How high should one relocate the front LCA pivots? - Ford Mustang Forums : Corral.net Mustang Forum
 
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post #1 of 17 Old 07-14-2015, 12:11 AM Thread Starter
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How high should one relocate the front LCA pivots?

I moved mine up 3/4" per the Mathis book, but I see cars at the track that have theirs up what looks like 1-1/2", and the aftermarket crossmembers go even higher. How high is too high, and what determines how high one should go? Tire temps are near even across the tread, and the outside tire is near vertical in hard turns, but that requires about -2.5 camber, and the inside edge of the inside tire is just touching the ground. Would a higher LCA pivot point allow less camber, give both tires a better contact pattern with the ground, and more importantly, the actual goal, give better grip and faster corner speed?

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post #2 of 17 Old 07-16-2015, 01:35 AM
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Moving the front LCA mounting point upward in the Mathis book was not equal front to rear...so if you went up 3/4" on the front & rear mounting points, then you probably didn't get the intended result. In the Mathis book, the un-equal upward change was intended to help improve anti-dive percentages.

Moving the LCA mounting point in-board & then using the 3/4" longer A-arm from the Thunderbird was intended to help improve camber gain ratio.

Moving the LCA mounting point upward as much as 1-1/2", without considering the relation of the spring perch (if your still using a stock spring location), or the change in the angular relation of the strut (or coil-over) to the LCA would be un-wise.

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post #3 of 17 Old 07-16-2015, 11:41 AM
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Depends on where you want the roll center. I've seen 15-30% of COG height thrown out a lot, or about 3-4" off the ground.
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post #4 of 17 Old 07-16-2015, 12:12 PM
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Originally Posted by EJthePJ View Post
Moving the front LCA mounting point upward in the Mathis book was not equal front to rear...
The uneven raise is seen as 'old fashioned' now-a-days. It creates unsteady feelings during braking etc. I undid mine a few years ago, making them even.

Aftermarket crossmembers raise it much higher. I think they go as high as 3 inches.


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post #5 of 17 Old 07-16-2015, 02:55 PM
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Originally Posted by 375instroke View Post
I moved mine up 3/4" per the Mathis book, but I see cars at the track that have theirs up what looks like 1-1/2", and the aftermarket crossmembers go even higher. How high is too high, and what determines how high one should go? Tire temps are near even across the tread, and the outside tire is near vertical in hard turns, but that requires about -2.5 camber, and the inside edge of the inside tire is just touching the ground. Would a higher LCA pivot point allow less camber, give both tires a better contact pattern with the ground, and more importantly, the actual goal, give better grip and faster corner speed?

The answer to your camber question is "probably", but you should take some measurements. You need to measure the camber gain relative to ride height as your car currently sits. Then plot it out. Shift the plot by the amount you want to relocate your pivot points and compare the slope of the gain curve. If it's steeper after the plot shift, you can reduce static camber by roughly that amount, (if you are running the same ride height before and after) to achieve the same contact patch under lateral loads.

Of course, consider the other impacts as already mentioned by those above before making any changes.

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post #6 of 17 Old 07-19-2015, 12:16 PM
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Quote:
Originally Posted by terryr View Post
The uneven raise is seen as 'old fashioned' now-a-days. It creates unsteady feelings during braking etc. I undid mine a few years ago, making them even.

Aftermarket crossmembers raise it much higher. I think they go as high as 3 inches.
Damn...it sucks when you find out your old & behind the times... LOL

We only ever modified one K-member, but it never gave us any braking woes.

If your running a stock spring location I would think you'd be significantly limited to how far you can move the LCA mounting point up.

Since almost everybody runs coil overs once they change K-members, the spring location no longer limits the mounting point of the LCA.
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post #7 of 17 Old 08-03-2015, 07:32 PM
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I modified a '89 K member to install in my '93 Cobra race car back in the mid 90's. I did it per the Mathis book and later found that the increase in anti dive was to much. If I was doing it today I would raise the mounting points the same. There is some antidive designed into the K members from the factory and the amount varied over the years some having less and some more.
When you go with CO springs and you up the spring rates over street rates for road racing the need for anti-dive is reduced. Add in a torque arm and I don't see the need to add any over what is stock.

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post #8 of 17 Old 08-10-2015, 07:17 PM
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I ended up moving just the rear hole out and up a 1/2" each direction. Wasn't a huge change but could feel the difference definitely under braking. I'd like to do more but just did it as a quicky job to test it out while the K member was in the car without having to make any huge modifications to anything. The main issue was my splitter would drag along the ground under braking after I made a few changes in the off season which was a TQ arm with stiffer Tq arm springs, and about 1/8" lower front ride height. The car nose dived terribly and could only brake just over 1G (supported by AiM data) which is terrible. Made the change, car didn't nose dive nearly as much, added coil overs to raise the front that 1/8 again, and can now brake around 1.4-1.5G's.

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post #9 of 17 Old 08-16-2015, 07:24 PM
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AJ, if you were going to do more, what would you do? That sounds like a significant improvement in braking! If I could replicate that, it would be huge for auto-x'ing, I'd imagine.
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post #10 of 17 Old 08-17-2015, 07:07 PM
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Originally Posted by aurdraco View Post
AJ, if you were going to do more, what would you do? That sounds like a significant improvement in braking! If I could replicate that, it would be huge for auto-x'ing, I'd imagine.
I don't think I'd go more with the rear hole, but move both upwards equally from that point to raise the front RC. But I would then have to raise the rear I would assume. But like anything, making one change, often effects several other and would have to do some testing or races to figure out where my car is happy.


