NEW WHEELS FOR YOUR 928 Phil Chadwick. p
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1. The very low profile tyres required for 17 wheels were just too expensive a common outcome when you try to increase metal wheel diameter by more than 2 Had tyre price been a really serious issue I would likely have settled for 15 wheels I found plenty of 16x7 16 diameter 7 wide and some 167 5 third party new Golf wheels advertised the Internet with tyre sizes up to 225 40 16 This set my nominal upper limit on the width of 16 tyres for wheel arch clearance at 225 mm To go further would have required professional advice and fitment had a small budget and wanted to stay within the common commodity wheels and tyre sizes I then got fitment data from http www tyresave co uk fitment html The summary for the Golf was Golf III PCD 4x100 Offset 32 38 C Bore 57 0 All wheel suppliers have these tables These data were essential to figure out what fitment was required for the Golf and what other cars could supply used wheels for the Golf The Golf requires a wheel with a centre bore of 57 mm i e a hole of 57 mm diameter in the centre of the wheel This is not an issue with most commodity wheels The manufacturers make such wheels with a large centre bore and produce removable adaptor rings for each type of car that the wheel is likely to fit The Golf pitch circle diameter PCD is 4 x 100 4 studs centered on a circle 100 mm in diameter a very common setup for small front wheel drive cars The Golf wheel offse2. 275 40 17 s on a 9 rim with a 55 mm offset You can go a fair bit wider than this If you can get professional advice and or are able to experiment with offsets the best starting point I have is sourced from Kim Crumb Rennlist mailing list Offset Max Tyre Width Front 65 to 68 mm 265 mm Rear 52 to 60 mm 285 mm Table 2 Offsets for Maximum Tyre Widths The widths shown above are the maximum possible for a standard outside tyre diameter of 25 for non GTS models by custom fitting wheels without modifying the guards Table 2 tells you every thing you need to size your new wheels and tyres provided that you maintain the sia basic assumptions namely the outside diameter of 928 tyres needs to be very close to 25 that 9 5 is the absolute rim width limit at the front that 11 is probably the absolute rim width limit at the rear and also provided that the tyres are wider than the rims More recent Rennlist postings by Kim Crumb and David Roberts have been more specific In particular Dave posted saying Here is what I currently use on my street 90GT with no clearance issues and ride height below spec 9 5 x18 front with 68 5 mm offsets and 255 35 18 tyres 11 x18 rear with 54 mm offsets and 295 30 18 tyres This information allows us to derive some useful clearance limits on late model S4s and GTs with factory rolled guards as long as the tyres are always wider than the rims Th
3. Note that changing the overall tyre diameter will change the gearing of your car and the accuracy of the speedometer A 2 5 variance in the diameter is usually acceptable but check with your local authorities to determine what is legal and remember that your speedo will read low if the tyre diameter is increased See Appendix II for a list of tyre sizes comparable to the original 928 equipment Ey BMW 7 series devotee Chris Koch http www bmwseven com advised tire sizes such as 245 45VR16 and the mathematical relationship the sizes indicate hasn t been true for at least a decade You must ignore the math of 245 mm wide with a sidewall height of 45 Instead you must follow the published specifications for each tire size within each tire model and pay particular attention to the revs mile This is good advice and should be followed where ever possible Revs mile is a much more accurate comparator for tyre diameter than the nominal geometry Sadly revs mile published specifications may not always be available for the factory fit tyres in your User s Manual 13 Scrub Radius STEERING oo jm TRUE VERTICLE INCLINATION iala INCLUDED M hiu ci ANGLE Sez Ba SCRUB RADIUS Figure 2 Included Angle Steering Axis Inclination and Scrub Radius Source http www familycar com alignment htm Steering Axis Inclination SAI is the measurement in degrees of the steering pivot line when v
4. like I did If I didn t own a 928 I d say the Golf handles pretty well 2152 19 APPENDIX II COMPARABLE TYRE SIZES FOR PORSCHE 928 Table 4 shows the original equipment Porsche 928 wheel and tyre sizes The table below shows other fitments within 3 25 of original equipment size for every tyre in Table 4 You may use this table to find all the close fit tyres for your car Tyre sizes are expressed as width profile diameter e g 215 60 15 means that the tyre is 215 mm wide 60 of 215 mm 129 mm high and made to fit on a wheel 15 in diameter The formula for tyre diameter is outside tyre diameter wheel diameter width x profile x 2 The height of the sidewall is width x profile i e the total diameter of the tyre is the diameter of the metal wheel plus two times the height of the sidewall Note that tyre width is in millimeters and wheel diameter is in inches so we convert sidewall height from millimeters to inches by dividing by 25 4 Also profile is always expressed as a percentage so we have to divide by it 100 to use it as a fraction For 215 60 15 tyre outside tyre diameter is 15 215 x 60 100 x 2 25 4 25 16 The tyre sizes below are generated form a formula They are a guide to obtain a starting point Please be sure to independently check that tyres you want may be safely and legally fitted to your rims and are actually made in the size you want Good tire size calculat
