F2x/F3x and F8x Damper Interchangeability?

[FaRKle!]

I know most of the time it's F2x/F3x folks looking to put F8x parts on their cars, but in examining the interchangeability of the dampers, I actually found a case where it could be preferable to use F2x/F3x rear dampers on the F8x.

Based upon the rear damper gas forces, I might end up using some F2x/F3x, instead of F8x, Bilstein rear dampers as donors for my FCM custom damper build...

Originally Posted by Index

-0:00 Intro
-0:39 Front struts comparison
-3:22 Rear dampers comparison
-7:39 Rear damper shock shafts and gas forces
-10:49 Closing Remarks

[N1rve]

Originally Posted by FaRKle!

I know most of the time it's F2x/F3x folks looking to put F8x parts on their cars, but in examining the interchangeability of the dampers, I actually found a case where it could be preferable to use F2x/F3x rear dampers on the F8x.

Based upon the rear damper gas forces, I might end up using some F2x/F3x, instead of F8x, Bilstein rear dampers as donors for my FCM custom damper build...

But why would you want to go down to a F2X/F30X part? The F8X parts are tuned for this car and the F2X/F3X is most likely tuned for comfort.

[FaRKle!]

Originally Posted by N1rve

But why would you want to go down to a F2X/F30X part? The F8X parts are tuned for this car and the F2X/F3X is most likely tuned for comfort.

If your aftermarket monotube rear dampers use a large piston shaft the gas force may be much higher than desired. Going to an F2x/F3x one can alleviate that with its smaller shaft. Having dyno'd F2x/F3x Bilstein B6/8 rear dampers, I can tell you they'll easily handle F8x spring rates. You could bump your rear spring rate higher than stock and they'd still damp appropriately.

A lot of aftermarket companies' tuning is crap. Way overdamped or "one size fits all." Let's take Bilstein for example, the B6/8 damper tuning (front and rear) is the same no matter if you have a lightweight 4cyl RWD 2-series or the heaviest car of all the F2x/F3x/F8x lineup, the F33 6cyl xDrive. There's a 900lb curb weight difference between those cars...

[gds]

Originally Posted by FaRKle!

If your aftermarket monotube rear dampers use a large piston shaft the gas force may be much higher than desired. Going to an F2x/F3x one can alleviate that with its smaller shaft. Having dyno'd F2x/F3x Bilstein B6/8 rear dampers, I can tell you they'll easily handle F8x spring rates. You could bump your rear spring rate higher than stock and they'd still damp appropriately.

A lot of aftermarket companies' tuning is crap. Way overdamped or "one size fits all." Let's take Bilstein for example, the B6/8 damper tuning (front and rear) is the same no matter if you have a lightweight 4cyl RWD 2-series or the heaviest car of all the F2x/F3x/F8x lineup, the F33 6cyl xDrive. There's a 900lb curb weight difference between those cars...

I totally agree.. I have installed the Ohlins RT BMS00 in my 2015 f30( 320i ed lci with N13 1,6 turbo 170ps ,eu spec) and the ride is awful, very harsh . Looking at the FCM flat ride catalog i understood that only the 6 cyl models can acheive flat ride with these coil overs .Before reading the catalog i tried to make the ride more compliant putting 120 n/mm rear spring (instead of 160n/mm Ohlins rear spring ) and things gone totally the wrong way.. the ride is still harsh (due to the very stiff front spring 60n/mm for the 905kg front axle wheight ) and the rear became soft an floaty -no flat ride at all.

[swagon]

I was wondering if the M2 F87 M Performance coilovers would be okay on an F80.

[Racer20]

Good insights, and it's cool that you're digging so deep on this, not many people go down the rabbit hole like this. A few comments:

Monotube vs two tube vs adaptive:

The piston rod diameter takes on a different significance in each of these different damper designs.

Monotubes generally like smaller piston rods because with a smaller rod you can use lower gas force AND it leaves more of the piston face open to accept pressure, allow oil flow, and build damping more efficiently. Generally the only reason*** to have a large rod on a monotube is for bending load or to pass a wire down, it even then its possible to gun drill a 14mm rod.

For twin tubes, the rod diameter basically determines how much compression force the damper can build. A small diameter rod doesn't force much oil through the bottom valve, and a low flow rate bottom valve can't support as much compression damping on the piston. There are ways to get around this, but they all have trade-offs either in compromising rebound tuning, increasing the complexity of your valve flow path, or increasing damper dynamic pressure, which hurts ride.

For adaptive dampers, getting the piston rod diameter right is absolutely essential. The explanation is too complex for here, but basically in addition to balancing rebound and compression against each other across the speed range, you have to ensure *that* balance remains consistent across the entire *amperage* range. If the piston rod is too small the damper WILL be rebound-heavy under some conditions, if it's too big it WILL be too compression heavy in some conditions.

