Steering geometry, installing Rack and Pinion

[flt-blk]

The car is a 55 Studebaker with a SBC/TH350
I installed a Diplomat-Volari-Aspen torsion bar front clip years ago then
abandon the project when I moved to Texas.

I am finishing the project now.

Space was tight so I pulled out the Box and idler arm steering with
the intent of using a R&P.  What I want is someone to check my
engineering.

I am using a 94 Ford Escort manual Rack and Pinion.
- I had to extend the rack so I made sleves to lengthen the arms and use the Dodge tie rod ends.
- The rack knuckles are the same width as the lower controll arm bushings so it should move in the same plane (no bump steer)
- I mounted the rack 2" forward of the spindle arm, the tie rods angle straight out and slightly rearward.

I need to check ackerman on the steering arms as the wheelbase of this car is a little different.   Does anybody see any problems with this setup.

The car is being shipped from Seattle to Texas by BenD and will be
making a stop at B'Ville on her way to me if you want to have a look.  I
plan on making it back to the salt in this car next year.


TZ

[Bob Paulin]

You might want to get yourself a large cup of coffee before starting......

What I want is someone to check my
engineering.

I am using a 94 Ford Escort manual Rack and Pinion.
- I had to extend the rack so I made sleves to lengthen the arms and use the Dodge tie rod ends.

So far, so good.....

- The rack knuckles are the same width as the lower controll arm bushings so it should move in the same plane (no bump steer)

There is more than a single geometric plane that must be considered to deal with bump steer.

The inner tie rod pivot must fall on or intersect a line projected through the upper inner control arm pivot and the lower inner control arm pivot.

If the rack is mounted at the same height as the lower control arm, having the inner tie rod pivot in direct alignment with the lower control arm inner pivot would probably be okay.

If the rack is mounted higher than the lower control arm - which is the most likely case - then the inner tie rod pivot will likely have to move towards the outside of the car - depending on where the upper inner control arm pivot is located, and where the projected line falls.

Now, just having the inner tie rod pivot fall on the projected inner control arm pivot line isn't quite enough.

The outer tie rod pivot has to fall on a line projected between the upper and lower outer control arm outer pivot points - the ball joints. If you are using the steering arm that came with the suspension that you are using, it's probably pretty close....but, if you have changed the steering arm, all bets are off!

Now, just to throw you one more curve, the horizontal line projected between the inner tie rod end pivot and the outer tie rod end pivot must intersect at a point where lines drawn through the upper control arm inner pivot and ball joint...and the lower control arm inner pivot and ball joint intersetc,

From the front, the projected lines between the upper and lower control arms will look like a "V" on its side, and the projected line from the tie rod should hit that "V" right in the middle of the point.

<--

Anything other than hitting the "V" dead center will result in bump steer - even if the other parameters have been met.

All three parameters are necessary.

- I mounted the rack 2" forward of the spindle arm, the tie rods angle straight out and slightly rearward.

ront-steer car, the rack should be mounted slightly behind the point where the tie rods attach to the steering arms, so the tie rods extend forward from the rack to the steering arms.

This will promote Ackerman when steering.

If the tie rods angle backwards from the rack towards the steering arm, this will result in reverse Ackerman - the outer wheel will turn a sharper angle than the inner wheel in a  turn.

- I need to check ackerman on the steering arms as the wheelbase of this car is a little different.

As far as checking Ackerman goes, the old rule-of-thumb was to project a line from the ball joint or king pin to the center of the rear axle. The tie-rod socket on the steering arm should fall on this projected line.

Nowadays, it is understood that Ackerman needs vary according to a number of factors including speed of the vehicle and tire design.

There isn't a universally agreed way to express how much Ackermann (toe-out increase with steer) a car has. The closest thing we have is to take the plan-view (top-view) distance from from the front axle line to the convergence point of the steering arm lines, divide the wheelbase by that number, and express the quotient as a percentage. If the steering arms converge to a point on the rear axle line, that's said to be 100% Ackermann. If they converge to a point twice the wheelbase back, that's said to be 50%. If they converge to a point 2/3 of the wheelbase back, that's said to be 150%. If they are parallel, that's zero Ackermann. If they converge to a point twice the wheelbase ahead of the front axle, that's said to be –50%.

Supposedly, with 100% Ackermann, the front wheels will track without scuffing in a low-speed turn, where the turn center (center of curvature of the car's motion path) lies on the rear axle line in plan view. This is actually not strictly true, even for the simplest steering linkage, which would be a beam axle system with a single, one-piece tie rod. With either a rack-and-pinion steering system or a pitman arm, idler arm, and relay rod or center link, we can't fully predict what the Ackermann properties will be at all, merely by looking at the plan view geometry of the steering arms. The whole mechanism affects toe change with steer.

Even knowing what instantaneous toe we want in a specific dynamic situation is not simple. We don't necessarily want equal slip angles on both front tires. For any given steer angle, the turn center might be anywhere, depending on the situation. All the infinitely numerous possible situations will have different optimum toe conditions. Therefore, there is no relationship between steer and toe that is right for all situations.

The toe we have at any particular instant results not only from Ackermann effect, but also from static toe setting and toe change with suspension movement (roll and ride Ackermann).

Because of these complexities, there is no single obvious way to define what constitutes theoretically correct Ackermann. It is possible to come up with a rationally defensible definition for your own purposes, but there is no standard rule, and it is unlikely that there ever will be.

Having entered these abundant caveats, I use the following guidelines on race cars...

1. In plan view, at zero steer (straight-ahead position) the steering arms should converge to a point somewhere between the rear axle line and the midpoint of the wheelbase. In traditional parlance, that's somewhere between 100% and 200% Ackermann. The tighter the turns, the higher the percentage.

