BMC's All-New Fourstroke Has a Self-Dropping 'Automatic' Seatpost
BMC have released an all-new Fourstroke, their full-suspension cross country platform. The biggest news is the race-ready Fourstroke 01 model has what BMC call "the first ever fully integrated, automatic dropper post."
Pinkbike photographer Andy Vathis spotted this prototype version back in 2019.
By "automatic", they mean it can extend and drop at the push of a button, without the rider having to compress the seatpost manually by squatting down on the saddle. BMC call this Autodrop, and the advantage is obvious. Come into a descent and need the saddle out the way? Just press the button and it's gone. No need to stop pedalling, squat down on the saddle and stand back up before you can attack the descent. That's probably not a big deal to the average rider, but used many times over the course of a World Cup race it could add up.
 | During a race, we activate Autodrop up to a hundred times allowing us to focus entirely on the track and giving us the gold-medal-winning advantage. |
A normal dropper post gets the energy it needs to return from a spring, which is compressed by the rider pushing down on the saddle. In technical engineering terms, dropper posts are ass-powered. BMC's Autodrop post, however, is air-powered. There's a tank of compressed air in the down tube, which is inflated via a Schrader valve to 14 Bar (200 psi). When the remote button is pressed with the saddle up, the compressed air pushes on a piston which forces the post down and compresses a small coil spring. When the button is pressed again, the air is released and the coil spring returns the post to the extended position.
BMC say the seatpost can do this up to one hundred times; when the pressure drops below 4 Bar (50 psi), it can no longer drop automatically, but it will still function like a regular (ass-powered) dropper post. BMC say that "During a race, BMC riders activate Autodrop up to a hundred times", suggesting they might use the full Autodrop capacity over a long race, perhaps finishing with effectively a "normal" dropper.
The seatpost offers 80 mm of travel and only has a high and a low position - it cannot be set to an intermediate height. There's no word on the weight, but it's based on BMC's RAD seatpost, which shares the same integrated design and oval cross-section. That post is claimed to weigh just 345g, but BMC admit the air tank alone adds 120 grams to the frame weight.
The top-spec Fourstroke 01 LTD's total weight is claimed to be 10.5 kg (23.1 lb). That's not the lightest by race bike standards, but the company acknowledges this. They say their dual-link APS suspension system is heavier than the flex-stay-single-pivot layout that's become nearly ubiquitous in the category. But BMC say their approach more than makes up for the weight penalty: "Extensive testing shows that the Fourstroke and APS suspension are faster than lighter bikes that feature weight-saving suspension systems, proving that superior traction and pedalling efficiency are more important than weight." Obviously, take any company's claims of "extensive [internal] testing" with due scepticism, but it's interesting to see BMC openly sacrifice lightness for overall performance in the XC category, both with the suspension design and the Autodrop feature.
Models
The top-of-the-range Fourstroke 01 LTD
BMC's naming convention is more than a little confusing. The Fourstroke 01 is the no-compromise race bike, and is the only model to use the integrated Autodrop seatpost. It has 100 mm of travel at each end, with 32 mm stanchion forks. There are three spec levels under the Fourstroke 01 banner, all of which offer Autodrop. These are the Fourstroke 01 LTD (€ 12,999), the confusingly-named Fourstroke 01 ONE (€ 10,999), and the Fourstroke 01 TWO (€ 8,999). There's also the Fourstroke 01 FRS V1 frame, shock, fork and seatpost (€ 5,499).
The Fourstroke ONE. (Not to be confused with the Fourstroke 01, obviously.)
Then there's the Fourstroke (minus the "01" part). This is similar to the Fourstroke 01, except it has a regular 31.6 mm diameter (round) seat tube, and so cannot take advantage of the integrated Autodrop or RAD droppers, which both have an oval cross-section. There are four spec levels: ONE (€ 7,499), TWO (€ 6,499), THREE (€ 5,299) and FOUR (€ 4,299).
The Fourstroke LT LTD
Finally, there's the Fourstroke LT, which of course stands for long travel. With a longer shock and fork, it delivers 120 mm of travel at each end but is otherwise similar to the regular Fourstroke. There are three spec levels: LTD (€ 11,499), ONE (€ 5,899) and TWO (€ 4,499).
Geometry
All three versions have similar geometry. For an XC race bike, the numbers are decidedly on the progressive side, with a 66.5-degree head angle and a 76.7-degree seat angle. The frame geometry of the Fourstroke and Fourstroke 01 are identical, though the cockpit proportions of the 01 are more aggressive. The LT version is near identical too, with no change to the frame angles. Instead, a longer fork and shock create a 15 mm increase in bottom bracket height when the suspension is fully extended.
Availability
The Fourstroke 01 LTD and ONE are available from June 2023, TWO from October 2022 and FRS VAR1 from February 2023, and cost 12,999 (13,499 USD), 10,999 (11,499 USD), 8,999 (9,499 USD) and 5,499 (5,999 USD) euros respectively.
Fourstroke ONE is available from October 2022, TWO from April 2023, THREE from June 2023 and FOUR from September 2023, and cost 7,499 (7,999 USD), 6,499 (6,999 USD), 5,299 (5,799 USD), and 4,299 (4,699 USD) euros respectively.
