BMW 1 Series Coupe Forum / 1 Series Convertible Forum (1M / tii / 135i / 128i / Coupe / Cabrio / Hatchback) (BMW E82 E88 128i 130i 135i)
 





 

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      07-24-2009, 11:19 AM   #1
dr.bmw
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2008 125i  [5.00]
Exclamation First tyre puncture, considering swap for non RFTs

Sensor picked up a flat the other day
went to Mobil and checked tyre pressures - about 16 -20 psi all 4 wheels. Can't figure out which one was causing the sensor to go off.
Pumped all up to 32 psi and a week later sensor goes off again
Can't tell which one's flat by pushing on tyres, no visual clues what so ever!
tyre pressure gauge picked up 16psi right rear, rest are at 32 psi

Took the car to Bridgestone tyres only to be told RFTs can't be fixed and i need to buy a new tyre!

Took the car next door to Bob Jane and got the tyre repaired.

Apparently Goodyear RFTs can be repaired at the technitian's discretion as long as they dont' show any signs of damage from prolonged driving on flat tyre. Others, like Bridgestone RFTs must be replaced.

Also heard that replacing the tyres with non-RFTs is problematic because
1. suspension set up will not work adequately with softer tyres, creating a significant "wobble" effect
2. Certain insurance companies will not honor any claims as this is counted as a modification and the car is designed to run only on RFTs

All sounds like a pile of bullshit to me

Now, does anyone know for certain of any obligatory conditions placed on tyres for BMWs? i.e. any insurance or warranty issues when changing from RFTs to non RFTs

How many of you have done the switch and has anyone noticed any negative side effects?

How do you cope with a flat on a non-RFT tyre?

And lastly, for 125i what are the pressures for front and rear in psi? manual gives different units and I'm confused as to recommended pressures. Tyres are stock RFTs 205/50/17 and 225/45/17 front/rear.

Cheers!
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      07-26-2009, 05:22 AM   #2
sparoz
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The BMW spec includes Non-RFT as specifications - there shouldn't be any problem with warranty claims as the tyres are correct any specifications.

I am more and more suspicious with the claim that the suspension is "design" for RFT. It is certainly tune to give a better ride to the runflats - I recently had a drive my wife's 125i with RFT, and the bounce and wobble is a lot worst than the non-RFT. The NRFT seems to able to dampen the bounce on the road a bit better than the RFT, and I have got PZeros which is one of the harder casings

On a straight road it certainly doesn't bounce more, but it is the body roll that I thought it enhances. To my surprise, the 125i still have the same body roll initially, but it doesn't seem to extend as far as the 135i.

My theory is that the suspension is just too soft, it just can't cope with the now stickier tyres of the non-RFT. It has got nothing to do with it's design to work with RFT - it's more like design to give a better ride with RFT.

The reason why I said this is that there's a 360 degree up ramp on a regular route I take. The fastest car I was able to take over was a IS250. The E46 was about 5ks slower, but you can hold onto the understeer as it is quite progressive. The biggest difference is that these two cars actually had much lower entry speed that the 135i. The 135i can approach with a much higher entry speed, able to have a good initial turn-in and than understeer mid corner, and as you dial down the speed - it snaps back with grip. By far, the 135i with run flats was about 10ks slower than the E46, and there are lots of understeer, and as you take the accelerator off, it snaps back in, making you have to adjust the steering to keep the line. The 135i with non-RFT, the point where you understeer is only about 3-4ks below the E46 where I am comfortable with.

What does all these means? I AM NO EXPERT, but this is my opinion:

The IS250, and the E46 have a much flatter stance as it goes through the corner - which gives a much better progressive understeer. I would say there might be a little less understeer for the IS250 so it can carry higher corner speed, but since I have only test drove the corner twice, and I won't know for sure. The E46 318i had the least engine out of the lot, and the understeer is progressive and hard to detect. As you slow down, the understeer gradually, decrease ie, you might seems to have a better control of the car.

The E82's problem with RFT is that it has least grip out of the lot to start off with. The car actually handles the best - giving it the best entry speed. However, as it goes in the corner the suspension loads on one side until the inner front looses grip and understeer. I am certainly you can maintain or increase the speed - but your confidence certainly decrease when you feel the body rolling so much in the corner and you back off. What scares you is then is the car starts to go where you are pointing to, which is towards the gutter into the corner.

The 135i with non RFT gives even higher entry speed, and able to hold at a higher speed through the corner. It didn't have as much understeer at mid corner probably because of the better grip level, but you still have similar problem as the RFT. What shows now though is the amount of body roll you have through the corner, even worse than the RFT. Why it actually gives more body roll - my assumptions is that it has better grip. The body doesn't understeer until later and not until the body roll further until; the tyres also have a little more give hence the extra body roll before the weight transfer finally unloads the inner tyres too much and it understeer.

I could be making all these up - but that's what I experienced and that's my conclusion - but other more experience drivers can correct me if I am wrong - but be gentle.

Pressure wise it will depends on the tyres you choose. I am on PZeros and they suggest lower pressure because of the harder casing 36/34. I found those pressures work quite well - but I might increase the pressure by probably 1 psi up the front to give a better turn in.

As usual.. I have talk too much on something I don't fully understand.
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      07-26-2009, 05:26 AM   #3
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A shorter piece on OP 1 and 2.

1. The car actually performs better with non-rft. It just shows more of is inadequacies. When they say the suspension is designed for RFT, they actually means they design them to ride better with rft. Non-rft with current suspension performs better and ride better. You feel more body roll because you have better grip.
2. As non-rft is also part of the specs as BMW can't force the consumers to use RFT, the insurance should not be a problem. I have checked with my dealer and they say warranty is not a problem as well. But if you want to be sure, you better check with your insurance company.
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