whitedevilTA

iTrader: (8)

So here I am constantly hating my factory gearbox steering, and thinking to myself that it's really the only downfall on my car after the full build. I have driven plenty of modern cars and know how rack and pinion steering feels and love it. I know some people will argue that it's "not as great for road feel" and "gearbox steering can be just as good" yada yada...but the truth is, no it can't. At least in my eyes, rack steering is superior in almost every way other than durability. It's lighter, much more simple, less moving and wearable parts, and the feel to me is amazing over the crap gearbox setup.

I have been doing nothing but research on this conversion, mostly on hot rodder pages that go into specifics on ackermann angle and all the important areas. I have seen many fellow third genners attempt it but so far nobody has come up with a 100% proven combo that is actually better than the factory setup. I think a lot of people have been just throwing a rack on there with little to no research on the topic which in turn gives horrible bump steer or a crappy turning radius.

I'm going to touch on a few main points, and then I will be making it my winter goal to design and have a fully functional power rack setup that is leagues better than stock. Please don't expect this thread to move fast, because I don't even have a rack picked out yet. I'm still in the measuring faze, and need to find a rack that exactly fits my needs, or I will get a custom or universal one. Theres going to be a lot of R&D in this but I hope I can shed some light on this much talked about topic, and if any of you have info to add, please do so! I am still learning about steering geometry and will take all the tips I can get.

Anyways, on to the basics...

With our third gen's macpherson strut style front suspension, the best way to size a power rack that will have minimal bump steer and great geometry is to follow the lower A-arms and crossmember exactly. This means that the main body of the rack must fit between the A-arm mounting points, and ideally, where the tie rods "flex" on each side of the rack should be exactly at the A-arm bolt location. The pic below shows about how much space on my setup I have to work with, and I will use these measurements as a general guideline when sourcing out a rack to fit.


This next picture shows a general idea of how the rack is supposed to look when installed. You can see that the rack body follows the K-member, and is right in line with the A-arm mounts. The joints on both sides of the rack need to be where the lower A-arm bolts are on the K-member (green "X's"). This will ensure that you retain the correct geometry when setting up rack steering.


Like I said, this is still the very early part of this, but I have every intention of having a very useable rack and pinion setup by spring time. I will update on here whenever I make progress. My biggest concern at the moment is getting the correct throw out of the rack. If I have to, I will shorten my steering spindles however I'd prefer not to. Flaming river now offers a universal rack with over 6" of throw, so this may be a definite possibility!

LX_SS

The housing width is not the same as the pivot point on your rack end.
If you take this into account, your last picture is incorrect, the housing should be narrower.

Its where the issues are with flaming river racks, off the top of my head the FR rack was 500mm/20" wide (housing only, no boots).
Front mounted/Rear mounted makes a difference as well.

Twin_Turbo

iTrader: (25)

The rqack pivots ideally should intersect with the imaginary plane beween the 2 lower a arm bushings and the strut mount. (since we don;t have an upper arm w/ bushings), this means that even if the rack width is a little off you can adjsut for it by moving it up or down although there is not a whole lot of room to work with. End take off, front steering rack would be ideal. Anyone have the dims of the frt susp? If not I will measure myself. depending on rack placement, the tie rods should level or thereabouts, easily taylored with adjustable ends & spherical rod ends. (bumpsteer kit)

A rear mounted rack has some drawbacks, biggest one is deflection of the lower arm bushings under hard corner lolading. This puishes the arm a little inboard and with a rear mounted rack that translates to oversteer as the wheels see more input than the driver is giving. Rear steer setup is pretty much impossible with the pan and the exhaust running there.

LX_SS

Rear mounting of a rack on our vehicle is impossible due to the lower arms.
This is not why you cannot use a rear mount rack in front mount position, the reason why you can't is because it reverse's the ackermann angles. Its why you can take a RHD rear mount rack, and use it in a LHD front mount rack, same as using a LHD rear mount in a RHD front mount, but LHD/RHD Rear mount to LHD/RHD Front Mount doesn't work and same as front to rear.

Depending on engine mounting position, sump clearance also limits your height. Also the tie rods whilst the vehicle is sitting under its own weight shouldn't be level, they should remain parallel to your control arms for ideal placement of the rack, However you will have to make a compromise somewhere with the conversion, otherwise there would be plenty already going around with it done.

