I'm contemplating getting a set of rims second hand and was wondering if anyone can confirm the factory offset for a TJ? Most posts I've checked talk about getting the right offsets but what does it actually mean? :shock:

I've done a little homework on this (:nuts:) and would like your thoughts.

All terms in this message are adopted from http://www.rsracing.com/tech-wheel.html (pretty good website with diagrams)


I understand that the legalities of offset and wheeltrack (the distance from the centreline of each wheel), seem to be this:

1. Your car's wheeltrack cannot be reduced (affects stability). Your offset cannot be legally increased beyond the factory offset - ie. by increasing the offset, you push wheel centreline towards the centre of the car, thus reducing the wheeltrack.

2. Your car's wheeltrack cannot be increased beyond 25mm (1") from the maximum factory settings. This means you can reduce your offset by 12.5mm from the max factory offset, which pulls each wheel centreline away from the centre of the car, thus increasing wheeltrack.

The first seems easy to work out - don't increase your offset beyond factory.

The second seems a bit more difficult to work out, but I think the answer is don't decrease your offset by 13mm. The pits check your offsets by looking at the actual wheeltrack and comparing it with the factory wheeltrack. Eg. for a TJ Magna 2000 sedan the manual says 1545mm for the front and 1535mm for the back. The sheet I got from the WA pits state a max wheel track of 1571mm for the front and 1561mm for the back.

That means you can increase your wheeltrack by 26mm front and back - meaning your can decrease your offset (and consequently pulling the wheel centreline away from the centre of the vehicle) by 13mm from factory (13mm x 2 wheels = 26mm).

Now what is the factory offset? I have no idea and my searches of this forum hasn't pulled up an answer.

Using the info from the above website, there's a method for working out offset and I did it as follows:

1. I measured 138mm for the backspace (about 5.5") I did this by using electrical tape across the 'inboard flanges' (see website diagram) and then running a measuring tape perpendicular from the 'hub mounting pad' - where it intersects the electrical tape is your measure.

2. My rim width is 6" (factory alloys, not steelies).

3. The chart says my offset is about 51mm.

(I'm sure this technique can be used to obtain the offsets for any secondhand rims someone is attempting to buy. However, it might be difficult to measure rim width as there seems to be a slight difference between 'rim width' and the distance between the inboard and outboard flanges. The latter giving a slightly larger reading.)

If this offset of 51mm and my analysis above is correct, then I can't increase the offset beyond 51mm and I can't decrease it beyond 38mm (51mm-13mm).

Does this sound right? :doubt:

I suppose the ultimate test is to put the wheels on and check that it doesn't extend beyond the guards and that it doesn't foul the calipers and any part of the wheel arches or suspension. But not every second hand seller would be willing to put up with you doing this in their driveway while you holding the dough.

Naturally you gotta check the PCD, spigot diameter, total rim width also.

Whew. Hope someone can confirm this. Tek? Vlad? From your posts, you guys seem to know quite a bit about this. I don't want to end up buying a set of wheels that somebody says will fit a magna when in fact it doesn't :nutkick:

The VR-X 17's are 46P.

Just called Bob Janes - 46P is the factory offset for a TJ. Seems that the lowest we can go 33P (46mm - 13mm) and the max is 46. Is this right?

Don't need a tape measure, just look on the back of one of the wheels, or your spare if it's an alloy). It will be stamped on either the mounting face or on the inside edge of the wheel somewhere as ETxx

Don't forget whenever you change the offset of your wheels you are altering the forces placed upon the wheel bearings, steering knuckle, possibly tie rod ends. This can lead to premature wear or failure of any of these components. Anything up to a max of 20% change in offset is fairly safe, so with +46 wheels you wouldn't want to go lower than +37 or +55.

Going wider in width for wheels/tyres also gives poorer wet performance + greater tendency for tramlining (the car wants to follow the little cracks and divets in the road instead of where the steering wheel is pointed).

tek:
i drive a tr and the offset is +48 is that good?

also i dont really agree with saying wider = worse in the wet, quite the contrary actually.
when you say tramlining do you actually mean the actual tram tracks?

Don't need a tape measure, just look on the back of one of the wheels, or your spare if it's an alloy). It will be stamped on either the mounting face or on the inside edge of the wheel somewhere as ETxx

Don't forget whenever you change the offset of your wheels you are altering the forces placed upon the wheel bearings, steering knuckle, possibly tie rod ends. This can lead to premature wear or failure of any of these components. Anything up to a max of 20% change in offset is fairly safe, so with +46 wheels you wouldn't want to go lower than +37 or +55.

Going wider in width for wheels/tyres also gives poorer wet performance + greater tendency for tramlining (the car wants to follow the little cracks and divets in the road instead of where the steering wheel is pointed).

Thanks a million :D

tek:
i drive a tr and the offset is +48 is that good?

also i dont really agree with saying wider = worse in the wet, quite the contrary actually.
when you say tramlining do you actually mean the actual tram tracks?

+48 is negligible change, you shouldn't notice much difference. It's moving in the other direction that most people do, +38 for example like mine. When you decrease offset you are widening the track, or road footprint of the car. This is great for stability however makes the steering effort harder.

Re wide tyres are worse in the wet... I think you're used to crap narrow tyres, compared to good wide tyres, in which case you won't notice the difference. *Usually* when people buy big wheels and low profile tyres you move away from the all-round tyre compounds and into more performance-orientated tyres. Except if you buy Sava tyres of course in which case you might as well run no tyres at all and have better grip from the wheel rim. :badgrin:

I mean tramlining as on any road, not necessarily tram tracks. I think that's where the term came from though because tramtracks are suicide for bike riders.

i get your point.
that explains why my steering like jerking here and there when there are bumps and groves in the roads.

oh yeah i had 2 monoform, 1 yokohama & 1 dunlop onmy steelies, 2 of them buckled. so i know what you mean by 5h|t.
but now it's a different story, 225/50......dunlop sp sport 3000a
and they are way better, in the wet and in the dry.
i thought the wider you went the better it is, is that bullsh|t or what?

i thought the wider you went the better it is, is that bullsh|t or what?

Narrow tyres tend to cut through the surface water and get contact with the road better than wide tyres because the greater serface area of wide tyres trap more water under them, meaning they have to displace a larger ammount of water to get contact with the road in the same ammount of time.

While the quallity of the tyre makes a big difference, the above is the general effect of tyre width :D

What people seem to forget is that wider tyres do not give greater contact patch area onto the road. It changes the contact patch shape. When you go wider in size the patch is wider but shorter. This changes the handling and water dispersion characteristics.

The only way to increase contact patch area is to lower tyre pressure (to an extent), or increase the weight of the car.

What people seem to forget is that wider tyres do not give greater contact patch area onto the road. It changes the contact patch shape. When you go wider in size the patch is wider but shorter. This changes the handling and water dispersion characteristics.

You havn't been reading through the pistonsheads forums by any chance have you? :D

I only really have time to read AM forums these days as at least then the information is relevant. I used to frequent PerformanceForums, Autospeed (before they closed down and everyone moved to PF), CarAudioAustralia.

cool, it's just that there was an explination on tyre dynamics on that forum explaining exacttly what you said :D