I have upgraded the wheel size from 18 inch to 19 inch. Here is what I have notice. The front end of my 2003 996 TT doesn’t seem as stable at speeds above80 mph? It almost feels like the front end has a mind of its own, its not that the car drifts but it just feels not as tight or controlling as when I had the 18’s on the car? During high speeds turns, I don’t feel like I am in as much control as before?

My car probable needs a realignment? It slightly pulls to the right, but not too badly. Could this make the car behave in this matter?

My car came stock with these
225/40ZR18
295/30ZR18

I have replace the tires and a set of factory 997 TT rims with these tire sizes.
Michelin Pilot Sport PS2
235/35ZR19
315/25ZR19

I kept the height ratio the same when comparing the front vs the rear because I heard some of the 19 inch tire sizes have a large enough differential that you can destroy the all-wheel drive system on your 996 Turbo.

Does anybody have any idea what could be going on here?

 

Texas 996 Tt , I Have The Same Exact Problem It Seems. The Best Way I Can Describe It Is The Front End Seems Very Light And Twitchy. I Noticed This To When I Switched From 18" To 19" Wheels. It Is Very Noticable Under Wot. I Am Replacing My Shocks With Pss9 Coilovers And Adding H&r Sway Bars, Then Having It Aligned To Gt2 Specs. I Will Let You Know If This Gives My Car A More Planted Feel. Pauliewalnuts

 

You need to do a realignment most likely (adjust Toe). Try first changing your tire pressures, lower both front and rear pressures a bit (from factory reco) and narrow the gap between fr. and rear (try 2PSI apart only).

 

+1 (after all Jean is the master)

You should be stable @ 160 let alone a silly little speed like 80.

MK

 

Jean, Would The Aligniment Change When Switching From 18" To 19" Wheels ? I Have 36psi In Front(235 Ps2) & 38-40psi In Rear (315 Ps2) Does This Seem Right? Thanks For Your Help. Paul

 

Paul

The suspension geometry changes with the larger wheels. Your front suspension setup will stiffen causing understeer and and a feel of instability, mainly on uneven pavement or at high speeds. Slightly more toe-in and softer springs will help, together with slightly lower pressures on the 4 tires, which will reduce the 'nervousness". Your pressures seem fine.

If you want to cure understeer, you will need to increase the camber in the front, but make sure the toe-in goes with it (slightly more toe-in) regardless of whether it increases understeer since it will be offset by the incremental camber, or you will end up with a twitchy car for the street.

There are more radical ways but these are the initial things I would do for a 100% street car.

You might want to ask a professional shop, I am simply a user, but this works for me. I don't think it will take much to improve your handling.
Cheers

 

Your car looks great!!

 

I am having the same feeling. I just upgraded from 18 to 19's. The tires were originally set to 40psi cold. I have PSS9's which are set to 3 in the front and 5 in the back. I'm thinking I need to lower both front and back to stiffen the ride up. Was looking into some kinda sway bar as well.

 

I have the RUF coil overs and my car feels stable with either my 18's or 19's, although I do have an aggressive allignment setup for the track....

 

Yes - MAJOR UNDERSTEERING I am experiencing - pretty scary around 100 mph....

Thanks Jean, I hope this can be improved and fixed with some trial and error. I am going to do the alignment and have the mechanic drive the car first and see if they can adjust anything without having to upgrade the springs. . I am making the assumption these wheel alignment companies that only do wheel alignments tune understeerring .... adjust toe-in and offset the camber so we will see what happens. I am just very surprised that going from 18 to 19 makes a major handling difference.

Thanks

 

Texas, welcome..
I would do the following by order, for financial and simplicity reasons (please check with a good shop, this is my personal, non professional opinion):

- Experiment with the tire pressures front and rear (delta)..don't go below 36 or so (cold) for the street however
- Soften the front compression (if adjustable suspension), stiffen the rear one
- Increase slightly camber in the front
- Increase (very slightly) toe-in to make less nervous, eventhough this will not help reduce understeer
- Softer front sway bar adjustment or buy softer bar (thinner)
- If all else fails, go with 245 front tires.

I am assuming the tire thread is OK across all 4 of them.

 

Question: Do any of you who are experiencing the same problem have spacers in you wheel set up? I have 5mm spacers on all 4 tires.

Could this be the root of the problem?

 

Alignment issue...as well coil overs will bring the car down if it is not already lowered and definitly help the front end...remember even though the TT has the axel up front the majority of the weight is rear biased due to engine placement my regualr NA 996's were both floaty until I hunkered them down with H&R CO's and some sway bars...then they were like a rock...

On a further note 19's are not the right way to go IMHO stick with 18's

 

I havn't aligned it yet but trying to setup a date for next week with Arling. I'm not running any spacers. Also looking into sways. Which ones did you guys have? and how much?

Thanks!

David