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01-03-2015, 11:43 PM | #1 |
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OE Rear Shock Construction
For those that might be interested in the internal workings of the OE rear shock, I cut one apart to have a look. Of course I relieved the pressure and drained the fluid before doing so. Exploded view:
Construction is twin tube, gas pressurized. Rubber bumper on the shaft cushions full extension. Piston has a single disc controlling compression damping and another single disc controlling rebound damping. The foot valve in the cylinder controls the flow of fluid between the outer tube and the cylinder. The compression disc is controlled by a star-shaped spring. Under compression fluid passes through the outer slots in the orifice plate. The rebound disc is controlled by a conventional wound spring. Under rebound fluid passes through the inner circular holes. The orifice plate also has 4 small cutouts that represent a common fluid passage that permits fluid flow in both compression and rebound directions when shaft speed is insufficient to open the discs. No surprise that this is a very basic shock absorber. |
01-04-2015, 06:36 PM | #2 |
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This is great, can you take a picture of all the pieces lined up in the order that the go on the damper body? I'm a little confused on how everything pieces together.
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01-04-2015, 10:11 PM | #4 |
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Nice mad science lab stuff
What gives this shock the "long life" characteristics to be an OEM part?
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128i Sport 6MT converted to Euro 130i spec, 3.73 diff, tuned by evolve ~220 whp 207 wtq(ft-lb) SAE
In-progress: //M front arm, M3 rack, e36M lip Wishlist: Coils, n55 mnts, headers, LSD, e60 finn diff "The 1-series is the last car that BMW engineered before the Germans, as a car-making culture, fell out of love with driving." - R&T 2013 135is |
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01-04-2015, 10:47 PM | #5 | |
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01-04-2015, 10:52 PM | #6 | |
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My motivation was actually to salvage this part from the bottom of the shock: So I could make a fixture to test the stiffness of the lower shock-to-camber-arm mount. But that is another subject ... |
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snub-nose 2856.50 MightyMouseTech4341.50 |
01-05-2015, 03:01 AM | #7 |
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fe1rx Very nice! We look forward to more of your posts.
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01-05-2015, 04:22 AM | #8 |
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I want to see how strong those flimsy lower shock mounts are lol! subed!
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01-05-2015, 09:30 AM | #10 | |
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01-05-2015, 02:48 PM | #13 |
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Here is the stiffness of the OE rear lower shock mount in compression:
To know what this means you have to know the maximum damping force is. I did a back of the napkin calculation based on these assumptions: 1) 800 lb corner weight and 1.8 Hz undamped rear ride frequency give an effective spring rate of 45 N/mm (265 lb/in). This translates to the actual spring location as 144 N/mm (by dividing by the MR^2) and is close to my spring rate of 120 N/mm plus the added effects of the rubber suspension bushings. 2) I assumed critical damping, which will over-estimate the damping force. 3) I assumed a velocity of 19 in/sec, simply because this is the highest speed on a typical shock dyno, which presumably has some basis in reality. These assumptions produce a damping force estimate of not more than 900 lbs. If all these assumptions have any merit, the deflection in this mount is not likely more than ±0.04" (±1 mm), which to my eye is not much. I am going to test the stiffness of the upper shock mount as well, as that one is soft enough that you can see it move even at very low suspension velocities as you jack or lower the car. To have a complete picture I wanted to look at both ends of the shock mounting, but the upper end is actually more interesting and easier to modify. |
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01-05-2015, 10:36 PM | #14 | |
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An interesting point that is not visually apparent because of the dust boot normally covering the top of the OE shock - the 135i rides on its rear bump stop at normal ride height. The bump stop is very soft and fairly linear in its initial stiffness, but it adds to the total spring rate at the rear and means that the rear has a progressive total rate, even with a linear spring. Any load going into the bump stop also loads the shock upper mount, so that thing gets a good workout. You can see how soft this upper mount is simply by jacking the suspension and watching it move. I plan on measuring the actual stiffness of this mount also, but have to make another fixture. In contrast to the OE suspension my Ohlins suspension at ride height has about 27 mm of shock travel from ride height before the bump stop engages. This is "normal" for a performance oriented suspension, where a linear spring rate is desirable in the steady state working range of travel. I am planning on installing Powerflex upper mounts to stiffen up the upper mount. I will measure the actual stiffness of this configuration before I install it also. I think the soft upper mount chosen by BMW is an inexpensive way of softening the damping under high frequency low amplitude inputs. Any large amplitude input would immediately bottom out the upper mount and bring the shock into play. With a good shock, I don't believe this gimmick is necessary. |
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01-06-2015, 12:01 PM | #15 | |
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I just installed the Dinan mounts recently. That annoying E9x/E8x secondary oscillation after bumps is reduced--I suspect due to not bouncing off the bump stops all the time. Very interesting topic, fe1rx.