5. 15 225 45 17 diam 24 97 0 76 235 45 17 diam 25 33 0 68 245 45 17 diam 25 68 2 07 205 50 17 diam 25 07 0 36 215 50 17 diam 25 46 1 19 225 50 17 diam 25 86 2 78 205 55 17 diam 25 88 2 86 235 35 18 diam 24 48 2 70 245 35 18 diam 24 75 1 63 255 35 18 diam 25 03 0 52 265 35 18 diam 25 30 0 56 205 40 18 diam 24 46 2 78 215 40 18 diam 24 77 1 55 225 40 18 diam 25 09 0 28 235 40 18 diam 25 40 0 95 245 40 18 diam 25 72 2 23 205 45 18 diam 25 26 0 40 215 45 18 diam 25 62 1 83 225 45 18 diam 25 97 3 22 225 50 16 24 86 265 40 16 diam 24 35 2 05 275 40 16 diam 24 66 0 80 235 45 16 diam 24 33 2 13 245 45 16 diam 24 68 0 72 255 45 16 diam 25 04 0 72 265 45 16 diam 25 39 2 13 215 50 16 diam 24 46 1 61 225 50 16 diam 24 86 0 00 235 50 16 diam 25 25 1 57 245 50 16 diam 25 65 3 18 215 55 16 diam 25 31 1 81 265 35 17 diam 24 30 2 25 275 35 17 diam 24 58 1 13 225 40 17 diam 24 09 3 10 235 40 17 diam 24 40 1 85 245 40 17 diam 24 72 0 56 255 40 17 diam 25 03 0 68 es Tyre Size Diameter Other fitments within 3 25 of original 265 40 17 diam 25 35 1 97 275 40 17 diam 25 66 3 22 215 45 17 diam 24 62 0 97 225 45 1
6. 175 mm 205 mm 8 175 mm 205 mm 235 mm 9 195 mm 230 mm 265 mm 10 215 mm 255 mm 290 mm 11 240 mm 280 mm 320 mm Table 1 Optimal Tyre Width for Various Rim Widths Make sure that you check what s recommended by the tyre manufacturer and also what s legal and recall that the absolute maximum possible tyre width with un flared guards is 265 mm front and 295 mm rear 8 Wheel Offsets Offset is the distance between the plate in the centre of the wheel that bolts onto the hub called the mounting pad and the imaginary plane which bisects the tyre into two equal doughnut shaped halves See Figure 1 Offset determines scrub radius on the front of the car see Scrub Radius below In conjunction with tyre width offset also determines both inner and outer wheel arch clearance of both front and rear tyres Cars almost always have positive offsets i e the metal wheel is roughly saucepan shaped without handle or lid with the base of the saucepan facing outwards Negative offsets which are common on trucks and 4 wheel drives occur when the mounting pad plane is closer to the centre of the car than the centre line plane i e the bottom of the saucepan is pushed in towards the vehicle more than half way I have from reliable here say Ed Ruiz Rennlist mailing list that the following work on ordinary 928s i e without the flared guards of the GTS front 235 45 17 on an 8 rim with a 70 mm offset and rear
7. bolts already on the car are pretty ugly when exposed by removing the hub caps Also they don t fit into the bolt head recess in the new mags I bought 16 chrome bolts from one of the local wheel retailers The other slightly cheaper option was to buy 16 studs and lock tite the chrome nuts to the studs to form a bolt However this would have been less convenient less reliable less safe and maybe not legal I was lucky to be able to buy high tensile chromed M12 1 5 thread bolts off the shelf to fit the wheels onto the car These bolts are commodity in this size because so many small cars use them However since wheel attachment hardware is rated high tensile and custom made high tensile hardware is very expensive you would be very wise to sort out the attachment hardware needs and costs before committing to buy wheels The new wheel attachment hardware adaptor rings and chrome bolts cost me Aus 100 That did not include locking nuts to deter thieves 14 I should have waited for the new Koni shocks to arrive so that the wheels could be balanced and aligned at the same time as the shocks were fitted But I couldn t wait The rear wheels required a 6 mm thick washer to be inserted between the wheel and the 3 wide plastic Viper decorator cap in the centre of the wheel I used 1 mm gasket material from Super Cheap Auto to make the washers 6 layers Check for this lest you crack the cap when bolting on the wheel
8. 18 235 35 18 diam 24 48 0 81 245 35 18 diam 24 75 0 28 255 35 18 diam 25 03 1 42 18 Tyre Size Diameter Other fitments within 3 25 of original 265 35 18 diam 25 30 2 51 235 40 18 diam 25 40 2 92 225 45 17 24 97 265 35 17 diam 24 30 2 68 275 35 17 diam 24 58 1 56 235 40 17 diam 24 40 2 28 245 40 17 diam 24 72 1 00 255 40 17 diam 25 03 0 24 265 40 17 diam 25 35 1 52 275 40 17 diam 25 66 2 76 215 45 17 diam 24 62 1 40 225 45 17 diam 24 97 0 00 235 45 17 diam 25 33 1 44 245 45 17 diam 25 68 2 84 215 50 17 diam 25 46 1 96 265 30 18 diam 24 26 2 84 275 30 18 diam 24 50 1 88 225 35 18 diam 24 20 3 08 235 35 18 diam 24 48 1 96 245 35 18 diam 24 75 0 88 255 35 18 diam 25 03 0 24 265 35 18 diam 25 30 1 32 275 35 18 diam 25 58 2 44 215 40 18 diam 24 77 0 80 225 40 18 diam 25 09 0 48 235 40 18 diam 25 40 1 72 245 40 18 diam 25 72 3 00 215 45 18 diam 25 62 2 60 255 40 17 25 03 265 35 17 diam 24 30 2 92 275 35 17 diam 24 58 1 80 285 35 17 diam 24 85 0 72 295 35 17 diam 25 13 0 40 245 40 17 diam 24 72 1 24 255 40 17 diam 25 03 0 00 265 40 17 diam 25 35 1 28 275 40 17 diam 25 66 2 52 245 45 17 diam 25 68 2 60 2
