alignment after new wheels?

[SoCal]

When will I need my wheels aligned after replacing my 18" wheels for another set of 18s as soon as possible after delivery?

www.WheelEnhancement.com advised me that they will balance the new wheels but that alignment will not be necessary since the car will have been aligned before I accepted delivery.

The new wheels will be the same widths as the prior front and back, but different offsets. The rear wheels will use spacers, and all new wheels will be about 7mm wider.

I thought it would be a good idea to drive the car 50 miles or so and then get the alignment checked back at the dealer. Is that truly unnecessary, if I don't notice any mis-alignment?

SoCal, what wheels did you get? I asume you got your new car!

What size spacers are you going with?

[mjw930]

As strange as it seems wheel width and offset will not effect alignment. But, different wheel specs respond differently to various changes to alignment so if you are competing with the car you might want to experiment with different settings.

Example, 18" wheels and tires may be able to get acceptable cornering with less camber since the slip angle of the tire is less.

[MNBoxster]

Hi,

You may or may not need an alignment once the wheels arrive, but be aware of any unusual feel or behavior and if found get the Car aligned ASAP to prevent premature wear to the Tires.

I assume you bought a Wheel and Tire Package? If so, be sure that they are dynamically balanced. Wheels and Tires, believe it or not, are not truly round. They are only round to within a certain degree ±. Dynamic Balancing is done on a Computerized machine which first Road Tests the Wheel and Tire by spinning it to 50MPH and then pressing it against a Roller to find it's Runout (eccentricity). It is not the same as Spin Balancing a Tire. Ask if they are using a Hunter GSP 9700 (best) or 9600 (they should be) Balancing Machine. Also, ask them for the printout for each Wheel/Tire, the machine will do this printout.

The Tech will then mount the High side of the Tire to the Low wide of the Wheel, or vice-versa before determining the Mass High/Low side of the Combo and adding a weight to compensate. If this is not done, especially on a Performance Car, Suspension, and Tire, you can get some vibration at a given speed which is not due to, or eliminated by, alignment.

And, there is an acceptable degree of runout and Mail Order Tires are notorious for being out of spec (do Manufacturers sell-off these Tires and keep the ones in Spec for their own Authorized Distributors? Who can say) , but the incidence of out of spec and near out of spec Tires is higher with these MO outlets. I have bought many Tires through such outlets as Tire Rack, Discount Tire, and such, but I always tell them I will have the Tires Dynamically Balanced and reserve the right to return any which are not within spec. I had to do this once. Hope this helps...

Happy Motoring!...Jim'99

[Brucelee]

I would realign and rebalance the car if I replaced tires AND/OR wheels.

Given the cost of our tires, I think this is prudent.

[SoCal]

Originally Posted by bmussatti

SoCal, what wheels did you get? I asume you got your new car!

What size spacers are you going with?

My projected build date is October 24, so still waiting.

I'm getting the 18" 996 Carrera 5-Spoke wheels that were optional on the 2003 Boxster S. click here

Those wheels come in 8" and 9" widths -- the same widths as the 987 18" Boxster S wheels, which I ordered.

Since the diameters and widths are the same, I'm planning to take the tires that come with the car and have them installed on the new wheels. (What do you think I should do with the factory Boxster S wheels? Wheel Enhancement offers to take them as a trade in for a $900 credit.)

According to Wheel Enhancement:

The wheels on the 2006 Boxster S have the following offsets:

8x18" 57mm
9x18" 43mm

The 2002 996 Carrera 5-spoke wheels have the following offsets:

8x18" 50mm
9x18" 52mm

Using the 8x18" 2002 wheel with a 7mm more aggressive offset on the front is not a problem, the wheel will be well within the fender on the car. The rear 9x18" 2002 wheel sits too far inboard without using a spacer, affecting the handling as well as having a negative affect on the appearance. Using a 17mm Porsche Factory wheel spacer (and silver anodized lug nuts) will bring the rear wheel out 8mm further than the stock wheel, compatible with the front wheel's position.

