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

[mik_ok]

ORIGINAL: pikey7
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.

The latter point would be valid if you were trying to squeeze enormously wide rims in, but you aren't.

Hence the former point takes far greater importance, and to achieve this you need 30mm spacers all round. Unless you want a slightly "squatter" look, in which case go by my comments above. (The front can take an extra 5mm with no ill effects on the steering, and the rear can take a little more, 'cos it doesn't (usually!) steer).

 

[944turboS]

The front can take an extra 5mm with no ill effects on the steering, and the rear can take a little more, 'cos it doesn't (usually!) steer

Now if we were talking 911s this might be different.

 

[Guest]

Mike, The calculation on the front shows that with no spacer, the outer edge of the front wheel moves outboard by 42mm. by adding a 30mm spacer it'll move inboards by 12mm. This is half an inch inboards over standard! Pointless! Especially when the wheel as standard has a gap to the arch. It'll also fig you issues perhaps with the rear face of the spokes clattering into the brake caliper!. I'm sorry, but you're most definately wrong on that point.

The rear with a 30mm spacer will move the outboard face of the wheel 14mm further out than standard. While i'm not 100% familiar with a pre-85 car, 14mm seems like a lot to me, and may cause issues rubbing on the arch.

While I agree that getting the centre of the wheel in the same place SHOULD take precedent, if the wheels dont fit using this philosophy, then they don't fit. Period. You can argue with theory, but you can't argue with big lumps of steel.

Yes, you will only get rubbing on he suspension components using massively wide whels (IIRC 9's front and 11's rear on 944s), but this isn't the point of my post (If you'd quoted the whole lot)

 

[Fen]

This is all getting needlessly complicated.

The ET on a pre '87 car is of the order of 23, the ET of a post '87 car is of the order of 53, therefore there is a 30mm difference. That IS a big spacer, but on the same rim width it would leave both inside and outside rims in the same place relative to the arch.

There is plenty of room for 14mm more rim outset on a car that started out with 15" rims, but I don't know where that comes from. With a circa 30mm spacer you're only going to get 14mm more projection of the outer rim if the new wheel is 28mm wider than the old - that is a little over 1". As a guide I replaced 7.5" fronts and 9" rears on my Turbo with 8" fronts and 10" rears in an offset that pushed the outer rim out slightly. The arches are very full. but they don't rub.

Porsche made (as far as I know) the wheels pre and post '87 to sit in the same places but taking the different hub face into account, therefore the difference in ET between -'86 15" teledials and '87- 15" teledials of the same width is the exact spacer size needed to put later wheels on the early car with exactly the same fit as on the later car they were designed for. That means that by fitting a spacer as close as possible to that size you get the 16" cups (in this case) to fit exactly as they would have on the 968 they left the factory on. It is possible to tweak the position for aesthetic reasons by adjusting the spacer size. This is what mik is saying and I still think he is 100% correct. For info he runs small spacers of about 15mm on the back of his S2 with standard wheels, which clearly pushed his rim out by 15mm, and he has no problems (and knows what he is talking about).

 

[Guest]

While I see that the difference in offsets is clearly 30mm, are you saying that the change in wheel width makes no difference?

 

[Fen]

No I agree a change in wheel width makes a difference to the rim position. In fact half the difference in rim width on each side, so if you go 2" wider you will have 1" more inside and out, assuming the spacer and rim ET equate to the old ET.

 

[Guest]

OK. So in Pauls position.....

Assuming there is a 14mm gap between arch and rim as standard, does that not mean that to swap a 7" for a 6" front wheel with 30mm more offset means that:

+30mm to correct offset
+12.5mm (half of the inch reduction) to correct the width
+14mm to move the wheel to the arch (optional)
means a 42.5 (or 56.5)mm spacer is needed on the front

And again assuming a 14mm gap between wheel and arch as standard, and for the rear going from 7 to 8":
+30 to correct offset
-12.5 (half of the inch increase) to correct the width
+14mm to move the wheel to the arch
means a 17.5 (or 21.5)mm spacer is needed on the rear.

Does that add up?

 

[pae]

ORIGINAL: Paul Fraser

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

That's exactly the same size tyres I fitted (same style of wheels too) on my 924S . I think you'll find those sizes are near enough identical to the 'original' 195/65x15.

Mine was a late one that already had the 52mm offset hubs, but it could still afford more spacing even on the 924 body. So it's should be possible to space them wider.

Phil

 

[Paul Fraser]

Mike, I think you are overcomplicating it. If you go from a 23mm to 53mm offset the wheel moves inboard by 30mm. To get the wheel back to the 'correct' position you use a 30mm spacer.

As I am using a 6J rather than 7J wheel on the front, the outside and inner faces will be 13mm nearer the centreline of the wheel than previously, because it is not as wide. If I wanted to put the face of the wheel where it was previously for cosmetic reasons I'd use a 43mm spacer. This is question that really concerns me -- what will the front wheel face look like 13mm inboard from its present position? How will the car handle (answered previously).

