Parallel front leafs....help needed....BADLY

ok, my 31 chevy has about 800 miles on it, and the front had been giving my problems recently.

it's running stock non rebuilt 32 chevy p*** car front springs, mas drop axle with 54 chevy p*** car brakes. there are new king pins (just checked no slop) mustang box setup as traditional steering, not cross steering. nothing is loose or worn on those parts it's all new. the caster is 2 1/2-3 degrees, toe is set at 1/4".

what is happening is the front is shaking left to right(steering wheel too) and the wheel's bouncing side to side. this is only at low speeds 15-25 (although since it's gotten worse i haven't gone any faster) but i also noticed that it happens when hitting small bumps or cracks in the road.

I am running bias plys....which i did check and they did have a flat spot in it, swapped a different set on...same problem, so it's not the tires, wheel bearings are new and tight, no slop.

what am i missing?

 

1. Did you properly adjust the steering box before you installed it? That is by Factory specifications and procedures?
2. New toe rod ends all around?
3. You might try bumping your caster up a bit to 5-7 degrees positive, that is the top of the axle tilts to the rear 5-7 degrees from vertical, and changing yout toe-in to 1/8".
4. This low-speed shimmy is symptomatic of bad kingpin bushings and worn front end components at the low speed and slightest bump sets it off.
"Low-speed shimmy is usually caused by under-inflation or looseness of wheels, wheel bearings, worn spindle bearings or bushings, tie rod, drag link, steering gear or spring mountings usually in conjunction with too much caster or toe-in.
Low-speed shimmy is often caused by the same conditions that cause high-speed tramp.
Either too much caster or too little caster may cause low-speed shimmy, whereas only too little caster causes high-speed tramp."
from 1936 Ford wheel alignment and service manual

 

letts see some pics of this chevy

 

35 years ago, I bought an old roadster rod with a mopar tube axle & twin springs and the side drag link. First it was fine but I then put deep dish mags on the front instead of stock narrow ones. If I hit a bump with both front wheels it went into convulsions at 25-40 mph. It was dangerous to drive.

It had the old style drag link with adjuster ends to make the joints stiffer, and tightening them made it worse.

I look back now and I'd have to guess the drag link angle was way off, rather than parallel with the frame. Just my guess.

 

1. didn't mess with the box, 68 mustang box, any ideas what to do to adjust it?
2.yes new everything in the front...but a used steering box, but it's tight.

3. that was my first idea....

4. kingpins and bushings are new and tight...well greased, wheel bearings are new, greased, and to the proper tightness, tie rods ends and drag links are new with no slop in.

 

That seems to me to be a LOT of toe-in!
May be time to consult the old manuals, but perhaps experiment with "finding" the tread centerlines, marking w/sharp chalklines and then measuring carefully to set a lesser value?

 

Pics would be good for sure...

How far away from the rear spring eye is the draglink tierod end compared to where it would be with the factory box?
Must be higher than stock due to the configuration of the Mustang box correct?

Shackles in front or out back of the front spring?
Rear shackle...forward running draglink...flexible spring with a good arch...
The axle moving back on the shackle due to a soft spring could cause some onwanted steering input...not sure it would cause a wobble to start though.

Is the Mustang box centered in turns of the steering wheel when the car is aimed straight ahead?
That could be a little difficult to check because you need to check it with the draglink pulled clear to be sure. (NOT that you need to DRIVE it like that!!! )
Boxes are machined extra tight at the center point to give better tracking and to allow for a wear adjustment point.
If the car is going straight with the box in the wrong position you will have a loose condition that could cause the wobble.

How stiff and resistant to deflection is the box mounting area?

F&J mentioned offset rims causing his problem. That would be due to the increased scrub radius of the tire to the kingpin line at the ground.
Hows yours?

Just some thoughts....

 

Pitman...yah that does seem like a lot of toe, but reading "How to build chevy hotrods....Book 1" which i've been using as a reference, it acutally states...."something like 3/8" toe-in will work........(and goes on to say)...sometimes, no amount of toe-in works seems to remove the shimmy fore a front end, expecially one using a solid front axle. Try as much as 1/2" toe-out." yah....un fricking beleivable!!! i have not tried toe-out, but i'll be messing with the toe in the next day or so....


"How far away from the rear spring eye is the draglink tierod end compared to where it would be with the factory box?
Must be higher than stock due to the configuration of the Mustang box correct?"
yah, it's higher, but i have no idea as to the stock config

"Shackles in front or out back of the front spring?"

out back

the box is centered, sure on that...if it wasn't it would have the tires bouncing off the headlights

"How stiff and resistant to deflection is the box mounting area?"

that i would say is overkill on the truck....yes it's in very good...

"F&J mentioned offset rims causing his problem. That would be due to the increased scrub radius of the tire to the kingpin line at the ground.
Hows yours?"

I'd have to measure, but they are stock 49 chevy 15", so very little offset


dialup ****s....these are the best pics i can get as of now...hope it will help, show/explain....
fyi, i did pull the p***enger side front apart, bearings are good and the kingpin has no slop in it...drivers side tomorrow, and searching for 2 degree shims for the springs to get the caster to 6 degrees..

 

The drag link looks parallel & correct. The angle of the pitman arm should be 90 degrees to the ground when the wheels are straight; but that wouldn't cause that shimmy.

One thing in the 2nd pic; the centerline drawn through the kingpin down to the ground, should end up under the center of the tire...in theory.. but, I have never found that to be true when looking at hundreds of early ford rods. Theory says that if the line IS in the center of the tire, then hitting a hole won't cause the wheel to try to turn out.