Twin_Turbo

iTrader: (25)

I know a little about this. I designed my own front steer end take off rack and pinion for my c3 vette.

Ackerman is simply a relation of the steering knuckle geometry. Rack width has nothing to do with it.

The diff between a frt and rear mount rack is the placement of the pinion over the rack shaft. A rear mount rack extends away from the rotation direction. If used in frt. mount sreering left will make the car go right.

To get a rear mount setup if it would work would require moving steering knuckles inboard to get the ackerman back where it needs to be.

As for parallel tie rods. That's a rule of thumb. In reality it needs to be in correspondence with the suspensions instantaneous center. It's only parallel if the rack and tie rods sit in the same horizontal plane as the lower arm. In reality the arm bushings and balljoiint center are what's important.. anything higher or lower does mean it doesn't require parallellism. Also some bump steer is not always a bad thing depending wether it translates to bump over or understeer and what the driver prefers.

I have some drawings somewhere to illustrate. Ill dig them out tomorrow.

LX_SS

Not quite sure what you want me to say about your rack, never seen it.

I never referred to the ackermann's + rack width's, i'm thinking your getting confused there.

The point i was making with front mounted vs rear mounted racks, if the OP is looking at oems racks, make sure he looks at vehicles with the rack mounted infront of the x-member, not rear mounted as there is differences.

Diagrams do help more so to translate things, imo things can get very skewy sometimes talking about steering stuffs.

Twin_Turbo

iTrader: (25)

You said you can't mount a rear steering rack in the front because of the ackerman angles. The rack has no influence on the ackerman angles because it;s a relation between the steering knuckles and the kingpin axis. If you mount a rear steer rack on the front, turning the wheel left will make the car go right & vice versa.

Here's a couple articles I once did:

------------------------
Bump Steer or toe steer, called that because it is caused by a change in toe-out, is caused by a mismatch in geometry between the tie rods and the upper and lower A-arms.
More precisely, they do not share the same instantaneous center and /or the pivot points may not line up properly.

This figure displays the desired geometry.


The combined geometry of the upper and lower a arm dictate the arc that the wheel scribes when moving up/down. The path that the wheel center scribes is the same as the path that the steering arm scribes, only in a different/offset position. they are however fully parallel because these parts are rigidly attached to each other.
To avoid the tie rod interfering with the toe setting during suspension movement it has to be fully compliant with this geometry. This means that is has to be compatible for:
1) Outer tie rod end height
2) Inner tie rod end height
3) Tie rod length (The tie rod length directly relates to the pivot points)

In the above picture, the tie rod under the lower control arm is similar to how it is set up on steering box systems using a center link to provide the inner pivot points (and the ackerman effect by the angles of the pitman and idler arm). The other situation is what can be found on C4 and later corvettes using a front steer end take off rack.
The popular rack & pinion conversions for steering box systems all use end take off rear steering racks with tie rod positions more or less in the same configuration as stock (under the lower control arm) or about level with the lower arm.

Because of the above 3 conditions there can only be a couple of design issues that can cause excessive bumpsteer due to incompatible geometry.
They are of course, outer pivot point too high, outer pivot point too low, inner pivot point too high, inner picot point too low, tie rod too long or tie rod too short.
This brings us to a point that is often overlooked. The relationship between caster and bump steer. The kingpin inclination is a fixed parameter that is part of the design of the spindle. However, the actual angle is a combination of the inclination and the caster setting. Changing the caster setting means that the actual position of the upper ball joint changes in relation to the lower. This also means that with this the tie rod length and/or the outer pivot points ideal location has shifted slightly.
------------------------------------

scrub radius:

If you draw a line through the upper and lower ball joint you will get an imaginary line. This line is the steernig axis. The angle of this line with vertical is called the kingpin inclination and is a design feature of the front spindle.

Extend the steering axis to ground level and the distance from the wheel centerline to where the steering axis meets the grouns is called the scrub radius, also called kingpin offset.

Reducing the scrub radius reduces steering effort, this is especially important for cars without power steering. It's easy to see the scrub radius in effect when steering a car while standing still. Notce how the wheel moves.
Another advantage of reduced scrub radius is that the effect of cornering forces is lessened. Any force applied to the tire, being it bump or cornering forces, can exert a force on the wheel directly proportionate the scrub radius. The scrub radius acts as a force arm for these forces.

