Choosing the right wheel spacers
 

Hi Guys...

I recently purchased an 01 Boxster S. Okay, a bit of a lengthy one but I want to get this right the first time to avoid wasting money on un-useable spacers and I know some of you guys have a lot of experiences with running spacers on the boxsters..so I have included all of my workings so far

It is currently fitted with the following wheel and tyre combo:

Rear - 255/40/17 ET47 9 inch wide rims
Front - 205/50/17 ET52 7.5 inch wide rims

I have bought a set of Turbo twist wheels and tires as follows which I am hoping to fit:

Rear - 285/30/18 ET65 10 inch wide rims
Front - 225/40/18 ET50 8 inch wide rims

Obviously I will need spacers to make it fit but I also would like to fill the wheel arch out as much as possible...

So to get the new wheels to fit in the same place as the current ones I have calculated I would need spacers as follows:

Rear - ET65 - ET47 = 18mm spacers
Front - ET50 - ET52 = 2mm spacers

Now regarding filling the arch I have measured the current set up and the available space in terms of gap from the outer tread of the tire to the wheel arch...

Rear = 25mm
Front = 40mm

So this makes the spacers total as follows:

Rear - 18mm+25mm = 43mm
Front - 2mm+40mm = 42mm

Now considering that the new wheels are wider by 25.4mm (1inch) at the back and 12.7mm (0.5 inches) at the front, this would mean that they will fill the arches by a further half of these amounts and so we should take off 12.7mm from the rear spacer and 6.35mm from the front spacer. So our totals are now as follows:

Rear - 43mm-12.7mm = 30.3mm
Front - 42mm-6.35 = 35.65mm

And now to shave off 3mm for tolerences and safety

Rear - 30.3mm-3mm = 27mm (rounded)
Front - 35.65-3mm = 33mm (rounded)

So my calculations are resulting in 27mm at the rear and 33mm at the front...

Does this look right or have I miscalulated somewhere? I feel like the front spacers would be way to big at 33mm as others have only used 15mm...would I run in to issues turning the wheels? Also does the rear seem right...

I am also wondering whether I should be factoring the tires in to my calculations as the tyre sidewalls are sticking out wider than the rims themselves?..

Thanks in advance

Wow, that's a lot of calculating! Your prospective fronts are about stock. I would add a 10 or 15 mm spacer to push them out a bit. Don't go overboard as this could cause handling problems. You need 18 mm to bring your rears out to stock. Again add on 10 to 15 mm to push them out a bit, so roughly 30mm should do the trick for the rears.

Thanks NewArt.

I have read of some guys going with 23mm spacers at the rear, so in my case could I not go with 18mm to bring me out to stock + 23mm to fill the arch, giving a total spacer of 41mm at the rear?

Do I also need to consider the fact that my new set up is running 10" wide with 285 tyres, or is that irrelevant?..

No, it is not irrelevant. You are adding 20mm over stock tire width, so that's 10mm on the outside. There's more to this than just the aesthetics. Changing the effective offset of a wheel will also affect your suspension geometry and could cause handling problems. I stand by my previous recommendations.

also too large spacers would increase early bearing failure..

If intend to drive the Box you will not like the 285 tires.

Clearly this is more to 'show'....

i drive plain o 2.5 hard and 255 rears are plenty, and I won't think about 18" unless I can reduce overall sprung weight and in fact my 97 is not designed for 18" anyway so little temptation.

And spacers will change suspension geometries/ scrub, effectively soften the spring rate and add more load to wheel bearings. Winder tires pushed out will add more wind drag at speed too.

It will look aggressive I'm sure but you would need to live with functional compromises when you are inside the car and actually driving it.