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[afuiw]

On my 128i with Sport Package and 18", I changed out the OEM RFT's for Pilot Sport AS2's. Had them for about a year now, but haven't been too happy with the loose rear end. Even going over minor bumps or man holes in the road during "normal" straightline driving will cause the traction control to momentarily kick in. Rear tires are at 38 psi as per door jamb recommendation.

Anyone else have the same problem with traction control kicking in all the time?

[imported_mega]

38 psi ?!?

[Dackelone]

Look at the rear tire sidewall. What is the build date ? I bet they are really old. BMW says no replace tires after six years old. Over here in DE the tuv says every five years or less than 4mm thread remaining. 1.2mm is min to pass TUV I think. But everyone replaces tires after 4mm or so bc of the aquaplaning on the autobahns.

[Tom K.]

Originally Posted by imported_mega

38 psi ?!?

Unless you habitually exceed 160 kph with a full load, try 32/35 psi as a starting point and see if that helps.

Tom

[boostm3]

Originally Posted by Tom K.

Unless you habitually exceed 160 kph with a full load, try 32/35 psi as a starting point and see if that helps.

Tom

Arent those high inflation pressures that you find mentioned in the owners manuals, etc, applicable only to run flats? And if you change out to conventional tires, go with the more modest inflation pressures of up to perhaps 34 or 35 psi tops?

[Tom K.]

Originally Posted by boostm3

Arent those high inflation pressures that you find mentioned in the owners manuals, etc, applicable only to run flats? And if you change out to conventional tires, go with the more modest inflation pressures of up to perhaps 34 or 35 psi tops?

BMW doesn't say as they don't recommend non RFTs. I've always used the same OEM pressures when switching to non RFTs with no problems. 32/35 is the recommended pressure for 18" wheels on a 128i with normal loads and speeds. My ZSP 128i with 17" Conti DWS tires handles bumpy corners quite well at 32/35 and provides a comfortable ride to boot.

It does seem that some of BMW's recommended pressures, especially those with large F/R differentials such as 32/39 for an E91 wagon, are primarily to encourage more understeer.

Tom

[igom3]

Originally Posted by afuiw

On my 128i with Sport Package and 18", I changed out the OEM RFT's for Pilot Sport AS2's. Had them for about a year now, but haven't been too happy with the loose rear end. Even going over minor bumps or man holes in the road during "normal" straightline driving will cause the traction control to momentarily kick in. Rear tires are at 38 psi as per door jamb recommendation.

Anyone else have the same problem with traction control kicking in all the time?

38 cold?

[boostm3]

Originally Posted by Tom K.

BMW doesn't say as they don't recommend non RFTs. I've always used the same OEM pressures when switching to non RFTs with no problems. 32/35 is the recommended pressure for 18" wheels on a 128i with normal loads and speeds. My ZSP 128i with 17" Conti DWS tires handles bumpy corners quite well at 32/35 and provides a comfortable ride to boot.

It does seem that some of BMW's recommended pressures, especially those with large F/R differentials such as 32/39 for an E91 wagon, are primarily to encourage more understeer.

Tom

BMW recommendation for the 135i rear stock runflats is also 39 psi. I dont know but ive never heard of such high recommendations for non runflats.. Does a higher rear psi encourage more understeer? I know wider rear tires than fronts does that, but didnt know about the pressure differential in that regard.

Who knows.. maybe this is why you hear comments from 135i owners about the harsh ride over bumps with runflats. On my E36 M3, ive found the best blend of performance and comfort comes with about 32 psi in all tires. My 135i is supposed to be delivered in about a month, and Ill be sure to try pressures of around 35 psi for the rears, if I find the runflats too jarring.

[RPM90]

Originally Posted by boostm3

BMW recommendation for the 135i rear stock runflats is also 39 psi. I dont know but ive never heard of such high recommendations for non runflats.. Does a higher rear psi encourage more understeer? I know wider rear tires than fronts does that, but didnt know about the pressure differential in that regard.

