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PLATNUM Supporting Member
Do not make the same goof up I did.
I let the guy who did my Corvette alignment do my 818. He is a good guy but the lateral links were not parallel. This caused a terrible condition under heavy acceleration that had my 818 all over the road above 80. I came close to wrecking the car twice. Thanks to Wayne from VCP I got it fixed. He was able too tell me what to check over the phone. The lateral links need to be parallel from the chassis pickup to the knuckle ( the one with the long bolt for the lateral links). Mine were way off causing a toe change under squat. Sort of a bump steer type problem. I used a digital level to make the arms parallel. I adjusted the arm above the lower trailing arm to get them correct. I have them both correct now and took it up to speed with no problem at all. Other than a neighbor called the cops on me. I was on a desolate back road and no one was in danger but me. I do not suggest this kind of behavior to anyone. Lucky the cops could not catch or find me in time. I won't be doing that around here again. LOL. I hope this is a help to someone else. I am also getting the alignment double checked again to be safe.
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818 builder
I did the alignment and what your talking about is trust angle. The links need to be even so the tires are even, or you will get what feels like a nasty toe out at speed. I wish you mentioned this before cause I would have told you.
Btw I got pulled over the second day I drove it , wanted to know what it was lol, let me go
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Metal, I think you mean "thrust" angle...yes?
-j
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Senior Member
I'm pretty sure there's more to it than just thrust angle... If I'm understanding Tom correctly, the thrust angle could be spot-on (perfectly symmetrical), but if the linkages aren't parallel, Tom's issue could still be there: excess toe change with compression. Note that it's impossible to have this condition with a solid rear axle, but TA is still an issue in that case. The root cause seems to be that with all the adjustability in the rear of the 818, it's an under-constrained system; i.e. there are more degrees of freedom than constraints. There are multiple solutions which all produce the same camber, toe and thrust angle, so a computerized alignment machine doesn't have enough info to go off of; the reading could all be right, but the geometry not as intended. The missing piece of info is as Tom posted, using a level to get the lower arms parallel, then, when the camber toe and TA are dialed in the geometry is correct.
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I think metalmaker's response is what is happening. If the rear link is higher than the front, then it will contribute more pull-in than the front. A perfectly horizontal member moves straight up and down, but the outer end of a member inclined at 10 degrees will move in 0.17 inches for every inch of vertical travel. So, under an inch of squat our hypothetical 10-degree-off setup adds 1.2 degrees of toe out.
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Senior Member
Originally Posted by
Jaime
I think metalmaker's response is what is happening. If the rear link is higher than the front, then it will contribute more pull-in than the front. A perfectly horizontal member moves straight up and down, but the outer end of a member inclined at 10 degrees will move in 0.17 inches for every inch of vertical travel. So, under an inch of squat our hypothetical 10-degree-off setup adds 1.2 degrees of toe out.
Agreed... it sounds like that's what's going on, but that's technically more than thrust angle. A solid axle can have the TA be off, but obviously the toe doesn't change as it compresses. I'm kinda harping on it, since even if the thrust angle measures zero in a static condition, you could still have this issue. So, if you bring it to a shop, have them set camber, toe and TA (all the machine can measure), it doesn't necessarily mean it's right.
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818 builder
Originally Posted by
Santiago
Metal, I think you mean "thrust" angle...yes?
-j
Yea lol, thanks. Thrust!!! See I can spell
Let me clear this up. If you try to set thrust angle with the upper rear arms it will be off, and I am guessing that's what he did. You simply have to set them to the same length to set the spindles the same dictance from frame on each side. I was just guessing were it went wrong, and obviously Tom figured it out. Should be good now, have fun, I am.
I set ride height 4.5 , which I thought was staying constant but when I got on rack it was 4.5 rear and 4.25 front. I kept it there since it was settled.
I than set up the car on the machine. I started with the rear upper control arms and made sure they were the same distance from the frame and that the center of the spindles were also the same distance from frame. I than set rear camber to .50 and toe to -3/32:-3/16 total. I than moved to the front and set camber to .50 and caster to +4. I than set front toe to -1/32:-1/16 total
I hope this makes sense
Last edited by metalmaker12; 04-18-2014 at 06:14 PM.
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Administrator
What was happening was roll steer and on Tom's car it was toe out under compression. With the car squatting under acceleration it would toe out and it would pull to which ever side had more traction. Tom's car had been aligned on a laser machine and the thrust angle was set.
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818 builder
Ok we did set the trust angle, then camber and toe in the rear. I missed that part lol
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Senior Member
Originally Posted by
Wayne Presley
What was happening was roll steer and on Tom's car it was toe out under compression. With the car squatting under acceleration it would toe out and it would pull to which ever side had more traction.
That is super scary. Rear toe out is sketchy to drive when you know you've got it. But with toe out being induced under compression, you've got a recipe for disaster. Turn a corner? Toe out. Accelerate? Toe out. Drive over a bump? Toe out.
Last edited by Rasmus; 05-18-2014 at 11:06 PM.
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Just surveying you guys: which of you do your own alignment and which depend on a pro alignment with a laser machine? The adjustability of the rear suspension of the 8181R is remarkable, for the reasons wleehendrick covers above. I find it very difficult to find 10/10ths alignment guys who are facile with these types of suspensions. What do you guys do with this problem?
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Administrator
I do my own alignments, I have wheel hub stands with lasers, and the billet digital longacre caster/camber gauge.
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Wayne - With that setup, how do you ensure the parallelism of the lateral link/toe link as discussed above - the level trick?
Thanks
Edit: mind my asking which wheel hub stands you use?
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Administrator
I put a .500 steel rod through the R upper lateral/toe links frame mounting holes. I use the digital angle gauge on the rod and then lay the gauge on the ends of the links by the knuckle and then adjust the upper trailing link until they match.
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Thanks; do you put the angle gauge on the bolt running through he knuckle or use another steel rod?
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Wayne should make a video on how to adjust the suspension or better yet GRM should do it as part of their build articles.
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Administrator
Using the bolt would be hard to get the angle correct. I lay a rod across the links and put the gauge on the rod.
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Thanks, Wayne; which hub stands did you go with?
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