| } "if you think about it" the average of the high speed and low speed should
| } come out to a just about right speed, certainly not 25% high anyway.
|
| Ah- this might be true, if the speedo needle is as likely to go down as up.
| But, lets throw in some damping on the return stroke. This will allow the
| needle to quickly rise, but fall slowly. (this allows quick response,
| without jerky action. If you have smooth, constant rotation of the cable,
| you can calibrate the system- the rise is proprotional to the speed
| of the cable, and fall times are slow, as real speed changes. If your
| cable gets sticky, you begin to get a series of fast pulses, which can
| make the needle go up rapidly, but not down, due to damping. This would result
| in a high reading. If the pulsing is excessive, or the damping fails, the
| needle will jump more.
| Seeing as how speedos aren't direct-drive mechanisms, there is damping in
| the system, so this failure mode is quite possible. The *rate* of damping
| in the speedo is designed for slow, gradual transients. When we get
| large, rapid transitions (Pulsed cable rotation), we exceed the systems
| capibilities.
|
A good explanation. Do you have personal experiance or info to support it
or is it conjecture?
actual practice?
My experiance tends to run the other direction: Symmetrical damping would be
easier to design. Further, most british and other car decelerate faster then
they
accelerate and so if the damping needs to be asymmetrical you would expect
it would be in the other direction.
In the last 35 years I've owned lots of cars and driven lots of others
I've seen lots of bouncy speedos and can't recall any that read way high on
a regular basis. Some read low. Ive worked on a few speedos and I don't recall
any thing that would explain a very high reading better than wrong gearing.
:^)
PS has anyone established that the speedo in question bounces?
best to all/dickn
|