Bilstein shock travel, bump stops and spacers

[Solamar]

I'm mid process upgrading my suspension (T-bars, shocks, etc).

The shocks I'm using (re-valved Bilstein HD's) have both bump stops and a spacer. A quick search on the spacer led me down a brief rabbit hole in an attempt to understand the purpose of the spacer. Most search results flat out said to toss it, some even said it was just packing material. It is not packing material, but rather an important design element to prevent you from internally bottoming out your shock, which is bad. Several other threads mention that depending on your ride height you should cut it down in size, eliminate it or trim a ring off the bump stop.

I imagine some of the confusion comes from a change in the provided bump stop. At some point in the past the provided bump stop was one piece, about the same length as the current bump stop/spacer combo.



My car will end up just below Euro height, 25.5" f / 25.0" r. Should I eliminate or trim the bump stop/spacer?

I had some time to kill (procrastinating actual work) and decided to model the shock at different positions. The standard Bilstein HD has ~ 7-3/8" total travel and at my ride height about half the travel has been compressed.



I'm glad I drew it up, visualizing the parts helps me understand the relationships. I 'think' ride height actually has nothing to do with whether or not to trim the bump stops. It looks like Bilstein designed this assy so that the bump stop would have to compress to about 1/2" before internal shock components crashed into each other, add some safety margin and it's realistic that this was designed for the bump stop to compress to ~1" WITH the spacer. Ride height has nothing to do with this.

If you run you car much lower than stock and are likely running out of compression travel in your shocks, trimming your bump stops just makes it easier to internally damage your shock. The solution is to have the shock body and shaft shortened.

Sound correct? Or have I turned left when I should have turned right?

[smokintr6]

If you can get an extra one of the soft BSRs and send it to me, I can check the rate curve on it, and give you an fairly accurate solid height. If I was to make a guess I would say it will probably go solid around 20mm. Until you know that, you can't really make an informed decision about how much you can trim.

[Solamar]

Originally Posted by smokintr6

If you can get an extra one of the soft BSRs and send it to me, I can check the rate curve on it, and give you an fairly accurate solid height. If I was to make a guess I would say it will probably go solid around 20mm. Until you know that, you can't really make an informed decision about how much you can trim.

That would be good to know but I dont have an extra bump, shocks going back in shortly.

An interesting bit of info I just found is that most production sport cars are designed to corner ON the bumps and the bumps are designed as an integral, progressive part of the spring rate. That would make the location and spring rate of the bump transition fairly important.

And I thought picking T-bar rates was challenging

[smokintr6]

Originally Posted by Solamar

That would make the location and spring rate of the bump transition fairly important.

Yes and no. If you are using a stock torsion bar, then yes, you want the engagement point right if you're using the Bump stop as an auxiliary progressive spring. I recently found out that my ~87 911 was setup to ride on the bump stops, and it's been that way for the 10+ years I've had the car. Typically I don't like cars that are on the stops. It makes tuning counterintuitive, because softening a main spring can actually reduce grip at that corner if it allows the suspension travel to crash onto the stops harder and shock the tires. Personally I like cars with a stiffer main spring that don't rely so heavily on the stops, but then you give up some ride quality because the initial motion is on a stiffer primary spring, vs stock soft springs. You'll probably just have to try it out and tune it subjectively. If the gap is too large you *may* experience too much body roll or the rear may feel over stuck. at that point you could try to lower the ride height or adjust the BSR gap. If you get the gap too short the rear could feel a little unpredictable or peaky.