lower a-arm geometry

[rrelco]

so i have installed my lower a-arms with ats tall spindles and i am a little confused, if i measure at the pivot point were the a-arm attaches to the frame to the ground then measure from the ball joint pivot point to the ground my measurements are different shouldn't they both be the same?


66 elcamino a-body

[monteboy84]

Not necessarily. It's a general "rule-of-thumb" that these should be level and the lower control arm should be level, but if the measurements are relatively close and your roll-center is where you want it, I wouldn't worry about it. AFAIK, the primary impact LCA has is on roll center height, though it does impact your camber curve on a smaller-scale.

Also, if the car is driving fine, don't worry about it.

-matt

[MrQuick]

Not necessarily. It's a general "rule-of-thumb" that these should be level and the lower control arm should be level, but if the measurements are relatively close and your roll-center is where you want it, I wouldn't worry about it. AFAIK, the primary impact LCA has is on roll center height, though it does impact your camber curve on a smaller-scale.

Also, if the car is driving fine, don't worry about it.

-matt

agree but I just wanted to add that the arm should be close to level in relation to the center of the ball joint ball pivot and the center line of the front and rear bushing axis.

Vince

[JRouche]

so i have installed my lower a-arms with ats tall spindles and i am a little confused, if i measure at the pivot point were the a-arm attaches to the frame to the ground then measure from the ball joint pivot point to the ground my measurements are different shouldn't they both be the same?
66 elcamino a-body

The same, or close. When the ball joint side is higher it creates some semi-dramatic changes in camber. The reason you would want the control arms level is because there may be a lil bit of "kick" in the camber profile if the ball joint side is higher than the inboard side.

What happens is during compression, and thats more important than rebound, is if the ball joint side is higher the control arm acts like it is growing in length. Till it centers out becomes level, then as it passes over center and into an downward travel its going into a shortening lower arm length. So you get a flip flop of the camber gain as you are going through a turn (the loaded tire). It goes from some camber gain till it passes center, then goes into decreasing camber gain as the lower control arm starts to pull the bottom of the spindle in.

And thats if the ball joint side is higher than the inboard side of the control arm at ride height.

A better case is if the ball joint side is lower (if level is not possible) than the inboard side at ride height. Then the control arm is always in a shortening direction. It wont go through the transition of a the arm getting longer then getting shorter as it passes center. It will just be getting shorter for all the travel during compression. And thats why we like to start off with the control arm level with the ground so we get the maximum amount of "long" arm at ride height and it goes into shortening during compression. It becomes predictable. We can set up our camber with the upper arm at this point, and thats how it should be. Set the camber with the UCA when the LCA is in a predictable curve if it starts out at level.

And you might think, only concerned about compression? Yeah, not only, but mainly. In a turn the tire that has the most control of the car is the one in compression, the outside tire. Its taking most of the loads so its the one that gets the money.

Ideally we would like to have the inside tire take some of the loads. And it will, its not the king in a turn though. The inside tire on a heavy turn goes into rebound if the car has any compliance to it. A REALLY stiff race car can get away with less compliance and then its time to see what the inside tire is doing.

Once again, the inside tire will go into a similar shortening of the lower arm (just like the loaded side) if we start out with the arms at a level point. It will reduce camber gain the same amount as the loaded side. So its uniform, equal and most importantly, predictable.

The lower control arms arent meant to dictate the camber curve or roll center. They are there to support the car. Thats their main purpose. They are the load handlers and the tuning arms are the uppers. And because of that you want them to be as neutral as possible.

And so far, for an average street car neutral means having them as close to level at ride height as you can get them. JR

[rrelco]

[QUOTE=JRouche;762535]The same, or close. When the ball joint side is higher or lower it creates some semi-dramatic changes in camber. The reason you would want the control arms level is because there may be a lil bit of "kick" in the camber profile if the ball joint side is lower than the inboard side.

What happens is during compression, and thats more important than rebound, is if the ball joint side is lower the control arm (lower) acts like it is growing in length. Till it centers out becomes level, then as it passes over center and into an upward travel its going into a shortening lower arm length. So you get a flip flop of the camber gain as you are going through a turn (the loaded tire). It goes from some pretty drastic camber gain till it passes center, then goes into decreasing camber gain as the lower control arm starts to pull the bottom of the spindle out.

And thats if the ball joint side is lower than the inboard side of the control arm at ride height. And thats usually the case where folks are trying to lower the car without looking at the over all system.

A better case is if the ball joint side is higher than the inboard side at ride height. Then the control arm is always in a shortening direction. It wont go through the transition of a "short" arm getting longer then getting shorter as it passes canter. It will just be getting shorter for all the travel during compression. And thats why we like to start off with the control arm level with the ground so we get the maximum amount of "long" arm at ride height and it goes into shortening during compression. It becomes predictable.

