Before I lose my mind, AST 4150 Setup Opinions

[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.