Changing wheels / tyres - Calculator

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Changing wheels / tyres - Calculator

Postby DTH » Wed May 21, 2008 9:14 pm

If you are changing your wheels / tyres worth having a look at this to see how it will affect your speedo etc.  Seen it linked on the other site but think its worth posting here

http://www.miata.net/garage/tirecalc.html

imo its worth making a sticky.
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Postby bigginger » Wed May 21, 2008 9:38 pm

&IMHO2 - so it is one :D
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Postby DTH » Wed May 21, 2008 10:06 pm

Thank you Andrew!
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Postby DTH » Sat Jun 28, 2008 9:30 pm

Actually, Just worth a reminder that the norm for minor tyres is 145 80 R14.   Sure everyone knows but worth mentioning.
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Postby Orkney » Sun Jun 29, 2008 6:48 am

Must take a pic later - went in the garage yesterday and because the wing is off one side get a really interesting look at the lack of clearance on the tyre inside to the leg.
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Postby Davidward » Tue Aug 05, 2008 7:28 pm

I've been looking at tyres recently for the morris.  

The original tyre for cars from about 1956 onwards was 5.20-14 crossply.  The original manufacturer was Dunlop and the model was D75. Earlier cars had Dunlop c49 tyres 5.00-14.  Some police cars etc. had radials the size of which I'm unsure probably 145/80/14.  Anyway the D75 had a circumference of 610mm.  A 155/80/14 tyre such as the toyo 310 155/80/14 is 605mm which means that the speedo is less than 1mph fast for road speed.  

a 145/80/14 means that the spedo is nearly 6mph wrong.

Sorry anorak time....

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Postby Ned Ludd » Wed Mar 09, 2011 7:39 am

More anorakkery, I'm afraid, but perhaps useful: if I remember correctly the old "standard profile" tyres had an aspect ratio of 82%. The radial equivalent to a 5.20x14 works out to 155R14, with a diameter of 609.8mm, or just a smidgen over 24".

(The first tyres advertised as "low profile" were the Firestone "Wide-Oval" of the late '60s, with an aspect ratio of 78% which, in theory, gave 5% more tread width.)
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Re: Changing wheels / tyres - Calculator

Postby Beagle » Thu Nov 13, 2014 10:41 pm

Unfortunately it isn't as simple as measuring the outer circumference of a tyre to calculate accurately the effect on the speedo, gearing etc. The effective 'circumference' is not at the surface of the tyre but closer to the 'belt'. In a typical radial tyre there is a triangulation between the radial ply and two further plies laid at opposing angles of around 20 degrees under the tread. These form a quite rigid belt and keep the tread area on the road surface. The sidewalls have only one radial ply and are quite flexible. A radial tyre looks flat at the bottom when compared to a cross ply tyre, a result of the true radius of a radial tyre being closer to the belt than the surface of the tread.
The aspect ratio of the tyre also depends on the manufacturer, to a degree, and for example a 145/80 14 has a nominal 80% ratio but has a tolerance fixed by international standards. It would be necessary to get the data from the manufacturer to do an accurate comparison.
Hope I don't sound too pedantic!
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Re: Changing wheels / tyres - Calculator

Postby Plin » Fri Nov 14, 2014 12:05 am

Oh we don't mind helpful pedantic explanations! :D Welcome to the forum Beagle - do you possess a classic car?
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Re: Changing wheels / tyres - Calculator

Postby Custard » Sat Nov 15, 2014 9:59 am

Welcome to the forum Beagle, but I think you have overcomplicated matters.

I agree the effective radius is different due to the deformation of the tyre, but that has no difference on circumference.

As the tyre deforms it ceases to be round but the length all the way round has to be the same unless you actually remove rubber from it.

So whether the tyre is round, oval or square the circumference is the distance travelled for each turn of the half shaft and that is what matters as far as calculating overall gearing and speedometer accuracy.

If the tyres are on the car easiest way is mark the tyre and the floor push the car one complete revolution and mark the floor again, then measure the distance between the gaps.

I managed to do that, and use the diff ratio and calculate and program an electronic VDO speedo to 2% accuracy at 70 mph.
(VOSA figures from an SVA and they do have grippy rollers so no slip at all).

Obviously it changes as the tyres wear as if they have 8mm tread new by the time they have 2mm left on them they have lost 12mm of their diameter which is over 36mm off the circumference.

