The Correct Settings

I had a visit from a Bristol 403 today (not forgetting its owner) for the fitting of new rear shock absorbers – or to use the more technically correct term: dampers.  One of the things we discussed was the intital setting of the new units, the answer to which is – at least in theory – simple:

The theoretical and precisely accurate amount of damping required is critical damping as shown in the graph:  If you press a car down on it’s springs it will describe one of the shapes shown in the graph above.

As it happens few of the settings to do with driver experience ever seem to be best left at the theoretically correct, and this is largely because we have expectations of them which are very much human and  we use terms such as “handling” , “feel” and “character” to describe them.  Consequently there are plenty of sports car drivers who set their cars up to be very much overdamped and at the other extreme plenty of American luxury car drivers who drive around in an underdamped condition (not to mention that of their cars).  The choice seems to come down partly to what suits the type of car and its intended use, and partly to the preferences of the driver.  It often takes me at least a week of driving before I’m absolutely sure that the settings are the most appropriate, but that may say more about my personality than it does about damper settings…

There are plenty of instances where automotive engineering becomes tailored to human experience rather than to theoretical perfection and another such example is steering geometry: Most people who design steering modifications – particularly rack and pinion conversions – take great care to calculate a geometry which will avoid any bump steer, and I am no exception to this rule. Bump steer is a condition caused by alteration of wheel track as the front suspension deflects, and as it happens many modern cars are deliberately not devoid of the phenomenon.  The idea is that if a car is designed to give a moderately increased toe in at extremes of driving then it will be easier to handle by less experienced drivers:  It serves as a sort of early warning system when the car is getting “near its limit”.  In this case most enthusiasts prefer the theoretically perfect setting of zero bump steer and have no desire for the handling to be compromised in order  to reduce the neccesity for driver skill.  Interestingly both Jowett and Riley used to provide a height adjustment for the steering tie rod ends so that one could set the car up with precisely zero bump steer.

Personal preference is a huge determining factor: Many enthusiasts over the years have told me that vintage cars “handle better” than later designs although clearly modern cars go round corners an awful lot quicker.  I suspect the truth of this is that to these individuals the cars just feel nicer – and I can quite understand why. A good vintage car is incredibly predictable on the road, it gives plenty of warning when it is about to slide and when it does do so it is very easy indeed to control.

Which conveniently brings me back to the issue of damper settings:  When, years ago, I used to compete in an Alvis 12/50 I treated the car to a long overdue overhaul of it’s Hartford shock absorbers in time for my entry in the winter driving tests at Enstone airfield.  The car was one which I believed I knew back to front but the newly re-conditioned and carefully set up shock absorbers had altered things quite significantly so that the panache which I intended to display in a controlled rear end drift was quite destroyed by the resulting 4 wheel slide into a bunch of cones. Come to think of it I should be waiting for people to forget about incidents like this instead of blogging about them….

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