I am happy to answer to you
@rafiki. These cut down stems were invented in the 90ies when there was no Brompton bar apart from the "M"-model. Steve Parry wanted to have straight bars due to more stiffness and sportiness on his modified Bromptons and so he created the mod, initially by welding a longer tube onto a cut down Brompton stem and adding the straight bars via a ahead-stem to it (lower and thus sportier than M was possible). Later he created the adjustable version consisting of two parts, lower part being a cut down M-stem, typically in combination with a suspended post-moderne seat post as upper part which offered a kind of front suspension as a bonus. The same mod was later made and sold by Ben Cooper of Kinetics and, in a slightly different version, by a Dutch bike shop (forgot the name), with the Dutch guys (in opposite to Steve Parry) having in mind higher bars than the M on the stock Brompton offered. The latter is what is now sold by Vincent van Eerd and I suspect he may have been the maker of the Dutch version from day one on.
Today we have a variety of Brompton stems, offering different bars and heights, including the S with a straight bar and the H which is higher than the M. So the need for this mod has somewhat vanished. It still has it's advantages today (along with some downsides).
I do own two of those stems, both of them bought second hand, one made by Steve Parry and one sold by the Dutch bike shop ages ago (it is based on a MK2 stem, so really old). What's important to know is that these mods are based on the M stem before 2017, other stem models do not work due to the stem angle as far as I know (I may be proven wrong) and I do not know if the newer M-stems from 2017 on will work.
edit/correction: Following the current page of Vincent van Eerd ist looks as if the S-stem would also be an option as a basis for the conversion. I am pretty sure someone told me that the P-stem would not work, so this would leave the H-stem as an open question. /edit
The M-stem is cut slightly above the screw that is holding the plastic knob that attaches the stem to the forks in folded state - until there both mods are identical and here's where the differences start. There are two of them: Steve Parry brazes on two lugs that clamp the upper part to the lower part using hex bolts while Vincent van Eerd is using a quick release lever, simpler, but as effective. The second difference is the inner part: Steve Parry is using a machined metal shim to bring the tube to the right inner size while Vincent van Eerd is using a machined plastic tube. Both end up with 27,2mm inner size (at least my two do - it may be the case that more current Eerd-conversions use a different, possibly smaller size).
As a consequence with the Steve Parry Version you set the height once and if you want to change it you need tools. With the Dutch version you can adjust on the go through the quick release. I consider this to be an advantage as you can adjust the bar height based on your daily mood (or the head wind that approaches you) and can furthermore use higher bars than on the SP version as you can lower them before folding w/o additional tools. The SP version clamps however a tiny bit better due to the two bolts.
If you want to modify yourself you'll be having a hard time finding a stem with 27,2mm - almost impossible to find ourdays. Suspended seat posts in 27,2mm are plenty available, to find one that is able to take a handle bar and at the same time offers a suspension level that works on a stem and does not have side play is a bit of a challenge. I ended up buying a bunch of different seat posts until I found one that works ok enough in all aspects. There is another issue: As Vincent van Eerd writes on his webpage the amount of height variation is somewhat limited: for one by the maximum what the stem or seat post is offering regarding max. height and secondly by what the Brompton stem is able to take regarding lower position. Depending from the bars that you are using the upper limit is possibly more relevant as longer stems are hard to get hold of and, as they are made of steel, they are heavy as hell. You can use 25,4mm stems (that are easier to get hold of) by adding shims but this affects stability a bit. Seat posts are available in 300mm and 350mm (with the usable length/extension being considerably less), so less of an issue here, but still limited. Long classic stems are rarely to be found in our days, so max entension is somewhat limited with those.
When you are using straight bars on a converted M-stem you are going to end up with an overall height considerably lower than the stock M - probably around the 100cm mark with either of the two options. When you are using a conventional, long stem it is about the same with straight bars. When you are using a riser bar the overall height depends from the rise of the bars but when going considerably higher than stock M chances are that the bars will hit the ground in folded state. Thus the combination of suspended seat post and stock M-bars will not be possible as it would end up way too high. So depending from your desired height you may have to hunt for an appropriate riser bar that is lower than the factory Brompton bars such as the Joseph Kusoac bars or alike - apart from these models of this kind that fit properly onto a Brompton can be challenging to find.
