Driveline vibrations can be tricky to diagnose, because you can’t get under the car and see what is going on while it is driving down the road at 60 MPH. If you have read my first article on the three types of driveline vibration, you have a pretty good way of narrowing down the list of possibilities based on if the vibration is engine speed related, vehicle speed related, or engine load related. Here are some other considerations to keep in mind when diagnosing a classic car vibration.
The first thing to consider is the overall current situation. If this is a new build that has never been run before, then everything is suspect. You may have total faith in your engine builder, but could one of his tools be out of calibration? Is the engine supposed to be externally balanced, and you ordered the correct flywheel but actually received one for an internally balanced engine? Are your driveshaft angles out of spec? Could you possibly have a defective pinion or axle bearing, a bent axle, a warped wheel, or a tire out of balance? Is your exhaust touching the body or another component at any point? Are all the suspension bolts, body to frame bolts, and steering linkage nuts/bolts tight?
If this is a modified car, there are so many more things to think about. If you have 1968 Camaro (my favorite year!) with a Total Cost Involved front end, a late model LS3 engine with home-made engine mount towers, a Keisler transmission, a Currie 9″ rear end, and a Detroit Speed rear spring and shock package, there are a number of different opportunities for incompatibility, not to mention the fact that the car is almost 40 years old and the production tolerances back then were considerably “looser” than they are now. Aftermarket companies usually design their parts to fit a completely stock car, and their parts will usually fit an otherwise unmodified car very well. When you start to combine aftermarket parts from different manufacturers, you sometimes run into problems. The aftermarket front end may be designed to improve cornering ability which changes some geometry that the aftermarket transmission system also changes due to tunnel interference with the transmission, and then the rear end company also takes a few liberties with their design to make the rear end fit several different cars, and the rear suspension company modifies the geometry for maximum traction when drag racing. Each part by itself works perfectly with an otherwise stock car, but when all are combined you can end up with a car where the transmission touches the car body and the driveshaft angles are unacceptable. This of course isn’t always the case, but if you run into a vibration problem on a heavily modified car you need to take this possibility into account.
If it is a recent restoration that was vibration-free before, then first look at anything that was changed, and the relationship all those components have with other systems on the car. If you just swapped a Ford 9″ rear end into your Chevy, the rear end itself may be fine, but you could have altered the driveshaft angles. An angle that was barely acceptable before might be slightly beyond the limit now, creating a vibration.
Sometimes you have to “think outside the box”. I once corrected a “vibration” in a two year old, 25,000 mile Buick Lesabre that had a vibration complaint since day one. It had been to three different dealerships, where they had rebalanced and replaced the tires and wheels several times, had multiple wheel alignments, and even had the shocks and struts replaced. I noticed on a test drive that the vibration was only in the steering wheel. I could not feel it in the seats, the armrests or the dash, or see it in the mirrors. I took it back to the shop and ran over some speed bumps at about 10 MPH to see what happened, and the steering wheel moved about three inches up and down! I was a service writer at the time, and all of the technicians refused to believe that the steering column could be loose from the factory, so none of them would work on it. I dug into the dash and found that two of the four bolts that secure the steering column to the dash were about three turns shy of being even finger tight! It had come that way from the factory, and none of the previous technicians had taken the time to pay attention to what was actually happening with the car. I tightened the loose bolts, and I had a customer for life! Sometimes you have to open your mind a little, and throw all assumptions out the window.
The bottom line is to take your time and consider all the possibilities. Assess the overall situation, drive the car, and think about what is happening. Don’t make any assumptions, and realize that aftermarket parts from different reputable companies aren’t always designed to work with each other. Have an open mind, and with patience any vibration problem can be diagnosed and corrected.