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Lowering a 308 Using Custom Lower Spring Perches

Discussion in '308/328' started by Brian A, Apr 4, 2020.

  1. Brian A

    Brian A Formula 3

    Dec 21, 2012
    2,202
    SanFrancisco BayArea
    Full Name:
    1983 US 308 GTS QV
    I have lowered my car 25mm (1 inch) in the front and 19mm (¾ inch) in the rear using custom lower spring perches. It is an inexpensive and robust way to lower the car.

    What custom lower spring perches do is simply move the bottom of the spring down the shock absorber (damper). When loaded with the car’s weight, the spring compresses the same amount as before, but since the spring is lower on the shock, the car lowers down a fixed amount.

    I installed the custom perches in conjunction with a major renovation of all rubber suspension components on my car (A-arm bushings, shock silent blocks, sway bar bushings, etc.). My car rode particularly high even with the old sagging rubber in place. I reckon my car was probably sagging ½ inch from my worn-out shock bushings. I worried that ride height would be even worse after I redid all the bushings.

    I used the spring perch design of FChat member Steve Magnusson. With his permission, his design specification sheet is posted below. Steve says many people have used his design for their cars. Also below are photos of my custom spring perches, my shock/spring/perch assemblies, and my car before and after. The tops of my tires are now 2 ¾ inches below the center of the wheel arch. My frame, just in front of the central jacking point, is now 117 mm (4 5/8 inches) above the ground. It seems there is variation in stock ride height for our cars with some cars above and below these numbers.

    Because the shocks are set diagonally, the drop of the spring and the drop of the car are not the same. In the front, for every 1 cm you drop the spring, the car drops 1.7 cm. In the rear, for every 1cm you drop the spring, the car drops 1.4 cm. In Steve’s drawings, the critical dimension is the L1. In analyzing the various L1s for the front and back, I came to realize that using the same L1 = 23 mm yielded a 25mm (1 inch) drop in the front and 19mm (¾ inch) drop in the rear. That seemed perfect so I ordered 4 identical lower spring perches.

    Any machinist can make the perches. Mike Maier Inc. (a Mustang suspension specialist) (https://mikemaierinc.com/) is near me in Livermore CA and he agreed to make the perches for me. Though he has a CNC machine, he elected to hand-turn them on a lathe because of the low volume. Three of us, “Brian Harper” and “kcabpilot” and myself, ordered sets. Brian and Paul have yet to install theirs.

    Installation of the spring perches requires substantial compression of the springs to install them on the shock. Reinstallation of the rear shock/spring assemblies also requires some compression of the assemblies. The fronts do not. There is more clearance between the A-arm and spring perch so there is room to lower the car more. I believe Steve Magnusson’s drawing indicate the maximum drop. If the perches are too low, large washer-like rings could be stacked on the perch to fine tune the drop a little.

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    BLACK HORSE, EP328 and thorn like this.
  2. Banzairacer

    Banzairacer Karting

    Aug 24, 2017
    233
    San Diego, CA
    Full Name:
    Sanjeev Thohan
    Hi Brian that is an awesome solution. If you don't mind my asking - what did it run you and would he be willing to make more? The one thing that I don't understand - as a newbie likely, how is the perch held in place? how can you lower it more if needed? does it necessarily stiffen the ride?
     
  3. Brian A

    Brian A Formula 3

    Dec 21, 2012
    2,202
    SanFrancisco BayArea
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    1983 US 308 GTS QV
    Mike asked me not to post the cost. He has to evaluate his production time to figure out a retail price. I think he would consider doing more, but his focus is Mustang stuff, so you'd have to discuss with him directly.

    The perches are held in place by two stock half rings. Below is a photo. They slip into the groove on the shock body and the spring perch traps the two half rings in the groove.

    To lower the car more, you would need to make spring perches with more drop than I specified. There is more room to a limit. Likewise, there are other ways to lower the car such as having threaded collars welded to the outside of the Konis or by using other aftermarket shocks such as QA1. I knew what I wanted, don't need to corner-weight the car, so selected a simple solution.