9. 2 3 928GTS Cup 1 72 x17 225 45ZR17 65 0 1992 1993 9 x17 255 40ZR17 55 0 35 mm 928GTS Cup 2 7 2 x17 225 45ZR17 65 0 1994 1995 9 x17 255 40ZR17 55 0 35 mm Option M347 7 Slot Forged 7 x16 225 50VR16 65 0 928S Platinum Anodized 7 x16 225 50VR16 65 0 Option M393 Club Sport Forged 8 x16 225 50VR16 60 0 928S4 GT Magnesium 9 x16 245 45VR16 60 0 17 mm Option M395 7 Slot Forged 7 x16 225 50VR16 65 0 928S Silver Anodized 7 x16 225 50VR16 65 0 Option M400 Design 90 7 2 x16 225 50ZR16 65 0 928S4 GT Cast Aluminium 9 x16 245 45ZR16 52 3 Option M401 Phone Dial Cast 7 x16 225 50VR16 65 0 928 Aluminium Alloy 7 x16 225 50VR16 65 0 Option M415 928S spacers 21 mm Table 4 Original Porsche 928 Tyre Fitments 10 Determination of Offset Genuine Porsche wheels have the offset cast on them Look near the valve seat for markings like ET65 7 5 which indicates a 65 mm offset and a 7 5 wide wheel The ET stands for Einpress Tiefe offset Theo Jenniskens Rennlist mailing list You may also find cast markings on the inside of the wheels the area hidden by the spokes that s very difficult to clean It is possible to measure the offset with the wheel removed from the car You will need a good straight edge and a 3 foot rule or metal tape measure place the wheel on the ground street si
10. 65 30 18 diam 24 26 3 08 275 30 18 diam 24 50 2 12 285 30 18 diam 24 73 1 20 295 30 18 diam 24 97 0 24 245 35 18 diam 24 75 1 12 255 35 18 diam 25 03 0 00 265 35 18 diam 25 30 1 08 275 35 18 diam 25 58 2 20 245 40 18 diam 25 72 2 76 19 20 ACKNOWLEDGMENTS Original Porsche 928 wheel and tyre sizes http homepages apci net slyjay Pagel10 html What you need to know about wheel fitment and the terms used www halibrand com surefit fitment pdf http www geocities com johnsautos WHTY html Generic fitment data for a wide variety of vehicles http www tyresave co uk fitment html but their offset data on the Porsche 928 is wrong Graham Bates Australian landshark mailing list Math Creemers 928uk mailing list Kim Crumb Rennlist mailing list Glenn Evans Australian landshark mailing list John Cumming Australian landshark mailing list Martin Dunks Rennlist mailing list Mark Edwards Australian landshark mailing list Greg Gray Australian landshark mailing list Doug Hillary Australian landshark mailing list Jon Holdsworth 928uk mailing list Theo Jenniskens Rennlist mailing list Michael Kelly Australian landshark mailing list Chris Koch pers comm Mark Moss 928uk mailing list John Pirtle http members rennlist com pirtle keepers html David Porter 928uk mailing list Mike Readman 928uk mailing list David Rober
11. 7 diam 24 97 0 44 235 45 17 diam 25 33 1 89 215 50 17 diam 25 46 2 41 265 30 18 diam 24 26 2 41 275 30 18 diam 24 50 1 45 225 35 18 diam 24 20 2 65 235 35 18 diam 24 48 1 53 245 35 18 diam 24 75 0 44 255 35 18 diam 25 03 0 68 265 35 18 diam 25 30 1 77 275 35 18 diam 25 58 2 90 215 40 18 diam 24 77 0 36 225 40 18 diam 25 09 0 93 235 40 18 diam 25 40 2 17 215 45 18 diam 25 62 3 06 245 45 16 24 68 295 35 16 diam 24 13 2 23 255 40 16 diam 24 03 2 63 265 40 16 diam 24 35 1 34 275 40 16 diam 24 66 0 08 285 40 16 diam 24 98 1 22 295 40 16 diam 25 29 2 47 235 45 16 diam 24 33 1 42 245 45 16 diam 24 68 0 00 255 45 16 diam 25 04 1 46 265 45 16 diam 25 39 2 88 235 50 16 diam 25 25 2 31 295 30 17 diam 23 97 2 88 255 35 17 diam 24 03 2 63 265 35 17 diam 24 30 1 54 275 35 17 diam 24 58 0 41 285 35 17 diam 24 85 0 69 295 35 17 diam 25 13 1 82 235 40 17 diam 24 40 1 13 245 40 17 diam 24 72 0 16 255 40 17 diam 25 03 1 42 265 40 17 diam 25 35 2 71 235 45 17 diam 25 33 2 63 255 30 18 diam 24 02 2 67 265 30 18 diam 24 26 1 70 275 30 18 diam 24 50 0 73 285 30 18 diam 24 73 0 20 295 30 18 diam 24 97 1
12. NEW WHEELS FOR YOUR 928 Phil Chadwick p chadwick ugq net au Revision 7 5 1 Caveat I have presented the information below in good faith However I take no responsibility for errors omissions or fitness for any purpose This paper is intended to be a guide for what might be possible It is your responsibility to determine what is safe and what is legal Caveat emptor Tyre and wheel suppliers should be able to provide assistance though the local rule makers e g Transport Department or enforcement entity e g Police may also need to be consulted You should also contact your insurance company to see what they will and will not cover 2 Preamble This is a work in progress New versions will be issued as I gather new information Please email me with any feedback you may have I am particularly grateful to the many contributors The Acknowledgments are summarised at the end of the paper 3 Introduction The issues involved in fitting both standard and non standard wheels to a Porsche 928 are discussed The following are considered pitch circle diameter PCD wheel centre bore diameter and width offset and its measurement original wheel and tyre sizes the use of spacers tyre size and profile scrub radius sources of replacement wheels and issues with the wheel lug nuts Appendix I looks at the principles of wheel fitment by practical application It is applicable to
13. a hight of 45 of 245 mm 110 25 mm If the tyre was a 225 50R16 it would be 50 of 225 mm 112 50 mm This two digit figure is referred to as the tyres series For example 45 series tyres Next is usually the letter R This identifies the tyre as having a radial ply construction If it was the older style cross ply construction it would be identified with the letter D With the prevalence of radial ply tyres it is not unusual to see the speed rating see below in place of the R e g 245 45ZR16 Next is the diameter of the metal rim Our example size was a rim diameter of 16 Rim diameters are always measured in inches Next comes the load index This refers to the maximum load for which tyre is designed Below is a table of Load Index values Remember these are the maximum load for each tyre with the car fully loaded Our example has a load index of 94 so it is designed to carry 670 kg per tyre Index Load Index Load 75 375 kg 76 400 kg 77 412 kg 78 425 kg 79 437 kg 80 450 kg 81 462 kg 82 487 kg 83 487 kg 84 500 kg 85 515 kg 86 530 kg 87 545 kg 88 560 kg 89 580 kg 90 600 kg 91 615 kg 92 630 kg 93 650 kg 94 670 kg 95 690 kg 96 710 kg 97 730 kg 98 750 kg 99 775 kg 100 800 kg Table 5 Tyre Load Index Values Finally is the speed rating which describes the speed at which the tyre is designed to operate The following is a list of speed indexes as found on passenger car tyres Our example has a speed ratin
