PDA

View Full Version : Speedo Puzzle

tony draper
3rd Oct 2008, 19:11
Yacking to a old pal yesterday and he tells me he has acquired four new wheels for his old horseless carriage, well the wheels are second hand but in good nick apparently,anyway he say the problem he will have will be that his speedometer which is calibrated for the old larger diameter tires will not longer register a true speed for his vehicle as said new wheels are equipped with low profile tires resulting in said wheels having smaller diameter therefore they will have to rotate more times to cover a given distance and his speedo will register a lower speed than he is really doing.
We discussed this problem for a while and twer my opinion that the opposite would be true, with smaller diameter wheels his speedo would register a speed greater than his vehicle was actually traveling.
Who is right?

Flash2001
3rd Oct 2008, 19:15
You're right! Good heavens Drapes do you have to ask somone?

PaperTiger
3rd Oct 2008, 19:15
As with most subjects, an answer is readily available on this interthingy: Speedometer, Gear Ratio, and Tire Size Calculator (http://www.clarks-garage.com/shop-manual/trans-01.htm)

Lost man standing
3rd Oct 2008, 19:16
You are. It will over read.

G-CPTN
3rd Oct 2008, 19:18
I'm with Lord Draper . . . :ok:

The things to check, though, are the diameter of the rims and the section of the tyre(s), as it is feasible that, despite being low profile, if they are fatter then the height of the section might be almost the same.
There should be tables of rolling circumference available from the tyre manufacturers.

tony draper
3rd Oct 2008, 19:25
Well one suspected on was right of course but one has had little to do with horseless carriages as one noted early that one tends to get exceeding filthy pottering about wi them so one was not even sure how a speedmeter functions one assumes it is connected to the wheel in some way and a given number of turns registers a given speed.
:)
Err one did indeed consult the tinternet thingy Mr Tiger, behold above.:rolleyes:

PaperTiger
3rd Oct 2008, 19:32
one did indeed consult the tinternet thingy Mr Tiger, behold aboveHuh ? What ? Where ?

tony draper
3rd Oct 2008, 19:36
One's first post Mr Tiger,does not Prune dwell upon titnternet?,and one tends to get swifter and more accurate answers on Jetblast as it is a veritable font of wisdom and one is always eager to provide intellectual stimulation for the denizens of Prune.
:rolleyes:

Rainboe
3rd Oct 2008, 19:39
Borrow a GPS and calibrate the speedo with the new correct speeds. Sounds like he may need one of those nice sticking plasters for cuts, cut into sections and placed around the speedo at strategic locations, with an arrer drawn in pencil with correct speed next to it. Then he's sorted.

tony draper
3rd Oct 2008, 20:13
There is a large screen on one of the side roads not far from here that displays your vehicle speed as you approach it,I suppose he could use that.
:)

The Flying Pram
3rd Oct 2008, 20:19
It depends entirely on the exact size and type of tyres on the "new" wheels. Just because they are low profile doesn't automatically mean they will have a different overall size. It's normal to go up a size or two in the section width to compensate. To show my age the old pretty much standard 155/80R13 size could be replaced with 175/70R13 and still have the same overall diameter and number of revs per mile. Tyre companies should be able to supply this information.

Storminnorm
3rd Oct 2008, 20:53
Fer FECKS sake! Just refit the old wheels!!!!:ugh:

frostbite
3rd Oct 2008, 20:56
Most towns have at least one set of half-mile marker posts for use by the old bill in checking their instruments.

If the good cause is explained, I daresay they will oblige with directions.

Blues&twos
3rd Oct 2008, 21:18
I just drove in front of a friend of mine who thought he had a dodgy speedo on his car and switched on my hazard lights briefly a pre-agreed speeds (40, 50, 60 etc) on a not very busy dual carriageway.

Just for you Mr. Draper:

HowStuffWorks "Eddy-Current Speedometer" (http://auto.howstuffworks.com/speedometer2.htm)

G-CPTN
3rd Oct 2008, 22:55
If you got oil into the speedo-drive cable (instead of grease) then it could get 'pumped' up (through rotation of the cable inside the outer) into the speedometer head (the instrument fitted to the dashboard that indicates speed) then the drag of the aluminium cup and magnet increases such that the needle over-reads and 'flutters' (and eventually goes full-scale and rests against the back of the zero stop).
Had a Riley RME that did that . . .

SyllogismCheck
4th Oct 2008, 01:11
Most modern speedos are pretty linear in their pre-set error and it's generally not much at that, so all he needs to do is calculate the percentage reduction in the tyres rolling circumference and deduct that percentage from the speedo's actual reading at any given speed to get the true, or as true as pre tyre swap, speed.

