How to calculate wheel offset for wider wheels

[Burdekin]

I'm looking at going from 4" rims running 165 tyres to 6" rims running 195s. I have 195s on my 142 and 205s on my Amazon estate. The 142 has Volvo GT wheels but they are a different PCD and the estate look great but are 17" and a but too aggressive for what I'm looking for the 131. I'd like a Dunlop D type style wheel but not decided yet. Torque thrusts or slots are also options. But the thread was about calculating the correct offset in relationship to the old wheels and suspension. What is offset and how to calculate the difference is straight forward, I just wanted to know if there is any science in what offset to go for.

 

[Burdekin]

Ok, but it's not clear from your first post, and was only trying to help but from your reply, I guess you don't really need it!

I was just being clear, look at the other tools above who wanted to take it off topic. There's some helpful posts above which are appreciated and then there are guys who just need to post.

 

[Fastdruid]

I just wanted to know if there is any science in what offset to go for.

Yes. As I said before. The offset in combination with the SAI/KPI and the tyre diameter will dictate your scrub radius.

EDIT: See diagrams. If you keep the diameter the same and decrease the offset you will make the the scrub radius more negative, if you increase it you will make it it more positive.

If you increase the tyre diameter then you will make the scrub radius more negative and conversely decreasing it will make it more positive.

Note that I have chosen my words carefully there because it is possible to go from positive to negative scrub radius.

To copy from Wikipedia:

Large positive values of scrub radius, 4 inches/100 mm or so, were used in cars for many years. The advantage of this is that the tire rolls as the wheel is steered, which reduces the effort when parking, provided you're not on the brake.

The advantage of a small scrub radius is that the steering becomes less sensitive to braking inputs. More scrub radius adds to road feel by pushing the inside wheel into the ground.

An advantage of a negative scrub radius is that the geometry naturally compensates for split μ (mu) braking, or failure in one of the brake circuits. It also provides center point steering in the event of a tire deflation, which provides greater stability and steering control in this emergency.

Finally the KPI/SAI will change if you change the ride height, how much will depend on how much camber gain you have (and that will depend on your suspension type), mcpherson struts are typically crap for camber gain (and some layouts actually lose some) but that can be advantageous in not changing SAI.

 

[Hopefuldave]

Speakiing from experience, a major change in scrub radius will give a noticeable amount of kickback through the wheel on rough surfaces, can be quite unpleasant or even unstable...

Dave H. (the other one)