2. At all steering positions, the rack or relay rod should be either slightly behind the outer tie rod ends or even with them. This applies to both front steer and rear steer cars. With rack and pinion steering, it means that at zero steer, the rack should be a bit behind the outer tie rod ends on a front steer car, and about even with the outer tie rod ends on a rear steer car. Purpose of this is to assure that tie rod angularity adds Ackermann at large steer angles, rather than subtracting.

3. Angle between any arm and any link in the system should never be less than 30 degrees or greater than 150 degrees. This helps to assure that the mechanism cannot snap over-center due to deflections of the components. Alternatively, over-centering can also be prevented by provision of stops at appropriate points in the mechanism.

For most street use, "close enough" IS, actually, close enough on Ackerman. This is one of the few times I will ever say that.

A prime example is the S-10/15 chassis that ranges from a 100-inch to a 123-inch wheel base, yet uses the same steering components throughout the line.

Does anybody see any problems with this setup.

Well, I certainly don't....but, I'm way over in the Northeast, so the chances of you wandering into an oncoming lane in front of me are quite slim.....

LOL!!!!

But, seriously....I had a friend who mounted a rack incorrectly - against my advice - on an S-10 chassis in order to accomodate a 462 Lincoln. After a year of screwing around with it, he converted back to a steering box/idler arm setup - but, in spite of my offers to help, he didn't take the correct geometry into consideration, and it is feathering tires.

He's now so frustrated, that he's now trying to sell the truck he was once so excited to be building.

You are smart in attempting to build-in the correct geometry rather than try to fix it later.


Bob Paulin

[Dingus]

I think the rack should be moved toward the rear if possible. The angle between the line from the ball joint to the tie rod end and the direction of the end of the tie rod should be way less than 90 degrees. If you look down on a front steer car it is visable that the linkage is to the back. My 60 Chev is a good example.
To check for bump steer, assemble the front end without springs. put a piece of plywood vertically near the outside of the wheel(no tire) Move the suspension up and down and take measurements at the front and back of the wheel They don't have to be the same but the  front and back measurements should maintain the same differences as it moves.

[flt-blk]

Wow, great feedback, thanks Bob.  
I was planning on slightly bending the factory steering arm so the
projected line hits the pinion, but now I may not bother.  That is a great
explanation of how it works.

Too bad you're not closer I would love to discuss my mental track car
project suspension with you.

Dingus,
I don't know where you read 90 deg.  The rack is 2" forward of the
steering arm (Rear steer).  The angle from the inner knuckle to the steering arm is 10-15deg on a tie rod that is maybe 14" long.

I forgot to mention the car is a partial custom, so suspension travel will
only be 3-4" with marshmallow tires helping ride quaity.

Is there any benefit to having the rack mounted directly between the tie
rod ends?

I wish I had pictures to post.  Anyone else have comments/feedback?
TZ

[Dingus]

My bad! I somehow decided it was a front steer deal. Too many MII's  I guess. All sounds good.  By!  :oops:

[flt-blk]

It's OK,  funny story about that though.

My first crack at this project (I was very young and had never done
anything like this before, this was almost 10 years ago) I tried to use a
Mustang rack.  Everything was tacked in place and looking good.  Picked
up a shiny new Borgeson U-joint for the steering and hooked it up.

I had a friend jump inside and turn the wheel to make sure there was no
binding or clearance problems with the exhaust, no front end on the car, no wheels, just spindles and rotors.

I watched him turn the wheel left, then right, but something wasn't right.

He left and I reached into the window and turned the wheel to the left and
watched the spindle turn to the right...........  %&*#^... and a couple
more adjectives came out of my mouth.

Moral of the story is don't try to use a front steer rack on a rear steer clip.   Anyone want to buy a nice MII power rack.   :oops:  :oops:    
Shhhhh, don't tell anyone though.  
TZ

[Bob Paulin]

Wow, great feedback, thanks Bob.  
I was planning on slightly bending the factory steering arm so the
projected line hits the pinion, but now I may not bother.  That is a great
explanation of how it works.

Too bad you're not closer I would love to discuss my mental track car
project suspension with you.

My shop/office phone number is in my member profile.

If I don't pick up by the third ring, hang up and save yourself some money because the answering machine will usually pick up after four rings.

Bob Paulin

[SKR8PN]

It's OK,  funny story about that though.

My first crack at this project (I was very young and had never done
anything like this before, this was almost 10 years ago) I tried to use a
Mustang rack.  Everything was tacked in place and looking good.  Picked
up a shiny new Borgeson U-joint for the steering and hooked it up.

I had a friend jump inside and turn the wheel to make sure there was no
binding or clearance problems with the exhaust, no front end on the car, no wheels, just spindles and rotors.

I watched him turn the wheel left, then right, but something wasn't right.

He left and I reached into the window and turned the wheel to the left and
watched the spindle turn to the right...........  %&*#^... and a couple
more adjectives came out of my mouth.

Moral of the story is don't try to use a front steer rack on a rear steer clip.   Anyone want to buy a nice MII power rack.   :oops:  :oops:    
Shhhhh, don't tell anyone though.  
TZ

On the 1970's Dodge 4X4's,they used TWO different ways of mounting the Saginaw steering box's. First way was right on the frame rail. Second was a bracket mounted to the frame,and the box laid on the bracket,kinda sideways.......PHYSICALLY,the box's will interchange,and BOLT UP.
Do NOT try to interchange the two.........
Ya get the SAME effect.................
Don't ask. :oops: