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      04-26-2013, 10:33 AM   #1
E89Tardis
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New convert to Michelin Piolet Super Sport

I've finally joined the ranks of the Non RFT community with the 18" PSS.

I'm interested in what tire pressure is recommended for this car with these tires. Given the RFT's have such a stiff sidewall, I'm guessing BMW spec'd the nominal tire pressures with this in mind.

Does anyone know of any BMW spec's for the non RFT with a softer sidewalls and radically different roll geometry in our cars? Has there been a consensus of the ideal PSS pressure or some way calculating it based on tire size and load?

Also I'd be interested in any suggestions about the value of the original tires. All four look excellent to me with lots of deep tread. Should I put them in the forum classifieds or craig's list and what do you think would be a fair price range.
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      04-26-2013, 11:36 AM   #2
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I can only answer your last question, but am very interested in the answer to your first question, so I'll look for replies.

My experience in selling used tires (and others can testify to this as well) is that you will get no replies on this forum unless you're also selling wheels. Craig's list is a better option, IMO. At least that way you don't have to ship them ($100). As far as price goes, it's dependent on mileage, any damage and what you think is fair price.
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      04-26-2013, 11:54 AM   #3
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Quote:
Originally Posted by E89Tardis View Post
I'm interested in what tire pressure is recommended for this car with these tires. Given the RFT's have such a stiff sidewall, I'm guessing BMW spec'd the nominal tire pressures with this in mind.
Perhaps someone knowledgeable at Tire Rack can get you in the ballpark.

For a definitive answer specifically for BMW Z4, I'd recommend calling the Performance Center in Spartanburg SC (Greer, acutally). They go through some serious rubber in their various schools and have a contract with Continental, I believe, to supply the mountains of tires destined for demise. I can't imagine PC would be using RFTs on the track, so they should know all about non-RFT inflation pressures. Worth a try...
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      04-26-2013, 02:37 PM   #4
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Thanks for the suggestions. I spoke with Tire Rack and he said they use the "Bennett Guide" that recommended 32 and 35. That sounds low. I called Michelin and they recommended the door sticker 36 and 42 or if your looking at the yellow supplemental sticker 38 and 45 (18" 225/255).

For now I guess I'll go with 36 and 40 for no particular good reason.

As far as the old tires, I will go the Craig's list way.
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      04-26-2013, 03:15 PM   #5
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Quote:
Originally Posted by williakz View Post
Perhaps someone knowledgeable at Tire Rack can get you in the ballpark.

For a definitive answer specifically for BMW Z4, I'd recommend calling the Performance Center in Spartanburg SC (Greer, acutally). They go through some serious rubber in their various schools and have a contract with Continental, I believe, to supply the mountains of tires destined for demise. I can't imagine PC would be using RFTs on the track, so they should know all about non-RFT inflation pressures. Worth a try...
Having been to Spartanburg, I can attest that they do indeed use non-RFT's. In fact, I don't think Conti makes RFT's anyway. But that's a great idea, contacting them for inflation pressure.
I would be careful about buying Conti Extreme Contact DW though. I believe they're not making them anymore and after inventory is out, that's it. A quick check at Tire Rack shows they're on special, which usually is code for "close out".
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      04-26-2013, 03:20 PM   #6
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I am on 17" PSS and get 39/42 nitrogen from Costco. Seems to be about the right pressure for my car as I don't see a lot of tire wear and the mileage is decent.

Never tried performance driving on them, apart from my brief skirmish with a Z06 yesterday.
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      04-26-2013, 03:30 PM   #7
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Conti does make run flats, for my X3 35i with staggered wheels anyway - they cost a darn fortune and only last 20-25k miles but they do perform well compare dto what was OEM on our e89.
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      04-26-2013, 03:32 PM   #8
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i run 32 all around. I've gone through two sets of pilot super sport on my car and they wear evenly so i don't think i need to go higher on the psi. i have a 5 series and i pumped it at 36psi and now the center is more worn out than the outside so I don't think it needs to be so high for cars w/o run flats
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      04-26-2013, 05:57 PM   #9
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Quote:
Originally Posted by E89Tardis View Post
For now I guess I'll go with ... for no particular good reason.
If you're not going to heed the experts, at least admit YOU'RE the expert. Nothing wrong with that - I know more than almost anyone else, too!
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      04-26-2013, 07:09 PM   #10
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Quote:
Originally Posted by E89Tardis View Post
I've finally joined the ranks of the Non RFT community with the 18" PSS.