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Re: Changing wheels / tyres - Calculator

Postby Beagle » Sun Nov 23, 2014 10:32 pm

Sorry to disagree, Custard, but unfortunately it is much more complicated than that! The rolling circumference of a loaded car wheel is the Static Radius SR (measured from the wheel axis vertically to the contact patch) x 2 pi. Relative to the axis the contact patch describes a perfect circle of 360 degrees. This is the accepted calculation in tyre technology and for a typical radial car tyre this radius approximates to 0.96 x the unloaded tyre. There are many articles supporting this on the internet though many more sites providing calculators which are incorrect as they ignore the fact that tyres 'have a flat bit at the bottom'. They are fine as a comparison of radial tyres giving an approximation of the effect of changing wheel/ tyre sizes but beware of going further!
It becomes further complicated:
- as speed increases the Dynamic Radius DR increases and hence the circumference (DR>SR)
- varying tyre pressure alters the radius
- varying loading alters the radius
Calculating the circumference of a tyre by using (rim diameter + 2 (section x aspect ratio)) x pi assumes that the aspect ratio is exactly as stated on the tyre whereas it is a nominal value and manufacturers can use the tolerance to advantage when standardising tyre carcases to reduce costs and optimise production.
I have little knowledge of cross ply tyre aspect ratios but over the years they appeared to reduce from 100 to around 80. I remember having 2 Austin 7 tyres with the same size markings but wildly different in diameter!
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Re: Changing wheels / tyres - Calculator

Postby Custard » Mon Nov 24, 2014 9:29 am

It is fine to disagree, I enjoy a well thought out discussion.

I just think focussing on the radius is misleading no matter how many people may do it on the internet.

I agree entirely about the radius changing, especially when measured from the centre of the wheel to the floor, as when the tyre is inflated it increases initially as the flat spot lessens and if you inflate to silly/dangerous pressure the tyre would balloon across its tread width too.

But you would get to a true radius of a circle instead of a fictional radius of a circle with a flat spot.

So I agree that measured radius changes and therefore have to agree with you that calculated circumference changes.

I do make the assumption that a steel belted tyres construction does not allow the fabric to stretch and that the radius is only changing because the flat spot at the bottom due to flex under load and inflation changes.

Therefore as no material is removed or added to the tyre as it is inflated the true circumference, the distance it rolls forward in one revolution remains the same. And that is the measurement that should really be used in calculations.

The more calculations you use to get to that point like wheel size, width aspect ratio the more errors creep in.

I think it would be great if the manufacturers actually published the circumference as measured, but I imagine there are not enough people swapping sizes to warrant it and diameter lets people quickly measure what it looks like in the wheel arch so that is what seems to get used.

e.g.
http://www.etyres.co.uk/tyre-size-calculator/

Next time I have a wheel off I may just roll it along the ground and measure then put it on the car with 10psi in it and again with 40psi but I honestly cannot see it being any different the higher inflation just minimises the flat spot at the bottom and therefore lessens the flexing of the tyre on each revolution and therefore the heat generated in it.

Also,

I did have the same issue as you in not being able to find a comparison to cross ply tyres, but I have in the back of my head from years ago that cross plies were 88% aspect ratio and that an unlabelled radial was 80% I went up to 15" wheels on that basis with a common (cheap for what they are) size 195 50 15s and still have a smaller circumference than the tyres I took off.

The other car is still on crossplies (only fit for keeping it off the floor) which are massive in comparison.

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Re: Changing wheels / tyres - Calculator

Postby Beagle » Tue Nov 25, 2014 8:40 pm

Thanks, Custard. At the risk of becoming a real 'nerd' I have been searching out tyre technology articles and found one that may be of interest:
https://www.tut.fi/ms/muo/vert/11_tyre_ ... ndling.htm
On about the 6th page it goes into rolling circumference and variation due to load.
A common constant (96%) for comparing radius of a loaded wheel assembly to an unloaded wheel in applications like ours is shown in: http://blog.mechguru.com/vehicle-design ... n-example/
From the ETROT manual (European Tyre and Rim Standards Manual), they use a value of 3.05 rather than pi when calculating rolling circumference from design tyre diameter assuming max design load and standard pressure. This equates to 97%.
So the rolling circumference of a tyre under load cannot be measured externally as it is below the tread and approximately at the 'belt'.
Tyre/ Wheel calculators are OK as a guide but should not be taken too seriously.

I can see why you raise the question of where the rest of the circumference goes to when a tyre rotates with a flat bit at the bottom: "as no material is removed or added to the tyre as it is inflated the true circumference, the distance it rolls forward in one revolution remains the same". I did a tyre tech course many years ago and struggled to get my head round what the theory had proven as being correct. That's without getting into steering, motive force, part used tyres.....

Tyres to many are a distress purchase and people wonder why 'pouring rubber into a mould can be so expensive' but some tyre manufacturers used to claim that there is more technology in a tyre than in the rest of the whole car!

Love the red saloon, often fancied going for wider Minilites on my Traveller but for my use and old fart style of driving I'm asking Santa for a set of 145 R13 Bridgestones to go on my standard steels.

Hope this isn't boring the pants off everybody....promise to be lighter in future.

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Re: Changing wheels / tyres - Calculator

Postby TFM150K » Tue Nov 25, 2014 10:13 pm

Is the moral to this story simply "If you want to know how fast you are going, buy a satnav"?
:happy8:
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Re: Changing wheels / tyres - Calculator

Postby 123 » Sun Feb 22, 2015 9:55 am

TFM150K wrote:Is the moral to this story simply "If you want to know how fast you are going, buy a satnav"?
:happy8:


http://www.ebay.co.uk/sch/Vehicle-Parts ... peedometer
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