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E92 M3 Bits F: Sway/Control Arms/Tension Rods R: Subframe Bushings | BMW Perf. Springs | Koni Sports | Dinan Upper Shock Mounts | 225/245 PSS LUX 5000K | BMW Shadowline Grilles |
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01-09-2015, 09:47 PM | #16 |
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For those that are not familiar with the upper shock mounting point, here is a look at the hole where it mounts in the body, looking from below. The pressed in steel insert fully supports the lower half of the upper shock mount.
The same mounting point looking from above shows that the support area for the top half of the mount is reduced by a counterbore and large countersink. For the bench test of the upper shock mounts to be representative the test fixture needs to have this feature. My fixture block has been adjusted for bore diameter and thickness to compensate for the dust boot that is normally installed on the lower mount. I did not want to test with this boot in place because it obscures what is happening. Here is the test fixture assembled finger tight with the OE microcellular urethane mounts at their uncompressed thickness. The orientation is as installed in the vehicle. Here the assembly has been fully torqued showing the urethane mounts compressed to their installed thickness. An internal bushing controls the amount of installed compression. Because the lower half of the mount is fully supported and the upper half is only partially supported, they compress different amounts when installed. The top mount loses 38% of its uncompressed thickness, while the lower mount loses 20%. Here is the test setup for the OE mount. Applied force is measured by an electronic load cell and deflection with a dial indicator. Here is the test fixture assembled finger tight with the Powerflex yellow urethane mounts at their uncompressed thickness. Here the assembly has been fully torqued showing the Powerflex mounts compressed to their installed thickness. The top mount loses 28% of its uncompressed thickness, while the lower mount loses 19%. Here is the test setup for the Powerflex mount. This is at maximum applied force of approximately 1800 lbs. Load vs Deflection was plotted for each of the mounts and a linear regression line was fitted to give the stiffness. Below the stiffness of the OE lower shock mount, OE upper shock mount and Powerflex Yellow upper shock mount are shown in Imperial units. As can be seen, the Powerflex mount is comparable in stiffness to the OE lower mount and is approximately 7 times stiffer than the OE upper mount. Same again in Metric units: |
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01-09-2015, 10:01 PM | #17 |
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i wonder what the dinans are. i was thinking about getting those to pair with my koni yellows and bmw performance springs. how the ride for you? i see you went with the ohlins.
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01-10-2015, 10:28 PM | #18 | |
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It appears to be a cast polyurethane part, like the Powerflex mount, but thinner. In my experience cast and microcellular urethanes are quite happy being compressed to half their original thickness. They will compress more, but may temporarily take a set in the process. They will recover their original thickness with time. Accordingly I figure that under normal conditions they shouldn't be called on to compress more than 50%. The OE lower mount is about 14 mm thick in its relaxed state. That means it should not be called on in service to compress to thinner than 7 mm (if you buy into my logic). It is compressed on installation to about 11.6 mm thick, giving a usable stroke of 11.6 - 7 = 4.6 mm. I actually tested the upper mount to a bit over 5 mm stroke from the installed state. I didn't include this data point in the previous graphs because it falls in the non-linear range of the mount, but here it is below: Dinan claims that their mounts "increase shock travel by 10 mm for improved ride quality and performance". The only way they could increase shock travel by 10 mm is if the lower mount compressed to a thickness of 11.6 - 10 = 1.6 mm. Since (I am assuming they are) made from cast urethane they would need to be only about 2 mm thick to increase travel by 10 mm, and this would only be true if the OE mount itself was compressed to 2 mm in service (which is extremely improbable). As usual, the marketing hype is overblown and vague. |
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01-12-2015, 03:59 PM | #19 |
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Another interesting post.
From what I recall, briefly setting the two mounts side-by-side, the lower portion of the Dinan mount is approximately 1cm thinner and the upper part is spaced out to compensate, for an equal "stack height." Compression of the OE mount is something I hadn't considered as I compared the two, however.
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E92 M3 Bits F: Sway/Control Arms/Tension Rods R: Subframe Bushings | BMW Perf. Springs | Koni Sports | Dinan Upper Shock Mounts | 225/245 PSS LUX 5000K | BMW Shadowline Grilles |
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01-18-2015, 12:47 PM | #20 | |
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What bother me about the Powerflex busing is how much load would be put on the shock shaft when articulating. I do believe this is about 10 degree angle maximum. The added friction to the system might not be worth any benefits? |
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01-18-2015, 06:33 PM | #21 | |
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The top mount only articulates about ±1 degree, which is surprisingly small and results in quite manageable articulation loads even with the Powerflex upper mount. The bottom mount does go to about 10 degrees at the extreme of bump travel, as you observe. This takes 35-40 lbs of side force at the rod guide bushing. I haven't tried to characterize the friction resulting. I see you have an M lower camber arm and thus a rod-eye bushing at the end, which possibly results in lower side forces on the bushing. |
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01-19-2015, 10:38 AM | #22 | |
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Last edited by Orb; 01-19-2015 at 02:49 PM.. |
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