The same happened to me while driving to SoCal....it started to shake and I slowed down, I started going faster and it shook so hard that I thought either it was going to flip over or the whole rear-end was going to fall off.
It turned out that one of the tires was starting to separate.....fuuny now...not so funny in the middle of the desert.
P.S. make sure your spare is filled with air....some people (like me) have to learn the hard way.
I had the same problem on a truck at work, and it ended up needing new shocks. One shock went bad or broke, and it would cause the exact problem you are describing. It could also be a tire as suggested.
Do you have a steering stabilizer? If so check it. This is the leading cause of violent front end shaking (almost acts like a basketball bouncing). If you do have a stabilizer, remove one end and it should act as smooth as a shock in both directions.
Hmmm, Violent shaking is definetly not much fun. Fortunatly for you, it's pretty easy to diagnose and some of the repairs can be made by a weekend warrior.
First, you need to check your tires for inflation, cracking, seperation and most importantly balance. If you have a single tire up front loose a balancing weight, it thows off the whole geometry of the steering. At a certain speed you come to harmonic flux, which is the speed at which an unbalanced tire resonates (frequency) and creates friction and viola', shaking. Since most people never really maintain their tires, it'll probably save you money from now on to have your tires rotated twice a year and when the tire shop rotates them, have them balanced. This if free for people that buy tires from reputable dealers.
Now, if your tires are all in good shape, properly inflated and balanced and there are no signs of seperation, I would then look at the drag link, and it's components (inner/outer tie rod and ends) if anything looks bent, it's time to have a mechanic continue the work. If everything looks straight, give the tie rod ends a good tug in all directions, if they wiggle, they're bad and need to be replaced. While you can do it yourself, you will need an alignment when finished.
While I'm on the subject of shaking, it's important for you to understand that shaking is an alignment issue. There is a part of the vehicle or tire causing the wheels to spin out of alignment and that causes the different frequency that causes shaking.
Anyway, if your tie rod ends are good, your drag link is good, your tires are good, your original alignment is good, then it's time to check for wheelbearing freeplay. Lift the wheel at the axle, shake it, if it wiggles but the ball joints don't move, replace the wheelbearings, if the ball joints are wiggling, then replace them.
In end, heres a list of things that could cause this, and it's a pretty short list, I'd suggest learing tricks to test each and take them in a logical order.
The good thing is that if you were to have an afternoon and a friend that's mechanically inclined, these are all things you can do yourself, even the alignment but you need a really smart friend for that one. The prices for the wheelbearings are cheap as ever, inners on both sides are $10 ea, outers are $12 ea.
Drag link, tie rod ends etc are cheap too, about $80 to replace the whole deal.
Upper/lower ball joints are pretty cheap too and you can replace them all for about $100 yourself but you'll need to have alot of faith in your mechanical ability for these, they suck on Bronco's and are a BIATCH for a nonpro especially the first time.
Let me elaborate on this. I have balanced all my wheels. The truck has a 4" lift with 35x12.50-15's on it. I am sure that the tie rods are good (I specialize in Alignments at a local Goodyear). My next thought was stabalizer shock on the front end and now it seems to feel like a wheel bearing. I was thinking that the cause could be a 7.5' Meyer Snowplow that I mount on sometimes when it is going to snow. The problem started out small and not often and now, it happens at about 35 mph and is getting more toward 30mph. I bought all new wheel bearings for it and am going to install them this weekend and the next thing will be the stabalizer shock on the steering. If it happens after that then I have problems. After all this I am going to replace the shocks anyway and install airbags for my plow in the coil springs.. I know the tires are good and will rotate them to test if thats the problems.
Please keep the ideas rolling because I am out after all this I described.
Well, now that you say it's shaking at 30mph, I'd say check those wheelbearings. They fail often on Bronco's and especially on Bronco's in the snow with plows. If you were to jack a tire from the axle, and shake it vigoriously, you'd know if it was those bearings. It'll feel the same as a bad ball joint, but the ball joints won't be moving, you might need a friend to shake it while you start at those balljoints. If there is any play or wobble and it isn't the balljoints, replace the wheel bearings immediatly.
One of the main reasons I know so much about this subject, is I had a passenger side outer bearing fail going 80Mph on the highway. Not only did I ruin my hub, rotors, calipers, spindles, axles etc. But the tire shook so bad that it blew, and took out the entire fender and peeled back the door skin about 4".
It's the scariest thing that's ever happened to me on the road and It's something that has the potential of causing death, not only to you but your passenger and anyone else on the road next to you when the bearing fails.
Replacing the bearings is pretty easy, just remove the hub and it's just like on a 2wd vehicle. Removing the hub assys. is a pain the first time but if you take your time, you'll be fine. Anyway, let me know, I've very curious about this.
BTW, unless it's completly frozen, it's not the steering stab. I've heard of them getting very loose and causing a loud rattle but never heard or seen one cause serious vibration in the past 20+ years.
Do you have radius arms on your year vehicle? Check the bushings on the frame of the radius arms. They will wear out and cause the front end too move side to side causing the front end to shake. Hope this helps. Rob
Okay, so when my steering stabilizer went and refused to compress smoothly may be I should have put it on display Now that everyone has told you to check, replace, grease everything in the front end you seemn to have your work cut out for you.
Humor me....check the stabilizer (I have seen many go over the years) and now add this to your list
Check your universal joints...all of them to include the rear drivshaft. They should all be free (they do freeze up if you run in salt road conditions and have not been well lubricated) and lubed properly. A rear u-joint that is frozen will raise hell with the vehicle and you will feel it in the steering wheel.
Well, I know that it isn't a u-joint because it is definatly in to front end and the front ujoints wouldn't be rotating when the hubs are free. Tommorrow I start bright and early on the job and I will let you all know.
Well everyone, I spent my Sunday replacing wheel bearings and shocks. I did all this just to find out that that wasn't my problem. It ended up being that my lift mount that connects the axle and frame cracked then broke. Well I took it off and had someone weld it. I then put it back in and gave it a good alignment and that fixed it right up..
Congrats on finally finding the problem. Now consider why it might have happened? I too run a plow on my Bronco and it does tend to rattle the hell out of everything not to mention the pounding it takes. I am constantly tightening bolts and mounts related to the plow. This only happens in the wintertime when I have the plow in use.
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