Tweener: make a small bike big, or a big bike small

I'm 176cm metric, 5'9 freedom units,  and for my last two bikes i went up from M to L.

For me the biggest difference was the first ride, as reach was about 30mm longer, but after that, i never missed the M bikes. I also run 40mm rise bars at 760mm width. 

A lof of the sizing is what you re used to, but for me who rides all kind of trials, sizing up was the right thing to do.

Have you tried going with a slightly wider handlebar or even a bit more forward roll?

My enduro bike was 465mm, I think this is the best fit for me at 178cm/5'10. Similarly, to you I have trouble finding bked with this reach.

My last ebike was 455 reach, but very slack headangle (62.8), so I was able to run a 45mm stem rather than my usual 40mm with no change in steering feeling. Went up to 40mm riser bars also with barely any spacers, which as above will give you the ability to play with a few extra mm using bar roll.

My new ebike is 475mm reach, but it overforked by 20mm which reduces the reach a bit, and I'm using loads of spacers with a flatish bar which again gets my bars in forward position more similar to a 465mm with standard +20mm bar.

This is an awesome topic!

Approaching this from a generalized perspective, things to consider are where you live, style of trails you ride mostly, and what travel category bike you're referring to. I think on one hand longer travel bikes are easier to make a larger size feel smaller in the cockpit/rider area due to more sag and travel. But, typically already have a longer wheelbase that can feel a bit more cumbersome from a maneuverability standpoint. Whereas shorter travel bikes tend to feel longer in the cockpit/rider area as the geo doesn't change as much due to having less travel. And the wheelbases are typically shorter that can make a larger size feel more maneuverable. 

Bar roll can make up quite a bit of reach differential, especially as you consider taller bar height. Also, if you ride clips, you can cheat your cleats fore/aft 10mm which fundamentally affects your reach. 

Eg: MD bike, stem mounted lower on steertube + taller bar with an upright mounted position + cleats cheated forward10mm - vs - LG bike, stem with a few more spacers under it + shorter bar SLIGHTLY rolled back + cleats cheated 10mm rearward can almost make up a frame size difference. Add in a +- 5mm stem length and you have a lot to play with. Next consideration would be wheelbase differences related to style of trails you prefer to ride (Open and fast, vs tight and tech).

-Lars

I'm 5'10", so in a similar situation to you. Up until about about six months ago I've spent my entire riding career on size mediums. Two years ago I built a sweet Transition Spire size M (460 reach) and rode that for a season and a half. Well I cracked the front triangle and decided to go to a size Large (485 reach). Long story short, I absolutely love it, and I have no plans to go back to size mediums. 

The large feels noticeably more stable, especially through chunk, but I can still throw it around really well. I didn't notice a major difference in playfulness going to the large, maybe some moves require slightly more input but that's about it. The biggest difference is that it feels like I have a lot more margin for error on the large. With the medium, it felt at times like a slight shift in my body position would cause a big difference, especially in corners. With the large, I feel like I have more freedom to move my body around without upsetting the bike. Overall it just feels more natural.

If you're curious, I run 760 bars at 40mm rise, a 45mm stem, and 5mm of spacers under the stem. I ran this setup on both the medium and the large; I didn't change a single thing. I want to experiment with higher bars, but unfortunately I cut my steerer too short and don't want to pay for higher rise bars. 

I'm 6'1 (185cm) with a 32" inseam, so long torso and a 6'3ish arm span. I always used to ride Larges, 460-485mm reach depending on the year and bike, but I'm going to be going to XLs for my next bike, 490mm+. With how I've been raising up my stack height, I've started to feel cramped on a lot of bikes, like when I get down into my ready position in the bike, I'm curling up into a ball instead of getting ready to take impacts. The few bikes that I've ridden with 490mm+ reaches have felt way more comfortable to me and like I'm in a good position to ride aggressively. Currently on an over forked large Norco Sight Gen 4, so a 480mm reach, I've got about 35ish mm of spacers under my stem, a 50mm stem, and a 780 wide 38mm rise bar. That being said, I'm hesitant to go up to XL for bikes that are 140mm of travel or less, since I'll set it up differently from my big travel race bikes. 

I just built up a hardtail where I sized down. I wanted maneuverable and playful. It won't hit chunk at high speed, so it's been all smiles. 

My squishy 5010 is much longer, and it feels much better at speed in the rough, but semi-playful and stable. The previous 5010 I had was shorter. And it was much more playful on trails, like everything was a stunt. So fun. I could still go fast in the rough, but a soon as I started to get tired, it became a handful at speed.

I've hit 55mph on a short-ass road bike on pavement.

All of these words to say, choose length for your riding style and where you ride and what makes a ride fun for you.