14. all cars 4 Pitch Circle Diameter PCD Porsche 928 wheels are fitted on to five high tensile studs centered on a circle 130 mm in diameter i e the Pitch Circle Diameter PCD is 5 x 130 All Porsches to date except the 924 and including the Cayenne have a PCD of 5 x 130 The only other vehicle I know of with a PCD of 5 x 130 is the Mercedes Sprinter The Sprinter is a minibus or camper van You probably don t want minibus wheels on your 928 though I have heard of 928 wheels looking good on the Sprinter Since the PCD is almost unique you won t be able to use wheels taken from another make of car Measuring the bolt circle PCD Offset Information Negolive offset Positive offset Car side Street side Figure 1 PCD and Offset Source http www tyresave co uk fitment html For additional diagrams and explanations see also www halibrand com surefit fitment pdf 5 Centre Bore All Porsche wheels to date except the 924 not sure about the Cayenne have a centre bore of 71 5 mm this is the diameter of the hole in the centre of the metal wheel It has to be wide enough to accommodate the grease cap 6 Wheel Diameter This is the diameter of the metal wheel at the point where the tyre bead sits which excludes the flanges which hold the tyre beads in place Porsche didn t recommend 17 wheels until they introduced them on the GTS Porsche do not now recommend wheels greater than 17 diamet
15. at it means there s no availability of any inexpensive wheels that would properly fulfill that 928 front wheel application sorry This from Mark Edwards on the Australian landshark mailing list The first wheels that I had on my GT courtesy of the previous owner were Mille Miglia cup 2 replicas ET47 amp 52 I was not happy with them as they were scratched and I believed much heavier than the originals I got a real set of cup 1 s and couldn t believe how much more stable the car was I have had a wide range of other negative comments about instability when the offset is too small They have ranged from no worse than Weisach axle steer to so dangerous I wasn t game to go over 60 kph Straight line stability may be impacted by quite a few things apart from offset e g tyre choice especially in relation to sidewall strength wheel weight wheel alignment shock absorber type and condition springs and their matching with the shock absorbers steering rack condition and mounting sway bar bush wear and other front end component wear So it s difficult to sort all the issues quantitatively but I think that tyres shocks and springs are the big ones But we can make some conclusions The smallest front wheel offset Porsche ever shipped on a 928 was 60 mm on the early GTs and Club Sport So 60 mm should be OK but do note that the factory only ever did this in combination with very light magnesiu
16. at spacers always reduce the effective offset of 928 wheels Please bear in mind that wheel shape is an issue most particularly in relation to brake caliper clearance Also manufacturers stated tyre widths and profiles may not be absolutely reliable or closely comparable especially between brands See Using Other Porsche Wheels below for more information on offset 9 Original Porsche 928 Wheel and Tyre Sizes Table 4 shows the details of the wheels tyres offsets and spacers shipped from the factory with the various 928 models Factory fitting of RDK tyre pressure warning system commenced with the introduction of the digital dash in Model year 1989 commencing August 1988 though possibly not standard for all countries and ceased at the start of Model year 1994 August 1993 Wheel Tyre Offset Spacer Dre Type Size F R Size F R mm Size 928 Phone Dial Cast 7 x15 215 60VR15 65 0 1977 1982 Aluminium Alloy 7 x15 215 60VRI5 65 0 928S 7 Slot Forged 7 x16 225 50VR16 65 0 1980 1986 7 x16 225 50VR16 65 0 928S4 7 Slot Forged 7 x16 225 50VR16 65 0 1987 1991 8 x16 245 45VR16 52 3 928CS SE GT Club Sport Forged 8 x16 225 50VR16 60 0 1988 1989 Magnesium Alloy 9 x16 245 45VR16 60 0 17mm 928GT Design 90 TY x16 225 50ZR16 65 0 1990 1991 Cast Aluminium 9 x16 245 45ZR16 5
17. de down place the straight edge across the wheel so it sits flat on the metal rim measure the distance from the mounting pad to the straight edge this is the backspacing call it B turn the wheel over car side down and repeat the process measuring from the same now lowest point on the mounting pad as used previously this is the frontspacing call it F The total width of the wheel call it T is given by the equation T B F This will be greater than the conventional width because it includes the flanges which hold the tyre beads in place The offset call it O is given by the equation O B T 2 It s possible to use this procedure even if the straight edge is held proud of the rim by the tyre To measure backspacing just rest the straight edge on the tyre Measure the distance from the mounting pad to the straight edge Then measure the distance from the rim to the straight edge do it at each of the two points where the straight edge crosses the rim and take the average To get the backspacing subtract the second measurement from the first Apply a similar procedure to get the frontspacing 11 Using Spacers You can use a spacer between the wheel and the wheel hub to reduce a positive offset This will move the wheel away from the centre of the car closer to the wheel arch body panel and increase clearance between the wheel and the brake calipers Spacers come in two versions namely si
