Technical Rack and Pinion steering questions

Guys I'm in the process of installing a Cavalier R&P on my 1953 Oldsmobile 88 and wanted some opinions before I made the final rack mounts.

First off I've lowered the front by putting Fatman 2.5" drop uprights and cutting a couple of coils off the stock springs. The instructions for the uprights say I need to heat and bend the steering arms down 2.5". I know its pretty a pretty common thing to do but I just didn't like the idea. SO I HAVE NOT DONE THAT YET.

My thinking was since I'm installing a R&P anyway I would just raise up the inner pivot to match the now higher outer pivot. Due to oil pan clearance I can't quite get the angle of the tie rod to perfectly match the angle of the lower a arm.

Question #1) is the angle of the tire rod to the A-Arm shown below OK or does it have to be perfectly parallel?

Each tire rid is 24.625" long if that matters.

At full drop the lower A-arm is at a 15deg angle and the tire rod is at 11degrees.


At a little above ride height(lower than it will sit but the direction the suspension would move when it hits a bump) the lower a-arm is at 0 deg. and the tire rod is at 2 deg. (see below)



I'm as high as I can go on the center pivot. I can still heat and bend the steering arms but I'd like to avoid that if possible (I can always come back later if it drives bad and lower the outer pivot.)

Question #2 Is this wide enough on the center pivot. / is the center pivot correct like I have it?

The center pivot V/S what the stock steering had is throwing me off. From what I've read on this sight the inner pivots should have been in line with the pivot axis formed by the inner pivots (upper and lower). They were not even close! I don't have any pictures but it was a standard box on the drivers side with a pivot on the passenger side. The inner pivots were 24" apart and directly in line with the lower inner pivot (height wise). So they were not in the plan of the upper and lower inner pivot.

below is what I came up with. The center of my Hiem joint is higher then the lower inner pivot and slightly wider than it would need to be if you continued the inner pivot back to where the hiems are. NOT THE BEST PICTURE I KNOW BUT WITHOUT A LIFT IT WAS THE BEST I COULD GET.

See picture below.





I have every article I could find or build thread at used this set up! I think I'm pretty close with this but figured I'd post and ask if someone who knows can tell me if this will work or not. I can't move the rack any further back or up due to oil pan clearance's.

THANKS

 

You will have some bump steer. Can't tell you how much. It's really easy to bend steering arms, especially as you don't need very much bend, and done all the time. I would take the time to do it right the first time. Bump steer can be very disturbing.

 

So if I move the outer lower tire rod pivot down and the lower A-arm and the tire rod stay parallel I will have zero bump steer correct?

 

You will probably never have "zero" bump steer, especially with the tie rods pivoting that far inboard of the lower control arm pivot axis. It is not all about having the tie rod parallel to the lower arm but also about the arc that the arms and links swing through. With a rear steer package like you have the overly long tie rod will have a tendency to cause toe in if anything, the reason being the the spindle will tend to move inboard a little more than the tie rod end as the steering articulates due to the difference in the arcs. Plot it out on a piece of cardboard or tin and you will be able to determine which is the best case scenario. If you do that, remember that the fact that the lower arm inner pivot axis is not parallel to the longitudinal axis of the chassis means that the effective pivot point is midway along the length of the shaft that the arm pivots on which puts it considerably outboard of the rear point shown in your last photo (adjacent to the tie rod pivot)

Roo

 

Thanks Rooman! To make sure I'm understanding you correctly you are saying that the factory swing set that had the inner tie rod pivot well outside the "longitudinal axis of the chassis" is what I need to mimic with my inner mount? What about the height of the outer mount?

I will certainly do some more measurements before I move any further. I'm going to put a large diameter disk on the wheel mounting surface and measure for toe changes as I cycle the suspension up and down.

In my head it seemed that if the tire rod stayed parallel to the lower a arm that I would have no bump steer. I hadn't taken into account the fact that the spindle does move in and back as it travels up.

 

If you are willing to spend the time to make a drawing of where all of the relevant points are --inner pivots for both upper and lower arms, upper and lower ball joint location at the spindle and the steering locations--I will run the numbers with some software that I have. I would need the locations in all planes--x, y and z axis.

Roo

 

You can see how much bump steer you have with the springs removed. Move the spindle up to it's highest point and mark where the spindle is pointing. Then go to the lowest point the suspension will go and mark where the spindle is pointing. The difference between the to points is the degrees the wheels will turn when going through a really big bump. The inner tierod mounting points should be the same distance apart as the inner mounting points for the lower A frames. And the tie rods should swing through the same ark as the outer tie rod mounting point does. Another words when the spindle goes up and down it should not steer the car in any direction but straight ahead in a perfect world.

 

Thanks guys. I have the springs out already so I could check how the tire rod angle changed in relation to the lower a arm.

My plan for tonight is to hang a plumb bob of the front and rear of the rotors. Do a red neck alignment at approx. ride height and then cycle the suspension up and down measuring the changes. That should tell me for sure how much bump steer I have. Then the question will become how much is acceptable on an 11" rotor....LOL

I'll check back in the morning with the results and photos

 

Set your toe at -0- with the lower a-frame at ride height. Lock your steering in place, put a jack below the lower a-frame and run it thru normal range of travel on each side. Stand something up that is beside the tire and parallel to the face of the tire and you will be able to measure your toe as the wheel travels up and down.

