suspension geometry question

DUCE400

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Hey all, I am currently installing the BMR LCA relocation brackets on my 2012 gt500 and I was calculating the appropriate length to set my BMR adjustable LCAs to. I dusted off my high school geometry skills and calculated a length of 19.5 inches overall (using the lowest spot on the brackets). This is 1 inch longer than the stock LCAs. Mathematically this makes sense however becuase this is a load bearing system having a LCA length that Mathematically makes sense may not be the correct answer. Therefore my question is what length have some other members of the community used with a similar set up and how did it work out for you? Thanks.
 
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Black Cobra '99

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Well, what are you adjusting the arm length for?
These arms are adjustable so you can dial in your pinion angle, BMR's instructions says to start with stock arm length and then adjust so that your pinion angle is correct. As for the holes on the mount, that would depend on if you are lowered or not.
 

SCGallo2

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My initial setting on new LCA relo brackets would NOT be the bottom hole. In most cases for lowered street cars, the middle or top hole is a better starting point. Generally, you want the LCA rear mounting points on the axle to be lower than the forward mounting points on the body by 1" +/- 0.25" with the suspension loaded.

I have an adjustable UCA and adjustable LCAs. I primarily use the UCA to adjust my pinion angle. I only adjust the LCAs to zero thrust angle during alignment and to perfectly center the rear axle forward and aft in the wheel well. As stated above, start at stock LCA length and adjust as needed.
 
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DUCE400

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Thanks for the input gents, I forgot to mention the car is lowered. I ended up going with 19 inches overall .5inches longer than stock. Test drive went well, any other input would be great thanks guys
 

Black Cobra '99

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I would take it to an alignment shop to make sure your thrust angle is correct, and if you can do it at home check your pinion angle as well.
As for which hole on the bracket, you should have the control arm as close to parallel to the ground or lower on the axle side. The lower it is on the axle side the more anti-squat you'll have, but if you don't have the tires for it you won't see any real benefits. Thats why I prefer stock geometry in the rear.
 

Jam421

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Track guys or rear geometry experts can correct anything I might say that's wrong because live axle UCA/LCA geometry was explained to me 20 years ago. In essence with suspension loaded on ground LCA's shld be angled slightly upward & the UCA's slightly downward. If one draws imaginary lines extending CA lengths where the two lines intersect should be forward on the car . That intersect point is Instant Center. On my old FFR Cobra kitcar the best traction Instant Center crossing point was just in front of the door rear seams. Sounds weird but the chassis shop back then told me my UCA/LCA geometry before tuning had effect of extra weight " behind the car". Front used to pop up but it got squirrelly once it hooked. After adjustments it went dead straight...traction or no traction. Also, max straight line traction geometry can have negative effect on handling in the twisties. My goal was street driving.

On my '13 LCA's are stock with BMR UCA's set at middle hole . Can't recall first setting but I was a bit disappointed so I went back for a UCA position reset. Whatever they did when I left that shop traction was awesome ! Mustang race shops or even better a chassis shop can tune your rear end geometry.
SCGallo2 & Black Cobra '99's advice can likely save you from bad rear suspension reactions with a heavy foot stomp. So yeah.....if you want to keep her straight under power with two straight lines and no wheel hop....live axle rear end geometry is a big deal. Perhaps even a call to BMR would help as well. At work while being paid of course :)->).
 
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DUCE400

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Guys this is great stuff thank you very much. I had realized that the set up I'm currently running is certainly not ideal for circuit style driving. My personal goal is simple street and drag which is why I used the lower mounting point. I should say though my upper conrol arm is set to stock length to maintain the stock pinion angle (dont want to bust a U joint). Additionally I replaced the two piece drive shaft in the car with a single for performace shaft. Thus far driving around with the LCAs at 19 inches on the lower mounting point has felt really good. Car seems planted well and I have had zero drivability issues, car goes straight as an arrow as well. Anything else you guys have is greatly appreciated. Thanks
 

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