Shock Valving, Spring Rates

[The Dark Side of Will]

I think I'd estimated ~100 lbs per corner based on individual measurements of various components and estimates of wheel and tire weight. Good to see I wasn't too far off.

With the wheel and tire being between 40 and 50% of the unsprung weight, light weight wheels and tires are very important.

[Series8217]

I picked up a pair of Monroe Sensa-Trac 5906 (C3 Corvette) shocks for the front my 88 Fiero as backups for my Koni reds, one of which I found to be leaking today. These shocks have the same upper and lower mounting provisions as the 88 Fiero front shocks, but are designed for the 1982 Corvette.

The Monroe 5906 shocks have an extended length of 14.625 inches according to the Monroe catalog, but 14.375 according to my measurements. The compressed length is 9.375 inches according to the catalog, and I have confirmed that spec.

The Koni shocks for the 88 Fiero have extended and compressed lengths of 12.25 inches and 8 inches respectively, according to my measurements. With the spring removed, I compressed my suspension to the bump stop (which I trimmed back some) and the Koni shock is around 3/8" from full compression.

For reference, the Monroe 5821, which is listed for the 88 Fiero, has extended and compressed lengths of 11.750 inches and 8 respectively, according to the catalog.

To use the Monroe 5906 shocks, they need to be mounted at least 1.375" lower on the bottom, or higher on the top, in order to maintain the same compression travel as the Fiero Konis and the Fiero Monroe shocks.

With the Konis so close to full compression when I'm on the bump stops, it's possible that my leaking shock was damaged from over-compression. I think it would be worthwhile to space the bottom of the Konis down to increase the compression travel at the expensive of some droop travel.

I'm going to build a shock bottom spacer and see if there's enough clearance in the lower shock hole to run the Monroe 5906 shocks that way. If they fit, then the Corvette Koni and Bilstein shocks should as well, which gives a lot more options for valving without a custom revalve.

[Series8217]

The Bilstein motorsports AK-series dampers have the right attachment styles for the Fiero. If the length and stroke are correct, it may be possible to pick one out with the right damping for some proper springs. Page 9 of the catalog shows the valving and dimensions. The 88 Fiero uses "T-bar to pin" style mounting. Unfortunately, the AK series are not rebuildable so they can't be revalved either. Luckily there are a lot of valvings available from Bilstein.

Monroe's OE replacement shocks for the front of the Fiero have extended/compressed lengths of 11.750 and 8 inches respectively. There is a note in the Monroe catalog about the extended length being a bump stop length. My guess is there's an internal bump stop in the Fiero shocks to limit droop without damaging the shock. Other available shocks may or may not have this.

The Bilstein AK1051 should be a direct fit for the front of the 88 Fiero. This shock has an 11.93" extended length and 7.95" compressed length. This is pretty much the same as the Monroe OE-style replacement shock for the 88 Fiero. It would be wise to use up the extra extended length to lower the bottom shock mount a bit and gain some compression travel. I measured my Konis at very close to full compression when my suspension is touching the bump stops, which I trimmed a bit off of. It's a bad idea to bottom the shock.

Here's a list of all Bilstein shocks that may work, including the minimum length of required bottom-mount spacers to ensure the effective collapsed length is short enough for the 88 Fiero suspension. The first line is the Monroe shock for the '88 Fiero.

Code: Select all

Valving is in pounds, measured at 10"/sec.
The first number is rebound, the second is compression.
All other dimensions in inches.
AK series shocks are "sealed" motorsports shocks. They can be converted to take-aparts.
B46-ATA* are ASN-series motorsports shocks. They are designed to be user-rebuildable and revalvable.
B46-**** are aftermarket replacement shocks. They can be converted to take-aparts.

