Just ordered a Bird Aeris, and am rather excited. Spec has been nailed down, and is listed below. But bit stuck on the stem length issue.

My last bike was a Large White T130, with a 55mm stem and 750mm bars. It felt ok, bit twitchy at first, but got used to it. Reach was good with the 609mm toptube combined with my 5’10” height, although in terms of reach I could have easily gone a bit shorter on the stem.

The Aeris seems optimised around a 35mm stem (I ordered a Medium, which has a 620mm toptube). Thing is, I find it quite hard to imagine dropping another 20mm on the stem! I’m therefore minded to go either 40mm or even 50mm.

Aeris owners; what stem length have you gone for? Do you find that the slack head angle makes a shorter (say 35-40mm) stem feel more natural, than on a bike such as the T130? Do you find yourself twitching all over the place with a 35-40mm length?

Frame – Medium (green)

Fork – Pike RCT3 150 solo air

Shock – RS Monarch RT3

Wheels – DT EX1501

Rotor Front – Shimano 180

Rotor Rear – Shimano 160

Tyre front – Maxxis Minion DHF

Tyre back – Maxxis Ardent 2.25

Headset – CC40

Stem – Easton Haven (either 40mm or 50mm)

Bars – Raceface Next Carbon 760mm, 20mm rise

Grips – RF Half Nelson

Brakes – XT

Shifter – XT

Mech Rear – XT

BB – Raceface Turbine 30

Crank – Raceface Turbine Cinch X1

Cassette – XT 1×11 (11-42T)

Seatpost – RS Reverb Stealth (150mm drop)

Saddle – Charge Scoop

Seat Clamp – Hope QR

Total – £2,738.95 including delivery. Not bad!

My son, who’s 5’10” has a large with a 30mm stem. He loves it and it looks right on him. I’ve ridden it and it rides as well as my SC Bronson.. Steers well doesn’t flop etc – are you sure you’re medium?

I’m 5’8″ on a medium aeris, I put on a 40mm stem and it suits me fine.
I added ett to stem length on my 456 and went for similar total length.

I’m 5 9 on a good day and Bird recommend a medium and I got the 35 mm answer stem . I would speak to them and see what they reckon as they got it spot on with me .

You will be better off comparing the reach measurement, as this will make the stem length easier to understand

I can’t find the reach measurement for the t130 (only the aeris), so it’s difficult to compare.

Bird recommended a medium for my 5’10” height, unless I wanted a “really big” bike. They also advised two staff at 5’10” rode the medium and felt it well suited.

As I didn’t want a barge, and wanted to run a 150mm reverb, with loads of stand over, medium seemed the right one for the job. It’s still longer (4cm in wheelbase), slacker and lower than my old t130 in large. So I’m hoping I made the right choice :/

By chance, I ended up hiring an Aeris at FOD today. Had no idea they’d be in the hire fleet.

Fully expected medium to be right size, but actually, with the 35mm stem, the Large was a better fit. On the medium I just felt a bit “over the front”. Hire staff said the same. 5’10” but with long arms. I’d definitely have to run a 45-50mm stem. And given that the bike is designed around a 35mm stem, I think I’m going to change order to a large.

Medium was a bit more agile through turns, but large felt better overall. Oh and its sooo stable off drops and jumps- felt incredible in the air.

Tried a 40mm and preferred the 35mm. No problem with the bike feeling twitchy.

Compare the reach measurements on the two bikes and go from there.

I’m 6’3 on an XL with long arms and legs.

I’m 5ft11 with long arms on a Med with a 40mm stem. For me the Medium was absolutely the right size, even with gibbon like proportions the long is massively long. Can’t say I’ve felt over the front on mine. Still we’re all different 🙂

I don’t see how you can run a 150mm dropper on a 19.5″ frame if you’re actually 5’10”? If you feel too ‘over the front’ on a bike with a 448mm reach I suspect you have issues with hamstring tightness and hip hinge mobility.

Large is an 18.5″ seat tube, so could probably run 150mm dropper (the demo bike had a 100mm dropper and I had it about 5cm out of the collar).

I’m torn. All I know is that with the 35mm stem, the Large felt more roomy and right when pedalling. The staff also said large looked like a better fit from the car park test.

