yow

I'm possibly either going to repair or replace my front rims, which may be bent. (I'm getting vibration.) If I replace, I want to go wider. The previous owner of my c6 widebody rear put z06 spyder wheels from www.factoryreproductions.com on there. See link -

http://www.factoryreproductions.com/...n-Grey-c78.htm

He used z06 size in the rear but base c6 in the front (18x8.5). If I replace the fronts, I would rather go wider because I HPDE my car. Would I use the +40mm offset or +57mm offset? What's the difference? Enlighten me, please.

What's the widest you can go up front? I wouldn't mind exploring the idea of getting a square set-up for the track at some point. Would it be possible to go 18x11 all the way around or is that too wide in the front?

Kubs

Offset refers to the mounting surface (where the wheel meets the rotor/hub) in relation to the centerline of the wheel, in millimeters. An easy example is, if you have a 10" wide wheel and a 0 offset than the mounting surface is on the centerline, or at 5" of backspace (the distance from the back edge of the rim and the mounting surface). If that same 10" wheel has a +25 offset, then the moounting surface moves 25mm towards the face, or outer portion of the wheel and the backspacing is now 6". If it has a -25 offset, the mounting surface moves toward the back of the wheel and has a 4" backspacing.

Basically, whatever the offset is you have to add or subtract that amount to 1/2 of the wheel width to find out where the back of the wheel will be, and to see if it will rub.

One other thing you must remember, is that wheel widths are measured INSIDE the lips where the tire sits, and does not account for the thickness of the lip. For instance the stock 18x8.5 front wheel you mentioned is actually 9.5 inches wide outside to outside. You need to account for that in your backspacing measurement.

dbratten

I seem to recall that backspacing includes the lip of the wheel as a straight edge is placed on back of the rim and the measurement taken to the mounting face. So, in your example of a 10" wheel with zero offset wouldn't it have a 5.5" backspace?

--Dan

Kubs

Read the last paragraph I wrote, I addressed that as well. The 10" wide wheel example did not specify if it was 10" overall or 10" in between the lips, it was purely to introduce the concept of offset.

dbratten

I did read the last paragraph but in the first paragraph you gave the backspacing as 5". Wheels aren't sold as 10" wide meaning they are actually only 9" inside the edges. I get that you know what you're talking about but this was confusing.

Scooter70

http://www.1010tires.com/Tools/Wheel-Offset-Calculator

This is a great calculator to determine how changes in offset will affect clearance to the fender and suspension. It also helps to visualize if you don't completely understand the concepts of backspace and offset.

Kubs

The intent was to introduce the variables one at a time. The first example was made with only the concept of offset to show what effects it had. Then by introducing backspacing, it becomes more obvious how the two relate, rather than throw everything in at once.

I have seen wheels sold a number of ways. Some would list a 18x10 wheel with 0 offset and 5" backspcing, and some would list the same 18x10 wheel with 0 offset and 5.5" backspacing.

yow

So...

Will the 18 x 9.5 +57mm fronts fit on my base c6 front?

dbratten

Yes.

In this case the 9.5" wheel has close to the same offset as the OEM 8.5" wheel (+56 mm) meaning that the 9.5 sits .5" farther to the outside and .5" farther to the inside as the width is split as Kubs explained.

For race tires I run a 18x10.5" front wheel with a 58 mm offset giving 1" more to the outside and to the inside.

--Dan

yow

Thanks!

yow

Is this on a base c6? If so, I'll get an 18x10.5 for mine, for sure.

dbratten

Yes, on my base C6. BUT ... at full lock the rim rubs against the tie-rod and the lower a-arm (due to the 1" additional backspacing) so you need to be aware of this in parking lots and pit areas. I use these larger wheels for 305x30x18 race tires although they also get run on the street getting to and from events.

--Dan

yow

Great! Thanks for the info. I just ran Atl Motorsports Park and am disgusted with my narrow fronts on street tires (NT05s). I plowed a bunch. Next up is some slicks on 10.5s.

froggy47

Don't forget some neg camber goes a long way with a pushy car.

0Anthony @ LGMotorsports

Cool little tool to let you compare..

http://www.rimsntires.com/specs.jsp

yow

Do your 305s rub the front fenders? I am thinking of buying c5z rears that already have 315/30/18s. Will those rub the fenders?

Thanks!

dbratten

Only in the most extreme situations -- like turns 6 & 10 at Laguna Seca where it dips into the apex.

But, rubbing will depend on a lot of things. I have COs which are a heavier spring than stock. I have heavier sway bars. Negative camber will tuck in the top of the tire and give clearance. And, of course, your ride height matters. So, mine aren't a problem, but YMMV. That said, many run the 18x10.5s on the front with 295 to 315 tires.

--Dan

yow

Great! Thanks for the input. I owe you big time.