MII Directional Stability

I bought a 54 Chevy with an M II front end under it. At first, it was really scary. After some examination, I determined that the rack was lower than the tie rods and I was getting some ugly bump steer. So, I replaced the springs and presently the tie rods are parallel to the ground. I was afraid to cut another half round off the springs until they settled. I also put new napa M II shocks in the front and gas shocks in the rear.

I took it to the alignment shop Friday and it was aligned to the followings specs. 2.8 degrees of caster, zero camber and .2 degrees of toe in per side. I don't know how .2 degrees of toe correlates to 1/8" toe in but it should be close.

The car definitely drives better but the straight ahead stability is still not comfortable. When driving on a secondary road, it tends to drive itself to one side or the other. Also, when you start to steer off center, it changes direction pretty quickly. The steering effort feels fine to me, not over boosted.

Where did I go wrong? How can I get better Straight line stability?

 

Would you please snap a picture, or two, of the setup, and post it?

 

If power steering I usually set up about 4-5° of Caster, and .5° negative camber, 1/8 toe in

 

If powersteering ,ford rack and a gm pump,sometimes you have to reduce the pump pressure

 

It is a power steering setup. Sort for forgetting to include that. I was also told it was a Fat Man set up with 2" dropped spindles. The spindles are in the center of the yoke, if that tells you anything.

 

If it is an actual MII power steering rack, for sure. If it is a later, becoming more common, late-model T-Bird rack, now shipping with many kits, the flow rate/pressure should be okay with the Saginaw pump. Need some pictures.

Also, I once found that a prior installer had re-tapered the spindles so that the tie rod ends could be mounted the other way. I have no idea why, other than to possibly make use of the wrong tie rod ends. After correcting this, and re-aligning, it was just fine. Crazy sh!t happens.

 

Ugly bumpsteer stated in the opening post suggests a problem with placement of the rack and suspension components.

A picture of the front end would resolve the questioning.

"If" the front end is set up correctly, there should be very minimal if any bumpsteer with the Mustang II suspension throughout the entire suspension travel range.

 

ELpolacko, the tie rods are dead level with the ground now. Before, the rack was lower than the steering arms.

 

That's fine, but what is the relation of the rack to the lower control arms.

The inner rack pivot should be about two inches above the lower control arm pivot. If you squeeze the rack boot and feel for the knuckle of the inner tie rod joint, it should be slightly outside that same pivot.

 

If you draw a plumb line down from the lower control arm pivot, it should intersect the tie rod knuckle at the rack. Both sides. Does it?

Are the rack mounts nice and tight?

Any slop in the steering ***embly (with wheels off the ground, garb a tie and give it a push/pull to see if anything looks "loose")?

 

There is no slop in anything. Here are some pics.

The better lit ones are with the car up in the air.

 

One of the things that kit builder do to make these front ends fit without much effort is spread the upper and lower control arms vertically. This makes the mounting points of the control arms taller than the spindle. This produces positive camber gain under compression and manifests as low directional stability.

Have your suspension alignment guy change your camber to .2 degrees negative on both sides. See if that gives you a better on center feel.

 

.2 degrees is .0977" over 28".

 

Would a little more Caster help ?

 

^this.


I have MM2 on my '49 chevy truck and its a dream.

You need more caster to get better straight line handling and the camber can be a little pos or a little neg, but I never set it at "0".

 

swaybar..

 

You need more caster in the MII setup. 6 degrees is what I have run in the past.

 

FYI . I've been working to better tune my MII for highway driving . I'm running 4 degrees of caster with the later Mustang ( NOT MII ) power rack. I've installed the shim pack to reduce pump pressure and it does seem to have helped some. But I'm convinced the problem with the overly quick response is not a pressure issue , but rather, a flow issue. I've been going to my local junk yard and have about a half dozen GM pumps from trucks and cars. I've taken the pressure and flow valves from each and there is absolutely no difference . They each have one shim on the pressure valve, and in the flow valve, the flow orifice and return orifice are the same size.
I plan to modify one of the flow valves to reduce the flow orifice size by about 50 % and increase the byp*** ( to tank ) orifice by about the same. This should reduce flow to the rack while at the same time allowing the excess flow to dump without dumping over the relief valve.
I'll try to remember to post the results.

 

More caster will give you better straight line driving stability. I ran into this when setting up my stock cars years ago. I'd start with 4.5 degrees and go as high as 6 as recommended above. It'll also steer a bit easier.

I don't thing the toe is your problem, I'm not used to seeing it described in degrees though. With bias ply tires, go with a total of 1/8" in, radials you can go with a 1/16" in to zero toe.

Bob

 

I'm far (really far!) from an expert, but shouldn't the tie rods be parallel to the lower arms.
To me it looks like the rack needs to be moved higher up.

 

To a point, anything beyond 4º doesn't help but can contribute to caster shake.

Maybe, if he was concerned about body roll. No mention of that though.

You will find few IFS designs that go beyond 4º positive caster. The way most Mustang II front suspensions are designed, going past 3-4 degrees will lose caster under braking and can cause an increase in brake dive.

Nope, relation from tie rod to lower arm looks decent.

 

A sway bar will not fix what I am experiencing. Thanks El. I will try a little neg. camber. Is .2 degrees toe in about right?

 

Should be.

Depending on how quality your tires are, toe may not be all that important. 1/8" is usually enough. Decreasing the toe in or going toe out can help some instability issues but it's hell on the tires.

 

the other thing is over-boosted combo from GM pump ad MII rack can lead to what feels like instability. They become really sensitive and the least little input from he tire hitting a bump, groove, or irregularity in the road causes input at the wheel and it is very easy to overcompensate without realizing your'e doing it, and you think it's bump steer or alignment issues.
I have "cured" the problem for a couple of guys by fixing the mis match pump to rack problem

 

I have found the best set up for Mustang II steering in my opinion is a full sized stock steering wheel that most cars came with, and a manual rack. Like I said, that's just my opinion. Most that I have driven with power steering have felt a little "flighty" for my taste, and it's suprisingly easy to turn a non power car with a 17 or 18" wheel even with fairly large tires on the front.

 

??? It will steer easier with more caster ???? I believe the more caster the harder the steering .

 

No necessarily.

 

reduce the pressure of p/s pump down to the correct pressure of your rack and it will be a little more civilized

 

I have just had this over sensitive steering issue with a 37 Chev we built with a JW Rod Garage set up. I was using the type 2 GM small pump but it was putting out around 1100 psi. Car was like a roller skate and scary to drive. Got a reducer orfice from JW that dropped it to 700 and all is well. Car drives like a dream with around 2.5 in caster FWIW.

 

have you checked the level of the m2 crossmember front to rear ? I cant tell from the pics but it does seem to look like its not kicked back at the top.