Who knows.. maybe this is why you hear comments from 135i owners about the harsh ride over bumps with runflats. On my E36 M3, ive found the best blend of performance and comfort comes with about 32 psi in all tires. My 135i is supposed to be delivered in about a month, and Ill be sure to try pressures of around 35 psi for the rears, if I find the runflats too jarring.

This is a good question.
I'm running 39psi rear and 37psi front, cold, with no problems.
Recommendation is to increase tire pressures if you're doing more sustained highway driving.

As far as under steer, you'd have to try it out.
Generally, you want to "soften" the suspension on the end that is slipping, or "harden" the opposite end.
So, if your front is sliding, like under steer, you'd want to soften the front relative to the rear, decrease front pressure, or increase rear pressure.
Tires are part of the overall suspension tuning.
It stands to reason that if you increase the rear tire pressure, then relatively, you are softening the front, which could help reduce under steer.
But, it depends. So, you have to experiment and see what you like.

I don't see why RFT's would need greater pressure than non RFT's.
After all, RFT's have ultra strong/stiff sidewalls to begin with.
If you want stiff sidewalls on a non RFT you have to go with higher pressures.
Too low of a pressure at sustained highway speed, I consider it 80+mph, and you run the risk of the sidewall getting too hot, which could lead to failure. 32psi cold, seems awfully low to me. I can see that low pressure for increased traction in snow, but for general driving on clean roads, there would be increased wear.

Plus, if the tire pressure is low, the sidewall is softer. The lower pressure might increase tread grip, but it can also lessen turn-in response and transitional quickness. A higher pressure results in a stiffer sidewall, which can quicken turn in response.
Again, it's all relative to all the other factors. So, you have to experiment to see what happens, and adjust to what you like.
There are obvious trade offs in any tuning.
Softer sidewall-more grip. Softer sidewall-slower steering. And vice versa.

Tires usually list their maximum pressure that is safe. I think my non RFT Conti's list 50psi. I think that's cold, but I'll have to double check.

If my pressures are too high I have no problem going lower. But, I'd like to know more first about what BMW recommends when using non RFT's.
Maybe I'll try 36rr and 34ft to see what difference it makes.

[boostm3]

Well, 32 psi is not all That low.. Actually, on the E36 M3, recommended pressures are 33 psi up front and 35 in the back.. thats for 225 tires in front and 245 tires in back. I have 245 all around on Fikse rims, and with the Dinan Koni suspension, I find that pretty harsh around town, and 32 psi works much better for general use .

I find it kind of weird that on the 135i, supposedly, the wider tires in back are a nod to Promote understeer, yet the rock hard rear tire recommended inflation pressure is supposed to counter understear?? Its like they want to promote understeer with the larger rears than fronts, and then counter that with the harder rears than fronts....?

[MOCKBA]

I do not like how car feels even at 35psi, so I 38psi recommendation looks fair enough. My Benz has non RFT, so recommendation is 30/35 or 33/42 for fully loaded.

[Sit]

Just as a side note, we had the RFT (on fancier rims) from a 135 put on our 128 at delivery. Once they got the tires off, the found that two of them had bulges in the side walls, so we had to wait on delivery so they could put new tires on. My wife's boss has a 6 series and she had ultimately replaced 4 RFT due to bulges in side walls. So much for thicker tougher side walls.

As for recommended tire pressure from BMW vs tire manufacturer, this comes up alot with motorcycles too. The psi on the car is what is recommended for the OEM tires. Once you switch away from those the PSI can change due to the actual tire you are running. Check with the tire maker and use their PSI. That's what I would do.

[RPM90]

Originally Posted by boostm3

Well, 32 psi is not all That low.. Actually, on the E36 M3, recommended pressures are 33 psi up front and 35 in the back.. thats for 225 tires in front and 245 tires in back. I have 245 all around on Fikse rims, and with the Dinan Koni suspension, I find that pretty harsh around town, and 32 psi works much better for general use .

