brjak

I know Porsche has a service updating highly suggesting 17" as the largest rim you can put on a 97 Boxster. I understand it puts undue pressure and stress on the suspension.
I am a Sunday driver, I don't drive the car that hard. I am curious with my driving habits if 18" would be ok.

Macster

It is not the suspension per se that limits the early models to 17" or smaller wheels, it is the strength of the car's chassis/tub.

My info is the items strengthened in later models to accommodate 18" (or larger) wheels were the rear wall crossmember, the lower engine bulkhead, and the rear axle mount support.

Even if the car doesn't just collapse in a pile of crumpled metal at the first corner there is the concern about what the extra loads of these larger tires/wheels could have to the feel of the car, on its running gear?

Car makers do not like to add weight to a car and to modify the above mentioned items to accommodate larger wheels strongly suggests the mods are really necessary.

I would not be that willing to fit larger non-sanctioned wheels/tires and find out.

Last but not least the smaller wheels/tires are better for the Boxster in my opinion. My 2002 came with 17" wheels/tires (I would have preferred 16") and these are as large as I would go. Both my 08 Cayman S and my 03 Turbo came with 18" wheels and the ride was harsher.

17" tires are cheaper than 18" tires, too.

ep3_lol

Shouldn't you be more concerned about the weight of the tire and wheel put together than the wheel's diameter? That is just one factor in the whole package. I have had 18s on my 2002 for the street for a couple of years. I have a few track days and autocrosses on them and haven't seen any problems yet. They are partially forged so they are not as heavy as a standard cast wheel, I think they are around 20lbs each if memory serves.

Macster

The weight of the larger tire/wheel combo is a problem mainly for the suspension, the spring/shocks, and mainly the shocks I think.

The shock is responsible for helping to control the motion of the wheel/tire. A heavier wheel/tire combo would make the shock work harder and could lead to a shorter life. The car's grip could be affected if the shock is not able to keep the wheel/tire in contact with the pavement.

Regardless of its weight, the larger wheel/tire though still results in more loading of the suspension components and to the places/areas where these connect. This is due to the larger wheel/tire combo's greater leverage. It might also be from higher G forces as the larger tires may provide more grip than the 17" tires.

This is clearly a concern as it was in these areas Porsche made the changes to support using the larger wheels/tires.

I'm sure there are owners of the earlier cars that have fitted 18" wheels/tires (maybe even bigger) and the car has not crumpled into a ball of tin from the loads.

The questions is will every car so fitted with the non-sanctioned larger wheels/tires be unaffected? Now it is not the sudden failure of something during the first test drive after fitting the larger wheels/tires that would be a problem, it is the higher loads/stresses that over time could cause cracks to form, thus weakening the car in these vital areas, and at some point then something could let go. The risk too is these cracks could be places for corrosion to gain a foothold so there is that to consider.

I would never fit 18" (or larger) wheels/tires to an early Boxster or fit any unsanctioned wheel/tire combo to any model of Porsche and I would never buy a car that had had this done.

ep3_lol

What you say is all true, but the fact still remains that wheel diameter is only one piece of the puzzle. If you hypothetically had a wheel/tire combo that weighed the same and provided the same size contact patch and same tread compound with the only difference being wheel diameter, it's probably not going to make much of a difference. The suspension will be forced to absorb more from the road surface as there is less tire sidewall to do so, but if that is a big concern then you should probably stay away from sporty 17" tires as well.

Macster

If you hypothetically had a wheel/tire combo that weighed the same and provided the same size contact patch and same tread compound with the only difference being wheel diameter, it's probably not going to make much of a difference.

How you can say this and with any authority is beyond me. Porsche clearly cautions against using 18" wheels/tires on these early models and Porsche makes no distinction regarding wheel/tire weight, tire patch size, tire compound, etc.

The wheel/tire size is the only piece of the puzzle that matters.

The issue is not the stiff sidewall but the increase in forces arising from the larger wheel/tire combo. The distance from the center of the wheel/tire to the pavement is more. During side loading, this subjects to wheel attachment points to more loading/stress. This may be increased even more due to better grip these larger tires provide, but as I mentioned above Porsche puts no qualifications on its guidelines against fitting larger wheels/tires.

