The following case study is a typical one and tells the story with example vibration traces of a 1966 Ford Mustang that had engine, prop-shaft, wheel and brake drum unbalance. The proud owner just knew it had too much “Shake, Rattle & Roll!” Firstly, we have to get a quick lead into measuring and interpreting vibration as not everyone is use to this “stuff”. You know the car is vibrating because you can feel it through your seat and steering wheel. You can probably tell it’s getting worse as you speed up or rev the engine. But where is it coming from and what’s causing it? Well that is the job that Vibration Free staff have been doing for over 25 years and use some fairly sophisticated equipment to help them, but let’s break it down into easy bite sized stages. First off a quick test drive, this just lets the VF technician understand and relate to the description given by the owner of the vehicle of what is happening and the level of noise and vibration present on the car. Then the vehicle is put on a special four post ramp that can support it on its suspension but still allow the wheels to turn. The engine is run and the vehicle is put into gear to drive the wheels at a speed that reproduces the vibration felt on the test drive. The vibration is measured and recorded at the engine, gearbox, axle and wheels. Often print outs are taken to show the frequencies present and the direction of the greatest movements. It is from this data that helps diagnose the faults present and from that information the customer can make an informed decision what he would like to have done next. This stage will have taken around two hours, depending what is required to be done and could be any or all of the following;-
- In-situ engine balancing (single or two plane) 2-4 hours
- In-situ prop-shaft balancing 2-4 hours
- On-car wheel balancing 1-2 hours
There are other common faults that get a little more involved such as ;- Gearbox alignment, engine crankshaft bearing changes, two and three piece prop-shaft alignments, brake disc and drum balancing (off the car). So now we can consider the case of the 1966 Ford Mustang Coupe. It was a lovingly restored car and of show standard, but spoilt by the driving experience. The steering wheel shuddered from wheel unbalance but the noise and shake of the whole car at any reasonable driving speed was just “nasty”. The vibrations just got worse the higher the engine revved or the faster the road speed got. It didn’t take long for the guys at Vibration Free to work out what was wrong (about 2 hours) but it took a little longer to put it all right and hand back a smooth and a lot quieter Mustang, which was then a delight to drive and enjoy. Here is how it was done;
Trace 1, with the engine running at 2050 rpm and in 5th gear, the gearbox vibration shows a collective (overall) reading of 12.6. This is made up from two main sources, the prop-shaft (4.9) and the engine (5.36). Straight away it looks like the engine and the prop-shaft are out of balance tolerance, but further readings confirm exactly what is going on.
Trace 2, with the engine running at 2500 rpm and in 4th gear (1:1), the vibration on the differential shows the difference between the prop-shaft level (3.0) and the combined wheel and drum unbalance (1.5).
Trace 3, the rear wheels and brake drums are removed while the engine is run at 3100 rpm and the prop-shaft is balanced in-situ with a temporary weight (jubilee clip). Everything behalves as it should and the reading drops to 2.0 even though it is at the higher speedof 3100 rpm.
Trace 4, On to the engine, which is giving out an overall level of 11.4 at 2200 rpm, of which 5.9 seems to be due to the unbalance. Some further check speeds show how alarmingly this grows with revs!
Trace 5, taken as above but at 3000 rpm gives an overall of 12.6 and unbalance peak of 6.8.
Trace 6, taken as above but at 4000 rpm gives an overall of 20.1 and unbalance peak of 11.7.
Trace 7, taken as above but after balancing at 2000 rpm gives an overall of 6.9 and an unbalance peak of 0.89.
Trace 8, Taken as above at 3000 rpm but after balancing giving an overall of 10 and an unbalance peak of 2.0.
Trace 9, Taken as above at 4000 rpm but after balancing giving an overall of 13.8 and an unbalance peak of 3.5. Further to the engine balancing, the prop-shaft was removed and made slightly longer so that it had better support from the gearbox tail-shaft bearing. Of course it was suitably balanced and gave no vibration once refitted to the car. The rear brake drums were balanced on a balancing machine, then all four wheels were balanced on the car to give the best wheel balance available. If you’re of the “need to know” type, the vibration levels quoted were in velocity mm/sec RMS. Should you have any questions, please don’t hesitate to call or email Vibration Free.