Technical GOD AWFUL FRONT SHAKE COMING OUT OF TURNS

My roadster has since it was built had a negative caster and I could not resolve it. It handled like crap. So I pulled the wishbones and had the angle changed on them, to 10 degrees. I did this because I figured in the end, it would come out to 6 or 7 degrees. Well, I was way off because I also added a new 2 inch drop leaf spring and I am sporting just about 11 degrees. Steering is great now unless I happen to turn a corner and when I do, at around 15 mph or more, as I come out of the corner, the front end begins to shake side to side (heave is more like it, like the front axle is possessed. Everything is tight, I have aligned all to the best of my ability (yea, scary thought) but it still does it. I set the toe in at about 1/8 inch. I look forward to some input before I begin to replace a bunch of parts including the bones and the axle.

 

Not sure what's going on with the drag link, but it appears that it has been modified (possibly lengthened?)

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By "negative" I'm assuming you mean the top of the kingpin slants toward the FRONT of the car? If so, who built the thing and what happened?

You are probably looking for a "positive" caster. See the various diagrams from google.
This should be an easy fix with the parts you have. Looks like you have A model bones or some such. That's what I used on my 32. But they had to be "pie cut" so they would bend up slightly. You want the top of each kingpin tilting towards the BACK of the car. In my case I went for 7 degrees (ya got an angle finder?) but plan to add another 2-3 degrees or so. Once your axle is tilting BACK you can pie cut each bone so it will again plug into it's frame mount. And re weld them.
And have the car on the ground at ride height when you measure the degrees at each king pin.

This is how I did mine......................

You got more pics of the setup?

 

Looks like there is not much down travel on the shocks. Does it bottom out sometimes?

 

It had a negative caster when I got it, took it apart to rebuild and it still did. It is real positive now at about 11 degrees. I guesstimated and had thebones pie cut to 10 degrees. Is 10 or 11 too drastic is what I am wondering.


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It had been.


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I suspect it does but I can't find shorter shocks. I wonder if that could make things go nuts in turns.


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Maybe you could try a panhandle bar. It helped my cross steer setup huge.

 

Panhard bar. Friggn autocorrect.

 

I got shock mounts that mount to the bottom of the spring perch. That will give the shocks more length.

 

hard to tell from those photos but is the ackerman correct?

 

Here are some more pics of all parts of the front end.

 

And I am wondering if that alone could cause things to go crazy if one side bottoms out in a turn. I will have to look around to find some like that because it never did what it is doing until I put the lower spring on it.

 

Every Ford style front end I've done I shoot for 6-7 degrees positive caster, 11 might be too much. Check ackerman also, altho it looks OK in the pictures, shocks could use a little more travel. Bet its the caster combined with the ackerman if its wrong.

 

OK, help me outhere. What is an ackerman?


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Search for Ackerman angle.

https://www.jalopyjournal.com/forum/media/ackerman_angle.540235/



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Need the bottom mounts lower down.
You can buy shock mount studs which can be fixed to axle, between perch bolt and kingpin

 

Either get it back to around 5 to 6 degrees positive caster, or mount a steering dampener.

 

Draw an imaginery line from the centre of the kingpin back thru the centre of the hole for the tierod end in the steering arm.....this line if continued rearwards should intersect with the centre of the rear axle.........if this is correct then when you steer the car the inside tyre in a turn will have more of an angle as it is turning thru a tighter (imaginery)circle and the outer tyre at the same time will be turning thru a less tighter circle...........this is the ackerman principle..........more or less......if the line doesn't go thru the steering arm hole to the centre of the rear axle then you will have steering problems and/or tyre wear...........andyd

 

Difficult to see from the pics but the caster doesn't look excessive, but that's no substitute for proper measurement!

The additional caster has started a fight with the shock mounts - they've got quite a bit of bind / misalignment which can't help the situation, but probably isn't the cause of the shake. However, presumably the same bind will be present in the spring and that can cause issues. Angle shim the spring if there's room in the crossmember or consider self aligning perches? Either way, removal of the shackles first will tell you if there's misalignment between the spring and axle - take care dismantling.

As stated by others the shocks have insufficient bump but the lower mounts look to be strange hybrids and possibly a bit flimsy? The lower style mounts will probably result in the existing shocks being too short! Budget for lower mounts and shocks.

Steering arms look to be aftermarket bolt on's and shouldn't therefore cause ackermann issues - discuss! But the tie rod is fitted underneath and there's now a ton of unnecessary clearance below the bones, and what looks like verging on scrub line problems (might be the pics) - might the tie rod be fitted incorrectly and therefore not properly snug in the arms? Aren't the arms normally taper reamed for the tie rod ends to fit from above? I know things can be modified to fit but looking at the way the shocks have been done i wouldn't rule out anything!

Chris

 

I couldn't for the life of me get the shake out of my 31 Chevy. After checking, tightening, and rebuilding everything (including installing tapered bearings and installing a So-Cal stabilizer) when installing driver side front tire, happened to spin the tire/wheel before pulling the jack-stand. Low and behold, I notice a very slight wobble in the sidewall of the tire............Long story short, The tires were coming apart and had a slight lump in the sidewall and tread. After checking the date code on the tires i found they were about 7 years old (my, oh-my how time flies)!...........New tires installed and the ol' Chevy drives like new...........no wobble, shake, nothing.......just smooth driving, at any speed.

Don't overlook bad tires.........they don't last like they used to.

 

I used these mounts. As Happydaze said you may have to remeasure for shocks.
http://www.speedwaymotors.com/Forged-Lower-Shock-Mounts-Plain,2133.html

 

If you suspect the shocks, you could take them off and try a slow test ride.

 

You have all given me a whole lot to think about and examine. I just learned a bunch about front ends that I did not know! Thanks for the help and I will post a follow up here after I go over all of this stuff and test drive again.

 

From looking at you pic the Ackerman is way off!!!

 

Thanks to you, I know what an ackerman is and bet you are spot on with it. I will check and correct that, add shock mounts to the bottom of the perch pins for better shock travel, and rechecked everything else. Them maybe I will be back on the road safely. Thanks for your help.

 

the one thing I saw in the photos is that the drag link is bent is this done for tire clearance when turning left? With a bowed drag link could it be flexing under a load? I just don't know.
you say the car handled like crap before the caster was changed, so now it handles better? just has the shake comming out of turns, right.

 

Could there be a possibility of a separating tire? I've had that problem and have seen a lot of it at the alignment shop I worked at.

 

I concur!

There's literally dozens of "death wobble" threads here on the H.A.M.B. ... and the majority of these issues have been resolved by correcting the caster and/or installing a dampener.

PS: Give the Stell got my deth wobbel thread a read ... It's still my favorite ... not for its technical advice ... but for its very humorous content!