AQnd they were 5% unless you had a precision movement. mP{recision and cars dont match
I actually designed in the rev counter guts from MG midget style cars into some apparatus. A cheap form, of 270degree meter. 5% at best.
Steppers are good if they are used, yes.
WEll its moot if you are physically limited to 90km/h
So what? Its not a circle. Radius is meaningless.
I'm not sure that's true. The question is one that could be resolved by observation and experiment and really doesn't bear discussion, but a tyre, off the car has its shape changed by inflation pressure, eg the flatness of the treaded surface is decreased as it is forced into more of a doughnut shape by internal pressure. That would say that the circumference is changed. TW
The point is that that cannot happen, without the tyre actually slipping on the rim.
So long as the tyre is NOT slipping on the rim,. what counts is one revolution of the tyre which MUST equal one revolution of the wheel.
The wheel is NOT rolling directly on the ground. It has a tyre in between.
No,it wouldnt.
Any more than a tank track changes its length as it goes over humps.
Think of the tyre tread as a tank track whose speed is determined by how long it takes to get back to its starting point...which has to be the time for one revolution of the wheel. The tyre tread is always moving faster than the wheel rim, by the ratio of the tyre circumference to the rim circumference.
You can see this in action with very low pressure balloon tyres.
It i8snt paricularly clear here, because there is loads of wheelspin, but you cab see how the carcase deforms here: The lines of tension show that the tyre tread has lagged behind the rim and is being pulled to 'catch up'
BMW MINI S, R56 .
On the link below under the section indirect TPMS is a description of how it works and there are many other sites which describe how it works in the using much the same description.
"An indirect TPMS typically relies on wheel speed sensors that the anti-lock brake system uses. These sensors measure the rate of revolution each wheel is making and can be used by on-board computer systems to compare with each other and to other vehicle operation data such as speed"
The bit where it says compare with each other is what I think could be called differential rotation.
I've had a couple of false warnings on long fast journeys where I have wondered if going around lots of roundabouts may have influenced the system . And a lot of wheel spin on say gravel or driving like a teenage prat can also go towards setting it off, but presumably they have designed the system to ignore most of temporary situations like that out. Somewhere on some forum ISTR that for a small change of rotation speed compared to what it was such as a slow puncture may cause needs about 10-15 miles to trigger a warning. But I'm happy to be corrected if I've misunderstood things.
G.Harman
Indeed. My older car has a fairly early electronic speedo. Pulse generator sender, pulse counting electronics which drive a compensated moving coil meter. The accuracy of which depends on how well the pointer is positioned on the spindle. But once set for a true 30 mph is also spot on at 70 - or at least as accurately as you can read a non mirror needle.
My newer car has both needle and digital display. Again, both spot on. Or within 1 mph at 70, since the digital display doesn't have decimals.
There were mechanical speedos in the 50s with similar accuracy. Let's face it you would laugh at a clock with a 10% error.
In article , snipped-for-privacy@yahoo.co.uk writes
When I had Jeep Cherokee putting new tyres onto one axle made sufficient difference to fool the 4x4 system and it locked the centre diff.
How much do I expect?
Well maybe they all use the same speedo manufacturer.
Just the sort of figures given by a journo who can't use a calculator. Perhaps Dave will use his and give the actual speed error at 30 mph of say a 4mm difference on a wheel turning at 6 times a second? Could prove interesting.
An over inflated tyre will wear the centre of the tread first, which does suggest the overall dimensions change slightly. But as regards a speedo reading, a drop in the ocean.
bert submitted this idea :
As already said, the inaccuracy is not in the speedo / the moving coil instrument, it is a deliberately introduced error in the firmware which drives the instrument.
Not relevant - the tyre is no longer a circle as someone else pointed out.
The correct way to envisage the problem is:
a) The tyre has an unvarying circumference (due to steel braiding);
b) One revolution of the wheel means the tyre surface will contact the road along its full circumference (assuming no slip);
c) Therefore the car will move forward one circumference of length (on average, as there might be a bit of wobble between the axle and the tyre/road contact point).
At no point above is radius relevant at all.
Interesting. How can I tell if my speedo is a moving coil or stepper motor display?
Only for one based on GPS observations.
The "car expert" sounds like it is aimed at innumerate morons.
Nominal 220mm radius wheel a change of 4mm is a shade under 2% change in the indicated speed irrespective of the actual speed of the vehicle.
Indicated speed is typically more like 5% high as a default today. You can test a speedo against measured miles (now fairly rare), GPS or fixed radar your speed indicators (not always properly calibrated).
Is the one on Blackpool north cliff promenade still there?
A stepper motor would jerk slightly? I'd also expect it to do a 'find zero' after being powered down.
Measuring the time to travel a measured mile has an inherent error which needs to be taken into account: the reaction time of the person who is operating the stopwatch as it passes the start line and the end line. Also the driver needs to be able to drive at a constant speed without having to slow down as the come up behind a slower vehicle ahead - you need a quiet motorway and a cruise control (or a very steady right foot).
The error in timing over a measured mile could be around 2%: if you are travelling at 60 mph it will take you a minute to cover the distance so an error of 1 second will be 1/60 * 100 %.
I remember the very first time I travelled on an HST 125 train: my grandpa had drawn up a list of times between one 1/4 mile post and the next, and the corresponding speeds. When we were up to full speed I got out the stopwatch, timed from one post to the next, referred to his table and arrived at a speed of 140 mph. This led on to a long lesson from grandpa (he used to be a teacher!) about error and accuracy: that it was better to measure over a longer distance/time to reduce the timing error. I've never forgotten that lesson! What would he have made of GPS and the ability to measure speed directly - always assuming the carriage body/windows didn't block GPS signals!
It was possible to make a speedo accurate (in terms of reading an analogue needle type display) long before GPS was thought of. And electronics make it very much easier today.
Speedos used to be called clocks. You'd laugh at any clock which was as inaccurate as the spec for a speedo - a 10% error allowed? Set it correctly at midday and it's over an hour out at midnight? They could do better than that hundreds of years ago.
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