18. e same limits should be applicable to earlier models but it would be necessary to roll the guards The clearances are Front 9 5 x18 ET68 5 wheels 255 35 18 tyres Protrusion from mounting pad to inside tyre wall 68 5 mm 127 5 mm 196 0 mm Protrusion from mounting pad to outside tyre wall 127 5 mm 68 5 mm 59 0 mm Rear 11 x18 ET54 wheels 295 35 18 tyres Protrusion from mounting pad to inside tyre wall 54 0 mm 147 5 mm 201 0 mm Protrusion from mounting pad to outside tyre wall 147 5 mm 54 0 mm 93 5 mm Table 3 David Roberts 1990 GT Wheels and Tyres The above clearances are derived from the offset and half the tyre width It helps to draw a diagram of the wheel and tyre to understand the geometry Note that tyres with rim guard beads may have reduced arch clearance The positioning of the tyre relative to the car is determined by offset and tyre width alone within the bounds of our basic assumptions above In particular the diameter of the metal wheel is not relevant to wheel arch clearance Armed with offset and tyre width the protrusions for any wheel and tyre may be calculated and compared to the above There is no particular offset requirement for the rear of a 928 apart from that dictated by aesthetics and wheel arch clearance which is certainly the issue in Table 3 Also on the rear any combination of wheel offset and spacer may be used to achieve the stated effective offset but note th
19. elf 928 specific wheels They can be found at 928 specialists e g www 928gt com www 928intl com and www devek net If you have the cash you can also head straight for the 3 piece wheel manufacturers They can custom build what ever you want In Australia http www simmonswheel com au range html is worth a look given that you can specify offsets widths etc and the weights are very good 10kg Have a look at the model FR17 especially the colour options Less than Aus 600 per wheel John Cumming Australian landshark mailing list 12 18 APPENDIX I GENERAL FITMENT To demonstrate the general principles that usually apply to acquiring new wheels I have set out in this section the process I went through when buying used mag wheels for my VW Golf III This was a learning process for me but it was fun and I got Aus 2000 worth of slightly used tyres and wheels for Aus800 00 A worthwhile outcome Use what ever resources you have to figure out the metal wheel diameter width and style you want First call your insurance company and find out what they will allow Then get brochures talk to friends go to car meets talk to professionals search the Internet and don t forget to price suitable tyres The Golf had 14 wheels fitted in the factory I wanted the big wheel low profile look suggesting 16 or 17 wheels but was constrained by the usual need to maintain tyre diameter with in the 2 5 variance from stock
20. er for the 928 17 upgrades to older models are now common 18 less so but certainly exist Fitment for a 19 wheel requires an extremely low profile tyre see Tyre Size and Profile below e g 255 30 19 to 275 30 19 These are rare and likely to cost serious cash The lower the profile the less the sidewall is able to flex to contribute to a supple ride the more likely the rim is to be damaged by pot holes and the more expensive the tyre So spend some thought and check tyre prices and availability before you choose wheel size 7 Wheel Width This is the width of the metal wheel measured from bead seat to bead seat this excludes the flanges which hold the tyre beads in place Original wheels ranged from 7 to 9 wide See Table 4 To The maximum possible metal wheel width rims are 91 2 front and 11 rear These are pretty aggressive and require expert fitting The maximum tyre width I have heard of on the front on stock non GTS non flared guards is 265 mm It s 295 mm on the rear However 255 mm and 285 mm might be safer working limits Tyre width has to be matched to the metal wheel width Marc Thomas from Devek Rennlist mailing list said that he thought the ideal ratio for tyre to wheel width was in the range 0 85 1 to 1 15 1 with the 1 1 likely to be ideal for best handling though some will certainly prefer maximum tyre width Width of Rubber Tyre Rim Width Minimum Optimum Maximum 7 150 mm
21. erences from happy users and some replicas e g the Porsche factory licensed replica cup wheels are extremely heavy so check the weight You will want to get the lightest and strongest wheels possible to minimise the unsprung weight Heavy wheels rob horsepower and cause sluggish handling Aim for no more than 10 to 12 kg per wheel without tyre John Cumming Australian landshark mailing list My design 90s with tyres weigh 19 7 kg front and 20 7 kg on the rear Early flats on a S4 weigh 18 kg My sons expensive lightweight 18 wheels and tyre on his FTO weigh 22 8 kg however he replaced them with a forged set of 17 x 7 with Kumhos 712s that weigh 17 kilos The difference in performance was remarkable Graham Bates Australian landshark mailing list 16 Wheel Lug Nuts The 928 wheel lug nuts are a M14x1 5 14 mm thread diameter with 1 5 mm pitch and made of aluminum They are of very good quality but being aluminium means that they require special care The workshop manual page 44 19 specifically states that wheel nuts may only be loosened and tightened with Special Tool P 300 in perfect condition and that the aluminum nuts could shear through use of unsuitable tools wheel nuts held by only about 2 3 of total depth excessive tightening torque erratic loosening impact tool and missing or unsuitable lubricant Special Tool P 300 is a 14 mm hexagonal socket w
