Cup wheels on pre-85.5 944, spacer size...?

[Paul Fraser]

Just got a set of cup wheels (cup 1s I think but the later thin spoke ones that you see on the 964 and 968 I think). The offset wheel calculator I used shows:

7J15 ET23.3 to 6J16 ET52 moves the inner front wheel 16mm inboard and the outer 42mm inboard
7J15 ET23.3 to 8J16 ET52 moves the inner rear wheel 42mm inboard and the outer 16mm inboard.
PS I know the rear offset on the 944 is 17mm but that is what the PET gives as the offset for the wheel (they are all the same).

So what size of spacers should I use? 30-40mm on the front (how will this affect handling?) and 20mm on the back?

Also noticed that the rolling diameter of the 16s is greater than the 15s that are on the car at present so I assume the speedo will underread? (Should make it about right, it is 5-8% over at the moment.)

 

[Fen]

That'd be Cup 2's then. There is no rocket science to offsets. If you have the offset for the wheels you bought, and know the correct offset for that size of wheel on your car then the difference is the spacer size you need in mm.

 

[Paul Fraser]

I don't think they are cup 2s as I know what they look like, but I'll post a pic later and we can see.
As per your comment about offset difference, this would give me 30 and 35mm, but this would leave the front wheel face 12mm inboard of its current postition and the rear 19mm outboard -- what would this look like cosmetically?
Also, how do 30mm spacers on the front wheel affect the handling of the car...?

 

[mik_ok]

are they Cup2s? I thought the 16" Cup1 was the thin spoke varient (?) [&:]

As per fen - don't worry about inner and outer edges - if your previous wheel was ET23 and you now have ET52, you've moved the centreline of the wheel 29mm inboard. You want a spacer as close to that figure as possible.

I'd be surprised if the rolling radius is significantly different - what are your before & after tyre sizes?

 

[mik_ok]

ORIGINAL: Paul Fraser

As per your comment about offset difference, this would give me 30 and 35mm, but this would leave the front wheel face 12mm inboard of its current postition and the rear 19mm outboard -- what would this look like cosmetically?

the rim is 25.4 mm wider. With the same offset (via a 30mm spacer) the inner edge of the rim will be 12.5mm (ish) further inboard, and the outer edge of the rim 12.5mm (ish) further outboard.

30mm spacers should allow the handing to stay (essentially) the same.

 

[mik_ok]

PS - the "face" of the wheel has no specific relation to the centreine of the wheel, or the ET number.

 

[Fen]

Doh! I didn't spot the 16" bit. There was only one 16" Cup design as mik says.

 

[Paul Fraser]

Photo as promised.

Fronts 6J16 ET52 944.362.112.00 with Uniroyal 205/55 ZR16
Rears 8J16 ET52 944.364.116.00 with Uniroyal 225/50 ZR16

Car currently on cookie cutters with Yokohama 205/55 R15 87V

Got the wheels for £260 on Ebay, which I thought was pretty good. Front tyres are good and so would the rears be but for the fact that they are down to about 2-2.5mm on the inside (uneven wear).

 

[mik_ok]

Well you want 30mm spacers all round (and personally I'd go for 35mm on the front, and 43mm on the rear ~ purely for the cosmetic appeal).

I thought the cookie cutters should have been shod with 195/65 tyres(?), so sounds like your previous tyres were non-std (?).

 

[Steven944]

original cookie cutter tyre size(for pre 85 cars) was - 185/70 VR15.

 

[Guest]

There are 2 points to your question. Handling and fit.

To get the handling characteristics the same, you want to get the centre of the wheel in the same place as the old ones. to move the centre changes the king-pin offset (the distance between an imaginary line from the upper and lower mounting points to the hub, extended down to the road, and the point on the road at the centre of the wheel.)

The problem with this, is that if you go with a wider wheel, this moves the inner and outer edges of the wheel inboard and outboard respectively. This scenario means that the outer edge of the wheel can rub on the arch during steering and/or suspension travel.

What you are aiming for is to get the wheel as far outboards as possible without rubbing on the arch. The chances are that the wheels on the car are already at the point where if you move them any further, then they will rub. So, given your figures, you want a 42mm spacer on the front, and a 16mm spacer on the rear. Moving a couple of mm extra probably won't make all that much differece if you can't find spacers with those exact numbers.

Yes, it will affect the handling slightly (increasing the king-pin offset will in theory make the respective axle more skittish), but given the increase in rubber, it'll probably offset this characteristic.

 

[Fen]

There's enough room to park a double decker bus between the tyre and the outside of the rear arch with 15" wheels.

 

[Guest]

so you want 42 & 16mm + bus width spacers then!

 

[Paul Fraser]

Thanks for all your responses: I'll have a think about what has been said and be back with more dumb questions when I get closer to fitting them.
PS those were the tyres that were on the cookie cutters when I got them. What difference does the 205 vs the 185 make?

 

[Steven944]

Hi Paul,

I put the 205s on just for the extra width/grip...and the slightly more 'rubber band lower profile look !' The rolling radius is the deciding factor(i.e. the distance travelled for one revolution of the wheel/tyre) the extra width of the 205 is compensated by the lower aspect ratio of the profile (55%), and this keeps the rolling radius/gearing/speedometer reading at the same level as the standard tyres.

all the best

 

[mik_ok]

The only problem is that a 205/55 doesn't equate to a 185/70 in terms of rolling radius ~ the gearing drops by just over 5% (even a 205/60 would be a smidge undersize)

the good news is that your 16"s with the tyres on them bring you back to the correct gearing.

 

[Paul Fraser]

Is that why my speedo is overreading between 5-10% then (or part of it...)?

 

[Fen]

5% over-read is about standard, more is possibly down to wrong tyre size.

 

[mik_ok]

I'd say a little more than possibly. []

 

[Fen]

Pedant [] Go check your email.