An ugly fix would be to try a steering damper shock on the tierod?? A VW bug will do the exact same convulsions if the original factory damper is bad.

 

Just got my new So Cal catalog in the mail yesterday..pg. 19 ...dampner, black #001-60770 $ 34.95.
this jewel has cured many hot rods !
dave ;-)

 

We're all pulling for ya Jeff.

 

Start witht he easy ****. What is your tire pressure? Over about 25 pounds on a light car and it is too much.

 

You said it in the first post, Then no one has covered it since...

you need to start by setting the king pin angle FIRST !!!

2~3 Degrees is not enough......... You need to kick it back to 6~8.

If yer king pins are standing straight up, its kinda like when you were a kid & would turn the handle bars on yer bike around backwards, And it was real hard to steer it..... imagine riding yer bike at 50 with the bars turned around

START WITH THE CORRECT KING PIN ANGLE !

 

It's a Chevy, not a Ford.
It doesn't necessarily need the higher caster angle Fords ran.
look somewhere else, like loose and/or worn wheel bearings.

 

I think you should bring it down here this weekend. I think it a trip to dixie fried would have cure things.

 

Hmnn I was gonna suggest the same thing in terms of increasing the inclination first.

What makes a Chevy require less inclination than a Ford???

 

i dont give a **** about inclination, most of you idiots run way too much pressure.

 

WOW....tires are higher than that, but that will be the first thing to do after work. that makes total sense, hard tires+bump=bounce?

and thanks to greenmtboy he lead me to superlift pt #003 3 degree shim, that will be the next step.... that will set it at 6.5 degrees.

Tman, it's a light vehical, but will it make a difference with a heavy 401 buick up front? just a random thought...

 

You can buy various angled shims from NAPA. I get'em there or at a BIG truck spring and alignment shop. Come in handy for setting pinion angle.

Lowering tire pressures will usually improve straight line directional stability, especially with radials.

Make sure your heim joints don't go into a bind. Is the back side of the bolt chamfered a small amount? You may want to use a small spacer between the heim and the steering arm to make sure it's bind free. Heim joints transmit every front end motion back through the steering wheel. There's no compliance in them, which is good for precise steering, but bad for damping motion.

Reduce the toe-in to 1/8" max., caster at 6-7 degrees as previously mentioned.

 

Mt T has a 348 up front with a cast intake. I run about 22#, if I hit 25+ it bounces like a superball. About 90% of the customer cars that came into the shop all had issues until we dropped the pressure.

 

IS THAT RIGHT ?? :: MOTORS MANUAL FROM 1963 SAYS...

SOLID AXLE GM REQUIRES 6 1/6~8 1/6 Degrees

It Doesent Matter if it a Frod, Chevy, Or a ****in '32 Auburn...

You stand the king pins almost straight up & down, the ****er will hop & bounce all over the place.

I have had several '49~'54 Chevy trucks, I know King pin angle is very important for the car to go down the road smooth & straight.

Mayb its not his problem, But 2 degrees is not even enough.

 

I have 29 chev, whick is the same body style and frame. I run a stock chev i beam ...year unknown but suspect 40's. No low speed issues, a little shimmy at 60+ (tire hop) but i dont have front shocks mounted up there yet either, i hope that the dampening from the shocks will reduce this. What shock set up are you using?


Leon

 

I had a similar problem with my 28 Pontiac, same set-up except a Vega cross steer box, and radial tires.... (and it was black with two doors and it was a '67 that my uncle.... oh sorry, wrong post...)

Check the eyelet bushings on either side of the spring as well as the play in the shackle bolts.... Also the amount of endplay between the shackles and the spring... I found mine by jacking the car up on the frame, which took the weight off the spring... I could then move the axle a little bit- I could feel the thump, which was probably only .020", but when it is amplified by the 5' wide tread it made the car only aimable, not steerable. I then could move the shackle side to side enough to feel it..... I figured it came and went with speed because of the loading of the spring at different speeds...

hope this helps....

Andy

 

ok today's....excitement...
tires down to 25 psi. (thanks Tman) and the side to side bouncing is gone. I'll try to find the shims tomorrow and get the caster to 6+ and redo the toe/play with the toe...thanks so far guys!

 

Well then!
Good to hear man.........just help someone else down the line, too many guys make that mistake. It IS A PET PEEVE of mine

 

Not positive, but might be because they are parallel leaf spring beam axle instead of the transverse spring and wishbone Fords ran.
Most (Chevy) 30's years and models range from +1 3/4º to +3º caster.

(And that's according to MY '35-'53 Motors Repair manual)

Today I exprimented with tire pressure on my '40 GMC to show how right I thought Tman was.
I put 33 lbs in the front tires and it dribbled like a globetrotter with two basketballs down Pacific Coast Highway, dropped it down to 26 lbs and it was fine.

 

Right on Doc! Now go out to the unwashed m***es and spread the word!

 

Yep! Thanks Tman for the other tips offered!

 

I don't know how you are measuring your caster, but in our shop we used 1/2 neg to neutral caster. toe in about 1/8" or even less. you said problem recently, what did you change or is something loose and changing itself?

 

Bringing up an old post, have been searching past posts on this, but what are the specifications for a drag link? My old problem is much better, but every now and then.....it's back.

Should a drag link be horizontal or is there an amount of angle to stay within? Right now mine is sitting at almost 7 degrees, lowest part being at the pitman highest part being the steering arm.