Twin_Turbo

Guys,

I am trying to figure out what the ideal (or as close to it) length of our spindle is. This because of Norvals work and him saying it was a spit ball deal. I'm trying to nail ti down as good as I can.

For this I need to get an idea of the ideal swing arm length. The swing arm length (which is a virtual concept for IFS derived from the rear swing axle susp) is the horizontal distance from the wheel hub center to the instantaneous center at ride height)

Doing some searching I came to the conclusion that it's about 100-150 inches. The longer the swing arm the less the horizontal component in the suspension travel changes, make it really short and you'll get a jacking effect like on the true swing axle suspension.

From this guesstimate (take 125" as ideal) we can calculate the ideal spindle height for the stock suspension. Because the instantaneous center on the stock setup lies outside the car there's no question about it that the spindle must be extended to get it on the inside of the car.

So, does anyone know a better guesstimate for the swing arm length?

Twin_Turbo

maybe I should have named this topic H(*)(*)ters or something like that, bet that would have got some attention :crazy:

Twin_Turbo

........................................ ........................................ ...........
:cry

norvalwilhelm

Sorry Twin Turbo but like I said I have been skimming over the posts lately and missed this one. I searched for the name instead this morning.
Roger my race car friend said I want my instantanous center on the inside of the opposite front tire not well beyond the car like it is now.
You chose 125 inches which is over 3 meters. Our cars are alot narrower then that. Why 125 inches then if the instantanous center should fall within the opposite front tire?
Our stock spindles I beleive are 10.5 inches and I increased them to 12 inches.Spindle extenders of 2 inches are available on the market.
The longer spindles increase the roll center height and decrease the gravity affect on the car.
The car will lean alot less going into a corner.
But leaning is considered elastic because the springs and sway bars absorb some of the force when the car is turned but it also slows down the turning.
A high roll center decreases the elastic effect and the turn is fed more into the tires and linkage making for a quicker turn but scuff the tires.
The longer spindle does crease camber change. I had an example where a 12 inch spindle vs a 10 inch spindle camber change is 1 degree less for the longer spindle.
Once again Twin turbo sorry to have missed this post, It is one of my favorite subjects.

MikeC

I'm a little bit confused when you say swing arm, I assume you mean trailing arm for the rear end..... The spindle height makes me think you are talking about the front suspension?

So what is it? Then I can figure out if I actually have something useful(doubt it) to the conversation.

norvalwilhelm

Mike I answered assuming he meant an imaginary swing arm on the front like the ford trucks had. A long swing arm going from one side of the truck where it pivoted to the other side with the tire on it.
Our A arms form a swing arm, we are just missing part of it but the imaginary center is still somewhere out there.
If we knew where we want the pivot point and could carefully measure 3 off the 4 pivot points we alread have fixed on our cars the 4th, upper ball joint point could be calculated and we would know the spindle length required.
After a different spindle is installed the bump steer( I know a bad word around here) needs to be addressed all over again.
Since I believe I am the only one so far sorting out the bump steer it is not a problem for anyone else.

Twin_Turbo

yes Norval, that's exactly i, an imaginary swing axle. I got the 100-150" guesstimate from a book, it didn't specify why that was chosen, therefore I posted this topic. I too thought that it would be best if it's somewhere in the center of the opposite wheel. The instantaneous center on our stock suspension actually lies outside the car instead of inboard near the other wheel.

We don't want the swing arm too short either, that will give a lot of camber gain but it will also cause a jacking effect, not good at all.

I will measure the distance to the other wheel then and will calculate from that. Will report what I find, I hate spit balling something without having made an attempt to figure out what is best suited.

As for bump steer, I'm still working on that and then some.