You are going to have a lot bump with the inner pivots of the rack being so far off the imaginary line between the inner pivot point of the upper and lower a-frames. You can also add shims on the spindle end of the tie rod if needed to help bump steer. The idea is to get the inner pivot points of the rack t fall on that imaginary line of he inner pivot points of the a-frames AND the tie rod running parallel to the lower a-frame. That's not going to happen with your rack spacing.

Would it be possible to add a second plate where he inner ends of your tie rods bolt to he rack? That plate could bolt to the existing inner tie rod holes on the rack, be wider and have the holes drilled so the inner heims on your tie rods fall on that inner pivot a-frame line and adjust your height at the same time to keep the tie rods parallel to the lower a-frames.

I think your rack is going to cause a problem you can't fix for bump steer with the inner pivots so far in.

SPark

 

That sounds like a GREAT idea if you have the room.

 

Blase, Here is some info worth reading.
http://www.jalopyjournal.com/forum/threads/49-51-mercury-power-rack-pinion.510106/

 

A string line is your friend here

 

Well , i put one in my 50 ford and all i did was made sure my tie rods where level with the control arms. Those dropped spindles are making more work for you and i get that. Why is one tie rod level and the other isn't in the pics. The inner tie bracket distance looks ok to me as is the back a forth of the rack mounting. You say you can't raise your rack anymore because of the oil pan. Can you get level if your inner tie rods are raised on the bracket if there is room? Make a new bracket.
I can't tell in the pics but are you using a heim on the steering arm? Mount on top or bottom for gains as well.

 

Let me see if I'm up to speed on this.

You are running hand fabricated uprights made from about 8-10 various bits all welded together, but you don't want to heat and bend your forged steering arms as the instructions tell you to do.
To compensate for this you are totally ignoring the instructions and raising your rack to a point where you are having potential oil pan clearance issues, your inner pivot points (not to mention your input shaft from the steering column) are vertically and horizontially completely in the wrong place to work acceptably without a bunch of experimentation, conjecture and Mathamatics.
All this despite the fact that at the suggested rack mounting location they can simply mirror the exact location of the original inner tierod ends.
No experimentation, conjecture or Math involved.

Soooo...this is all in the interest of "safety" I suppose???

 

Did I miss anything good?

 

Ok everyone first off thanks for all the replies! I assure you that I read everyone and take them all into account.

As to the not wanting to heat and bend the arms. All I was saying is since I was adding a R&P it made sense to try and place the R&P so that it wasn't needed. I can see now that theory will be harder than I thought!

Ok below is what I did last night to measure bump steer. The results are as most of you expected.


The above is ride height. I measured from string to string in the front at ride height, full droop and full compression. *NOTE HOW THE HEIM IS MOUNTED TO THE STEERING ARM IN THE PICTURE*

To show where ride height is in the suspension travel I measured from the ground to the center of spindle.
23.750" is ride height
27.750" was full compression
20.750" was full droop.

At ride height it measured 60.750"
At full droop it measured 58.9375"
At full compression it measured 60.875"

Knowing that is way too much bump steer I did the only thing I could last night to see if it helped. I lowered the outer tire rod pivot just as a test. It will not stay like this!!


Moving this down 1.25" (all the bolt I had) produced a tire rod that was much closer to parallel with the lower a arm. I understand that doesn't mean a whole lot but its a gage of sorts.



I then ran the same test. below are the results.

At ride height it measured 60.750"
At full droop it measured 59.4375"
At full compression it measured 60.750"

Much better but still not right. BTW I'm measuring from one string to another so the change per side is 1/2 of the total amount.

SO. As I expected you guys are correct. Heating and bending the arms would correct some or maybe all of my bump steer. Another benefit is I can make them slightly shorter which will get me back some of the turning radius that I've lost with this rack.

In order to see exactly how much I need to bend them I'm going to pick up two longer bolts and I'll continue to drop the outer heim until I'm happy with the results.

BUT since I've got someone's attention I'd like to know if my new theory is correct. Say I again like a hard head did not want to heat and bend the arms. Would widening the inner pivots basically do the same thing? Seems like it would be easier to just drill 4 new holes in a wider pc of plate v/s heating and bending those arm an exact amount.

Again guys I'm young and just like to make sure I completely understand the consequences before I alter a part I can't replace (steering arms in this case). I am a machinist by trade and I tend to be overly accurate with measurements. Worse case I could make custom steering arms if I messed these up but its much easier and simpler to move in inner pivot if that is the acceptable.

Any thoughts on if that alone would work?

 

I think you will need to do both, widen the plate and bend the arms.

 

Fabricate a new bracket to mount on the rack that sets the tie rod pivots at the same points as the lower control arm. Since you are using the Fatman dropped uprights, you will need to bend them down as the instructions state. This rack system was used by Butch's rod shop and I have drawings for the bracket for the rack to get the tie rod pivots where they belong and mounting the rack to the chassis. You could have stepped your lower control arm and used 53-54 uprights and arms and I think eliminated some of the issues with about the same drop.

