Technical Front Suspension problem

Hi folks, I have been building a 31 roadster on a 32 frame, and I have a problem. During test drives, under deceleration or light braking, I get a shimmy which quickly grows into an awful shake, both front wheels, it's like some sort of harmonic or something. Scares the bejeezus out of you. It has nothing to do with the brakes. I am using a front solid dropped axle, with a split model A wishbone, F1 box. I took the whole front suspension out of it the other day and found that my spring shackles were too long, but I didn't find any other problems. Caster is about 6. Toe has been set from even to 3/16" in, seems like it's worse when toe is more in. Have any of you all come across this? When it happens, you basically have to pull over and stop to kill the harmonic. And you don't have to be going too fast, maybe 30. I am at my wit's end!

 

Ackerman?
Someone brighter than me will chime in.

 

^^^^^ Fastest way to see if ones steering arms are for front steer or rear steer without going into full blown science.

 

New tires? If yes see if a friend can lend you a set from their car and see what happens.

 

You said shackles,too long ? A photo would help a lot. But only since you talked about shackles; If your shackles are not at about 45* with spring loaded{car just at rest but with driver lbs. in seat;} So if not,then you must add a panhard bar from axle to frame,as a way to hold axle on center.{there are other kind of fixs,like VW steering shock an fix spring to right size so shackle is at 45*, Other things can make deathwabble also***,but #1 is that front axle can move side to side,that starts harmonic.

 

@dana barlow I was going to suggest that but stopped because if I understand OP's correctly this is not a cross steer set up. So my question is this. Side steer can suffer from side shifting as well?

 

Scrub radius wrong? How do you know it's not the brakes? Does it keep shaking after you let off the brake pedal? Does it diminish when you take your foot off the brakes? Does it get worse if you give it more brake?

 

65 mustang box on my A coupe daily, four bar with panhard front and rear...drove good. no speedo, but got a ticket on the interstae, 94 mph...

 

Side steer can suffer from side shifting as well?

Yup

 

OK let's work off this since my question is a fixed one. OP states "like some sort of harmonic". It's not a road condition or sharp turns causing pendulum affect to begin happening. Why it "grows" rather than "go away" as soon as shift happened as in most cases is what needs to be found. A panhard would only mask it would it not?

 

A panhard would only mask it would it not?

Correct, first thing I do is try to find out what is loose. I will secure both wheels from turning and attach
a dial indicator to the frame and against the steering arm. Turning steering wheel will give me slop in the system. Then I figure out what parts are worn or loose and proceed to repair. Now this is if camber, castor, toe, ackerman is correct. Spring shackle alignment plays a big part of it also.
'Death Wobble' has been beat to death on the Hamb, but there are many good suggestions on the posts about the subject.

And just to add steering box needs to be adjusted properly

 

I too am of the notion that something is loose. If the front end is tight, then the harmonics can never get a chance to begin. IF the tires are good, toe is within specs, ect... unless you have a new four wheel drive diesel ford pickup, then all bets are off. LOL.

 

I had the same thing on a long ago build. I installed a steering damper, and set the toe at 2 degrees out!
No more problems, and no tire scuffing. I was running a side steer Mustang box. I made many out of state trips to the Nats in that car.
Can’t hurt to try.
Rich

 

Both times it happed to me it was a bad center link...

 

Caster and toe can compliment each other or fight each other. I'll let someone that would like to do a whole write up explain. But yes, toe out does work when required.

 

johnny:
I did not make that answer lightly. I had all new parts, the king pins had been honed for fit,
the steering box was a almost new 65 Mustang box. i went over every thing many times to
no avail. I had a pro toe adjusting tool from work. I would hit a bump at 30 mph ,and it
would rip the steering wheel out of hands. I found the toe out made all the difference in the
world. I than installed a steering damper which stopped bump steer.
this may not be the answer for him, but it's a inexpensive avenue to try.
Rich

 

I agree Rich, however from what OP posted I believe he has a few issues that he needs to address before he goes to a damper. I also found that toe out works better than in.

 

I had a similar situation but only at around 30 MPH after I tried a lower tire pressure to soften the ride. Aired em back up and went away.

 

Do the wheels have a lot of offset?

 

Thanks. Will take a hard look at that. I think the spring angle is ok but side to side wobble might be it. I have disassembled the whole thing and didn't find anything loose or out of sorts. A panhard bar would be fairly easy to do, especially since everything is already off.

 

Had not tried toe out. I will! Thanks. Also will post some pics when it's all back together.

 

Wheel offset will magnify any issue in a front end no matter what the style of front end.
Nothing was mentioned about shocks and how they are installed. You do have real shocks and not those cute home made things or undersize sporty car lever things. Had those on my T and they didn't work worth a damn.
I've got to agree with AngleDrive in that a dial indicator to check movement that you can't see helps a lot in these situations. I used to love the old even then John Bean Visualiner that I had had in the Firestone store I worked in in Waco in the 70's because you could often see the movement projected on the screen that your eye or hand didn't pick up.
Unless you are having issues with the thing tracking wrong or dragging a corner Ackerman is just something that clueless people throw out to sound knowledgeable and has little else to do in the real world. Big word that sounds important. If you changed the wheelbase by a great amount like using a MII front end up on a 130 inch wheelbase rig then it might be a big deal.
One thing that might be an issue is the diameter and strength of either the drag link or the tie rod if they have been changed to something that didn't come with the axle. My T had some issues and I was sure that It was partially due to the long skinny drag link that may have been flexing. When the car acted up I was too busy getting it under control to look though. It also had crome reverse wheels on it even though they weren't very deep.

