Tuesday, 12 December 2017

Wheels and Tyres

This post is going to provide an explanation on why I intend to replace the current 18 inch alloy rims on the 4x4 with 16 inch steel rims.

Our Isuzu MU-X My17 4x4 came with 18 inch alloy wheels and 255/60R16 AT tyres.  This figure 255/60R16 is a combination of metric, percentage and imperial measurements.  The R16 means the diameter of the wheel rim the tyres fits is 16 inches.  The 255 is the width of the tyre tread in millimetres.  The 60 means the height of the sidewall of the tyre is 60% of the width of the tread.  So the height of the sidewall is 153mm (255x60%).  Converting 18 inches to millimetres =  457.2mm.  The overall diameter of the current wheel and tyre is therefore 763.2mm.

In Australia you are legally allowed to increase the ground clearance height of a vehicle by 50mm without requiring any special permission.

Some people believe you can increase a vehicles ability to operate off sealed roads by fitting wider tyres with aggressive looking knobbly tread patterns.  Whilst these tyres are good when climbing steep rocky terrain or driving through mud, they perform poorly on sand or out in the desert.  For this type of environment a narrow tyre with high sidewalls is better.  When travelling on soft sand or desert tracks it is important to reduce the air pressure in the tyres.  This increase the tyre tread “footprint” on the ground thereby spreading the vehicle load over a greater surface area. 

The footprint of a “fat knobbly’ tyres looks like this 


Whereas the deflated footprint of a ‘skinny high sidewall’ tyre looks like this


The “skinny” tyre footprint looks very similar to that of a tank or bulldozer track and that is what improves it’s performance in sand or soft dry terrain.

Our Isuzu has been delivered with rims and tyres that are a compromise between bitumen and off road work.  The majority of 4x4 vehicles never leave the bitumen and therefore the manufacturers fit rims and tyres for that environment.  I therefore need two different sets of rims and wheels.  The original tyres and rims will be for normal use.

for off road use my solution is to fit smaller rims by replacing the alloy 18” with steel 16”.  This will then enable me to fit “skinny” tyres with a high sidewall.  That way I can deflate the skinny tyre to achieve the desired caterpillar footprint.

I’m rather fortunate that many base model light utility vehicles are sold with 16 inch steel rims.  Steel happens to be cheaper than alloy.  Moreover many young male utility owners don’t like the ‘basic’ look of 16 inch steel rims and replace them with larger after market alloy rims and big knobbly tyres.

Today I was able to purchase four 16in steel rims from a young male utility owner who now has larger alloy rims.


In my opinion a steel rim is superior to alloy for off road movement.  The alloy is certainly lighter and just as strong a steel.  However if you hit a rock with a rim it can bend and steel is much easier to bash back into shape with a club hammer of axle head.  So for off road work I prefer steel.

A 16 inch rim diameter means I can increase the tyre sidewall height.  I wrote a small spreadsheet and calculated the best tyre for the 16in rim is a 235/85R16.  At 235 it is 20mm narrower than the current tyres on the 4x4.  The diameter of the combined rim and tyre is 801mm which is an increase of 38mm over the existing combination.  It’s also less than the maximum allowed 50mm.   So this should result in the desired narrow and long footprint when the tyres are deflated. 

There is one other advantage of using a ‘skinny’ tyre.  It means the sidewall of the tyre is less likely to be damaged (eg, staked or punctured) when travelling in the tracks of a previous vehicle.

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