Reach is an important number when considering a bike fit. I decided to go from 460mm to a bike with 478mm reach with the intention of gaining a little bit of stability.  The only other change in setup was that I went from a 50mm stem to 40mm in order to make it not feel so big. I am 5'9".

From day one I had an issue with front end grip with this larger size. I tried different stem spacers, from 25mm to 38mm rise bars, and then finally put on a 50mm stem to put more weight over the front. The longer stem certainly helped the most, but lacked front end grip and confidence in comparison to the previous bike. The two bikes were same model of different years, but had similar geometry to each other. Keep in mind that chain stays do not grow much with this particular bike when going up in frame size.

For me, I will go back to a reach of approx 460mm. I found my current limit in relation to trail bike geometry as it is now. There are more upsides to the "shorter" frame reach for my riding.

Go large if it works. Feel free to jam the saddle forward on the rails if the reach feels too stretched out on climbs. Most bikes these days have enough clearance [via dropper and top tube] that unless you have particularly short legs you'll still be able to move the bike underneath you. For reference, I'm 5'10".

Depends on the bike. I could go L or XL. XC bikes seem to fit me better in XL. Longer travel/slacker bikes seem to fit me better in L. 

(5'11 with short arms and long 35" legs)

Depending on the head angle some headset spacers can make quite a difference as well. I don't think you can talk about reach with out discussing stack. I had a V1 ripmo and really struggled to weight the front end. It gave me a real aversion to the 470-480 reach range  and I spent a few years trying to find "small" larges.  I'm now on a Patrol and I initially planned to put a -ve offset headset in it to reign in the reach but I've not needed to. I think a slightly higher stack and a couple of headset spacers have brought the bars up and back enough that I don't struggle with front end grip.

The way most companies scale their geo numbers with larger sizes these days, I would have to say it's better to go smaller.  Hopefully soon these big name companies will start making size specific geometry for reach and chain stay length on every size sadly only a few companies are doing it these days (forbidden, frameworks, banshee, etc.) but it will catch on and then they will change the wheel sizes and we'll have to start all over again...

6"1 here, 35" inseam. I ride large with a long stem. Feels like a bigger bike but don't have to deal with an XL wheelbase on the tight and technical trails I have here.

One thing that i don't think has been mentioned is that a longer chainstay can help make a bike feel smaller if you've sized up, in certain ways. If you don't need to lean forward onto your bars as much to get the right amount of weight onto the front wheel, then you can get away with a shorter stem/more rearward hand position, and your neutral position in terms of where you feel strong and comfortable will be better matched to where the weight distribution of the bike is supposed to be.

If you're in between sizes for a bike that has chainstay adjustment, that can shift the decision on size to be more of a "do I want my bike to be more stable, or more agile?" rather than "do I want my bike to be too big, or too small?" And if you're looking at a couple different bikes with similar overall geo but one has a longer stay, it might just feel smaller... It really depends on how you ride and what trails you ride. 

I wish more Brands did an Extra Medium like Ibis. Myself and most other 5'9"-5'11" people I know go between M/L depending on brand or bike feel, and from the one time I rode an Extra Medium at an ibis demo I felt it solved the "which size" dilemma. 

I'm between a small and a medium, with relatively short arms and legs. I generally size down to a small for my enduro / all-mountain bikes, since I like riding slower techy stuff. But when I go to bike parks, the bike starts feel a bit small on the fast open flowy trails. My wife is the same height as me, but with longer legs and arms, and she definitely fits better on a medium. So ape index definitely makes a difference, as does riding style. I guess that means you get the standard answer of "it depends"

I'm 5'9" with long-ish arms and legs.  Rode mediums for years, but have been on larges for the last 5 or so.  Currently on a large Raaw Madonna and feel like it's the "right" size.  Getting on a medium feels small and twitchy.  Somewhere around a 475-480 reach, 510-515 effective top tube, and 445-450 chainstay feels good for me.

As much as anything, I think trails have gotten straighter and flowier over the years.  20 years ago, lots of trails were slow, tight, and janky a maneuverable bike was an asset.  Now most bike oriented trails are considerably faster and there's a berm or a rut on a majority of corners.  Even the techy, janky stuff usually has a pretty fast line through it.  Which means it's easier to get away with a bigger bike; for most of what I ride, I'd rather have some extra stability over a smaller, more maneuverable bike that's better at very low speeds.  

I try to size down wherever I can. Steeper seat tube angles have made that difficult. I’m solidly XL in torso and arms but with L sized femurs. 

It depends on your proportions.  You really can't just do it based on height.  Example:  I'm 5'9" with short legs.  My torso and arms are long, so I have to fit a bike to that.  I can ride M bikes just fine, but I have to use a considerably longer stem to not feel cramped.  L just fits better for my upper body. 

