Hi Julien!
Your question is a good one and one I had asked myself as well. I use my size 590M Nomad with drop handebars. Despite the longer "Medium" (M) top tube, I find I can achieve the same position as my other drop-bar bikes by fitting compact drop 'bars with less forward reach and also a shorter reach (60mm) stem. I've found no detriment in handling or steering in my use. However, like you I was initially concerned about anything that could increase my reach to the handlebars because it was just right with my setup and I didn't want longer. So far, all has been well. I will explain why below:
The Thudbuster LT (ST also) is a parallelogram design, so the distance between saddle-top and bottom bracket remains pretty constant as the clamp declines down-and-rearward under bumps.
While this keeps saddle height effectively constant (but does change effective distance behind the bottom bracket in a dynamic fashion as you hit bumps), it does allow for a change in effective top tube length. I use the word "effective" because it is a dynamic process. The top tube doesn't actually change length but it feels a little like it because the saddle moves rearward a little as it moved down. That's a key point: It moves "a little" and only while responding to a bump, the distance it moves depends on the size of the bump.
The thing is, the Thudbuster 'posts in my actual use do not have the full travel promised -- they bottom out against the rising-rate of the compressed elastomer stack or rubber puck (depending on LT or ST model) before full travel can be reached. The designer and Cane Creek confirmed this with me in correspondence. This means the difference in reach to the handlebars throughout its travel at at the extremes is also less than you would think.
On various bikes in my fleet, I have two LTs and two STs. Despite my best efforts using things like tearable tape at the limits and so on, I have not been able to accurately measure the actual travel on my 'posts, so I cannot give you any accurate figures. In one test, I mounted a thin wooden dowel on my rear rack to measure where the travel extremes should have been, rode the bike on a rough road and found the markers undisturbed, so I know travel was something less than that. Mounting a camera to the rear rack and taking video it appears the travel on all the seatposts is less than the full stated range, but I'm not sure how much less because it is so hard to measure in practice.
This I do know: the ST (Short Travel) Thudbusters are ideal for my randonneur-road touring bikes in terms of taking the edge off rough chip-sealed pavement and concrete expansion joints and do a lot for making me feel more comfortable on long road rides. The longer the ride, the less fatigued I feel at the end and this really shows up on rides of 300-400km/day. However, the short-travel posts really do nothing for me on things like sharp frost-heaves, deep potholes and roots that have sharply bulged a paved surface. I have to use the same care in those circumstances as I would with a rigid seatpost. Hit something like that hard and you'll know it!
In contrast, the LT (Long Travel) Thudbusters work much better at damping much larger bumps than those mentioned above because it has more travel. I use mine on rough gravel and ballasted logging roads, though potholes and over root- and frost-heaves and they really help in that regard, making a noticeable difference and greatly reducing the number of times when I would otherwise stand to ease the road shock transmitted from the rear wheel up through my neck.
A couple things to keep in mind regarding my use and generally:
1) All my bikes have drop handlebars and I prefer setting my bikes up so I have a 45° back angle. Because of this, I place less body weight on the THudbusters than I would if sitting more upright. Because the 'posts are supporting less weight, I have found I need to go with a softer elastomer stack or rubber buck than my weight alone would indicate. Otherwise, the recommended durometer is just too stiff to get much benefit. This is a huge factor in how much actual travel I get from these parallelogram suspension seatposts. If your bike is setup so you're sitting much more upright, then you'll also get more travel, both downward and rearward.
2) Thudbusters don't mean you can or should remain seated over all bumps! I still post (stand up on the pedals) when I expect a big hit and this avoids any recoil effect where it might feel as if the 'post is launching me. It doesn't take long to learn what size bumps can be ridden seated and which require a quick standup.
3) I would much prefer to fit the LT model to all my bikes as it is more tunable (elastomer stack vs single molded rubber puck) and has a greater range of shock absorption. However, the limiting factor is the space available. The longer travel requires longer link-arms and not all my bikes have the clearance to allow this. That's why the road bikes (with horizontal top tubes) get the ST models; they simply have no room to fit the LT version unless the saddle were raised too high to be comfortable for me.
3) The Thudbuster pivots are highly polished stainless steel tubes riding in bronze bushings. This is appropriate for fretting loads. It also means the pivots should be lubed periodically and kept clean to reduce wear. I ordered the fitted neoprene sleeves for all my Thudbusters when I got them, fitted them at installation, and have found the pivots stay really clean with much longer lubrication intervals than if the posts were open (compared to the experience of friends who left theirs open) -- a bit like fitting a Hebie Chainglider vs an open chain.
4) Fitting a saddle to a Thudbuster is not so straightforward as with a conventional rigid 'post, so if you are switching, I would recommend taking careful measurements of...
a) The usual difference in height from bottom bracket center to saddle rails along the seatpost centerline.
b) Distance between steerer center and saddle nose.
c) Distance between underside of saddle at nose and top tube.
...so you can duplicate your position on the Thudbuster. Installation is fussy because there are two bolts to clamp the saddle rails. The forward one is fitted with a ring to serve as a thumbscrew. You first get that as tight as you can with finger pressure, then torque the rear bolt to spec with an allen key bit. Trouble is, as that rear bolt is torqued, the saddle tips rearward, so your initial adjustments will likely need to be redone a couple of times to get the saddle where it was on your old rigid 'post.
5) When you first sit on the saddle, your body weight will compress the elastomer stack or rubber puck, causing it to settle ("sag") and decline down-and-rearward a little. This can be addressed in one, the other, or a mixture of two ways: Either set the seatpost height a little higher than with a rigid 'post or set the saddle nose forward a little more than with a rigid 'post.
I chose the latter, pushing my saddle forward 4-5mm to serve as a preload for my body weight. Thinking about it more closely, I think this is a partial answer to your question, Julien: If your bike were setup like mine and with similar weight-elastomer settings, you could expect an increase in static reach at rest of about 4-5mm - easily compensated for by moving the saddle forward by that amount to put it back the same. Any increase in reach due to travel could be figured as a rising rate calculation (based on the compression curve of the damping medium, whether elastomer stack or rubber puck). I just went out to the garage where my Nomad is parked and draped my chest over the saddle, loading it with more and more of my body weight. I would =guess= I managed to achieve about 1cm of rearward travel and 1cm of downward travel total under as much load as I could apply to simulate a really big bump. That would total 2cm of travel, but only 1cm rearward and then only momentarily at close to full effective travel.
Is it right Dan ? Do you think it will be a real problem ?
I really don't think it will be a big problem in actual use, but it will depend on variables like your body weight, how upright you choose to sit, how close you are to not reaching the handlebars, how big a bumps you expect to hit while seated and the specific elastomer durometer you fit in the stack.
Maybe is there a counterpart ? from the family of telescopic suspension seat post ? ..
I haven't gotten along with telescopic seatposts to date, finding they tend to stick to some extent at points throughout their travel and work less freely and my knees don't tolerate the difference in effective saddle height so well as with a parallelogram design. That said, there are other parallelogram seatposts on the market and they work very nicely as options to choosing a Thudbuster. I went the Thudbuster route because it is widely distributed and replacement parts are available for when/if they wear out, unlike some others and there is good warranty coverage. I had to make use of this on one LT model when I made a warranty claim for a clicking pivot. After I described the steps I'd taken to isolate and try to address the problem, Cane Creek promptly sent me a replacement pivot and the tool to fit it, all free of charge. Terrific service. My other three posts have so far been problem-free.
Hopefully, something in the above will prove helpful to you.
All the best,
Dan.
