OT: Digital Speedometer

[nightjar]

Street lights used to work well as an indicator when the only speed limit was 30mph. These days, you need signs to tell you when you are subject to anything other than 30mph or the NSL.

[Dave Plowman (News]

And the low traffic areas. Got caught by one in Fulham. A previous much used alternative route suddenly resident's only - and the signs half way along a road. Where there are no residents. Appealed and got let off, though. As did many others.

[Tim+]

<sarcasm> No, none at all. </sarcasm>

Fecking stupid comment Dave. There are of course some moving components. Literally hundreds fewer though.

Cabs whine. My car doesn?t. What?s your explanation? I assume you know.

Tim

[Dave Plowman (News]

My IC engined car doesn't thrash. What's your explanation for that?

[nightjar]

...

Also a problem for human drivers, which is why, although there is no legal requirement for speed limit terminal signs in the UK to be erected in pairs, the DfT strongly recommend that they are. That also reduces the problem of them being obscured by a vehicle in another lane. Where a single limit sign is erected, it is a requirement that it is subject to an enhanced regime of maintenance. A missing or obscured sign is a defence against prosecution. An inaccurate database is not.

Which is why, where a slip road joins two roads with different speed limits and the limit needs to be signed, the signs are erected at the end of the slip road, rather than at the start. That way anybody reading a sign, whether on the main road or on the slip road joining it, is subject to that limit.

About as likely as the moon being made from green cheese.

[Dave Liquorice]

Any non-motorway road with street lights(*) and no small repeater speed limit signs is 30. "Urban" or "built up area" isn't relevant.

Any variance requires the repeater signs showing what the limit is.

Motorways have hard shoulders and a different set of rules to other roads. The default limit for cars is the NSL, currently 70 on motorways, with or without "street lights". Again variance from NSL needs repeater signs.

(*) "Street lights" not "footway lights", though how you can tell the difference whilst driving or without fairly detailed local knowledge I'm not sure. Around here "street Iights" have numbers on them and are maintained by the County Councils Highways Department. "Footway lights" which can be roadside don't have numbers and are maintained by the District Council, though that responsibilty is being devolved to the Parish Council.

[NY]

The problem is making sure the master copy of the database is up to date and that this updated file gets propagated to all satnav devices that use it.

I've come across numerous examples of the Garmin-supplied database on our Honda having incorrect speed limits - usually too high where a blanket 60 limit on a road has been reduced in specific places to 50 or 40. So I should have updated the car with the latest database. But I did - the latest database from Garmin is several years out of date as regards new roads and modified (reduced) speed limits.

Fortunately I use the speed limits signs on the satnav map as corroboration of what I think is the last speed limit signs I've passed, and I opt for the lower of the two until I get definite info such as a repeater speed limit sign. I don't trust the satnav limit as being more true than my own memory of the last sign I've passed.

[charles]

Our Parish Council doesn't have street lights - we have a "dark skies" policy. As a previous Parish Council Chairman said " If you want street lights, go and live in Surbiton."

[Tim+]

Because you?re simply deaf to the noises it makes. You?re *used* to all the thrashing. Seriously, virtually the only noise in my car is tyre noise.

Once you actually drive a car without an IC engine you?ll realise.

Tim

[NY]

I'm not sure I can think of any road which have lights that only shine on the pavement and not on the road - which I presume is what "footway lights" are. But now I know that the concept exists, I'll have to look out for them!

Is it still the case that you are allowed to drive with sidelights only (no dipped headlights) in a street-lit road, or has that idiotic get-out clause been removed from the legislation now? Must you always have the headlights on at night (as defined by lighting up times etc) when you are moving, irrespective of whether the road is lit?

[Vir Campestris]

Then why is it that a common performance tweak is to fit wider tyres? They too will result in less force per unit area.

Andy

[Vir Campestris]

The simplest ones have an electric motor connected to a shaft to each of the drive wheels.

The only moving bit in the motor is the rotor. I think there's a CV joint on the inboard end of the shaft, but I won't swear to it. There will probably be one on the outboard end.

All the complicated stuff is solid state electronics.

An IC engine has a gearbox, differential, and a bunch of valves pistons conrods camshafts chains belts generators pumps...

Andy

[nothanks]

I agree with a lot that you've said about this (and about the people who seem to have fixed opinions that are based on hearsay rather than actual experience) but not that final bit. In Eco or Hybrid mode, and at reasonable speeds and power demands, my PHEV stops and starts its ICE whenever it feels the need, and frequently the only indication is that the rev counter wakes-up or goes to sleep, or that the haptic feedback on the accelerator pedal changes.

