after installing about 3/8" of shims in my rear right swing arm, it still too negative.

here's my questions:
1. is there a max amount of shims allowed or can i get longer bolts and continue?

2. the frame/body is pretty rust free so is it safe to assume the swing arm is bent?

3. is there a way to measure and verify?

4. can the inner arm bushing be worn and effect the camber?


Thanks guys and gals. Yer da' best!

Roger

There are lots of possibilities, but the swing arms are designed to bend before the pickup points to reduce body damage in the event of of a mishap. The holes will oval, the rubber creeps, bodies do get twisted, and oh yeah the arms do BEND. BTDT.

I have more than 1/2" of shims. got longer bolts. The lower you go the more shims you need. I am just too stuborn to raise my ride hieght. I am at -1 degree in back. I think my slicks should like this angle

All conditions above have to be dealt with before a good alignment can be done.

what joe, no adjustable perches for the rear??? =-)

so i'll get longer bolts then.

thanks guys! =-)

huh?

i don't see how lowering/raising the rear could effect camber as the trailing arm does not pivot in that plane ...
Andy

They're semi-trailing arms, and they pivot in more than one plane. The further up into the body the arm goes, the more negative camber you get, I believe. Up to a point, of course.

Someone posted camber curves here a while back, didn't they? Or was that on the Bird board or the Bird's Racing board? I forget now where I saw that....

--DD

I think Mr. Groot has the numbers...

They do gain more negative as you lower the car.


B

Sir Brad,

How much shim would you go, max?



Yes, my car is now lower than stock.

i thought joe was talking about if he shimmed his swing arm more he would lose his lowered stance.

when you shim it you raise the rear end.

=-)

EDIT
removed my obnoxious pic. =-)

Whoah, Roger! I think you've got everything backwards...

When you shim the outer suspension pickup point, you don't get much change in the ride height at all. Probably less than half the change in the shims!

But the question was, when you change the ride height, what happens to the camber? In other words, if you leave the shims alone and just swing the whole trailing arm upward some on its pivot, do you gain more negative camber?

The answer, due to the pivot axis of the trailing arm (which is technically a "semi-trailing arm" because the pivot axis is not straight across!) is that you do gain some.

--DD

ahhh, yes.

thank you dave.


no worries.
=-)

so no one want's to post who's got the largest amount of shims???????????

LOL

It doesnt raise the car.. because thats just a pivot point... the coil over assembly is that the wheel rests on... it alone controls how low the car is. Lowering, or raising the pivot points just allows the trailing arm to be at the appropriate angle..


adding the shims just makes the tire tilt left to right..ie camber.. looking from the back.

I got a 1/4 in in both.. and will take an 1/8 out..

Go with however much shims you want, given you have long enough bolts to screw in enough... Your going to have to fix the car sometime...

Its obvious that your body or trailing arms are twisted.

Andrew


PS.. how much is to negative? I like 1.0 on the rear and 1.5 on the front....

according to most of my readings....
it's the tires that dictate the camber. and of course the application.

some are designed for more neg camber than others.


these pilot sports i run supposedly don't like exaggerated neg camber.

i still wish there was a measurement i could take on the trailling arms to find how far out they really are......

ok, then where does the trailing arm rotate left to right when the car is lowered?

simply be deforming the rubber bushings?

in order to gain/lose camber, there has to be a rotation in the vertical front/back plane somewhere ...
Andy

i know andy, it's confusing... that's why i posted the pic... to help me visualize what's hapening when the suspension is lowered.

the outer bushing is much more forward than the rear.
so when the car is lowered the negative camber is increased.
higher or lower it toes out.... i think lower. LOL

toe is *not* camber ...

the trailing arm has to "pivot" somewhere for the camber to change and i just can't see how.
i don't have a car to play with, but with both the outer and inner mounting of the traling arm being fixed, it can only rotate up/down but not left/right at the same time.

so, how does it change camber ??? where are the engineering types when you need them? mueller?
Andy

if the swing arm shaft was mounted perpindicular to the length of the car then there would be no increase of neg camber.

right? =-)

Andy.. sadly... the pivot is on the rear suspension ear...

Just think about it... Its the only part of the suspension that does not have an adjustment. The second pivot point is adjustable (you new that... but still)

Andrew

Roger... "could you rephrase the question?"

wrong ... it doesn't matter if the trailing arm is mounted level or not. if it's not level (like you using shims), you set a initial camber, a solid mounted arm would simply move up/down on that fixed camber setting ...

in order to *change* camber while moving up/down, the arm needs to pivot somewhere.

simply moving it up/down at an fixed angle (camber) will not change that angle ...

change of angle means rotation ...