Making comparisons across the three different architectures like you've done here can lead to some not-quite-right conclusions.

For modern cars F80-sized and larger, a 12 or 14mm piston rod is too small for a rear twin tube. Most of the tuning challenges my team deals with are due to OEM's that specify an undersized piston rod on one or both ends of the vehicle.

For a monotube, 11-14mm is the right size for this (and most) vehicles.

I missed the part where you said what specific shock you were going to put on your car, but if it's a twin tube, lower gas force is not enough of a benefit to offset the valve tuning challenges that will leave you with.

That said, Shaikh at FCM has a good tuning philosophy. Since he is an advocate of using more compression and using it properly, I'd be surprised if we was pushing to use a smaller rod in a twin tube. But if he is, I'm sure he has his reasons and I would trust him.

A couple other things:

Are you sure that the rear piston is larger than the front? Which dampers are you referring to here?

With respect to gas force making the damper not act like a damper, that's exactly right, and something I teach people. But it's not exactly a solid member either . . . In actuality the damper is acting like a spring under those conditions: returning energy to the system rather than removing it.

Great content, great presentation. Subscribed to your channel.

***another reason a monotube might use a larger rod is if it's using the piston rod to displace fluid through another valve or hydraulic circuit, like some adjustable dampers. Damper designs can get pretty crazy so take all of the above with a grain of salt.

[FaRKle!]

Originally Posted by Racer20

Good insights, and it's cool that you're digging so deep on this, not many people go down the rabbit hole like this. A few comments:

Wow, thanks for the insights! There's so much of this that's "black box" if it's not your day job (and mine has about 0 to do with the auto industry), haha.

Originally Posted by Racer20

I missed the part where you said what specific shock you were going to put on your car, but if it's a twin tube, lower gas force is not enough of a benefit to offset the valve tuning challenges that will leave you with.

That said, Shaikh at FCM has a good tuning philosophy. Since he is an advocate of using more compression and using it properly, I'd be surprised if we was pushing to use a smaller rod in a twin tube. But if he is, I'm sure he has his reasons and I would trust him.

I'm going to put the Ohlins monotubes on my car to start, and eventually move to an FCM setup based off of Bilstein dampers (also monotube). Interestingly for Bilstein's B6 passive damper the rear uses an 18mm shaft, which I presume is to keep the same thread size as stock. I wonder if the F8x B16 rear damper also uses an 18mm shaft, or if they use a 14mm instead since their rear shock is oriented upside down so the top mount thread and piston shaft are uncoupled.

Shaikh isn't pushing me towards twin-tube for my build, and is doing his usual build off of Bilstein donors.

Originally Posted by Racer20

A couple other things:

Are you sure that the rear piston is larger than the front? Which dampers are you referring to here?

With respect to gas force making the damper not act like a damper, that's exactly right, and something I teach people. But it's not exactly a solid member either . . . In actuality the damper is acting like a spring under those conditions: returning energy to the system rather than removing it.

For the Bilstein B6 dampers the rear piston is larger than the front. It wouldn't surprise me if Ohlins was similar since they both have to fit the damper body inside the outer strut body (instead of the strut body being the damper body).

Originally Posted by Racer20

***another reason a monotube might use a larger rod is if it's using the piston rod to displace fluid through another valve or hydraulic circuit, like some adjustable dampers.

Hrm, is that why MCS uses such large shafts?

[Racer20]

Originally Posted by FaRKle!

Wow, thanks for the insights! There's so much of this that's "black box" if it's not your day job (and mine has about 0 to do with the auto industry), haha.



I'm going to put the Ohlins monotubes on my car to start, and eventually move to an FCM setup based off of Bilstein dampers (also monotube). Interestingly for Bilstein's B6 passive damper the rear uses an 18mm shaft, which I presume is to keep the same thread size as stock. I wonder if the F8x B16 rear damper also uses an 18mm shaft, or if they use a 14mm instead since their rear shock is oriented upside down so the top mount thread and piston shaft are uncoupled.

Shaikh isn't pushing me towards twin-tube for my build, and is doing his usual build off of Bilstein donors.



For the Bilstein B6 dampers the rear piston is larger than the front. It wouldn't surprise me if Ohlins was similar since they both have to fit the damper body inside the outer strut body (instead of the strut body being the damper body).



Hrm, is that why MCS uses such large shafts?

With a monotube the rod size isn't quite as critical to the tuning as a twintube . . . you just need a bit more packaging length and you get more gas force pushing the car up like you mentioned. So yeah, the thread size could be the reason, or some other loading or parts sharing reason.

Monotube fronts would definately have a smaller piston than the rears due to the strut tube. I thought you were referring to the stock dampers, in which case it's the opposite.

Good luck!