Do they have as much affect on handling as a mountain bike stem does? Adding 20mm to my mountain bike stem would ruin the ride. Is it the same on a road bike or is it just whatever you need to get the reach right? What happens if it's too short/long? Does the fact that I load the rear end (panniers) make a difference?

Just trying to work out what frame size I want for my next commuter and the two sizes are either side of what I ride now.

just whatever you need to get the reach right

In my opinion..

Don't forget that with drops you can also adjust the reach by choosing compact bars or 'normal' bars.

[Pedantry]
You mean effect
[/Pedantry]

I would say it's more important on a road bike as you tend to move about less so your position needs to be spot on to keep comfortable for long periods. I know it gets trotted out all the time on here, but if you're spending a lot of cash it's well worth doing some decent test rides if possible.

I realise a test ride would be the best thing but it's a Tripster frame I'm looking at and I don't know of anywhere that has one built up. I run a 90mm stem at the moment. So go with the smaller frame and a slightly longer stem isn't going to upset anything?

A 'normal' length road stem would be between 100-130mm, unless your frame is very small. I actually use a 140mm stem, as this allows me to go down a frame size (56cm instead of 58cm), and haven't noticed any handling difference.

When you're sizing a road frame, look at reach (top tube length plus stem length) and headtube length: I went for a smaller frame plus longer stem so that I could have a lower front end.

Crikey, although compact bars may have a slightly reduced reach compared to standard bars, the difference is primarily in the depth of the drops (compacts are not as deep).

If you look at road stems, they are typically quite a bit longer than their MTB equivalents – e.g. 120 isn't at all unusual.

However, you can't really compare any road frame to an MTB frame, so I'd avoid using your current MTB as a starting point.

I changed my stem down 20mm handling the same but comfort loads better. I took my bike to a local roadie specific shop, we put the bike on the rollers and the guy measured me and looked at my postion then changed my stem, made a real difference in comfort but ime road bike don't really do handling not as if you swerve in and out of trees on a ride. Maybe different if you commute though and swerve inbetween traffic?

Well, I beg to differ to most of the above in that when I swapped out to a smaller but otherwise identical road frame, with a slightly longer stem than before, handling particularly on fast feisty decent was much more stable with better turn in to corners. I put this down to more weight over the front wheel. On my ss road bike, Ive just gone down 10mm from 110 to 100mm and the steering is noticeably quicker on the turn in, but less stable mid corner.

So yeh, I think the subtle relationship between frame size and stem size & bar reach has a very noticeable effect.

well, i'm more than happy with the reach I've got on my current bike. I just want to replicate it on a new frame. It's looking like the smaller frame with a longer stem should be the way forward rather than the longer frame with an even shorter stem (seems my 90mm is on the short side already).

even shorter stem (seems my 90mm is on the short side already).

Yes it is, unless you are very small 90mm is very short for a road bike.

Ah, KT, I remember the days of you starting out as a roadie, asking about fitting a 90mm stem to improve your road bike's handling when going round small roundabouts 😉

As above, because you 'steer' a road bike much less than an mtb, stem length is much less critical – I've got a 140mm on my one and it handles brilliantly – Most confidence inspiring bike that I've had and I've had a lot…

(Caveat, I'm tall so a 140mm stem is relatively less long than it might sound).

Ecco, the difference in reach using normal and compact bars is significant; compact bars allow a 20 mm longer stem for the same reach to the hoods..
I know because I've done it.

Was easier in the days before people came in three sizes.
I wonder how dearer bikes would be if the came in sizes like they used to, instead of us having to buy the wrong size and make it fit?

My humble opinion is that it's easier to build up a frame/bike than build it down.

Ah, KT, I remember the days of you starting out as a roadie, asking about fitting a 90mm stem to improve your road bike's handling when going round small roundabouts

Indeed, which puts me in a good position to offer advice.

Ecco, the difference in reach using normal and compact bars is significant; compact bars allow a 20 mm longer stem for the same reach to the hoods..

Eh?

Shorter Stem-twitchyness increases and so does comfort for some.

Longer stem, smooth handling but slower turn in and allows aero sitting posture-not great for everyone.

Best getting the right size frame and using an average stem 100-110 but it varies for everyone like shoes.

Agree the reach on some factory Giant bars I had was so huge it put the hoods almost out of reach. Whilst the hoods on my carbon wing bars start at the curve on the tops.
Had pics somewhere as it was so mad.

I was trying to find a picture, but it works like this;

Reach is measured from your seatpost or your saddle tip to the hoods.
It is made up of; toptube, stem, reach of handlebars, hoods.
The reach of handlebars describes how far forwards they go from the tops (the bit that clamps into your stem) to where the hoods are fitted.
Compact bars have a shorter reach, allowing you to run a longer stem for the same distance to the hoods.

That's it, on my compact bars the 82mm measurement is actually 90mm, whereas on my standard bars, its 120mm, allowing a longer stem but not moving the bit I hold onto further away.

Ah I see, I never knew that, I always thought compact refereed to the drop not the reach as well.

Yeah mine are 80mm reach and 125 drop. But if you can imagine my hoods start paralel with where that guys knuckles are.

I find you're in less situations where you'd notice a shorter stem, but find it's the angle of the stem that makes the biggest difference. On my bike I've swapped from a 110 7 degree drop to a 90mm 17 degree drop and the drop has made the biggest improvement to handling- it's a lot better being lower.

Anywhere between 90-130 is fine.

Hmmm, so did I, then I had a chat in my LBS and did a few measurements and realised I could use an oversized stem that I had lying about with a compact bar, and get the same reach as my normal setup. The advantage was a slightly stiffer bar/stem combo and a shallower drop with the same reach.
Longer stems also tend to look a bit better on roadbikes.

It's a useful tip if you are concerned about reach to the hoods; gives you 20-30mm of adjustment depending on the setup you have and the bars you choose.

Deffo prefer the longer stem as it puts the whole bar further over the wheel, which is for me anyway nicer when youre racing on the tops.

I'm no roadie, but I do have a rpad bike and ride it now and then. I swapped the 100mm stem for a 70mm to give a more relaxed riding position and it feels much better. Mind you, 'proper' roadies probably pee themselves when they see it. That and the triple chainset, mtb pedals and helmet peak. Probably why they never speak …..