In a few month's I plan on purchasing a set of wheels. I am looking at the Rota line right now, specifically the Subzero.

I was told by a friend that Rota's are no good for autocross use because of the 40mm offset (my car is front wheel drive). He recommended that I go with something with a lower offset.

My question is,is there ay truth to this? Does it make that much of a difference, if so? Thanks.

 

i have rota Torques and i just did the sunriders auto x over the weekend and they were just fine.

 

Your friend has left out a huge chunk of the answer to your question.

Offset is the distance from the center line of the wheel to the boltface (the part that sits against the hub). A positive offset has the boltface closer to the outside of the car. A negative offset wheel is often called "deep dish".

Of primary concern to most people when considering offset is backspacing, if you choose a wheel that is wider than stock, then you'll need to adjust your offset to ensure that the tire doesn't rub on suspension components. The backspace is how much of the wheel/tire package is inboard of the bolt face. If your aftermarket package increases the backspacing too much, your tires will rub on your strut housings. Depending on wether or not you are also lowering the car and the width of tire that you are trying to fit, you may need to be concerned about rubbing on the fenders as well. If you are trying to get the maximum amount of tire shoved under your car, then getting just the right offset (using spacers or just buying the right wheel) will be key to your success.

Something to consider:
Stock xA wheels: 15"x5.5" +38 offset
Rota Subzero: 16"x7" +40 offset

5.5" is about 140mm, that puts the centerline at 70mm, a +38 offset leaves you with 108mm backspace
7" is about 178mm, putting the centerline at 89mm a +40 offset gives you 129mm backspace

Do you have 21mm of space between the inside of your factory tires and the strut housing to spare? Do you have 17mm of space between the outside of your tires and the fenders, even with the front wheels at full lock? If so then you can run this wheel.

Wheel offset is much more a matter of fitment than it is a matter of performance. There are very few people that would be able to capitalize on the performance gained by having track width that is a few mm wider.

I got my information regarding the stock Scion wheel size here. There's also a listing of different wheel and tire combinations that people have managed to shove under their Scions. Good luck.

 

Your offset requirements are determined by your suspension and where the manufacturer placed the hubs. (and to some extent, how they engineered the bearings to deal with the stresses put on them) So, the question is: What offset are your factory wheels?

It's perfectly okay to deviate from that by 5, maybe 10mm. Moving the tire outward (less negative offset), is preferred to get a wider track. I think the net effect is a reduction in body roll due to changing the geometry. (my brain hurts when I try to comprehend why)

If you deviate too far from the factory offset, you can cause premature wheel bearing wear.

Bottom line: Most FWD cars come with an offset somewhere between -40 and -50. Most FWD aftermarket wheels come in offsets of -35 to -42, any of which will work just fine.

If you're getting wheels that are strictly for autocross competition use, then you might want a more extreme offset. Why? Because they're going to be much wider than your street wheels, and if you don't change the offset, they'll scrub your struts, inner fenders, control arms, etc. For instance... if you wanted ultimate performance, you might fit a set of 13x8's... with an offset of -20 or so.

 

Wow, thanks a ton for the info.

I plan on running the 15 inch subzero, 6.5 indes wide. I don't think I will have any fitment issues as people have managed to squeeze much,much larger wheels under the car (19 inches) without problems.

The offset won't be too drastic of a change at all. The guy I did talk to was running Hoosiers with his xA, so that might be why he recommended such a low offset. These will be dual purpose wheels though (street/autocross), so I don't need something that drastic.

 

Loren's right about the wheel bearings by the way, but he's got the wrong sign on his offsets. FWD are typically high positve offsets, where historically RWD will have low or even negative offsets. You definitely do not want anything with a negative offset for your Scion. The 13x8 example he gave with a +20 offset would yield a 117mm backspace, only requiring that you give up 9mm clearance between the inside of your tire and the closest suspension component. Of course then you have to determine if a 13" wheel will fit over your brakes, most likely in the rear, hard to say for the front.

 

Hehe. You'll notice that Nunyo and I are referring to negative offset backwards from each other. He's more correct, though it USED to be the other way around. Somehwere along the line, the industry changed the way they look at things, I'm not sure why.

Check this out if you're curious:
http://realbig.com/detomaso/1998-02/1319.html

Whichever way your wheels are offset, making the number lower is bringing the rim closer to the centerline of the hub mounting flange!

 

Nunyo, I understand about backspacing, but I don't quite understand how you obtain how much space is available between the tire and the shock housing.

 

I usually reach back there and guesstimate based on finger widths. Calipers or even a ruler would be a more scientific approach. The width change that you're considering is pretty minor, I suspect that you'll be just fine.

 

It's also a good idea to turn the wheels all the way in each direction and check for other clearances, as well.

If you have 15x5.5 now with a 38 offset and you went with a 15x6 with the same offset, you would lose 1/4" of that clearance (1/2 of the added wheel width). If clearance becomes an issue, moving the offset outward will add to the clearance.

Don't forget to consider fender clearance if you start getting WIDE and pushing the offset out, though!