Old AST 4150 Journey 3/17 Nitrogen Checked and Filled

[samthejam]

Hey everyone,

Yesterday I switched from an old Ground Control (GC) coilover setup to a set of used AST 4150s on my E46 M3. My car is a full-weight (~3400 lbs) track car that I also daily, putting 15k miles a year on it, including long drives to Road Atlanta and Barber from Houston. Previous Setup (Easy to Drive, but Worn Out)

• GC Single-Adjustable Coilovers
• 550 lb front / 650 lb rear springs
• GC front inserts & very tired OTS Koni Sport Yellow rear shocks
• Car was super easy to drive—neutral, predictable, great at the limit.
• Rear shocks had almost no gas pressure left, but still had some damping effect.

I planned to keep driving on the GCs until they got really bad, but a used set of AST 4150s with Swift Springs popped up for $800 from an owner upgrading to JRZs. They were in good shape (no leaks), so I swapped them in.

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*Note car is at same ride height as it was before height recommended here: (https://nam3forum.com/forums/forum/s...uspension-mods)

New Setup (AST 4150s – Rear Feels Way Too Stiff)

• 11k (616 lb) front / 14k (784 lb) rear Swift springs
• Front feels fantastic—sharp turn-in, soaks up the road well. No complaints.
• Rear is brutally stiff—beating the hell out of me and the unibody. Daily driving and backroads are rough.

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Next Steps – Looking for Input


I’ve played with the damper settings, but the rear still feels too stiff.

• Front: 2 clicks from full soft (feels perfect).
• Rear: Tried full soft to 6 clicks. Right now, 4 clicks feels best, you can tell I'm, in the ballpark of the where the damper is happy for the rate but the spring rate is just wrong. How it feels to me anyways.

Plan:

1. Swap the 14k rear spring back to my old 650 lb spring and see how it feels.
2. Check bump stop engagement and possibly swap to softer bump stops for big compressions from my old GCs.
3. Fine-tune rear damper settings if needed.

Does this sound like a reasonable approach? Anyone with AST 4150 experience have input on whether 11k (616lb) front / 650 rear would be a better match? Or can share where they ended up with their ASTs. Just a sanity check before I start swapping things!

Thanks!

[Tbonem3]

That's a really stiff front spring. Try 400lb (7n/mm or 7kg/mm).

I'm running the 784lb in the rear on my e90 and LOVE it, but that car is 3600lb. Try the 650 or 685 (120n/mm or 12kg)

[CrisSilberGrau]

What Tyler said. I'm on FCM H&R shocks and 400lbs front and 784lbs rear (car weight is 3240lbs wet - 3440 with me in it). Feels like its on rails and rides better, more compliant, more comfortable (even on some poorly paved roads) than my TCKline dual 400F/600R lbs .

Another point to mention is perhaps the shocks are in fact tired and my need a rebuild, not sure if there's a way to check that without going to an authorized rebuild center.

[9kracing]

You should contact AST and see about upgrading those to the 5100s

But to answer your question, yes, that's way stiff in the rear, try 600-650

[bigjae46]

Those rates are quite high. I'd guess that you're riding on the springs and getting bounced around.

If you're running MSR Houston, you will want softer springs.

[samthejam]

Those rates are quite high. I'd guess that you're riding on the springs and getting bounced around.

If you're running MSR Houston, you will want softer springs.

I wasn't having any issues on the 550/650 a bit stiff but car seemed happy

[samthejam]

So I put it in the 650s, the 800s genuinely felt like a go kart even on the highway. Wife said she wouldn't ride in it again.

So yeah swapped those out, I ended up just really liking them at two settings.

1. Full Soft
2. 6F and 4R

I am probably going to leave at full soft and forget about it. The roads here are awful. It handles dips and dives much better than the old setup at least.

That knob definitely has cross talk and does not just effect rebound. I built coilovers for my E28 and went through the rebound fun with the universal race inserts, so I am pretty confident in saying definitely increases the compression whenever you turn the knob.


I had ISCs on one of my old E46s and that's kinda what these are like tbh. A little disappointed but they were cheap.

https://www.bimmerforums.com/forum/showthread.php?1660186-AST-4100-Owners-What-Are-Your-Settings

I found a plot for the 4100s and it seems to agree as far as the cross talk.

[bigjae46]

I wasn't having any issues on the 550/650 a bit stiff but car seemed happy

Under heavy braking is where stiff springs will hurt the most. Especially diamond's edge both directions where the pavement is rippled.

My car did pretty good with 600/700 springs before pulling out a bunch of weight. I think 550/650 is a good spot for a full weight car.

[Tbonem3]

I don't understand these companies (mcs, ohlins, ast) having rebound increase along with compression. If i increase spring rate, I want to slow rebound a bit more, why would I want more compression? The heavier spring is more compression. Maybe the monoshock just is that way? Twintube ots koni yellows have rebound only adjustment to match your springs.

[samthejam]

Alright so this is moreso notes for myself, hopefully it doesn't get M3Forum'd.

It needed a lot more rebound which makes sense since the springs are back in.