I am seriously looking into converting to front steer rack and pinion because of several things:
* Rack & pinion is lighter, not necesarrily more responsive than a good setup recirc. ball system (like yours, sounds like it's really tight, the only big drawback I see is the power steering valve, the movement of the knucle gives the drag link a non rigid point at one side that you can't eliminate)
* I want front steer for:
- the input shaft (steering shaft) will have a much better angle, no messy u joint and bearing setup that can bind. No flimsy brackets to mount in stock location that will bend
- more room under the oil pan
- and most importantly it's just way better than the rear setup. Not only is the end take off rack a good way to eliminate bump steer (you need the correct width rack) but the front steer has advatages over rear steer. This has to do with suspension deflection. If there was no deflection front or rear steer would perform equally. But with deflection under cornering the side with the load on it (outboard) will actually be pushed inboard (bushings deflect), this moves the spindle further inboard as opposed to the steering arm, which is mounted almost rigid with ball joints. This creates torque steer. On a rear steer setup this gives more steering input causing oversteer. On a front steer setup the deflection would give less steering input or understeer. Understeer is much easier to handle and anticipate, it just gives a better feeling over the car.

Then there's the rear too, those bushings deflect a lot and that gives torque steer too, also rear oversteer. But, I will tackle that later since the solution there is a lot easier.

Marck

norvalwilhelm

I took a quick look at mounting a rac in front of the crossmember and it is not easy. The lower A arms really stick out causing a problem where to mount the rac. The A arm pivot points are right where the tie rods want to be. The rac would need to be pushed way out in front.
As for bump steer mounting the rac is very critical, tie rod length is critical and steering arm curvature is important for ackerman.
Too many variables that are too hard to adjust.

Twin_Turbo

I found a BMW E36 rack that is the correct size to be mounted on the frame with the correct pivot to pivot width, it's about 16,5 inches wide. I mocked it up a bit and the rack won't have to go that far to the front at all, it's close but I think it can be done. It's possible to mount the rack so that the tie rod is over the lower arm's cross shaft:

The 2 will never touch each other if done correctly since the pivot points are on the same line from lower to upper cross shaft, the tie rod would be at the same angle as the lower arm.

When you swap the steering arms left & right you'll see they extend forwayrd quite a bit, there's enough room to mount a rack here (I think)



Only problem is the sway bar, that will have to move either to the rear or to the upper arm and be mounted on some kind of boxed up front.

MikeC

I did a project on this in College...... too bad that was a ong time agao and I don't remember all of it. But I believe Norval to be correct, You want the IC to be somewhere on the opposite wheel, I just forget the ideal location. I'll look it up tonight.

As for the front steer R&P That'll be a tough job... not a lot of room there and there are a lot of components in that area. With Norvals urging I am looking at fabricating my own R&P setup, hoping to save some cash.... maybe I'll look into a front steer setup. Unfortunately a BMW rack would be extremely expensive here so I'll Have to locate a compatible version..... hoping a latemodel F-body might be the ticket.

norvalwilhelm

Twin Turbo are the steering arms interchangable. I looked at that a few weeks ago and thought the mounting holes would not work if reversed but in the picture I see you did it already. Are yours bolted on or just set in place for the picture?

Twin_Turbo

Norval, the spindles are symmetrical with the centerline being the mirror line.
This pic is for another little project but look at the 2 mounting bolts under the spindle center, they both are at 2,81" from the centerline. The only consequence is you have to rotate the caliper & bracket too (or simply put the left spindle assy right and vice versa), this makes the brake setup trailing instead of leading, it also improves caliper cooling since it won't be shrouded by the rim. You will need new lines but they are easily made.

oops forgot the pic:



[Modified by Twin_Turbo, 10:09 AM 9/25/2003]

norvalwilhelm

Thanks Twin Turbo. My car is going into storage or the shop in a week or two then I will start pulling everything apart. I will look more closely at a rac. Who knows I might just try to modernize more.