And you might think, only concerned about compression? Yeah, not only, but mainly. In a turn the tire that has the most control of the car is the one in compression, the outside tire. Its taking most of the loads so its the one that gets the money.

Ideally we would like to have the inside tire take some of the loads. And it will, its not the king in a turn though. The inside tire on a heavy turn goes into rebound if the car has any compliance to it. A REALLY stiff race car can get away with less compliance and then its time to see what the inside tire is doing.

Once again, the inside tire will go into a similar shortening of the lower arm (just like the loaded side) if we start out with the arms at a level point. It will reduce camber gain the same amount as the loaded side. So its uniform, equal and most importantly, predictable.

The lower control arms arent meant to dictate the camber curve or roll center. They are there to support the car. Thats their main purpose. They are the load handlers and the tuning arms are the uppers. And because of that you want them to be as neutral as possible.

And so far, for an average street car neutral means having them as close to level at ride height as you can get them. JR[/QU

[rrelco]

thanks guys the car rides great i was just concerned about them not being level the lower ball joint is about 1.0 -2.0 higher and JR thanks for the info

[ArtosDracon]

If that's 1-2" inches higher, that is A LOT. Do you mean centimeters?

[rrelco]

If that's 1-2" inches higher, that is A LOT. Do you mean centimeters?

i will get a better measurement this week-end

[rrelco]

so after re-reading your post JR your saying that the control arm should be parallel?

[JRouche]

so after re-reading your post JR your saying that the control arm should be parallel?

Yes, as close to parallel as is possible. And by the way. I totally F-ed up my explanation of how I think it works. I fixed my post. I had it bass-ackwards. With the ball joint HIGHER it will go through the transition. I had it the other way around.

Call it too many beers, not enough attention to the details or what ever. I was wrong with my first post. LOL I could be wrong with my fixed post.

Yes, try to keep the lower arms level. Cause at level they are basically neutral. And any suspension movement from there means the lower arm will be "pulling" the lower part of the spindle inward "shortening control arm". And thats the bummer of the suspension.

When you pull the lower part of the spindle in it acts to give you more positive camber (not good in a turn where you want to get camber gain which is actually negative camber). Even if you have a nice camber curve for the upper arm, the lower arm will be counteracting the upper arm.

The ideal setup is where the upper arm overcomes the lower arm slightly (maybe thats one of the reasons they are shorter). If the upper arm is shorter then it will pull faster than the lower arm is pulling in, camber gain (increasing neg camber) in a turn, as slight as it might be.

Comes to another issue. Roll center migration. Too short of a lower arm allows the RC to dive deep into the ground. Really long lower arms will keep the RC out and not bottom as fast. You get a fast diving front RC and you end up with dive and understeer with a short lower arm. Lower RC makes the car feel heavy and it is, the cars weight will make it push in a turn. And we try to fix it with a large front sway bar, and it works.

I dont know really. Im just shooting from the hip. And after seeing how I F-ed up my other response Id say listen to ANYONE else LOL To be honest, Im just as confused as you, maybe more so at this point. JR

[ArtosDracon]

I'll expand on JR's confusion by adding, that the shorter upper control arm, while fixing camber gain, screws with the roll center because it causes the instant center to move both more quickly and to move more laterally than if the arms were the same length. Which them bring kinpin inclination into play. That's the angle off of pure vertical between the lower and upper ball joints, having a higher KPI will mean a shorter UCA with the same length LCA. KPI however also effects scrub radius, which effects how a tire takes a lateral load, which will alter tire deflection, completely ruining your previous calculations.

I'm going to upset some peopel with this statement, but a situation like yours is EXACTLY where drop spindles would hugely benefit suspension geometry, so long as the hub is the only part moved. You could switch to a taller spring, can frame clearance at the lower control arm, get your lower arm leveled out and keep your CG at the same height, that's win-win-win.

[rrelco]

SO WHAT AM I TO DO...... I spent a lot on this suspension its from Speed Tech its there coil over conversion with ATS tall spindles, when I first installed the front the car sat way too low even with the coil all the way adjust, so they sent out 550lb spring and now i have ride height but my lower a-arms are not parallel. If i put a floor jack under it and go up on the suspension to get the a-arms parallel ride height is ugly...unless i install 24" rims it might work....F___K

[parsonsj]

I'm going to disagree, but only a little bit. Putting the lower BJ slightly below the LCA frame pivot allows the LCA to provide some of the camber gain. The idea of the top of the tire tucking in being better than the bottom of the tire pushing out is true, but we are talking about differences way under what the rubber compliance of the tire supports. In full race-car mode, the inside tire in a turn hardly matters at all, to the point that most professional drivers aggressively drive over the apex curbs on the track because hitting them doesn't upset the car, and lets the car back into acceleration more quickly if they can straighten the turn a bit.

