Find your ideal tyre width for given Rim width

[pascal]

Upgrading Wheels and Tyres .... Small Guide

The rim width should be approx 85% of the tyre width profile.



To find the correct tyre profile for a given rim. (J = rim width, ie 7")

J x 25.4 x 1.1765 ..............

(25.4 converts J to mm. 1.1765 converts the 85% rim to 100% tyre section)


Here are some samples of widely used setups:

IE: 6.5J wheel ..... 6.5 x25.4 x 1.1765 = 194.24mm (the nearest tyre size is 195)

IE: 7J wheel ........ 7 x 25.4 x 1.1765 = 209.18mm (the nearest tyre width is 205)

IE: 8J wheel .........8 x 25.4 x1.1765 = 239.06mm (the nearest tyre width is 235)

IE: 8.5J wheel...... 8.5 x 25.4 x 1.1765 = 254mm (the nearest tyre width is 255)

PS: Usually you can go one/two sizes bigger than the above ideals without a problem.
PS: I hope I got my sums correct:Oops:

 

[pascal]

Now for a wheel upgrade

Sample...........

W124 with standard 6.5J x 15 rims & 195/65R tyres.

First we establish the total circumference of our setup.

The rim is 15" x 25.4 (convert to mm) = 381 mm (diameter), divided by 2 = 190.5 mm (radius)

The side profile of a 195/65 tyre is 65% of 195mm = 126.75mm

The wheel radius + the side profile equals total radius: 190.5mm + 126.75 = 317.25mm total radius

The tyre circumference is therefore R x 2 x 3.14159265358979323846 (radius x 2 x pi)

For simplicity we'll use 3.1416 as pi. C = 317.25 x 2 x 3.1416 = 1993.35mm

So for our upgrade we will be aiming to get to within 1% of 1993.35mm rolling circumference.

So we have bought a set of 16" alloy with the correct ET for our car.

They are AMG 7.5J 16 ET31 rims.

From the formula on last post we get J x 25.4 x 1.1765 ...... 7.5 x 25.4 x 1.1765 = 224.12 mm as an ideal width.

As these wheels are marginal fit for the front of the W124, we will fit narrower tyres to the front.

So we decide to put 225s on the rear and 205s on the front, and need to know if this is possible.

Front: 205/55 16. RIM radius: 16" = 16 x 25.4 divided by 2 = 203.2mm
............................Sidewall: 55% of 205 = 112.75mm
.......................Total radius: 203.2mm + 112.75mm = 315.95mm
New Rolling Circumference: 315.95mm x 2 x 3.1416 = 1985.18mm

Rear: 225/50 16. RIM radius: 16" = 16 x 25.4 divided by 2 = 203.2mm
............................Sidewall: 50% of 225 = 112.5mm
.......................Total radius: 203.2mm + 112.5mm = 315.7mm
New Rolling Circumference: 315.7mm x 2 x 3.1416 = 1983.61mm

Conclusion: Original rolling circumference: 1993.35mm
.....................New Front circumference: 1985.18mm
......................New Rear circumference: 1983.61mm

Well within our aim of 1%

Note: The above upgrade is not allowed on the E500/E420 or the Cabriolet.
MB recomends wing extensions with this application

 

[pascal]

ET Numbers (wheel offset)

Now we will discuss why the ET number of a wheel is so important.

The ET number is measured in milemetres, and is the distance from an imaginary plane down the centre of the wheel to the fixing face plate of the hub that takes the wheel.

This drawing shows a wheel with no offset

This drawing shows a wheel with an inset

This drawing shows a wheel with an outset


To be continued later

 

[turnipsock]

remember the law doesn't allow your tyres to stick out past the bodywork.

My thoughts on tyres...You can fit rubber band tyres if you drive on race tracks all the time...but we live in the UK...have you seen the state of the roads?

 

[pascal]

That is why wing extensions are required as stated above.

A wing extension is considered part of the bodywork

 

[c13tay]

have a look here - calculates everything for you !!!!!!

http://www.chris-longhurst.com/carbibles/entrypoint.html?menu.html&tyre_bible.html

 

[pascal]

Yea, Ive seen Chris'es site.

He has the fomula for tyres to wheels, but none for wheels to tyres.

There also seems to be an error in his calculations for tyres over 50 profile.
He uses a 70% rule for these, instead of the normal 85% rule. Using his calculator, 6.5J x 15 wheels whould be fitted with 235 width tyres

Have contacted him re this.

 

[pascal]

Chriss agrees that the change from 85% to 70% for tyres over 50 profile seems innaccurate. This information was supplied from Michelin, who has at this time, still not yet replied to me.


For now I will be using the 85% rule for all sizes ( ie multiply rim width in mm by 1.1765) for ideal tyre width


Pascal

 

[mike65]

remember the law doesn't allow your tyres to stick out past the bodywork.

Really? (me rushes out to check....) Not sure what the law says here (Ireland).

Mike.

ps I'm just okay