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Thread: Suspension geometry analysis

  1. #11
    Registered Member chevynut's Avatar
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    Rick, I got close to the right SAI with my new dimensions, mostly confirming their accuracy. It's a little less than 3.5 degrees but with a couple of shims it would be right on. Actually the SAI is variable, if you define it to the ground so maybe that's why they specify a range in the specs. Every reference I've seen for SAI shows it from "true vertical" not relative to the spindle.



    I agree that it might be best to fix the lower a-arm, locate the upper balljoint at the 9.75" and 3.5" SAI, and then locate the upper a-arm pivot from there. That will eliminate any ambiguity regarding shims.

    Here's where the SAI is shown as 3.5-4.5 degrees: http://chevy.oldcarmanualproject.com...esto/56031.htm
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  2. #12
    Registered Member chevynut's Avatar
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    A little more info....from the center of the stock tri5 lower balljoint it's 2 13/16" up to the center of the spindle. Also, the tri5 wheel mounting surface is 4 7/8" outboard of the lower balljoint. The stock wheel has +9/16" offset and the front track width is 58". This with the 13.36" rolling radius will help locate the tire contact patch.

    (edited balljoint to spindle dimension)
    Last edited by chevynut; 02-22-2016 at 09:14 AM.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  3. #13
    Registered Member chevynut's Avatar
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    Using all the stock measurements, here's what my CAD layout shows for a STOCK tri5 (55-56 tire dimensions) with the lower a-arm level.

    The front suspension instant center is located 215.78" from the car's centerline on the same side as the tire in question. This puts the stock roll center at 1.61" below the ground at the center of the car.

    My 56 sedan appears to sit at a bit higher than stock height based on the lower a-arm pivot height around 10.5". I have read that the CG should be about at the camshaft centerline, which looks to be about 20-22" above the ground on my car, as good as I can measure it. I'll call it 21" for this discussion unless someone has better data.
    Last edited by chevynut; 02-19-2016 at 12:02 PM.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  4. #14
    Registered Member chevynut's Avatar
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    I just looked at the scenario using a stock spindle with 1/2" taller upper balljoint, and a lower a-arm that's level, 1" and 2" above level at the lower balljoint. These are all STARTING POINTS with the camber set to zero. The car is assumed to be at ride height in each situation.

    For the level lower a-arm, the instant center is at an infinite distance from the center of the car because the top and bottom a-arms are both level. The roll center is indeterminate (no instant center) but will be somewhere at or very slightly above ground level. I'm assuming camber is zero in this case.

    For the 1" off-level (up) a-arm, the instant center is 341.05" from the center of the car on the opposite side of the car. The roll center is 1.05" below the ground at the center of the car.

    For the 2" off-level (up) a-arm, the instant center is 177.26" from the center of the car on the opposite side of the car. The roll center is 1.83" below the ground at the center of the car.

    I have not yet looked at camber curves for any of these situations.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  5. #15
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    Ok I've crunched several scenarios on stock based suspensions and think I've learned a bunch. I used Chevynut's latest dimensions which make sense.

    Here's a summary of my results.

    Roll center height
    Stock -1.63
    Stock w/ 2" spring drop -4.02
    Dropped spindle -1.20
    Extended ball joint w/ 2" spring drop -1.07
    C3 spindle w/ 2" spring drop -0.8
    Dropped spindle + extended ball joint 0.96

    Roll moment arm
    Stock 22.63
    Stock w/ 2" spring drop 23.02
    Dropped spindle 19.47
    Extended ball joint w/ 2" spring drop 19.34
    C3 spindle w/ 2" spring drop 19.07
    Dropped spindle + extended ball joint 17.31

    Camber gain in next 1" compression travel
    Stock -0.21
    Stock w/ 2" spring drop -1.00
    Dropped spindle -0.21
    Extended ball joint w/ 2" spring drop +0.69
    C3 spindle w/ 2" spring drop +0.77
    Dropped spindle + extended ball joint +0.33

    I used Cnut's suggested 21" CG height for the initial setup, then let it move with the lowering. I think that's maybe low because what we're talking here is sprung CG not overall. Sprung is going to be a bit higher because all the unsprung is down low. But it's the same for all examples so you can see the trend.

    Except for the stock and stock w/ 2" spring drop listing, I used the rolling radius Cnut posted for a 225/50/17 tire.

