47-55.1 bump steer

[nvrdone]

I've been thinking about installing power steering in my'49 using the kit that POL sells. It mounts the p/s box ahead of the axle giving more room for headers etc. Yes, I do have a dropped I beam axle
I have read comments that mounting the steering box forward of the axle causes serious bump steer, but I haven't found any info verifying this. The only reason seems to be that with the drag link going forward and the spring shackles on the forward end of the spring that causes the problem.
My understanding is that bump steer is caused if the drag link is not parallel to the ground. I cant find any info indicating that the mounting location of the spring shackles causes this problem.
I have seen class 8 big rigs with the p/s box and shackles on the front of the frame.
Can any one enlighten me on this be for I go and spend the $$$ for the kit.
Thanks

[leegreen]

I believe it will cause bump steer if the spring flexes much at all. As the leaf flattens out the spring gets longer and moved towards the shackle end, the axle moves back and forth about half as much as the total spring changes length. if the steering box is at the shackle end as the axle moves relative to the steering the wheels will turn from side to side a bit.

look at an 1952 frame: rear steering box and shackles in the front drag link pivot is very close to fixed shackle
look at a 1970s leaf spring 4x4 frame, front steer and shackles in the rear

People will argue I am wrong and the axle moves relative to fixed end of spring too. I could be wrong. Maybe someone here has the same repositioned box and can comment from actual knowledge of your proposed setup

the semi truck you mention....maybe there is just not much spring flex? or the spring is pretty much flat anyway so it won't change length much?

drag link not parallel to ground also cause bump steer. the more angle it starts at the more back and forth movement there is as it travels through it's arc of movement

[dsraven]

here is a thread with some options
https://67-72chevytrucks.com/vboard/...d.php?t=658542
bump steer sucks
the box on the outside of the frame ahead of the axle limits the right hand turn due to tire rubbing the box
the rack attached to the solid axle means there is a lot more weight attached that is unsprung plus the steering column shaft needs to do a lot of extension/retraction, so not awesome. power steering hoses also nee to flex a lot, like every time the truck goes over a bump
personally I would stick with the stock location and adjust the drag link and pitman arm so that the drag link is level at ride height. move the box outboard, move the engine to the right
money spent on a quality steering system that works is money well spent. funny if you think about it and compare prices for other items on the project. guys will hem and haw about some things, like a part for the foundation of the truck, but then not think twice about a few thousand dollars in wheels and tires or a quality sound system.
thats just me though

[dsraven]

here is a link for the cpp version in the stock location. a motor trend upgrade for a 55-59 series truck but they basically have the same set up. the later frame is wider though so a little more room between the frame and the engine.
https://www.motortrend.com/how-to/po...traight-axles/

[1project2many]

I'd love to see pictures of the 47-55.1 installation.

Generally the steering box is connected to the knuckles through linkage that has minimal length change as the axle moves. One way to do this is to connect the steering box to the RH wheel through a long drag link then use a long tie rod to connect the LH wheel. Jeep used this scheme in multiple vehicles.



This kit provides an aftermarket improvement for vehicles with more suspension travel and lift:



Ford has also used a version of this on F series trucks for years. This image is from an '05 to '13 F series:



The stock TF system appears to have been designed with interesting geometry interaction during front spring compression. When the spring compresses from stock height the axle is moved forward, potentially causing the wheels to turn slightly right. But at the same time the end of the drag link traverses an arc which potentially causes the wheels to turn left. If the box is moved to the front of the axle this interaction can be reproduced but it would take some planning. Do you have any pictures of your truck currently?

I would look at the "at rest" angle of a stock drag link when the vehicle is parked and compare it to the "at rest" angle of the drag link when installed on a front steer conversion on a truck with lowered axle. If the "at rest" angle of the modified truck causes the drag link to be parallel to the frame or even higher at the box end than at the knuckle end, I would expect unwanted steering changes and the need for frequent corrections.

[leegreen]

Originally Posted by 1project2many

The stock TF system appears to have been designed with interesting geometry interaction during front spring compression. When the spring compresses from stock height the axle is moved forward, potentially causing the wheels to turn slightly right. But at the same time the end of the drag link traverses an arc which potentially causes the wheels to turn left. If the box is moved to the front of the axle this interaction can be reproduced but it would take some planning.

thank you for that!
I had noted this angle of drag link as significant before when looking at specific setups.
Moving the steering box to the front would reverse the counter affect of the drag link and amplify bump steer unless the angle of the drag link is also reversed. So if the stock drag link angles down from rear box to axle with a front box you'd need it to angle up from box to axle.

