Redrilling LCA pivots

[kyle]

Hi guys- I've been reading here and posting at zcar.com today on the subject of getting some camber.

 

It seems that the cheapest and easist mod would be to move the LCA pivots up and out.

 

A 3/4" raise sounds like a good choice to raise the roll center and reduce bumpsteer.

 

My question is this:how far out should to move the LCA pivots? Moving them up 3/4" would lose some camber... and I'm going to lose some of the new camber to the TC rod as well. I'd like to get 2* as a baseline.

 

I'll likely either put my TC ball kit back in or shim the PU TC bushings to get the caster back.

 

Thanks for any help.

[kyle]

http://www.analyzemath.com/Geometry_calculators/cosine_law_calculator.html

 

.5" of tilt on the 15" wheel lip gives 2*.

[jeromio]

I moved mine up by 13/16" and out by 1/4". I now have 1.5* of static camber on a car that is lowered such that the arms are almost flat at ride height.

 

BTW, this is good for turn in, but terrible for street driving. The car follows in and all road irregularities and my tires wear excessively on the inner tread.

[kyle]

Thanks for the reply, jeronimo.

 

Been driving it this week getting reaquainted with the rear sway bar; normally it just gathers dust until the next track event.

 

Car is basically driven to the track, at the track, and home again, so street performance isn't a real big deal. Getting better turn in is the key.

 

Did you pull a spring and set the suspension at normal height to locate the rear hole? What did you do with the TC rods? How much caster did you end up with?

 

//edit, checked your site, still have a couple questions

[Twoeightnine]

Try this one on for size.

 

http://forums.hybridz.org/showthread.php?t=103886

[kyle]

Good link, thanks.... been reading a lot of past threads, and somehow that one escaped me.

http://forums.hybridz.org/showpost.php?p=569246&postcount=3

suggests that moving the LCA pivot is preferable to the spacer approach... I didn't see any feedback on that one way or the other. Or is this a CLSD vs. Torsen type debate?

[JMortensen]

I had somehow gotten confused when I wrote that FAQ and made a big mis-statement that spacers could be used to fix bumpsteer. They absolutely cannot fix bumpsteer. They change ROLL CENTER. If you look at the diagrams, Rob was showing why the spacer doesn't change bumpsteer at all, and how moving the control arm pivot DOES change bumpsteer.

 

You weren't initially interested in bumpsteeer, you were interested in changing your camber. Moving the pivot up or down doesn't do anything to change your camber. I'd suggest if this is a street car and you're not racing that you look at camber bushings as a cheap way out and don't bother with bumpsteer.

 

If you are planning on road racing then you need to take all of this crap into consideration, and I would worry about bumpsteer and roll center by drilling or slotting the crossmember and then worry about adjusting camber with plates or with adjustable control arms or both.

[kyle]

Hi Jon:

Always nice when a mod takes an interest in a n00b's thread.

 

The car is pretty much strictly a track car, although it's street driven to and from track days.

 

This whole saga started with this thread at zcar.com:

http://www.zcar.com/forums/read.php?f=1&i=703149&t=703149

which was an folow on to previous wonderings about getting the car to hook in a bit better.

 

Added the rear sway bar as suggested, and that made the balance more neutral. Still not quick enough turn-in for my tastes - the Miata's kinda spoiled me that way. And now it's unloading the inside rear tire: LSD time.

 

And it's obvious to all that I need more camber for the track... I'm trying to get there without camber plates or going to coilovers. LCA pivots is a free mod, and looks to get me the camber I want.

 

I'm not worried about bump steer, since the car is really stiffly sprung and has big rollbars; never even gave bumpsteer a thought, truly.

 

But if I'm drilling to gain camber, it's a good opportunity to get the roll center back up, reduce roll couple, all that good stuff.

 

Thoughts?

[JMortensen]

Biggest things to change for turn in: CASTER and TOE. Set your caster to about 6 or 7 degrees and your toe to something like 1/4" toe out (don't drive to the track with the toe set that way, change it at the track) and that car will turn in a whole lot better.

 

The more you get into this the more you'll realize that it all works together and any change you make to one part is going to affect everything else.

 

If you are building a track car you will want to take bumpsteer into consideration. It may not have been a problem thus far, but eventually it will be. A couple years ago I started to get going fast enough that it suddenly became a problem for me. I was running Buttonwillow clockwise like I had before without issue. This time was different though, I'd come over the Grapvine I guess they call it and when the car hit that rise there at the exit of the turn the car would do a really abrupt twitch, almost like an emergency lane change maneuver to the right. Then going through that long sweeper I'd be sawing at the wheel and the car would be taking a steady line around the corner. After that I determined that I needed to fix the bumpsteer. Did the slotting and that was it. Measured the bumpsteer, raised the LCA pivot until it was gone, went back to the track and I could go through the Grapevine foot to the floor and it didn't twitch at all.