Part Number        Valving           Extended Length    Collapsed Length    Travel    Spacer Required   Price
'88 Fiero Monroe    unknown              11.75              8.00             3.75          None           
AK1051            392/172 linear         11.93              7.95             3.98          None           $70
AK1043            358/219 linear         13.48              8.60             4.88          0.65           $70
AK3050F           131/206 digressive     13.48              8.60             4.88          0.65           $80
AK3060F           110/233 digressive     13.48              8.60             4.88          0.65           $80
AK3060COBF**       110/233 digressive     13.48              8.60             4.88          0.65            ?
AK4040F           197/154 digressive     13.48              8.60             4.88          0.65           $90
AK5555F           245/190 digressive     13.48              8.60             4.88          0.65           $90
AK6565F           325/220 digressive     13.48              8.60             4.88          0.65           $80
AK1053*          565/164 linear         14.11              9.21             4.90          1.21           $70
AK1195*          577/359 digressive     13.86              9.21             4.65          1.21           $80
AK2072            381/233 linear         14.02              8.92             5.10          0.92           $65
B46-ATAD4          custom digressive     14.21             10.45             3.76          2.45           $220
B46-ATAL4          custom linear         14.21             10.45             3.76          2.45           $220
B46-1104          427?/120? linear         14.09              9.05             5.04          1.05           $100
B46-1516          110?/149? linear         14.25              9.82             4.43          1.82           $85

Valving is in pounds-force @ 10 in/sec with rebound listed first, then compression. This is NOT the same as the motorsports manual valving which is tens of kilonewtons @ ~20 in/sec.

AK1053 and AK1195 have larger shock bodies with 46mm pistons. The body diameter is ~50mm. These shocks some minor grinding/filing of the hole in the lower control arm in order to fit. The rest of the AK-series shocks have 36mm pistons and bodies fit through the existing hole in the lower control arm.

Some notes on spacers:
"Lowering balljoints" will reduce the spacer length required. Subtract the height of the new balljoint from the height of the original Fiero balljoint and multiply the result by 0.53 to amount to reduce the spacer length by. For example, a 1" longer balljoint reduces the required spacer by 0.53".

Trimming your bump stops will [i]increase[/i] the size of the spacer required to keep the shock from bottoming out.

The stock 88 Fiero suspension needs a shock with a maximum compressed length of 8.00 inches. If the bump stop is trimmed, the compressed length needs to be lower. The spacer used with the above shocks needs to be increased in length to reduce the effective compressed length.

Note that most of the AK series shocks have a 36mm diameter body and piston, but some are 46 mm. For example, the AK1053 is actually based on the B46-1104 shock body, which uses a 46mm diameter piston. The AK1195 also has a 46mm body. If converted to take-aparts by drilling a hole in the gas chamber area and adding a Schrader valve, these shock bodies will accept standard 46 mm Bilstein shock parts, including pistons and valve stacks. I've read that 36mm parts are available, but I don't know where to get them.

The alternative to using an AK-series shock is the $200 ASN-series B46-ADAL4 race shock, which has a threaded aluminum body and a digressive piston and is designed to be rebuilt and revalved easily. It's the only Bilstein take-apart that is available with a 4-inch stroke. I don't know if it's practical to shorten the shock bodies on the other shocks.

If you start with a digressive piston AK-series shock (AK1195) and don't need the coilover body, the AK series is more economical than the ASN-series.

It should be far easier to install an AK-series in the Fiero -- many of them bolt right up with no modifications. The AK1053 or AK1195 might provide a shorter 46mm-size body than the ASN but I'm not sure.

Adapting the ASN series is a bit more involved. At the top, the rod end needs to be unscrewed and replaced with a "pin" style extension (long threaded extension like the Fiero shocks have). Extensions are available commercially but the thread diameter is too large to work with the stock bushings. The ASN is about an inch longer than the 46 mm AK shocks when collapsed, and about the same extended. If the ASN is longer only because of the shaft, then the shock could be converted to use a remote reservoir with a separator piston, allowing removal of the separator piston from the ASN body to get some more compression travel.

* I have an AK1195 so I personally confirmed that it has a 46mm piston. Here's a source indicating that the AK1053 is 46mm: http://www.impalassforum.com/vBulletin/ ... ostcount=5 Note that "B46" at the beginning of the part number means 46 mm.

** I'm not sure what the difference is between AK3060F and AK3060COBF, but I suspect that "COB" refers to Bilstein's COB digressive piston, which contains a check valve. Although the AK3060 has more compression than rebound damping, it's easy to reverse that by flipping the shimstack and putting the check valve on the compression side.

[Series8217]

Since I have to send in one of my Koni Red shocks (it's leaking oil...) I guess I might as well send in both and have them revalved for the 400# springs eh?

Will, I think you mentioned that you had some Konis revalved before. Did Koni provide dyno charts of the shocks after the work?

