I'm getting ready to buy some new tires for spring/summer. Looking at the PS2s most likely.

I know the stock size is 235 up front, but can I go any wider(same diameter at 19) without any risk to the differential? I have read that deviating from stock sizes can burn the differential, but don't know if thats just related to diameter and not width? I was hoping to do 245 or 255 up front in an effort to help reduce understeer. Thoughts?

 

Changing the width, and based on the second measurement (which is read as a % of the width, which is rim to rim) changes the diameter. It is almost impossible to keep it the same as the sidewall sizes are increments of 5% which is a lot. 25 vs 30 vs 35.
So a 235x35 is a lot different from a 245x30... Be extra careful.
There are suspension modifications and alignment specs which can help with understeer...

 

Thanks, that makes sense. If I kept everything stock, which alignment specs would help me with the understeer?

 

You wouldn't go 245/30, you'd go 245/35. From what I read that stays within the safe range for non-OEM widths.

Now, if you try to do something like a 325 rear... well, you won't have to look far to find the thread where a guy roasted his differential because it was too far out of spec

 

Good point and this was actually done by a European tuner (not sure, I think Techart??, or Cargraphic??) a couple of years ago in that tuners' shootout that is held yearly in Europe. They specifically used wider tire up front to reduce understeer and this was mentioned in an article in Excellence Magazine. I don't remember exactly what tire they used to keep the diameters within specs so the diff doesn't get fried, but vaguely recall the front width was increased by 1/2 to 1 inch. BTW this is a rarely discussed "side-effect" of a staggered wheels setup with front narrower than rear: understeer.

As for the reduction of understeer there are a number of things you could do, but perhaps the simplest, cheapest, and most commonly used is to increase front negative camber - in my car it is increased to the max allowable, around minus 1.1. More front neg. camber is IMHO one of the most significant mods to our Turbo and I recommend it very highly.

Another very common approach (used by pro's and amateurs alike) is to adjust your sway bar. You will have to buy adjustable sway bars such as ones by GMG. To reduce understeer, you soften the front bar and stiffen the rear bar.

Another approach is to adjust tire pressure: soften front, stiffen rear tires, but of course there is a limit as to how much you could do this. I quoted a table that lists all the approaches here: https://www.6speedonline.com/forums/...-turbo-12.html

 

Thank you for your responses.

Cannga,

First off, thanks for all the informative posts you have contributed. They are an excellent resource and will weigh in heavily on the decisions I will be making with my car.

Secondly, right now everything in my suspension is stock. How much neg. camber can I go with in the front and do I need to do anything with camber in the rear?

 

A "neutral" setup on a car delivers the best grip and neutral is defined as being when the front and rear tires have the same degree of slip angle to achieve maximum traction. Tires need to slide a little bit to achieve maximum grip. What you don't want to have, say, is 10 degrees on the front and 18 degrees on the back. This would result in a great deal of vehicular yaw, say 14 degrees, which is termed oversteer. Similarly, you don't want to flip those front and rear numbers around and end up with too little yaw, say 4 degrees, which is the understeer you refer to. Both are "slow" solutions and require active driver intervention to correct. Ideally, you might want 10 degrees on both the front and rear wheels with perhaps 8 degrees of yaw in the car in order to achieve maximum speed with minimum drama. The appropriate slip angles depend upon your car, its tires, suspension setup, how the tires are tuned, precise throttle control and the use of the available terrain (off camber, positive camber, dips, crests, etc.). Consequently, while going to a wider tire up front will undoubtedly change things, you are just as likely to exacerbate your understeering problem as remedy it!

For a much more complete discussion on this topic, please read, Going Faster; Mastering the Art of Race Driving. It's written by the Skip Barber Racing School and is available on Amazon. You wouldn't think this topic requires such an elaborate explanation but, alas, I'm afraid it does. Hope this helps.

 

Great responses. To add my 2Cents, there is a reason why cars (particularly in the US) are dialed in with understeer from the factory and that is because most folks enter a corner too fast or when feeling that the car is sliding they either lift off the gas, lift off the gas and break and/or turn the steering wheel more into the turn. These actions are somewhat negated by the built in understeer.

Coming off the throttle during cornering in any 911 is not a good idea (i.e., the Off-throttle oversteer nature of the car). Pay close attention to your driving and see if on the on-off ramps or turns you tend to come off the accelerator. If you do, my suggestion is to improve the driving first as taking away the understeer gets the car closer to oversteer and that is more dangerous in these cars than understeer.

