ARNTZ COBRA---- BUMP STEER PROBLEM

[cobie]

HI, I OWN A ARNTZ COBRA . AT CERTAIN TIMES I GET VIBRATION IN  STEERING WHEEL i HAVE AN MG STEERING SETUP.  BUMP STEER ? ANYONE OUT THERE WILLING TO SHED AND SHARTE SOME ADVICE ON THIS NAGGING PROBLEM ? COBIE

[Moderator]

Welcome to the forum, Cobie!  I'm very pleased to note that when you activated your UserID earlier tonight, you became our 1500th member. (722 members have posted on the board so far, and the other 778 are lurkers.)

I think it might help us if you describe the vibration in a little more detail.

Bump steer is pretty common in cars that have been lowered from the ride height their suspension was originally designed for. A little bit of bump steer is almost inevitable, but some cars have it much worse than others. I don't really think of bump steer as a vibration. Rather, the symptom is simply that a wheel steers left or right (i.e. toe-in changes inward or outward) when subjected to an upward or downward movement (e.g. a bump or a pothole.) Obviously, this condition makes the car want to lurch left or right. The cause is simply that the steering rack and the steering arms (on the uprights) aren't in proper alignment.  I'm not familiar with how your Cobra is set up, but if bump steer is the correct diagnosis you may be able to correct the problem by shimming the steering rack mounts. Bending the steering arms is a common solution, but not an very good one unless you have the capability and knowledge to heat treat them after you bend them. Alternatively, you might be able to fit spacers between the tie-rod ends and the steering arms. It would help to have a jig for measuring bump steer... a picture would be worth a thousand words here, but I don't have the right picture handy.

If your vibration continues when you're traveling on a flat surface, it isn't bump steer.

If your vibration comes and goes based on what speed you're traveling, it's much more likely to be a tire problem (e.g. a tire that's out balance.)

If your vibration is all the time, park the car immediately and don't drive it until you figure out what's loose.

[mgb260]

Curtis is right on, generally you want your lower A arms as close to parallel to the road as possible and the tie rods parallel with them(same angle).

[ex-tyke]

Just went through this same issue on my conversion...vibration was worse between 60 -70 mph. A friend of mine had access to an old tire spinup machine which identified that the vibration was coming from the right hand side.
I knew from a past experience that a bad brake rotor (Asian aftermarket) contributed to an earlier balance episode, so I removed the hub and rotor and had my local machine shop check the balance. The rotor was off by 26 grams (almost an ounce), so I replaced it with a better used rotor (still, 13 grams unbalanced.
The other item I had rechecked was the tire/wheel balance and between the two items, the car is much improved.

[bplus]

If you have heim joints at the tie rod ends, as some cobras do, you can also shim
under the the ends that connect to the hubs.

[roverman]

Cobie,Sorry for late response, but we're kinda fixated on MGB front supensions now. If you can tell if your steering wheel tends to vibrate up/down or quivers left/right ? Left/right, is usually insufficient toe-in. After driving the car in a straight line for few miles, check tire temp differences on the tread, inside to outside,(finger tips are accurite enough for this test). Any difference ? If noticeably warmer on inside,"viola", there's your problem,(toe-out). Also, I supect your Cobra has more wheel base than MGB ? If so, standard "Akerman" will be overbored and will tend to add excessive toe-out,(dynamically), to inside tire. Now would be a good time to check "scrub radius", another problem maker.  Good Luck, roverman.

[Bill Young]

I'll add one more thing, bump steer doesn't usually display as any sort of vibration, instead it shows up as the car wanting to change direction when the front wheels are deflected on a bump. What you describe does sound like a tire or brake disc balance problem as those are usually more speed sensitive. I had a vibration in my steering for years that I thought was related to the toe setting although that checked out. I installed new tires and rebalanced those and the problem cleared up.

[BlownMGB-V8]

We were donated an Arntz/Butler front suspension for the MG-Roadmaster project. It used very nice greasable inner bushings on the lower control arms and  shorter, stiffer springs and that was about the extent of the modifications as far as I could tell, aside from the hole through the crossmember and the galvanized coating. And the upgraded brakes. So all MG specific info should apply. How many miles are on it? The MG king pin bushings do wear and have to be renewed from time to time. This is a job that requires a hydraulic press, a couple of specially made pressing pins, and a stepped reamer. I've done this job and it's not particularly difficult if you have the tools, but if not, it's reasonably economic to just buy the reconditioned assemblies exchange. The bushings can be honed to size but they wear out pretty quickly afterwards, BTDT.

From what I can tell the Arntz/Butler sits at a lowered height, an inch or so below stock. Meaning the lower control arms sit angled upwards from center a bit but the steering links are still in proper alignment. So using the upper part of the suspension travel, a bit of bump steer might creep in but it shouldn't be enough to cause much of a problem unless you are just real sensitive to it. Curtis may be able to give a more refined perspective on that. But it won't cause vibration unless something else is loose somewhere such as worn bushings.

JB