Thread: 4 link rear alignment

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  1. #1 4 link rear alignment 
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    Using various chassis/body parts (OE LCA mounts, plumb bob dropped from holes spaced equally front to rear and side to side, ect) I centered my rear axle in the wheel well. This is with springs/tires installed. Once centered I set the PA to 11.8, rechecked the axle center, made minor adjustments, rechecked PA, more minor adjustments. Measured the length of the UCA's and LCA's to ensure equal adjustment. Rechecked placement and angle of all link tabs. Nothing is more than 1/16 out.

    When measuring my wheel base, I get 105 7/8 on the PS, and 106 1/4 DS (WTF!)? Do I need to get this on a rack to make sure my wheels are straight ahead, before adjusting the axles to exact parallel (thrust angle)? Should I even worry about this? Should I just set the axle to center and square using chassis parts as a point of reference (POR)? Just to be sure I didn’t pick one chassis POR, I used several chassis locations, in front of the axle, and behind.

    How do you align your 4-link rear? To chasis square, or to axle square? On a rack, ect?
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  2. #2 Re: 4 link rear alignment 
    My avatar isn't animated Lifetime Supporter SirFuego's Avatar
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    Dumb question, but are your front wheels pointing straight ahead? If they were turned slightly (which may not be visible to the naked eye), that could account for the wheelbase difference (assuming you were measuring from hub center to hub center).

    EDIT: I guess it's also possible that your front axle isn't aligned, either.
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  3. #3 Re: 4 link rear alignment 
    Senior Member IndyZJ's Avatar
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    Make the axles parallel to eachother or it won't drive right down the road. Ideally that would be square to chassis, too, unless something is tweaked or you mismeasured somewhere. It's likely that your tires are not pointed exactly straight ahead, so measure off the axle tubes/ inner Cs.
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  4. #4 Re: 4 link rear alignment 
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    I checked, and double/triple checked everything, including my front axle square to the chasis. Im assuming at this point it must be that my steering is just slightly off to one side, and its so little that I cant even see it. Which is why I think the only way I can square the axles to eachother (thrust angle) is on a rack, where I can verify my front wheels are dead straight.


    Last year about this same time I was installing my front LA's while I was taking an alignment class getting ready for my ASE, so it worked out nicley, I could set it up in my shop, then take it to school to verify my set up on the rack. Knowing whats involved with setting up the rear on a rack, I could get it done very quickly if I had access again. Im afraid if I take this to a shop, they will take one look at the suspension and either multiply there normal price x10, or just tell me GTFO.....
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  5. #5 Re: 4 link rear alignment 
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    Quote Originally Posted by IndyZJ View Post
    Make the axles parallel to eachother or it won't drive right down the road. Ideally that would be square to chassis, too, unless something is tweaked or you mismeasured somewhere. It's likely that your tires are not pointed exactly straight ahead, so measure off the axle tubes/ inner Cs.
    FAWWWKKK!!! This is a no shit moment for me....... Yea Ill drop a bob down from the front tubes, that will eleminate any issues with the wheels not being perfectly straight..... Thanks for pointing this out for me.
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  6. #6 Re: 4 link rear alignment 
    Senior Member AgitatedPancake's Avatar
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    is your front axle alined to be dead center with the track bar? If your off in that way it miiight skew the measurement, dunno
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  7. #7 Re: 4 link rear alignment 
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    Quote Originally Posted by AgitatedPancake View Post
    is your front axle alined to be dead center with the track bar? If your off in that way it miiight skew the measurement, dunno

    Im not for sure what you mean by "dead center with the track bar"? Im pretty confident that with the suspension at rest my front axle is straight side to side. For reference about half way down here is a good pic (half way down the page) of how my TB is set up on my front.
    Last edited by albersondh; 04-18-2011 at 01:55 PM.
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  8. #8 Re: 4 link rear alignment 
    Senior Member Jeeptech01's Avatar
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    Your wheelbase will measure different on the left and right side based on the amount of caster in the front end also. Most front lca brackets will be set up with the rt slightly in front of the left to compensate for road crown. HTH
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  9. #9 Re: 4 link rear alignment 
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    Quote Originally Posted by Jeeptech01 View Post
    Your wheelbase will measure different on the left and right side based on the amount of caster in the front end also. Most front lca brackets will be set up with the rt slightly in front of the left to compensate for road crown. HTH
    Why wouldnt they just turn the "C" on the DS to a bit more negative caster than the PS? This would have the same affect (compinsating for road crown) while allowing the axles to be perfectly paralell. This would elleminate crabing and dog tracking that occurs if the axles are not paralell? It just seems very odd that the axles would not be paralell by design?
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  10. #10 Re: 4 link rear alignment 
    Senior Member Jeeptech01's Avatar
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    Honestly I dont know why but Im sure there is a reason. I can say with 100% certainty that my factory lca mounts are offset and the rt side includes an adjustment to offset it more. Also almost every solid rear axle vehicle I have aligned had the lr tire slightly ahead of the rr the measurement of how much is referred to as thrust angle.
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