https://www.bikeradar.com/news/2023-bmc-fourstroke/

For a mere 13 grand, you can have your seatpost drop without putting your weight on it.

Jokes aside, interesting development (which has taken 3 years) https://www.pinkbike.com/news/bmc-autodrop-dropper-post-lenzerheide-world-cup-xc-2019.html

that might become the norm in the future? Integrating within the seatpost itself might be useful, but BMC might be usingthe whole downtube as a storage tank.

When I saw the pics my first thought was it was charging it via the rear suspension which would be neat. Charging it via a pump seems a bit limiting.

The 3 yearold prototype was apparently charged with a (road) floor pump, so quite likely ~100psi ish?
I wonder what the volume is, is a shock pump is going to take you hours?

It still functions like a normal dropper when the air runs out. Took me a while to figure that out from the design which initially seemed overly complex and upside down – but using the air to drop and coil spring to raise means that you can use your body weight to lower it if required. Also means it failsafes to a regular dropper.

Using the rear suspension would be really dificult to do, using itself though – ie you do 2 regular arse drops, and it charges the system enough for an auto drop? plausible.

I just don’t get using a pressure reservoir.
AXS and it’s ilk is in existence. Yes, it will use significantly more power to run a motor, or linear actuator, but the space regained by ditching the pressure cartridge would free up a fair bit of space for that. Granted, additional motors and batteries are heavy, but why hasn’t it been done? Especially when you look at the weight of e bikes, it’s small fry. Anything where you need to keep recharging an air cartridge is doomed to fail.

Using the rear suspension would be really dificult to do, using itself though – ie you do 2 regular arse drops, and it charges the system enough for an auto drop? plausible.

You’d need multiple pistons/air chambers. A small diameter one to charge a reservoir then a bigger one to move the post? Might make it a bit complicated, and you’d need a 2nd lever.

Other idea, remember those bionicon bikes a few years ago, how about reducing the fork travel and using the extra air spring pressure to keep the post up on climbs, then reverting on the descents? Might not work though as you’d need quite a lot of spring to push the air back to the forks, and they’d need to be unweighted during that time before you went downhill.

I just don’t get using a pressure reservoir.
AXS and it’s ilk is in existence. Yes, it will use significantly more power to run a motor, or linear actuator, but the space regained by ditching the pressure cartridge would free up a fair bit of space for that. Granted, additional motors and batteries are heavy, but why hasn’t it been done? Especially when you look at the weight of e bikes, it’s small fry. Anything where you need to keep recharging an air cartridge is doomed to fail.

Makes more sense, without the need for an airspring there’s no load on it so a motor could spin a ball screw pretty quickly? And without the need for pistons, seals and hydraulics you’d save some weight to go towards batteries.

but why hasn’t it been done

I had a brief look a while ago at the concept of an electric motor with a worm gear type arrangement. As in, a thought experiment looking at the required power, sizes of motor, battery capacity etc., I would have no idea how to design, prototype, manufacture, market or sell such a thing.

To get the speed needed to match the “up” performance of current posts, it was quite a lot of power and motor. ebikes obviously remove the battery concern if integrated.

But my main stumbling block would be misuse. People sitting on it while expecting it to go down either because they have trained themselves to do that with a normal dropper, or because they conform to the lazy and mechanically unsympathetic ebiker stereotype was going to put a lot of force through moving parts.

I think this might turn out to be a mistake by BMC.

If I were in the market (and I almost am) for a top end XC bike, I would look at that system and think i couldn’t be bothered recharging the system every ride. 200psi for 100 drops of the post, that going to be every ride isn’t it? I’d probably not bother charging it and use it as a conventional dropper post. Can’t think i’ve ever thought “I wish i didn’t have to push this post down with my own backside”.

If you don’t charge it, you’re then left with redundant material (weight) in the frame, and a post that has only two positions, which i would find unacceptable. Lots of XC riders/racers like to use a very short drop, maybe 1-2cm, so they can still pedal fairly well when seated, when riding through choppy stuff on the flat. I have an 80mm Divine SL and rarely put it all the way down.

then the “lower” and considerably less expensive bikes (Fourstroke as opposed to Fourstroke 01) make a lot more sense. (The Fourstroke Two looks fantastic, to my eyes anyway).

A bit odd that, with the LT version, there is such a gap between the £12k LTD and the £5.5k LT One, which is an SLX level bike. No XT/XTR or GX version at probably £6.5k to £7k?

If I were in the market (and I almost am) for a top end XC bike, I would look at that system and think i couldn’t be bothered recharging the system every ride. 200psi for 100 drops of the post, that going to be every ride isn’t it?

Every race you mean. It’ll already be getting tyres checked, freshly charged AXS batteries and (probably) shifter batteries, freshly scrubbed and lubed drivetrain, brakes checked over, suspension checked and pumped up… so pumping a reservoir up to save energy and pedaling time is hardly a massive extra task.

Can’t think i’ve ever thought “I wish i didn’t have to push this post down with my own backside”.

I’ve been using one a couple of years now. Two things i dislike about droppers, i need two or three interim stops (or a shorter total drop) and interrupting pedaling to push the saddle down is sometimes a pain and slows you down.

Can’t think i’ve ever thought “I wish i didn’t have to push this post down with my own backside”.

Same. It’s about as inconvenient as changing gear.

Same. It’s about as inconvenient as changing gear.

I can see the use for it for other people. Not as a trail rider, but an XC racer; approaching an uphill or flat obstacle, or even a corner… one press and the seat is down while they continue pedalling uninterupted until the last possible second.