[Closed] Curious. You're right in the middle of bike sizes....

Down, providing I can easily set saddle at my correct height.

I go smaller personally

I would check things like seat tube length to make sure I could put a long enough dropper post in. Compare reach, stack and effective top tube to other bikes I get on with, what stem length I would be using etc. Is it a bike that has relatively long top tube?

Depends what the bike is, if it would be better to have a smaller or larger size and if you can get the best position for the purpose.

I have only bought 3 bikes over the last 10 years and all were different. First one (hardtail) i sized down, over the years i felt a touch small so I sized up on the next hardtail and much preferred it. When i was getting my full suss bike I sized down again and it feels right. So I guess there is no right answer for me.

Depends on geo and your riding style. Assessing seat tube height as a measure of bike size doesn't really mean much these days.

Old skool dogma is to size down for 'chuckability'.

New skool thinking is to choose a bike based on reach (in the main), which often means you'll fit on at least two sizes because the seat tube is so short its irrelevant.

Road I go down generally, mtb I go up generally. But it depends.

Compare to other bikes you have owned. One companies medium is another's large. Look at the difference between the bikes. In some it is tiny. Being long legged I'm often on the max seat post length for a medium so it's never an issue on a large. For a mtb if it is a little too long then shorter stem isn't an issue. For a road bike I find the opposite, a little too short is better as long stems work well.

The general rule used to be that it was easy to size a bike up than down. Stem length, bar rise, long, layback seatpost. The opposite, making bike smaller is harder.

I can ride a bike that's a size too small. I can't ride one that's too big. Dropper posts help a lot, that's why reach has increased so much recently, but you still need to be able to ride it seated so a top tube that's too long will leave you having to stretch too much to reach the bars.

In that situation my first consideration would be can I fit a minimum 180 dropper in there?
If yes on both I'd go bigger.
Kind of depends on what kind of bike it is and what you're going to be doing with it though.

Very much depends on the bike.

I much prefer too small too big, I would definitely go down a size

I went from a large Capra to a ML Bird AM9. The Capra was in-between the M and ML AM9s so I sized up to the ML but I do sometimes wonder if I'd have been better going with M AM9 instead. I think you can fall into the trap of constantly sizing up with each bike and after a few bikes you're on one that's maybe too big.

Compare them against your current bike and how that fits.

“ Compare them against your current bike and how that fits.”

This! Don’t look at reach on its own, stack matters too. ETT as well - a few days ago I hopped on a mate’s new Privateer 141 which is slighter longer reach and similar stack vs both my bikes and it felt way too small because the seat tube angle is so steep that the ETT is much shorter.

for me slightly too large rather than slightly to small but it really does not seem very important. One of the bikes I ride regularly is a bit small, one a bit large.

I’d say it’s a choice between fun and fast. Smaller is more fun larger is faster downhill generally. I think reach is the key measurement. I like 450mm

Depends what the bike is, if it would be better to have a smaller or larger size and if you can get the best position for the purpose.

This. For some purposes I'd go down a size, for some I'd be happy to go up.

However, it's also possible that the particular model you are looking at just won't be right in either size and that you should look at something else that actually fits.

I’ve gone down to Med on both of my current bikes, means I have to have a slightly longer stem than is on trend. The bikes did feel a bit short at first, but you get used to it pretty quickly.

I'm probably odd, at 5'10" I tend to be on the boundary between 'Medium' and 'Large' depending on the make.

For road and gravel, I've tended towards going up, but being prepared to run a shorter than standard stem.

On MTBs I am probably happier sizing down.

The main things I seem to notice are front centre and wheelbase length, too short on a road/gravel bike and I feel cramped, too long on an MTB any I find it barge like, especially since geometry started getting LLSer.

My full suss I went up (I had demoed it though). I then had a panic as I took it out for a ride and it felt awful, achy neck, shoulders and I thought I’d made an expensive mistake.

