Thread: D2 tyre pressures

Not that I'm likely to convince many of the "same psi all round" crew, I thought I'd share a link to a paper on reduced load tyre pressures.

http://dawsengineering.com/linked_fi...ducedloads.pdf

The thing that I found interesting was the information on the effect of pressure on the stiffness of the tyre and the amount of deflection of the tyre for a given loading.

Working from the basis that the load/pressure curve is linear - with the maximum load and maximum inflation giving the upper end point of the curve - it is possible to calculate the pressure required to support a particular load.

Using formulae 3, 4 and 5 from the paper it's possible to gain a better understanding of why the D2 has a 28:38 pressure ratio, and why it is a good idea to maintain that ratio.

The vertical stiffness of a tyre (Kz) is described by

Kz = Pressure * F + 3.45

where F is a function of tyre geometry. F takes into account width, aspect ratio, and rim diameter.

F = 0.00028 * SQRT((1.03 - 0.004 * AR) * Sn * (((Sn * AR)/50) + Dr))

where:
AR = aspect ratio i.e. 70% = 70
Sn = profile width in mm i.e. 235mm
Dr = rim diameter in mm 16" = 406.4mm

So given a 235/70R16 tyre with a maximum inflation of 353kpa (51psi) we can calculate that:

F = 0.10080554
and therefore
Kz = 38.83 kg/mm

The Kz result indicates that this tyre would deflect by 1mm for every 38.83kg load when inflated to maximum pressure.

Another example would be a 245/75R16 with maximum inflation of 551kpa (80psi):

F = 0.10417054
Kz = 60.84 kg/mm

Which shows that at full pressure this tyre would deflect by 1mm for every 60.84 kg load.

We can also calculate how far each of these tyres deflect when loaded to their maximum rating:

Tyre 1 has a 925 Kg load rating:

925/38.8 = 23.82 mm

and Tyre 2 has a 1379 kg load rating:

1379/60.91 = 22.64 mm

So what does this mean for a D2 owner? The maximum rear axle loading for an SLS equipped D2 is 1800kg or 900kg per tyre and the front axle loading is around 1200kg. Normal running weight is probably closer to 1050kg (f)/1350kg (r) in standard trim.

Fortunately formula 5 gives us the answer

Pressure = ((( load/maxload) * (maxpressure * F * 3.45)) - 3.45) / F

With the stock 235/70R16 tyre and 1050/1350 loading:

Pressure F = 184.4 Kpa (26.7 psi) Deflection = 23.82 mm
Pressure R = 246.9 Kpa (35.8 psi) Deflection = 23.82 mm

The author notes that these figures are the minimum and manufactures usually specify a slightly higher figure.

On the D2 placard are pressures 1-2psi higher and give slightly less deflection: Front = 22.91mm, Rear 22.60mm.

If we apply the same math to the 245/75R16's (Bighorns):

Pressure F = 189 Kpa (27.5psi) Deflection = 22.7 mm
Pressure R = 252.7 Kpa (36.6 psi) Deflection = 22.7 mm

Again these are minimum and an additional 1-2psi is probably worthwhile.

CGT indicated that he was running 36psi F and 42psi R with these tyres, then dropping to 20psi all round for off road.

Deflection:

36psi Front = 17.9 mm
42psi Rear = 20.1 mm

This indicates that the rears are under-inflated relative to the front tyres, and will have a larger footprint on the road.

20psi Front = 29.5 mm
20psi Rear = 37.9 mm

It's immediately apparent that "20psi all round" results in the rears deflecting 8.4mm more than the fronts.

Ultimately my calculations are based off estimates of the weight distribution based on stock tyre pressures, curb weights, and maximum axle loadings. The only way to determine spot on figures would be to put the laden vehicle on a weigh bridge and measure front and rear loadings.

cheers
Paul

Hello,

Heres my 2 cents worth.

I run 42psi front and 44psi rear. This gives me even tyre wear and good handling. Anything less in the front tyres and there is excessive wear/scrubbing on the tyre shoulders.
The pressures are adjusted every tyre rotation. This also gives me the magical 4psi increase once the tyres are warmed up. I add 2psi front and back for highway travel.

All the scientific/mathmatical formulas in the world don't always equate to real usage and wear characteristics.

Cheers,

Jason

Great stuff Offtrack - an A for effort, and I also agree with the thrust of your argument ie that tyre pressures need to be chosen to suit a range of factors and same front and back is not only simplistic but wrong most of the time.