Rolling circumference calculator here. (http://www.alloywheels.com/tyrecalc.asp)

BlueDiamond
4th Oct 2008, 04:16

A A Gruntpuddock
4th Oct 2008, 05:00
Hmmmm... I had always thought that the 'effective' diameter should be used for such calculations, ie twice the height of the wheel centre from the road surface. Any comments on this?

kansasw
4th Oct 2008, 05:58
I am not familiar with "effective diameter" but it may well be equivalent to "static loaded radius," which I think is a familiar term in the tire industry. I believe this to mean the height from the ground to the axle center with a given load. It is useful to determine proper tire pressure for a vehicle with a given load.

ZEEBEE
4th Oct 2008, 09:08
Most modern speedos are pretty linear in their pre-set error and it's generally not much at that,

Sorry, but that's not really true. I believe that the specs actually allow +7% and most manufacturers run pretty close to that, if not slightly over..

There are some REAL advantages in this for vehicle manufacturers AND dealers. Even some for the owner actually.

1.) The car appears to be quieter at an indicated speed (people think that's good)
2.) Individuals who run slightly over the speed limit don't get clobbered by the keen eyed fuzz.
3.) People think they get better fuel economy cos they think they've travelled further than they have.
4.) people who speed are actually a little safer because they're not really going as fast as they think.
5.) Services come around a bit quicker...Good for dealers.
6.) Warranty runs out quicker. Good for the dealer amd manufacturer.

and you get my thread drift.... With all those, why wouldn't a manufacturer run to the edge of the spec?

So, your friend (who lost the argument by the way) should be aware what an over-reading speedo actually costs.

BombayDuck
4th Oct 2008, 09:20
Like Bluey, I was wondering what Herr Draper had to say about swimming trunks!

srobarts
4th Oct 2008, 10:39
A GPS with speed display is easiest way to check actual speed vs indicated speed. My worst car was a Jeep Grand Cherokee which was 8mph too high at 70!! My Road Angel keeps me on the right side of the law.
I have seen this gadget advertised: GlobalTop HG-100 Bluetooth GPS + HUD Speedometer.

Buster Hyman
4th Oct 2008, 11:01
I'm with you Bluey...this is the mental image I got when I thought of Admiral draper in Speedos...

http://www.thetop10everything.com/files/www.thetop10everything.com/img/56/speedos.jpg

MY EYES! MY EYES!!!:eek:

Effluent Man
4th Oct 2008, 11:03
In my feckless(er) days I used to charge about the highways and byways in a Riley 1.5.At the time I worked for PYE television and colour had just come out.I acquired a set for a local garage owner at a discount.I used the staff shop although some rather less scrupulous colleagues used a row boat from the wharf at the back of the factory.My reward for this was a set of 5.5inch wheels with radial tyres that made the old Riley stick to the road like shit to a blanket.Despite being 13inch rims compared to standard 14inch the 185 section tyres made it under read a bit.However having a rev counter and being astute at my 19 times table I always knew the true velocity and therefore avoided being apprehended by yonder bureau.

BombayDuck
4th Oct 2008, 11:24
Buster, just because you had a horrifying mental image does not mean you should project it onto the interwebs and blind the REST of us! :ooh:

G-CPTN
4th Oct 2008, 12:12
The Riley 1.5 of which you speak is, no doubt, the overbodied Morris Minor (though the 50% increase in engine capacity more than made up for that). IIRC it was inclined to excessive bodily roll unless additional devices were installed.
Rot around the front wing (fender) to bulkhead seemed to be the killer of what was a commendable vehicle.

SyllogismCheck
4th Oct 2008, 13:29
Sorry, but that's not really true. I believe that the specs actually allow +7% and most manufacturers run pretty close to that, if not slightly over..

Whilst accuracy no doubt varies, I've clocked my car at a solid 84mph on GPS while set to 85mph on the digital cruise control and indicating exactly that on the speedo. That means it over-reads by 1.2%. If it over-read by 7% my indicated speed would have been 90mph at a true 84mph.

Liking a true 85mph as a nice 'keep the licence clean without hanging about on quiet motorways' speed, I've checked a few vehicles and not yet found one where I had to exceed an indicated 90 to get a true 85. Based on the few I've bothered to check, I'd say average errors seem to be more in the region of 3% than ≥7%.