 

There is a ton of info on camber mods here and you should read all the stickys and FAQs that concern it, but here's the bottom line for me (and I'm sure some will differ with my opinion):

 

The "best" way to adjust camber is with a combination of adjustable control arms AND camber plates. The reason is that our cars are almost never square so if you have for example -1 degree on the LF and +1 degree on the RF and you redrill the holes, you'll still have a lot more negative camber on the LF. If you install adjustable control arms you would need to adjust 2 degrees on one side just to square them up at -1 degree, so that doesn't work either. Plus when you have them squared up the RF control arm is say 3/4" longer than the LF, so that wheel isn't inline front to back anymore. Making the front track wider than the rear pays off in a big way, so you might want to adjust the track with the control arms for that purpose. With both adjustments available you can set the track where you want it with the control arms, then adjust the camber independently with the plates on top.

 

There are other issues too. Lengthen the control arms out too far (or redrill the holes too far out) and you'll run out of threads on the tie rod ends. Redrill the holes farther out also exacerbates bumpsteer issues. It goes on and on.

 

I'd suggest you do like I said and read the FAQs and all that, then also search the archives and read, pick up some books on the subject, and try to take as much into account as you can.

 

If you're looking for a quick fix I'd take Dan Baldwin's advice, lengthen the control arms 9/16", slot the strut tops (you might be able to even the camber out left and right with the slotted tops). By the way I totally disagree with Dan's bit about rear toe in. In my experience my lap times were fastest with about 3/16" total toe in at the rear, and I tried everything from 0 to 3/8" toe. You can always do the crazier mods later on.

 

At some point you need to decide to take the plunge and go all the way. I started with a pretty mild street car and changed parts and more parts and more parts, I think I've been through 4 TC rod modifications now, 3 front control arm mods, 3 rear control arm mods, etc. If I had known then that this was going to be a track only car, I would have foregone all the BS and went all the way the first time, and I would have saved a hell of a lot of money in the long run.

[kyle]

Been reading and searching a lot here this week, absorbing what I can. That's what lead me to the LCA pivot as a preliminary modification.

 

No argument that coilovers, plates, adjustable arms and rods, rear offset bushings, etc etc etc are the way to go. That's the reason I'm not screwing around with offset front bushings or offset hats; that stuff goes out the window with the next round of upgrades.

 

Between last year and this, I've done the rear bar, seat and harness, LSD en route, and up next some camber and toe... that's got to be enough tweaks from one session to the next, right?

 

My plan for this season (4-8 track days, if I'm lucky) is to get more consistant and go to Rcomps and hotter pads. No sense in building a car that exceeds my ability. More major suspension stuff next year, perhaps... and power the year after that.

[GrommitZ]

Hi Jon,

 

I just want to say that your posts are GREATLY appreciated.

[JMortensen]

Been reading and searching a lot here this week, absorbing what I can. That's what lead me to the LCA pivot as a preliminary modification.

 

No argument that coilovers, plates, adjustable arms and rods, rear offset bushings, etc etc etc are the way to go. That's the reason I'm not screwing around with offset front bushings or offset hats; that stuff goes out the window with the next round of upgrades.

 

Between last year and this, I've done the rear bar, seat and harness, LSD en route, and up next some camber and toe... that's got to be enough tweaks from one session to the next, right?

 

My plan for this season (4-8 track days, if I'm lucky) is to get more consistant and go to Rcomps and hotter pads. No sense in building a car that exceeds my ability. More major suspension stuff next year, perhaps... and power the year after that.

The one thing that I think you'll find is the the DOT tires will make you a lot faster, but they'll also be SCREAMING for more camber. I would still caution you against redrilling the hole because it will mess with the bumpsteer if and when you decide to worry about it later on. I'd run the DOTs as is if you have to, but you'll have to run a lot of pressure in them to prevent them rolling over onto the sidewalls. As a stop gap for the meantime I'd probably go with some camber bushings in the front (even though I really don't like them) vs redrilling the hole. Or lengthen the control arm like Dan did, which isn't too hard to do. Nobody says you have to take my advice though.

 

I just want to say that your posts are GREATLY appreciated.

Thank you sir.

[kyle]

Did not know that about the tires, food for thought, thanks Jon.

 

I just hate to spend money for offset bushings when I know their use is temporary. Then again, it's only $70. FWIW, MSA says they're good for an additional .75*. Hacking the arms is an idea, but somehow I don't much care for the idea of slicing and dicing such a critical piece.

