Trying to go fast, riding steeper and faster trails - Large

More about having fun, jumping out really tight trails - Medium

 

This ⬆ ⬆ ⬆ is good advice.
If you know which type of riding you favor &/or whether your fit preferences lean toward smaller or larger frames, you've got your answer.
Personally I prefer to size up but I believe I'm in the minority on this.
The good news is you could probably flip a coin and once you've had the bike for a few rides & get used to it, you'll likely be happy with it regardless.
Being in the middle isn't so bad -- it usually means you're good to go in either direction.
=sParty

 

I’d also say that your body dimensions other than just height play into the determination as well. If you have a long torso and short arms and legs, a medium would probably work better. Long arms and legs, large would probably be better.

Is it possible to look at the geometry of the Polygon and see if any bikes at your LBS come close to the same numbers? It wouldn’t be a perfect way to compare sizes, but it may get you close enough for you to make a more informed decision. You also might find that you really like one of their bikes in the process. IMO, it’s often better to go with what you can demo until you determine what geometry you like, then move on to a direct-to-consumer brand instead of taking a shot in the dark on what you might like based on what people that you don’t know on the internet say.

For what it’s worth, I’m 5’9.5” with average torso and arm/leg lengths and went medium on my current bike, thinking along the lines of it being easier to maneuver over being at the bottom of the large size range. I’ve spent a lot of time changing stems, bars, adjusting bar and seat positions because the cockpit feels cramped to me. I have made a big improvement after multiple changes, but I still feel like I have a very small area to adjust my body position while riding. I wish I had gone with a large.

 

I’d also say that your body dimensions other than just height play into the determination as well. If you have a long torso and short arms and legs, a medium would probably work better. Long arms and legs, large would probably be better.

Is it possible to look at the geometry of the Polygon and see if any bikes at your LBS come close to the same numbers? It wouldn’t be a perfect way to compare sizes, but it may get you close enough for you to make a more informed decision. You also might find that you really like one of their bikes in the process. IMO, it’s often better to go with what you can demo until you determine what geometry you like, then move on to a direct-to-consumer brand instead of taking a shot in the dark on what you might like based on what people that you don’t know on the internet say.

For what it’s worth, I’m 5’9.5” with average torso and arm/leg lengths and went medium on my current bike, thinking along the lines of it being easier to maneuver over being at the bottom of the large size range. I’ve spent a lot of time changing stems, bars, adjusting bar and seat positions because the cockpit feels cramped to me. I have made a big improvement after multiple changes, but I still feel like I have a very small area to adjust my body position while riding. I wish I had gone with a large.

You just contradicted yourself in your own post.

The length of the bike is the least adjustable part of it, so get that right first. If you have a long torso and/or long arms, you are going to feel a more comfortable on a long bike. So size up. If you have a short torso and/or short arms, then sizing down is going to give you more range of motion.

If standover clearance becomes a problem for a bike that's the correct length, then you should look at a different frame that has better standover clearance for your size, instead of sizing down, because that's going to cause different problems with a cramped cockpit.

standover clearance should never be a determinant of which frame size you should ride. If standover clearance is a priority for you, then use it to filter different bikes that DO fit in the important metrics.

 

I'm 5'10".

This is me thinking about how buying a modern large (because everybody thinks that's what a rider my size should ride) ended up working out:

 

The smaller size is most definitely going to be more playful as long as the cockpit fits you.

 

I am 5'9 as well and fall in between sizes too. What has worked for me in the past was to go with the medium frame bike and size the cranks down by 5mm. This change will give you more room by pushing you up and back just a little bit. Also, you can try going with a stem that is 10mm longer. The crank change will make the greatest difference.

My current bike is a 2021 Stumpjumper Evo size s4 (Large). Unfortunately the S3 (medium was just too cramped and I felt that a crank swap wasn't going to make enough of a difference. I purchased the S4 size for this reason and figured that I could make some changes to shrink the cockpit reach a bit. I ended up going with a 35mm stem positioned lower on the stack to bring reach in even further then went with 770 bars with a 38mm rise to regain the stack height. The fit is much better with these changes however it still feels a tad bit stretched and not as comfortable as the medium with crank swap.

Given my experience, I would advise going with a Medium and doing the crank/stem swap as this will probably make for a more comfortable bike to ride.

 

I always go with the smaller size. Slide the seat back. Kept the stock stem instead of a shorter one like most guys do and you should be fine

 

What yzedf stated earlier is similar to how I would perceive sizing, but with a slight change. I'm 5'9". .Smaller size is playful and twitchy
and larger sizes are more for stability at speed! I, being an older rider, prefer the larger size bike as it let's me to be able to ride faster
without needing to be "on" all the time. It's more forgiving to rider input!

 

Four years ago the rule was size up. I don't think that holds any more.

