SoloRacer

I have a 1989 944 Turbo with the M030 Option. From what I read I have 7.5 x 16 wheels up front with a 65 mm offset and 9.0 x 16 wheels in the back with a 60 mm offset. If I go up an inch or two in rim size (17 or 18 inch) does my offset have to stay the same? What if the rim depth is different? I'm considering going to a set of 18 inch rims (18 x 8.5 and 18 x 10) but I don't know what offset I need. Even with the correct offset will these rims fit nicely on my car and not rub?

Also what size tire should I go with on a 17 or 18 to keep my speedo correct?

TomH

Try this site for off sets:

http://www.cantonia.com/wheel_offset.html

SoloRacer

Thanks Tom. I have already seen that site and that is where I got the offset for my car. My question was not so much what the stock offset on my 951 is but instead does the offset change when you change rim size. Do you have to buy rims with the same offset regardless of size? If not, what offsets are ok?

TomH

My bust. I'd say that if you don't go with the same off set then you have to deal with spacers. And from what I have seen those can get rather pricey.

keith

you can use your current wheels as a starting point to measure clearance for additional width.

Turn the fronts lock to lock and check it out for clearance.

Past that, Danno is the resident wheel fitment expert. I know that 18x10 and 18x12 will fit on these cars with careful measurements...

Danno

Ughhh, FORGET that table on the cantonia site for wheel offsets. It's from the 944FAQ and it's completely BOGUS!!! This table has caused more confusion about 951 wheels than nothing at all!

Ok, for a thorough understanding of rim widths, offsets and backspacing, read the "Measurement Basics" page under the 944-WheelsFAQ section of my website:
Racer-X website

SoloRacer, yes you are correct about there being a relationship between offset and rim-size. However, if you keep the offset the SAME as stock, but increase the width (7.5" to 8.5"), then you split the increase evenly between the inside and outside edges (maintain same bearing loads as well).

However, here's the catch in your situation. If you look at your front fender, the tire is pretty much flush with the fender. Your stock 7.5" front wheels have 65mm offset, while those 18x8.5" wheels have 50mm offset. What this means is that ALL of the extra 1" goes on the OUTSIDE the rim. Nope, it won't fit.

The rears, however, will fit. The extra 1" of width will be split with 0.70" going on the inside and 0.30" on the outside.

Here's a recent thread on using those same wheels on an '86 951:
http://forums.rennlist.com/scripts/rennforums/ultimatebb.cgi?ubb=get_topic&f=20&t=0000 95

This guy can actually use those wheels because his stock configuration has much less offset than yours, so he can make up the difference with spacers.

As for tires, here's some correct sizes for around 24.87" diameter:

225/45-17
255/40-17
285/35-17

225/40-18
255/35-18
285/30-18

SoloRacer

The wheels I'm looking at are:

8.5 x 18 w/163 mm offset w/225/35 tires
10 x 18 w/175 mm offset w/285/30 tires

My car is a 1989 944 Turbo w/M030 Option

Will these work? Also, what is all the "illegal" talk about 18 inch wheels on a 944 Turbo all about?

rangelica

FYI, Here is the Porsche Tech Service Bulletin on 18 inch wheels.
Rick
'86 951

18-INCH WHEEL INSTALLATION NOT APPROVED
Article Text
1987 Porsche 944
Tuesday, August28, 2001 11:31AM

ARTICLE BEGINNING

TECHNICAL SERVICE BULLETIN

INSTALLATION OF 18" WHEELS NOT APPROVED

Model All Porsche Models
Group 4
Bulletin No. 9305
Date April 15, 1993

Concern:

Retrofitting 18" wheels on Porsche vehicles which do not have 18" wheels as original equipment is NOT ALLOWED.

General Information:

Endurance tests have shown that equipping certain Porsche vehicles with 18" wheels in an application not already approved by Porsche Cars, N.A. may result in loosened bolts and /or threaded connections on the front wheel carriers. This, in turn, may cause fractures in these components. Fractures may also occur in the front strut mounts and in the body in the area of the rear suspension as a result of the installation of 18" wheels not already approved for installation for this specific application.
Such damage can occur without warning and the vehicle may become very hard to control/steer as a result of the improper installation.
Porsche dealer personnel coming in contact with a vehicle that has been retrofitted with 18" wheels in an application not approved by Porsche, should inform the owner of the potential for damage to their vehicle and loss of steering control. Porsche Cars, N.A. should also be notified of any vehicles retrofitted with 18" wheels.

END OF ARTICLE

Danno

I think you are referring to BACKSPACING here, the distance from the mounting-surface to the inside-edge of the rim. OFFSET is the distance from the mounting-surface to the centerline of the wheel. With those backspacing measurements, we have:

18x8.5" 163mm backspacing = 45mm offset
18x10" 175mm backspacing = 40mm offset

Nope, they'll stick out about 1" past the fenders, with about 2" of clearance on the inside. Also with these extreme differences in tires, you'll have a humongous amount of understeer.

