Do I need spacers?

[msterling]

I am prepping my stock 88 Carrera for a little more serious AX this year. I plan to get some runs under my belt with the higher traction R-compound tire. I have 4 15x7 Cookie Cutters and Hankook 225/45-15 Z214 C51 tires. I will have alignment and corner balance redone and look for some more negative camber. The wheels are ET 23.3. Do I need spacers to move the rear wheels outboard? I see that 8" 911 Fuchs have ET 10.6 and I realize that narrower wheel makes a difference. I don't have tires mounted yet so I can't judge how they will sit.

[KTL]

You would be wise to space out the rears if you want to keep the car balanced the way it way it was previously. You can go with a 930 spacer and long 72mm studs to get the rear spaced out.


Assuming you started with 16x6 front, 16x7 rear

Stock SC/Carrera
1361mm front (53.58 in.) 6 in. front wheel with +36mm wheel offset
1367mm rear (53.82 in.) 7 in. rear wheel with +23.3mm wheel offset

With ET 23mm 7's on all four corners your track widths are theoretically as follows:
Add 1 inch of track width in front which = 1/2 in per side, when each front wheel width grows to 7in. wide

Add the reduction in positive offset when switching to the 7 in. wheel, which = 36mm - 23mm = 13mm additional track width increase per side = 26mm total.

So the net result is a 1 inch + 26mm additional outward offset of the wheels. Call it 51mm total.

Now you've got a front track of 1412mm and the rear track width remains the same at 1367mm. So the front is 45mm wider than the rear.

To deal with that, you should install at least the 930 rear spacers (28mm thick) on your 7's , for a total track width increase of 56 mm, and then you end up with the rear being 11mm wider than the front. That is, if you want the track widths to be nearly the same difference as it was originally before you changed the wheels.

Or, if you want the car more tail happy for AX, leave it as is. I don't know AX setup since i've been primarily road course driving my cars.

[msterling]

Kevin,

Thanks for the well thought out reply. I am actually going from 7x15 et23.3 Fuchs in front and 8x15 et10.6 in back. I think this means the fronts will be same but the backs will be narrower and have a lower ET. It doesn't change your math. It sounds like I need spacers and longer studs.

[newms]

It is very unusual to run same size tires f and r. Though you will enjoy the Lower gearing from those short tires. You don't need spacers for the wheel/tire to fit but the car might handle better with wider tires on the rear. Or get 205/50/15 up front and the 225/45 rear. The 911 is tail happy enough!

[msterling]

When I did the math I came up with about 1/2" narrower track on the rear when switching from 15x8 ET -10.6mm to 15x7 ET -23.3mm. Rim is 1 in. narrower but negative offset is ~1/2 in. more which would put the center of the rim 1/2 in. inboard and the outer edge of the rim 1 in.more inboard than with the 8" rim. The tires are the same 225 width and will bulge more past the 7" rim so the outer edge of the tread would be 1/2 in. inboard than with the 8" rims. I think there is plenty of room for this in the rear. A 1/2 in. wheel spacer would make the track the same as before. A 21mm spacer would widen the track by about 1/2 in.

Front wheels are changing from Fuchs to Cookies but rim width, diameter and offset (ET) are the same. The tires are wider and lower aspect than the current tires. New are 225/45-15 versus 205/55-15. I am hoping that the wider tires won't rub. Car is not lowered less than Euro ride height.

I am getting conflicting opinions on same size tires at 4 corners (225/45-15) or staggered (225/45-15 rear, 205/50-15 front). Same size will allow me to flip tires on rims and go front to back to even out the wear.

[newms]

Running spacers on the rear will help though what the car wants is more tire. If you do go spacers don't forget you need longer wheel studs.

[KTL]

I used to run 225/50-16 on the track with 16x7 or 16x8 all the way around. It let me juggle the tires onto any axle. I did it because I had the wheels on hand and the tires were cheap closeouts. I didn't do it because I thought it would be the fastest setup.

As you can see, I was dealing with a little bit of oversteer in some places on the track. It was still fairly quick though.

Octoberfest BHF 10-6-07 - YouTube