I'm interested in what tire pressure is recommended for this car with these tires. Given the RFT's have such a stiff sidewall, I'm guessing BMW spec'd the nominal tire pressures with this in mind.

Does anyone know of any BMW spec's for the non RFT with a softer sidewalls and radically different roll geometry in our cars? Has there been a consensus of the ideal PSS pressure or some way calculating it based on tire size and load?

Also I'd be interested in any suggestions about the value of the original tires. All four look excellent to me with lots of deep tread. Should I put them in the forum classifieds or craig's list and what do you think would be a fair price range.

There is no correct answer. The car mfg put a sticker on the car to cover their assets/asses over the most extreme possibilities of speed and weight for liability reasons. Remember the Firestone tires on the Ford Exploders (not a typo) - substand tire design + low tire pressure + bad vehicle geometry = blowouts, rollovers, and possible death.

Normal use

I know this is a jeep with big tires but the same principals apply for any tire outside the mfg specs.

http://www.youtube.com/watch?v=ojGsavK0Uwc


Auto cross

Put in extra 4-6 psi in your tires over mfg ratings. The reason for this is to keep your tires from rolling under during hard cornering. But how much is too much? Put chalk on the edges of your tire, in three places around the diameter, and you can see how far over the tire was going during your runs. Bleed out a little if the chalk is still showing on the tread, or add a little more if the chalk has been worn off down the sidewall. The line of worn chalk to remaining chalk should be right at the corner of the tread and sidewall. Keep notes on how many psi you ran, and where the chalk line was, for your next event.

Search of "chalking tires autocross" on the web = Lots of different ideas.

If you want the easy, or lazy, answer go with the mfg spec tag inside your door no matter what tire you put on.

Otherwise you have to experiment depending on your type of driving.

You stepped outside the box and bought non RFT's - the journey is not over. Go have fun and experiment. Personally I run 34/38 psi F/R on my 235/40x19 & 265/35x19 F/R PSS Non RFT's for normal around town and short single person trips. I bump them up by 2-4 psi depending on max load (2 people fully loaded for a trip out of town - packing my Conti Kit and Stop n Go Plug kit) or 6 psi for hard driving/auto cross.
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      04-26-2013, 07:30 PM   #11
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Quote:
Originally Posted by HerrK View Post
Put chalk on the edges of your tire, in three places around the diameter, and you can see how far over the tire was going during your runs. Bleed out a little if the chalk is still showing on the tread, or add a little more if the chalk has been worn off down the sidewall. The line of worn chalk to remaining chalk should be right at the corner of the tread and sidewall. Keep notes on how many psi you ran, and where the chalk line was, for your next event.
So what's your prediction for chalked run-flats inflated to factory-specified pressures? Shouldn't this be the baseline, control, default setup? Yet has anyone done it? Crickets...
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      04-26-2013, 07:58 PM   #12
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Good question, I'd like to know too since I dunno too much about PSIs. Generally what does more pressure do? How about less? I'm still running 36/42 f/r w/ my Michelin PSS.
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      04-26-2013, 09:18 PM   #13
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If a each tire had a design "constant" = C and you could say that the pressure required for the ideal footprint would be a function of the weight on that tire then you could calculate PSI=Weight x C 36psi=850lbs x .0423 I think that would assume that the function is linear which I bet it's not... but I'll bet the designers has a graph of each tire somewhere of the load-PSI relationship that maintains the ideal footprint. Then again I'm probably over simplifying it.

The Chalk seems like a real practical approach and makes sense but I wonder if we did it with the stock tires, how close we would be to the door label specs.

I know there are people in cubicles in some buildings somewhere that really know the answers and the reasons behind them but for now, my car rides much nicer with the PSS. Between the tires and my newly purchased 3 year Platinum Warranty, I feel like I just got a new Z4....
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      04-26-2013, 10:18 PM   #14
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Quote:
Originally Posted by williakz View Post
So what's your prediction for chalked run-flats inflated to factory-specified pressures? Shouldn't this be the baseline, control, default setup? Yet has anyone done it? Crickets...
Quote:
Originally Posted by tranquility View Post
Good question, I'd like to know too since I dunno too much about PSIs. Generally what does more pressure do? How about less? I'm still running 36/42 f/r w/ my Michelin PSS.
Mea culpa, mea culpa, mea maxima culpa. Mea culpa, mea culpa, mea maxima culpa. .......The gods honest truth is it's not that simple
J. Buffett - Fruitcakes. It's a Latin phrase or if you're a good Catholic you should understand.