Well, in certain countries with strict quality standards like TUV, retrofitting 18" wheels on Porsches that didn't come from the factory with them is indeed ILLEGAL. Check out these recent threads...

http://forums.rennlist.com/scripts/r...&f=15&t=001277
http://forums.rennlist.com/scripts/r...&f=15&t=001274
http://forums.rennlist.com/scripts/r...&f=15&t=001319

rangelica

Also,
Here is the Porsche Tech bulletin for adding 17" wheels.


Rick

'86 951
APPROVED WHEELS & TIRES: NEW SPECIFICATIONS
Article Text

ARTICLE BEGINNING

TECHNICAL SERVICE BULLETIN

17 INCH APPROVED WHEELS & TIRES

Model(s): 1987-On Porsche 944 (with M30 option) 1987-On Porsche 944S (with M30 option) 1987-On Porsche 944 T (with M30 option) 1988 Porsche 944 Turbo S
1989 Porsche 944 Turbo
1989-On Porsche 944 S2
Group: 4
Bulletin No.: 9303
Date: September 28, 1993

SERVICE INFORMATION

Concern

Installing 17" Porsche wheels on vehicles previously equipped with 16" wheels.

SERVICE PROCEDURE

Installation Information
1. Front and rear fender well lips must be reworked and/or checked for adequate tire clearance. See Fig. 1.

2. Suspension travel must also be checked. Parts such as brake lines, hoses and brackets must have adequate clearance.

3. New caster eccentrics, rear mounts and nuts must be installed in the front control arms. This change is necessary due to a change in caster specification on vehicles equipped with Porsche 17" wheels. The specification for vehicles equipped with 17" wheels is 3'-15'-45. See Fig. 2.

4. Vehicles must have a rear stabilizer bar.

5. When adjusting vehicles to new caster specification, all alignment values should be checked at that time.

6. When summer tires are used on vehicles with 17" Porsche wheels, snow chains cannot be used. If winter tires are used, only snow chains with Porsche Part Number 911 361 924 00 may be used.

7. Tire pressures for front and rear 17" Porsche wheels is to be set at 2.5 bar (36 PSI) at approximately 20'C (68'F).

8. When using this rim and tire combination, a 17mm spacer plate must be installed on the rear wheel hub. The customer must be informed that if the emergency wheel is needed, the spacer must be removed.

9. Not for Cabriolet vehicles.

APPROVED WHEEL & TIRE TABLE

Tire size Rim size Offset Position Note

225/45 ZR 17 7,t Jxl7 AH 65mm Front Summer tires
255/40 ZR 17 9 Jxl7 AH 55mm Rear (See installation
information
A through G.)

225/45 ZR 17 8 Jxl7 AH 52mm Front Summer tires
255/40 ZR 17 94.@ JX17 AH 68nm Rear (See installation
information
A through I.)

205/50 R 17 7;-i Jxl7 AH 65mm Front Winter tires
225/45 R 17 8 Jxl7 AH 52mm Rear (See installation
information A through G.)

NOTE: When using this rim and tire combination, a 17mm spacer plate must be installed on the rear wheel hub. The customer must be informed that if the emergency wheel is needed, the spacer must be removed.


PARTS INFORMATION TABLE

Description Part Number Quantity

Spacer Ring 951 331 611 02 2
Wheel Bolt 904 341 671 00 10

Wheel Nut 999 1 82 003 36 10
APPROVED WHEEL PART NUMBER TABLE
Size Offset Part Number

7;-i Jxl7 AH 65mm 96536212405
8 Jxl7 AH 52mm 96536212601
9 Jxl7 AH 55mm 96536212800
91.@ Jxl7 AH 68mm 96536212805


APPROVED TIRES AND SIZES (SUMMER VERSIONS) TABLE

Size Makes

225/45 ZR 17 Front Bridgestone Expedia S-01 Nl
255/40 ZR 17 Rear Continental Sport Contact CZ 91 NO
Pirelli P700-Z NO
Yokohama A 008P NO
Michelin MXX 3 NO or MXX NO Pilot SX


APPROVED TIRES AND SIZES (WINTER VERSIONS) TABLE

Size Makes

205/50 R 17 89 H M&S Front Bridgestone WT05
225/45 R 17 90 H M&S Rear Continental TS 750
Pirelli winter 210 Performance


SUSPENSION PART NUMBER TABLE


Suspension Parts

1. Rear Mount


Part Number Quantity Required
951 341 023 00 2 each


2. Eccentric 951 341 431 02 2 each
3. Nut (torque to 100 Nm) 900 910 115 02 4 each


END OF ARTICLE