The mod as sold by Vincent van Eerd seems to use a shorter conventional stem in combination with the stock Brompton M-bars, so you avoid a lot of issues and research (but some possible gains as well).
As he alternatively seems to use an S-stem as a basis and this is cut higher from the ground than the M you might be able to go a little higher with that.
Both stems lack an orientation helper or ruler - not much of an Issue with the SP version but if you regularly rise or lower your bars with the Dutch version it is annoying to constantly get the thing in line with your front wheel w/o any support, especially if you have a front bag mounted to the carrier block.
I am using the Dutch (quick release) version along with a suspended seat post and a 8cm riser bar (which is the max possible in this setup) on my touring Brompton and like the setup in general. It is however a bit fragile as, being a sporty rider, with massive bumps the seatpost may sack into the stem a little bit sometimes (really rarely and maybe caused by the fact that the stem and more important the plastic shim inside have been in use for probably 20 years and thus the shim probably suffers from some wear), and the need for (re)adjustment of allignment with the front wheel each time after changing the height is enerving. Furthermore the construction adds weight and, as the seat post is obviously straight and misses the angle of the original stem reach is shorter and has to be reestablished by the angle of the riser bars in my case which looks a little strange. Using a suspended seat post way from it's intended and warrantied purpose in a slightly weird construction to hold your bars (and thus your control of the bike) is psychologically nothing for the fainted hearted as well.
So, would I recommend this mod? Depends. With the SP Version you'd typically end up with a max height of shortly below 100cm height plus bars, so basically what the S-model is offering from factory. If you are using a suspended seat post as upper part and are not too tall (so that this height or at max, the height of the original M-Model is enough) it may be an option. If you need to go higher than M overall (including bars) this hinders the stem from folding properly. W/o the use of a suspended seat post I do not see an outcome that you could not achieve with one of the factory stems easier, cheaper, less fragile and with less weight.
With the Dutch version as it comes from Vincent van Eerd you can go higher than M, but this can today far easier be achieved by buying a H-stem that furthermore does not need to be lowered before folding. That leaves as advantages the optional possibility of suspension and of rising and lowering the stem on the go which I like on my touring Brompton. But then again the latter is a more complicated way to achieve more or less the same variability in grip positions that is offered by the P-stem and bars (that are no longer available since mid 2019 and apart from that are not very aesthetic).
So today this mod is of very limited use in my opinion, but for those who appreciate exactly that use or have special needs (like usage of the bike by multiple riders of massively different height) it may still be worth it. A huge advantage in my eyes is that it is possible to find out which bar height you like best by testing it in very granular steps w/o extra effort and have the possibility to quickly adjust (whereas with the stock stems you are set and limited from the beginning and, while experimenting with different bars is possible, is a lot more effort each time you change, the adjustment steps are bigger and it quickly starts to become pricey and annoying as well).
One should be aware that the page about the stem mod on Vincent van Eerd's website is no longer reachable via the sites navigation but only through the direct link
https://www.eerdermetaal.nl/telescopische_stuurpen.html and a link from within some text on his homepage which probably means he has stopped offering it. He may however still be open to build such a stem, it just seems no longer to be part of his primary offering, maybe because of some of reasons I outlined above or simply due to lack of demand - I don't know.
If you want to create a mod like that yourself: I think in the picture archives of the bromptontalk mailing list that just moved over to groups.io is a picture story of someone who did it himself, including a parts list and detailed description of the work; I think he was using a Batavus stem for his mod.
Management summary:
Positive:
+ very granular adjustment of the bar height w/o extra effort
+ adjustment of the height on a daily basis according to your mood and needs
+ possibility to add front suspension
Negative:
- adds weight
- cost
- a little loss in stability
- one extra step for folding/unfolding when using high bar positions
(- advanced setups need massive fiddeling and research of parts)
(- using a suspended seat post as a stem replacement is a little bit frightening)
Neutral:
° today different (fixed) bar heights can be achieved easier, lighter and cheaper through factory stems and/or 3rd party handlebars and/or risers like the "aberhallo" or the Tern Andros
° still limitations regarding the possible amount of adjustment, especially when using the suspension mod.
° front suspension on a Brompton can today far simpler be achieved by using suspended
velospring handlebar grips, way less frightening than the seatpost solution but unfortunately not cheap either.
