

When I compare my mtbs to my drop bar bikes, the distance from the saddle to the center of the bars on my XC mtb always ends up longer than the saddle to center of the bar tops of a road bike, but shorter than the saddle to hoods position.

Hopefully that helps.

[coylifut]

give us more information about the bike you are considering. mountain position and road/cross position are completely different these days. Seat tube angles have gotten very steep to ward off wheel flop. top tubes long, dropper posts and stems micro short with wide bars. the bike may feel upright, but not so much when it's vertical going down a rock drop.

[Jaybee]

There are some additional considerations beyond "am I comfortable with this reach while pedaling?" These include "Can I comfortably get behind the seat on sketchy descents and drops?", "Can I provide adequate weight to the front wheel for cornering purposes.", and "can I maintain a loose, elbows bent, body-as-suspension position over the chattery stuff." Probably a few others I'm missing.

I know you were looking at an El Mariachi - for a bike with 2010ish XC geometry, I find that a stem beyond 110mm was a little too much front weight, and stems below 70mm made the steering a little too quick. Play within that range, considering also your bar width, where that puts your elbows relative to shoulders and how upright you want to be (probably more than your CX position).

Leelikesbikes is a good resource for MTB fitting/position for newbies.

[benb]

Originally Posted by KonaSS

When I compare my mtbs to my drop bar bikes, the distance from the saddle to the center of the bars on my XC mtb always ends up longer than the saddle to center of the bar tops of a road bike, but shorter than the saddle to hoods position.

Hopefully that helps.

That's been my observation as well.

Modern MTBs have super different geometry though I would not be afraid of really short stems at all, that's what they come with. I don't have a modern MTB, my wife does, it might have a stem as short as 50mm IIRC.

Most of the online stem mantras are mythical more than factual because there are so many variables that actually go into steering response.

And I think there are also a lot of people who have posted things over the years that said things like "Stem below Xmm will be unstable in my experience." Well that experience could have been with another variable in the danger zone, like the saddle so far forward that weight distribution was disturbed regardless of stem length.

For MTBs if you throw too long of a stem on you could find yourself spinning your rear tire on steep hills or something as well, not really something you worry about on a road bike. Saddle fore-aft is more likely to cause those problems IMO but for the most part I think with MTBs it is a goldilocks situation.. your saddle position + stem setup in combination sets the fore-aft location of your center of gravity and it can't be too far forward or two far back or things will go really wrong. Too far back because of the saddle being too far back and/or the stem being too short & too upright you will be more likely to pull wheelies and/or have a very difficult time turning the bike around on steep uphill switchbacks. Too far forward due to saddle position or too long of a stem and you may find your rear tire spinning on steep uphills. On downhills a position too far rear may cause you to wash the front tire (BAD) when cornering. A position too far forward on downhills could cause the back wheel to wash out and/or cause you to feel like you're going to go over the bars on steep descents.

[John H.]

Assuming your road and/or cross fit is good- I recommend this.
Reach- longer than tops on road bike, shorter than hoods.
Drop- start at half of road drop. Don't be afraid to keep raising- There is no right answer.
Bike should feel like it stays put on steep climbs. From wheel does not wander or want to pop up. Back wheel doesn't easily slip out or loose traction. Upper body should feel relaxed.
If you are running wide bars you want a shorter stem than you would use with say 680-700mm bars.

[Lewis Moon]

Holy Crap, I just bought a XL Salsa El Mariachi.
I ride my "off road" CX bike on a lot of single track, and it feels pretty good, so that is probably a good place to start. The MTB bars will probably be somewhere just over 700, as I could never get used to super wide MTB bars. I'll need to look at the sweep back of the bars and do some trig to figure out how long the stem should be to put me in a slightly more upright position than on the Ridley. Experiment, assess, repeat.