Put a shorter stem on, rode it more and got used to it and not it’s perfect. My wife had the medium and whenever I sit on that it feels tiny, far too small for me to want to ride any distance.

Depends on body shape too.
My height very much puts me in most companies medium sizes, however my height is mostly in my upper body so feel better on a large as long as I can get it with a short enough seat post and run a shorter dropper than most.

Bought a new hardtail earlier this year unseen and untested and went with figures similar to my full sus that I got after trying loads, just needed to be careful which measurements I compared due to squat and stuff. Feels about right and hopefully gives some consistency between bike cockpits.

New skool thinking is to choose a bike based on reach (in the main), which often means you’ll fit on at least two sizes because the seat tube is so short its irrelevant.

Something like this although more than just reach. And you could find two manufacturers making frames of pretty similar geometry where the sizing chart linking them to your height differs in quite a big way. I did. So I wouldn't buy anything without pouring over geometry a fair bit.

My current two frames (Bird AM160 and Pipedream Moxie) are both "sized down" a bit I suppose, but then they're both known to be long frames, so a big pinch of salt there. 10+ years ago I sized up for the length but had to look for something with good standover, but I think that constraint ceased to apply some years ago.

I go up as long as the standover and seat tube are low enough

Depends on the bike but as long as I fit, and can get a proper seatpost in it, I'd go up. (my main bike is 2 sizes up on recommended, just because it's old)

Up, for sure, as long as I can still get a good 170mm dropper to fit.

I’m probably odd, at 5’10” I tend to be on the boundary between ‘Medium’ and ‘Large’ depending on the make.

For road and gravel, I’ve tended towards going up, but being prepared to run a shorter than standard stem.

On MTBs I am probably happier sizing down.

I'm 5'10" and the opposite.

Down a size on road bike, so 52/54/medium depending what it is and likely a 110mm stem with 400mm bars.

Larger end on an MTB so a large, then something like 800mm bars on 40/50mm stem (full susser with relatively modern geo).

I did however try a large Whyte full susser in my last bike search and it was like steering the Ever Given. Ended up with a different brand.

Up for me. I have short legs and long body for my height, so need the extra reach.
As stated earlier, beware of reach on its own. I have two bikes with the same reach, but one has a steeper seat tube and feels more cramped when pedalling than the other one. It's an ebike though, so doesn't affect climbing performance and once out of the saddle going down it feels amazing.

5’ 9’ and depending on brand could be S,M or even L (if Genesis) but usually M in MTB, yet wishing for just a little more readh.

55/56 road

Regular/leg, longish body, arm short as per velociraptor forelimb.

Having spent years sizing down because they look better I've come to realise that 'up' is actually the right answer for me.

I've always been in-between large and XL and have had L that seem to fit, L that have been too small and XL that have been too big but since stems went short and bars wide I've settled on XL as even the L bikes that have felt spot on have been absolutely on the upper limit of all the settings where as the XL's can usually be tweaked to suit. Long seat posts and short stems give little room for manoeuvre.

Road; down
MTB; up

I like a small road frame and long stem. For mtb I like the longer wheelbase and a shorter stem. But I’m absolutely average anyway so it’s not really an issue. I did Run a Small road bike with 130 stem to test the theory for a winter. It was just fine.

Lets also say you cant get a demo ridet so have to take an online punt, or you’ve demo’d both sizes and they both work

Those are quite different circumstances. The former has a risk of buying an unsuitable bike, the latter is just getting the best of two good options.

I'd size down be default, but look at the geometry numbers.

When I replaced my 26" though, things had moved on so much that numbers were incomparable so I hired bikes of similar modern geometry/sizes from trail centres.

In the olden days it was always a rule to size down, mainly due to standover increases being an issue, but nowadays with the lack of any standover issues on most frames it's really up to what you are after, more reach, or less, again depending on bike, reality is you can fiddle with a few other things, such as stem length, bar width, etc to fine tune it whatever way you go.

