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I'm 5"10 with a 30" inseam - I'm looking at a couple 27.5 all mountain carbon bikes and I guess I'm in that tweener category between medium and large for my favourite choices. (Recommended M sizing is 5'8 +- 2.5 inches, and L sizing 6'0 +-2.5 inches...I'm perfectly placed to overlap, haha.) Top tube effective centre to centre is 600mm for the M, and 625mm for the L.

In riding both sizes, the M felt okay and the L felt, well, a little bigger - certainly was not too uncomfortable and I would likely get used to it quickly.

Is there any generally accepted wisdom in this instance? I have heard that the smaller frame is usually the better choice, but no idea if that is still a rough guide, or if it applies to all types of biking. Many thanks.
 

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I'm about the same size as you (5'10" and 32" inseam). I've tended to go with the medium frame BUT I now look for frames with longer top tubes. The last bike I bought also has more rise in the stem (14 degrees IIRC) to keep the handlebars ~ same height as the saddle. A good shop will do a fit and figure out what you need and if a size needs to be tweaked with a different stem. Also check crank lengths - sometimes going from a medium to a large can mean going from a 172.5 to a 175 crank arm length. That made a big difference for me. The 175 was "faster" (average speeds were higher on similar courses) but caused me knee troubles.
 

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I'm 5"10 with a 30" inseam - I'm looking at a couple 27.5 all mountain carbon bikes and I guess I'm in that tweener category between medium and large for my favourite choices. (Recommended M sizing is 5'8 +- 2.5 inches, and L sizing 6'0 +-2.5 inches...I'm perfectly placed to overlap, haha.) Top tube effective centre to centre is 600mm for the M, and 625mm for the L.

In riding both sizes, the M felt okay and the L felt, well, a little bigger - certainly was not too uncomfortable and I would likely get used to it quickly.

Is there any generally accepted wisdom in this instance? I have heard that the smaller frame is usually the better choice, but no idea if that is still a rough guide, or if it applies to all types of biking. Many thanks.
Ride the smallest frame you are comfortable on.
See if the LBS will swap stems for your test rides. 10mm longer or shorter can make a big difference in the fit without too much change in the handling.

You may also need to go with a difference brand to get the fit/feel you prefer.
 

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Ride the smallest frame you are comfortable on.
See if the LBS will swap stems for your test rides. 10mm longer or shorter can make a big difference in the fit without too much change in the handling.

You may also need to go with a difference brand to get the fit/feel you prefer.

As a tweener, I would say this ^^ is pretty spot on, but would caution that, where you really feel the smallness of the bike is when pointed downhill IMO. So keep that in mind if/when parking lot testing. Always best to trail demo, if you can.

I have walked away from bikes I really thought I wanted on paper, because the actual fit was just too much of a compromise... small as the difference between sizes may be. If I had to, I could make either work, but neither felt perfect, even with stem and/or post swaps.
 

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Brands do vary quite a bit, I am a tweener for small/medium at 5'6" with 31.5" inseam. As I am all leg and no torso I typically buy a 400mm seatpost in order to get above the minimum mark. I'll run anything effective top tube 22"-23", nothing more, nothing less, adjusting with stems. Recently I am tending to like a little longer top tube with the shorter stems. Plug your measurements into competitive cyclist and see what they recomend and compare to previous "favorite" bikes. Once you find that sweet spot (top tube length), look at brands that fit you best because they all make pretty good stuff.
 

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Fat-tired Roadie
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IMO, the real goal in sizing a frame is to get the right weight distribution and handling. Too small a frame blows on the way down. Too big sucks to climb on.

Try to ride the bikes up and down the steepest grades you can find around the shop and some curbs and stuff. Trail demo would be even better if you can do it.

Stem length can make a big difference. Too short a stem makes it hard for me to weight the front wheel, which is problematic on the way up. Too long makes the bike nosedive. I think the frame size and stem size work together, but stems come in 10 mm increments, cost $15 if you want them to, and swap out in a couple minutes.
 

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I'm the same dimensions as you 5' 10". I felt too crampt on a M so went with a L with a shorter stem (90 or 70mm) and a no-setback stem. It all comes down to how you like a bike to fit, I'm a little short on standover clearance bit I don't build a bike to stand over it, I build it to ride it and I like the fit that a large frame gives me.
 

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Is there any generally accepted wisdom in this instance?
Yes. The more space between your jewels and the top tube in the unfortunate event that you come off the seat....the better. So "generally" speaking going smaller is better.

Not being an expert I can only share with you what others "experts" have done to help educate me. It's your trunk size not your leg size that could make the difference when discussing comfort / efficiency / position on the bike.
 

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Same boat. I really think it depends on the bike. The bike and model I bought I ended up with a medium and a longer stem. Which was a SC tallboy. Funny thing is the large super light 29er felt like a better fit. Similar geometry but just the way the bike rode, it felt better to me in a large.

I haven't test ridden any 27.5, but I do know on most 26" a large feels better for me.

I also think a lot has to do with what kind of bike you are getting. HT or FS and how aggressive you like to ride.

All that said generally speaking go smaller.
 

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A smaller frame will be more playful, but a larger one is more stable and less likely to fling you OTB when going downhill.

I don't see how it would climb worse, unless the contact triangle (pedals, saddle, handlebar) was adjusted differently. Obviously I'm assuming that this triangle remains untouched (fit to the rider) so the only difference of the large frame will be longer wheelbase and taller standover height.

There are folks who say 'tweeners should go smaller and folks who say larger is the way. Personally I think there is no better choice, only a better choice for you.
 

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Fat-tired Roadie
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A smaller frame will be more playful, but a larger one is more stable and less likely to fling you OTB when going downhill.

I don't see how it would climb worse, unless the contact triangle (pedals, saddle, handlebar) was adjusted differently. Obviously I'm assuming that this triangle remains untouched (fit to the rider) so the only difference of the large frame will be longer wheelbase and taller standover height.
Easy - the front wheel has less weight distributed on it. Start tipping it up and it gets less planted and wanders around. It becomes difficult to keep it down on short steep bits and harder to control for getting up a ledge.

Since the OP wants it for AM, maybe this is a non-issue though.
 

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5'11 3/4" and 30" inseam. All torso, no legs. I am an inbetweener like the several here. I tried a medium, and it was a bit too small, even with a longer stem. Tried an 18" hardtail with a setback post and a 90mm stem, and it was almost there. Then a 19" large with a 90mm stem and it was perfect.

No one on this forum can tell you what is right for you, as you have to decide that. The fit needs to be on, either big or small, or you won't ride it.
 

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Easy - the front wheel has less weight distributed on it. Start tipping it up and it gets less planted and wanders around. It becomes difficult to keep it down on short steep bits and harder to control for getting up a ledge.

Since the OP wants it for AM, maybe this is a non-issue though.
It has less weight on it because the wheel further forward. As long as the triangle "pedals-saddle-handlebar" remains the same and chainstay length is not affected, the front should not tip up more easily with a larger frame. On the contrary, it will be a slight bit harder to lift the front.
 
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