As you can see below, there are other factors which have an influence on the scrub raduys. These are wheel width, wheel offset, wheel diameter, overall tire diameter and wether spacers are used or not. Also rotor hub thickness can play a part here.

Some scrub radius is still to be desired as it gives the steering feel.

The kingpin inclination offers self centering of the steering, even when standing still. Think about what happens when you turn the wheel. With the wheels straight the inclination is @ maximum in respect to the front axis. When you turn the wheel the spindle turns and the vertical plane though the spindle in which the inclination is present rotates away from teh axis. The effective ball joint distance has to change and as a result the suspension has to move in or out. This can sometimes be seen on vehicles, whens teering at standstill the front corner will actually move up or down. This effect puts an increasing load on the steering as it gets further off center, giving self centering and an on center feel.

The more kingpin inclination, the more feel you have. However, because of above effect the tire contact patch will also move in or outboard, causing it to wobble about which is undesirable. A commonly used maximum kingpin inclination angle is about 9-10deg.



-----------------------------------------------

roll center & IC:

This drawing shows how to find the Roll Center for an Unequal Double A-Arm front suspension.


As you can see the upper arms are moving in a plane that is angled with respect to the lower arms. This means that the above drawing is not actually correct, it's just a simple illustration.


The actual plane where the lines should be drawn is this:


Showing the actual plane (angled because of caster setting)


The only way to plot this accurately is to make accurate measurements and to use a software package like Suspension Analyzer by Performance Trends



Here's how to find the Roll Center on the 63-82 Corvette IRS


This method is similar for the C4 IRS. The only difference on the C4 system is that the 2 trailing arms on that system form a virtual swing arm, whereas the C2/C3 system has a fixed trailing arm that represents the swing arm.



The virtual swing arm extends from spindle center to the instantaneous center formed by the intersecting point of the 2 extended lines through the trailing arms. It's quite obvious that as the suspension moves, this virtual swing arm moves about as the IC moves, length is also not (necessarily) a fixed length, it depends on the geometric relation between the 2 trailing arms. Anti squat is one of the factors which is influenced by these but also other aspects like how smooth the suspension negotiates an obstacle is influenced. For example, if the instantaneous center has a z height below the spindle height, if the wheel has to run over and obstacle the suspension moves up but the wheel also moves slightly forward because of the arc it swings with the IC as it's center point. Moving forward means the suspension moves into the oncoming obstacle. Having the suspension move up and back makes for a smoother ride.

The Roll Axis of the car is simply a virtual line through the front and rear roll centers.
The roll centers are found by the intersecting line of the car centerline and the virtual arm through the instantaneous center and the wheel centerline to ground intersection point. This can be done like this because these cars are built with symmetrical suspension corners. On a car with a staggered setup the roll center will be offset from the car centerline, however there's no need to discuss that here.


-----------------------------------



Now, these are done for a double a arm suspension. For our strut suspension the upper arm falls off the equation and you simply draw a line through the camber plate perpendicular to the shock axis to give you that virtual plane.

this is the (unfinished here) rack setup



I had to swap the spindles left and right to make the steering knuckles point to the front (originally a rear steer car) and fabricate adapters (like guldstrand bumpsteer blocks) to move the tie rod hole a bunch outboard, inside the wheel to correct for ackerman.


All of this obviously is not an issue for our cars since it's already front steer. What is important is to select a rack with a pivot to pivot width that is close to that of the bushing distance between the 2 a arms. A little off is not that big a deal since the angular change of the tie rod ends through suspension travel is pretty minimal so the change in effective horizontal component (effective steering length) is minimal also.

LX_SS

My apologies im getting my words mixed up, haven't been getting much sleep lately. i'll have read when im fresh and post then.

whitedevilTA

iTrader: (8)

The picture was more just a quick drawing to sort of explain my point. I'm not too concerned about the actual body width, as long as I can find a rack to fit my goals that has the pivot points where the lower A-arm bushings are. I know in reality that the body is a bit shorter.

I have heard that a rear steer rack when mounted in the front position will make the wheels turn opposite of where you turn your steering wheel. That would be fun to try to drive!