    I think ride height could be investigated some more. I used a level lower arm for "uncut" springs.

    I realize I should have made a table in Word or Excel and imported it, hard to read what I posted. Will do tomorrow. I've put too much time in it today already to fix it now.

    What I see is that drop spindles get you better roll center and roll couple moment arm than other choices. The extended ball joints or C3 spindles get you more camber gain and improve the roll center a little, assuming lowering with cut springs. The combination of drop spindles and extended ball joints is an interesting mix.

    The main benefit of the C3 spindle is the scrub radius. That and an easy way to get 12" rotors and big calipers. Otherwise the results are similar to a 1" extended ball joint, which I expected. I guess you can consider whether an extended ball joint is strong enough but it probably is.

    As I've always said, the drawback to 2" lowering springs is the reduced travel.

    I'm going to put mine together with the C3 spindles/brakes and see how the suspension travel deal works out, as well as seeing whether I'm happy with ride height. If I'm not satisfied, it's pretty obvious which way to go.

    Edit: I made an attempt to make the data more readable.
    Last edited by Rick_L; 02-20-2016 at 09:07 AM.

  6. #16
    Registered Member rockytopper R.I.P 5-13-2017's Avatar
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    Please present data to support assumptions that stock trifives have level lower arms other than cartoons that are assumed.

  7. #17
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    I'm waiting for you to do that. Will I have to wait a long time?

  8. #18
    Registered Member rockytopper R.I.P 5-13-2017's Avatar
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    Quote Originally Posted by Rick_L View Post
    I'm waiting for you to do that. Will I have to wait a long time?
    I will help you to determine it yourself. If the arm is horz at stock ride height then my car setting in my shop at curb weight at least 2 inches down from stock is defying the laws of phyics. Do either of your models that are now perfectly correct per the fame gm diagram have the ground line 9.5 -10.49 below the lower pivot? I bet not.....
    Last edited by rockytopper R.I.P 5-13-2017; 02-20-2016 at 06:17 AM.

  9. #19
    Registered Member chevynut's Avatar
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    I laid out the early C4 geometry this morning and it all seems to work out fairly close. I located the lower a-arm pivot and balljoint, set the SAI at 8.744 degrees (not sure that's right), and located the upper balljoint. Then I worked back and got the inner pivot using the upper a-arm length. The distance between the upper a-arm pivots came out to 30.6" which is a bit wide but close to what I measured on my late suspension. With a little negative camber it would be closer. My C4 "Vette Specs" book says the camber should be "0.8, +/-0.5 degrees" for all years. Does positive camber make any sense? I would have thought it should be -.8 degrees so it must be a typo...on every page .

    The late C4 is another story. I laid out the 15" lower a-arm using the same lower pivot point since early and late lower pivot spacings are the same. Then I located the upper balljoint using the spindle height and the same SAI of 8.744 degrees (from my specs book). After locating the upper a-arm pivot, I ended up with over 34" between upper pivots which is over 4" too wide . If my measured dimensions are correct, then the late suspension SAI must be a lot more to end up at 30" between upper pivots as I measured on my Nomad chassis. So I used my digital level on my Nomad spindle and found that the SAI for the late suspension is actually closer to 18 degrees per my measurement!!! If I use my measured 30" at the upper pivots, SAI turns out to be 18.04 degrees so it all works out. I'm surprised there's that much SAI on the late C4 spindle.

    Also, the center of the early hub is 5 5/8" above the lower balljoint and the wheel mounting surface is 5.25" outboard of the lower balljoint, with the disc installed. The center of the late hub is 5 3/4" above the balljoint and the wheel mounting surface is 4" outboard of the balljoint. The tires are 245/45-17 which roll 810 revs per mils (BFG G-Force Comp T/A). That makes the rolling radius 12.45".

    To validate the measurements above I added the lower a-arm lengths and the balljoint to WMS distance to get 18.5" (13.25+5.25) total for the early suspension, and 19" (15+4)for the late suspension verifying the 1/2" difference that I've measured in WMS width. So everything seems to be checking out.
    Last edited by chevynut; 02-20-2016 at 10:02 AM.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  10. #20
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    18 degrees doesn't sound right at all. Does your layout let you measure scrub radius? Is it close to zero with the layout you have?

    The 8.75 degree SAI with the early suspension is a bunch more like I expected.

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