 

How do you set toe by degrees ?? Toe is a lineal measurement...

 

Be sure to measure wheelbase on both sides of the car.It needs to be the same.

 

The T in my avatar had, and still has, a similar wobble problem but I did get it to lessen. My car acts up in a slightly different way though. I seems mine has a "window" of speed in which the wobble occurs, 15-25 MPH or there-about. On acceleration or deceleration I try to get through the speed range quickly by quicker acceleration or firm braking. At typical highway speed (60+ MPH) I don't think I've ever noticed the behavior.

I tried various toe-in settings and found it less pronounced with less toe-in. I increased caster, which didn't help, maybe even made it more pronounced, and finally reduced it again which made the wobble less severe. Since the car is very light (~1400-1450# wet) I was running low tire pressure, maybe 18 PSI or so, and bumped it up to the mid 20's and again the wobble became less pronounced. It has "shorty" tube shocks, but they are mounted at a fairly steep angle which would "soften" the dampening they offer. My Corvair steering box has more play than I wish it did but replacing it would pretty much be a last resort since installing a different one would require quite a bit of re-working of the whole steering column with the way it's set up. Tie rod flex is a possibility, although I think that is unlikely. The tie rod and drag link have spherical rod ends that are all tight. In my case I might try a tiny bit of toe-out as has been suggested since that's an easier thing to try than switching steering boxes. I just need to rationalize in my mind the effect of toe-out and how it could help eliminate the problem.

I realize I probably haven't helped the OP (junk_yard_jack) any at all other than sharing my experience in trying to solve a very similar problem, although I think I've lessened the problem on my car with various changes. I'll certainly be watching this thread with interest.

Lynn

 

I have fixed many straight axles with this problem by changing the caster.Thats after you make sure everything is tight and right!
I had three identical brand new trucks( straight axles are straight axles) , one would do the wiggle thing, checked everything, everything was exactly the same on all three, but only one would wiggle. My old time front end shop with the old timer mechanics told me I need to what I thought was increase the caster, when they gave me 2degree shims for the axle.I installed them, problem when away! When I told them, they said I should have DECREASED the angle. But changing the caster a little off specs solved the problem on this new truck. I have solved many straight axle wobbles that front ends shops couldn’t fix, by changing the caster a little. Sometimes everything can be “ right” and still have a problem. Just my experiences.








Bones

 

I'm not making light of it. I'll spell out the basics to what I meant. Toe is measured typically half way up the tire's height right?. Now let's use 1/8" either in or out. This measurement is being done at a true horizontal plain of earth. Now if one went back and remeasured toe after being set but now at 90 degrees of caster angle, well guess what. It's not 1/8" any more. So are you truly toe'd out? Then there is this. Tire diameter does not mean 1/8" of toe on a 26" tire is the same as 29" tire. Look I'm not disbelieving you. I'm just saying this stuff is more complex than saying everyone needs this caster and that toe. Your proof of this.

 

There's a number of threads on this topic but why not visit this again. It would really help if you could post some good pictures of your set up. Lots of sharp guys on here to spot any engineering or geometry problems.
Troubleshooting this begins with very carefully inspecting the whole front end for anything loose, worn or flexing (bending). Have someone turn the steering wheel while you watch, then watch as you try to turn the front wheels. A little slop here and a little flex there can get to be a big wobble. I have found, like Boneyard51 said, if everything's tight, (1) excess caster, (2) excess toe (in or out) can cause the symptoms you have.

 

About spring shackles, read this entire thread: https://www.jalopyjournal.com/forum/threads/tranverse-springs-tech-info.1112840/

I can believe that. More caster should give you less wobble at the expense of heavier steering. Many will insist that it's the other way around because of shopping trolleys. It never made sense to me so I tried it recently in a supermarket. The trolley I had had a slight wobble in one wheel, so I tilted the trolley so the two wheels on the other side were slightly off the ground. As soon as the weight of the trolley was on the wheel which had been wobbling, the wobble went away and stayed away regardless of how fast I pushed. Ergo, wobbling shopping trolley wheels are not caused by too much caster but most probably by slight twists in trolley frames lifting one wheel slightly off the ground. In any event a shopping trolley is a bad illustration of this.

@junk_yard_jack , I'm convinced that a lot of instances of "death wobble" are due to harmonic bowing oscillations in the tie rod. Try this: tape something heavy, like a piece of steel bar stock, to the middle of the tie rod, then see if the wobble either goes away or happens at a different speed. If that is the case, a larger-diameter i.e. less slender tie rod should solve it.

 

no.