As for the parallel tie rods, my front suspension is honestly almost straight across. The A-arm itself has a bend in it, but where the actual ball joint is in relation to the A-arm bushing is basically horizontal.

I was actually told by a tech at flaming river that if I wanted a decent rack setup put simply, just mount it to follow my existing steering dimensions. Meaning the body would be where the centerlink is, and the tie rods would follow the lines of my existing ones. In the pic, my current factory steering is also almost straight across as well.

I will be using a front mount rack for the setup. So far I have my hands on an SN95 mustang GT rack, and I will be meeting with a buddy soon who has a spare 4th gen LS1 Z28 rack laying around. I'm going to be doing a lot of measuring! I have another friend who has an LT1 4th gen rack (they are different) and then my brother has a 96 BMW with a front mount steering rack on it. Basically, I plan on measuring all the front steer racks I can find for pivot point width and steering travel until I find one that fits the bill.

And heres an article I pulled off a hotrodder page that was very helpful in explaining the conversion to me. It explains the difference between doing a macpherson style steering rack and a double A-arm rack.

http://hotrodphotos.co.uk/Rack%20opinions.htm

crazy_hotrodder

Subscribe.

I applaud your effort, please take lots of pictures and post to completion.

Quick idiot's(myself) question for the masters.
Why do all of this work with the stock K-member when there are tubular k-members on the market that have prefab'ed rack mounts already installed?

whitedevilTA

iTrader: (8)

I plan on documenting it very thorough with lots of pics because I'd like to get sort of a "how to" on this controversial conversion, especially with all the half completed ones and the others that turn out crappy.

As for the K-member, I do have a BMR tubular K-member in my car. The only one offered with rack mounts already on it is for the pinto rack, which when used with stock steering arms gives you terrible steering angle, and it's also manual steering which I will never do on a street car. From the get go I planned on eventually doing a power rack conversion which I figured would be all custom fabbed anyways.

The techs at BMR assured me their K-member was structurally sound for any type of power rack conversion I wanted to attach to it, so that aspect is all set!

IROCgiraffe

iTrader: (3)

(subscribed) :-)

TTOP350

iTrader: (8)

Good luck with this, this is something I started on years ago but never really found a good rack that would work properly.
I talked to one company about it and he laffed at me and said its not that hard to do and would have one offered soon. I then pointed out all the issues with it, I still haven't seen his kit yet.

anesthes

iTrader: (12)

I did a power rack about 10 years ago. required 2 u-joints in the steering column, a heim joint to hold it. It sucked, because I couldn't get full turning radius out of it. The travel on the mustang racks was too short. Nobody made aftermarket spindles with shorter steering arms.

-- Joe

tvc 15

Have you researched any racks yet? In my eyeball calculations it looks like the rack could be about 2 inch's longer than the hole to hole measurement on the original drag link.

TTOP350

iTrader: (8)

Yes, that was the main issue I pointed out to him. He said It doesn't matter. I said, yeah, it does to the people that drive/race the cars. I just smiled and walked away. I pretty much wrote him off at that point. I was very surprised because he is a fairly well known name.

I don't think it would be 2 hard to mount the spindle in a jig, cut the arm back, drill and tap it for a heim joint and go from there.
Or machine the back of the spindle flat, cut off the arm completely and bolt a billet replacement on there kinda like the mustang 2 suspension kits are using.

anesthes

iTrader: (12)

Not enough material in the arm to drill out a hole farther back, or at least when I measured it I said no way. With a 5/8" hole it didn't leave enough room on either side.




What I did at the time was cut and weld them shorter, but then I got all chicken about running them.. Not that I've never driven a car with a broken tie rod before, but not at 120 mph...


This was about the time I got real annoyed with the car and parted it out and bought a vette. I had spent countless nights in the shop trying to make the car handle like a sports car. Funny thing, once I had a vette with rack and pinion, 275mm tires, etc I hated it. Every little problem with new england roads would grab the steering wheel. Every little bump I could feel in my tail bone.. bah!

-- Joe

tvc 15

On your diagram above you show it as shortening the steering arm, but if you put the hole in the arm like shown you would have a bunch of ackerman. The hole would need to be right where the two lines intersect in order to have neutral steering. I have made and I'm using a set that I have shortened. On mine the shortening was a side affect my goal was to get rid of the anti-ackerman. On mine the pivot point would be where the two blue lines intersect on your diagram.