At 5'9" I have similar proportions: long-ish legs, short-ish torso with wide hips and shoulders, and I am cursed with T-rex arms. I prefer to size down, ride with a 45mm stem + 800/35mm raiser bar, and I also try to cheat the geo with Outlier Pendulums that increase the virtual stack a bit. When I switch bikes with my taller buddies, their large frames just don't appeal to me.

One of my friends has t-rex arms and ended up getting professional help with his on-bike posture/setup/fit. He ended up on a wild Dak-style setup with ~80mm risers on his enduro bike and said it was like night and day, didn't realise how unbalanced he was or how limited his movement on the bike was.

I really don't like how short some manufacturers cut their steerers. IMO they should always come with the maximum amount of stack between the headset and stem as dictated by the fork manufacturer. 

5'10 here.  I go with whatever is closer to a 465mm reach and that seems to be ideal for me.  I prefer to err on the bigger side and think I could go up 475mm with a shorter stem or maybe narrower bars.  So your answer, I think, lies in reach, stem length, and bar width.

Over the past several years i've had a 2018 YT Capra Large (465mm  reach) with 40mm stem, 2021 Specialized Enduro S3 (464mm reach) with 50mm stem felt good.  I have a Specialized Demo s3 where the reach is 446 and I have a 50mm stem.  That one is quite small, but I got a screaming deal on it a couple years ago.  It works and i'm faster on it than I was on my Enduro.  Its playful, but I really want an s4 with a 466mm reach.  Then I got a YT Decoy in XL that has a 469mm reach.  It does feel a bit bigger but its manageable with a 32mm stem.  The enduro did start to feel small after riding the Decoy for most of last year.  I did couple rides on a Large Evil Wreckoning. It felt too long with a 50mm stem and much better with a 40mm stem.

5' 10" here, short legs (30" inseam pants, longer torso)- on a S4 Stumpy EVO (470-475 reach depending on settings).  I'd prefer 460 reach, but it was either S3 445 or S4 475.

Running seat slammed forward with slightly modified clamps and 35mm stem and lots of spacers.  Need a few more rides on this setup to see if I have enough front end grip but would probably be fine with a 40 or 50mm stem.

My current (1st gen) wife is a S with a short reach. I’m considering updating to something newer and longer with updated geometry as well. However, I’m concerned about the price tag and new tech requirements despite the improved (and quieter) ride quality. FWIW I’m 47 years old and 198cm

Just chiming in to say my body is perfect in every way. 

SHORTER. ALWAYS SHORTER. CASE CLOSED.

I was presented a similar opportunity- I rode a size large 2018 sentinel for years, and when I got it I thought “Finally! A bike that fits me!” Last year i snagged an xl spire and it feels great. 

I have a pretty positive ape index, and longer torsos relative to my legs. I think that’s what makes a longer reach and eff top tube of the spire work for me, even though I’m in between large and xl on their size charts. 

Since I've been reading all the comments, I wanted to clarify that there's a world of difference between these two genres of responses that keep popping up in this thread:

"Previously I'd never ventured beyond the manufacturer's recommended size for my height, but then I tried sizing up and it's my preferred frame size now". 

vs.

"I've tried many different frame sizes, including bikes that are bigger and smaller than recommended, and I'm pretty confident that I know what my preferred size is. However, I'm stuck choosing between two sizes on a bike I'm looking at, one that I know is slightly too big and one that's slightly too small."

If you're in the first camp, by all means try different sized bikes and see what you like. After rereading the OP, I think he's in camp #1. There are legitimate reasons why you might prefer a larger or smaller than usual bike fit, and you'll never know unless you try. Bonus points for demo days or borrowing a friend's bike instead of dropping $5000+ on a bike and not liking the fit.

However, if you're in the second camp and you already know your preferred size, but you're stuck choosing between porridge that's too hot or too cold, I one hundred percent recommend sizing down as opposed to up. I'm in camp #2, and it's hard for me to ever feel fully relaxed and comfortable in technical situations when I feel like I'm too stretched out on a bike. The only time the "too big" bike feels good is when I'm going in a straight line, which is already the easiest part of mountain bike riding. On the smaller bike it feels like I'm 2% less comfortable going fast in a straight line and 50% more comfortable everywhere else.

6'1" - 36" inseam. Pretty much always going for 475 - 485 Max. Currently on a S4 Stevo, 40mm stem 30mm rise bars at 775mm. 240 OnUp dropper slammed with a drop best saddle clamp. Fit is spot on imo. Had an XL Norco Range at 510mm, thing was wicked good on fast straight trails, but pretty much a turd everywhere else. I lean more towards the playful side of riding over outright stability/speed.