(Edit: If you got the 2002 996 Carrera wheels as an option on the 2003 Boxster S, you got 7.5" wide on the front and 9" wide on the rear. The 996 wheels for the Carrera were 8" front and 10" rear. But since the 18" Boxster S wheels for MY2006 have 8" front, and 9" rear, I'm going with that same combo.)

Hey SoCal:

Sounds good!

Why don't/didn't you order your car with the Carrera Classic 19" wheels. This is option code #405. The cost is $1,940. The wheel is very similar to the ones you have selected from Wheel Enhc. I suspect that you did not want 19's"

If you sell your Boxster 18" wheels on your own, I would suspect that you can get more than the $900 that Wheel Enhc. is offering you. Post your sale here...looks like stuff sells pretty fast.

Good luck!

[SoCal]

Originally Posted by bmussatti

Hey SoCal:

Sounds good!

Why don't/didn't you order your car with the Carrera Classic 19" wheels. This is option code #405. The cost is $1,940. The wheel is very similar to the ones you have selected from Wheel Enhc. I suspect that you did not want 19's"

If you sell your Boxster 18" wheels on your own, I would suspect that you can get more than the $900 that Wheel Enhc. is offering you. Post your sale here...looks like stuff sells pretty fast.

Good luck!

True that I didn't want 19" wheels. I think the 18s provide a better ride and better handling.

[MNBoxster]

@SoCal,

Watch that the change in Offset (a component of SR) does not affect the Scrub Radius of the Front Suspension. Too much change to the SR can make the steering real twitchy after turns (return-to-center is affected) or when going over even the slightest bump. There is no alignment cure for improper Scrub Radius. The resulting instability is often improperly attributed to Tire imbalance or such, but isn't and cannot be cured. Otherwise, they look great! Good Luck!...

Happy Motoring!...Jim'99

Originally Posted by SoCal

True that I didn't want 19" wheels. I think the 18s provide a better ride and better handling.

SoCal, I understand what you are saying here about the better handling...but how did you determine this???

Thanks.

[SoCal]

Originally Posted by bmussatti

SoCal, I understand what you are saying here about the better handling...but how did you determine this???

Thanks.

For ride comfort, I can notice myself that the 19's are significantly stiffer. For handling, I'm relying on the opinion of Walther Röhrl and many others.

[SoCal]

Originally Posted by MNBoxster

@SoCal,

Watch that the change in Offset (a component of SR) does not affect the Scrub Radius of the Front Suspension. Too much change to the SR can make the steering real twitchy after turns (return-to-center is affected) or when going over even the slightest bump. There is no alignment cure for improper Scrub Radius. The resulting instability is often improperly attributed to Tire imbalance or such, but isn't and cannot be cured. Otherwise, they look great! Good Luck!...

Happy Motoring!...Jim'99

I understand the concept of what you're saying, but I have never even heard of "Scrub Radius" before.

Is there any way the SR can be affected by only 7mm more width (for each wheel) on the front? If so, what can be done to determine whether I'm at risk, or how to address it? (I'm getting PASM too, if that matters.)

Can the SR be affected by adding width to the rear also?

[MNBoxster]

@SoCal,

Hi,

It's not unusual for someone to have never heard of the scrub radius because it is essentially a fixed (not readily adjustable) imaginary point. This point is fixed by the configuration of the suspension geometry and will not change unless a piece of the suspension breaks and/or is replaced by an aftermarket piece, or by a collision or by altering the Wheel Offset.

The point at which the SAI contacts the ground is the fulcrum pivot point on which the tire turns. The location of this point within the contact patch has a great effect on steering effort, feel, and stability. A true plumb line, placed at the center of the wheel at the point of road contact, and the projected line created by the strut, or the upper and lower ball joints, determine SAI (Steering Axis Inclination). The two lines will intersect at a point just below the road surface on most vehicles. On some front-wheel-drive vehicles the point of intersection will be just above the road surface (this helps to reduce 'Torque Steer'). The distance between the projected line and the vertical line at the road surface is called the scrub radius.