TTOP350

iTrader: (8)

I was going to drill/tap up the spindle arm 2 add a lil spindle arm length adjustment to it. Buuuut I didn't know what kind of side loads the heim joint threads could take.
Know what you mean about how the rack steering feels, weird.

TTOP350

iTrader: (8)

That's a good Idea, have you thought of encasing more of it and thru bolting it?? (4 safety) Would that help or hurt the spindle arm?

whitedevilTA

iTrader: (8)

Thanks for the ideas guys, and TVC, thats a great pic of a custom spindle! I have been curious how I would weld the spindles if I did have to shorten them, and I like that setup.

As far as ackerman, the angle of the steering arm where the tie rod would bolt from my understanding basically has to be relatively in line with an imaginary line drawn from the rear axle pinion through the front ball joint. If the tie rod mount area is close to that, you should have no ill ackerman effects. Haven't really mapped anything out yet but it is a work in progress. I'm still waiting on measurements from a buddy on the 4th gen rack, and I would like to measure my brother's BMW rack tomorrow possibly.

Twin_Turbo

iTrader: (25)

Not the pinion but the center of the rear axle so that the steering is over that imaginary turning point right in the middle of the rear axle. The pinion is offset/.

tvc 15

There are two bolts that hold the piece on. One uses the dust cover hole and the other I drilled a hole by the lbj. I fit the pieces very well and the last weld I did I preheated my bracket before welding it. When it cooled it was so tight it would not move on the spindle.

84 1LE

iTrader: (9)

Subscribed....Great work guys.

How tall is that ball joint?

whitedevilTA

iTrader: (8)

Thanks for the correction. Should have looked at my diagram before posting, but it was late and I was tired haha.

FlyDoc

WhitedevilTA
I like the fact that you have committed to putting a Rack in a third gen and not just doing it half assed!
before I throw some pennies in the debate, I have not read every post in this thread, but working on it.

I think that you need to invest in a suspension analyzer program.
but the longer the suspension arms ( upper / lower arms, tie rods) the larger the Radius, which effects the amount of camber gain/loss, and bump steer. so i was going to just build a bracket that just mounts the inner tie rods as close to the center as possible, to make the tie rods as long as possible.
But I like the way your going!

tvc 15

In my minimal amount of research I find no stock racks that are short enough. Sweet makes some shorter power racks but I don't know if they have enough travel.

BlackTopKing

iTrader: (1)

Whoa.....I'm watching.
And twin turbo....that just blew my mind...lol

whitedevilTA

iTrader: (8)

I'm sure I can find a rack that is short enough to fit where I need it...my brother's BMW is front steer and it is not a wide car. The problem will be the travel amount and where the inner tie rod pivot points are. I am very prepared to shorten the spindle arms if I have to, but would prefer not to if thats an option. I have plenty of buddies that are into the drifting scene with custom steering on their nissan 240's, and they all have modified spindles with no issues. Most of them drive the cars on the street a ton as well.

tvc 15

The width of the car is not the whole concern. Even if you find a narrow car, if it has short A-arms it makes the rack wide. The thirdgen has long A-arms so it might be difficult to find a production rack. But keep looking.

Banks4004

these are nice

whitedevilTA

iTrader: (8)

I'll keep searching high and low. But like I said before, if I can't find a factory one, I'll go with a custom/universal one. Flaming river isn't cheap, but they have some nice quality stuff!

I've seen those...while they seem nice, I'm not dishing out $2700 for a power rack lol...I don't care how nice it is! I work at a machine shop, and I could honestly probably machine one from scratch for less than that, haha.

TTOP350

iTrader: (8)

I was looking at the trailblazer SS or the smaller truck racks but never got farther than that.

tomsaddy

iTrader: (4)

I really want to do something similar with my project (currently in the middle of a rotisserie resto-mod, a lot of which was inspired by your build). I've come to the same conclusion that the trick seems to be finding a rack that's narrow enough and still has enough travel.

