Petza914

I'm planning to change the wheels on my '79 Spyder and have a set in-mind. What I'm looking for is some input on offsets and then tire sizes.

Currently on the car a CII Lightweights -
FRONT 225/40 ZR18 mounted on 8Jx18 ET 50 wheel
REAR 285/30 R18 mounted on a 10Jx18 ET65 wheel

I don't seem to have any issues running these, but believe the 50ET offset on the front is less than ideal from what I've read, although I think from the info in my Manual that the car originally came with 7" x 16" et 50 Offset with 225-60/16 on all 4 corners. So the firtst thing I'm curious about is whether the OEM sizing info I have here is correct or incorrect for a '79 5-speed. If these combinations don't usually fit on an OB928, remember that my car, because of the convertible conversion has some non-standard bodywork (rear fenders are fiberglass) which is why I posted the info about what's currently on the car.

The wheels I'm planning to put on are:
Front - 18x8 - offset 57
Rear - 18x10 - offset 65

So here are the questions:

1. With these wheel widths and offsets, do you think I will need to run any spacers on the front or rear, and if so, how thick?
2. What size tire should I run on the front - seems like most recommend a 225 or 235 with a 35 aspect ratio. From online research, it looks like a 245 width is the max I should run on an 8"wide wheel. If my OEM tire sizes were correct, the revolustions per minute is actually best with a 245/40 tire than either of the previous sizes - will these fit?
3. What size tire should I run on the rear? Again, if my OEM sizes were correct, the closest match to this for tire RPMS would be a 305/35, but this size appears to only be available in drag radials so I might have to downsize 1 notch to 295/35. Will these fit?
4. One option I have would be to discmount the current tires and remount them onto the new wheels but if there are more appropriate sizes, I can just burn these current ones up and change to the new ones when it's time or sell these current wheels with the tires.

I wanted to get some preliminary information from you experts before buying any tires or test fitting new wheels to the car. The attached PDF has additional detail as well as some chart printouts from the "will they fit" wheel and tire size calculation tool.

Thanks for the help.

76FJ55

no help with the rears, but I would go 235/40 on the front. I have 18X8 -59mm Offset Panamera wheels on mine and 235/40 fit with no issues, with enough clearance that the extra 2mm form a -57mm offset wheel shouldn't be an issue. No spacers on the front. Spacers only make it worse.

Petza914

Anyone have input on the rears for me? Thanks.

jetson8859

I have 18x10 with 62mm offset on rears Am running 285/35s and they tuck in very nicely. As others have said, there can be quite a bit of difference based upon tire manufacturer. I am running Bridgestone Pole Position S-04s. Its an 87S4.

FredR

No 928 I have read about had a ET50 up front but then I do not know too much about your model year. I am under the impression it has similar front geometry to my car but achieved with different components.

The wheels you are proposing should work. I am under the impression that all 928s need a front offset of 65mm to achieve the NSR the steering was designed to give. ET 57 falls short of that mark but you should be able to get a 245x40 or possibly a 245x35 section to work. You are looking for a rolling radius ideally 25 inches and not more than 25.4 inches.

For the rear you want either a 285x30 or a 295x30 section. ET60 on a 10 inch wheel works fine, not so sure what the effect of an additional 5mm will be- I suspect it will be fine- worst case scenario is a spacer and new [longer] wheel studs.

Cannot comment on the impact of your fender mods but assuming they increase flare you can play around with the spacers to suit the available dimensions

Rgds

Fred

Petza914

Fred, thanks for the input. I ran some additional combinations on willtheyfit and have the following follow-up questions.

It looks like a 245/40 puts the rolling diameter (I assume you meant diameter and not radius) outside the range you recommend at 25.7".
A 235/35 is 24.47" - a 235/40 is 25.4" - a 245/35 is 24.75". In looking at my car with the 50ET wheels it looks like I've actually caught the inner lip of the right front fender with the tire as it has a little pucker. I have a fender roller that I'll use to smooth this inner lip out and also get it out of the way. Once I go to the 57 ET wheels, it will move the whole wheel in by 7mm correct? This means I could go up to the next width of 235 and still be 2mm further away from the outer fender lip, right? If that's the case, which of the 3 sizes above is my best bet for rolling diameter as once the fender lip is rolled, I think I could fit any of the 3 of them.