22. g of W so it is designed to operate at a speed not to exceed 270 km h Symbol Meaning N speed not to exceed 140 km h 87 miles h P speed not to exceed 150 km h 93 miles h Q speed not to exceed 160 km h 99 miles h R speed not to exceed 170 km h 106 miles h S speed not to exceed 180 km h 112 miles h T speed not to exceed 190 km h 118 miles h U speed not to exceed 200 km h 124 miles h H speed not to exceed 210 km h 130 miles h VR for speeds over 210 km h 130 miles h ZR for speeds over 240 km h 149 miles h V speed not to exceed 240 km h 149 miles h W speed not to exceed 270 km h 168 miles h Y speed not to exceed 300 km h 186 miles h ZR W for speeds over 240 km h but not exceeding 270 km h ZR Y for speeds over 240 km h but not exceeding 300 km h Table 6 Tyre Speed Ratings The VR and ZR speed ratings are largely obsolete but still retained by a few tyre makers for what I assume are marketing reasons The nominal outside tyre diameter is equal to the diameter of the metal wheel plus two times the height of the sidewall e g for a 245 45 16 tyre it is 16 plus 2 x 45 of 245 mm which is 16 2 x 110 25 25 4 24 68 When changing wheels and tyres it s very desirable to maintain outside tyre diameter within 2 5 of the diameter of the original equipment tyres Thus if we increase the diameter of the metal wheel we must reduce the height of the tyre sidewall lower the profile in sympathy
23. hat s a bit techie for me I understand what you are all saying but don t know how to find out what offset they are I m sure someone out there will know Tyres 225 40x18 Front and 265 35x18 on rear The fronts fitted fine while I need 18 mm spacers on the rear fitted by a independent Porsche garage The rears just fit with minimal clearance on the _9_ wheel arches I can get my fingers between the tyre and wheel arch The wheel arch has not been changed or modified Also this from Martin Dunks on Rennlist I have just put Boxster 996 18 7 5 amp 10 BBS sport classic II s on my car Bolted straight on no spacer no problems David Porter from the UK 928 mailing list in response to my request for more data points Just fitted a set of 17 996 wheels 55mm offset on Contis to my S4 Phil no clearance bump steer or straight line stability problems Bought them from Porsch apart who allowed me to try before buy as I was a little concerned See http www 928sg com for the design So some people use 993 996 and Boxster wheels on the 928 apparently with great success However some very well informed people are quite negative on the issue For instance Kim Crumb well known and respected on Rennlist is adamant that 911 wheel geometry is wrong for the 928 52 offset or something so the tire has been moved out an half inch that doesn t work on the front of 928s one implication is th
24. iewed from the front of the vehicle Included angle is the angle formed between the SAI and the camber Scrub radius is the distance between where the SAI intersects the ground and the center of the tire Where the centre of the tyre is outside the line of the SAI the scrub radius is positive as in Figure 2 The scrub radius on the 928 is negative with the centre of the tyre inside the line of the SAI see Figure 3 Commencing about 1986 1986 5 in the US the S3 in Australia and certainly by the introduction of the S4 in all markets 928s were fitted with 4 piston Brembo front brakes With the big brakes came revised suspension geometry In particular the scrub radius was reduced from 16 mm to 10 2 mm See Figure 3 The negative scrub radius is imparted by the large positive front wheel offset The relationship between offset and scrub radius is direct i e for each 1 mm reduction in positive offset negative scrub radius magnitude is reduced by 1 mm The negative scrub radius imparts a self correcting force on the steering when the front wheels have unequal drag e g when you put a wheel onto the soft shoulder of the road or one Figure 3 Porsche 928 Scrub Radius wheel into a patch of water or when the ABS Model Year 1986 and later brakes one front wheel and releases the other On cars with big brakes the negative scrub radius will be lost if a front wheel with an offset of less than 54 8 mm is fitted On olde
25. m wheels 9 kg without tyre John Cumming Australian landshark mailing list Club Sport wheels are really quite rare but at 60 mm they do set a very significant precedent and beg the question how low can we go I have not heard a single complaint from anyone actually using 55 mm front offsets there are really quite a few of them and I have been asking everyone I found However some of the problems associated with reducing the scrub radius and front offset may only appear in adverse driving conditions The complaints commenced at 52 mm often in conjunction with heavy wheels and were numerous at 47 mm This is entirely consistent with the scrub radius information provided independently and much later by John Cumming and Glenn Evans To get 65 mm factory spec wheels you either can buy a wheel made specially for a 928 or you can make one from a 3 piece wheel These are the only options and both are relatively expensive If I was paying list price I would insist on minimum 65 mm offsets However if I got offered genuine light weight 911 Boxster wheels that I liked with 55 mm offsets at the right price I would probably take 10 them but only on approval I would absolutely not use front offsets less than 55 mm 15 Using Replica Porsche Wheels The same advice applies to replica wheels as to the genuine article There are two additional specific issues check manufacturing quality get ref
