Overlapping frame sizes - go larger or smaller?

Interesting discussion - thanks everyone.

@sensible

80mm stem on a modern road bike is a touch too short - it can work, but it also tends to make the front end too light whether climbing, or more importantly when descending as it makes it harder to weight the front wheel for grip, you can also find yourself snagging your shorts on your saddle tip when moving from seated to standing!

90mm is probably the minimum length stem to put onto a road bike, if you need to go shorter to get the reach correct (observing proper saddle position setup so as to not compromise pedalling efficiency) then its likely the frame is too long / large for you

experienced fitters would always recommend getting the smallest frame that "fits" you as it lets you lengthen or shorten the bike fit to suit your needs

buying a bike that is one size too large, generally means you are having to shorten the bike fit to get it right, and can end up with compromised handling or even long-term problems with bike fit leading to injuries

I am 5'10" with 32" inseam and long arms and found a 56cm allez too long (after purchase) so sold it on, then got a 54cm tarmac (same geometry as allez) and its just a little long but ok with 90mm stem, I could easily ride a 52cm tarmac with 100-110 stem and ideal fit.

I've had 2 Specialized Body Geometry "fits" on the Allez and Tarmac and have played with setup including multiple stem lengths / stack height and pushing the limits of the saddle / cleat setup to address reach distance

You may not want to hear it, but I would look at other bikes as well, from what you're saying you are unsure, from my experience when I have tried bikes. I have a positive "this is the one feeling" you don't want to end up with bike that could end up left in the shed or being sold because you find it uncomfortable on rides. A decent bike shop should fit you up. my lbs did a bike fit on all my three choices. See if they will do both and see which/if either fits properly.

5' 11" 1/2 here too and am a definite M/L in Giant sizing. As for the medium feeling faster uphill, you must be incredibly in tune to be able to tell the difference. You must feel lightning after having a piss.

+1 for the Competitive Cyclist fit calculator - millimetre perfect for me.

Remember, top tube effective length is the most important measurement and the hardest to change.

I'm the opposite. 5'11" and would take the medium every time. I find the m/l slightly out of proportion and can't make the adjustments necessary to make it just so. With a medium, a few tweets and I'm sorted

I am 5'11" aand went to Giant shop to be measured, then to test ride a bike. I ended up with a M/L Giant TCR Advanced. I later changed the stem from 100mm to 80mm. I am now very happy with the fit.

If you cannot be @rsed to go and get a proper fit , then at least give www.competivecyclist.com a go and do theirs. You'll need someone to measure you up. On the face of it, I'd reckon you'd need M/L but as you say, only just so. I'm 5ft 10 1/2 and would be on the medium - just. Having too long a stem can make the steering suffer and being too low can make your back suffer. 20 mins rechecking some details on there could be worth it.

At the risk of confusing matters, I'm about 5'10", but I have a long ish back and short ish limbs (inside leg 30"). I went for the m/l for my defy, in Oct 2012. I've ridden it for a few thousand miles, including London to Paris in 2 days (86 miles day 1, 136 day 2). I've had no trouble at all, and love riding it. I'd go with the comfortable one

I'd go for the smaller one every time - stiffer, lighter. Can always put a slightly longer stem on if you find you're too scrunched up.

I'm slightly taller than you, have an inside leg 36" arm reach 24" & have the Defy, bought it last June
I tried both medium & the medium/large frame
Although the medium was better for me in terms of reach - Top tube length, I had to put the seat post right up & the bars were too low.
So I opted for M/L.
After a few attempts to get the reach right, I had a bike fit done this month, which resulted in only a few minors tweaks being made - The bars tilted up slightly & saddle moved forward to get the correct reach.

I would recommend getting a proper fitting done, the comparing it to the actual bike

When I test rode my current bike (a 46 compared to my previous 52, slightly shorter TT length 1.5 cm but also just different manufacturer sizing as Pinarello do their measurements differently than Kestrel) it felt twitchy and skittish at first, especially over cobblestone. On the second test ride and descents I realized it was just really responsive compared to what I'd been used to. I could lean hard into it and pop right back up to stay on a straight line vs my Kestrel where once I leaned over at the same angle it pretty much committed me to making a turn. Also, a bigger frame has a longer wheelbase which can give you a more stable feeling on descents which may be the sensation you were experiencing with the M/L frame. I would do a couple more test rides on both (your LBS shouldn't give you flack for wanting to feel comfortable with deciding between the two) and see if the twitchiness you feel levels off as you get used to a more responsive M frame or that you're more comfortable with the slow climbing, more stable descending M/L frame.

Personally I would always say go for the smaller size as you can always raise the seat height or get a longer stem etc. It is often more difficult to do the opposite if you have already set to minimum settings.

However, deep down you need to go with what felt most comfortable, especially taking into account your surgery. Friends have bad backs and they find smaller bikes uncomfortable as the seat is often quite a bit higher than the handle bars.

I don't really feel I have answered your question, but if it was me I would 'size down'..but you need to take into account your own circumstances and what feels right.