On the rears, I have a good bit of space between the outer edge of the tire and the cars fender - more on the outside than inside, especially considering the wheel should tuck up and in a bit as the suspension compresses. Since the new wheels have the same offset as what's currently on the car, I thnk I'm going to use aset of 7mm hubcentric spacers that I have to push it out a bit and that should let me move to the 295 section width rear without any clearance issues. My last question is am I trying to hit the same rolling diameter target of 25" on the rear that was discussed for the front? If so, a 295/30 is 24.96" and a 295/35 is 26.13" so I'm assuming the 295/30 is the better fit, but let me know.

FredR

Pete,

One of the considerations I made was to ensure I did not move far from the rolling "diameter" of my stock wheels thus needed to be close to 25 inches. Another consideration was to ensure the difference between front and rear did not upset the anti lock braking system- 25.4 front and 25 rear works fine. I have no idea what difference would cause the ABS to pack up.

Your current front wheels should work fit wise but when I had similar on my late S4 on the track they did not work at all well lap time wise. Like your wheels they came from a 911 or Boxster, fitted well, looked great but the stop watch said otherwise so flogged them to a friend with a 911 where they were better suited. If you have had a rub with these then I would want to know why.

If you go to a ET57 front wheel then the centre of the wheel indeed goes inboard 7mm but remember, the 245 section tire is nominally 10mm wider from the centreline so you are going to lose 3mm- not good if you already have problems. Rolling the fenders should give you a few mm and depending on your current alignment, putting some sensible camber into the mix will also gain you something. Thus you have a risk based choice- the 235 will be safe the 245 seemingly marginal.

For the rear the 295x30 is a no brainer. If you have to fit spacers remember you need longer wheel studs to compliment. 7mm is the kind of thickness that may tempt owners to leave the studs "as is"- not good.

Given that you presumably invested a lot of the green stuff in your build, would it not make sense to get some wheels that compliment the build with a correct front offset? It would make a somewhat unique vehicle more complete I suspect. Porsche did a great job designing the suspension so why corrupt it?

Rgds

Fred

Petza914

Fred, thanks again. Mine is a '79 so no ABS issues to consider. Like you say, safer is better so I'll go with the 235/40 on the front and 295/30 on the rear. That will pick me up 2mm of front clearance and I'll have an alignment done once the new wheels and tires are mounted. I've read that 9 threads is the magic number that should be engaged by the lug nut onto the hub threads. Would you agree with this. If so and I have fewer than that, I can look at longer rear studs. I may also be able to go with a 3mm spacer to create enough inboard clearance that I don't think would require stud replacement. I'm hoping to have it put together in a couple weeks and will post some pics.

GlenL

I'd caution against the 295 in the rear. That'll be tight. A 285, or 275, will be really big and it'll fit well.

As people note, each brand/model of tire will be slightly different for a given size and the cars also vary between model years and the individual cars. (S4s seem to take a bit more tire in front) Nothing worse than getting those $350 tires mounted, made unreturnable, and finding they rub. I got some tires off CL to check general fitment. You might look for take-offs to check it out...and then expect your final tires to be different!

FredR

Pete,

The front section width is what limits the 928's cornering performance but that may not be a prime concern for you. A 235 works reasonably well up front but nothing like a 265- horse for courses.

With a 235 up front you do not need a 295 at the rear for cornering but if I remember correctly you have some serious grunt from the motor you had built- correct? If so you may need rubber to handle that.

There should be enough room in there to take a 295x30 providing it is offset correctly but ET65 is a bit suspect without a spacer. I have seen a number of such examples on this list running 295 on the rear so logic suggests it is possible.

