Lets read into what the response is saying.
(Howdy,
I appreciate your inquiry. From my research, there aren't any rebuildable Bilstein HD or Sport available for your car. The B4 'OE' Bilsteins are twin tube and not serviceable. So unless you find someone to the contrary, the best you can do is bump stop tuning or see if a Bilstein from another vehicle might fit (which is doubtful since your Smart is a very unique vehicle).
For bump stop tuning, you can utilize the 47mm 'strut' style bump stop available at 5Xracing.com, or a 58mm 'red' bump stop for the rear shocks. That's my free advice. Wish I had better news but other than a custom-designed front strut or rear shock (which could run $2,000+) I don't see a way to improve the damping on your vehicle. Also, I only work on front AND rear dampers, never just one end of the car. It's called symmetry, and it matters.
Good luck,)
So this is the the "free" advise bumpstop recommendation for front.
https://5xracing.com/i-21093011-5x-racing-47mm-bump-stops.html?ref=category:1105233
What they are recommending is that you should use the longer bumpstop in the rear. The reason for this is that any time the bumpstop comes into play, the spring rate increases. When that happens, grip is transferred **away** from that end of the car. Longer bumpstops are usually manufacturered to be either smaller in diameter/volume or of a lower durometer material, and typically designed to be compressed over a greater relative percentage of its overall length. This means it provides a more gradual ramp up in spring rate. The longer length means it is in contact sooner, so you don't all of a sudden get this unexpected increase in spring rate when the spring is more highly compressed, such as if you were cornering moderately or hard already. This longer bumpstop is designed to come into play much sooner so you don't get that surprise when you're already cornering hard.
The shorter front bumpstop is probably recommended because they're trying to keep you off the bumpstops as long as possible, and also because there are more situations the front bumpstops can be engaged, like under heavy braking. Also, if the bumpstops come into play and grip is transferred to the rear, you're more likely to survive loss of front grip rather than loss of rear grip.
Just a couple of other things. You said in another post that you said:
I feel once the chassis was properly lubed the suspension stopped being bouncy as the unsprung weight is allowed to move without making the springs react to everything.
The springs have to react to everything, because you do not have suspension movement without the springs either compressing or extending. The reason a car feels bouncy is either because something like the shocks are seized and the suspension is unable to move, thus transferring all pavement undulations through to the chassis, or that the shocks are unable to dampen the movement caused by pavement undulations. Either case is like those people who lower their cars without changing shocks - they bounce their way down the road. Shocks are usually the answer, but good shocks cost lots of money. There's really no way around that.
Lastly, a bumpstop should definitely not be in play when you hit a pothole. The wheel entering the pothole extends the suspension, so unless the car is sitting heavily on the bumpstops at rest, the bumpstop is not in play when the extended wheel hits the backside of the pothole and you get that severe jolt. That jolt is due to the shock not having good high speed compression ability. It's expensive to make a shock dampening slow motions well, yet also not be too rigid when asked to compress quickly.
If you're talking about driving along and hitting something like a speedbump, then yes, a good bumpstop will make a ton of difference. I ran the Fat Cat Motorsports bumpstops in my Miata and the transformation in behavior over the stock bumpstops was significant. Very plush when compressed, with a bit of hysteresis upon decompression so it didn't totally overwhelm the shock.
