Tyre inflation - non-standard

[martyn668]

Calling all maths graduates and tyre engineers:

 

I have just upgraded the tyres and wheels on my 90 Td5 CSW to BFG KM02 MTs on Dotz Dakar steel wheels. So now I need to work out what I think the tyres should be inflated to. I found this calculator http://www.uklegacy.com/forums/index.php?/topic/98525-calculate-correct-tyre-pressures/  

 

Here are my figures

 

Weight 1870kg (source https://workshop-manuals.com/landrover/defendertd5/general_specification_data/eec_vehicle_kerb_weights/) (Note EEC kerb weight = Unladen weight + Full fuel tank + 75 Kg driver (165 lb))

 

+ roll cage 70kg (source https://www.devon4x4.com/90-defender-station-wagon-full-internal.html - not the same as mine is fully external, but close enough. My guess would have been heavier than this, but hey ho.)

+ winch 10kg

+ spare wheel carrier 20kg

+ larger wheel / tyres (not sure if I need to include this unsprung weight? At any rate, the rims are 1.5kg heavier each than standard. I guess the tyres a bit more, so for an extra 15kg total I’m going to ignore them here)

 

Load rating of tyres, as taken from the sidewall

Max load (single) 1380kg at 450 kPA(65PSI) cold.

 

My total GROSS WEIGHT OF VEHICLE as above = 1,970kg

 

1,380 x 4 tyres = 5,520kg (max load)

 

5,520 (max load rating) / 1,970 (vehicle weight) = 2.80

 

65PSI (max tyre PSI) divided by 2.80 = 23.2psi

 

Then as a comment on pistonheads at https://www.pistonheads.com/gassing/topic.asp?h=0&f=23&t=1573098 (if link works) (which pointed to this topic) said that this doesn’t take account of the fact that the front and rear axle will have different loads, so if I take Land Rover’s own 28psi front / 40psi rear ratio, that’s effectively the rears at 43% higher inflation than the fronts.

 

So if I assume I need an average of 23.2psi all round, the fronts should go down the same amount the rears go up, and I get:

 

Front 19.3psi

Rear 27.8psi

 

Doesn’t seem a million miles away from the 28/40-psi for 7.50r16s as per LR specs that I had before.

 

Anyone care to comment on my calculations?

 

 

 

[Ex Member]

Find a weigh bridge and weigh each axle to get real weights.  Then do the calcs using those numbers.

[martyn668]

Hi Red90,

Absolutely. You're 100% right. That would make my back / front balancing more accurate, but Land Rover can't have been far off with their 28:40 ratio. Obviously they didn't know about all my accessories. And also, how many Land Rover drivers weigh 75kg (11st, 11oz). Seems a little flattering, or is it just me who's overweight?

[SteveG]

Or in this nice dry weather, get a piece of chalk, (most real mathematicians have this to hand  ) draw a line or multiple lines across the tread of the tyre. Drive down an even-ish piece of tarmac and check for wear. If it’s even you’re ok, if not adjust air pressure up or down accordingly. 

[miketomcat]

Put new tyres on at standard pressure drive along a dusty road. You should see a strip of dust down the middle of the tyre the out side edges will still be clean. Reduce the pressure, clean the tyres and repeat. After a few goes you will see the dust right the way across the tread. At this point your about there you may need to do the front or back more but you get the point. This how we used to set slicks on my brothers race car. For reference I run about 25 front and 30 rear in 285/75x16's on a 90. I do tend to run them on the high side as I drive quite enthusiastically.

Mike

[martyn668]

According to the Internet, there's even a weighbridge in my village. The Internet says its public, but it belongs to a food company? Presumably for weighing food trucks full of carrots to check the loads. Not sure if they'll let me weigh my Defender, but I'll give them a call. 

[martyn668]

Hi Steve. I'd read about the calk method, but the MTs are very knobbly. Not sure if that would give reliable results. 

[zardos]

Or just inflate to 8psi because you want the extra grip in the mud
And the will be sliced to shreds by stones / perished before they wear out.

[Arjan]

Weigh the vehicle and take it from there.

Chalk works very well.

 

[landroversforever]

14 hours ago, martyn668 said:

According to the Internet, there's even a weighbridge in my village. The Internet says its public, but it belongs to a food company? Presumably for weighing food trucks full of carrots to check the loads. Not sure if they'll let me weigh my Defender, but I'll give them a call. 

It's public as in anyone can turn up throw them some money and be given a weight.

[toenden]

...10kg for a winch seems a bit low? -have you remembered the winchtray and stuf like that?

People always seem to be a bit low on the expected weight of their car... 😀

/mads

[landroversforever]

5 minutes ago, toenden said:

...10kg for a winch seems a bit low? -have you remembered the winchtray and stuf like that?

People always seem to be a bit low on the expected weight of their car... 😀

/mads

You're right there! TDS Goldfish is 35Kg with rope and ali fairlead..... 50Kg if its steel and roller fairlead.

 

[Ex Member]

16 hours ago, martyn668 said:

Hi Red90,

Absolutely. You're 100% right. That would make my back / front balancing more accurate, but Land Rover can't have been far off with their 28:40 ratio. Obviously they didn't know about all my accessories. And also, how many Land Rover drivers weigh 75kg (11st, 11oz). Seems a little flattering, or is it just me who's overweight?