 

Let me correct part of what I just posted, I thought it was a Chevy. The upright statement is wrong, but I would think you could step the Olds lowers like a chevy and pick up about an inch and a half drop. The rack to tie rod bracket to get the pivots correct would be jut like the one that Butch came up with some dimensional changes for the difference between the Olds and 49-54 Chevy.

Wanted to correct this before the hate postings started

 

The bending of the steering arms is for the stock steering with the dropped spindles. Once you change 1 piece of that equation, the bent arms go out the window as far as I'm concerned.

On dirt cars we change our spindle end like this.
http://www.speedwaymotors.com/Pinto-Spindle-Bump-Steer-Correction-Kit,52425.html

We also raise/lower racks to ma bump do hat we want it to. Inner pivot, outer pivot, length of tie rod vs length of control arm will all have an effect on the ump steer.

You are playing with what you need to. Since you have the rack as high as you can (it still needs to be higher with the results you are seeing), changing length is another option.

Keep playing with it. It's a lot easier to change at this point than it is safer you're driving it and wreck it after you experience big bump steer.

Here's how to adjust, good reading here.

http://www.longacreracing.com/technical-articles.aspx?item=8162

SPark

 

That for the response and the link! I am still playing with it but last night I took and lowered the outer tire rod mounting point 2.5" (just using a lower bolt for testing) and it made it worse. Seem like with where my current inner mounts are that 1" drop is the sweet spot.

I have made a testing mount so that I can move the inner tire rod mounts wider and find the sweet spot there. I for see lots of trail and error over the holiday weekend to find the spot that works best for my set up.

On the bump steer kit posted above....how does that work exactly? A buddy told me about something like this but couldn't explain how it worked. Does it just allow the heim to float up and down?

I'll be reading the other link shortly.

 

My 85 corvette had bump steer , the engineers figured its a compromise....speedway kit looks cool....any use it?

 

The heim does NOT float. The kit just relocates the pivot, away from its original location.

 

The tapered/straight piece is just a spacer. The heim rides on the bottom edge of the straight part of the shaft. Once you find what you need for length (by using washers as spacers while finding what works best), you trim the bottom straight shaft part of the spacer to the exact length you need. The wide flared part sits against the spindle for a more solid base to help prevent flex. You may find the 2 sides need different lengths of spacers to get exactly what you need.

We use these all the time in circle track racing. They work well. We usually have a 5/8" heim on the outer end of the tie rod and that's what these are made for.

SPark

 

These heims are just for mock-up, right? You'll use tie rods with greaseable ends on the final assembly.

 

No I am running Heims. They will get Seals It seals on them for final Assembly.

http://www.speedwaymotors.com/Seals...-Rod-End-Seals-3-4-Inch-Hole-Set-6,28021.html

These are not junk tractor supply heims. These are high quality chromoly , Teflon lined, plated joints that I've had good luck with.

 

Ok so daily update. I spread out the inner tire rod mounts so that they are just a little (1/2) wider than the lower a arm pivot point. Still have almost 1" of bump steer. Pictures below.







I tried spacing the outer tire rod down 1" and 2" with this new inner mount and the best it got was 1" of bump steer. 60.750" at ride height and a nice progressive movement to 59.9375" at bump and full compression.

below are some pictures of the stock set up.



The swing set it self was 25" pivot to pivot.

The inner tire rods pivoted at 16.50" from each other. My current set up is only 10.25". Is the fix to mount the inner tire rod at 16.500" from each other? 8.250" from center?

From the article posted above it says I should lengthen the tire rod.
Symptom 3. Always toes in both compression and rebound. Cure 3. Lengthen the tie rod as it is too short.

 

The pivot point of the lower control arm is not the zerk by the tie rod end. It's the average of the center of the lower control arm front and back pivot points since the lower mount is angled. It would be a couple inches out from that rear zerk in the control arm.

Unless you copy the height of the stock inner pivot points, the old parts have no bearing. If you set your inner pivot points (the rack) at the same height as the drag link, you could copy the width of the original drag link inner pivot points. Once you move the rack up or down from stock, you have to start all over and throw the stock stuff out.

Another thing to consider, when you move the LF wheel up and down, it the right front moving at all? When we do bump measurements on race cars, we use a bump fixture that is independent of the car. By measuring off the right hub you may also be adding any movement of the RF corner as the LF wheel moves.

You may always have some bump with the setup of the front suspension. Long lowers and short uppers create some issues in themselves. It would be interesting to see what bump was with the stock setup. You may not be that far off.

SPark

 

Thanks that is VERY helpful! I will try again tonight with the inner points moved out another hole.

I don't know what is acceptable. Reading the dirt track guys pages they are measuring bump steer in thousands of an inch......I'm still in inches! LOL.

The rack can't move. Its as far back and up as it can go. So at this point I'm just looking for the best I can get it with steering arm mods (bending or spacing) and tire rid length.