 

I have two datum points for you. I am 5'8" with long torso and short legs (Wear 29 inseam pants). I have a Medium 27.5 D7. I would say it fits but it's closer to being too small than too big. IIRC 29er D7 is a little bigger.

I rode a Large size T7. It felt too big for me mostly in the vertical direction. 170mm dropper was way too long. I thought the stack felt way too tall but some people like the big stack like they are sitting up straight on a beach cruiser. Doesn't feel as effective boosting on jumps since I don't get maximum leverage and extension at full boost. Length was OK and manageable. Knowing the T7/8 is a little bigger than the D7 I might lean towards the Medium on the T7/8. I will favor a smaller bike over a larger bike 9 times out of 9 though.

 

Don't use the sizing chart, use actual numbers.

I just bought an XC bike, sizing chart had me on a medium. However, the numbers showed that the small was nearly the same size as the two medium bikes I already had that I was comfortable on. The medium was about the same size as the large frame bikes I had in the past that were too big.

I bought the small, and it fits me well. I even have the seat slide forward.

 

Depends on how that bike model is sized. Some bikes are sized small and some sized big. I go based on geo numbers but if you don't know what you're looking for I'd post this question in the Polygon sub forum and get responses from actual owners of the model you're looking at. I think using generalities about sizing up or down is a bad idea. I have a friend who is between sizes and he sizes up or down depending on the actual reach, stack, etc of the bike.

 

I also had the same experience when ordering my 2022 Trek Top Fuel 9.7. I am 5' 10.5" in height with 32" inseam and 35" sleeve length. Originally I ordered a M/L, but a couple of weeks later I tested a 2022 Trek Top Fuel 8 in M/L and L. For me the M/L was cramped and after riding it on a dirt path for about 20 min my lower back was bothering me. In contrast, the L felt more natural and was more comfortable.

If at all possible try to ride both sizes before making the purchase.

 

If at all possible try to ride both sizes before making the purchase.

That would be nice, but around here bike shops will just let you sit on a bike in the store. Asking to actually ride it, even for a couple of minutes in the parking lot - forget it.

 

I think the shop was spot on with their recommendation. At least generically speaking. But as others have said, your specific body dimensions (inseam, torso, and arm length specifically) and your style of riding could change that one way or the other. But I'd say 5'9" is pretty darn close to the spot on for a medium.

The Medium T8 has 460mm of reach, and 607mm of stack.

Anecdotally, I have a buddy who is 5'8" and rides a 2018 Kona Process 153 in size medium... That bike has 450mm of reach, 607mm of stack and he's actually wanting to downsize to a smaller bike lately.

 

I kinda go with the theory that its easier to make a large bike 'smaller' to fit you (stem, seatpost etc), than to make a small bike 'larger' to fit you (stem, seatpost etc)' without messing up so much the intended 'geometry/balance'?, so in your position id take the large and be prepared to change a couple of parts if i really could not get along with it

 

That's assuming the larger bike isn't actually too big, which sometimes they are if just going off the mfg's site.

Example: I just bought an Ibis Ripmo AF a couple weeks ago. By the size chart, I'm between a LG and XL, when in reality the LG was actually a little big for me. The large I bought actually came with a 10mm shorter stem than it was supposed to, which ended up being perfect. I could have made the XL work, but the LG with the shorter stem fit way better.

Bike sizing can get kind of confusing. It's better to know real measurements that work for you and match those to the bike than rely on the sizing sticker.

 

I’m 5’8 with a 32.5” inseam (no shoes), so more legs than torso. Neutral ape index. I ride a medium T8 and it fits me very well. If you have a longer torso, I can see you potentially with a large, but with my measurements, I would not want to be on a large. For comparison sake, the T8 medium fits me better than than the intense primer medium.

I did need to size to a 200mm dropper with about 2-3 inches of seat post showing though.

 

I’m 5’8 with a 32.5” inseam (no shoes), so more legs than torso. Neutral ape index. I ride a medium T8 and it fits me very well. If you have a longer torso, I can see you potentially with a large, but with my measurements, I would not want to be on a large. For comparison sake, the T8 medium fits me better than than the intense primer medium.

I did need to size to a 200mm dropper with about 2-3 inches of seat post showing though.

Hi - I'm pretty close to your stature (5'9" with a 33" inseam) and thinking a bit about the T7/T8. Your dropper seems quite long, do you use that much? They offer a 170 in the large/xl, which I would have thought would be more than sufficient. Did you upgrade your dropper yourself, or were you able to custom order?

Did you get a 29er? (I'm thinking I want the 27.5+ if it can go 2.8 wide enough on the tires. I can't seem to find that spec).

Thanks.

 

I say depends what you like. Personally I prefer a bike a tad too small vs too big. If I was bombing downhills I might prefer the larger size but for general trail riding I think the smaller size is easier to maneuver and toss around, and prefer the lower seat/top tubes and stack height.