Andy

so you're suggesting the rubber bushing on the inner ear allows for camber gain when the suspension gets loaded ???
or the ear flexing itself?

Andy

Roger,

I would run those PS's with -1.25 front and rear with a zero toe front and 1/16th toe in on the rear.

Andy,

They dont gain a lot, but *somehow* they gain. I play with this every time I have a car on the alignment rack that I'm personally aligning. It will gain -.25 in 2inches of bump with plastic bushings. The inner ear does flex alot if not gusseted (it has to somewhat)

What does change when adding/removing shims = roll center.

Plus: adding shims can change the ride heigth.


B

that's a lot ...

i personally think it's a combination of flex on the ear and maybe even the arm itself ...

are the upper and lower shock mounts in the same plane or are they offset? if so, one would think camber gain would put a lot of stress on the shock as well, with the solid lower mount ...

it seems, if you added spherical bearings for the upper and lower shockmounts you could reinforce the trailing arm mounting to a point where there would be zero camber gain during suspension travel ...
Andy

Nope. Or rather, that's not much of it, if any.

The inboard mount point for the pivot is aft of the outboard one, right? But the arm still faces directly back (pretty much). So, do what I did a few years ago for a MUSR tech session: Build yourself a mock-up trailing arm out of paper towel tubes. Exaggerate the angles, though--make the arm come out of the pivot at a ~45 degree angle. Stick some paper or a flat piece of cardboard on the side of the "trailing arm" to stand-in for a wheel. Then tilt it so the arm is straight fore and aft, and rotate the whole thing about the pivot. You'll see that the wheel does indeed change camber as you rotate the arm.

Click to view attachment

--DD

Andy and Roger and anyone else who is a non-believer,

As you lower the back end of a 914 (pivot the trailing arm) up into the body, you will increase the negative camber.

Have you ever put your car up in the air with wheels on and looked at it from the back? When lifted, the rear wheels will tip in at the bottom and out at the top. This is due to the axis of rotation and the 3-D relationship of the hub to the axis of rotation of the rear trailing arm.

In the same manner, the rear toe also changes as you raise and lower the back end.

If you still don't believe or understand, pull your rear shocks/springs and jack your car up and down with the wheels touching the ground and watch the magic.

These are the reasons that you need to re-align the rear end when you change the rear ride height.

Similarly, the front end will change camber and toe when raised/ lowered. It is just the nature of the beast....

You could always take a class at a community college in suspensions/alignments. You could use thier equipment to boot.

Good luck.

John

Well it makes sense if at a small degree of compression camber is one angle and as you lower the car it gets to be more camber... which is the case for anyone who ever checks their car after lowering it....

But also... Doesnt the wheels have positive camber when you let them dangle freely from the car being on stands or a jack?

PS.. Brad... Roller bearings rock!!! Smooth ride.. its not even unbearable with 275 springs on the back and 18's on it... but I havent taken any corners because I dont have a sway bar on it up front... (working on the nascar one..)

i'm a total believer... LOL

i actually did almost exactly what DD did. only out of a piece of paper and a pen. but same thing was observed.

lowering increased neg camber and toe out. thank you dave.

andy, the swing arm shaft is level but not square with the length of the car. the outer bushing is forward of the rear. this means that when the tire goes up/car is lowered, the top of the tire goes inward a bit and the front of the tire goes out a little bit.

thanks for your help ...


as to dave's pictorial, i guess i didn't realize that the trailing arm mounting and the wheel mounting are *not* in an 90 deg. angle.

i somehow assumed they were, in which case there would be no camber gain ...
which explains a lot ...


so, what's the advantage of not having a 90 deg. angle?
Andy

The advantage is that it gains negative camber in a corner. The toe changes are also to help with stability when the suspension is loaded up (while cornering). This is for stability.

This is by design.

Race cars are typically designed differently with long upper and lower a-arms to provide for little alignment change as the suspension travels through it's limited movement.

However, race cars are not designed for the street and would not do as well on irregular surfaces as they do on nice smooth tracks.

as brad mentioned, roll center is stabilized more with increased neg camber in turns. when taking a right hand turn more pressure is pushed down on the left corners and the chassis wants to tilt to the left.
the tilting induces the wheel geometry to a positive camber position.
by combining the pre-dialed in neg camber, and geometry that supports additional neg camber under load, makes for a more preferential roll angle.

i visualize the old movies where you see an old ford pickup truck taking a turn at speed and watching the front tire roll over in an extreme positive camber state. ... those were the good old days... good riddance. LOL

That's the point I was getting at when I mentioned "semi-trailing arm" versus "pure trailing arm".