Don’t get me wrong, medium felt good on the trail too. Ergh, don’t know what to do now.

Give them a call in the morning… I had a long chat with Ben and Dave before choosing the medium. I’m not sure I’d get hung up abour running a slightly longer stem either – it’s not like e.g. 50mm is much longer.

Ah, that must have been the pre-production geometry chart I was looking at. In that case I’d go with your gut feeling.

FWIW I’m 5’10.5″ (with 6’2 arm span) and riding a bike with 427mm reach and 50mm stem (but with riser bars rolled back so it’s effectively a 40mm stem). One of my mates is the same height and riding a bike with a 403mm reach and 70mm stem. Both similar animals to the Aeris but the Birds are 449 and 468mm reach.

Don’t forget that you’ve easily got +/-5mm and even up to 10mm leeway on reach (and effective stem length) by rotating riser bars back and forth.

Thanks for the useful input. As the reach is fairly long in each respective size, perhaps a medium with a 40mm stem will be the “right” size. Maybe car park tests can be deceptive. On the actual trail, it felt like either bike could fit and do the job well. I guess the advantage of the medium is more agility in corners and definitely being able to run a 150mm dropper. it’ll still have a relatively long wheelbase and slack head angle.

Interestingly though, have you seen the new Whyte geometry? On a medium 901 hardtail it’s 637mm top tube and 40mm stem. Pretty much an Aeris in large! And that’s recommended for heights 5’6″-5’10”. Similar with the G160.

Likewise, you get Danny Milner of MBR riding a Large Reign and Strive in the reviews, yet he’s 5’9-5’10”.

Did any of you 5’10” folk try a large Aeris? Did it feel definitely too big?

I think the problem you run into as bikes get longer and longer in their front centre, whilst chainstays stay short, is it becomes harder and harder to get enough weight and thus grip on the front tyre, especially on flatter trails. You only have to read of experiences with the first Mondraker forward geometry bikes which had 10mm stems as standard, to see that you can definitely have too much of a good thing.

I don’t really understand why journalists refer to longer bikes as having more room to breathe when pedalling – your lungs work better when sitting more vertical with less bend at the hips or in the trunk – it’s only for aero reasons that road bikes have that low stretched out pedalling position. And aero doesn’t matter at all when seated pedalling a MTB. Modern geometry full sus non-XC bikes are usually long when standing but shorter feeling when sitting, which seems like the best of both worlds.

Yes, that is a key concern actually; keeping enough weight on the front tyre. I did notice actually on the relatively brief demo of both sizes, that with the Large, I was very conscious of getting my weight forward, in an aggressive stance, when carving corners on the Verderers trail final descent. It was an instinctive thing, to offset the chance of the front washing out. Because I’d be right at the bottom of a Large in terms of sizing/fit, with the shortest possible stem (35mm), that behaviour would be at its most exaggerated (relatively speaking).

Ultimately, I think I could probably go with either size. Large would be a bit more stable, bit more roomy, and faster in a bomber-straight line kind of way. Whereas the medium would be slightly more agile in the twisty stuff, with more standover and the potential to really slam the saddle right down if/when required.

Had a good chat with Ben at Bird about it yesterday, and on balance, I think Medium with a 40mm or longer stem is going to be the best option. My riding is generally limited to trail centres and BPW, rather than big, wide open tracks taken at top speed. So a bit more manoeuvrability will be welcome. I was definitely surprised how small the bike looked in the flesh though! I guess it’s from coming from a Large Whyte T130, which although shorter and steeper than a medium Aeris, had a ‘bigger frame’ due to the higher standover and taller seat tube.

My issues with the Whyte T130 in large were: 1. The seat tube was 19”, so even with the saddle completely dropped, it still felt a little bit in the way on the steeps. 2. I craved a bit more slackness. 3. While it was wonderfully playful, it definitely lacked for/aft stability, especially in the air. The Aeris on the other hand, even in a medium, is 11mm longer in the toptube, 40mm longer in the wheelbase, 1.5 degrees slacker, and with acres of standover. While still being quite playful, due to the poppy suspension.

Hopefully it’ll be the right choice. :\