    The modification does not change the ride quality in any way (i.e. not stiffer or softer etc). It reduces the travel of the shock by the amount you add to the spring perch so that is an issue. Mike Maier says that it is becoming increasingly common to deal with shorter shock travel with specialized bump stops. Apparently Penske Racing makes a bazillion of them. I'll see, but all reports from Steve are that there remains sufficient travel for the lower perches to not cause problems.

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  4. thorn

    thorn F1 Rookie

    Aug 7, 2012
    2,647
    Tallahassee, FL
    Looks great, Brian.
     
  5. Banzairacer

    Banzairacer Karting

    Aug 24, 2017
    233
    San Diego, CA
    Full Name:
    Sanjeev Thohan
    Thanks Brian- I appreciate the cost issues and Mike’s main focus. Thanks for explaining the install etc. It’s a great option.

    Thanks. Sanjeev
     
  6. tuttebenne

    tuttebenne F1 Rookie

    Mar 26, 2003
    3,003
    Bay Shore, NY
    Full Name:
    Andy
    Elegant solution.
     
  7. ferrariowner

    ferrariowner Formula Junior

    Feb 21, 2014
    622
    Mansfield, TX
    Full Name:
    Ron
    I would like to perform this mod. However, I want to keep my front spoiler intact. It barely clears a lot of drives now.
     
  8. mk e

    mk e F1 World Champ

    Oct 31, 2003
    11,502
    The twilight zone
    Full Name:
    The Butcher
    Nice job Brain!

    I think about the first mod I made to may 308 was new perches for the factory shocks that allowed aftermarket 2.5"ID shocks to be installed on them, which allowed a little wider tire on the rear as well as the ability to change the ride, something you were tying to avoid I guess. Anyway, NICE!
     
  9. Alex308qv

    Alex308qv Karting

    Jul 1, 2016
    205
    PA
    Full Name:
    Alex
    Great well-written post, Brian. A suspension rebuild for my '85 is in the plans for next winter, and your (and Steve Magnuson's) posts appear to be just what I need to preserve originality while ending the periodic "gee, it looks like it's riding high" comment. Question about strength... these are machined with (what appears from the 3rd picture) a sharper 90-degree from the sleeve section to the flange (perch) section. From that same picture, the original part appears to be a forged item with a smoother transition from sleeve to flange. It sounds like a lot of owners have done this modification, so am curious if there have been any issues with strength, for example, any signs of stress cracking at the flange given the shock loading these receive plus (rears anyway) supporting about 1,000 pounds plus pre-load.
     
  10. Brian A

    Brian A Formula 3

    Dec 21, 2012
    2,202
    SanFrancisco BayArea
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    1983 US 308 GTS QV
    It is a good question about the strength of the flanges with sharper angles. We (I!) shall see if it is an issue. Steve's drawings do specify curved transitions rather than the sharp corners.

    I don't think there is really much hard impact on the perches though. The tires and springs smooth all shock/spring acceleration forces. Likewise, the load is only the load of the car; any preload is relieved when the spring receives the load of the car.

    On the safety side of things, if a spring perch were to fail, the spring would move less than an inch down and rest on the A-arm. Not good and needing repair, but probably not catastrophic.
     
    Alex308qv likes this.
  11. Steve Magnusson

    Steve Magnusson F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    19,148
    Texas
    Full Name:
    Steve Magnusson
    For those reasons, I increased the thickness of the machined design to 3.5mm (the stock Koni lower perch is 2.5 mm or 3 mm thick material -- I can't recall which ;)). Also, the reason for using the 3.5mm internal radius where the flange joins the body to reduce stress concentration. No reports of any failures that I'm aware of. Even though the load seems high, having the spring in the situation makes it a rather "soft" situation (versus a more ridgedly bolted situation).
     
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  12. Nuvolari

    Nuvolari F1 Veteran
    Sponsor Owner

    Sep 3, 2002
    5,332
    Toronto / SoCal
    Full Name:
    Rob C.
    I did exactly the same thing almost 18 years ago. No problems with the perches built to Steve Magnusson's specifications.
     
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  13. wildcat326

    wildcat326 Formula 3
    Silver Subscribed

    Dec 10, 2012
    1,600
    Chicago, IL
    Full Name:
    Justin
    FYI to others interested in such an outcome, I bought replacement springs from Superformance, which achieved the same effect (I recall someone mentioning they have one less coil than stock).
     

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