The slight advantage to letting the lower control arm run a bit "downhill" is that it gives your suspension a bigger sweet spot (longer run of linear camber gain), which means you can allow more body roll and compliance. An even smaller point is that this also puts the LCA (and the UCA for that matter) into a better position for resisting bending forces at max lateral acceleration.

jp

[ArtosDracon]

I totally agree that once you get past level the difference between .25" up and 2" up is actually just going to be fewer shims on the UCA to get a good alignment. And that both tire and busing deflection are going make much larger changes than just about any suspension differences you're going to make from here.

We should also talk about relationship between upper and lower control arm angles here too. If your UCAs are above level(as they should be but only slightly) then your camber gain is giong to both good, and increase fairly slowly which is ok. If they are at a sharper angle (20°+) then you are going to get into camber gain much faster.

My take, if it's a street car, see if you can raise it .5-1" and not hate it as it will give you more travel before the bump stop and most likely be a positive change for the geometry.

[JRouche]

The slight advantage to letting the lower control arm run a bit "downhill" is that it gives your suspension a bigger sweet spot (longer run of linear camber gain), which means you can allow more body roll and compliance. An even smaller point is that this also puts the LCA (and the UCA for that matter) into a better position for resisting bending forces at max lateral acceleration.

jp

Yup, I get where you are coming from. Its like a lil "down hill" for the LCA is good cause it kinda sets the suspension up during a full compression.



But the thing is, it will still have to do the "swap". As the LCA comes up (on the wheel side) it will still be swapping from being a short arm to a long arm THEN the swap happens as it passes over parallel and goes from a long arm to a shortening arm. All the while the UCA is shortening the entire time, thats the key, the UPC is always moving in a certain direction, its not on the down side of the circle, its always on the upside, always moving in the same direction.

It really is a major transition and thats why we like to have our lower control arms parallel at ride height. JR

[rrelco]

Thanks for the input it looks like i opened up a can of worms on this. i will raise it and go from there, my bump stop is about a .5 inch away from hitting as it sites now so raising it might be better

[monteboy84]

But the thing is, it will still have to do the "swap". As the LCA comes up (on the wheel side) it will still be swapping from being a short arm to a long arm THEN the swap happens as it passes over parallel and goes from a long arm to a shortening arm. All the while the UCA is shortening the entire time, thats the key, the UPC is always moving in a certain direction, its not on the down side of the circle, its always on the upside, always moving in the same direction.

I'm more concerned with what the camber is doing, and as long as the camber is always going further negative as the suspension travels downward, I'm okay with the LCA length changing. As the UCA angle increases, our camber curve increases rapidly, so allowing the LCA some time before it's angled upward can be desirable.

Like I said earlier, level is a rule of thumb, but it's not a hard fast law of suspension geometry. Every car and application is unique, so there are a lot of factors to consider. I typically like them level or with the ball-joint end angled upward for the reasons you've stated, I'm just saying there are places where the LCA's can be angled downward for whatever reason the builder may have. The whole front end setup needs to be considered, which is why I'd suggest to the OP that he leave it alone and just trim his bumpstops if they're close to touching ;)

-matt

[silver69camaro]

Even having the BJ 1" higher than the frame pivot is quite a bit. Keep in mind if the BJ is taller than the frame pick-up points, it puts more wear on the ball joint as well.

[rrelco]

ok so there is endless info here on the bad of my problem i measured it and its 8.0 inches from the ground to the pivot to frame and 9.0 from ground to ball joint pivot, i raised the coil over up until i couldn't go any more and that is it no more adjustment.


SO GUYS HOW DO I FIX IT???????

[Ron.in.SoCal]

OK, let's recap here. You have the full Speedtech coilover treatment w AFX spindles. To the good, level LCA's and no more than 1" diff in pivot point vs BJ is acceptable. You are also adjusting your shocks all the way up which may not be ideal. If it were me, I'd do two things: 1 - find a happy medium that keeps all suspension components within acceptable opertating range (hopefully this is a nice compromise on ride height) and go for a seat of the pants test and 2 - take some pics and send them to Speedtech for further review. I know you changed springs based on their reccomendation but it's their design, they should know where improvements can be had without sacraficing geometry.

I remember Penny had uphill C6 LCAs before Rupp changed spindles and the car handled pretty good...