Modern bike sizing

I think the decision may come down to bar height. 

I'm slightly shorter than you but with crazy long ape index and have found 440 reach to be at my outer limits for and enduro or DH bike set up (longer travel fork is the big key here) as you will never run your bars super low. 

I love my small Ibis Ripley with 425 reach for trail riding - but the fork is only 140 travel so I can run my bars really low (5mm spacer with 15mm rise) and make the cockpit roomy enough. 

On my enduro and DH bikes I'm good at 435 with 10mm under the spacer and either a 20mm (enduro) or (30mm) DH bar for the increased fork travel. 

The big question is if you go shorter will you be able to go lower in the front stack to make up for it w/o it feeling like you're always getting pulled forward in steep terrain. Going to a 50mm stem could help on the small bike but that changes your steering feel significantly. 

Maybe grab a 5mm reach adjust HS and reduce your current bike to see how that feels. You can then run it in the longer position on the new small frame to bridge the gap a bit more. That's a lot cheaper than buying the new frame right away. 

In your shoes, I'd have a close look at the small. It's challenging, as you're basically between sizes. If there's any possible way you can throw a leg over a small, it would be worth it to see if the reach is more comfortable. Good luck!

At 5’9” I used to ride a large (SC nomad when it had a 460 reach).  Now riding a medium Giant Reign and find 450ish has been good for me.  Seems like some people love huge bikes and some people love smaller bikes.  Both have pros and cons, just what you prefer really.  I personally enjoy the smaller bike feel as it seems more interesting to me than riding in a large wheelbase, but to each their own!

What modern bike have done is move more of your weight through your hand (longer reach) while moving rearward your center of mass in relation to your wheels (longer front center, but similar rear end). It pushes you to put more weight through your hand to keep traction on your front wheel.

What you can do to still benefit from the modern geometry is to put a higher rise (50 to 80mm) bar to enable you to push your front wheel without behind in an uncomfortable position. Also, choosing a bike with a longer chainstay will allow more of the weight you put through your feet to weight the front wheel and relieve some of the pressure you need to put with your hand.

Does it make sense to you?

I used to basically only/mostly look at reach for sizing, but you have to be careful with overall wheelbase now that some bikes are so slack:

My previous YT capra was 465mm in reach, but "only" 1220mm wheelbase with a fairly standard 64 head angle, 160mm fork, 440 rear end. I had an ebike in similar sizing and both fit perfect.

My current bike is a Giant Reign e+, I was between sizes for reach, decided to go down to 450mm reach rather than up to 480, thank god I did! With the 62.9 head angle (flip chip in high!) coupled to a 180mm fork, and an extra 10mm of rear end, the overall wheelbase is a massive 1270mm. As you can imagine, it is stable at speed. I put a 45mm stem rather than my usual 40mm and taller bars, position feels great.

By the way I'm 5'10, quite surprised you would go all the up to frames my size to be honest.

5'6" with negative ape index and I run a small Pivot Switchblade and small Canfield Balance both with ~430 reach with a 40 and 50mm stem.  I tried the medium switchblade (~460 reach) in the parking lot and could barely even get the front off the ground.  I also rode a 430 reach older bronson and was very easy for me to ride.  So I feel pretty comfortable with this reach but haven't spent much time on ~450 to compare.  My trails a bit more tight and twisty than straight and fast. 

That said I've gone from a 395 - 415 - 435 reach over the past decade and every time I think "oh this reach gets outrageous", butI adapt and all is fine.  There is likely a tipping point, but demo'ing goes a long ways as a reach number is only part of the equation and a bike could feel a lot different.  

My current opinion (no demo thoughts) is a 430-440 reach combined with a 420-430 rear center is my sweet spot for my style which is more fun than race.

This is a great topic, and it's been something I've been pondering for a couple seasons now because I feel a size small wheelbase is now plenty long enough to provide enough stability at higher speeds.  There are fewer handling trade-offs than in the past.  I'm 5'6", so we are close in height, and Santa Cruz bikes have been my primary brand and I have been able to ride most of the models over the years.  I did have the gen 1 Megatower that I raced DH on a few times and did plenty of all-around riding with.  The ass-buzz, is one of the main reasons I stopped riding that bike.  It hurt me good one time on a small rock drop that I'll never forget.  Thought I was bleeding to say the least.  The overall size of that bike also made most of the trails I ride fairly boring because it just rolled through everything.  It was great on DH stuff, but I didn't like it for much more.  I went back to 27.5" wheels until Santa Cruz started making only mixed-wheel bikes. 

