So based off the "Math Check" thread and chatting with Wheel Enhancement, the 18's I'm looking to get will fit without issues
So the wheels are 18x8" ET50 and 18x10" ET47

Current DD wheels are 19x8 ET57 and 19x9.5 ET46

This is going to reduce my front track by 7mm on each side and increase my rear track by ~6mm on each side.

My guess is that this should induce extra understeer, any other comments as to what I could expect?

Cheers!

Apolo

Keeping the front width at 8" while increasing the rear to 10" should give a little better grip at the rear, so if you're right about the track changes adding a bit more understeer, the wider rears should add a bit more understeer to that. But if you've got too much oversteer now, your problem might be solved.

You can vary this by the Tyres you put on. Try to keep to the '30' rule when you choose Tyres.......... There is a large variation available..............

Front Tyres = Optimal size to the Rims
Rear Tyres = Front Tyres + 30mm

This will give you balanced adhesion on a rear wheel drive car.

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Cheers Karel
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'Power is Nothing without Control..........'

kareldeeley,
I was planning on running with 235/40 18 and 265/40 18. So following the 30 rule should keep me in line, but based off the math with my track widths since I am altering front and rear tracks by what I mentioned is that all i should be expecting? Reducing Front track due to new offset (increases understeer), then Increasing Rear track due to similar offset of a wider wheel (also increases understeer) ? But thats about it?

If you can put 245's on the front rims - you can put 275's on the rear

You can also think about the 5mm Spacers from Porsche (or Aftermarket Suppliers) - Needs Longer Wheel Bolts...........

Giving greater Traction / Balance in General Terms.

You need also (on the track) to adjust the pressures for optimal response.

In real terms the felt differences for most drivers (including me) are theoretical and minimal.

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Cheers Karel
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'Power is Nothing without Control..........'

You've posted of this supposed "30 rule" before. Do you have any printed authority you can refer us to for this "rule" which is in conflict with the actual findings over many years and different cars experienced by many seasoned track drivers here?

The general assertion that "[t]his will give you balanced adhesion on a rear wheel drive car" is incorrect for too many RWD cars to list (e.g., 911s, BMWs, etc.)

Apolo
Are you going to track your car? If it's for street driving only, I wouldn't worry about the marginal addition of understeer to the already understeering Cayman. I seriously doubt you'll notice any handling difference in even spirited street driving (i.e., 6/10ths). If you do, adding spacers up front is no big deal. You can also reduce understeer a bit by decreasing the Porsche recommended F-R pressure differential.

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Dan

STLPCA,

I do plan on taking my boxster out, soon. I just boght it last fall and with all my business travels this year havent had time to do anything. OEM Warranty expires shortly so I was planning on getting one last good lookover as it was used when i bought it and then I'll start coming out to some of the local PCA events

You didn't mention if you're a track novice or more advanced. If you're at the novice/beginner stage I still wouldn't sweat your wheel changes at this point. If you get hooked, then you may want to consider steps (and there are several) to get your handling closer to neutral and also reduce what you'll find to be excessive outer edge front tire wear as you increase your track speed.

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Dan

Complete Novice, although in my mind I dream as though I could be good one day if I had the time to practice often. I figured that I wouldn't notice the difference too much, if at all. I just wanted to make sure I understood the change that I am making as I shift to the 18s. I'm sure once i get into it I can see where I feel the car is and make adjustments as needed.