The Land Rover book weight figures are miles off in my experience.....  Just go weigh it.  

[Arjan]

Weighbridge should be accurate for your vehicle within a 20 kgs.

Place it in the centre of the bridge and have a look...

Chances are you'll be getting a somewhat different reading than you'd expect reading the manual - put the weight with the date  / or the weight ticket - with your car paperwork so you can find it.

We weigh all our vehicles & trailers to make sure we work with real world figures when loading them etc.. Makes talks with uniformed people so much more relaxed as at times we like to push it right to the max.

 

 

 

[landroversforever]

16 hours ago, martyn668 said:

Hi Red90,

Absolutely. You're 100% right. That would make my back / front balancing more accurate, but Land Rover can't have been far off with their 28:40 ratio. Obviously they didn't know about all my accessories. And also, how many Land Rover drivers weigh 75kg (11st, 11oz). Seems a little flattering, or is it just me who's overweight?

75kg is less than 3/4 of a Ross!  

[Eightpot]

I think you're wildly over thinking this - a circa 2 ton Defender with MT tyres is very regular, just do them 30 front, 40 rear and adjust slightly if it feels it needs it.  19psi on the front is very low unless you're mostly driving sand tracks? 

 

[western]

my 110 runs 255/85 KM2 MT's my fronts are at 28psi & rears at 40psi [carries 400kg of motorsport recovery kit most of the time] tyre wear is minimal. my previous set of mt's did well over 50,000 miles

 

[Ed Poore]

On 07/06/2018 at 12:20 AM, zardos said:

Or just inflate to 8psi because you want the extra grip in the mud
And the will be sliced to shreds by stones / perished before they wear out.

Wot he said. Still got 10mm+ on all my KM2s but the sidewalls are slashed to the point replacements are unfortunately imminent. 

[Blanco]

+1 for 19 being a bit low, I think there must be a difference between the 'curves' for regular car tyres (eg Legacy) and the 4x4 tyres we are concerned with???

[martyn668]

I have a sports car! I took my Defender to the local weighbridge, and it turns out my Defender has nearly perfect 50:50 front:back weight distribution. Next stop the Nurburgring...

 

So my initial weight guesstimate was weigh off. Total weight including driver is 2300kg, split 1140kg front 1160kg rear. Must be why it handles so well...

 

So reworking my sums, I get

 

5,520 (max load rating) / 2,300 (vehicle weight) = 2.40

 

65PSI (max tyre PSI) divided by 2.40 = 27.1psi

 

And that would be the value front and back. I  guess Land Rover have 28psi/40psi as standard with 7.50/16s so you can load up the rear whenever you like. Probably too hard for unloaded. I think I'll run it 27 front and rear, and pump up the rears more if I'm loaded heavily. At least I'll try it like this for a while and see what it's like. Might tend to go higher for on road use, obviously (much) lower for off road use.

[Eightpot]

Out of interest, why are you basing your tyre pressures on a calculation based on the tyres max inflation pressure and max weight divided by kerb weight?? 

I can't see how that will work out the correct footprint and sidewall stability? 

[Bowie69]

I confess I was confused by that too, one suggestion, try contacting the tyre manufacturer?

[martyn668]

Hi there Eightpot ad Bowie69,

I figured there must be a calculator for this, rather than the suggested (and proven, I'm not saying there's anything wrong with them) methods of chalking the tyre, or driving down a dusty road).

When I searched for one, I found this calculator http://www.uklegacy.com/forums/index.php?/topic/98525-calculate-correct-tyre-pressures/  

Not saying the formula is correct, but it's the only one I found, and it seems logical to me. If my vehicle was at the maximum permitted weight carrying capacity of the tyres, I'd pump them up to the max to avoid the sidewalls collapsing. The lighter I am, the less I pump them up, to give some flexure in the sidewalls. My original question was really whether anyone had any comments on the validity of the formula. One poster above (Blanco) asked whether "normal" tyres might use that formula, but it might be different for knobbly tyres. Good question, and I don't know.

Happy to hear if anyone has a different formula.

[Ex Member]

The method is correct....

Yes, an empty 90 is nearly perfect 50:50 weight distribution.

Now, I would suggest 30 front and 35 rear.  The vehicle dynamics are better when the rear tyres have a slightly higher rolling radius than the front with the full time 4WD.

[Eightpot]

Couple of things- 

I don't believe that's the correct way to arrive at optimum tyre pressure anyway,  particularly as it's mixing metric weights with imperial pressure, but running with it for a moment.. 

The difference between max gross weight and unladen weight in a Defender is huge, so you change tyre pressure depending on if you're running empty or full, it doesn't make sense to average it. 

The max tyre pressure has nothing to do with the application, only tyre construction.  

A Michelin X 7.50 has a max pressure of 65psi stamped on the sidewall, a general grabber TR is rated at 83psi,  and a michelin 205 AT I've just checked on a range Rover Classic is 36psi.  

So using those calculations, a range Rover Classic should run at 19psi - which is way off. 

And gross vehicle weight for a 90 is 3050kg, not 5520kg..