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      01-04-2015, 01:43 AM   #1
fe1rx
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Arrow 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:

Name:  1 Exploded.jpg
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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.

Name:  2 Piston.jpg
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The foot valve in the cylinder controls the flow of fluid between the outer tube and the cylinder.

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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.

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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.

Name:  5 Orifice Plate.jpg
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No surprise that this is a very basic shock absorber.
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      01-04-2015, 08:36 PM   #2
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Thumbs up

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.

Good stuff
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      01-04-2015, 10:13 PM   #3
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This is great thanks for posting. Not too many folks out there take the time to make these types of detailed threads.
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      01-05-2015, 12:11 AM   #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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      01-05-2015, 12:47 AM   #5
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Quote:
Originally Posted by chris82 View Post
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.

Good stuff
I thought that was fairly apparent. This should help.

Name:  7 Exploded V2.jpg
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      01-05-2015, 12:52 AM   #6
fe1rx
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Quote:
Originally Posted by andrey_gta View Post
Nice mad science lab stuff

What gives this shock the "long life" characteristics to be an OEM part?
I think the primary relevant characteristic is that it is cheap.

My motivation was actually to salvage this part from the bottom of the shock:

Name:  8 Needed.jpg
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So I could make a fixture to test the stiffness of the lower shock-to-camber-arm mount.

Name:  9 Test.jpg
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But that is another subject ...
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      01-05-2015, 05:01 AM   #7
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fe1rx Very nice! We look forward to more of your posts.
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      01-05-2015, 06:22 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, 10:32 AM   #9
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I like this idea, a lot.
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      01-05-2015, 11:30 AM   #10
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Quote:
Originally Posted by fe1rx View Post
I thought that was fairly apparent. This should help.

Attachment 1138571
Thanks! nice work!!
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      01-05-2015, 01:10 PM   #11
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When I get to a computer I am reppin this thread, OP.
Great read!
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      01-05-2015, 01:24 PM   #12
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Yes, an engineer presumably, with access to some testing equipment. This should be good. I love seeing actual data.

Keep us updated.
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      01-05-2015, 04:48 PM   #13
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Here is the stiffness of the OE rear lower shock mount in compression:

Name:  Rear shock lower mount stiffness.jpg
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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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      01-06-2015, 12:36 AM   #14
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Quote:
Originally Posted by HooningB2G View Post
I want to see how strong those flimsy lower shock mounts are lol! subed!
So you can see from the above that the lower shock mount is not particularly flimsy.

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.

Name:  10 OE Rear Shock at Ride Height.jpg
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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.

Name:  11 Ohlins Rear Shock at Ride Height.jpg
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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, 02:01 PM   #15
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Quote:
Originally Posted by fe1rx View Post
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.
I suspect this is the key to the ride improvement from the Dinan upper shock mounts--that the car no longer rides on the bump stops.

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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      01-09-2015, 11:47 PM   #16
fe1rx
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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.

Name:  1 Upper Seat.jpg
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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.

Name:  2 Upper Seat from above.jpg
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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.

Name:  3 Fixture Block.jpg
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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.

Name:  4 OE Relaxed.jpg
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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%.

Name:  5 OE Installed.jpg
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Here is the test setup for the OE mount. Applied force is measured by an electronic load cell and deflection with a dial indicator.

Name:  6 OE Test.jpg
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Here is the test fixture assembled finger tight with the Powerflex yellow urethane mounts at their uncompressed thickness.

Name:  7 PF Relaxed.jpg
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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%.

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Here is the test setup for the Powerflex mount. This is at maximum applied force of approximately 1800 lbs.

Name:  9 PF Test (loaded).jpg
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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.

Name:  10 Rear Shock Bushing Tests.jpg
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Same again in Metric units:

Name:  11 Rear Shock Bushing Tests Metric.jpg
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      01-10-2015, 12:01 AM   #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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      01-11-2015, 12:28 AM   #18
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Quote:
Originally Posted by mike082802 View Post
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.
The Dinan upper mount uses a thinner lower part per the attached image from the Dinan website:

Name:  12 Dinan Upper Mount.jpg
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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:

Name:  13 OE Combined.jpg
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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, 05:59 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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      01-18-2015, 02:47 PM   #20
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Quote:
Originally Posted by fe1rx View Post
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.

Attachment 1140746

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.

Attachment 1140747

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.

Attachment 1140748

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.

Attachment 1140749

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%.

Attachment 1140750

Here is the test setup for the OE mount. Applied force is measured by an electronic load cell and deflection with a dial indicator.

Attachment 1140751

Here is the test fixture assembled finger tight with the Powerflex yellow urethane mounts at their uncompressed thickness.

Attachment 1140752

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%.

Attachment 1140753

Here is the test setup for the Powerflex mount. This is at maximum applied force of approximately 1800 lbs.

Attachment 1140754

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.

Attachment 1140755

Same again in Metric units:

Attachment 1140756
Interesting data. You mentioned that the lower OEM and Powerflex bushing are plotted but I only see the OEM bushing.

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, 08:33 PM   #21
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Quote:
Originally Posted by Orb View Post
Interesting data. You mentioned that the lower OEM and Powerflex bushing are plotted but I only see the OEM bushing.

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?
Lower is OE only. I am not familiar with a Powerflex option for the bottom mount. And given how stiff it is in the axis of the shock already, I can't see the point.

Name:  10 Rear Shock Bushing Tests.jpg
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Size:  78.8 KB

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, 12:38 PM   #22
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Quote:
Originally Posted by fe1rx View Post
Lower is OE only. I am not familiar with a Powerflex option for the bottom mount. And given how stiff it is in the axis of the shock already, I can't see the point.

Attachment 1144966

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.
I misread this. I assumed the top mount had the lower and upper parts (2 part to the upper mount) tested separately. I was not able to observe the shock top mount in practice so assumed it articulated about 5 degrees if not more. A 40 lbs force side load is reasonable. Thanks for the extra details.

Last edited by Orb; 01-19-2015 at 04:49 PM.
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