Pete79L82

Marck / Norval; I hope you two are happy now. I have just spent the last 5 hours measuring my car so I could plot the I/C and R/C so we know where they are. And I had a lot better thing to do than that. :banghead:

These measurements are from my stock 79 with FE7 suspension and P255/60/R15 tires on factory aluminum wheels. Obviously any changes from that will change all dimensions. These measurements are done roughly with a tape maeasure with everything in the way so they are not exact.

With stock spindles:

Instant Center: -6.92" high / 239.58" outward from center of wheel.
Roll Center: -.67"


With 2" taller spindles:

Instant Center: 13.17" high / 23.18" outward from center of wheel.
Roll Center: 4.76"

Hope this helps. :thumbs:


[Modified by Pete79L82, 7:53 PM 9/25/2003]

norvalwilhelm

Makes me happy Pete :) I wanted to measure mine but with the big blower sticking through the hood I can't run a straight edge across the top to get measurements from the top down.
You confirmed that I am in the ball park. My aim was for a 4 inch roll center and with the 2 inch extension you get 4.76.
I extended my spindles 1.5 inces so it will be close.
Thanks again Pete and nice to hear from you. Of all of us you are the most knowledgable and twin turbo and I just bounce ideas back and forth hoping you would chime in.
Thank you.
Now help me with picking a shock. :) :)

VegasJen

i'm glad i found this thread before it made it to the 2nd page. i have a question for you guys regarding this. what difference would it make if the inner pivot point were moved down as opposed to the outer pivot point moved up? also, what difference would it make if that upper A-arm were alot shorter than it is now? we've all been kicking these ideas around for a while and i'm thinking, "what if we were to create something similar to the c-4 where the A-arms are mounted outboard of the framerails (mainly the just the upper one for us) and lower to give us the effect we want?"
lately, i've been playing with a generic template and drawing with a ruler and protractor different combinations altering the pivot points and lengths. don't have anything conclusive yet and i will let you know what i find.
just thought i would throw that out there for you.
also, isn't it ironic that the rear suspension came built in with the kind of camber changes we would typically find desirable for the front and yet the front is just about how we would want the rear? maybe we should just learn to drive our cars really fast in reverse! :rolleyes:

VegasJen

oh yeah, one other thing. the way the factory rear suspension is, it does exactly what we want by gaining negative camber with compression. but does it matter that the lower 'A-arm' is pointing down while the upper 'A-arm' is horizontal? does that make any sense? actually, it seems like exactly what we would want. if you imagine the suspension under compression, it seems like the intersection of the two planes would stay consistenly in line with the opposite tire. am i wrong here?
:confused:

norvalwilhelm

Moving the inner pivot point down has the same affect as a longer spindle. Thje camaro guys drill holes lower down for the inner mount or remove the inner mount entirely and weld on a new 1/2 inch plate. They have lower inner mounting holes plus they move the hole 1/4 or 1/2 inch to the rear to improve caster.,
As for making the upper arms shorter I would worry about too much Camber change as the shorter arm move through the arc of travel. While this could be good the excess camber change could wear out tires.
You want camber change but only so much. The shorter the arm the more the camber change.
The longer the spindle the less the camber change.
My back is 6 link so I have almost no camber change.

Twin_Turbo

clutchdust, mounting the upper arm on the outside of the frame and then using a shorter (adjustable) arm is what I have been saying for some time, now, that will change the isntantaneous center without the need for a longer spindle. With the adjustable arm you won't have the problem that the C4 has and that is a max. to the amount of caster you can set is determined by the frame, shaft against frame is max. nef. camber.

norvalwilhelm

I am having trouble seeing how moving the inner upper A arm pivot point further towards the outside changes the Instantanous center.
As you know th einstantanous center is the line projected through the upper ball joint, through the inner mounting point and projected towards the opposite side of the car. Where the lower projected line meets the upper is the instantanous center.
Moving the upper inner mount in or out should not change the pivot point along the projected line, up or down changes the line but not in or out.
Newer cars went to longer spindles even though the mount was moved outside the frame.