JPSC Forum

Hi,

I've got a question about changing the wheels on my IRS Fury to 15's, which is something I've considered before for road use. 13" CXR's are great wheels, light and strong and quite neat, but there's something about 15's on a Fury that looks just right. Not sure if anyone remember's the blue Fury in PPC a few years ago by 27/47 engineering, but it looked the business.

So, my question relates to how to determine what offset is correct. If anyone can supply some answers I'd be grateful, but the most important thing is that I'm not interested in how much the wheels stick out from the arches. I'd like to know the technical 'right' approach to detemining the required offset, from a suspension perspective, and I'll deal with the body work later if necessary.

With 13x6's on, the recommended offset is 19mm or thereabouts. If I decided to go to 15x7's, am I right in thinking that the aim is to keep the centre line of the new wheel in the same place as the old wheel? In which case, I'd need an offset of 19mm PLUS 1/2 of 1", i.e. 19mm + 12.5mm = 31.5mm?

Advice appreciated,

Ian

The ET is the distance from the centre line that the hub face is set. To keep the centre line the same you need the same ET (ie. ET19 if thats what you've currently got). The extra 1" would be split 1/2"each side.

adrian

Are you changing the rolling radius, ie overall diameter of wheel tyre combination? If so then you will need to do some maths. What is important with wheel offset is the effect on scrub radius. This is the measurement between the center of the tyre contact patch to the point at which the wheel rotates around at the contact patch. Hard to explain without drawings so do a search or visit a library and get a book out on the subject. At the non steering wheels the offset is not so critical.

Hope I have confused you

Ian Kelly wrote:With 13x6's on, the recommended offset is 19mm or thereabouts. If I decided to go to 15x7's, am I right in thinking that the aim is to keep the centre line of the new wheel in the same place as the old wheel? In which case, I'd need an offset of 19mm PLUS 1/2 of 1", i.e. 19mm + 12.5mm = 31.5mm?

As someone said, the important point is keeping the scrub radius the same. However, if the centre of the tyre is in the same plane as it was before then the centre of the contact patch should also be the same and the scrub radius will be the same.

The problem is the offset calculation. You're going to need wheels where the hub is more inset with respect to the rim. ET31.5 has the wheel more outset with respect to the rim. (Modern FWD wheels tend to have the spokes near the outside edge of the rim, they're typically ET38.) So, I think (and please check this because it always confuses me) that you'll need wheels that have an offset IRO ET 6.5.

Tim

The ET is the offset from the centre line so whether the wheel is J5 or J7 the offset, of say ET18, remains the same and if the diameter of the tyre remains the same the scrub stays the same also. The problem is that the wider tyre is now not only sticking out further but also in further and whether you have enough clearance.

adrian