I'm looking for an inexpensive way to feed ride height data into a
commercial data logger in order to test different aerodynamic pieces
for my race car.  Ideally, I'd be able to detect changes in ride
height in half-millimeter (or less) increments, which the data logging
software could 'clean up' and compare.  I've looked into laser ride
height sensors and both linear and rotary potentiometers for it, but
don't really have enough knowledge to put a cheap and effective system
together without your guidance.  Durability over time is less
important than accuracy for a few days of testing.  To offer more
data, the car has 600#/in springs in the back and 300#/in springs in
the front.  It generates about 250 pounds of positive lift at 100 mph,
and I'd like to be able to see differences in negative lift in at
least increments of 10 pounds.  If strain gauges would be a better way
to go, I'm all ears about that, too.

Thanks!

There are so many ways you could do this that it's not even funny. The
build-or-buy tradeoff will probably be the decision-maker for you. If
you have $1k to spend on the sensor (exclusive of data recording
means), I would be thinking some more about that laser sensor (the
so-called triangulation sensors from Sick and others). A potentiometer
as one leg of a Wheatstone bridge could certainly be made to work, but
will require some effort to get it adequately linked to the
suspension, and calibrated so that you can relate the measurements
back to the ride-height answers you're looking for. Same goes for an
LVDT. Even with the engineering to integrate it into the suspension,
the potentiometer might be the low-cost solution. An LVDT would be a
good substitute for the potentiometer if more durability in the face
of environmental contaminants (dust, water) is needed.

Strain-gage sensing of the deformation of the springs is an
interesting idea, but it seems like overkill in light of easier
solutions, above. There might be gotchas, too, in the presence of
strain in the spring that doesn't arise strictly from spring
compression and extension, i.e. strain that comes from deformation of
the suspension parts themselves and that ends up twisting (or in other
ways deforming) the spring independent of changes in ride height.

I'm not sure of the sensitivity, but any car with air suspension has
ride height sensors which feed the suspension computer.  Lincoln
Continentals are the vehicles I've seen the most of this on, although
other manufacturers also use air suspension.

They normally have 2 sensors (one for each front wheel)in front and a
single sensor for the rear. They connect between the suspension and
the body via a rod. These are just variable resistors that change
value as the suspension goes up and down.

Of course you would have to come up with a/d conversion and software for these.

another idea: the vehicle control computers on the later models MAY be
able to read the ride height values out via a scan tool.  I'm not sure
about this: yopu could check out the manual on one of the late models.
 If so, you could install ride sensors with their computer and just
read out the values with a scan tool. you could probably get
everything from a junkyard cheaply.  i'd be interested to know how it
turns out!

econofix

I circle track race and know how expensive it can be. A wise man once
said the way to make a small fortune in racing is to start with a
large fortune! Anyway i have also used laser gauges and lvdts @ work
and they are big$$$$. These items from mcmaster-carr may do the trick.
The circuit would be a simple 3 wire poteniameter for either device
and your data would be easy to scale to the stroke length of the input
device. My only concern with the string pot would be if it would
retract (spring loaded) as fast as your suspension travels.
data logger-6395k56
linear string pot-6440k2
or
linear transducer-7383k16

good luck