Binning the Run Flats

[WLH]

Kewl video....

[AlanL]

Ok, just put 250 miles on the new shoes. Ran for about 85 mile and stopped for a picnic at the lake. Let the tires cool a bit. Outside temp when we left the beach 72F, through the valley up to 92F, at the lake 85F. On the way up the hill it is quite curvy. The tires seem to slip in the corners (front end). Checked to make sure traction control not activating, True, turned traction control off, same feel of slipping. The return home coming down the hill same thing.( coming down the hill through the curves, 91F, through the valley 100F home 81F). Reaching home , checked the tires and the rears have road dirt to within 3/8 of an inch from the tread pattern ( outside of the tire). The fronts have road dirt all the way to the outside of the tire pattern.

So, my question is, which end of the car needs a tire pressure adjustment?

A little hard to tell from your description so far. Out on a track the best indicator of optimal tire pressures is the temperature of the tread as you come off the track measured at the inside shoulder, the middle, and the outside shoulder. The goal is a fairly uniform temp between the center and the shoulders. Depending on the camber of the wheel you might find the outside shoulder to be a little hotter but the goal is still to make sure that the center is never the hottest.

On the track we have the advantage of fairly uniform sets of turns and straights we can measure for while driving out on the highways is a mixed bag and how precisely we execute turns is far more variable.

Our Z4s are supposed to have a little understeer built into the basic design as do most all modern cars (including Porsches) because it is a safer design than purely neutral or slight oversteer. Assuming that your driving tends towards neutral your car might be doing exactly what it is designed to do where the front end slips first.

Did you measure the tire pressure both cold and hot after driving a while? You might want to try that to see if it gives you an indication that either the fronts or rears are heating up more or if they are about the same.....

Lots of variables in this equation to add to your head scratching

[IMZ4N]

AlanL
Thank you for the input. I will do the tire pressure before and after and see what that is. I take it that if there is a greater difference between the front and rear of the cold and hot readings, the end that has the greater difference ( getting hotter ) needs some more air.

Example front rear

cold 35psi 40 psi
hot 40 psi 42 psi

If that was the case, I would need to add 1-2 psi to the fronts?

[AlanL]

I'd be somewhat inclined to remove some air from both. I am currently running about 35psi in both front and back hot and it feels just about right.

[IMZ4N]

AlanL

I have 35 in both front and rear .

The example above was just to see if I understand what I might see and need to do if the example happened. The information above was not actual readings.

[AlanL]

35 hot or cold?

If the front is getting hotter than the rear, do let about 2psi out of the front so you can give the sidewalls the ability to flex a little more. This may seem counterintuitive, but the problem in the front is that our cars have less negative camber up front and when cornering hard the inside and outside edges of the wheel rim on the outside wheel in the turn are no longer parallel to the ground. In a hard right turn for instance this is the driver's side front.

If the tire has too much inflation the tire sidewall can't flex enough to allow the tread surface to stay on the ground and the result is that the tire goes over on its edge and shows the wear pattern you first described. Run flats with almost no sidewall flex at all are a joke on this count. Less inflation allows the rim to float a little more while leaving the tread still square to the ground and offers better grip.

The other correction for this phenomena is to add more negative camber up front, but this can lead to more twitchy steering on the streets and uneven tire wear. This is a common tweak for heavily tracked cars but the tradeoff is always straight line steering and tire wear versus better cornering traction.

[IMZ4N]

Thank you for all the input. Just coming from RFT tire to real tire, I want to get tire pressure correct.

Bob

[DrakeZZ]

Well , The Tires are changed.

The decision was spelled out for me yesterday when I was passing a Home Improvement store and picked up a nail in the rear. The air pressure monitor let me know that yes, Houston, I had a problem. Yes it was a sidewall and yes the car does continue on runflats.

I was also within 1/2 mile of the tire store (America's Tire) , a division of The Tire Rack. I rolled straight in and they said yes dead tire and what would you like to replace them with. Past the wear bars on the rears and at them on the fronts.

In 30 minutes I had 235/45 R17 (225/45-R17 stock) replaced. BF Goodrich. $660 otd.

Wow, from the first turn out of the garage I noticed the difference in smoothness and handling. Road noise is dropped >50%, rides like a luxury car not a pickup. Turing effort was also reduced. The twitchness was gone under power. A dream on any road and no tram-lining. I don't feel the expansion joints in my back or bum either.

I did get informed that a rim did have a good dent but still sealed fine. It was bent enough that it could not be balanced. at 75 I did not feel any vibrations but I will need to watch for any wear issues.


All in all , I am pleased.

[IMZ4N]

I find switching to non-RFT a diffident + . After this weekend and another 200 miles on the tires, I feel the car soo much better and the way it turns. I do find that getting the tire pressure correct to be a huge +. Coming from RFT's to Dunlop Max Sports the tire pressure has changed. With the RFT (18" staggered set-up ) 34 psi in front and 37 psi in the rears to the non- RFT's 32 psi in front and 35 psi in the rears to hold the road very well. Make sure you scrub those new ones in and the play with the pressure a bit to see what works for your style of driving.

The difference is amazing when you actually have sidewalls that provide a function.

Mark, get a plug kit and some fix-a-flat and a compressor !

[DrakeZZ]

Mark, get a plug kit and some fix-a-flat and a compressor !

BAH I have AAA and a cell phone. I never need to be anywhere is that much of a hurry.