I haven't tried a small Nomad, but am also curious.  I do have a current gen medium Nomad as well as a Bullit.  I feel the sizing is great for what they are intended for, but I'd still be curious to know how a small handles.  Last season I built up a size small current gen 5010 and it's been great.  I've also spent a good amount of time on a small Heckler SL, also super fun.  Overall the small frames have been really fun to jump and throw around, yet they still have been plenty capable on DH runs and resort riding.  No major trade-offs that I'm personally aware of.  If I was going to race dh or enduro, I'd likely opt for the longer bike if given the option, but if I only had a small then that would be fine as well, because I'm not a serious racer and my skill would be more of a factor for my results.  

Essentially, I think both sizes work well, but if you want a more fun and nimble bike, then go with a small.  If you want high-speed stability and max control, then the medium is probably the way to go in my opinion.   

If you're concerned about the lower stack, try a 35mm rise bar with the small frame maybe?  I've got a 30mm rise 800mm length bar on my small 5010, an my medium Nomad has a 20mm rise 760mm length bar.  It's funny because the Nomad feels smaller than the 5010 when you first get on it.  People always think my Nomad feels small just because of the bar.  I didn't set my bikes up like this for any reason other than I was swapping parts and didn't much care at the time, but both setups work fine for me so I haven't messed with them. 

I'm 5'8" on a medium Nomad 6, and if I were 3" shorter I'd definitely size down to a small.  Sizing down will give you more room to move around on the bike.  You'll still be able to get plenty of stability by playing around with bar width, stack, and stem length.

I'm 6'1, and I like trail bikes with slightly shorter reaches, I was on a large Niner Rip 9 Rdo for a while and that had a 465mm reach, I ran a 50mm stem on it and an 800mm wide bar with 20mm rise, it was very fun and playful. I've ridden bikes with 480 and 490 mm reaches recently and they are comfy and fast, but definitely not as nimble as the niner. 490 reach, 50mm stem, 800mm 20mm riseBar, 435mm chainstay kona process x, and a NORCO Sight with 480mm reach 440 chain stay, 40 mm 3 degree rise stem, 800mm 20mm rise bar. 

184cm (6') and riding a Large Mondraker foxy XR. 2° angle set to get it to around 64° (probably just a bit slacker since it is overforked to 170). 35mm stem with 30mm spacers and a 20mm rise bar. The catch is I added about 35mm to the rear making it a 460 chainstay (Grinder and welder needed). The change that it made is awesome, feels more playfull and poppy... don't ask why.

imo the problem is (like mentioned by others already) that your CS usually is too short for the reach, moving you too much over the front to get grip. 

A 35mm increase to the CS seems massive.  They must have been way too short to begin with.  Can you share some pics of your handy work?  Now I'm just curious to see this.

Initiator was a crack at the caliper mount. Was aiming for 20 extra to start with, Don't really remember why I went all the way from the get go.

Here a link to google photos: https://photos.app.goo.gl/9scMkNTgTrZci3VG8

She aint pretty but everything is still in one piece after riding half the season on it, bikeparks and racing...

To top it of I also made my son a bike, from 2 other bikes (Vitus kids hardtail and a Radon Slide 160). That one has 510mm CS and 60° HA. reach is around 390 (11yo son). 130mm travel front and rear. He rips on it, winning a German enduro championship on it, jumping 7m doubles and stuff with ease... Photo

Point I'm trying to make is that you really benefit from a longer rear end.

Damn, nice work!  This makes sense now.  I pictured the back of the swingarm somehow being cut and re-welded, but you extended the entire thing.  Crazy.   Very cool story with your son's bike as well. You've got some skills. 

I fully agree with your point, but hearing of a 35mm increase in CS seems massive and hard to visualize.  I had a Cascade Link on a Bronson, which increases the CS by around 5mm, and I thought I was gonna hate that.  Turns out it was fine.  

interesting topic. not specific to anything but more on the industry in general: IMO the reach increases have gotten kind of crazy. having said that i'm burdened personally with short arms and legs and a longer relatively speaking torso. at 5'8.5 the mediums are my sweet spot, maybe 445-465 at most and to be honest more like 455 is my preferred limit, but it seems like medium reaches creeping to 470+. i know we all have diffrent experience of the world, but i feel like bikes are made for 5'11-6'2 people with 32" inseams and 34" sleeves.

I'm 5'-10" and have always felt more comfortable on reaches between 440 and 460.  That used to mean large frames a while back but now I'm on mediums (or S2's).  Most manufacturers would recommend a large or S3 for my height but the smaller frames are so much more fun to ride.  On modern large frames I always feel like the bike is taking me for a ride.

