Guidance on "correct" stem length

Back when the 500SE came out B'Twin's sizing was a little odd - their 60cm frame was more like a standard 57, and they also specced quite racy stems. I normally ride a 56/57 with a 100mm stem and found I had to switch from the 110mm my Triban 3 came with to get comfortable.

As others have said, 10mm difference on stem length is noticeable. I had a basic bike fit done due to having some lower back pain on longer rides and the fitter specced a longer stem which sorted it out. Somewhat counter-intuitive but it worked and at least it's easy/cheap/quick to swap stems

If you've got a turbo trainer or someone to hold the bike steady a good rule of thumb is to sit in the saddle with cranks and quarter to three and lean forward. The position you can support with just your lower back/core muscles is the right one, and then it's a case of placing the bars/hoods at the point where your arms aren't locked rigid.

Before you make changes to your position measure everything so you can get back to where you were if you go wrong. Given you're playing with saddle fore/aft and stem length measuring from the tip of the saddle to the split between the stem and the faceplate is a good one to do. You mention you feel like you're too far forward, that might be a saddle angle thing and easy to experiment with if you have a micro-adjust seatpost. If you make a change make sure you ride it a couple of times before deciding if it's better or worse, and try to only change one parameter at a time.

Some riders are tolerant of key setup paramaters being in wide range, for me I find I've got a very narrow envelope of comfort. My approach without a bike fitter on hand would be to get my legs feeling right in terms of saddle height and fore/aft and then sort out the bars. Just note that it will be individual based on age, flexibility, body symmetry, and historical injuries. There are rules of thumb but they're just that - they merely get you in the ballpark.

Small moves can make a huge difference. When I first started I was amazed how much faster I was after I'd put the saddle up no more than 10mm.

If you have a good relationship with your LBS, they will have a few different stems sitting about in their workshop, try a few, see what feels best, before you buy the one you really want.

Alternately, buy a cheap second hand one on ebay to see if it feels good, before you invest in the pricey one.

I had exactly this dilemma last month but on an endurance bike with quite a race position that left me uncomfortably stretched out.  Had the bike for 2 years but finally decided even the relatively minor discomfort was lessening the time I wanted to ride that bike.

Long story short - I dropped 20mm from 90 to 70 and inspite of some trepidation about making the bikes handling more twitchy I haven't looked back.

The nice thing about stems is that as long as they are removable faceplate designs they are one of the easiest parts to fit and change.

Looks like there's a few KOPS advocates on here, so it's time for the late, great Sheldon Brown: https://sheldonbrown.com/kops.html (although written by Keith Bontrager)

Peted76 - 10cm is way too big an increment.

1) You want to have your seat bones on the wide part of the saddle ideally, that's literally what it's there for. Yes the pro's and the 'racy types' will move onto the rivet when they are pushing themselves into the red.. however for the most part that's not about fit and more about how they are riding at that moment. It's the wide part of the saddle which is where you 'fit to'. 

2) Check your fore/aft saddle position. With cranks level, lower a plumb bob (tie a 30-inch piece of string to a rock if you don't have one) from the front of your kneecap to where the pedal spindle connects to the crank arm. If the line falls in front of or behind the spindle, slide your seat forward or back, then recheck.

3) Stem length should come after those two checks. Stems are cheap enough that you can experiment with and there's always loads for sale on facebook or ebay. Try 10cm increments, 10cm does make a difference. 

Without seeing you no-one can really say - and there's no obligation to sit back on the bum-shaped part of the saddle while actually riding (look up the phrase 'on the rivet').  Before buying a shorter stem though, it's usually easier just to move the saddle forward by 20mm (or buy a seat post with less setback).

My usual tip for the 'correct' reach to the bars is that the tops of the handlebar obscures the front axle.  But obviously that depends on how long your arms are compared to your body, etc... all things are variable and a professional bike fit (when possible!) should solve all these issues.

Sit on the bike where you feel is right, then reach forward to the bars - where do your palms "naturally" land? Try It a few different times. Adjust the stem length accordingly. 

10mm isn't a big difference in stem lengths, so I'd try a 90mm one.

However, I'd be more likely to play with the saddle position first - try moving the saddle forwards and experiment with the height a bit (measure the setup first so you can easily get back to your starting setup). If you don't have much success with changing your position, then the best bet is to pay for a proper bike fit though I don't know if they're still in business at the moment.

Nat Jas Moe - I've heard those recommendations before, but although they're a handy rule of thumb, they don't take into account different body shapes (e.g. long thigh bones) and flexibility.

From what I have read

First sit on saddle, cranks at 3 oclock, a plumb line from the front of your knee cap should be in line with the pedal axil.

Now hold the drops and the tops should be in line with front axil.

Apparently.