Solving Vibration Problems
It’s true, some classic sports cars did rattle and shake from new – but it’s wrong to think all old cars are like that. Despite the millions invested in NVH (noise, vibration, harshness) research and development, many modern cars have a more bone-jarring ride than most classics.
The obsession with ‘good handling’ – which these days seems to mean an ability to go round smooth-surfaced corners as if on rails – has brought with it rubber-band low profile tires with negligible compliance and anti-roll bars so stiff the suspension can’t do its job. Great fun on the racetrack, for sure, but with goverment slashing road repair budgets to cope with budget shortfalls, the shortcomings in normal road use become ever more apparent.
In the days before motoring journalists with a mania for performance (yes, we do accept some blame) had such a strong influence on car manufacture, the average car’s ride was softer and more compliant than it is today. Look at the suspension travel of an old Caprice – or older Pony cars like the Mustang and Camaro. Long-travel suspension soaked up the bumps, while compliant rubber bushings and high profile tires at relatively low pressures dealt with ridges and road surface imperfections.
The linked problems of vibration and harshness deserve deeper analysis if we’re to get the best out of our classic cars. We drive hundreds of classics every year: as many as 75 percent are below their original manufacturing standard in NVH – and owners think there’s nothing wrong. Everyone assumes that ‘old cars are noisy and uncomfortable, it’s part of the experience of owning one, but the styling, the performance, the looks they get, make up for it’. Rubbish: you can have your cake and eat it if you get the car working as it did when new. NVH is grouped together because the three are often inextricably linked: harshness leads to vibration, vibration leads to noise. So, harshness is a good place to start. It’s caused by a lack of the correct compliance between body/chassis and mechanical components. It’s most likely to result from the deterioration of rubber bushings that are there to absorb mechanical vibration.
Rubber bushings can wear, especially if the components they sit between are subject to excessive movement, but generally they deteriorate due to long-term exposure to oil, which softens and weakens the rubber. Cracking on the visible edges of bushings doesn’t mean they need to be replaced, it’s just surface perishing from daylight – the working parts will not be affected. Jack up the car to take the load off the wheels and use a big crowbar to load up and work the suspension, looking for any sideways movement in bushed joints.
Check engine, gearbox and rear axle mountings for bulging and sagging due to oil contamination. Even slight deterioration will have an effect and if it reaches the point where there’s metal to metal contact (which may not be at the bushings: with engines and gearboxes it can be several feet away), NVH will be significant.
Spend some time working through these areas and your classic will be transformed, becoming much more enjoyable and safer to drive: time and money well spent.
Rubber or polyurethane?
Polyurethane or nylon bushings were developed for racing, but in most case are too inflexible for road use. I fitted nylon rear spring eye bushings to my TR2 because everyone said I should; after I’d broken both main spring leaves, I realised the shock loading between axle and chassis was too much for the springs. Rubber bushings went back in. Polyurethane isn’t unsuitable in itself, but it must not be less flexible than the original rubber. Getting rubber bushings made to the original specification can be difficult too: you may have to hunt around for the ideal replacements.
There are always vibrations that were not there when the car was new and the most obvious come from the wheels. Balancing is the usual solution but don’t assume that because you had the wheels balanced when the tires were new, they will stay in balance. Wire wheels are a particular problem. But wheel vibration doesn’t just come from unbalanced wheels. Over-sized or lower profile tires transfer more shocks through the suspension to the body shell. Brake drums can be out of balance, brake discs become distorted and wheel bearings get worn, all causing vibration. Loose nuts do the same: ignore vibration at your peril – it may be a wheel is about to seize or fall off.
Recent research in the USA revealed that 86 percent of cars arriving in scrapyards still have their factory-fitted shock absorbers, despite the fact that virtually all telescopic shocks are well below their original efficiency by 60,000 miles and stiffer ones can fail by 30,000 miles – so it’s no surprise that many classics would be transformed by new shocks. Ideally you should go for the nearest to original equipment specification and quality. However, if you simply can’t get good-quality replacements then amass a collection of used items, pick the most effective ones out and check them regularly.
If there is vibration that is shaking the whole car rather than coming through the steering it may be from the rear wheels, but it is more likely to be the result of an out-of-balance prop or driveshaft. Driveshaft joints wear, center mountings can disintegrate or soften and driveshaft fitted with new joints may still need dynamic balancing to eradicate vibration. It’s well worth taking a troublesome driveshaft to a specialist for professional attention. Worn driveshaft bearings, splines and CV joints all bring significant vibration – and new components are rarely hard to find.
Published Dec 7th, 2015