Of course, there are many variables potentially involved. Not least of which, I would fully agree, is likely to be the manufacturer's greed. :suspect:

G-CPTN
4th Oct 2008, 13:39
(and more similar videos accessible from there)

chuks
4th Oct 2008, 17:08
Ze Germans always have a little table showing speedo error when they test cars in "auto, motor und sport" and motorcycles in "das Motorrad". Some Italian bikes are surprisingly accurate, e.g. 98 km.h. at 100 indicated. I have never seen one under-read, no.

Interestingly, we have these separate LCD displays for speed, fuel consumption, air temperature, Hitler's Birthday and all that sort of stuff in both our cars, a VW Passat and a BMW 3-series and in both the speed displayed on the LCD seems to be slightly lower and thus more accurate than on the speedo itself.

(German highways usually have distance markers every 100 metres, making it easy to check speed if you have powerful nerd tendencies, plus there is the GPS, of course.)

There seems to be something wrong with that film clip. The sound plays okay but the picture only runs in slow motion!

There are porn sites enough nowadays without bombarding us with gratuitous topless imagery, I find. Good God, Sir!

dazdaz
4th Oct 2008, 17:28
Just curious re: #1 What make/model/year is this "horseless carriage" ?

Daz

G-CPTN
4th Oct 2008, 18:27
Guess what Ł8000 would buy you . . .

More Motor Show nostalgia:-

Captain Stable
4th Oct 2008, 19:47
Damn it, misread the title again. I assumed this was something about why I can no longer fit into my Speedos and would have to find a brand of swimming trunks called "Comforto" instead... :(

M.Mouse
4th Oct 2008, 20:36
Interesting discussion.

I recall reading an authoritative document that speedometers must by law NOT under-read. Hence they all over-read. My Mercedes, checked against a a Road Angel GPS, over-reads by about 3 m.p.h. whatever the speed!

Various speedometer repair companies can re-calibrate speedometers, certainly the standard cable driven dial type, and tell you on their websites what they need to know to do so. I cannot be sure that it is possible with the electronically driven version but see no reason why not.

What nobody has mentioned is that if the horseless carriage was on a conveyor belt then would the speedo under-read or over-read?

dazdaz
4th Oct 2008, 23:33
Captain Stable...I know where your coming from.....Late 30s and the old scrote sack starts to dangle.Welcome to the club.:)

BeechNut
5th Oct 2008, 03:16
Sorry, but that's not really true. I believe that the specs actually allow +7% and most manufacturers run pretty close to that, if not slightly over..

There are some REAL advantages in this for vehicle manufacturers AND dealers. Even some for the owner actually.

1.) The car appears to be quieter at an indicated speed (people think that's good)
2.) Individuals who run slightly over the speed limit don't get clobbered by the keen eyed fuzz.
3.) People think they get better fuel economy cos they think they've travelled further than they have.
4.) people who speed are actually a little safer because they're not really going as fast as they think.
5.) Services come around a bit quicker...Good for dealers.
6.) Warranty runs out quicker. Good for the dealer amd manufacturer.

and you get my thread drift.... With all those, why wouldn't a manufacturer run to the edge of the spec?

So, your friend (who lost the argument by the way) should be aware what an over-reading speedo actually costs.

Not quite entirely true. While speedometers are allowed to be slightly optimistic, odometers are required by law to be accurate. A while ago in N. America, Honda was sued for having odometers that counted up too quickly.

I have a VW Passat (2007). I can GPS-verify that the speedo reads 5 km/h fast, but the odometer is spot-on accurate.

Beech

obgraham
5th Oct 2008, 04:36
What would be the correction factor if the car wuz on a conveyor belt?

2 Dogs
5th Oct 2008, 05:13
Lifesavers swap budgie smugglers for boardshorts on beach
Article from: The Courier-Mail Jeremy Pierce September 29, 2008 12:00am

BUDGIE smugglers versus boardshorts has emerged as a burning issue among image-conscious surf life savers manning Queensland's beaches. In a bid to keep more young people in the sport, surf life saving officials have introduced uniform boardshorts as an alternative to budgie smugglers and the David Hasselhoff Baywatch-style gym shorts. Boardshorts or budgies? Have your say. Pictures: Say goodbye to budgie smugglers Surf Life Saving Queensland boss George Hill said uniform was always a hot topic among younger clubbies. "The feedback was that they wanted some more comfortable boardshorts, so now we've given them the option," he said. An official from one Gold Coast club, who asked not to be named, said he had been trying to introduce a new uniform for years, but was met with resistance by club hierachy. "The surf carnivals are a perfect example," he said. "Kids hang around the beach all day in their boardshorts and they only take them off at the last possible second for a race and as soon as the race is over they put them back on," he said. "Bright red and yellow caps and club uniform speedos or those other daggy red shorts are not a good look for a kid who would probably rather be wearing a pair of Quiksilver or Rip Curl boardies." "But some of the old salts at our club don't want to know about it, which is a shame, because we are losing young kids to the sport." Mermaid Beach club captain Pete Degnian, himself a devoted budgie smuggler, said anything that helped keep kids in life saving was good for the movement. "It's probably cooler for kids to wear boardies than the old budgie smugglers," he admitted.
Patrol member Matt Williams, 14, said the new-look boardshorts definitely had appeal. "You do get paid out on the beach wearing speedos," he admitted.

