wheel offset and effect on car geometry

Question

I have a Subaru WRX which is about to have Brembo calipers and disks. The OEM wheels at 17x7J and 55 Offset do not clear the Brembo calipers.

After spending days measuring clearance from wheel hub, brake disk, center of hub, etc, I consulted with a few wheel manufacturers and was told that my best (and safest) bet is to go for a 17x7.5J at 35 or 37 Offset.

Checking on online offset calculators, I can see that:

1. the outer rim will poke 24.4mm more than before
2. the inside rim will move 11.7mm further away from the suspension strut

Aesthetics aside (I'm doing this to clear the brakes and for performance on road and track) the new wheels are actually lighter (forged) and the tires will be the same.

So what I'd like to understand is how much of an effect to the car geometry will a 11.7mm move away from the suspension will have, as in will it be positive, neutral or a negative effect.

Secondarily:

• will it affect handling (e.g., loss of grip with less warning)
• will it affect performance (e.g., better grip due to being slightly wider)
• will it affect component wear (e.g., wheel bearings or ball bearings)

I know that the same year Subaru STi, has a 17x7.5J at 53 Offset, and a slightly different design to clear Brembo brakes. Which in effect would mean:

1. 4.4mm closer to the suspension strut
2. wheel will poke out 8.4mm more than before

So there is a clearly shift in geometry, although I'm not sure if it is significant to worry about?

EDIT

I guess KPI/SIA and scrub radius will be affected. I don't know if the current scrub is negative, but I presume that by changing wheels it will be moved towards positive.

I'm not sure how it will affect SIA, or if it will at all. The fact that Subaru has made wide-track versions, and that the WRX has the narrowest track of all the performance versions, makes me think that the effect might be more not so grave?

The car is currently on OEM caster, camber and toe at:

• Camber -0.45 front left, -0.47 front right
• Caster 3.28 front left, 3.06 front right
• Toe -0.03 front left, 0.01 front right

EDIT - UPDATE

I went ahead and ordered 2Forge 17x8J with ET37 Offset. As suspected, the Wagon fenders are too small, even with severe pulling they simply won't fit the wheels. So the STi archer liners and fenders are needed. However, the wheels easily clear the Brembos (with about 15mm left to spare) and the handling has increased quite drammatically: - The car feels considerably steadier around corners, it has a very evident more planted feeling - The wheels sometimes tramlime (this was expected and doesn't bother me) - There is no rubbing (with fender folding and pulling) the rear wheels cleared the fenders with folding and some light pulling. - The car should now have a similar track width to an STi which has a slightly width (I believe 10mm each side) and whereas the new wheels offset inside (towards the suspension) only +5mm the outside offsets by 23mm Overall a very positive experience, even if it means changing fenders and arch liners. Steering wheel has become slightly heavier.

Answer 1

This is something I've thought about quite a bit. The biggest change involved here will be the scrub radius. I'm going to be a bit lazy here and quote myself from a previous answer.

To put it succinctly, scrub radius is the distance between the center of your contact patch and the intersection of the steering axis on the ground surface. The interaction between these two points works like this: any (longitudinal) frictional forces that act on your tires (ie. acceleration and braking) will result in a moment around the steering axis, and can affect stability and handling characteristics under certain conditions. In simpler terms, whenever you accelerate or brake, the friction of the tires will try to make them turn inwards (towards each other) or outwards, depending on the scrub radius. Therefore, a change in scrub radius will affect this behaviour.

Positive scrub radius is when the contact patch center is further outboard than the steering axis point, and negative scrub radius is the opposite. In addition to this, scrub radius can affect steering feedback - higher scrub radius provides more feedback, whereas zero scrub radius can make the steering skittish or unpredictable. Zero scrub radius can also increase steering effort when stationary. On the other hand, high scrub radius can result in tramlining, an effect where the tires have a tendency to follow grooves in the road.

There are more intricacies when it comes to high performance handling, and quite frankly it's hard to remember all the details, so I recommend that you look a little into this yourself. Racing Aspirations has a pretty good article on it here, to start (note that FWD and AWD will behave differently than the RWD that they describe here, I'll try to find a better article that covers them all). Generally speaking, most road cars will have negative scrub radius for stability, so when you increase your wheel offset, this can decrease your scrub radius (amount), or possibly make it positive. Despite all this, the effects may not be as dramatic as it all sounds. Plenty of people install wheels of varying offsets all the time, so this may only be a big deal to people building race cars or not using power steering.

As you've mentioned, decreasing your wheel offset will also affect your track width and bearing wear. I don't have any experience with busted wheel bearings, but I'm inclined to say there isn't much to worry about unless you're going with extreme offset decreases or your particular car is known for bad wheel bearings. The track width increase will decrease the amount of body roll on that axle when cornering. If you are only changing the offset on your front wheels, then this will very slightly decrease understeer or promote oversteer.

With all that said, I noticed in one of your comments that your car is lowered on coilovers. Lowering your car on coilovers has a much bigger effect on your suspension geometry than wheel offsets, so if you went ahead with that already, then I would say you shouldn't worry about this. Lowering your car changes your car's center of gravity, and depending on the type of suspension, can completely change your roll center, which in turn affects body roll, camber change on bump/while cornering, and jacking forces. Aaand this post is already getting ridiculously long, so I'll leave it at that for now.

Answer 2

Source the wheels from the STi, seems the logical choice.

I would suggest you get a full alignment done so that you make sure you get what you expect, as you have not mentioned caster or camber...