[Series8217]

Some old (1980's) shock dyno data for various C3 Corvette shocks:
http://www.corvettefaq.com/c3/RideControlNewShocks.htm

Unfortunately many (or all) of these shocks are no longer available.

[The Dark Side of Will]

Since I have to send in one of my Koni Red shocks (it's leaking oil...) I guess I might as well send in both and have them revalved for the 400# springs eh?

Will, I think you mentioned that you had some Konis revalved before. Did Koni provide dyno charts of the shocks after the work?

I've had the Koni custom shop shorten Fiero Konis (for that Fierorrari that I worked on), but I haven't had them revalve anything yet.
I @$$ume that they would provide such, especially if you specifically asked for it.

[Series8217]

T-bar for converting eyelet shocks to T-bar mounting:
http://www.daymotorsports.com/proddisp.php?ln=17631

Images of Bilstein AK series shocks, from Day Motor Sports

Bilstein AK-series shocks

Bilstein-Street-Shocks.jpg (100.67 KiB) Viewed 9433 times

From these images you can see that AK1053 and AK1195 have a larger shock body diameter. This might not fit through the lower control arm on the 88 Fiero.

[Series8217]

I found a somewhat local Koni rebuild center: http://www.propartsusa.com/shockdept.html

Their prices for rebuilds on the 8040 series (front of the '88 Fiero) are higher than the shocks cost new though... but perhaps the price is for revalve as well, or includes dyno printouts.

[Series8217]

Revalve and rebuild from Pro Parts includes dyno printouts of full stiff and full soft. I was quoted a very reasonable price for 2 extra settings since I want some in the middle as well. They also offer a standalone dyno service for a reasonable price. They were extremely responsive over e-mail.. I got replies in less than 5 minutes to each e-mail I sent. Looks like Jeff Wong is behind the company. He's an SCCA Solo National Champ.

I requested a quote for revalving my fronts, and getting a printout of 4 settings (full hard, full soft, 1 turn from each), as well as the same 4 settings for my rear struts (which I will not be getting revalved at the moment since I haven't settled on spring rates yet).

If the price is reasonable for the whole shebang, I'll go ahead with this instead of trying to make some Bilsteins work.

[Series8217]

Got a quote back, should be around $500 for everything depending on parts (gotta repair whatever is causing one of my fronts to leak).

I might add on a dyno of the one non-leaking fronts before the revalve, just to complete the baseline so others have a good reference.

[Series8217]

What's a good way to measure the motion ratio on the front of an '88 Fiero? I'm about to take my shocks out to have them revalved and I want to make sure I get it right.

[fieroguru]

What's a good way to measure the motion ratio on the front of an '88 Fiero? I'm about to take my shocks out to have them revalved and I want to make sure I get it right.

The distance the lower pivot of the damper moves is 52% to 54% of the lower a-arm ball joint for +/-80mm of suspension travel.
The Lotus suspension analyzer says the ratio is 1.866 for 30mm bump and 1.897 for 30mm droop.

[Series8217]

What's a good way to measure the motion ratio on the front of an '88 Fiero? I'm about to take my shocks out to have them revalved and I want to make sure I get it right.

The distance the lower pivot of the damper moves is 52% to 54% of the lower a-arm ball joint for +/-80mm of suspension travel.
The Lotus suspension analyzer says the ratio is 1.866 for 30mm bump and 1.897 for 30mm droop.

I appreciate the numbers. I can use them to verify my measurements.

[Series8217]

My measurements agree with those numbers.

[fieroguru]

My measurements agree with those numbers.

Bloozeberry provided the needed 88 suspension coordinates and Zac ran them for me earlier this year.

[The Dark Side of Will]

What's a good way to measure the motion ratio on the front of an '88 Fiero? I'm about to take my shocks out to have them revalved and I want to make sure I get it right.

Nominally, motion ratio is the square of mechanical advantage... MA would be the distance from the bushing pivot axis to the ball joint pivot axis divided by the distance from the bushing pivot axis to the shock mount pivot axis.

This isn't quite perfect, as the shock isn't perpendicular to the control arm throughout the range of motion, but it'll get pretty close.

[Series8217]

What's a good way to measure the motion ratio on the front of an '88 Fiero? I'm about to take my shocks out to have them revalved and I want to make sure I get it right.

Nominally, motion ratio is the square of mechanical advantage... MA would be the distance from the bushing pivot axis to the ball joint pivot axis divided by the distance from the bushing pivot axis to the shock mount pivot axis.