ZEEBEE
5th Oct 2008, 06:32
Not quite entirely true. While speedometers are allowed to be slightly optimistic, odometers are required by law to be accurate. A while ago in N. America, Honda was sued for having odometers that counted up too quickly.

All interesting stuff.

In my job, I get to drive a large number of different vehicles by virtue of being away from home a lot and having to hire them.

My exposure is largely related to Australian vehicles, but they're almost universally overeading. Typically, by averaging GPS readings 118klm/hr indicated returns 110 true(most of our hway limits).
Interestingly, a Jeep Compass I rented in the US recently was pretty much spot on throughout.

It's hard to see how the oddometer can be different from the indicated as they're driven from the same source and you'd be inclined to think that a systematic error would be consistent on both.

chuks
5th Oct 2008, 09:02
I suppose the explanation might be that everything is electronic nowadays so that you easily could get one speed displayed on the round dial and another one on the digital display and a third value on the odometer.

Yamaha was marketing a new motorcycle in the States with an engine that hit 17 thousand r.p.m. on the rev counter. It then came to light that the real value was a mere 15.5 thousand so that some disappointed customers sued! The machine performed as promised and there was nothing at all wrong with the engine, when you cannot hear the difference between the two values but... Yamaha had to pay out and then they re-phrased their advertising. There is just no pleasing some people...

A guy goes into a bar, asks for a drink and then takes a mouse out of his pocket, along with a tiny piano. The mouse sits down and starts playing and singing to its fascinated audience. Then one guy looks really closely and says, "Hey! This guy is a ventriloquist! This mouse isn't singing!" So they all turn away...

I was interested to see that our BMW car will only show 250 km./hour when it's maxed out. That's not the 250 shown in the papers but only about 235 km./hour! Instead of 155 it's only doing 145 m.p.h.! Well, never mind, anyone asks I just tell them "250" and leave it at that.

I met a Norwegian with a Suzuki Hayabusa fitted with luggage. (The Haybusa is today's fastest production motorcycle.) I asked him how fast it would go with the luggage fitted. "300 km./hour," was the answer. Well, yes, of course! Big increase in drag, shift in C.G., loss of stability and a 10-km./hour increase in speed; that makes sense to me...

I have a BMW bike that will do 310 (if you drop it out of an airplane).

Peter Fanelli
5th Oct 2008, 12:25
You are. It will over read.

Given that the speedometer is driven from the driveshaft and has no knowledge of what size wheels and tires (tyres) are installed I put it to you that the speedometer will not over-read but in fact the car itself will underspeed with smaller diameter tires fitted. :E

chuks
5th Oct 2008, 13:44
You measure the new tires and the old tires and you get the percentage difference in their circumferences. Then you put the car on the treadmill you usually use to keep your airplane on the ground and run it forwards at the velocity difference between the two sets of tires, thus correcting the speed of the car to agree with the speed displayed on the speedometer. (I suppose it would be best to have a feedback sensor linking the speedometer to the treadmill, using that same instantaneous acceleration feature needed with the airplane, for this.)

Of course you must have the treadmill correctly positioned between points A and B (origin and destination). If you then want to return to your starting point you just reverse the cables on the treadmill's propulsion unit so that you can go from B to A (new origin and new destination) with your speedometer still reading correctly. If you forgot to do that then you would make the return trip with the speedometer still reading correctly but the car's speed being twice as incorrect.

What, you may well ask, of the odometer error in this, though? Good question and I am still working on that one.

Too, what happens if you have a really gross error in your speedometer as I seem to remember Smiths speedos sometimes giving? Could even an infinitely powerful treadmill cope with the errors those can generate?

A kinked drive cable would be pretty bad, with those wild swings of the needle we have all seen. Zero to 100 and back again, so that the treadmill should have its work cut out.

Another potential problem must be total speedometer failure. Say you were doing a really high speed, for an English car like a Riley 1.5 something like 27 m.p.h. for instance. Twangg! Your speedometer cable breaks! The infinitely powerful treadmill reacts instantaneously to generate infinitely high G forces, reducing your beloved example of British craftsmanship to shredded tinfoil, chips of hand-rubbed lacquer and scraps of Connolly leather. You would probably be unharmed but your car would be reduced nearly to its molecular state. So there is that...

