I have a 35i with the original Potenza tyres 225/35 R19 front and 255/30 R19 rear. According to the plate on the driver’s door the pressures should be 34 psi (2.4 bar) for the front and 42 psi (2.9 bar) rear. These are much higher than any previous car I have had, especially the 42 psi at the rear so I wondered why they need to be so high. Is it anything to do with the run flats?
Why are the recommended tyre pressures so high?
- Thread starter MikeS
- Start date
I have the same car and wheels and checked the manual (Canadian version), WOW !!!, mine says 36 front and 42 rear, it does seem high and I'm glad you posted this 'coz it's something I missed. 
I have no idea why they are so high, you can't possibly load the back end of that thing unless you start hauling lead bars around......
I have no idea why they are so high, you can't possibly load the back end of that thing unless you start hauling lead bars around......
My guess is that as BMW strive for fuel econony they are seeking tyres that run at higher pressures thus for any given combination of weight of load, width, profile, etc. they run more 'round' the sidewalls distort less, reduce rolling resistance and thus increase fuel economy from wasted heat generation.
Improved suspension will compensate for any reduction in sidewall 'suspension' of using low rolling resistance tyres.
Of course they may not be lower rolling resistance tyres, so then I'd have to rethink
Improved suspension will compensate for any reduction in sidewall 'suspension' of using low rolling resistance tyres.
Of course they may not be lower rolling resistance tyres, so then I'd have to rethink
The puzzle for me guys is not to do with high pressures, but that the recommended front tyre pressures, bearing all the engine weight, should be significantly lower than the rears . .seems odd to me, I even began to wonder if the tyre pressure labels on our cars had been misprinted ?
As far as I can remember, my past rear wheel drive, front engined vehicles always had more air in the fronts than the backs . . . . . .
Ahh well, back to toasting by the fire after an arduous session shovelling snow and spreading road salt outside my igloo lol
DELBOY
As far as I can remember, my past rear wheel drive, front engined vehicles always had more air in the fronts than the backs . . . . . .
Ahh well, back to toasting by the fire after an arduous session shovelling snow and spreading road salt outside my igloo lol
DELBOY
DELBOY said:
The weight distribution of the Z4 E89:
30i 48% front and 52% rear
35i 49.2% front and 50.8 rear
So the rear of the Z4 is heavier. If BMW introduces a 4 cylinder turbo engine in ~2012, the difference will even be bigger, almost rear engine specs.
The pressures are indeed high, but don't forget, this car is heavy.
EdP2
BMWs are as close to 50/50 as they can get them and the engine sits rather far back in the engine bay.
Also, look at your brake rotors-very close to the same size. I saw a new Corvette in the parking deck yesterday and marveled at the tiny rear rotors-they're a larger diameter but "thinner" from hub out. Shows you Chevy doesn't know how to distribute weight in that monster--talk about nose heavy!
Also, look at your brake rotors-very close to the same size. I saw a new Corvette in the parking deck yesterday and marveled at the tiny rear rotors-they're a larger diameter but "thinner" from hub out. Shows you Chevy doesn't know how to distribute weight in that monster--talk about nose heavy!
Profile has little impact on pressure needed, TVR tamora is 27psi on 18" spiders with 35's!
Also, BMW have big rear rotors because they have drive hub and integral handbrake drum, vette will have a 2nd caliper for handbrake.
That on the 35i sounds mad! Let's see how your tyres wear... Hmmmm
Also, BMW have big rear rotors because they have drive hub and integral handbrake drum, vette will have a 2nd caliper for handbrake.
That on the 35i sounds mad! Let's see how your tyres wear... Hmmmm
I have had the following information from BMW about this subject: 'Essentially the pressures are increasing because they can be increased rather than because they have to be increased. Increasing the pressures delivers benefits in driving as outlined below but we have to balance recommended pressures against ride quality.
'The answer to why are tyre pressures slowly increasing over the years, when comparing a like for like tyre appears to be simply that vehicle manufacturers want as high a tyre pressure as possible but without compromising the ride quality.These figures are obvious but they are the only figures quoted by BMW.
Braking from a speed of 90 km/h to 70 km/h on wet roads (identical vehicle):
- 40 metres with the tyre pressures set to 2 bar
- 44 metres with the tyre pressures set to 1 bar
When comparing an identical wheel and tyre combination on the same vehicle:
- The vehicle negotiated the corner at a maximum speed of 96 km/h with the tyre pressures set to 2 bar
- The vehicle negotiated the corner at a maximum speed of 87 km/h with the tyre pressures set to 1 bar
The advancements in tyre manufacture have allowed the vehicle manufacturers to increase the tyre pressures without having to compromise ride quality, in other words without having to make the tyres really stiff/hard. Obviously the material has to be much stronger to safely contain the higher tyre pressures but the experience gained with very early RFTs has allowed this to be achieved without a harsh/noisy ride.
'The answer to why are tyre pressures slowly increasing over the years, when comparing a like for like tyre appears to be simply that vehicle manufacturers want as high a tyre pressure as possible but without compromising the ride quality.These figures are obvious but they are the only figures quoted by BMW.
Braking from a speed of 90 km/h to 70 km/h on wet roads (identical vehicle):
- 40 metres with the tyre pressures set to 2 bar
- 44 metres with the tyre pressures set to 1 bar
When comparing an identical wheel and tyre combination on the same vehicle:
- The vehicle negotiated the corner at a maximum speed of 96 km/h with the tyre pressures set to 2 bar
- The vehicle negotiated the corner at a maximum speed of 87 km/h with the tyre pressures set to 1 bar
The advancements in tyre manufacture have allowed the vehicle manufacturers to increase the tyre pressures without having to compromise ride quality, in other words without having to make the tyres really stiff/hard. Obviously the material has to be much stronger to safely contain the higher tyre pressures but the experience gained with very early RFTs has allowed this to be achieved without a harsh/noisy ride.
Also, from a tire manufacturer's perspective, since air (volume) is the only thing carrying a load in tires, the only way of carrying more load is to either make a bigger tire (air chamber) or increase inflation pressure.