Forgive me if you've been over this a thousand times already but Im in need of some education...

My 997 (05) was running 265/35/18 rear and 235/40/18 fronts with good results, nice handling and feedback. Had nail in one of rears, got it patched then had nail in other and lost all confidence in the tires. SO, i went with 285/35/18s in rear (Hankook ventus v12's) and am now experiencing understeer. A lot of understeer. It's probably due to the older, rubber on the front not being as responsive as the rears but also possibly the size/diameter difference and was thinking of going with 245s on the front. My basic calculations tell me that the diameter difference is 1.75% with 14 R/P/Mile difference. If I goto 245's the difference will be 0.55% and +5 RPmile.
My questions are - will my stock 997 8inch 18's handle the 245's without tire rub (factory rims with 57 offset) and is the new understeer more due to new/old rubber (also different manufacturer (I already know that I probs should just replace the fronts to match the rears)) or should I get 235's and hope the understeer just goes away with new rubber.

Any advice you can give is much appreciated.

Thanks in advance.

 

An 8" rim is right on the bottom of the rim width spec for a 245.40.18 according to Hankook's spec on the Ventus V12 eve K110. You are adding 5 mm per side in comparison to your 235. The 245 has a 0.2" greater section width. Worse case you might want to add a 5 mm spacer to move it out but I doubt you'd need it.

More rubber in the back all else equal would be expected to go in the understeer direction. Mixing tire manufacturers, as most will say, is not a good idea and yes new rear vs. old front will probably lean to understeer. You changed two things at once:

1. New size rears, larger than previous
2. New manufacturer for the rears

 

You're right, I did change two things at once. I (foolishly?) thought I could get away with it - but obviously not......

This is my first pcar and my learning curve is steep. I couldn't believe the immediate deterioration in handling and steering response. Took me by surprise to be honest.
I've done a handful of DE's and have learned how the car feels and when something is wrong. The symptoms I experienced were what I can only describe as massive understeer and then all of a sudden, oversteer. Car was very unstable and I do not feel in full control.
Not a good sensation thats for sure.

I ordered 245/40/18's (Same model as rears) from CJ's today. Will have them installed tomorrow and see how things are from there....

 

No wonder. You increased grip in the rear and the car understeers more.

I hate to repeat this... Weissach knows a thing or two about cars (Porsches and many others). Their choice of rims and tires for a 911 is a well researched item. Hy compromise?

Unless I misinterpreted it, it appears that you installed a different type of tire in the rear vs the fronts. That is another definite no-no.

 

I whole heartedly agree. However, when I bought the car, there were different manufacturer tyres on front and back and all was well. (Dunlop sp9000 back, Yokohama advansport front).
Both were aging and the rears had puncture repairs which I did not like the sound of afterwards (almost like a thumping sound on every revolution of tyre). I live and learn.... haven't got the new fronts installed yet, so time will tell what the handling is like.

 

Thumping sound = flat spots.

You are still changing tire sizes against manufacturer's recommendations. My advice is to stick with rims and tire sizes Weissach sanctions. But that is your car and I suspect tire installers tell you they know better than Weissach.

 

These cars understeer from the factory with matched tires. They oversteer with a tap of the brakes or a blip of the throttle. At least thats been my experience.

 

When you mix and match brands on a car you never know what you are going to get until you do it. The Hankooks are also a more open-treaded tire than a lot of others are, so I suspect that you are also getting feedback from the car on it. I am finding from others as well that if you put V12's on the car it HAS to be done on all four corners or the car is going to let you know. That tire does not at all like to be mixed with others. If you get another pair your issue should be resolved.

 

Thanks Damon,
Will find out tomorrow when the fronts get installed.

With hindsight, I maybe shouldn't have gone with 285's on the back but once they were installed I was kind of stuck with them... I now know about diameter ratios etc - I was impatient and didn't do my research properly and learned my lesson. C'est la vie.

Was a real eye opener though the way the car handled so poorly.

With 20/20 hindsight I'd have bought better glasses.....

 

Just out of curiosity, is there some other specific reason not to choose the Hankooks other than Weissach said so? Not trying to be rude, just wondering.

 

I was not referring to the tire brand, but the sizes. Porsche recommends certain sizes that work well. Changing those alter the grip balance, that is all. Not to say it can't be done, of course it can, but it takes effort/experience and money. For most of us with street cars, Weissach does a phenomenal job.

 

/Weissach link please

 

Weissach is the legendary Porsche R&D facility. Do a Google search and you will find several links.

Here are some links:

http://www.porsche.com/germany/dialo...heag-weissach/

http://www.google.com/search?q=Porsc...g&ved=0CFsQsAQ

http://www.cartype.com/pages/2403/po...er_in_weissach

http://origin.porsche.com/uk/videos/...es-in-weissach

 

Fronts installed today.
Still feel a little 'greasy' but will do a couple hundred miles this weekend and see if they bed in a little.
So far though, I'm not impressed. I run MPSC's for the track with 285'sR 235'sF with great results - car feels balanced and agile. But with the Ventus V12 the car feels as though its sliding sideways with every turn.
I'm beginning to worry that I've just wasted a lot of money on the Hankooks.

 

Don't you think it's a bit much to compare Ventus to MPSCs?