G-CPTN
5th Oct 2008, 14:00
The speedometer was invented by the Croatian Josip Belušić in 1888, and was originally called a velocimeter.
United Kingdom regulations:- the speedometer must never show an indicated speed less than the actual speed. However it differs slightly from them in specifying that for all actual speeds between 25 mph and 70 mph (or the vehicles' maximum speed if it is lower that this), the indicated speed must not exceed 110% of the actual speed, plus 6.25 mph.
For example, if the vehicle is actually travelling at 50 mph, the speedometer must not show more than 61.25 mph or less than 50 mph.

M.Mouse
5th Oct 2008, 16:25
Given that the speedometer is driven from the driveshaft and has no knowledge of what size wheels and tires (tyres) are installed I put it to you that the speedometer will not over-read but in fact the car itself will underspeed with smaller diameter tires fitted.

If the car is going slower than the speedo is reading then the speedo is over-reading!

ChristiaanJ
5th Oct 2008, 18:24
It's hard to see how the oddometer can be different from the indicated as they're driven from the same source and you'd be inclined to think that a systematic error would be consistent on both.I see nobody picked up on your remark...

I think the answer is simply that the odometer counts distance. So unless you fit tyres with a different circumference (never mind rim size, etc.) there is a close one-to-one relation between number of revolutions of the wheels and distance travelled, and that can probably be calibrated to a percent or so.

The speedo comes off the same source, but it just indicates how fast the speedo cable is whirling around ('cup and whatever' gadget) and you can get that to indicate whatever you like. As said above, it's usually calibrated to show a few percent above real speed.

CJ

G-CPTN
5th Oct 2008, 18:51
Many modern speedometers are electronic. A rotation sensor, usually mounted on the rear of the transmission, delivers a series of electronic pulses whose frequency corresponds to the rotational speed of the driveshaft. The sensor is typically a toothed metal disk positioned between a coil and a magnetic field sensor. As the disk turns, the teeth pass between the two, each time producing a pulse in the sensor as they affect the strength of the magnetic field it is measuring.
A computer converts the pulses to a speed and displays this speed on an electronically-controlled, analog-style needle or a digital display, the latter of which is more common nowadays. Pulse counts may also be used to increment the odometer.Calibration of the 'computer' is probably more precise than the accumulated errors associated with the eddy-current / needle return spring / dial location. Don't know whether it is possible to adjust the calibration 'at the dealers' to allow for alternative wheel and tyre combinations (this used to be done by swapping cable drive gear sets). With the introduction of tachographs (for recording professional drivers' hours and speeds), external correction gearboxes were introduced (as standard accuracy was no longer good enough), though digital tachographs have probably superceded all that.

spekesoftly
5th Oct 2008, 20:57
This thread has brought back memories of when the speedometer on my 1964 Mini-Cooper developed a novel fault. The magnetic drive locked solid and the needle would spin through 360 degrees! I eventually sent it away for repair to JDO Instruments in Keighley. After a quick Google search, I see that they are still in business some 40 years later, and they are able to recalibrate mechanical speedometers to customer requirements (change of wheel/tyre size etc).

chuks
5th Oct 2008, 22:15
The first motorcycle I really noticed (Harleys were just sort of there, part of the scenery) was a Norton which must have been a 500 International. All I remember now is that it was a pushrod single.

Its chronometric speedo must have had some sort of malfunction because it was jammed at the maximum reading, which was something like 110 miles per hour! My Norton-owner friend told me, "Oh yeah, it did that when I got it up to 120 or so..." I was extremely impressed! (I guess one in good nick could probably hit about 90 downhill with a tailwind but this one was shagged.)

cockney steve
5th Oct 2008, 22:39
..........and everyone's ignored the fact that new tyres have about 7millimetres MORE tread depth than ones worn to the legal limit.
As the "common " wheel-size now appears to be 14 inches, and the "common " tyre-profile seems to be "70"(aspect-ratio)you get a diameter of around 22 inches, nominal..... errr.... the circumferential difference between a worn tyre and a new one, is about 3 1/4 inches.

....circ, is about 2 yards....=880 revs =:eek: about 80 yards difference per indicated mile.

erm....I REALLY must get out more :uhoh:

Flash2001
5th Oct 2008, 22:54
Chuks

If it was an International it wasn't pushrod and vice versa.

The chronometric speedo and tach told you the speed and revs one time hack ago.

Edited to correct spelling