I'd like a peer review of the following pls...
E89 tyre pressures
I’m changing to non run flat tyres..should I use the same pressures as run flat tyres on E89s?
Almost certainly not.
The E89 used an early design of run flat tyre Bridgestone Potenza RE050A that used extensive strengthening of the side wall to support the vehicle in the case of pressure lost. This is extra strengthening mandated slightly high pressures to maintain the correct profile compared to a similar non run flat tyre. This extra pressure is required also to stop the sidewall flexing as much as this, over time, damages the strengthening that supports the tyre in the event of pressure loss. The combination of stiff sidewall and higher pressures combine to create a disproportional less compliant ride and at the same time the tread cannot flex as well in corners creating negative handling characteristics when compared to current non run flat tyres. Later designs of run flats have to an extent engineered their way around some of these issues.
What pressures should I run with x tyres with y profile?
A few basic principles. For each tyre there is a load rating and speed rating. The highest pressures will be required when the actual load on the tyre approaches the maximum rated load of the tyre. Tyre pressures are also increased to support both transient and sustained high speed.
Some manufactures keep it very simple for the public with only one set of pressures. On the Mini F56 they have two sets of pressures, one for upto 2 passengers and one for upto 4 passengers, Volvo on the 2008-2016 XC70 have 6 pressure settings, Lower pressures for lower loads and higher pressures for higher loads and additional pressures for higher speeds.
BMW on the Z4 G29 do the same for speeds but not loads, but not in all markets.
Typical speed ratings are designated by single letters and on performance cars are for example V for speeds up to 149 mph Y for speeds up to 186 mph and ZR means speeds in excess of 149 mph. For initial approval and possible insurance reasons it is necessary to have tyres with speed ratings equal to or greater than the maximum possible speed of the car.
Load ratings are more complex and using a three numbers to designate the maximum static load the tyre can sustain. For example 108 equals a max load of 1000kg.
For the same reasons as speed ratings the load ratings have to equal or exceed to original manufacturers’ tyres load ratings.
So a Continental Sports Contact 7 275/19 30 has a load rating of 96 ie 710 kgs and a max pressure of 50 PSI for that load. Relevant is a higher profile tyre has a higher load index..the same tyre in 35 profile has a load index of 100 which is 800 kgs max load.
Now if we pair this rear tyre 275/19 30 with a matching front tyre say a 235/35 19 then we see its load index is 91 ie 615 kgs at 50 PSI.
Therefore it could be argued that the correct pressure for a set of fully loaded tyres should be 50 PSI for sustained speeds in excess of 149 MPH.
However is we look at a simple example and say our Z4 E89 weights fully loaded 1,800kgs and if we say for simplicity that weight distribution is 50/50 then we can see that each wheel has to support 1800/4 = 450 kgs.
Now our front tyres can support 615 kgs and rears 710 kgs. So the tyre pressure to keep the correct profile will be reduced from 50 PSI to something less. Sadly there is no mathematical formula that gives that answer.
The way that either the car manufacturer or tyre manufacturer determines this is by heuristic texting; a posh word for suck it and see.
The tyres are set at a guestimate based on past experience and then the tyres are tested on track and road. The distribution of tyre temperature across the tread indicates where and how the temperature of the tyre varies, underinflated its generally hot and hotter on the edges, over inflated generally cooler with max temp in the centre of the tread.
Wear rates, wear pattern and how the tyre interacts with the complex mechanics of suspension and steering allied to subjective and technical measurements of ride comfort and handing then result in proposed tyre pressures.
So the correct tyre pressure is a compromised solution based on the anticipated load, the expected speed envelope and the required steering responses, ride comfort and wear rates required.
Related to the Z4 E89 it can be seen by both looking at the preceding E85 and the successor G29 Z4s that typically pressures for a wide range of tyres in the 17” to 19” sizes range from around 28-32 PSI typically fronts to 32-36 PSI typically rears.
Examples:Z4M 30/32, Z4 M40i 32/35