Good side view pictures of the stock setup at ride height would let us ponder this some more

[57tailgater]

On the boxes put in front of the axle I believe most kits use a Saginaw box like ones used in a square body truck or Blazer. The drag link winds up being pretty short which as mentioned before is more susceptible to bump steer. I also assume these kits are designed for stock height suspensions. If you start lowering things either with springs, axle or both your results will vary. The kit with the box in the stock location keeps everything relatively the same as from the factory but lowering will affect this too. I have never seen a cross steer kit available but I have seen only one truck someone converted but the owner had no details as someone else did it. Keeping things parallel and minimizing turning during the suspension travel arc is key for sure. While I have stock steering, I do know a guy who has the box conversion in the stock location and he says it works well. You also may want to research caster as it is pretty low on these trucks stock to help ease the turning effort at lower speeds.

[1project2many]

Moving the steering box to the front would reverse the counter affect of the drag link and amplify bump steer unless the angle of the drag link is also reversed.

Yes. Provided the fixed spring mount remains in the rear and the spring retains a traditional arch, the effective length of the drag link would need to decrease as the spring compresses.

Question: Is it possible to mount the steering box so the Pitman arm makes it's sweep between frame rails instead of fore and aft?

[leegreen]

the top picture of this kit looks like it will attempt to reduce bump steer to my eye, axle moves forward on bumps and steers right, pitman arc will counter that:
https://www.classicperform.com/power2.htm
that looks like a saginaw box from c10 or similar with the pitman arm reclocked

Originally Posted by 1project2many

Question: Is it possible to mount the steering box so the Pitman arm makes it's sweep between frame rails instead of fore and aft?

57tailgater posted a version of that in post 9. I don't think that example could be a squarebody steering box though as GM light truck boxes from IRS years are all front steer. This setup might not work with a v8, it takes up some space between the rails



Another option to consider is a center steer rack and pinion mounted to the frame, not the axle. Having the long tierods will reduce bump steer but not eliminate it.

here is a list of center steer racks I also lifted from a mustang forum:
Front Steer
Dodge Intrepid
Chrysler Concord
Eagle Vision


Rear Steer (what you'd need for a stock axle AD)
Saab 9000/Early 9-3
Honda civic
Cavalier


for front steer Dodge intrepid is one I have had my eye on, but the cavalier is very similar from what I have seen. (not sure all years cavalier use center steer)



Stole those pictures from somewhere, so I can't claim them as accurate

[57tailgater]

I found pics of the TF cross steer set up I saw. It looks like they use a square body box in the stock location which may not help. Tie rod looks like a square body 4x4 tie rod using the steering dampner/shock hole for the drag link.

[mr48chev]

I'm thinking that a lot of the bump steer from those front mounted boxes comes from the short drag link.

A rack has to mount on the axle or you will have horrible bump steer. You would also have a lot of tire wear because the toe in would always be changing. Then you have to have a slip shaft in the steering shaft.
A friend of mine had a Unisteer rack unit on his 32 that he really liked but he sold the 32.
There a partial rack that works as a cross steer unit.

i've been trying to figure out a simple cross steer unit for years and think that a Ford Mustang (67/70) or a Fairlane, Torino or Grenada manual box could fit on the inside of the frame rail and not take up a lot of space between the frame and the engine. The actual box holding the worm gear sits over the frame rail.
That might be a challenge with a dropped axle though.

[leegreen]

Originally Posted by mr48chev

A rack has to mount on the axle or you will have horrible bump steer.

I think if you had a centre steer rack mounted on the frame, dropped near the level of original cross link the bump steer would be minimal. You could also offset it by mounting the centre of the rack below the level of the outer tie rod end .

you are right that toe would change with suspension travel, so this would not work for a truck with a lot of normal travel - like a flexible 4x4, but for a lowered straight axle AD driven on street with maybe 2-3 inches of travel it might be fine.

if you want to keep straight axle and have power steering yet another option is column mounted electric PS. from a prius perhaps. superfastmat on youtube put one in his telsa powerd jag saloon. Maybe this is viable if your current steering gear is all in good shape to keep using

the most well documented way to get PS under and AD is probably a MII crossmember kit

[mr48chev]

Originally Posted by leegreen

I think if you had a centre steer rack mounted on the frame, dropped near the level of original cross link the bump steer would be minimal. You could also offset it by mounting the centre of the rack below the level of the outer tie rod end .

you are right that toe would change with suspension travel, so this would not work for a truck with a lot of normal travel - like a flexible 4x4, but for a lowered straight axle AD driven on street with maybe 2-3 inches of travel it might be fine.
t

Nope, It has already been proven times over that a solid mounted rack with a straight axle doesn't work right. Even the least deflection is going to change toe in. A lot of guys screwed their cars up back 30/40 years ago trying it.