26. mes before the cap of the closed nut hits the stud Ed Ruiz Rennlist mailing list 17 928 Wheel Fitment Summary For the 928 you need to specify a PCD of 5 x 130 with a centre bore of 71 5 mm Front offset is 65 to 68 5 mm and rear offset 40 to 65 mm The wheel diameter will usually be 17 or 18 Maximum sensible wheel width with professional fitting and without flared guards is 9 2 front and 11 rear but many will settle for less e g 9 front and 10 rear Your choice of wide wheels is almost always going to be limited by what s available for the front with a 65 mm offset Even though other Porsche wheels may have the correct PCD small front offsets may be a safety problem 17 wheels from the Porsche 993 996 Boxster are used by many but with spacers generally required on the rear and some limitations to tyre width Many attest to 55 mm offsets working fine However with front offsets lower than the 65 mm factory spec there is a very real prospect of stability problems The smaller the offset the bigger the problem is likely to be Front offsets smaller than 50 0 mm on older cars without 4 piston Brembos and 55 0 mm on newer cars with 4 piston Brembos may be problematic to the point of dangerous Watch out for replicas which weigh more than the original wheels You do not want to increase the unsprung weight Avoid wheels that weigh more than 10 to 12 kg without tyre One choice is to shop for off the sh
27. mple insert discs with the 5 x 130 hole pattern and thicker discs that bolt onto the wheel hub 5 separate nuts and have 5 bolts that stick out and mount the wheel using the wheel lugs Theo Jenniskens Rennlist mailing list Both types of spacers will generally require that the original studs be changed Porsche used spacers when they installed wide wheels inside the flared guards of the GTS i e they increased the rear tyre width by 10 mm used spacers to reduce the offset of each rear wheel and thus moved each rear wheel outwards from the car to more or less centre it within the newly flared guards It s worth repeating that using a spacer will always decrease a positive offset e g a 15 mm spacer will reduce a 65 mm offset to a 50 mm offset This is relevant because other Porsches generally have wheels with offsets smaller than those used on the 928 When such wheels are fitted to a 928 the use of spacers will only make that offset even smaller The moral of the story is that you can t use a spacer to increase a positive offset not at least without deploying some anti matter but read on 12 Tyre Size and Profile A tyre specification will usually look something like 245 45 R 16 94W The first figure is the tyre width In the example above the width is 245 mm This figure refers to the tyre width not the tread width The next number is the tyre aspect ratio It is the ratio of height width The above example has
28. n 2 5 of the diameter of the original equipment tyres Excellent The local laws say that the tyre diameter may not increase by more than 15 mm nor decrease by more than 26 mm So both sets of used tyres are sensible for my car and legal to boot It was now time to try fitting the wheels to the car There are several reasons you must do this before buying They are there s no substitute for eyeballing the final result I would never have chosen 16 spoke mags from a catalogue but they look terrific on the car you must check for brake caliper clearance front and rear you should check inside and outside wheel arch clearance front and rear and you must check that the bolts studs are the right size mine were M12 1 5 and that fitting hardware will or can be made to work You may need to remove the centre bore adaptor rings for the test if their inside diameter is too small for your car I bought the Vipers Both the wheels and the tyres were in excellent condition and the fitment was perfect At 205 mm wide they were well under 225 mm wide to comply with my notional upper limit for wheel arch clearance The Vipers are a common brand and centre bore adaptor rings are readily available I removed the old plastic 56 mm rings at the time of the trial fitting I purchased new 57 mm aluminium ones from a local wheel retailer The Vipers came with 16 chrome nuts but the Golf needs bolts The factory standard
29. ors can be found at http www miata net garage tirecalc html http gs tolan hoechst com thanks to Greg Gray and Michael Kelly for these Also don t forget that the manufacturers published specifications of the revs kilometer or mile for each particular tyre are generally a much more reliable indicator of tyre size Tyre Size Diameter Other fitments within 3 25 of original 215 60 15 25 16 265 45 15 diam 24 39 3 06 245 50 15 diam 24 65 2 03 255 50 15 diam 25 04 0 48 265 50 15 diam 25 43 1 07 225 55 15 diam 24 74 1 67 235 55 15 diam 25 18 0 08 245 55 15 diam 25 61 1 79 205 60 15 diam 24 69 1 87 215 60 15 diam 25 16 0 00 225 60 15 diam 25 63 1 87 205 65 15 diam 25 49 1 31 265 40 16 diam 24 35 3 22 245 45 16 diam 24 68 1 91 255 45 16 diam 25 04 0 48 265 45 16 diam 25 39 0 91 215 50 16 diam 24 46 2 78 2165 Tyre Size Diameter Other fitments within 3 25 of original 225 50 16 diam 24 86 1 19 235 50 16 diam 25 25 0 36 245 50 16 diam 25 65 1 95 205 55 16 diam 24 88 1 11 215 55 16 diam 25 31 0 60 225 55 16 diam 25 74 2 31 205 60 16 diam 25 69 2 11 235 40 17 diam 24 40 3 02 245 40 17 diam 24 72 1 75 255 40 17 diam 25 03 0 52 265 40 17 diam 25 35 0 76 215 45 17 diam 24 62 2
