Offset

[c13tay]

As you probably know im looking for a set of wheels (16 or 17") for my C class

I have a set of 7x15 with an offset of 35, a set of 7.5 x 17 with an offset of 37.

Does anyone know what the maximum and minimum offset values can be used on a c-class

Ive seen a number of sets of mercedes 5 hole alloys for sale (16") which i may be interested in - some have offset of 37 but some have offset of 40 something. I know the 37 would be ok but what about the 40 somethings ???

I assume that the 37's are from a c-class originally anf the 40's from an e-class ???

 

[pascal]

C13,

The offset is relevent to the with of the rim. Too high a number on 8J wheel will rub off the struts on the inside, but the same ET number on a 7J wheel may be just a perfect balance between inner position and outer position. If you have too low of an ET the wheel will stick out past the arch.

MB make wheels with paticular offset for a given J width for a given model. But it seems that some aftermarket non MB manufacturers make some wheels that have an ET number that falls between that of two different model.

According to MB the 7.5 Js you mention require body modification even with the 35 ET offset. This usually means arch extentions, so they may stick out a little. By increasing the ET to 37mm or even 40mm will move the wheel inwards, but may be very close to the strut. The size of tyres will also factor, as some combinations will have more of a buldge at the sides. 225/45 are the 'normal' size for that rim.


Pascal

 

[c13tay]

thanks pascal

my 7.5 x17s are et 37 and are fitted with 235/40/17's - no fouling anywhere although they are very close to the strut on the inside.

so a wheel with a higher et will sit further in to the arch - is that right ???

 

[tom7035]

I thought it was the other way round, but Pascal is usually very good at this. I've been wrong a couple of times lately, so I don't want to chance my mitt!!

 

[c13tay]

I thought it was the other way too - the higher the et value, the further out the wheel sits

 

[pascal]

No Guys

I explained this before. Almost all wheels on modern cars are inset (the centre line of the rim is closer to the strut side than the hub fixing plate) and outset is seldom seen. You increase this number, you move it even closer.

Can you all see now. The line nearest the the red, is the fixing plate. If you increase this distance, the wheel will move to the right (closer to the strut).

If you still have difficulty I will draw this in free hand for you.

 

[c13tay]

thanks again Pascal

Get it now

Is the et a value in mm ??

 

[Bolide]

Et

Yes, ET is a value in mm

It's difficult to explain but Pascal's diagram is worth 1000 words. ET is the offset between the wheel centre line and the hub face and it's normally a negative value

Most cars should run with the centre of the tyre contact patch at the same point whatever the tyre width (this relates to a notional line describing the centre of the suspension leg passing through the centre of the contact patch to maintain correct geometry) so wider wheels are offset more inwards toward the car to compensate

Two things work against that: the desire of manufacturers to "fill" the arch by oftsetting the wheel outwards to the edge of the arch (particularly with poverty-spec cars on narrow wheels & tyres) and the presence of the supension leg. You can't move a wheel & tyre inward past a certain point otherwise it'll rub the strut or spring

So with increasing rim width the offset will tend to reduce, and maybe go positive, as it reaches the maximum offset towards the suspension leg

That make any sense at all??


Nick Froome
www.w124.co.uk

 

[pascal]

I am so glad you understand now:lol:

I dont know whether it helps to call it a negative value, but its usually referred to as inset (set inwards)

If you had dished wheels (seldom seen these days) you may have an outset or positive value, and then an increase in ET value would move the wheel outwards, like this:

AN OUTSET WHEEL (sometimes seen on old modified minis etc)



If the centre of the wheel is exactly the same plain as the hub face, you get no offset at all, like here:

NO OFFSET



But as I said in above post, The offset in wheels is normally INSET as shown here again:

AN INSET WHEEL (very much the norm on modern cars)



Also see this thread: http://forums.mercedesclub.org.uk/showthread.php?t=12181


Pascal