Shock Valving, Spring Rates

Real tech discussion on design, fabrication, testing, development of custom or adapted parts for Pontiac Fieros. Not questions about the power a CAI will give.

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The Dark Side of Will
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Re: Shock Valving, Spring Rates

Post by The Dark Side of Will »

And I assume you can get whatever valving you want in that body...
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Series8217
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Re: Shock Valving, Spring Rates

Post by Series8217 »

The Dark Side of Will wrote:And I assume you can get whatever valving you want in that body...
It needs to be converted to a take-apart by adding a Schrader valve so it can be refilled. The inner workings are the same as all the other 46mm shocks. The shaft diameter is smaller than the ASN circle-track shocks I have, but the end of the shaft where the piston goes should be the same. The endcap on the AK-series shocks looks a little tricky to get off, so we'll see how that goes.

I intend to do a writeup on the take-apart conversion process soon.
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Re: Shock Valving, Spring Rates

Post by Series8217 »

I took apart the AK1195. I took a bunch of photos and will do a writeup on this later.

I'm now considering shortening the shock bodies and shafts instead of spacing the lower mount below the control arm. This will make them drop in to a stock '88 Fiero lower control arm after minimal filing of the shock opening.

The shock body is trivial to shorten... cut it down by 1.25" and cut a new clip groove. The shaft takes a bit more work, since it's hardened and chrome plated. I could either shorten it at the pin end or the piston/valve end. Either one would require rethreading. According to Dennis Grant's page on converting ShockTeks to standard Bilstein take-aparts, the Bilstein shafts are case-hardened ~50 thou deep to Rockwell 52 (C scale?).

If I remove the separating piston and add a remove reservoir (which then contains the separating piston & gas chamber), I can just shorten the body and leave the shaft as-is. The shock gains 1.25" of travel (confirmed by measurement) since the gas chamber and piston will be moved to the reservoir out of the way.

What do I need to consider when machining the case-hardened, chrome-plated shock shaft? Do I have to give any special treatment to the edge of the chrome to keep it from chipping, or is that a non-issue?
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Re: Shock Valving, Spring Rates

Post by Series8217 »

The 4-inch travel ASN series shock is a lot shorter than I thought once the rod end is removed. It's actually about the same as the stock '88 Fiero shock. However, the stud on the end is too thick (10mm) and too short to put bushings on. I need to pull apart my front suspension and double-check the compressed length of the Konis I have. If they're measured the way I think they are then the shock shaft is the right length and, I can probably just cut the end off the Bilstein shock and weld or thread in a longer stud. Shouldn't be too hard.

I ordered a motor controller and 1 HP electric motor for the shock dyno. Parts should be here soon. Now I need to design the structure that will hold the shocks, the motor mount, the slider, and the flywheel/connecting rod assembly.

I also ordered a pair of Mustang Cobra Bilstein struts. They have a very thick top shaft and a simple body, so it should be relatively easy to adapt it to the Fiero. I can cut off the existing mounting ears and weld new ones on, and turn down the top on the lathe so it matches the Fiero top hats. The travel is within 1" of the Fiero and the strut body is close in length too.

Bilstein used to make struts and shocks for the Fiero, and I found a picture of the struts which shows an inverted 36mm monotube, so I've posted a WTB on the other forum in the hopes that someone is willing to part with them. I don't know if they made shocks for the '88, but they did make them for the 84-87 Fieros.
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Re: Shock Valving, Spring Rates

Post by Series8217 »

Just a quick update... got the Cobra struts most of the way apart. I thought they were "crimp tops" but there was just a bump stop plate that popped out and revealed a standard snap ring and rod seal underneath. I need to pierce the gas chamber still. Thinking about gun-drilling the mount stud to vent the gas and then I can add a valve there to fill it.

Back to the ASN shocks for the front...
I ended up ordering some Bilstein shafts for 6" travel shocks (longer than what I have), so that I can turn down the ends to make them pin-style like the stock Fiero shocks. I just didn't see an easy way to adapt the M12 threads of the original ASN shafts without increasing the length of the shock significantly. An inline ball joint might be the best solution if there was room for it.

I will be modifying a set of '88 lower control arms to mount the bottom end of the ASN shocks and carry spring loads. I mocked it up and it look sturdy enough. The solution is to cut a 3" hole through the shock mount plate of the LCA, then weld in a 3" OD 0.188" wall tube at the top (inner lip of the original spring perch) and bottom (OD of the cut hole). The 3" tube is drilled for two weld-in 3/4" OD 1/2" ID bushings to accept a 1/2" shock bolt. Since the tube is welded to the original spring perch and boxed in with the bottom plate of the control arm, it should be sturdy enough to carry spring loads through the coilover shock.

