We've had a problem with front end shake in the shutdown area for the last 3-4 years. Car is a 64 Dodge Polara, 493 wedge, 727, 4.10 Dana, runs 9.90's-10.10's at 134. Car has monoleafs, Calverts, tubular upper control arms from Bondo Bob, quick release centerlink bushings from Roth. It has been set up this way for the last 10+ years. Since this problem started, I have changed the lower control arm bushings to polyurethane, new idler arm, new Calvert 90/10 shocks, increased caster from 2 to 4 degrees, reinforced steering box mount (added a strut), pitman arm is tight, center link has no play, toe is +1/16" to 1/8", front tires rebalanced (does it with other tires too), front brakes sent to Aerospace to be rebuilt, rotors checked for runout. It has some front end shake in the shutdown area from about 100 mph to about 60. Seems to be going into a resonance where if something is done like wacking the throttle or hitting the brakes harder will stop it. Competitors have noticed the front wheel bouncing in the shutdown. Drives fine on the track, problem is just in the shutdown. The only thing we haven't changed are the torsion bars (6 cyl; adjusters are cranked in pretty far) and tried a stiffer shock. Any thoughts? This is driving me crazy.

Check the front tires off the ground for run out, out of round, to see if that is hurting you
I like to see less than 1/4 inch (.250) run out, no more than that
I had some high speed shake issues on my old pump gas Duster, I had replace all the front end wear parts when putting it together, the "new" idler arm wore out in less 1000 miles I could move it up and down by over 1.0 inch on the drag link end , I replaced it again and fix it, no more issues after that

I would check the toe-in like the old mopar manual says, ride height, then lifted an inch and then pull the car down an inch. If the toe is going too far positive on deacceleration when the front end sinks, it can cause a shimmy. I race a 64 doge NSS car as well, and mine did it years ago. I found that setting the front end ride height to factory stock specs, and adding a little toe-in fixed my problem. Mine started doing the shake when I tried to lower the front of the car. I also installed a set of these type strut bars, really helped make the car drive nice. When the suspension goes through it's movement, any deflection of the strut rod bushings, really affects the caster, on the car. and these will let you dial in a little more positive caster if needed. And as mentioned, make sure all the suspension is tight, no looseness anywhere.


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Thanks. We usually set the toe with the front end raised an inch, and I did forget to mention that I did swap out the strut bars for the aluminum QA1's with spherical rod ends (didn't help). We'll check the toe change when we get it ready for the season.

If the font end is real low. It may be contacting the lower control arm snubbers on decell. Or it has drastic toe change thru the range of travel. Mine has followed seems in the pavement when it had toe out. But it's never shook, even at 148-150. How are the shocks? Ever see a car run down the freeway with a wheel hopping up and down? Wonder why the driver doesn't feel something? Lack of shock control can produce some weird harmonics.
Doug

How much does the toe change when the front end is not raised, or is compressed some?

quite sure you have a drastic toe change, so its going into bump steer. You need to follow the mopar chassis book on sweeping your toe change and correct it. jake

Shocks are new, haven't investigated the toe change on the car. What is odd is it didn't do it the first 7 years of racing. So, either the toe is changing, the shocks are too soft or the torsion bars are worn out. Will be looking at it later this month.

Are the tires bouncing up/down r shaking side/side ?
How much caster are you running ?
Some high-caster setups will wiggle the tires side/side, fairly common on Sprint cars.
Genrerally on decelleration, and in a window of speed, say maybe 60-45 MPH.

Sounds like you have some bumpsteer going on there.

Went from 2 to 4 degrees. Observers in the other lane noted the tires are bouncing up and down, but can't confirm this. Steering wheel shakes when this happens.

Had this same problem in my RoadRunner many years ago. Finally traced it to play in the (original) steering box. Not the usual play that you can correct with the adjustment screw on top. The
needle bearings on the sector shaft had worn/rusted a small amount allowing the shaft to move - very hard to diagnose. This started without out any warning during a 3 day weekend at beaver springs.

Watching with interest. Having the same problem . Doesn't happen every time but enough to be concerning . definitely got worse as I lowered the front end trying to achieve more travel . Was hoping it goes away now that I brought he rear down and will get it aligned .

If the steering wheel just vibrates or shakes in a up or down fashion it is in the balance or run out. If it shakes side to side its in geometry or worn parts.

Thanks for the replies. This gives us something to work on. Will check the tires, but it does it with two different front tires. Will update after the season starts in late March/April.

We pulled the car out Friday to get it ready for our first NSS race at Cecil on Saturday, 27 March. Made some measurements at various heights of the front end and noted a signifiicant toe change. We had lowered the front end after installing taller front tires when running a 0.500 tree, compared to the shorter tires when running a 0.400 tree with the 422 AllStars. With the toe set at 1" above static height at +1/8", toe-in was significant at 2" below static to simulate hard braking (almost +1/2"). So, we raised the front end 1-1/2" and the toe sweep was less dramatic. Just a tiny bit of shake at about 60 mph, but signifiicantly gone. Will make some more measurements and pour through the chassis book for solutions, may bring it to Jack Arnew or Ken Keir to see if changes to the geometry need to be made.

Thanks for the earlier comments. You steered us in the right direction.

BTW, we went 6 rounds and won on Saturday.

Well done . Good results all round .

Tex

We flipped the Tie rod ends (inner) to improve bump steer, using a couple of the bushes we got from a guy on here.

Success on both fronts!