01-04-2016, 10:54 AM | #1 |
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Why do rear tires require higher pressure?
BMW recommends 32 PSI front, 38 rear.
On other cars I've owned/seen its usually something like 33 front, 32 rear (I guess because front is heavier due to having an engine). Any idea why there is such a large difference between pressure in front and rear, and why rear requires higher pressure? |
01-05-2016, 09:06 AM | #2 |
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Don't have an answer but Mercedes C class is (was- since owned two) same (lower front pressure). All non German cars I have owned (AWD, FWD, and RWD) have been identical pressures (typically 35psi).
I think it is probably more a cultural need to be different than some genius design concept. |
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01-05-2016, 11:05 AM | #3 |
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Having higher pressure under the heavy end makes sense to me, having 6 PSI more in the rear - not so much.
Hope there is a reason for this, and it won't lead to faster/uneven tire wear. |
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01-05-2016, 12:22 PM | #4 |
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BMWs are usually very close to an even weight distribution so not sure that would be the reason. Plus most other cars that don't have an even weight distribution have equal tire pressure front and rear.
I think it's more to the rear drive bias of the car. The rear tires get more load under acceleration |
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01-13-2016, 07:19 PM | #5 |
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I tracked my X1 s28i M Sport with summer performance Pirelli P7's grippy rubber.
I used a laser temp gun to make sure I wasn't running over the sidewalls. This car has very close to 50/50 weight ratio because of base engine, etc., but when I put 40psi hot in front and 40 psi hot in back it felt like it didn't want to turn very easy. When I put 42 rear, it would rotate just right because I was getting less traction rear. My rears are wearing slightly more in the middle, and faster than the front. I need to lower psi in the rears to get perfect wear, and I'd not put more than 3 psi more in rear cold (35 front cold 38rear cold max). |
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01-18-2016, 05:34 PM | #6 |
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I run 35 all around with the all seasons, which seems to make a nice balance. I'm running higher rear pressure in the wintertreads, tho' I may adjust that.
I suspect the higher rear pressure is a CYA thing if the car has four passengers and a full trunk. My bride's A4 Audi called for more air in the fronts. Moon |
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01-19-2016, 08:24 PM | #7 |
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There's a discussion remarkably similar to this under "need new tires". There can be different reasons for different rates front/rear. Staggered sizes is one - the MSport x1 has these - bigger tires in the rear (at least ours does). My S2000 is the same & specs different pressures. Also generally speaking a harder tire will usually have better traction and you want that with the driving wheels. Can't really speak to Ow40x1; maybe there's an upper limit, maybe it's the driving style - that car is a rear drive & with that when the car is balanced & has power sometimes it doesn't take much to make it step out (again the S2k is same)
The A4 mentioned by halfmoonclip makes sense - isn't it mainly front drive & a little nose heavy ? I had an Integra GSR that also specd higher in front. The standard Integra was specd even but the GSR was specd differently (it also had more power & was more tightly sprung for those who aren't familiar with the GSR) The main thing is the cars were designed a certain way & tire pressures dialed in by the engineers. BMW has the rep for handling - I'm sure they're going to pay attention to that. They're saying "this is how it should be" & the cars drive very well; I would trust their specs. There's also liability if they spec wrong (Ford Explorers & Firestone). A note on varying the pressure more than a little bit - I've noticed that uneven wear-especially due to incorrect inflation- is intensified in lower profile tires. Just my thoughts . . . |
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02-15-2016, 05:43 PM | #8 |
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cdog, the Audis are primarily the VW corporate platform, which can be either front drive or AWD, so the higher front pressure helps to minimize understeer. Her car was a sweet handler on our twisty WestSylvania roads.
While the Blizzaks currently on the car don't handle nearly as well on dry road, I am running 38lbs in the rear, and I may try the higher pressure when the all seasons go back on. Moon |
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02-28-2016, 02:55 AM | #9 |
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I run 38PSI all round which was the conclusion after posting here a few years ago about uneven tyre wear. My car has staggered tyres (225 front, 255 rear).
Enthusiasts used to say that the lower front tyre pressure BMW recommend on their cars is to encourage understeer. Not sure if that's the case tho. |
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