30. r cars negative scrub radius is lost when the front offset is lowered to below 49 0 mm The negative scrub radius is a significant design feature of the 928 imparting stability and controllability to the car s steering especially at times when it is most needed Thanks to Glenn Evans amp John Cumming Australian landshark mailing list for significant contributions to the discussions on scrub radius and offset 14 Using Other Porsche Wheels On the front the offset determines the scrub radius and thus where the tyre contact patch is in relation to the ball joints For proper steering response regardless of wheel width you really need the offset to be as close to the factory 65 mm ET65 as possible Changing the offset of the front wheels can cause undesirable side effects Tram tracking instability etc are likely to be noticeable before you have clearance problems Offset is not really an issue on the rear wheels The S4 and later see Table 4 have smaller rear offsets and are easier to fit but it s the front wheels that will constrain the choice of brand and style Wheels made for the 911 and Boxster have the correct centre bore 71 5 mm and PCD 5 x 130 for the 928 but their offset traditionally 25 to 45 mm is too small for the front They may also foul the 928 brake calipers thus requiring spacers which reduce the offset even further However some of the 993 996 and Boxster 17 wheels have become available
31. rench which only touches the wheel nuts in the middle of the flats and which is deep enough to cover the entire nut Use one Never allow the lug nut be tightened with an impact wrench the air powered tools favored by tyre shops that sound like a jack hammer The nuts should be torqued to 130 Nm or 95 ft 1b In the 928 Workshop Manual Porsche recommend lubricating the threads with Optimoly TA paste Optimol a German Company was taken over by Castrol many many years ago Optimoly TA is now called Optipaste TA and is available in 400 g spray cans and 500 g or 5 kg containers from Castrol distribution outlets Another very similar product is Mobil s Greaserex RGM 2 Doug Hillary Australian landshark mailing list Both Porsche and BMW specified Optimoly TA for wheel nuts on wide variety of models A 100 g tube from an official parts outlet should not be too expensive and should last decades I got given about 50 g in 1974 to lubricate the spark plugs of my Mazda RX4 and it ran out in about 2003 Never use a copper based grease such as coppercoat The copper reacts with water and bonds the aluminium nut to the steel studs If your wheels have a thicker than normal mounting pad or you place a spacer between the wheel and the a te hub the lug nuts need to make at least seven full turns to be safely seated Also if you use extra long studs typically with non standard wheels the lug nuts can only be turned about twelve ti
32. t the distance between the plate in the centre of the wheel that bolts onto the wheel hub and the imaginary plane which bisects the tyre into two equal halves is typically 35 mm but the acceptable range is 32 to 38 mm I compiled from the fitment data tables a list of vehicles with similar wheel fitments to the Golf 4 x 100 PCD offset around 35 mm A list of suspects was ferreted from the Trading Post These included Mags 4 of 16x7 Viper 205 45 16 Falken FK451 tyres suit Honda Civic Mags CSA 16x7 5 spoke 195 45 16 Goodyear F1 tyres Suit Mitsubishi Lancer A quick check of the fitment tables shows Honda Civic PCD 4x100 offset 35 38 C bore 56 mm Mitsubishi Lancer PCD 4x100 offset 35 42 C bore 56 mm The offset of the Lancer might be as high as 42 mm and the limit for the Golf is 38 mm So the offset on 135 the Lancer wheels would need to be checked actually they are mostly 35 mm since that fits so many small front wheel drive cars Otherwise the fitment looks good for both options Two different types of tyre were sold as original equipment on the Golf These were 175 65 14 diameter 22 96 583 2 mm 185 60 14 diameter 22 74 577 6 mm The used tyres advertised with the mag wheels were Falken 205 45 16 diameter 23 26 590 8 mm Goodyear 195 45 16 diameter 22 91 581 9 mm See Appendix II for the method to calculate tyre diameter Both the Falken and Goodyear tyres are withi
33. ts Rennlist mailing list Ed Ruiz Rennlist mailing list Paul R Smith Rennlist mailing list Marc Thomas Rennlist mailing list 20
34. with 55 mm offsets This is tantalisingly close to the 928 s standard 65 mm and even closer to the 60 mm offset used on the front wheels of the 1989 928 GT I have heard that the wheel nut cover plates on some 17 Boxster wheels foul the front axle grease caps when fitted to a 928 Doug Hillary pers comm So much for theory but what are people actually doing I have seen many reports of 911 and Boxster wheels fitted to a 928 e g from Mike Readman on the UK 928 mailing list My experience and preference wheel tyre set up on the S4 is 993 rims 17 x 7 and 9 with 55 offsets 20 mm spacer on the rear The tyres I am using are Pirelli Pzero Front being 225 45 17 N2 The rears are 255 40 17 N2 Wheel arch clearance on the front is fine obviously due to the shape of the body On the rear it is certainly at the max but I have not experienced any bodywork contacts this is without removing the arch folding roll as some have suggested And this from Math Creemers also from the UK 928 mailing list 1987 S4 wheels came from a 993 size and offset are stamped next to the valve Front 7J offset 55 mm Conti 225 45R17 Rear 9J offset 70 mm and 21 mm spacers Conti 255 40ZR17 And this from Mark Moss also from the UK 928 mailing list T ve been running some 18 sports classic wheels taken from a 996 Boxster on my 89 GT The are 8Jx18 on the front and 10Jx18 on the rear Not sure on their offset values as t
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