Wheel ET.

[Quiggs]

Someone explain wheel ET to me. Where do the numbers come from, and what do they mean? I know if the ET is too big, a wheel won't fit on a particular car. But after that I get lost.

The reason I'm asking is I was looking at a set of Boxster twists on eBay, and the ETs are 55 rear and 50 front.

[Matra et Alpine]

ET is the offset in mm of the wheel relative to the centre line of the tyre.



It can adversely affect the handling if you get this wrong as the contact patch moves away from the designed point.

[Quiggs]

So if the final ET on my car is 35, and the wheels I want to run are 50 and 55, I'll need to pull and roll my fender lips? Otherwise the wheel lips would be poking.

Edit: I'd also have to run 22.5mm wheel adapters to get from 5x100 to 5x130.

[KnifeEdge_2K1]

offset will change the scrub radius, which is the distance between the center of the contact patch to the point where the steering axis intersects with the ground

[Matra et Alpine]

So if the final ET on my car is 35, and the wheels I want to run are 50 and 55, I'll need to pull and roll my fender lips? Otherwise the wheel lips would be poking.

Edit: I'd also have to run 22.5mm wheel adapters to get from 5x100 to 5x130.

No.

A positive ET figure eg ET35 means the hub face is 35mm OUTSIDE of the centre line of the wheel. So it's likely you'll foul suspension components and bodywork INSIDE the wheel and on trying to turn corners with the fronts.

So when you fit ET55 wheels then the offset now moves the centre line of the tyre INWARDS on the car. See if this helps

  +-- wheel to hub face
  |   +-- centre line of tyre
  |   |
  V   V
+----------+
|          |
 \ ________/
  |
  |   this is exagerated to show the different lines
  |_______
 /         \
|          |
+----------+

The distance between the 2 points are the ET. If the centre line is outside the hub line then it is a negative offset number.

So, the HUB face is a fixed point, so the ET moves the centreline of the tyre in and out for + or - et VALUES. So a LARGER postoive ET value means a larger distance between the 2 points and the centreline of the tyre moves inward ( because the hub is fixed ). Equally a small ET moves it out. For negative ET values it works the other way round. Just keep sketching the above diagram with the different ET values and look where the tyre ends up - remember the hub is fixed.

So how to solve your ET55 problem ? Well, cheat and don't make the hub a fixd point. Simple spacers will more the hub face out by the thickness of the spacer. So for an ET35 wheel setup for an ET55 you want to add a 20mm spacer. BUT, make sure this is donw with professionally supplied bolts and spacers to ensure you don't over stress anything and it breaks.

By adding the spacers the centreline of the tyre returns closer to the point the suspensions is designed for and the "handling problems" I warned of are avoided. These problems occur becase as KE says you case the scrub radius to change having moved the camber and caster angles and aslo the ackerman angles for the steering. All can be bad news for safe and consistent handling.

The larger ET wheel with the same width of wheel means the centreline of the tyre doesn't quite return to the same place, to do that you also should go for a slightly wider wheel. Less of an impact as the outright ET change but still can have an impact on handling. Dependant on the wheels availabel you can juggle the ET sizes and wheel widths to better reposition the contact point. This is why any serious car will have fully adjustable rose jointed arms so that the camber/caster/ackerman can be adjusted for different wheels.

FINALLY you have to worry about bodywork and might need to give more space on the outside and the inside. In the works cases you sometimes contact suspens arms/track control rods etc. These all need to be thought befoer you buy-and-try Best advice is to find someone who has fitted the same and inspect for any rubbing or modifications and get to test the handling of the resulting car. In many cases added "better" rims can make the hadnling worse by compromising the stock (UNadjustabel) suspension