To make suspension effective, it must decouple light-weight moving parts from heavy stable parts.
For this reason it is most effective to suspend the wheels - they will move fast up and down, while the frame and the rider will not move vertically.
A suspension stem is not very effective because it does not carry much vertical load.
A suspension seat post is better but will never work as well as wheel suspension.
You can observe the same principle in cars: light-weight aluminium rims are better for suspension than heavier steel rims.
In sort: suspension stems are ineffective for suspension, add weight and complexity, and potentially dangerous failure modes.
bicycles.stackexchange.com/users/30916/Christian Lindig
Former Alsop Suspension Stem rider here... as much as I respect everyone's answers above
I can say that Alsop suspension stem advocates would disagree with some of the assertions.
Alsop's motto was "suspend the rider not the bike".
So as far as having to decouple light-weight moving parts from heavy stable parts for suspension to work...
That is exactly what an Alsop does, decouples the rider from the frequency vibration of the bike.
Alsop stems, at least the later models, were spring loaded not polymer loaded.
They traveled as a parallelogram giving about an inch to an inch and a half of travel.
The parallelogram motion of the Alsop is what set them apart from competing suspension stems.
Other suspension stems (Girvin Flex Stem) used a swing/pivot point which limited the overall travel and rotated the handlebars along an arc during compression.
Limited travel and rotating controls led to the demise of the competition long before Alsop succumbed to the suspension fork.
At the time I loved my Alsop Frankenstem... the problem and what eventually caused me to walk away from the product was they tended to wear out fast.
Bushings constantly needed to be serviced.
Additionally, they were susceptible to catastrophic failure.
Over the course of 3 years I had 2 stems fail...
horrible crashes and stitches were the result
so I spent the better part of the next 20 years riding rigid bikes.
bicycles.stackexchange.com/users/23679/dafew
I see suspension stems making a comeback.
If you do some searching you can find lots of examples .
However, they aren't usually found on mountain bikes because they provide nowhere close to the amount of of travel that today's mountain bikers demand.
Most of them provide under 20mm of travel, while mountain bike forks usually provide at a minimum 80mm of travel.
However, the suspension stems are getting somewhat popular with the gravel grinding trend that is gaining popularity, or even in places where the quality of asphalt leaves something to be desired.
bicycles.stackexchange.com/users/163/Kibbee
I was thinking back to the 1990s when I was looking at these suspension stems and remembered why I didn't get them then : 1) Price.
I was a starving student and couldn't afford them.
Many were as expensive as my bike itself.
2) Brakes.
Like many mountain bikes at the time, I had cantilever brakes.
These required a fixed position above the brakes for the brakes to pull against -- for the front brakes, it was the handlebar stem.
So if I switched to a suspension stem, I'd have to get a brake bridge or go to another type of brake.
That would add to the cost.
Those were the reasons I didn't get them at the time.
Now, mtbs have switched to disc brakes or V-brakes, so the brakes aren't a reason to get them.
bicycles.stackexchange.com/users/11160/RoboKaren
They work more like a damping of the shock rather than absorbing it.
Shocks might weight more but are good for down hill.
I had a Girvin for my road bike and it was down to weight.
It worked well on poor bumpy road conditions and gave better control through damping.
bicycles.stackexchange.com/users/33611/Ian Croasdell
I remember in the early 1990s contemplating getting a suspension stem for my 1989 non-suspended Rockhopper MTB.    I'm surprised that suspension stems appear to have essentially disappeared. Why? They seemed a cheap and easy solution to give a bike front suspension. They could also be made lighter than suspension forks. What were the negatives?
Why have suspension stems disappeared?
bicycles.stackexchange.com
<mountain-bike><suspension>
bicycles.stackexchange.com/questions/45190
bicycles.stackexchange.com/users/11160/RoboKaren
S2: A suspension stem is not very effective because it does not carry much vertical load. suspension stems are ineffective for suspension, add weight and complexity, and potentially dangerous failure modes. On mountain bikes suspension stems provide less travel than a mountain bike fork. However, suspension stems appear to be making something of a comeback. Fork suspensions are more effective because they decouple the moving wheel from the heavier and more stable frame and provide much more travel than stem suspension, which usually only provides slight damping. In addition to the lower effectiveness, suspension stems can have dangerous failure modes and require more frequent maintenance than fork suspension. Despite this, suspension stems are becoming more popular with gravel bikes designed for uneven road surfaces rather than off-road use.
Fork suspensions are more effective because they decouple the moving wheel from the heavier and more stable frame and provide much more travel than stem suspension, which usually only provides slight damping. Suspension stems are becoming more popular with gravel bikes designed for uneven road surfaces rather than off-road use. In addition to the lower effectiveness, suspension stems can have dangerous failure modes and require more frequent maintenance than fork suspension.