Fourstroke LT LTD is available from April 2023, ONE from January 2023 and TWO from September 2023, and cost 11,999 (12,499 USD), 5,899 (6,399 USD), 4,499 (4,999 USD) euros respectively.
For more information, check out bmc-switzerland.com.
Author Info:
Must Read This Week
Stopping pedaling to use my weight to drop the dropper multiplied x's probably 25x's per lap , x'd 3 laps = I just tend to not use the dropper in races and only in casual riding.
Now the question of reliability will remain to be seen with bmc.
thats exactly what he described with more words
Practice more.
The last thing you want to do when your legs are burning, is to do a squat.
Second...I dunno, I'm just the idea man. But the fuzzy idea I had was something that provided pressure for it to stay up, probably with some kind of latch to keep it firm, then when you hit the lever the latch is released and whatever provides the pressure is isolated from the system, causing it to fall. (Edit: or some kind of motor.) But the smart folks at BMC have a much better plan. Wonder if they'll release an after-market version?
(I'm kidding, I don't really care, but that would be neat though)
www.pinkbike.com/news/bmc-autodrop-dropper-post-lenzerheide-world-cup-xc-2019.html
highly unlikely that it does both at the same time.
"It'll be interesting to see how you diagnose/access any air leaks on the integrated air tank."
Just a guess but BMC dealers will be supplied with a lap pool placed in the back alley. Toss the complete bike in and yell, "Okay Fourstroke, let's see you do the breaststroke!". Reminiscent of witch dunking in the 1700's, if it sinks you know it was fine.
It's not like other companies aren't slacking out their XC bikes, but most seem scared to even go below 68°. There are still a few bikes being sold with HTAs of 69° or greater.
I don't care about the XC side of things but for these new school 150-160 bikes, its perfect. Suspension gets much better simply because no longer do companies have to really focus on climb capabilities (as much at least) because this simple dropper post/damper system firms it up automatically. We'd get sprite climbing 160 bikes that go Full-DH with the dropper simply down. In the day where everyone is stupidly using sensors and Ai, the simply human controlled system using a damn lever I'm already pressing is a no-brainer.
Basically, it’s a normal post until you auto drop it, then it uses the compressed air to act like your body weight. When the post hits bottom, it releases the Autodrop air (small volume, high pressure) to atmosphere. I imagine the action or resistance of the drop is very light, and very fast return, and it locks at both top and bottom position.
Then the normal posts pressure pushes it back up.
If the post could act like a pump it would have a lot more resistance during Autodrop and likely require significantly more volume in the tank.
But, although this looks like a great dedicated World Cup race bike, I would be hesitant to buy, even as someone who almost exclusively rides race-y XC, because:
-80 mm drop is not enough for most users outside of a race situation, and many other XC bikes can now run 125 mm+ ultralight posts (e.g. the KS Lev Ci) that allow them to become virtual short-travel trail bikes that are very versatile.
-Despite moving the shock to the top tube, there's no second bottle cage in the triangle, which is a huge advantage of most XC bikes with this design (e.g. the Epic). One of my favorite things about the Epic is that I can leave my hydration pack at home for any ride under 3 hours.
As far as bottle cages - No mounts on the seat tube. Looks like I guess you could run two on the downtube? I've never seen that before and seems like it wouldn't work on small sizes.
Onto the negatives: Although I've got lots of respect for BMCs engineering efforts, I personally (as an avid hobby XC racer) don't really see any tangible advantage over a regular dropper post. And besides not really seing how this would benefit me, I would definitley not want to be locked into their fully proprietary integrated dropper post system. Especially since it can't be swapped with a regular dropper. Same reason why I would never buy a Specialized Epic or Trek Supercaliber.
I gotta wonder if they're overshooting the optimum at this point. My Epic has a 67.5° head tube and it doesn't feel like much of a liability in the twisties, but it definitely feels more ponderous than the 69.5° on my '18 Epic or the 71.75° on the '14 (that bike was TWITCHY). How far does it go?
A lot of people love their electronic shifting, but IMO as an AXS owner, it's a quality-of-life net negative. The benefits are usually bullshit. Less maintenance? What? I spend 10x the thought and effort charging than I ever did on running a cable every 1-2 years. And then there's "faster shifts" which is nonsense, since shift speed is all about cassette rotational speed, not actuation.
Mechanical MTB groups were so, so good before electronic, and the fact XTR continues to compete on equal footing with the SRAM robo-groups absolutely proves this. I would trade my AXS group straight up for the mech equivalent if it wasn't for resale and logistics.
Which is a shame as a self dropping dropper is a great idea if done right. But this isn't the way. I haven't come up with any better alternatives, but BMC are gonna lose money with this one. Soz
Pneumatics are cooler than electronics in a steampunk kind of way. #shimanoairlines Who wants *beep* *zzzzzzzzzzzt* when you can have cool *pshhhhht* noises. Also you can recharge your pneumatics on a ride with the pump you carry in your pack.
In HPV races for example ice vests were banned for that reason...
Wait, it would be still ass-powered
Seems that ideally you'd just want a post that alleviated your legs from the work of squatting but still gave you infinitely adjustable saddle position.
I will be curious to see if that is still true when I race around here tomorrow.
Best guestimate, about ¼ of the top 20 riders this morning had a pack.
youtu.be/rP0uuI80wuY