I got some 4" OD aluminum stock to make upper spring perch adapters. These will center on the upper spring perch in the crossmember and provide a 2 1/2" spring seat. Then, assuming the spring won't be too short (the adapter takes up height and the coilover sleeve takes up height too), I can run standard 2.5" ID springs. Otherwise a custom 2.5" ID bottom, 3.5" ID top conical spring should fit without a top adapter, and still provide adjustability by sitting on the adjustable coilover spring perch.

For those who want to run the Bilstein ASNs but don't want to modify the control arm, I'm working on a solution that allows them to be bolted into the original control arm. The top is still an issue -- custom shafts still look like the best solution. It adds ~$100 total to the cost. Damping rates will probably be somewhat limited due to the strength of the mounting area. It's not really worth adapting the ASNs this way though... shortened AK1195s are the way to go for a non-coilover solution. See below.

I could not come up with a simple solution to adapt the AK1195 as it is. I think the right way to make them fit is to just have them shortened by an inch. Then they'll bolt right on the car after some minor rework of the control arm with a file or rotary tool to clear the larger body diameter. Vendors such as Fat Cat Motorsports can shorten and revalve them. I might do some R&D for WCF so they can offer these, but their existing solution for '88 front shocks is pretty good, and should make some motorsports Bilsteins usable with just a shaft mod for the top mount.
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Re: Shock Valving, Spring Rates

Post by The Dark Side of Will »

How long before you're running 700# springs like some of the E30 guys?
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Series8217
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Re: Shock Valving, Spring Rates

Post by Series8217 »

The Dark Side of Will wrote:How long before you're running 700# springs like some of the E30 guys?
In front or the rear? Fronts should be on 700# springs within a few days of my return (I'm in China for work right now). I already have the springs. Rears will be a little under 500 until I fit some 1100# springs up front.
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Re: Shock Valving, Spring Rates

Post by Series8217 »

The Bilstein revalving manual doesn't list the volume for 4-inch ASN shocks, so I had to call them. The tech I spoke with told me that it's 179 mL.

Here is the full listing of oil volumes:

Code: Select all

P/N                 Travel (in)     Oil Volume (mL)
B46-0201 /0204        5                 239 
B46-0202 /0205        6                 272
B46-0210 /0211        7                 305
B46-0203 /0206        8                 320
B46-0209              8                 320
B46-GNTF1-19          8                 320
B46-GNTFR2-8          8                 320
B46-0207              9                 369
B46-DTA / LTA5        5                 239
B46-DTA / LTA6        6                 272
B46-DTA / LTA7        7                 305
B46-DTA / LTA8        8                 320
B46-DTA / LTA9        9                 369
B46-ATAD4             4                 179
B46-ATAD5             5                 239
B46-ATAD7             7                 305
B46-ATAD9             9                 369
B46-60BG8             8                 390
AK1195                4.65              200

All values are from the Bilstein valving manual except B46-ATAD4 which was obtained by speaking with a tech at Bilstein's Mooresville, NC facility, 1-704-663-7563, and AK1195 which I measured myself.
The valving manual is available at the bottom of this page: http://www.bilsteinus.com/downloads/pro ... formation/

UPDATE 2015-02-19: Added oil volume for AK1195
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Re: Shock Valving, Spring Rates

Post by Series8217 »

Seems like this thread is just sneaking by since I haven't been posting any pics.

Made some serious progress today by test fitting an ASN-series shock on the front driver's side. I modified the lower control arm to transfer all the shock loads through the original spring perch and boxed it into the lower plate.

The tube that was welded into the control arm is 3" OD 0.180" wall A36 steel. I cut a 3/4" hole through the side of it with an endmill, 42 mm from the top end of the tube. From there, welded in 3/4" OD 0.12" wall spacers, cut to size on a lathe so that the shock eyelet fits between them but there's enough meat sticking out to weld them on. I also milled a 22mm clearance hole for the schrader valve, since it sticks out a bit.

I cut out the original shock mounting area with a 3" hole saw, like WCF does for their Koni motorsports shock adapter.

I set the top of the tube flush with the inside of the original spring perch, and the poi welded it up for me.

Image

Image

Image

Bilstein B46-ATAD4 damper assembled with my custom shaft:
Image

At bump stop:
Image

From the top of the fender arch:
Image

At full mechanical bind (not sure what's hitting)
Image

Showing bump stop compression at full bind:
Image

Ride height at full bind:
Image

With the shock bolt removed I could still compress it a bit, so the shock is not limiting compression travel (good!)
Image

Maximum distance between spring perches. Actually it isn't even functional at this height; the perch is only threaded on by one turn. It needs to go a few more turns, and then the jam nut needs to be installed behind it. So the actual maximum installable spring length without disconnecting the top of the shock is ~7.5 inches. That's if the spring fits in the factory upper perch....

.. which this spring does:
Image

But it's a bit too long at 700 lbs/in and 8 inches uncompressed height. I'm going to need a shorter spring. I may just chop this one down for now, because I need something that works before Friday.
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Re: Shock Valving, Spring Rates

Post by Series8217 »

Should be able to get the other side knocked out tomorrow or Wednesday.