If the scrub radius is zero, the scrubbing action of the contact patch is equal on either side of the pivot point causing the tire to act like a car with a welded differential, inducing a condition called 'squirm'. In a straight line the tire tends to be stable and tracks well. As you turn though, the portion of the contact patch on the outside of the pivot point moves faster than the portion on the inside of the pivot point. Since the scrubbing area is equal on each side of the pivot point, yet the forces are different, the tire tends to fight itself and it becomes 'grabby' causing tire wear to increase and the steering to become unstable.

When you have excessive scrub, whether it be positive or negative, steering effort increases and road 'feel' increases, as the steering is more susceptible to road shock. You must also take into account the amount of sidewall flex your tire will encounter under hard cornering. When the sidewall flexes, the contact patch moves in relation to the SAI and can make a slightly negative scrub radius become zero, or a positive one to become larger.

Ideally, the scrub radius should be as small as possible. Normally, the SAI angle, the centerline of the tire and the wheel intersect below the road surface, causing a positive scrub radius.

With struts, the SAI angle is much larger than the long-arm/short-arm type of suspension (unequal length). This allows the SAI angle to intersect the camber angle above the road surface, forming a negative scrub radius.

The smaller the scrub radius, the better the directional stability. Installing aftermarket wheels that have additional offset will dramatically increase the scrub radius. The newly installed wheels may cause the centerline of the tires to move further away from the spindle. This will increase the scrub radius. A large amount of scrub radius can cause severe shimmy after hitting a bump and/or impede the return-to-center action of the steering after a turn.

Four-wheel-drive vehicles with large tires typically use a steering damper to compensate for an increased scrub radius. Scrub radius is not directly measurable by the conventional methods and so care must be taken when changing any of the things which affect Scrub Radius.

Whether a 7mm increase to Offset will be unmanagable I cannot say. But, if you get any shimmying or twitchiness, this is most likely the cause. I'd say to proceed, but not to sell off the Factory Wheels until you're satisfied that the increase in Scrub Radius by the '02 Wheels (and it will increase) is to your liking. Hope this helps...

Happy Motoring!...Jim'99

[SoCal]

Thanks for the education. Are you an automotive engineer?

Just wondering, do all of the various optional wheels offered by Porsche for the 987 have the same characteristics, so that the same suspension setup works identically for each? Or, does the factory adjust the suspension configuration depending on the wheel selection?

[MNBoxster]

Originally Posted by SoCal

Thanks for the education. Are you an automotive engineer?

Just wondering, do all of the various optional wheels offered by Porsche for the 987 have the same characteristics, so that the same suspension setup works identically for each? Or, does the factory adjust the suspension configuration depending on the wheel selection?

Hi,

Thank you for your kind words. No, I am not an Automotive Engineer. I am a former Naval Aviator and Navy Certified Test Pilot. In order to do this - fly really fast Airplanes and spend loads of Taxpayer Dollars , I needed an Engineering Degree. So I got a BS in Materials Science and one in Aeronautical Engineering with an MS in Aeronautical Engineering. After my time in the Navy, I never used it professionally.

I am a little older than many here and have been hanging around cars since I was 12. My Late Father was an enthusiast and owned several Porsches, the 1st being a 356 Pre-A (wished he'd kept that and passed it to me).

I started racing in my 20's (Spec Class, Formula Ford, Formula Vee) and have been around that community for some time. By osmosis, if nothing else, I could not help but learn a thing or two. A lot of really smart, generous, and patient people flattened my learning curve considerably over the years, I am proud to count Paul Newman and Mario and Michael Andretti among them.

So far as Porsche Optional Wheels, I really don't know, you'll need to compare the specs. Altering the Scrub Radius does not always have negative effects or negative enough to be percieved. I doubt that Porsche spends too much time on this as their Wheel options are more about Profit than Performance. Most people add them for the aesthetics rather than performance.

The only reason for my comments was to help you insure that you realize what you want to achieve by spending the extra dough. It would be a shame to spend the extra money and end up with a worse driving or performing car...

Happy Motoring!...Jim'99