Duragrouch

Yeah this is a big challenge on most folders. On a Bike Friday, less so, because they use a skinny long stem tube, parallel with the seat tube, and then topped with a conventional stem, I can't recall if old-style quill or new style outer clamp, but I've seen a pic with a 'Friday with a suspension stem. The problems with this system (to me) is a) the long skinny stem is more laterally flexible than the much fatter, tapered long stems on most folding bikes, and b) because the stem is parallel with the seat tube, it's more likely to be hit by your knees near the top when climbing out of the saddle (standing on pedals).

Most folding bikes have the long stem NOT parallel with the seat tube, more near vertical, so plenty of knee clearance, and, the advantage is, as the seat and handlebars (if a telescoping stem) are adjusted up, the effective top tube length increases, which is why many folders can accommodate riders 4'8" to 6'3" height. The downside of this is that most of those stems have the handlebar clamp integral with the stem vertical, so you cannot fit any alternate stem top, much less a suspension one. I have thought about perhaps on mine, if the stem diameter is correct size near the top, perhaps I might cut off the handlebar clamp and fit a suspension stem. But I just wondered now if anyone online sells a stem like that? I prefer a tall, one-piece stem because it is more laterally rigid, and I like my handlebars at that height (typically equal to the max adjustment height on telescoping stems). I don't see one on amazon, a tall stem without handlebar clamp.

Also in any search, be aware that some handlebars fold to the right, and some fold to the left, depending on whether you want the handlebars to be between the frame folds or outside them. Most folders fold to the left, and I have seen those with handbars both inside and outside the fold, when folded. Brompton frames (after swinging in the rear triangle/wheel) fold to the right, to sandwich the drivetrain between the folds, and then the stem and handlebar fold down outside the frame fold (I can't recall if keeping the fork pointed forward, or turning in backward facing).

My recommendation, is research the suspension stem that you want, and what size stem clamp it comes in, then see if you can find an aftermarket stem (or use your current one) if it has a tube diameter the same size, then cut off the integral clamp from the stem and put on the suspension one. Keep in mind that the stem was not designed for clamp loads if it has an integral welded or forged clamp, so this is all at your own risk.

EDIT: Here's a folding bike stem with conventional top section to accept a clamped stem:

https://www.amazon.com/Handlebar-Alu...dp/B07JPYR75V/