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post #11 of 17 Old 08-23-2015, 10:53 AM
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Could you explain which of the four holes you are referencing here? I can't tell if you modified the two rear holes (one for each FCA bushing), or just the rear hole of the rear bushing. Thanks.
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post #12 of 17 Old 08-23-2015, 06:24 PM
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If just the rear hole of each rear bushing were moved, how could the bolt be reinstalled?

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post #13 of 17 Old 08-23-2015, 10:29 PM
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That's why I am confused. If he moved just the rear bushing's holes, I would have thought that he would have had to make an equal but opposite change to the front bushing's holes.

Anyhow, Jack, since SCCA won't allow MM K-members in CP without a ridiculous weight penalty: care to share how you would move the holes if you were building a CP foxbody?
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post #14 of 17 Old 08-23-2015, 11:07 PM
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<rant on>

If I was racing a CP Mustang, I would do what Brett Madsen is doing. Run the MM k-member, take the absurd 10% weight penalty, bitch slap everyone else in the class (hopefully) and take home the trophy.

<rant off>

To clarify, the k-member has four holes on each side, numbered 1 through 4 from front to back.

If you use rubber FCA bushings you can get away with a lot in terms of hole placement since the rubber bushings don't have thrust surfaces that need to be parallel with the four k-member tabs. Only the end of the crush sleeve needs to be perpendicular to the tabs. Once the bolts are tightened, the sleeves will bend this small area of each tab into conformance.

As soon as you use polyurethane or Delrin bushings, things get much more complicated. The bushing is the thrust surface for the k-member tab. They must be parallel or you will get bind and/or lots of play. To properly relocate the holes TO ANY DEGREE it is a ton of work to do it properly when using a nonrubber bushing.

Assuming the car doesn't have ABS, I would keep the stock antidive angle. Note that as the car is lowered, the amount of antidive will increase even with the same FCA angle. If the car has ABS, you can get away with a little more antidive, but Ford already built this into the 1996-2004 k-members.

I would raise all four holes the same amount. The limit on how high you can raise the holes is going to be based on the OD of your bushing package, and how much work you are willing to do to the k-member to get the matching FLAT thrust area raised also. Holes #2 and #3 are easy because this entire pocket is just cut off and moved up. Hole #1 can be moved upwards quite a lot. Hole #4 limits everything since the metal on the back of the k-member curves up and forward just above the stock hole. To move this hole up very much, you are going to need to cut out metal on the chassis and install the rear FCA nut above the k-member mounting plane.

Once I installed some Delrin bushings into stock FCAs that went into a Griggs modified stock k-member. There was no way that the very rear thrust bushing was ever going to be able to rotate against the rear thrust surface of the k-member due to the curvature of the metal noted above. I ended up machining and grinding my own rear thrust bushing to match the curvature of the k-member metal. This kept the rear thrust bushing from rotating and forced the sliding surface to be between the front of this thrust bushing and the aluminum case pressed into the FCA. I had to grind every thrust bushing in the FCAs at an angle to give both FCAs nice parallel sliding surfaces. I had the k-member, FCAs, bushings on a workbench with air tools and this took about 8 hours to do. Have fun. When I was done, I could spin the FCA nuts on and off the bolts with my fingers.

For every inch you lower the car, you need to approximately raise the FCA pivot 1". You very quickly run into needing to move the FCA upwards far more than it can physically be moved due to the hole location limitation. Sometime during your 25th hour of grinding, cutting and bending, you are going to realize that it will be much faster to fill out the SCCA paperwork to request a rules change and then just purchase a complete k-member, than it is to pick all of the steel slivers out of your fingers and finish this project

The other consideration is bumpsteer. For ever inch you raise the FCA pivot, you need to lower the outer tie rod pivot 1". If you are going to use 16" wheels, forget it, your car is going to have massive, uncorrectable roll understeer. I know of several competitive CP cars setup this way, but they also have lockers in the rear, so probably the two screwed up things counteract each other and make the car driveable

I would run 18" wheels and as tall a tire as I could get away with.

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post #15 of 17 Old 08-24-2015, 09:00 PM
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^^^ Totaly agree with everything Jack said above. I never said how I did it was the "correct" way. Just did it to see what effect it would have and if it was a step in the right direction. I know I was inducing some bind into the bushings, but the control arms really weren't much more difficult to move up and down than when in the stock holes. The poly bushings do have some play. I'm also not sure if you guys are thinking about the rear "hole" or "holes". I moved holes #3 and #4 out and up. I kinda oblonged hole #2 so all the bolts would fit. Hole #1, forward most hole, wasn't touched. It worked. Made the car feel much different, so its something I will pursue. Now that I'm headed in the right direction, I will do it more properly so Jack can't yell at me.

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post #16 of 17 Old 08-25-2015, 09:01 AM
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Thanks for clarifying, AJ, I appreciate it. I've got the car buttoned back together so I'm in no danger of mucking with it until winter hehe

Jack, I appreciate the detailed reply. What you said about the work required when using non-stock bushings and raised holes was very enlightening. I just put some bushings in the FCAs. The p/s FCA went right in, smooth as butter. I had to fight the d/s FCA to get it in, and this was with the stock pickup points.

Let's hope the CP guys push for the change.
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post #17 of 17 Old 08-25-2015, 05:42 PM
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Let's hope the CP guys push for the change.
A rule change regarding the use of aftermarket K members was requested in the past and was rejected. This was maybe 4-5 years ago.

If I recall (and I could be wrong), I believe the argument was that this modification is not widely available to all cars in the class, e.g. Corvairs (which are rear engine), giving unfair advantage to Mustangs/Camaro's.

LOL.

But nothing is stopping anyone from writing a letter and trying again and making them re-examine the issue.

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