 

hello,
On the 997 twin turbo 2006 480HP, stock tires are
235/35 ZR 19 front
305/30 ZR 19 back.
If i would replace front wheels with the 265/35 ZR 19 that can be found on the porsche cayman S 295HP design edition 1 (as back tires) (and which are of the same design as the turbo tires), I would fry the differential ?
To correct things and get in the 5% tolerance, which tires should i put on those wheels ? 265/30 ZR 19 for instance ? 265/25 ZR 19 ?
does anyone know how to calculate the right tires to put on those 265 wheels in order to get in the 5% tolerance ?
thanks.

thanks for the book advice, just bought it.
(some say the main reason of the gtr going so fast round corner is balance, and reduced difference between front and back width in tires. explaining less slowing down approaching corners. couldn't 997 TT be more balanced if front tires were larger ? 265 for instance ? ...

 

Your question is very confusing because you are making some inferences which are not correct.

Tire size and wheel size are not the same. A wheel is characterized by three measurements. Diameter, 19 in, width, in inches which I don't know for your wheel and finally offset. Offset is the millimeters that the wheel is off center from the hub.

So, there are two reasons why the rear wheel from a boxter won't fit the front of a 997 width and offset.

Furthermore there is a way to read tire sizes as well. 265 is the width of the tire rim to rim in millimeters. 35 is the percentage of that 265 mm which is divided in the sidewalls top and bottom. Thus this is a variable number which affects the overall diameter of the tire size when added to the 19 inches of wheel. Note the blend of millimeters and inches a sure way to make this complicated.

The tolerance you mention is not 5% but 3%. Thus, the margin of error very small.

I believe that changing the front tire to 245 and the rear to 325 is what some people are considering but that would mean changing the wheel widths that you have from stock.

This is a very difficult exercise and you must not reinvent the wheel (pun intended!). The 911 is a different vehicle with different dynamics from the boxter or the GTR and a much larger staggered setup front to back wheels for a reason. It is not a mid or front engined car but a rear engined car, and that engine is hanging off the deep end! Yes in theory the wider front tire the better but there are significant limitations to how much you can go. Subtle geometry changes can greatly affect understear and it's the way to go.

This is a simplistic approach to try to explain to you the gaps in your thinking process as it is hard to tackle the question that you pose in the way that you do.

 

first, to be sure i understand you:

overall diameter of the 235/35 ZR19 in mm:
19*25.4+235*35/100*2=647.1 mm

overall diameter of he 265/30 ZR19 in mm:
19*25.4+265*30/100*2=641.6 mm

percentage of difference:
(647.1-641.6)/647.1*100= 0,85%

so, less than 3% so, diameterwise, it could work without frying the differential (i guess the central differential which distribute between front and rear wheels)

but the 265/30 ZR19 woudn't fit due to offset ?

 

One doesn't have to go far to hear all the "hoopla" about 50mm more width up front in the 991 and how much it helps stability and under-steer.

You can easily take stock or stockish fronts with 15mm spacers (on each side), add some front camber and a rear sway bar and you will be amazed at the amount of under-steer you can dial out (even without resorting to GT2/3 from arms.

 

i will try next week 255x35x19front 325x30x19 rear
i will let you know the results

 

you will keep the wheels and change tires (possible ?) or change wheels and tires ?
if you change wheels and tires, which wheels will you put ? (the one of the gt2 have a central bolt and not 5 bolts.. i think). Let us know what you choose exactly. thanks

by the way, thanks for both answers i had.

 

You are correct. Here is a site to help you with the calculations
http://www.alloywheels.com/Tyre_Calculator

However, here is the next question for you. How wide must the wheel be to fit a 265 tire? According to the above site, it has a minimum of 9in width. The stock wheel I believe is 8.5 width.

Finally offset. Factory front offset is 56. More means that the wheel will stick out further inside the well. You can go up to 61-62 after that it sticks out too much. Increasing the wheel width at the same time as the offset, gets a bit more confusing...

Here is the relevant tool to use.
http://www.1010tires.com/WheelOffsetCalculator.asp

You will see that moving from 8.5 to 9 and changing offset from 56 to 62 keeps the rim at the same spot on the outer side, but increases the inward wheel width by 12mm.

So the limitation would be: what would the wheel width and offset be to accommodate a 265 x 30 x 19 tire, and would it fit at the front of a TT?

Finally, to make a clarification, the ratio of 3% I believe is referring on the front to rear circumference measurement. So the next question would be what rear tires would you need to put to accommodate the 265x30 fronts?

I hope all of the above helps clarify a bit... However, the answers posted also indicate that as per my earlier post there are geometry changes that can actually have a significantly greater impact on front end handling...