For me, one of the more important factors is comfort. Slightly lower pressures make it more comfortable and doesn't shake the hell out of the car. Also, with the 50mm lift, higher pressures (over 40psi) tend to make the car wander a little at speed (due to caster angle change). Tyres are just about the lowest cost in owning my D2a, lasting at 4 to 5 years. Achieving an extra year by running higher pressures isn't worth it to me.

The front tyres on the D2a wear away on the edges if I use the factory placard pressures.

I always run 38 F&R on the bitumen but can see the advantage of having different pressures F&R for dirt/mud/sand etc to ensure you are getting a similar footprint from both ends.

Regards Gordo

Since doing a company sponsored advanced driving program some years ago I've stuck with the instructors recommendation of >36psi based on demonstrated reduced tyre deformation under heavy braking i.e. more rubber on the road at the higher pressures. They showed it empirically on the track and we all experienced it. That's the time it's needed and a little less comfort and wear are immaterial in comparison.

So I run 36 front 40 rear or 42 if loaded on the bitumen.

I generally run:

road: 32 fr | 38 rr (will add a 4-8 psi rr depending on load for all situations)
track: 22 fr | 25 rr - max speed 50 km/hr (more comfortable on 18's and less chance of spiking)
sand: 15 fr | 16 rr - max speed 40 km/hr (will do 60km/hr for a quick run up a large dune once in a while); I do lots of hard turning on soft sand and have never lost a bead using LR Hurricane alloys (255/55R18). Just reading about a second bead on those alloys makes be think I'll run 12 fr in future and see how it goes.

I check pressures regularly when travelling and airing back up and found that I'm getting the 4psi increase so am pretty happy with it all.

I do tend to take advantage of ACE and 18" wheels with the resulting slightly increased wear on the outsides of the fronts. After reading this thread, will go a little higher to about 34 fr but don't want to lose grip in the process.

I've noticed that the tyre places just put about 40-45psi in all tyres. Seen it for my Golf (departed), Jazz, RRC and D2 - all have wildly different tyres. I always adjust the next morning after getting any work done.

Wow some seriously high pressures here.

On the road I run about 30/32psi all round

265 75 r16 mtr kevlars.

The ride is shocking otherwise... it is still pretty hard even at around 30.

coil sprung, no ace etc etc

I'd point out that the pressures I came up with are weight dependent, hence the suggestion that to get accurate figures, you'd need to weigh the front and rear axle loadings on a weigh bridge.

Adding a steel bullbar, winch and dual battery under the bonnet will easily add 100kg to the front axle loading and as a result will require something like a 4psi increase in pressures above standard.

A lot of the recommendations for even pressures all round originate from vehicles with a far more even weight distribution that the D2. In standard trim a 5 dr D1 TDI manual for example has a 1006kg F/1047kg R. Add on a bull bar and winch and some recovery gear in the back and you'd be close to a 50:50 distribution, and even pressures would be the correct choice.

A 110 County Tdi has a curb weight split of 993kg F/ 1061kg R, add a bull bar and winch and you are getting close to 50:50kg again. However once you start loading up the back the rear pressures will need to be increased.

Maybe you don't need to worry about contact patches, but I saw a stark illustration of the effect of simply lowering to equal pressure all round the other day. With pressures lowered to 26psi front and rear, a D2 I was driving with had rear tyre bagged out as you'd expect, however the fronts looked like they were close to road pressure. This probably isn't surprising as the fronts were about -10% on what would have been appropriate inflation for the axle loading, where as the rears were closer to -30%.

Both Tom Sheppard in "The Land Rover Experience" and Michelin recommend running placard pressures (although as mentioned you'd need to adjust to compensate for bullbars, winches and under bonnet dual batteries and rear load). Both these sources also recommend similar deflation amounts:

Track and poor roads (TLRE & M): -20% and max speed of 65 km/h
Sand (M only): -30% and max speed of 20 km/h
Emergency Flotation (TLRE)/Mud (M): -40% and max speed of 20 km/h

For the placarded D2 pressures (which I'm running btw) this gives:

D2_placard_pressures.jpg


It seems the main the reason for maintaining similar sized contact patches is that you get even amounts of floatation and traction on each wheel. This means that driving force is evenly distributed to the ground through all 4 wheels.

cheers
Paul