To deal with these forces from larger wheels/tires, Porsche beefed up the rear wall crossmember, the lower engine bulkhead, and the rear axle mount support, that ultimately have to absorb whatever forces the attachment points transfer.

And how can you support larger wheels/tires that fly in the face of Porsche's guidelines and then turn around and not support wheel/tire size that Porsche clearly sanctions? Many early Boxsters came from the factory with 17" wheel/tire sets and it is just silly to think that Porsche would do this if there was any hint of an issue.

To put it plainly while I could certainly reject a wheel/tire combo due to cost, looks, increase in ride noise, vibration or harshness, tire availability, road conditions, etc., I would not give it a second thought to fitting any wheel/tire combo, from the smallest to the largest, officially sanctioned by Porsche, due to any concerns about the combo being unsafe in any way, shape, or form.

ep3_lol

The key there is tire size. That is where your overall diameter and width changes, but you can get tires for different sized wheels that have very close diameters and widths. The wheel size and material greatly affects the weight of the system, which is important, but a bigger wheel is not necessarily going to give you a bigger, heavier wheel/tire package. That is why I say that the wheel size alone is not terribly important if you are just talking about an inch or two.

Again, the overall size of the package isn't necessarily affected by wheel size. The diameter of the tires on my 18" wheels is nearly identical to the diameter of the tires on my 17" wheels- and I could buy tires for the 18s that would have a much smaller diameter. The distance from the center of the wheel to the pavement is essentially the same. The wheel attachment point and suspension stress levels are just affected by so much that you cannot single out wheel diameter as the make-or-break specification when it comes to choosing a wheel/tire package. All seasons on 18in wheels will put much less stress on all of those components than racing slicks on a 16in wheel if everything else is equal.

Did Porsche recommend staying away from high grip tire compounds as well? I think they made the recommendation because, in general, a layman picking out a bigger wheel at the local tire shop is going to end up with something heavier than the stock options. It is much simpler for the average owner to see that and stay away from an 18 than it is to try to explain to them what is actually going on. That's patronizing, but considering the volumes that the Boxster sold in, it makes sense to me.

brjak

i'm totally confused. I think i will just stay with the porsche sanctioned 17" rims for 97 boxster. Thanks for all the input.

Macster

Confused? Only because you allowed yourself to be confused.

You asked about what wheel/tire sizes you can run on your car and you received info that essentially says whatever Porsche says you can run, which allows for up to a 17" wheel/tire combo.

However, you can of course choose to ignore whatever Porsche says and run whatever you want. As it is with oil so it is with wheels/tires. If you chose to go off the reservation and ignore the guidelines Porsche has provided -- which I have to point is the only authority on this matter -- then you can listen to anyone's opinion on what is OK to run.

Now that to me would be confusing.

Macster

As I covered earlier Porsche provides no exceptions regarding sidewall to wheel diameter, tread compounds, hardness ratings, side wall colors or anything. It says 17" is the largest wheel/tire size that can be used on some early models of Boxster.

If you chose to ignore Porsche -- which like I pointed out in a reply to the OP Porsche is the only authority on this matter -- then you can run whatever you want and use whatever spin you want to justify your choice.

ep3_lol

Likewise, you should think about the "spin" and all the considerations Porsche made when creating the recommendation, some of which may not have been technical.

But I guess we will never know. If a car ever snaps in half because it ran an 18", I will permanently move on to another brand.

Paseb

i just switched to 18'' oz wheels on my race boxster
8.5 front and 10 rear, 245 and 275 rubber
jrz pro suspension

my friend is running the same setup and havnt had any problems

Xpit77

My 99 has 18" Turbo look rims. It came with it as an option ( 413 ). When did the suspension change to accommodate them ? And what was changed ?

brjak

The change came in 98. What was changed I'm not sure. But, for my 97. 17" is the largest.

Macster

My info is 18" wheels/tires were an option starting in the 1998 model year. (They were standard equipment on S models, which appeared starting in 2000.) As for what changed, my info is the change involved: rear wall crossmember, the lower engine bulkhead, and the rear axle mount support.

Essentially, these were beefed up to withstand the higher forces arising from using larger wheels/tires.