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  #1  
Old 06-14-2009, 02:34 PM
Spagolli94 Spagolli94 is offline
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Another Tire Pressure Thread

I'm starting another tire pressure thread b/c I have some specific questions and can't find answers using search...

My door jam and owner's manual say 32/26 for my 18" staggered sport package. However, this number is for the stock RFT. I'm running non-RFT so should I go higher, lower, or stay the same? To throw in yet another variable, I bumped up the tire size from stock. To bigger tires typically need more or less pressure?

I tried running some high pressures, but this made the tires feel a bit harsh and brought back bad flashbacks from my RFT days. Then again, if I go too low, the tires start to feel too squishy.

Given that most of my driving is done at 25mph over manhole covers and potholes (sad, but true), I'm more likely to error on the side of squishy. I had 35/37 in today and felt it was too harsh. I'm thinking of trying 31/33 - is that too low?

What are the rest of you non-RFT guys running? I'm sure if I lived out in the burbs, I'd go with a higher pressure 'cause the tires seemed really responsive with 35/37.
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  #2  
Old 06-14-2009, 02:47 PM
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Did you check the back of the manual? I recall they had some fairly long lists of options.

I don't think the RFT is different than the non-RFT when it comes to tire pressures.

I'd call somebody up at the Tire Rack and ask them. The guy I spoke to about something a while back was very helpful.
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Old 06-14-2009, 03:07 PM
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ish ish is offline
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[QUOTE=Spagolli94;4267400]I'm starting another tire pressure thread b/c I have some specific questions and can't find answers using search...
QUOTE]

Those pressures sound way to low for my taste. I'm running 36f 34r on 18" non rft on my wife's X3.

PS. Great color combination on your car.....LOVE it!
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  #4  
Old 06-14-2009, 03:21 PM
Spagolli94 Spagolli94 is offline
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[QUOTE=ish;4267475]
Quote:
Originally Posted by Spagolli94 View Post
I'm starting another tire pressure thread b/c I have some specific questions and can't find answers using search...
QUOTE]

Those pressures sound way to low for my taste. I'm running 36f 34r on 18" non rft on my wife's X3.

PS. Great color combination on your car.....LOVE it!
I know I think I tend to run much lower than most on this board. Then again it's all really dictated by my road conditions. Trust me, I wish I could run higher pressure and enjoy the benefits. In fact, if I ever plan a road trip out of the city, I plan on setting the pressure higher just for that.

I noticed all the combinations in my manual - but I thought those were for RFTs? Just for the hell of it, I tried 36/37 today. After a half hour, I measured the pressure and I was at 39/40. That's an acceptable increase (sun was out, roads were hot), but when they warmed up that high, they did become noticeably harsh.

Why is the stock setting 4psi higher in the rear? I've never understood this.
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Old 06-14-2009, 03:23 PM
Spagolli94 Spagolli94 is offline
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Originally Posted by ProfessorCook View Post
I don't think the RFT is different than the non-RFT when it comes to tire pressures.
Maybe I should just stick to the stock setting then? Might be the safest bet.
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Old 06-14-2009, 04:06 PM
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I would for now. But you did go to wider tires. I think that would make a difference in the pressure needed. Probably up from the standard pressures. I'm gussing 34/38.
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Old 06-14-2009, 04:20 PM
Spagolli94 Spagolli94 is offline
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So as a rule of thumb, do wider tires typically require higher pressure?
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Old 06-14-2009, 04:22 PM
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I think so. But I'm no expert.
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  #9  
Old 06-14-2009, 04:36 PM
Spagolli94 Spagolli94 is offline
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Yeah, but I heard you stayed at a Holiday Inn last night.
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  #10  
Old 06-14-2009, 05:35 PM
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I run 36 front and 37 rear and find this to be the best from a performance standpoint (and even tread wear for the last two sets of tires), at least with H&R swaybars, M3 front suspension parts, and some aggressive driving

I think the danger of running low pressures is that your wheel is more susceptible to damage if you hit a pothole or other unavoidable road hazard......
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  #11  
Old 06-14-2009, 06:00 PM
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The best test of correct tire pressure is to check tire wear. The loading of your car and style of driving will determine the optimum pressure. I watch for edge wear on front tires as a sign that they are too low in pressure and for centre wear on the rear as a sign that they are too high in pressure. 36 psi front and back sounds OK to me for general touring. On track days taking it up to 40 psi will stiffen the side wall for better cornering. I agree that wider profile tires need higher pressure to create a good contact patch but again it is trial and error to get it right, in my opinion.
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Old 06-14-2009, 06:18 PM
R1200 R1200 is offline
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The best test of correct tire pressure is to check tire wear. The loading of your car and style of driving will determine the optimum pressure. I watch for edge wear on front tires as a sign that they are too low in pressure and for centre wear on the rear as a sign that they are too high in pressure. 36 psi front and back sounds OK to me for general touring. On track days taking it up to 40 psi will stiffen the side wall for better cornering. I agree that wider profile tires need higher pressure to create a good contact patch but again it is trial and error to get it right, in my opinion.
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Old 06-14-2009, 06:23 PM
Spagolli94 Spagolli94 is offline
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Quote:
Originally Posted by wrh3 View Post
I think the danger of running low pressures is that your wheel is more susceptible to damage if you hit a pothole or other unavoidable road hazard......
This is what I'm concerned about. I like the idea of dropping the pressure for comfort. But then I'm less likely pay as much attention to the pavement and more likely to damage a rim. Especially because I have the Koni FSDs helping to hide the potholes as well. I finally got my car all sorted out, so that's the last thing I want to do.
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Old 06-14-2009, 07:01 PM
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[QUOTE=Spagolli94;4267494]
Quote:
Originally Posted by ish View Post
Why is the stock setting 4psi higher in the rear? I've never understood this.
Because higher pressure in the rear increases understeer and BMW wants to make sure no one spins out in their new car while "pushing it" on curvy roads. Tho they (apparently) sell The Ultimate Driving Machine, not everyone who buys one can drive... so they make sure everyone *thinks* there's some magic to having wide rear tires running higher pressures. The reality of it is, wider rear tires = understeer... increase the rear pressure on those wider tires, even more understeer.

Size of tire has nothing to do with what pressure to run. 30psi in a 16" tire is the same as 30psi in a 22" tire... it's 30psi. Big rigs run super high pressures to support the heavy weight of the vehicle riding on them, not because they are big-ass tires.
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  #15  
Old 06-14-2009, 07:55 PM
Spagolli94 Spagolli94 is offline
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Thanks Jeff. I think I'd like to try matching pressures, or even going somewhat closer to matched. Given my horrendous pavement I have to drive on everyday, I'm more inclined to keep the front at the suggested 32, and drop the rears to 33 or 34. My hope is that is enough pressure to still be responsive and resist rim damage, but not too much pressure to be harsh. Gonna try that tomorrow and then post what I think. Now that I got my suspension sorted out, tire pressure is the last variable to conquer.
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