A lot of people say lca's improve traction, now could someone explain exactly how they do this. I know what lca's do, like keeping the axle from rotating forward and backward(toward and away from the front of the car).But why does this have such a dramatic affect on traction like most people say it does.

 

the stock arms flex. Promoting wheel hop and other problems. I don't know about dramatic effect, but combined with LCA relocation brackets you will for sure notice the effect. Look at it this way, the axle is what pushes the car forward. What connects the axle to the body of the car to do the pushing? The lower control arms. They do alot of work.


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350 with stealth intake, holley carb, 470 lift cam. 700r4 with .5 boost valve, vette servo, tci lock-up kit, B&M megashifter. Richmond 3.73 gears, powertrax locker, timkin bearings, synthetic lube. Custom 3 inch single into 2 2.5 pipes. 1 1/2 drop springs, 1 5/16 solid front sway bar, 1 inch rear bar, custom subframe connectors, custom LCA relocation brackets. Kobel ground FX, current red metallic paint. Lots of other stuff...
82camaro

 

Because when it flexes, it allows the rear end to flop around wherever it wants to, instead of holding it firmly in place.

LCAs by themselves aren't a really huge difference, noticeable yes, revolutionary no. The single most noticeable improvement in my car was adding LCA relocation brackets. If the brackets had already been there before upgrading the LCAs, I'm sure the LCAs themselves would have made more difference.

Look at the geometry of the rear suspension. In most cars, the attachment point for the LCA at the frame, is higher off the ground than their attachment point at the axle. So, when the power from the engine starts to accelerate the car, the rear end of course is trying to push the car forward; but its "hinge", the LCA, is arranged in such a way that pushing the rear axle forward tends to make it rotate upward, thereby lifting the wheels off the ground. So they lose traction, at which point the pushing-forward force goes away, so the axle rotates back downward. The tires bite, forward force is restored, so the axle rotates up, so the wheels lose traction, .... in other words, wheel hop. The LCA brackets make the attachment point for the LCA to the axle to be lower than the frame end, so the forward force tries to rotate the tires into the pavement instead of away from it. Lowered cars have this situation even worse since the axle end stays at the same height but the frame end goes even lower.

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"So many Mustangs, so little time..."
ICON Motorsports

 

RB83L69 has the right idea. To extrapolate, the angle the LCA's are at determine where the roll center of the car will be for longitudinal acceleration. As they flex the roll center of the suspension changes, which changes the amount of traction the wheels receive. As they are set up stock the roll center is at the bottom of the car since the LCA's are almost parallel with the centerline of the car. The Panhard rod also has some contributing factor towards this but not as much. As the car accelerates forward the center of gravity will try to pivot around this imaginary roll center point. This causes a torque that pushes down on the rear wheels. This phenomenon is what is known as squat. Drag racers will move the axle end of the LCA down to reduce squat which results in more forward acceleration since less energy is put into rotating the car. For street use traction is the limiting factor, so most people want more squat for better traction. Unfortunately, the springs absorb too much energy by moving to far. To combat this harder springs are used to transfer more energy to the tires. If you have so much angle that the roll center is above the center of gravity, the car will dive on acceleration. This is why rear suspensions are still very importanat on front drive cars. Bottom line is less flex lets the control arms work the way they are supposed to.

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'87 Bird 350 .060 over 700R4 + Lots more

 

Ok, I can understand how the rear wants to rotate upward but I thought the torque arm lifts the front of the car which plants the back tires into the ground? Or does this wheel hop happen during the time it takes for the springs to compress? So are stiffer rear springs the order of the day too??Are there any books that I could read that may have some pictures and get a little more in depth. In the mean while do the lca relocation brackets work well on cars that are not lowered too and do they weld on or bolt on or are there both kinds, and where can i get some of these brackets. thanks for all the info guys.

[This message has been edited by AFRO IROC Z (edited July 12, 2001).]

 

1. Torque arm only locates pinion angle which is another story
2. LCA relocators are available through SPOHN and do work well, but require welding
3. Harder springs are a must to a point and then you start going backwards because you overwork the tires. A cheap way to go is to use spring helpers or spring rubbers to stiffen stock springs.
4. Read Fundamentals of Vehicle Dynamics or any other Vehicle Dynamics books.

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'87 Bird 350 .060 over 700R4 + Lots more

 

The OE LCA's don't flex (bend) that much. Certainly not enough to shorten their effective length by enough to result in any significant amount of axle motion anyway.

It's the soft OE bushings that allow the uncontrolled axle motion under acceleration, which translates into wheel hop.

Relocation brackets increase the amount of anti-squat in the rear suspension geometry, and that's what plants the tires. And good shocks make quite a difference too.

Norm

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1979 Malibu w/some cornering tweaks and a few other interesting things

 

The stock LCAs do flex significantly. You can actually bend them with your hands. When I replaced the ones on my car, which has had a 400 in it for some time now, one of them was in fact permanently balooned out from excessive flexing. And of course, when they flex, energy is stored up in them like little springs, so there's some extra motion in the system from that effect as well.

They are garbage. Boxing them in helps, since it changes them basically to square tubing, but the material is still ridiculously weak.

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"So many Mustangs, so little time..."
ICON Motorsports

[This message has been edited by RB83L69 (edited July 13, 2001).]

 

Flex or no flex, I boxed them with 1/8" flat stock the full length, and installed poly bushings, and got a noticable improvement in traction.

 

If by "flex" you mean twist, I'll agree. Stock LCA's are by design not stiff against torsional loads. Very likely this is an intentional design feature and part of GM engineering's solution to minimizing suspension bind. Twisting of the LCA's is more related to cornering loads than straight line acceleration, so this kind of flex doesn't particularly translate into poor traction. If GM had made the OE LCA's box section they would have been forced to use even softer bushings and the traction problems would have been worse.

About that "ballooned out" description. If the rolled up edges on the bottoms of the LCA's have been damaged (bent, cracked), improperly modified (flattened) or just plain overloaded by the combination of way more power, gearing, and grip than was considered in the original design process, then it's very possible for the channel shape to open up at the bottom under heavy acceleration loads. This is a buckling failure, not bending. You could probably fix this problem by boxing only about an inch at each end and 2 or 3 inches in the middle of each of the LCA's, just enough to keep the flanges from spreading apart, but not enough to turn the entire LCA into a rigid square tube. Better yet would be to use a shear panel inside the channel shape at mid-length plus the above mentioned amount of boxing.

I still say that LCA's won't bend enough to notice. And the buckling failure mode by itself will not involve storing and releasing of enough energy to make any difference in traction either. That much energy is bushings or anti-squat.

Norm



[This message has been edited by Norm Peterson (edited July 16, 2001).]