 

Maybe I'll put my old TC rod ball/cup kit back in, add some caster, and leave it alone for now, then throw in some adjustable arms/rods when I do the tires. For that matter, maybe I just need to show some sack and buy a decent set of arms. Sigh, decisions decisions. Maybe that can be a midyear upgrade. Don't need the wife getting hostile.

 

So far, the street tires on it have been wearing straight and even, though I haven't checked temps. They look to have at least two more days in them, maybe three.

[heavy85]

Moving the pivot up or down doesn't do anything to change your camber. I'd suggest if this is a street car and you're not racing that you look at camber bushings as a cheap way out and don't bother with bumpsteer.

 

Strickly speaking moving the pivot does not change camber but the end effect is that it will affect camber. Let me explain. I raised my LCA pivot about 3/4" and run 1" spacers. This was an effort to keep a favorable camber gain in bump ... i.e. the front control arms pointing up so that the tires scrub out in bump and gain a little camber instead of losing it if the arms were flat or pointing down. Keep in mind my car is low and this allowed me to lower it even further. At normal ride heights the arms are already pointing up. My point is that the net effect of moving the pivot up was to keep the arm pointing up which narrows the track a little and reduces camber over keeping the arms either flat or at a less steep angle as they would be if you did not move the pivot up. I now struggle to get 2 degrees camber with offset bushings and EMI camber plates (which I dont care for but that's another story).

 

Thanks

Cameron

[kyle]

I would still caution you against redrilling the hole because it will mess with the bumpsteer if and when you decide to worry about it later on.

How so? Will the outward movement of 1/2" (just for the sake of argument) really have a different effect than a longer LCA with regard to the bumpsteer?

 

Will the 1/2" different effective length of the LCA create a significant angle disparity through the range of motion likely to be encountered? (IE, lengthen the tie rod 1/2" but keep the LCA the same length, just move the pivot)

 

Put slightly differently, if bumpsteer comes from dissimilar angle, does it really make a difference where the LCA is located laterally? When you're turning, the LCA and tierod could be anywhere in a wide range of positions.

 

I have a funny feeling I'm going to catch a beatdown for this, but I'm confused and it's all your fault!

[JMortensen]

No, no beatdown impending. We're getting into an area that always confuses me, but I think this is how it works. Follow me here: If you lengthen the control arm, you need to lengthen the tie rod and equal amount to get back to 0 toe. They are still the equal length. If you redrill the hole, you still have to lengthen the tie rod to get back to 0 toe. Now they are not equal lengths. When the suspension moves, the two won't stay parallel because they are not the same length anymore. The shorter one will move in a more severe arc than the longer one, leading to bumpsteer.

[kyle]

I'm with ya. What I'm wondering (and wishing I had the math smarts to wrangle with competently) is if a .5" different length of the control arm vs the tie rod gives rise to significant bumpsteer.

 

I played with that math calculator, and got .016" bumpsteer or .6", so I know I'm doing something wrong.

 

...probably trying to run track days on a budget, that's probably what's wrong...

[JMortensen]

A wise man once said "Don't ever trust Jon's math". Maybe Terry (blueovalz) or Cameron will check your math, but I won't. Bumpsteer moves in an S curve when you plot it out on a graph, and it's usually more severe at the ends of the suspension travel. How that works out for you in this case I really don't know. Maybe somebody else here has a program that can figure it out and plot the bumpsteer curve for you. You might be right, it might not make much of a difference at all, but it will make a difference.

[260DET]

I moved mine up by 13/16" and out by 1/4". I now have 1.5* of static camber on a car that is lowered such that the arms are almost flat at ride height.

 

BTW, this is good for turn in, but terrible for street driving. The car follows in and all road irregularities and my tires wear excessively on the inner tread.

 

The problem with excessive tyre wear etc is not caused by 1.5 degrees of camber, look elsewhere. My S30 had 3.5 degrees, sorting the suspension as a whole meant there were no significant problems. Its true

[kyle]

Back with more math, to see if anyone buys this: it's not an angle issue, it's a right triangle issue.

 

http://www.analyzemath.com/Geometry_calculators/pythagorean.html

 

Say the tie rod is 12", and rises 3" from dead level. The level effective length of the tie rod drops to 11.61895. Net decrease is .38105".

 

Say the LCA is 11.5" and rises 3" from dead level. The level effective length of the tie rod drops to 11.101802. Net decrease is .39820".

 

The difference is .01715". If this takes place at the knuckle, and the knuckle is 4" long, that's 0.24565585*. Taking that angle out to a 26" diameter tire (?) gives 0.111475006" of movement, or less than 1/8" bump steer.

 

Anyone buy that?