This isn't quite perfect, as the shock isn't perpendicular to the control arm throughout the range of motion, but it'll get pretty close.

I originally calculated it that way, but I wanted a more accurate number, and I didn't want to rely on someone else's measurements. I decided to measure the motion ratio directly.

Here's how I measured it: I removed the spring and shock, made a rod with I a pointed tip on one end, and a plate with a small hole the pointed tip of the rod will set into. I fastened the plate to the lower control arm so that the hole was in line with the axis of the shock. With the rod set in place, it sticks up through the upper shock mount hole in the crossmember. I reduced the upper shock hole with a bushing so the rod didn't flop around too much, and then slid a retaining washer onto the rod until it stopped against the bushing. I marked some reference points on the fender and wheel, and on the rod (where the retaining washer is). Then I jacked up the wheel incrementally, taking measurements of the fender-to-wheel distance and marking the rod (after sliding the washer down) each time. Then I removed the rod, measured each increment vs the original mark, and plotted those measurements against the fender-to-wheel distances.

I noticed that the lower shock mount pivot point is ~1/4" outboard of the center of the spring, so the damping motion ratio is actually a little bit different from the spring motion ratio. I did make another hole in my rod retaining plate so that I could measure the spring motion ratio, but I haven't done it yet. I probably won't bother measuring it though.

[Series8217]

I also tried measuring the front unsprung weight, but my poly bushings bind too much to get a consistent reading. If I lifted up the wheel and let it settle, the scale would read ~80 lbs. If I pushed the wheel down and let it settle, the scale would read ~95 lbs. The real number is somewhere in the middle. If the binding is the same in both directions, then it should be about 88 lbs.

[The Dark Side of Will]

Here's how I measured it: I removed the spring and shock, made a rod with I a pointed tip on one end, and a plate with a small hole the pointed tip of the rod will set into. I fastened the plate to the lower control arm so that the hole was in line with the axis of the shock. With the rod set in place, it sticks up through the upper shock mount hole in the crossmember. I reduced the upper shock hole with a bushing so the rod didn't flop around too much, and then slid a retaining washer onto the rod until it stopped against the bushing. I marked some reference points on the fender and wheel, and on the rod (where the retaining washer is). Then I jacked up the wheel incrementally, taking measurements of the fender-to-wheel distance and marking the rod (after sliding the washer down) each time. Then I removed the rod, measured each increment vs the original mark, and plotted those measurements against the fender-to-wheel distances.

What were the numbers? How much did it vary throughout the range of motion?

[fieroguru]

Here is the damper and spring ratios for the +/- 80mm of bump.

Code: Select all

				
							
	Bump	Camber	Toe	   Castor	Kingpin  Damper1 Spring1
	Travel	Angle	Angle	Angle	 Angle	 Ratio	Ratio
	(mm)	(deg)	{SAE}	 (deg)	 (deg)	  (-)	  (-)
	(deg)						
							
	80	-3.0988	 0.0002	6.735	 8.5684	1.836	1.852
	70	-2.5639	-0.0466	6.5128	8.0258	1.842	1.854
	60	-2.0728	-0.0784	6.2899	7.5292	1.847	1.857
	50	-1.6245	-0.0964	6.0662	7.077	 1.853	1.86
	40	-1.2179	-0.1012	5.8418	6.6682	1.86	 1.863
	30	-0.8525	-0.0936	5.6166	6.3018	1.866	1.866
	20	-0.5278	-0.0741	5.3906	5.9772	1.872	1.869
	10	-0.2435	-0.0429	5.1637	5.6942	1.878	1.872
	0	  0	     0	     4.936	 5.4526    1.884	1.873
	-10	0.2024	0.0548	4.7074	5.253	  1.889	1.874
	-20	0.3626	0.1221	4.4779	5.096	  1.893	1.875
	-30	0.4794	0.2027	4.2474	4.9831	 1.897	1.874
	-40	0.5506	0.298	 4.0158	4.9162	 1.9	  1.872
	-50	0.5736	0.4097	3.783	 4.898	  1.901	1.869
	-60	0.5447	0.5405	3.5489	4.932	  1.901	1.864
	-70	0.4588	0.6938	3.3133	5.0233	 1.899	1.857
	-80	0.3095	0.8746	3.0759	5.1784	 1.894	1.847