Classic cars forum & vehicle restoration.

[oldtimer]

I feel my queries must be sorely trying the patience of many of you learned forum members but here goes once again.
Among certain sections of the classic car fraternity there seems to be an irresistible urge to change wheel sizes,invariably upwards,often for glitzy alternatives.My enquiry regards the impact such alterations have on,for example,the accuracy of the speedo.I accept designs vary,those between instrument and gearbox, those to the hub,but I feel that an increase from,say,15 inch to 17 inch wheels must make a difference.Is this one which would manifest itself at M.O.T.time?Could it be a problem with a close encounter with a speed camera?Perhaps it works in favour of these wheels at such times.I freely admit an abysmal lack of understanding of anything mathematical and suspect such skills are a prerequisite here,thus I must crave the indulgence of some kind person who has patience and know-how of Fibonacci Numbers or whatever.
My thanks in advance.

[Penman]

Hi
Theoretically a change from 15" to 17" wou;ld make a 12.5% difference in rolling circumference, however a lower profile tyre on the larger size might mean that you could keep the same outer circumference or much nearer than a 2" diameter difference.

Bigger outer diameter means speedo under reads whereas it should over read.

There can also be problems with wheel arch clearances, extra tyre width can also cause problems here
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Bristols should always come in pairs.

Any 2 from:-
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[oldtimer]

Hello Penman,
My thanks for your reply and not the first time you have done so either.It is a source of wonder how some forum members can succeed in supplying answers in clear,concise terms which are guaranteed to get through to people like me.In relation to the Black Art of numbers,I am still convinced that there is some sort of conspiracy when,many years ago,a Maths teacher asked "What is the half of two plus one? "Naturally I said "One and a half" but was rewarded with a pitying smile and told the answer was two.Things have never been the same since,numerically speaking.

[Keith D]

Mr Oldtimer,

Your query raises another potential problem when changing wheels.

A very important consideration is the actual position of the wheel on the car. The horizontal distance from the mounting face of the wheel to the inside of the rim (if it differs from the original wheel) will have important implications to the whole front end geometry of the suspension and steering, apart from clearance inside the wheel arches.

As regards speedo inaccuracies, provided one has done their calculations correctly, I understand that small nylon gearboxes are available that screw directly into the rear of the speedo between that and the cable, to correct the ratio.

Keith

[oldtimer]

Hello Keith,
Thank you for your input It seems there are a number of important issues in this whole business and,while it is not a project which holds any attraction for me,I wonder if those so inclined have ticked all the boxes and have included all the safety factors involved.
John

[47p2]

Here is a tyre calculator to work out the different sizes
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ROVER
One of Britain's Fine Cars

[oldtimer]

Hello John,
Yet another who has regularly come to my aid.Thank you.Hope all is well with you.
John

[Jim.Walker]

A little story about wheel changes.
Many years ago when the MOT included a road test to try the brakes and I was an MOT Tester, I was faced with a Mini with wider and larger wheels ( and possily spacers - though I do not remember). Driving out for the brake test I found there was no "feel" to the steering and the car very frighteningly continued turning on corners until the steering wheel was physically centred.
Scared out of my wits, as was my right I abandoned the test as "unsafe to continue".
The young owner asked what he could do and I told him to be sensible and to re-fit the original wheels.
He duly re-presented the car on wheels that I suspected were borrowed, but everything was fine so he got his certificate.
A few days later, he was spotted passing by with the lethal wheels back on!
Jim.
_________________
Quote from my late Dad:- You only need a woman and a car and you have all the problems you
are ever likely to want". Computers had not been invented then!

[Rick]

Years ago with my first Spitfire I learnt about different wheel nuts for different types of wheels. The car originally had steel wheels. I fitted period alloys, but re-used the original nuts. I hadn't noticed that the lug holes were a different profile, and required nuts with a different shoulder to them. Fortunately I wasn't driving too quickly when the o/s/r wheel made a bid for freedom

RJ
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[Jim.Walker]