As someone else pointed out, biggest difference could be the issues such as shorter seatpost insertion on smaller bikes, wish i'd worked that one out before sizing down for my ebike, biggest annoyance for me with it, the seatpost not being able to drop any further and have 60 or 70mm sticking out still.

I'm in the overlap between L and XL - prob more into XL than L. For my last two bikes (hard tail and full suss) I tried both sizes. L felt ok until I got on XL which was clearly the right fit for me.

So up for me.

I'd look for a different model where I wasn't in-between sizes.

But if you held a gun to my head, I'd size up.

Yeah I find it's mostly down to body shape, short legs and long arms/torso means I'd size up. Used to have to run 54cm road bike with 130mm stem though, custom Shand is very long and low and much better.

New skool thinking is to choose a bike based on reach (in the main), which often means you’ll fit on at least two sizes because the seat tube is so short its irrelevant.

This.

Also worth bearing in mind that there's a lot of disparity between manufacturers in terms of top tube lengths. It's not unheard of that one brand's XL might be similarly-sized to another one's M in terms of reach. So if you're contemplating a brand with really 'progressive' (i.e. long) geometry, I'd perhaps size down, but for the main I would (and did) size up.

I find it’s mostly down to body shape, short legs and long arms/torso means I’d size up.

I think this is where the confusion comes from. People look at the manufacturers' recommended sizes based on height and then discover that they are more comfortable on a larger or smaller size because their body proportions are different to the average.

My take on it is that you need to find a bike that is the right length, so your torso and arm length are the main thing, not your overall height. You have to be able to pedal the bike seated, so the ETT is still important, but this is shorter for the same reach for modern bikes with steeper seat tubes. If you take an old school bike with a slack seat tube and buy the same reach as a modern bike, the older bike will have a longer top tube, so you might struggle to reach the bars seated.

So, you need to find what reach you like and make sure that the top tube isn't too long. Regardless of whether the manufacturer calls it an "M", "L", or whatever, a bike where you can't reach the bars comfortably is horrible to ride, so a bike that is a little bit too short is much easier to set up than one that's too long. This depends on your torso and arm length, not your height, so there's no point paying much attention to what other people the same height ride.

Then, having found a bike that is the right length, you need to make sure that you can fit a dropper post in that is long enough. This seems to be less of a problem now that most bikes are designed with long droppers in mind.

I'd go down. Modern bikes are usually long enough. And smaller is funner 🙂

Hmm, just reading this thread.

I ordered a Stanton Switch9er recently. At 5'11" I am right in the middle of the 2 size options.

Last time I was in this situation was with a Transition Covert. I went down a size to a medium with the same philosophy as most above, more fun and chuckaboutable. Never got on with the bike and wished I had bought a large. Sold it after just 2 years (I keep my bikes as long as I can so that's a blink of an eye for me).

Not wanting to make the same mistake I have opted for the larger Switch9er frame (yet to arrive) I did crunch some numbers and compared the reach to my current large TR Scout and I reckon with a 35mm stem the reach figures on the Stanton will be around 10 - 15mm longer. Hoping the 2 degree slacker head angle and steeper seat tube will compensate. I am hoping the leap of faith serves me well.

Weirdly, the Covert is the only bike I ever demo'd before I bought and every other bike I have bought by looking at the numbers and angles. I liken the effect that cause the error back to my DJ days. If I rode my motorcycle to the record shop I would come home with a bag full of nose bleed techno, if I took the train or walked the bag of tunes would be a lot more chilled. The demo day on the Covert was winch up then steep chute descents, it was wicked for that but not suited to the trails I normally ride.

I've had a large MTB which felt too small, then an XL that felt too big, then another couple of larges which both felt too small. Now I'm on an XL again and it feels right. I think the HA made a difference to my feelings, the bikes that were too small had 69-70 HA and I always felt too far over the front. So, I'd say for me going up was the right choice.