Here's a puzzler for you

[MrAl]

I... reminds me of a myth from lond ago. When cars actually had real hub caps before whatever it is they have now, that placing aluminum foil in the hub caps would prevent getting a ticket from that then new RADAR police used to measure your speed.
...
Ron

there was a theory that it would induce a doppler shift and cause the radar gun to receive false signals. Silly. I think if there were tuned reflectors that were presented on the bottom half of the wheel (the part with lower relative velocity) but not the top the strong, slower echo would overide the echo from the vehicle body. A lot of ifs.

Hi philba,

How do you mount something to the bottom of the wheel and keep
it there?

[Bigglez]

How do you mount something to the bottom of the wheel and keep it there?

Double-sided tape...
Perhaps you should ask how do you keep something
attached to a rotating shaft, and keep it at or near
the bottom of the hub? Take a look at a city bus or
long haul tractor trailer with a hubometer.
The hub mounted odometer uses an off center
weight to remain stationary as the hub rotates.

[Bigglez]

I... reminds me of a myth from lond ago. When cars actually had real hub caps before whatever it is they have now, that placing aluminum foil in the hub caps would prevent getting a ticket from that then new RADAR police used to measure your speed. :smile:
...
Ron

there was a theory that it would induce a doppler shift and cause the radar gun to receive false signals. Silly. I think if there were tuned reflectors that were presented on the bottom half of the wheel (the part with lower relative velocity) but not the top the strong, slower echo would overide the echo from the vehicle body. A lot of ifs.

There are several RADAR jamming techniques, the crude
ones distort or absorb the radar signal so that no useable
return is detected. The B2 stealth bomber uses radar
absorbant paint and angular flat surfaces to reduce the
radar cross-section.

It was reported that workers at the paint factory used
some on a car and deliberately raced through police radar
traps to prove it worked.

Police speed radar uses the doppler shift to read velocity.
A jammer could send a "low speed" signal regardless
of the actual speed. Such devices were sold as kits.

The early Police radar was not very accurate and required
daily calibration with a tuning fork. Challenging the
radar operator to produce daily calibration records in court
was a popular way to get out of a ticket.

An urban legend started that a trucker with a jammer
forgot to turn it off when exiting the highway, and was
busted by a Police radar trap for doing "45 MPH" while
stopped in the rest area.

The first Police radar band is shared with X-band door
openers. Radar detectors have a "city/highway" setting
to reduce sensitivity when parking near the mall, etc.

Operating an X-band door opener on the highway certainly
identifies who has a radar detector in their vehicle.

I had a summer job, in a military contractor that made
military radars, as a student. The facility had a microwave
anechoic chamber.

A staff member was trapped by Police radar and protested
the ticket by bringing his car in on a Saturday and scanning
it in the chamber.

The test data was pressented in court and the ticket
quickly dismissed. He claimed that the grill and bumper on
his car (back in the day when these were chrome plated
metal) gave a false reading on the radar.

It was thought at the time that the ticket was dropped to
save face for the Police, who would not welcome publicity
that "metal grills" stopped speed radar. Another urban
legend was born.

[reloadron]

Sort of looks like mentioning and old urban legend I managed to derail a thread from ignition wires to radar as used by police to monitor speed.

Sorry about that.

Ron

[haklesup]

My first impulse (pun) is similar to MrAl's first reply. Even if the tiny increse in inductance due to a crudely formed air coil inductor did impart a benificial effect, I would have to think that any infinitesimal fuel savings would be far offset by the premature replacemant of the OEM plug wires with more expensive and longer wires and further diffused through the guessing process of installing said wires.

Well, inductance is impedance and adding more shouldn't improve a spark in any meaningful way (change perhaps, but improve...). plug wires alreay have tuned impedance (R, L, C) so why would anyone think that the car manufacturers wouldn't select the optimal values. Perhaps to favor universal replacement parts but with electronic ignition, the computer could no doubt do a better job.

In any case, with an inductor in series with a spark gap one would expect that some of the energy from the surge in current when the arc ignites would be stored temporarily in the magnetic field before callapsing and returning that energy presumably in the form of an extended current pulse manifested as extended arc time. However if R and C were sufficient the spark may just as well extinguish prematurely and you would be left with an otherwise useless voltage spike at the end of the surge with no associated plasma.

To do this as an experiment in any rational way would require at least a good oscilloscope with a current transformer and maybe a fast light detector to measure actual arc timing reletive to the measured waveforms. You would need to try various cable length, turns and coil radius combinations to even verify if you could control any useful parameters and you would need to correlate that with at least a basic understanding on how that ignites the fuel ratio in a compressed cylinder. All that is well beyond just guessing. and the cost benefit ratio has got to be way less than one if not negative.

If it can be made useful it would have to be on an older car with carburator and points. A modern electronic ignition can adjust the pulse at the source in real time thus compensating for changing conditions of the plug tip and wire as well as a myriad of other engine parameters depending on what car you look at.

The good news is that someone should be able to model and simulate this, at least the pulse and inductance relationship. A spice model for a spark gap would be required and the time step would need to be quite small to get a useful result. A simplified model might be just a resistor that toggles between high impedance and low part way through the simulation but that would have limitations.