For now I'm just going to throw stiffer springs in the rear to match the ride frequency. I don't have dampers figured out for the rear yet. I paid for some Fiero Bilsteins from goatvenom on Old Europe but he has been non-responsive since before my China trip.. hasn't been posting on the forum either. Hopefully he's alright and I can still get those.
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Re: Shock Valving, Spring Rates

Post by The Dark Side of Will »

Have plans to trim the tube any?
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Re: Shock Valving, Spring Rates

Post by Emc209i »

It's good to see someone using "real" aftermarket parts. As usual, it looks really professional, keep up the good work.
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Re: Shock Valving, Spring Rates

Post by Series8217 »

The Dark Side of Will wrote:Have plans to trim the tube any?
I may trim off the unloaded sides to make it look a little nicer and save some weight. So it'll look more like a rounded clevis. I have it that way in the CAD model I analysed it with.

It's close enough to the wheel that ground clearance isn't really an issue.

I just didn't want to spend an hour sculpting this part and then find out it wasn't going to fit..
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Re: Shock Valving, Spring Rates

Post by Series8217 »

The passenger side control arm has been welded, and I also machined the shock shaft for the passenger side.

I also drilled and tapped the upper control arms to M8x1.25 and installed bolts which function as adjustable droop limiters. The bolts contact the crossmember as the control arm moves down. I can adjust droop easily by adding washers under the heads to increase droop.
I probably won't be using droop limiting as a tuning aid just yet, but the bolts will keep my shocks from being the droop limiters.

Tonight I'll throw it together and fit the springs. I'm just going to cut down the QA1 springs for now because I don't have time to finish my upper perch adapters and acquire 2.5" springs.
So at the end of the day the springs will be a little over 700 lbs/in, and I'll have a front wheel rate of ~200 lbs/in with a frequency of ~1.9 Hz assuming a corner weight of 600 lbs with unsprung weight of 80 lbs.

I haven't finished my shock dyno yet, so I'm just going to pick a standard Bilstein valving that's close and assemble the shocks to that spec for now. Unfortunately, all the Bilstein valvings are rated for the force at 10 in/sec. That doesn't say much about the low-speed stuff (0-3in/sec) which is more important. The specs also give no indication of how digressive the valving is.
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Re: Shock Valving, Spring Rates

Post by The Dark Side of Will »

I thought both early and '88 front ends had a motion ratio of ~2.0 (mechanical advantage of 1.4)... Wouldn't a 700lb spring result in a 350lb wheel rate?
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Re: Shock Valving, Spring Rates

Post by Series8217 »

The Dark Side of Will wrote:I thought both early and '88 front ends had a motion ratio of ~2.0 (mechanical advantage of 1.4)... Wouldn't a 700lb spring result in a 350lb wheel rate?
There is mechanical advantage and the spring moves less. So you have to square the motion ratio.
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Re: Shock Valving, Spring Rates

Post by The Dark Side of Will »

Motion ratio *IS* the square of mechanical advantage...
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Re: Shock Valving, Spring Rates

Post by Series8217 »

The Dark Side of Will wrote:Motion ratio *IS* the square of mechanical advantage...
Incorrect. Motion ratio is the motion of the spring relative to the wheel. It's the same as the mechanical advantage.

On the '88 Fiero front suspension, the spring moves 0.53 inches for every inch the wheel moves. The wheel has a mechanical advantage of 1.886 over the spring. When the wheel moves 1 inch, the spring moves 0.53 inches. With a spring rate of x lbs/in the spring force increases by 0.53 * x for every inch the wheel moves (in compression). The wheel has an advantage of 1.886 over this force. So the wheel rate is 0.53 * x / 1.886, or 0.53^2 * x.
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Re: Shock Valving, Spring Rates

Post by Series8217 »

This is one of the best references I have used for spring & damper design: http://www.kaztechnologies.com/fileadmi ... ch_Tip.pdf
Refer to pages 10 through 12 for some discussion of motion ratios.

Here's a very good hands-on example of gathering motion ratio data for an E30 M3 and the subsequent wheel rate calculations: http://www.e30m3project.com/e30m3perfor ... _rate2.htm
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Re: Shock Valving, Spring Rates

Post by The Dark Side of Will »

Series8217 wrote:So the wheel rate is 0.53 * x / 1.886.
Which is x / 1.886^2 (IE, motion ratio is mechanical advantage squared...) The wheel moves 1.886 times as far as the spring with MA of 1.886.

Based on checking things out with a tape measure, I thought that the mechanical advantage for both early and '88 suspensions was 1.4, which puts the motion ratio at 2.0.

I've read some of the E30M3 Project pages before. He uses the term "motion ratio" differently than the way it was introduced to me. What he calls motion ratio is really mechanical advantage. (I got my def here: http://www.amazon.com/dp/1560918314/ so I'll stick with it...)
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