>
> | } "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
>
My personal experience is this: On my 66 TR4A the speedo started to read
80 MPH at 60, a few minutes later the cable broke. Replacing the cable
returned the speedo to normal.
--
Darrell Walker walker@hprpcd.hp.com
(916) 785-4059 HPDesk: walker (hprpcd) /HP5200/UX
|