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Old 11-04-2005, 09:07 AM   #1
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Oversize tires on 2001 Boxster?

Hi, guys. It is time for new tires for my baby. I currently have the 17" stock "Boxster" wheels on my 2001. The rim widths are 7.0" and 8.5". The tires are Conti SC2s, 205/50 and 255/40. I would like to install Michelin PS2s on the stock wheels. Per the Michelin website the 225/45 and 265/40 look to be good candidates.

I have a couple of questions you can help me with. The 265/40 requires a 9.0" rim width. Will there be a problem with this tire on a 8.5" rim? Has anyone tried this? Any experiences? What is the maximum mismatch in radius or revolutions/mile acceptable before the ABS starts to act-up?

Thanks in advance.

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Old 11-04-2005, 11:14 AM   #2
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Quote:
Originally Posted by cyclocross
Hi, guys. It is time for new tires for my baby. I currently have the 17" stock "Boxster" wheels on my 2001. The rim widths are 7.0" and 8.5". The tires are Conti SC2s, 205/50 and 255/40. I would like to install Michelin PS2s on the stock wheels. Per the Michelin website the 225/45 and 265/40 look to be good candidates.

I have a couple of questions you can help me with. The 265/40 requires a 9.0" rim width. Will there be a problem with this tire on a 8.5" rim? Has anyone tried this? Any experiences? What is the maximum mismatch in radius or revolutions/mile acceptable before the ABS starts to act-up?

Thanks in advance.
It is not recommended to put a 265mm tire on a 8.5" wheel. The Wheel can not property support the tire and it probably wont perform the way it should. The 225/45/17 in the front will work just fine. I had the PS2 on my stock 2001 17s before i opted for the 18" upgrade. Personally i felt the PS2 was TOO soft of a tire. Its not a as great as i thought it would be. If i could do it all over again for that set i would go with the basic Pilot Sport.

I had 225/45/17 in the front and 255/40/17 in the rears.


My favorite set of tires that i have ever purchased would have to be the Bridgeston S-03s. Absolutly best tire i have had.....And i go through a set every about every 3-6months...
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Old 11-04-2005, 01:36 PM   #3
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Hi,

Wider Fronts are OK to a degree, will help to neutralize the Understeer, but watch out for BumpSteer..

For the Rears, Don't do it. Wider Tires in the Rear will tend to increase Tramlining and the Rear will be lighter (fewer lbs./in.≤) and can Breakaway more suddenly, increasing HydroPlaning in the Wet. Besides, the Tire you describe won't work on the Wheel you have. Hope this helps...

Happy Motoring!...Jim'99
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Old 11-04-2005, 10:06 PM   #4
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What is bumpsteer ?

I know what understeer, oversteer, camber, caster, toe-in, toe-out, etc., etc. is but what is BUMPSTEER ????????

Mark.
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Old 11-05-2005, 06:37 AM   #5
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Originally Posted by markk
I know what understeer, oversteer, camber, caster, toe-in, toe-out, etc., etc. is but what is BUMPSTEER ????????

Mark.
Hi,

Here goes... What is Bumpsteer?

The technical definition of Bumpsteer is a change in Toe Angle caused by the Suspension moving up or down. Bumpsteer is built into the geometry of the Suspension and Steering system, and has nothing to do with turning the Steering Wheel. The effect of Bumpsteer is for the Wheel to toe-in or toe-out when the Suspension moves up or down. This toe change or steering occurs any time the Suspension moves, whether it is from Body Roll, Brake-Dive, or hitting a Bump in the Road. Bumpsteer is undesirable because the Suspension , not the Driver, is steering the Car. This effect can range from barely noticable to severe negative feedback to the Driver through the Steering Wheel. While only an annoyance in Normal Driving, in High Speed or Spirited Driving, the Car, which may already be at the Limit, could hit a Bump and go completely Off-Balance, which may result in a Loss of Control by the Driver


What Causes Bumpsteer?

The Front Wheels do not move directly straight up or down when the Car hits a Bump. Instead, the Wheel follows an arc, or curving path, that pushes the Wheel slightly inward (towards the centerline of the car) or outward (away from the car) in response to vertical Wheel movement. The Outer Tie-Rod (which connects the Steering Rack to the Wheel) also moves in-and-out in an arc as it moves up and down. If the rate which the Outer Tie-Rod arcs in or out does not match the rate the Wheel moves in or out, the Wheel will be turned by the Tie-Rod. This is Bumpsteer. The center point of the arc traveled by the Wheel (known as the instant-center) is controlled by the location and angle of the moving Suspension Links and the Center of the Tire's Contact Patch. This point moves as the Ride Height or Tire Width changes (since Side Wall height is determined by the Aspect Ratio, increasing the Width of the Tire also slightly raises the Side Wall height and therefore the Ride Height or more accurately, the Instant Center) . In contrast, the arc of the Outer Tie-Rod is controlled by the position of the Steering Rack, which is fixed.


