Figuring camber change

[JPhil]

I am now exploring suspension mods to improve my handling, notably figuring out proper camber change both front and rear. I am reading a lot but still confused about a lot of things.

Yesterday as I reassembled my passenger side rear trailing arm after replacing the wheel bearings, I did a camber change check.


(Tape measure clamped to fender lip to measure wheel center, 18" level fits perfectly against wheel, laser level taped to 18" level shining up to ceiling, floor jack to raise and lower wheel with spring disconnected)

Starting at 15" from fender lip to wheel center, typical ride height, I marked the ceiling where the laser shone. Then jacked up wheel 2" for jounce (which was 1" into bump stop--I have about 1" free jounce movement of trailing arm), marked laser dot on ceiling. Then dropped wheel 2" for rebound and marked ceiling.

Using an angle calculator I came up with 1.3426* negative camber change on jounce and .98465* positive camber change on rebound.

I have a 7/16" thick aluminum spacer between differential and stock strut rod bracket, as per Herb Adams or whoever, and heim joint strut rods.

This camber change seems to be not much, although the car has never broken loose, either front or rear, in a few extreme 'cornering events'.

I did a calculation figuring body roll angle (using track width of 60" for wheels and 2" suspension movement, jounce on one side, rebound on other side) and came up with around 1.9* of body angle change. I don't know if that is the correct way at all, but it gave me an idea of body movement. Seems fairly close for amount of camber change for body roll. Did I figure that correctly, or is that full of bullpucky? Should I have used half the track width to find the roll center length of 30"? That gives me 3.57* of body roll.

OK, that is for the rear end. Next week I get to installing my new front VBP offset front upper A-arms to increase caster. I would like to get my front camber change correct there as well since I know it is off due to using shorter 550lb springs. How should the front and rear compare to be balanced?

 

[JPhil]

Here are a couple charts from another thread from White 76 showing his front end bump steer & camber change with bump steer blocks and 3/4" taller upper ball joints. I like the bump steer a lot, but the camber change seems excessive, especially in comparison to my rear camber change. I think I understand the front would want a bit more to allow for center of gravity movement in cornering, but still, isn't that a lot?


Bump steer





Camber change

 

[rtj]

Buy one of these, worked great on my diff pinion angle and other projects. Harbor Freight:



Just stick this magnetized, digital angle finder on any surface - table saw blade, miter table, even use it as a level - then just read the precise angle in seconds on the large LCD digital display. You can "freeze" that reading on the digital display by pushing the “hold” button. Press the "0%" button to gauge the angle change from initial measurement. Includes "calibrate" and "on/off" button. Durable stainless steel construction, with magnetic base.

 

[phantomjock]

JPhil -
For modeling the front suspension - BonesApart.xls would give you a leg up on the effort - before buying a bunch of bits. I think I loaded it i the downloads section a while back (link to previous post:
http://www.vettemod.com/forum/showthread.php?t=7905

else you can get it on this page:
https://sites.google.com/site/porschersk718/step8a:design

Its a simple excel spreadsheet - but generates camber curves and roll center movements.

Cheers - Jim

Other software similar cataloged here: http://irsforum.boardhost.com/viewtopic.php?id=263

 

[BBShark]

Using an angle calculator I came up with 1.3426* negative camber change on jounce and .98465* positive camber change on rebound.

I have a 7/16" thick aluminum spacer between differential and stock strut rod bracket, as per Herb Adams or whoever, and heim joint strut rods.

This camber change seems to be not much, although the car has never broken loose, either front or rear, in a few extreme 'cornering events'.

I did a calculation figuring body roll angle (using track width of 60" for wheels and 2" suspension movement, jounce on one side, rebound on other side) and came up with around 1.9* of body angle change. I don't know if that is the correct way at all, but it gave me an idea of body movement. Seems fairly close for amount of camber change for body roll. Did I figure that correctly, or is that full of bullpucky? Should I have used half the track width to find the roll center length of 30"? That gives me 3.57* of body roll.

It sure looks like the body roll angle counters camber gain regardless of your methods.

 

[redvetracr]

measure all you like but other than a 5-link about all you can do is move your t-arm pickup point up an inch, not done on my racer but according to Dick Guldstrand it`s a big deal.

 

[JPhil]

I'm doing the front suspension this weekend, installing the new offset upper A-arms & replacing the rubber bushings in the lower arms with poly.

Before I took it apart I measured the camber curve & bump steer. Both were pretty good, I think. The camber curve showed me at -1.75* at 2" jounce, and -3.125* at 3" which is maximum into the bump stop. The curve itself is quite comparable to the rears, and I didn't check the caster effect of camber change.

(This was using my new Fastrax guage from Jegs instead of the lazer level shining on the ceiling.)