I find it kind of weird that on the 135i, supposedly, the wider tires in back are a nod to Promote understeer, yet the rock hard rear tire recommended inflation pressure is supposed to counter understear?? Its like they want to promote understeer with the larger rears than fronts, and then counter that with the harder rears than fronts....?

I'll have to try out some lower pressures to see how it feels.

But, I've tried 32 in front, and the non RFT Conti's don't like it very much, as they have a softer sidewall in comparison.
The steering is slow and overall feel is mushy.

BTW, for those interested, iy seems that Bridgestone and independent car testers, agree that the RFTs on our cars ARE harsher than conventional tires. The 3rd generation of RFT's from Bridgestone sound very promising.
Here's a link:
http://www.autoblog.com/2009/07/06/r...ck-out-of-run/

What's funny is that BMW kept saying that they specifically tuned the suspension for RFT's so that the card still have that BMW ride and road feel.
But, even that test demonstrates what a lot of BMW RFT drivers already know, the promise and reality are not the same.

Is BMW using 3rd gen RFT's in the new 2011 1 series?
If so, then they might be worth keeping.

[afuiw]

I have the stock 18" sport package rims in Canada. The OEM RFT's were 215/40 and 245/35. The Pilot Sport AS2's didn't come in those exact sizes so I'm running 225/40 and 255/35 -- slightly wider than stock.

Always thought 38psi was high for the rears. I'll give 35psi a try to see if it helps.

Interesting to know that the E91 has 39psi as the recommended rear presssure. Guess those are RFT's?

[Tom K.]

Originally Posted by afuiw

Interesting to know that the E91 has 39psi as the recommended rear presssure. Guess those are RFT's?

I just rechecked both manual and doorjamb of my '07 E91 and for the OEM 16" RFT all seasons, the manual states:

4 passengers + luggage up to 100 mph: 32/39
4 passengers, no luggage - all speeds: 32/36
4 passengers + luggage - all speeds: 36/44

The door placard doesn't specify loading, only 32/39 for "normal" and 36/44 for over 100 mph.

So as I normally drive with a light load (and under 100 mph), I find 33/36 works pretty well.

Tom

[Hops128i]

Originally Posted by RPM90

What's funny is that BMW kept saying that they specifically tuned the suspension for RFT's so that the card still have that BMW ride and road feel.
But, even that test demonstrates what a lot of BMW RFT drivers already know, the promise and reality are not the same.

That's a great article, thanks for posting the link. There are two points in the article that the contradict the 'official' line I get about run-flats. First, I've been told by the dealer and tire shops, a runflat can't be repaired and any circumstances (total nonsense). And the article states: "(when we experience a loss of pressure with a run-flat, we immediately pull over safely in the hope that the tire may be properly repaired before it is destroyed by driving on the vehicle)"
The other official line I get about run-flats is the suspension is tuned to the run-flat, and if I switch to non-runflat's my car will handle worse than an Oldsmobile Cutlass Ciera and my safety will be in question. Well, here in the article they are running non-runflats and runflats and I didn't hear them complain about poor handling.

[RPM90]

Originally Posted by Tom K.

I just rechecked both manual and doorjamb of my '07 E91 and for the OEM 16" RFT all seasons, the manual states:

4 passengers + luggage up to 100 mph: 32/39
4 passengers, no luggage - all speeds: 32/36
4 passengers + luggage - all speeds: 36/44

The door placard doesn't specify loading, only 32/39 for "normal" and 36/44 for over 100 mph.

So as I normally drive with a light load (and under 100 mph), I find 33/36 works pretty well.

Tom

The yellow sticker on my 135i is 36ft 38rr.
For 100mph+ sustained speeds and load, the rears are 42psi.

I split the difference and am running 39/40 rear 37/38 front.

Today, I lowered the pressures by 3lbs each.
I'll see in the next couple days.