As to the back wheels, using a 8J rather than 7J wheel means that the outside and inner faces will be 13mm farther away from the wheel centreline than before. I could correct back to the original face position by using a 17mm spacer instead of a 30mm one (and this will also move the inner face of the wheel 13mm further inboard). However, I wouldn't do this as the wheelface being 13mm outboard from previous will probably look better on the car.

Or is this what you are saying?

 

[mik_ok]

Mike,

Don't disagree with any of your numbers ~ I'm just coming to it with the following priorities :-

1/ Matching original offsets. This has to be the primary target to maintain overall "feel", loadings on bearings and (at the front) relationship to the steering pivot.

2/ Interference. If and only if the rim widths cause any interference with suspension or body, then you need to consider the inner and outer rim positions. There is so much room in a 944 arch that (even with an 8" rear rim) it should not be a concern - certainly not to the extent that you would consider compromising "goal 1" in order to maintain the original inner or outer rim position.

3/ Looks. As Fen states, I run std S2 Design90s with a 7mm front shim (so the fronts are effectively now 7J16 ET48) and 15mm rear spacers (so the rears are effectively now 8J16 ET37).
This places the outer edges of the wheels at the same positions as Turbo-sized Design90s. Give or take a mm or so.The above still allows lots of room to the outer arch (I know someone who ran 21mm spacers on the rear of their S2, so effecively 8J16 ET31 !) and the arch was close, but still offered enough space to avoid contact.
This is purely for aesthetic appeal, although I wouldn't have stuck with it if there were any ill effects on the handling or steering. The front could take a far larger spacer for example, but I didn't want to move the centreline of the wheel any further from the steering axis.

 

[Guest]

To my mind, by moving the wheel further out, not only are you improving the looks, but increasing the track. Increasing the track will in theory give you more stability.

With regards to the looks, by only adding a 30mm spacer, the front wheel will move in 13mm from standard. In my mind this'll look pretty stupid given that the gap is "already room to drive a bus through" By adding a 30mm spacer to the back, you'll be moving the rim 13mm futher outboards that standard creating a completely unbalanced look, and probably an unbalanced handling trait.

Yes, it will increase loads on the front bearings, but in my mid, it won't increase to an unacceptable limit.

Interference to the suspension parts is highly unlikely with a 6&8" wheel. I run 8&10"s and have no problem. However, I'm not convinced that the space from mounting face to caliper will be enough. In theory it will be, but I remain unsure. Put it this way, My front wheels are right at the arch, and I'm not too far away. Given that if Paul uses a 6" wheel with 30mm spacers, he'd have effectively 1" less offset than I have, it'll put the wheel unecessarily close to the suspension & brake components.

But each to their own. Moving them out would be the way I'd do it. Some people would do differently.

 

[Fen]

Remember Paul's brakes will be circa 30mm further inboard than yours, Mike.

 

[Paul Fraser]

Sidestream question: I've always been told that adding spacers 'knackers your wheel bearings'. Presumably, although using a 30mm spacer will correct my offset using the new wheels, the centre of gravity of the wheel moves out 30mm as well? -- I assume when the wheels are on the correct offset car the centre of gravity is over the wheel bearing?

 

[mik_ok]

Adding the spacers takes the wheel loads to (near as dammit) where they were previously - so no change to bearing loads.

Going beyond this (taking the wheels further out) does increase the load on the bearings, but to be honest I have no idea how much more quickly they will wear.

Mike - I do realise what you are trying to achieve, and don't disagree that 30mm spacers all round will leave the rear wheels visually "further oot" than the originals, and the fronts "further in", but with a mere 6" rim on the front there isn't a heck of a lot you can do to address this before you start running into problems (imho).

The ET number describes the relationship between the wheel centre and the wheel's hub mounting face. The position of wheel "spokes" and the outer wheel face are independant of this (although there are some constraints obviously!) so there is never a cast iron guarantee that the brakes will be clear of the inner spokes - even if you move from one wheel design to another but don't change wheel size or offset. Suck it and see or gain the knowledge from others who have sucked it and seen seems to be the only way.

 

[Fen]

This is lifted from the 968UK forum where I know not many of you will see it. The numbers involved refer to the 968 which is the same as the '87- 944. Hopefully it explains things (and hopefully you don't mind me borrowing it if you see this, Steve).

Just so you know, the correct width and offsets for 17" Cup1's and Cup2's as fitted to a 968 are ...
Fronts - 7.5" wide with an offset of 65
Rears - 9" wide with an offset of 55

For the standard 16" rims as fitted to 968 Coupes, these were
Fronts - 7" wide with an offset of 55
Rears - 8" wide with an offset of 52

All offsets (et) are measured in millimetres.
The attached two digarms explain all ...

You can fit different widths and offsets but you need to ensure that the wheel doesn't foul anywhere, particularly when on full steering lock.

 

[Paul Fraser]

Very useful, thanks a lot.

 

[pae]

Just so you know, here's the room available when you fit an 8" rear wheel (52 offset) with 225 tyre to a 924S. No spacers. Should be plenty of space on a 944.

Phil

 

[pae]

And just one more because it looks so lovely [] [] . (but I've sold it now....[] )

Phil