Right, here goes……………….firstly just to clarify chassis in my terms, the light chassis has a maximum gross of 3850kg and the heavy 4250kg. The heavy chassis from year 2007 (X250) to date has different brakes, suspension and hubs than those on the light chassis.
In 2015, Fiat and consequently many of the motorhome manufacturers made an option available to have 16″ wheels on a light chassis – this was only previously possible by buying aftermarket alloys with question marks over their strength and suitability. Basically Fiat began producing a 16″ steel and alloy wheel with the correct hub fittings for the light chassis. This was in response to end users wanting 16″ wheels for the improved road handling etc. So for any vehicles produced after 2015 both the light and heavy chassis could have 16″ rims. A pre 2015 light chassis on 15” rims could accept the post 2015 16″ rims but only those made specifically for the light chassis. Heavy chassis 16″ rims will not fit light chassis, either pre or post 2015. The easy way for owners to check would be to look at their Fiat chassis plate under the bonnet on the slam panel – if its 3850kg then they need wheels specifically for the hubs of the light chassis and if its 4250kg then they need those specifically for the heavy chassis. I, like many other dealers and owners ordered most of our post 2015 light chassis with the 16″ wheel option.
Effectively, to use a spare wheel ( if it did not come with the vehicle ) the owner would need to first check their wheel size, then check the chassis rating to identify the correct hub type required.
The second fly in the ointment is that different wheel bolts are required for steel wheels versus alloys and again between light and heavy chassis. Alloy wheel bolts are longer than steel wheel bolts. If they use alloy wheel bolts in a steel spare then there is every chance that they will protrude through the hub too far and damage the ABS sensor.
Also remember Steel wheels have conical nuts to sit in and secure the cone shapes in the wheel . Alloy wheels have flat nuts. Hence the two must never meet so, please get professional advice before purchasing any spare tyres or wheels.