Sorry for not providing you with the all the easy answers. Really guys! Get off your keisters, that's old guy talk for buttocks, and keyboards tomorrow. Get out and experiment. In racing circles (well road courses and even in a straight line) it is common to change the tire pressure by 0.5-10 psi from tire corner to corner to alter the handling characteristics of the vehicle be it a car, cycle, or truck. Your size, weight, how much load (stuff) you cart around, your driving style or purpose for the day. I gave you my numbers. My PSS's were too stiff when chalked at 36/42 for my style, as in high in the center.

Car manufacturer's and their legal departments are conservative for the most part. They are not going to publish or recommend tire pressures which achieve the best handling, at the risk of another Exploder incident. Because lazy people rarely check their tire pressures, especially now with the TPMS systems. Doah - Tire pressure must be OK - I don't have a warning message. Heck I get "On Star" messages in CA from my Yukon, now with my daughter in CO - so I can remind her to air up the tires as the cold weather closes in.

Enough said.
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      04-26-2013, 11:44 PM   #15
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Figured as much - tires is effing magic. Right.
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      04-27-2013, 03:13 AM   #16
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RFT - sticker
Non rft my preference atm is 33psi front, 32psi rear but as stated experiment a bit to find what works best for you
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      04-27-2013, 10:52 AM   #17
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Went to Discount Tire this morning for the free air check (since it's finally getting a bit warmer and nicer around here -- was even able to drive top down yesterday )

Well, anyway, guy says I shouldn't put in more than 40 psi into these tires, they are not rated that high, so the "correct" setup is 35 front and 39 back (instead of what the door sticker says 36 front / 42 back).

I have the RFT OEM Potenza, 18inch, 35i.

The side wall reads maximum of 52 psi... I didn't argue with him & just let him do his thing.
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      04-27-2013, 12:13 PM   #18
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Advise you check your tires immediately with a proper tire gauge. I can't tell you how many times the tire guys whip out their $1.25 pencil tire pressure gauge ready to check my tires. I just reach over and pull my Michelin digital gauge from the glove box and tell them "Here, try this." They always love it and ask where they can get one (Amazon under $20). And I get precise and accurate inflation pressures.
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      04-27-2013, 12:37 PM   #19
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Quote:
Originally Posted by CeEl View Post
Went to Discount Tire this morning for the free air check (since it's finally getting a bit warmer and nicer around here -- was even able to drive top down yesterday )

Well, anyway, guy says I shouldn't put in more than 40 psi into these tires, they are not rated that high, so the "correct" setup is 35 front and 39 back (instead of what the door sticker says 36 front / 42 back).

I have the RFT OEM Potenza, 18inch, 35i.

The side wall reads maximum of 52 psi... I didn't argue with him & just let him do his thing.
Yeah, these are the same guys who told me I'd have to get new rims if I was switching from RFT to non-RFt. Brainiacs, all of 'em.
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      04-27-2013, 08:33 PM   #20
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Quote:
Originally Posted by williakz View Post
Advise you check your tires immediately with a proper tire gauge. I can't tell you how many times the tire guys whip out their $1.25 pencil tire pressure gauge ready to check my tires. I just reach over and pull my Michelin digital gauge from the glove box and tell them "Here, try this." They always love it and ask where they can get one (Amazon under $20). And I get precise and accurate inflation pressures.
If they use the machine, I trust them (well, the machine really). I did check with my tire gauge once back home. Although the tires were already warmed up, it was still cool in the morning. Fronts showed about 36, backs were about 41. Since its supposed to be above 60 and maybe even 70's next week, it's close enough for me.

Btw, I really wished the Z4 would show the actual tire pressure in the vehicle status screen....
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      05-02-2013, 12:34 AM   #21
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When I had PSS on my last car, I used the same PSI as the stock RFTs. Didn't have any issues.
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      05-02-2013, 01:16 AM   #22
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Quote:
Originally Posted by CeEl View Post
Btw, I really wished the Z4 would show the actual tire pressure in the vehicle status screen....
I recently saw a post on another forum by some coder claiming he got tire pressures AND temperatures to display. Don't know the type of car he was coding (5-series I think), but it may be possible on the Z, who knows?
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