Is there a rule of thumb regards stem length when changing from flat bars to gravelly drops?
Same as when flat, shorter?

Drop bars will gain you nothing but a lack of control. Keep the flat bars.

On the same bike?

If so, you’ll need a shorter stem.
Most people tend to ride on the hoods or drops, so if you look at the drop bars you can see that the both those positions are quite a bit further forward than your position with flat bars.

You might want a higher rise stem as well.
Drops are a bit of a waste unless you can comfortably use all the positions.
I find I use the hoods for cruising along and the drops for climbing and descending.
Very rarely use the tops of drop bars.

Have a look at all the different shapes available too – some have shallower drops and are shorter from the tops to the furthest extent of the dropped section. Personally, I find these the nicest to use.

Search for monstercross conversions. +1 normally requires shorter stem with a lot more rise.

Have you also considered H bars, bar-ends, alt-bars (like the Crazy Bar) etc?

https://terbike.com/comfort-mountain-bike-handlebars-2

Drop bars will gain you nothing but a lack of control

Not true for me. Traded my monstercross 29er for a straight bar 29er I’ve found have been missing the monstercross. Can’t seem to get comfortable on the MTB for long hauls. Tried Geoff bars but didn’t get on with them. Too wide and no ‘hood’ position (pistol-grip) as per drops, so at the minute I’ve compromised with narrow flats and stubby bar ends.

If the grass wasn’t greener there would be no ‘other side’ 😉

There’s no magic formula – start with the basics of bike fit with your saddle position and work forward (plenty of online info). Road/gravel bikes tend to have a slacker seattube and some saddle layback placing you further back over the BB in comparison to an MTB. If you’re not used to drops, then expect to need a process of adaptation to a slightly more stretched out position, initially placing some strain on your core, shoulders and neck.

Drop bars will gain you nothing but a lack of control. Keep the flat bars.

Try riding into a strong headwinds with your flat bars…you’ll also learn to be smoother, picking your lines and mildly technical stuff becomes fun. Everything doen’t have to be gnarr.

Drop bars will gain you nothing but a lack of control.

They will gain you
– around 1mph increase in speed on flat road and gravel surfaces
– more comfort for long rides as hand position can be moved and hoods position is not as bad on the wrists and flat bar position

They will lose you
– An element of fun and bike control leading to more fun

If you go from wide flat bars to narrow drop bars I find that the stem length stays around the same as instead of arms being splayed out and body brought forward the arms are forward and body brough forward same amount.

When I rode fixed gear I used to swap between flat, risers, drops, bullhorns all the time and all on the same stem.

Went with a 20mm shorter stem with a bit more rise. Seems to work fine. Obviously not as much control as a set of risers but at the same time not too bad at all with 46mm Easton EA70 AX fitted. It’s not like the bike is going to be used on trails like the Golfie.

The plan is to convert my Felt single-speed, winter hardtail (steep, short, 90mm stem) into some sort of gravelish thing to see if I like it. I’m thinking some of those shallow flared drops – any recommendations? I like budget carbon for the thrill of it…
Before mountain biking happened in this country I was a roady, and we used to do all sorts of dumb sh1t on our winter hacks, so the lack of control will come back to me no doubt.
I’ll keep it single-speed for now, are there levers with no gears inside which work with xt callipers?
Lastly, the fork on the Felt is a Lefty, is there an adapter to fit a Lefty hub to a QR rigid (budget carbon!) fork these days? They used to exist but seem to have died a death.
Alternatively a budget carbon rigid Lefty fork?

I converted a XC style mtb to monstercross with midge bars instead of the 760mm wide flat bars. Reach 430 which is less than I would choose on mtb. Went from 100mm suss fork to a 490 a2c rigid fork. Kept the same stem 50mm I think. Probably a few more mm of spacers under it. Seems fine.
Try it and see?
Wouldn’t want to ride steep tech gnarr on it but ok for bumpy gravel type stuff and fun but slow on man made bermy flowy stuff.

Can’t seem to get comfortable on the MTB for long hauls. Tried Geoff bars but didn’t get on with them. Too wide and no ‘hood’ position (pistol-grip) as per drops, so at the minute I’ve compromised with narrow flats and stubby bar ends.

I’m using narrow high-sweep bars (On One Fleegle Pro) with Ergon grips. It’s a magic combo for me – a position similar to a relaxed road bike on hoods, but of course there’s no drop option. I might try inboard bar-ends. I may also get Salsa Bend 2 but trim them a bit.

I recently fitted a BBB BHS-25 high rise stem (70mm/35 degree) to my rigid MTB as the bars felt a touch low for lifting front wheel. I bought it from a trials shop, but looking at reviews elsewhere, most people seem to buy them for their road/gravel bikes.

I’ve an XL mtb frame set that i ran with 80mm stem and 760 flat bars, i’ve toyed with the idea of running drops for a monstercross set up but i think I’d need a super short 30mm stem to even get close to a comfy position and not be too stretched out.
I roughly worked it out on effective top tube as i don’t have a reach figure for it. ETT is 642mm + 80mm stem gives a dim of 722mm to the cl of the bars for the flat bars that were comfy. so drop bars would need a short reach set of drops say 70mm +30mm stem length + ETT gives 742mm so 20mm further out than the flat bars to the hood position… very roughly??
I’ll get a cheap stem and try it i think and report back!

There also those surly bars that allow use oyur mtb controls and have an effective drop bar type position?