I actually took car apart to gauge condition of the dampers (bad human shock dyno). They are all responding fairly evenly on different clicks as well as feel/sound the same except for RF. It is the only one that wasn't exactly the same but it is nowhere near fully blown.

F-R = clicks from fulll soft.

6-6 Rides the best seems to neutralize all the bumps and feel closest to front ride. However it never will be as it needs like a 1200lb rear spring something insane like that when I used the FCM spreadsheet. I would assume this is where the springs are critically damped, as to why it feels good for road imperfections, dips, impacts etc, car settling etc.

5-6 was what I settled with at MSRH, where the car performed very well on very bad tires. Drove how I liked it but felt underdamped at higher speeds moreso in the front. Which is only really annoying on the highway. However it had the front bite and rotated the way I'd like.


I actually installed the old rear spring so the 14k spring and it still didn't do the thing I wanted. The goal was to get more rotation at 6-6.

However COTA is gonna be this weekend. Currently at 6-7. It's a bit overdamped I think there but the turn in is how I prefer.
BUT it's on new tires and the old ones were very heat cycled out.


I wish the knobs weren't so wide in how they feel, now that I feel like I'm close and I don't have the resolution I'd desire.

Also.

My wife being able to read her Kindle on the highway has been a huge help to testing how good it rides lol.

[bigjae46]

Alright so this is moreso notes for myself, hopefully it doesn't get M3Forum'd.

Easy there buddy...I think the correct term is Epstein'd hahaha

Seeing your journey with these ASTs really makes me hesitate on changing out my TCKs. They're almost dead but still do a good enough job.

[samthejam]

Easy there buddy...I think the correct term is Epstein'd hahaha

Seeing your journey with these ASTs really makes me hesitate on changing out my TCKs. They're almost dead but still do a good enough job.

If it ain't broke...

However,

I believe you said you would be going with FCM route though. I believe that journey would be a lot better than me with my shoe string budget and 10 year old takeoff parts, and at least you can talk with him and get something exactly how you want/suited for your needs with high quality bilstein based dampers rebuilt.

[samthejam]

So I have come to like it at 5F 6R, with the 11k/14k rates. I can daily drive it like that over BAD roads.

The car was doing well at COTA, not my best driving but happy with everything for the most part.


Now the RF is the one I have been bitching about, at COTA coming down the hill or any long right hander by the end of the weekend I was getting a bounce through the steering wheel. That wheel was oscillating when unloaded. It also was very loose on long right handers, can drive around that but if I can fix that, that'd be great. I took it out and it still has the some gas pressure (by feel), there isn't a huge mess of oil etc. Knob is still doing shit.


These are no longer supported by AST/Moton, or any rebuilder I contacted since they cannot get parts from AST/Moton. To add these were a very limited run compared to the 4100s. So I cannot get them rebuilt by anyone.



I would like to go MCS, but it's $3150 just for the dampers (only single adjustable). I think my camber plates and other hardware will transfer over. I can't afford that right now, and I really want to stay with a high quality monotube damper suited for the rates I'm at now that have support easily in the USA. I would also like to keep driving and enjoying the car while saving up for MCS, or wait for a good deal on 2WNR (he will never learn the true cost of a good deal).


I remembered, some of my air cooled buddies from PCA were able to fill their shocks with nitrogen. They actually play around with it for tuning, as well as check it if the car has any weird handling differences side to side. A lot of remote reservoir shocks will give the user a nitrogen fill port (schrader) for servicing, if the reservoirs have to be disconnected. Some use it for tuning but it's intended purpose service is for service.



Here is a video of it being serviced:

https://www.youtube.com/watch?v=n87-l8hpmX8

Now I remembered when I was researching these, Vorshlag clearly said a port was a nitrogen fill.




I thought how the fuck did they fill that, and I saw the only way that shocks are filled must be with this tool. Basically you stick the needle in and pressurize and it self heals, like a basketball and holds the gas pressure that way. It is less likely to fail than a Schrader valve and noone will fuck with it, I'm assuming it is also heaper to do this.



While I was window shopping (salivating at MCS) used sets or new sets, I noticed they did have a similar port.



So I knew someone who used to work at MCS and asked them about it to confirm it was a self healing rubber/silicone type of fill under that bolt and it is on the 1WNR and 2WNR, due to the similarities I am going to assume the AST is as well.



I bought the tool and will try filling it up, I have the other side which seems good as a reference to match.

https://www.amazon.com/gp/product/B082FK271J/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

It is quite fiddly though, finally found a vid from a BC Racing Revalve that illustrated how to do it. I bought a cheap one from Amazon so pray for me.

https://youtu.be/mDukDHoMSJg?t=395

[samthejam]

https://nam3forum.com/forums/forum/main-forum/e46-2001-2006/296956-fcm-spreadsheet-questions-measuring-susp-freq-w-phone

Just linking this here

[samthejam]

Also targets to aim for he said

"AST . They run they kind of high, too high if you ask me. But 150psi front and 800-100psi rear is somewhat common. I start feeling the difference of 20psi especially in the rear. Also, the exhaust side will typically be 5-10pis hotter when tracking."