[1project2many]

nvrdone, have you considered using an assist piston with stock steering gear? These systems were installed on thousands of vehicles and they can give years of reliable service. The control valve replaces one end of the drag link and the assist piston attaches between the tie rod end and the frame like a steering dampener shock. This allows you to keep the stock geometry and the original steering column and box.

How hard would it be it would be to modify an old Mustang or Corvette system to fit an AD truck? It seems like these systems occasionally show up used on Ebay for a reasonable price.

[nvrdone]

Hey everyone: Thanks for all he input but I feel like my head is about to EXPLODE!!!
Furst let me clarify: I don't have room for a cross steer due to oil pan interference and I don't want to go with an IFS or a rack mounted to the axle.
So here's what I think I understand.
With the steering box in the stock location and the rear end of the front springs mounted solid (stock mounting) when the front axle moves up and down, that will cause the axle to move in an arc. That movement will be taken up by the front shackles.
If I mount a steering box forward of the axle with a drag link the same length as stock or longer, the arc the axle swings in should not change. Therefore, the bump steer should not change from stock.
I would appreciate any info on what I may be missing.
Thanks

[leegreen]

Axle does not move in a perfect arc around the fixed shackle, as the leaf spring flattens out the axle moves further from the fixed shackle and the steering box.

Your drag link -probably- came from the factory with a downward angle from horizontal so as the axle end of the arm moved up it would move towards the axle and away from the steering box - cancelling out some of the bump steer.

So if you move the box to the front, moving shackle end of spring you need the drag link to angle up the reverse amount it angled down before.

a side view picture of the axle, spring and steering at ride height with wheel off will help this discussion

[nvrdone]

Leegreen: thanks for that info. It clarifys alot. I had not thought about the spring flattening out and the axle moving forward under compression.
1Project: I like the idea of the piston on the stock tie rod.
My whole idea on doing this project is that my original steering box is really worn with about 2.5 in. of free play. Its adjusted as tight as I can get it .Also the box is spaced out away from the frame about 1.5" which clears the exhaust manifold, but I have to crawl under the truck to remove the #5 spark plug. I just wanted to clean things up.
Looks like my only solution would be to flip the fixed end to the front and go from there.
Looks like this may be way too much work, so I may get serious about an IFS.
Thanks everyone for the input and education.

[57tailgater]

Originally Posted by nvrdone

Leegreen: thanks for that info. It clarifys alot. I had not thought about the spring flattening out and the axle moving forward under compression.
1Project: I like the idea of the piston on the stock tie rod.
My whole idea on doing this project is that my original steering box is really worn with about 2.5 in. of free play. Its adjusted as tight as I can get it .Also the box is spaced out away from the frame about 1.5" which clears the exhaust manifold, but I have to crawl under the truck to remove the #5 spark plug. I just wanted to clean things up.
Looks like my only solution would be to flip the fixed end to the front and go from there.
Looks like this may be way too much work, so I may get serious about an IFS.
Thanks everyone for the input and education.

If you're intending to flip the fixed end, note the change in height differences that may occur and how caster would be affected. My one theory on newer trucks why they changed the shackled end to the rear is to help dissipate road shock inputs into the frame. With the fixed end on the rear these inputs are going directly into the frame bracket where if the shackled end is in the rear those inputs can be dissipated some by that end being allowed to move. I looked at doing this on a Willys 4x4 pickup I once had and the height differential was too great to change. Food for thought.

[dsraven]

there are bolt in options if the welding part is whats holding you back from doing an IFS. my personal opinion would be swap it for a good quality ifs that has what you want for brakes and suspension parts. not saying anything is wong with a straight axle, just saying that they are made to work at the stock ride height so if that gets changed it affects other stuff and also if you upgrade to a v style engine there is not much room for steering box and the box gets overheated from being so close to the exhaust. there are lots of IFS installs on here if that interestes you.