To eliminate Bumpsteer, both the length and the center point of the two arcs must be the same. But, since the instant center moves with Ride Height, Bumpsteer cannot be totally eliminated throughout the entire range of Suspension travel. Consequently, Suspension Designers concentrate on minimizing Bumpsteer within the range of movement closest to Factory Specs, especially Ride Height and Tire Width. Changing the Ride Height, Tire Width, or other Suspension components may move the Suspension outside this narrow optimized window.

To fix a Bumpsteer problem, you need to alter the height of Outer Tie-Rod relative to the Steering Rack or change the Instant Center. Small changes in this relationship can be made with Offset Rack Bushings. Making big changes requires Adjustable Tie-Rod Ends, often known as a Bumpsteer Kit. Many Aftermarket Companies offer Bumpsteer Kits such as Baer Brakes, Steeda Autosports, Competition Engineering (although I don't know if a Kit is available for the Boxster). When installing a Bumpsteer Kit, it is highly-recommended that you have a Suspension Shop install the kit. It is advisable that an Alignment Rack be used in the installation of the Bumpsteer Kit. Using an Alignment Rack allows the Technician to monitor the changes in Toe with Suspension Travel and make changes in the Bumpsteer Kitís settings to properly adjust it.

Hope this helps...

Happy Motoring!...Jim'99
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Old 11-05-2005, 07:53 AM   #6
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Great explanation, thanx.

Now I wonder, I have the factory installed sportsuspension on my S which sits, by heard, 10mm lower. So what happens to bumpsteer and what is described as the narrow window of optimalization there ?

I assume, because it was not just lowered but factory installed/option they corrected accordingly.

Mark.
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Old 11-05-2005, 09:46 AM   #7
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Quote:
Originally Posted by markk
Great explanation, thanx.

Now I wonder, I have the factory installed sportsuspension on my S which sits, by heard, 10mm lower. So what happens to bumpsteer and what is described as the narrow window of optimalization there ?

I assume, because it was not just lowered but factory installed/option they corrected accordingly.

Mark.
Hi,

I don't know for certain, but am fairly confident that Porsche did compensate and fully Test the SportSuspension before putting it into Production.

This is an area where people doing AfterMarket Upgrades are, IMHO, taking some risk. They either assume the AfterMarket Co. has done this Research and Testing, or they're ignorant of it altogether. Most people don't really understand Suspension Geometry and it's various associated parts like Camber, Caster, Toe, Ackerman Angle, Scruff Radius, BumpSteer and on and on, or how these are all affected and inter-related.

The Boxter is really very limited production, so much so that I doubt that any of the AfterMarket Springs or Shocks are purpose-built exclusively for it. There simply isn't enough Demand to warrant the R&D and Engineering expense for this. Consequently, the components packaged as a kit are likely to be Off-the-Shelf components which are used on a variety of Cars, so I suspect there are compromises being made by the AfterMarket Companies. Of course, it would not be in their better interest to report on this. I suspect they chose existing components which, through Trial and Error (but of course even this is an assumption), were found to be a Marketable Product.

People either don't know enough about Suspension in General to know it's not right, or are feeling the Placebo Effect - they spent all this $$ so it must be better and EVERYBODY else says it's the thing to do so it must be better.

For many owners, Suspension Upgrades are simply another form of Automotive Jewelry, many are motivated merely by the need to Customize, or Individualize their Car, or acquire Bragging Rights. We see many owners on these Forums who have done the expensive Suspension Upgrades on a Car which has never seen the Track or an Auto-X, whose fastest Speed Run is the DownRamp on the Freeway and whose tightest Twisty is a CloverLeaf. That's all OK, it's their Car and their Money. But, in truth, their riding on a serious OVERBUILT Suspension, which they'll never derive any real benefit from. I regularly both Track and Auto-X my Boxster. It's got the Stock Suspension and performs great. Any increase in Lap Times for me is gonna come much more from improving my Driving Skills than trying to better the already excellent OEM Suspension, and I haven't compromised the Ride Quality.

Porsche has some real Suspension Expertise, probably Second only to Lotus. This is why I would be very hesitant to mess with the Factory Set-Up and would only go with AfterMarket Components after doing a LOT of Research. Hope this helps...

Happy Motoring!... Jim'99

Last edited by MNBoxster; 11-05-2005 at 07:20 PM.
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Old 11-06-2005, 06:30 AM   #8
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oversize tires

Thank you for all of your input. Yes, the 8.5" rim is too narrow for the 265/40. I wanted to see if anyone had any experience with this combination.

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