As I understand the rear wheel arch geometry is pretty much the same as the S4 so there should be room in there but I note Glen's comment and he knows his way around those earlier models.

When I ran my current wheels on my late S4 a 285 fitted with lots of clearance and no spacer. To fit your combination the rubber will sit 10mm closer to the inboard point of contact and I suspect that will be too close.

The other approach is to get hold of a worn out tire, mount it on the wheel and do a trial fit.

When I was upgrading my wheels some 13 years ago I took all kinds of measurements in both the front and rear wells to map the geometry. The front was very tight but the rear less of a problem.

Rgds

Fred

Petza914

Glen, thanks for the input - in general, your advice about the rear is sound. My 928 is different though as it's a convertible and does not have the OEM rear fenders, but rather fiberglass versions that tie into the trunk, cowl, etc and they must be a bit wider than the OEM fenders on the OB cars. If I didn't already have 285s mounted on the car where I could see how they fit, I would play it safe and go with a 285, but I have space to the outside of them and my new rear wheel is the same offset as my current rear wheel so I shouldn't have any issue with a slightly wider rear tire if I move it out just a bit with a spacer and bringing the wheel and tire out just a bit will improve the "visual stance" a bit too.

There are 3 reasons I'm changing the wheels and tires - one is the current ET of 50 on the front wheels that came with the car that's not a good fit onhe 928. The other is that the front tires and rear tires are different brands and models and I don't really like to do this (came that way too) as it can result in odd handling behavior. The 3rd reason is that I have also found a wheel that I think will really complement the car that I've never seen on another 928 becuase they don't have US distribution, so it will add to the uniqueness of the car, since a 928 Convertible isn't unique enough

Petza914

With the roof structure removed from the car, maximum cornering performance isn't really on the table since I have body flex anyway, although climbing the mountain on some high speed sweepers I coiuld feel a little front end push so thought a little extra section width up front might help that.

On the rears, with the Stage 2 Supercharger setup I'm getting about 300 HP and 300 lb ft of torque at the wheels and since my car doesn't have the roof structure, heavy rear window, and no rear spoiler, there isn't much weight or downforce back there so I figured I'd try to maximize my traction there as well.

Going from a 285 to a 295 in the rear, shouldn't the tire only move 5mm inboard and 5mm further outward (not the 10mm). If I go with a 7mm spacer then I'd pick up 2mm inboard and be 12mm further out and in the static postion, my digital caliper shows that I have about 40mm between the edge of the wheel/tire and the inner lip of the rear fender (can't roll that onse since it's fiberglass).

Here's a shot with the camera perpendicular to the wheel shooting up into the wheelwell and then 2 more showing the front wheel and rear wheel as they sit today. The fronts are a bit too far out and the rears too far in, at least visually.

Keep the input coming as I don't want to order tires that turn out to be unworkable.

Thanks.

FredR

Pete,

You appear to have stacks of room at the rear and if I were to hazard a guess I would suspect you have GTS type geometry available to you- maybe more. That being the case I would look at running with something like a 24mm spacer [as I do] with suitable studs to fill the wheel well better.

The numbers I quoted for the rear proximity were comparative given the additional rubber width [5mm] and additional offset compared to my wheels [+5mm] to push your set up 10mm further in than mine.

Your front wheels are exactly where I would expect- very close to the point of contact.

The visuals suggest you have a lot of camber on the rear but not so much on the front but that could be just the photo/wheel arch geometry.

As you say chopping off a shed load of lard is going to change weight balance and stiffness but optimising what you have is still the goal.

Rgds

Fred

Petza914

OK, now that we've settled on tire sizes, anyone have any alignment spec recommendations for good tire wear on a street-driven car that will yield a good balance between tire wear, handling, and no rubbing.

FredR

For your needs I would try:
Front -1.25 degrees camber, 5 degrees caster and toe in close to lower end of the allowable setting range
Rear: -1 degree camber, toe in middle of the allowable range

Rgds

Fred