[NY]

My limited experience of electric vehicles, standing beside an electric car as it drives past, or sitting inside an electric bus, is that some of the noise is a fairly pure-frequency speed-related whine, in addition to tyre noise. But the whine is quiet enough that you can actually hear the tyre noise, which is usually drowned out by the noise of an IC engine. I *think* it's natural noise, as opposed to an artificial IC-engine-substitute sound added so blind people know a car is approaching.

I actually find the fainter motor-whine more intrusive than the louder but lower-frequency noise of an IC engine.

Do electric cars alter the number of windings that are energised as a car moves up through the range of speeds? I'm very familiar with the "gear changing" noise of electric trains (eg the Class 465 third rail suburban trains in London) whereby the motor note increases in frequency as the train accelerates from rest, then drops back to a lower frequency and increases again for higher speed, and then again as it accelerates up to full speed. Does the same concept exist for electric cars, or is there a single-frequency note, with no "gear-changing" discontinuities, which is directly proportional to speed? When I say "gear changing", I realise it's not a real change of mechanical ratio between motor and wheels!

[NY]

I presume that CV joints have to be fitted in pairs - where there is an outboard sprung one at the wheel, there will be a matching one on the inboard unsprung end at the motor. Otherwise the wheels will never be able to move up and down as the car goes over bumps. Assuming that the motors aren't built into the wheels, which would give a very high sprung mass and so wheels that are less able to move rapidly to accommodate bumps.

Do electric cars generally have separate motors for the two driven wheels or a single motor and a differential?

[nothanks]

In the M-B the motor/generator drives the gearbox (a lot of magic trickery to make it all work, methinks). I nearly bought a Volvo and they have 2 wheels e-driven and 2 driven by the ICE (somehow they make

4-wheel drive one of the modes - again, a lot of magic trickery).

[NY]

That requires the car to have an internet connection via mobile phone

3G/4G/5G technology, and a lot don't seem to. The only thing our car gets online (and it seems to be getting less and less correct and useful) is up-to-date knowledge of traffic jams. It often warns of traffic jams ahead that don't exist, even slightly, when you get there, while it fails to warn of traffic jams (sometimes complete show-stoppers) that *are* present. Google Maps on a smart phone (read by a passenger) is a better way of checking for traffic jams before we commit ourselves to a road with a central barrier and therefore no turning-round option.

We learned that lesson after getting stuck in a long traffic jam (road closed while an accident was cleared) on the A1 in North Yorkshire, on the long stretch between Ripon and Bedale, where the old road was left in place as a single-carriageway road for non-motorway traffic and a three-lane motorway was built alongside. We took the motorway and got stopped in the traffic jam for ages, while being able to see traffic whizzing past us on the single-carriageway relief road which runs right alongside. If only we'd checked Google Maps a bit sooner... About the time that we finally reached the accident and got past it, the satnav warned us of the delay - about 30 minutes too late :-(

Yes, Garmin should have implemented that on our Honda.

Agreed. At a pinch I can live with having to update the car's local copy of the map database manually, but I want to make sure that when I *do* update it (and it's a 4 GB download every time) it actually contains recent changes rather than the "latest" version being a couple of years out of date. The map on Garmin's site shows roads that still haven't made it into the satnav map database; you would tend to assume that the two are kept in step so you can see whether a specific new road has been included yet and therefore whether it's worth downloading another 4 GB file and installing it.

[NY]

Ah, so that has several gear ratios (or a single continuously variable one) like for an IC car, rather than a single fixed ratio and taking advantage of the torque of an electric motor being available (and fairly flat) right from zero rpm to the maximum speed of the car.

[nothanks]

Yes, it (the M-B) uses the 6-speed auto/manual gearbox. Because of the electric motor the max initial acceleration in sport+ mode is best described as "more than adequate" (80 HP electric + 200 HP ICE).

[Paul]

I sense a crash into a "torque versus horsepower" thread is about to start :-) That's what happens with motor heads and ICE vehicles. Endless arguments about whether HP or Torque drives the car.

The second set of "curves" is interesting, because it implies as the RPM goes up, the available torque for a given power input is dropping.

The plots on this page make a bit more intuitive sense. You expect the electric motor to lose its mind eventually. Even before the rotor throws a lamination, the motor output is limited in some way.

It's still an impressive power source, the electric motor. It probably could benefit from a gear box. Some cars use a reduction gear to get what they want. Some may have tried shaft drive (1:1), but perhaps that doesn't lead to highway patrol "stunt driving charges". I think there's some concern about wear and tear on an actual gear box, if one was to be used.

With electric motors, you can easily have silly ratings on things, but with a smoking battery pack as your reward. The lithium batteries have a limit as to how many amps you can draw. There's a tradeoff for a given cell size, of Ah versus peak_amps. You can make a Lithium Cobalt cell with twice the amps outflow (good for drag racing), but in the process, have half the capacity and half the driving range when not drag racing. Still, the tradeoff selected today seems pretty generous. As long as you only do one burnout per trip :-)

Paul