The bump steer was only maybe 3/16" at 3" jounce, and virtually none on rebound, with a very smooth gradual change.

I'll check it all again, including the caster camber change, when I get it back together with the new parts.

At this point, I'm not worried about changing anything, I just want to know what I have.

 

[JPhil]

Bummer

Ran into a problem today when I got it all put back together and went to set the alignment. The offset cross shafts on these VBP upper a-arms won't let me get to even 0* camber. Even with no shims at all I can only get to about -.5*.
And with no shims, the shafts don't seat completely against the frame, so I need a bit of shimming.

I want to go to 0* static camber for easy street handling. I have tried running negative camber but it's too darty on our crappy roads. The factory spec is something like .5* positive, and I would like to at least be able to go there too if need be.

All I can think of is to knock out the studs and rotate the cross shafts so they are facing the wrong way, but that is how I mistakenly installed them years ago on the stock a-arms and I had so many shims under them I was running out of stud threads.

I guess I'm just putting this out there to see if perhaps I'm missing something or somebody has an idea I haven't thought of. I'm pretty tired right now from working on it all day.

 

[phantomjock]

There is/was a template for welding on new plate and mounting holes (I recall). Seemed to be referred to as the "Chevele Mod" = maybe? Lots of brain cells firing here - but I'd be real tempted to call Mark Savtkse. He might have a good idea or 3. Good Luck!

Cheers - Jim

 

[JPhil]

If that's the one I'm thinking of it's called the "Guldstrand Mod" and it relocates the mounts for camber and anti-dive. I'm not going to get into such extreme mods, I'm not taking it to that level. Plus I don't want to tear it apart far enough to do that.

I did come with an idea though. I will dig up my old stock cross shafts and replace the offset ones with those. That should give enough leeway in either direction. The problem is I won't know if they will even fit these a-arms until I take it all apart again.

I'll have to mess with it next weekend, I have too many other things I need to take care of right now.

 

[BBShark]

Don't know if this is correct but the increase caster (for the VBP arms) is accomplished by the position of the holes in the cross-shaft. So, using the originals will reduce caster.

If you need longer knurled studs for the upper mount, look at NAPA #641-1268. They are 2 1/8" long.

 

[JPhil]

Don't know if this is correct but the increase caster (for the VBP arms) is accomplished by the position of the holes in the cross-shaft. So, using the originals will reduce caster.

If you need longer knurled studs for the upper mount, look at NAPA #641-1268. They are 2 1/8" long.

Wow, that never even occurred to me. From eyeballing it, they don't appear any different in that respect, but it may be so small a difference only measuring will tell. Perhaps I'll call VBP later and inquire.

Meanwhile, it occurred to me that the shafts are only offset about 1/8", so if I roll them over it will only make about 1/4" difference so I will have only 1/4" or so of shims. That's not too bad--I had almost 1/2" of shims before.

I dunno, I'll just try it first and see.

Edit:
Just got off the phone with VBP-

The caster is built into the arms themselves, not the shafts. The offset shafts are offset .120". I will just rotate the shafts. Stay tuned....

 

[JPhil]

Flipped the cross shafts Saturday morning. Didn't take long. The alignment came right in too, only took about 4 tries to get both camber & caster where I wanted them. I think maybe the poly bushings top & bottom helped, I remember whenever I did alignments before when it had the rubber bushings I would have to dick with it forever. Or maybe I just have more experience now. Or was lucky. Whatever the reason, I was pleased with how quickly it came together and stayed constant with jouncing.

I have a 1/8" shim under the front studs and 3/8" & 5/16" under the rear studs, so if I would have used my stock shafts I still would have probably run out of adjustment on the fronts.

Using my Fastracks bubble alignment guage, I now have within a gnat's ass of 0* static camber on all four wheels and 5.9* caster on the fronts.

My front toe is 3/16" but my rear toe measured out at about 1/16", which doesn't seem right because on the machine last time I had about 1/8" on the driver side and 1/16" on the passenger. When I replaced the rear passenger side bearing last month, I swapped a 1/16" shim to give me more equal toe between both, so I expected 1/4" or so total this time. Hmm. I know I did it the right direction. Whatever, I know it's more even than before.

In a week or two I will get the car back on the shop rack and see how close I got it all.

Driving: It feels more stable now. Not as twitchy. A bit more sensitive to steering which I attribute most to the poly bushings, but at the same time more secure and less sensitive to road anomalies. Tracking seems better, and I think it pulls into turns with a little more 'crispness' if you will, both of which I give to the increased caster. I only have maybe 50 miles on it so far, this week we are supposed to have better weather so I will be driving it to work and hopefully to Denver on Friday. That is a real test of handling.