Appleton looks to have some potential candidates, but they're not cheap either.
http://www.appletonrackandpinion.com/

whitedevilTA

iTrader: (8)

Glad I could be of some inspiration to ya! Good luck on your build. At this point, I may call up flaming river and give them some measurements and see if they have something that fits the bill. I'm getting tired of monkeying around and going back and forth between friends trying to get measurements off different factory racks. Sometimes it's just easier to dish out the money and not have to deal with all the running around!

cam406406

iTrader: (1)

Just have unirack build u one

tvc 15

I've looked this over pretty good and I don't see this working with out a tubular cross member and a custom rack. But I'm still interested.

tomsaddy

iTrader: (4)

Any updates?

LX_SS

Well i decided to bite the bullet on mine and just get a rack that suits, the bmw rack i was playing with was nice, but the modifications required to make it work meant if your ever had an issue it was almost impossible to replace easily.

So off the shelf rack and pinion steering rack with center take off pivot.
Similar to whats used in mustangs and the retro rack just without the 3k price tag.
Still waiting for the ship to arrive with the unit, so i cant take any pics then guessing around end of feb is when its meant to arrive.

Reason for using the CTO style rack, is so long as it has the travel, it will work, you don't have the issues you normally do with an oem rack as its just like removing all the original box + idler arms, changeover the drag link to the new rack one and thats basically it. Also since i'm redoing my steering column anyway, i'm not sure how this would suit a factory column, it would be fine i imagine just need another universal.

whitedevilTA

iTrader: (8)

Not yet. It's winter here in wonderful new england and I just have not had the desire to work on the car much, after building it for the past 4 years straight! I'm taking a winter off to relax and drink beer. Haha, I'm assuming come spring time, I will have much more motivation to get this done.

Please keep me in the loop on how this works out for ya. I'd love to see some pics when you start getting it installed and hear how you like the setup. I've been torn between CTO style racks or the typical factory style end take offs.

FIRECHICKEN

iTrader: (3)

I have the BMR K member as well with no rack mounts. I really really want a rack and pinion too. My box has started to break the frame. Subscribed.

LX_SS

https://www.thirdgen.org/forums/fabr...1-third-3.html

last 2 pics of post 100, CTO rack and pinion suit rhd

whitedevilTA

iTrader: (8)

Def let me know how this setup works our once you get it a bit more permanent! Thanks for the link and pics. Nice to see another one in the works.

tvc 15

Are the photos of the rack for a right hand drive car? But either way that rack is way too wide for car. The tie rods would be very short and would cause bump steer. Or am I looking at it wrong?

LX_SS

If your referring to my rhd cto rack, then your are looking at it incorrectly.

The rack width itself is irrelevant, what matters is the travel of the cto which has enough movement for our steering full lock to full lock (8 inches total the rack gives).

So imagine your tie rods + draglink, put a clamp in the center of your draglink, and connect it to the center of the rack. Thats the utmost simple explanation i can think of.

However if you weren't referring to my rack, then disregard lol.

tvc 15

Ahh I see... said the blind man.

d3v1l8oy

so did you ever get those measurements for the fourth gen rack if you need me too I can this weekend coming weekend ill be replacing ball joints on a fourth gen but when I got looking at it they almost look like they would be proper length not really sure about rack travel but those fourth gens have boat front ends like ours and should be about the same wheel base right? so if I took a fourth gen rack and pinion and put it up next to the current stock and it ended up being from inner to inner tie rod ends the same length then in theory shouldn't that work I mean of course after your build a custom mounting point for it on the k member but ya im just trying to get this strait as I need to do this soon cause I am without my car cause the whole steering box broke through my frame where its mounted and cracked it so now I half to weld it then re-enforce it.

L695speed

Subscribing, I'd be game to using this as a how to. Friend of mine drove my GTA said the biggest let down was the vague steering feel through the box. I'm considering the possibility of putting a rack and pinion on my 84 if we save it and maybe even an IRS. Definitely digging LOL.

91RamAir Formula

Subscribing to see how this ends up?

tomsaddy

iTrader: (4)

I've been looking at the Gen5 Camaro ZL1 electric power rack as a possible candidate, but I don't know if the travel and tie rod lengths are appropriate for our cars.

There's a late model Honda that has what appears to be a Center-Take-Off electric rack that may be the best of all worlds if the ratio is right. If it has enough travel, you should be able to set up the geometry any way you want.