At 5”4 I rode a medium mega tower and I struggled with it for 2 years and then I bought a medium bronson v4 on the recommendation of a friend well I loved it , but I recently snapped it and after sitting on a small in the shop I ordered a small crash replacement ( I had to wait 3 months for it to turn up)  but it’s the best thing that could have happened it’s so much more fun I’m less of a passenger I feels like I’m more in control of where the bike goes  

Before seat angles became 5-6 degrees steeper (or however much), I knew a lot of guys who were 5'9"-5'11" who rode mediums, but they sized up once the new geo started taking over and the cockpit became cramped in the seated climbing position.  That was my primary concern as well when sizing down to a small, but I found it pretty easy to adapt with little difference in climbing performance.  I'm also on the shorter size for a medium and the upper end of a small.

I will add that I also noticed a lot of these guys who sized up get faster on the descents, probably due to the overall improved geometry.  Longer wheelbase, slacker head angles, reach, etc.     

I'm 5' 11" with long legs and long arms (my wingspan is 3+" more than my height) and seem to do best in the 470-475mm reach range. For me, it's the overall wheelbase that is getting crazy. I had all three generations of the SC Hightower (V1 LT, V2, and V3) and the wheelbase went from 1195mm to 1231mm to 1245mm on the last one. For a do-it-all trail bike I'm reaching the upper limit of what's manageable. The trails closest to me are super janky, tight, and awkward and the change in length between V2 and V3 was really obvious in a not good way. I appreciated the stability on faster, more open trails, but for me personally the V2 wheelbase was the sweet spot.  

If large/S4 sized bikes get any longer in reach or wheelbase, I will have to look at sizing down next time around. 

Slammed 50mm stem and 50mm riser bar on Small Nomad.6 frame?--static stack is conserved, and reach maximized. The longer stem, lower leverage point, and longer rear center will allow your hips to stay lower when you're actively weighting the big front wheel. Your info points to the 1240mm bike being the experimental size rather than the 1210

I prefer modern sizing with full suspension bikes and older geo numbers with hard tails.  Hardtails I tend to ride over/behind the back wheel when descending while full suspension bikes I ride pretty centered.

A little off topic, but does anyone here ever they could try a modern geometry numbers with 26" wheels?  Being a shorter guy, I'd love to give it a shot. 

I'm 5'9" and currently riding a medium Norco Sight, although Norco's Ride Aligned settings suggest that I should be on a large. The large feels uncomfortably big, and when I'm descending, the medium feels perfect and manageable.

I went the opposite direction. For last 25 years i was lucky to change bike every season or two and while i was on mediums in the past (old geos had simply too long seat tubes to ride anything larger comfortably), currently (at 5’9.7” = 177cm) im on L frames or S4 (except the latest Fuel EX where i picked M/L).

I definitely prefer longer bikes for stability and active riding position. Ideally reach of 470, wheelbase at least 1235 and front center 800+. 
 

I do believe you can tame a bike that might be a bit bigger but you can’t comfortably tame bike that is too small for you.

Respectfully disagree. Here's a pic of 6'4" Steve Peat railing the piss out of a Sea Otter turn on a cartoonishly short 2007 V10 size "large," reach length probably around 435mm. The crazy part is that he even looks comfy on it. Now, whether or not you can go marginally faster on a marginally longer bike is a worthy debate, but you can most certainly tame a short bike. The first 35 years of MTB racing is evidence of that.

Since his elbow literally touches his knee in a pretty casual turn, I'm pretty sure we can all agree something is wrong there.

To properly steer a bike, you need to be able to move freely your body.

He seems to be making the point that current bikes are in-fact bigger and arguably faster, yet the tallest of men were still killing it on tiny bikes for many, many years before we got to where we are now.  Steve needed a bigger bike but he didn't have one, so he made due and won world cups on frames that were not big enough for him.  Many of us were sizing up back then.  Others here are stating that current geo now allows us the possibility of sizing down if ones prefers to do so.  Wheelbases have increased to the point where the bike Steve is on in that picture are nearly the same length as a current size small or medium Nomad.  I don't think that's too much of an exaggeration. 

But he is riding what was available at that time and most likely what was the largest geo imaginable for those time standards. 

The need for larger - longer bike came up from something. If not, we would not have this debate as we all would have been riding tiny bikes thinking that’s how it should be.

And that seems like its not how it went.

My original point was that i personally feel you can spread across larger bike even if too big to some decent outcome but you cant fit well into a tiny bike as your body weight distribution will be too off in certain situations. However I agree a lot of pros are showing that through adaption, you can tame small bikes for sure.

I guess it all comes to probably personal preferences, the usage and overall skills.

So I definitely see your point and makes sense.