Your Bug (unless it was a Super, then I'm not sure) had pure trailing arm suspension in the front. So did the 356es. That suspension has been called "the worst race car suspension design in history", by a noted 356 racer and builder. The biggest disadvantage is that every bit of body roll (like what your car does in the turns) translates to an equal amount of tire lean--in the direction of positive camber on the outboard (outside of the turn) wheels. Those are the ones that are the most heavily-loaded, as I'm sure you know from your recent autoX experience.

What you want is for the outboard wheels to stay close to upright, or even lean in (negative camber) as the car leans. So as the suspension compresses, you want it to gain negative camber.

There can also be "packaging" reasons for particular mounting points, e.g. you need some particular bit of space to mount something important.

Of course, you can make sure the body does not lean any perceptible amount by putting huge springs and gargantuan sway bars on, but then you've effectively removed the whole suspension and replaced it by solid blocks. I recall a Chapman quote (seen on the Bird Racing board)--"Any suspension, no matter how primitive, can be made to work reasonably well if one can simply keep it from moving."

--DD

so i guess the maximum amount of shims is can put in is about 3/4"???

with the longer bolts?

Sure.

But realize something is bent.

Your gona need to fix it some time.
Its just gona get worse...

It might be trailing arms... it might be body. Any rust?

i agree andyrew,

the arm and the body have very little rust.

the questions going through my head are how did it get bent and how can it be prevented in the future?

are these areas inheritently weaker and they need fortification?

or was it purely a collision that i'm not aware of?

i don't ming getting replacement trailing arms and beefing them up. anything more than that and i'm going to put her on a frame rack and get her straightened right. then 6 point cage and beefed up everything. (i guess what i know i'm doing now this winter... )

Those questions are secrets that belong to your car... Only getting it on a chassis machine, and having someone who knows 914's and collisions will be able to find that one out..


Preventing it? Beefing up the suspension points, adding reinforcements, beefing up the trailing arms, such like that.....

Your gona do the right thing... But I would have the chassis looked at, because you want to do THAT before you put on reinforcements...

Andrew

The rear suspension mounting points are at a 15 deg angle to a line that is perpendicular to the centerline of the car. This is what causes the negative camber gain. It also causes toe change in roll, thus causing roll steer problems. It gets worse as you lower the car.

One of the problems with cars that require lots of shims and with cars that people setup with lots of rear camber is you lower the rear roll center. This in itself will hurt the cars handling. There are other problems with lowered cars, but that is another discussion in and of itself.

Oh yeah rear camber gain is on the order of -1deg for every 20mm of bump.

roger,

are you doing this in your garage?

are you sure your floor is level and the car is parallel with the floor (side to side plane)

mike, that's a really good question.

i have a 4' masons level which should show pretty close where i stand.
i think the garage is level and slanting towards the garage door but i'll check tomorrow. all my fluid spills tell me that. =-)

BTW: i'll be at home all day tomorrow if ya' wanna' drop by. Let's go for a spin!!!!!!
(i've been waiting soooo long to say that) !!!

<quote>The rear suspension mounting points are at a 15 deg angle to a line that is perpendicular to the centerline of the car. This is what causes the negative camber gain. It also causes toe change in roll, thus causing roll steer problems. It gets worse as you lower the car.
</quote>
i think toe out, under load, in the rear quarters is actually a good thing as it assists in turning. meaning the fronts are not the only wheels assisting in the turn with this type of rear suspension. almost all of the newest high perf cars have it now.
some of the older trend setting cars are....
VR4
Celica's GT-R
Prelude 2.0
MX-6

Here's a tip I read on-line about leveling the car for alignment checks:
Use floor tiles under the tires to get the car level. Measure how far off level the floor is at the four spots you want your tires to wind up on. (Mark the floor next to the tires then pull the car out.) Stack tiles until you've got level, then you can mark on the floor the number of tiles needed in each spot. This lets you repeat your measurements later.

--DD

No modern car is designed with toe out in the rear suspension. It makes for a very unsafe unstable car. Modern cars all toe in to create stability under braking and turn-in.

One of the problems with the 914 is the front will toe in under roll and bump and the rear toes out. Thus creating weird handling.

The toe can go either positive or negative with a sem-trailing arm... depending on the angle of that arm as viewed from the side. Most people lower their cars enough so the trailing arm is in a nose-down condition (meaning + on the axle height scale), the rear toes in under jounce (like Brett says). The chart says it all.... when the arm is level, (axle height = zero) the rear toes out in rebound and toes in under jounce. When the arm is nose high you have the worst possible situation, where the rear toes out and then toes in as it travels through it's motion.

The camber gain of a semi-trailing arm is linear and depends on the angle and length of that trailing arm as Brett and Dave mentioned. 15 degrees on our cars equates to about 1 degree per inch of travel.