Lowering Tyre Pressure - Why Do it?

[Kell]

Ok. Now no one is allowed to laugh....I have never been told why I should lower tyre pressure before 4x4'ing and was wondering why it was done.....what does everyone usually drive at and what should the roza be sitting on? I'm running Cooper ST's 235/75R/15 on it.

Thanks guys

[Area54]

Lowering the pressures for general wheeling will increase the contact patch of the tyre on the ground, improving the traction as you have more rubber in contact with the terrain. Does give the effect of more 'wheel travel' and also improves the comfort in the cab. Prolly try around 20-25 next time you go out, this will give enough pressure to still drive a short distance to a servo to fill back up after a run.

[Fieldsy]

It lets the tyre mold itself around rocks and stuff and so gives more grip. Also provides flotation on sand and mud. I also recon it reduces the chance of something going through your side walls (I know someone is guna flame me for saying that)

Down side is that your tyres are more likely to "roll" off the bead.

I air down for comps ( to what ever pressure Hoonz has his deflators set to ) but I don't for general 4wding unless I have to, as I don't have onboard air and would rather get stuck in the bush than have a tyre come off the bead while driving to the nearest servo. (Getting stuck is character building)

As a general rule don't go under 15psi on rims with out bead locks. (Yes I have gone under that many times and haven't lost a tyre yet)

[-Mick-]

No one's laughin How do you learn if you don't ask

It makes the world of difference to traction. Also allows the rubber to 'mould' to irregular objects.

[Fieldsy]

Lowering the pressures for general wheeling will increase the contact patch of the tyre on the ground, improving the traction as you have more rubber in contact with the terrain. Does give the effect of more 'wheel travel' and also improves the comfort in the cab. Prolly try around 20-25 next time you go out, this will give enough pressure to still drive a short distance to a servo to fill back up after a run.

OH yeah and 20-25 psi would be about right.

Dam ya beat me to that one. their was no replys when i started typeing. must leard to speed up my 2 finger typeing

[MY45]

2106 post and you still dont know how to use the search button

[christover1]

I run 30psi on road and between 8 and 12psi offroad...I have 31" mudders. lighter cars with bigger tyres can get away with running much less air...as the larger tyres were designed to carry 3 ton of truck, so our little cars need a lot less air to hold us off the ground...I have never rolled a tyre off a rim ... tho it is possible...
the only down side is my steering gets a little heavy, especially with the front end locked...christover

[OVERKILL ENG]

I guess one of the best answers next time you go for a wheel do at normal pressures 30-40psi and then do the same track at lower pressures 10-15 psi depending on what you drive.And see the difference you will have a lot more traction and a lot smoother ride.
SAM

[POS]

Anywhere from 0-5psi

[murcod]

I guess one of the best answers next time you go for a wheel do at normal pressures 30-40psi and then do the same track at lower pressures 10-15 psi depending on what you drive.And see the difference you will have a lot more traction and a lot smoother ride.
SAM

Just out of interest Daihatsu only recommend 23PSI in the front tyres for driving on road (and 26 PSI in the rear IIRC for no load & on road).

[redzook]

I guess one of the best answers next time you go for a wheel do at normal pressures 30-40psi and then do the same track at lower pressures 10-15 psi depending on what you drive.And see the difference you will have a lot more traction and a lot smoother ride.
SAM

Just out of interest Daihatsu only recommend 23PSI in the front tyres for driving on road (and 26 PSI in the rear IIRC for no load & on road).

sierra only recomends 20psi but if i set my tires at that the outside of the tires wear way to quick

[V8Patrol]

As the guys said above ....... its for the tyres sidewall flex over "rough" terain.
Its best used in places like the sand on beaches or over rocky ground. By lowering tyre pressures the tyre "Ballons" thus adding width to the surface contact patch, and being "softer" in the side walls allows the tyre to mould its self around odd shaped obsticles.

As for lowering em down in the mud, wel there are two distinct lines of thought here and really its trial and error testing for the individual driver/rig combination.
I drive a lot of mud and find that thelarger the tyre the lower the pressure seems to work but on smaller tyre the higher pressure comes infront. the theory here is that again the larger tyre needs to Ballon to get the grip, while the tighter smaller tyre "cuts" through the mud to find the firmer ground thus getting more traction.

Try lowering the pressures down step by step over the same section of track and you will see what pressure suits both your driving style and your rig.
For my setup I have run........
38" Swampers @ 4 - 10 PSI
35" BFG's @ 10 - 15 PSI
32" Mudders @ 15 - 25 PSI

750X16 Dunlops on split rims @ 25 - 35psi
825X16 Dunlops on split rims @ 20 - 30 psi

trial and error testing time !

Kingy

p/s
I didnt laugh

[shortgq]

I agree with V8patrol in relation to lowering pressures in mud, in my experience wider tyres and lowering tyre pressures actually reduce traction in mud. Back when i first started 4bying i owned the first ever jackaroo model with standard skinny tyres. A mate of mine had a late 80's hilux with 31x10.5x15 tyres. Now i'd agree that the hilux is a vastly superior vehicle offroad but when in mud the jackaroo absolutely smoked the hilux, no matter who was driving each. we used to used the jackaroo to extract the hilux from big mud holes. The only conclusion we could think of was the jackarros tyres were bighting through to a firmer surface and getting traction where as the hilux was 'floating' and just throwing shit everwhere.

I guess the only oproblem with the theory would be if the mushy shit is deeper then the height of the tyres and it can't reach firm stuff before bottoming out???

[sierrajim]

Depending on the tyre lower pressures in mud can help the tyre "float" on a lighter vehicle and can also aid in the tyres self cleaning ability.

As the other guys said its trial and error. Personally i have found that less is best.

Just don't forget if you take air out you must have some way of getting it back in. So make sure you've got a compressor. Another handy thing to have in your kit is a ratchet strap, this will make it a lot easier to rebead a tyre after its popped off the rim.

[HeathGQ]

Feroza - light vehicle. I have 30 BFG muddies. Trail and error got it down to running 12 PSI for off-road and about 18 for beach stuff.

I would say 18-20 would be a good place to start, and then go lower if need be. It will depend on the 'bagging'



As stated, lower pressures allow tyre to 'mould' to obstacles. Rocks, gravel, and allows more traction. With high pressures, your contact area is limited cause the tyre cant compress.

[Big Red Toy]

well i now run about 15psi offroad and my tyres are 11.5" wide and they work great, its all about keepin the axles off the mud so bigger tyre is better also

[DFND90]

Im laughing

[Kell]

Im laughing

Bite me

Thanks guys I now know why and will do it next time I go out. Better to know all the reasons why than to not know at all

Kell

[AussieCJ7]

want to see the effects first hand

go to the $2 shop and find one of those little bals about the size of a tennis ball but have a valve you can inflate them with

first start with it at low pressure and roll it over your fist

notice the area that touchs your fist will be longer and will wrap around it

now inflate it you will notice that the ball only contacts your fist on a small point as it rolls over your fist


The term bagging is a little miss leading as the contact patch grows in length more than width at lower pressures


The more complex way to see the effect

on clean flat cement surface color in the tread surfact of your tyre with chalk and roll it forward so it leave a print on the cement then do the same at lower pressures and look at the print it makes

[droopypete]

Another angle, I am suprised no-one has touched on,
is, the lower the tyre pressure the lower the diff clearance,
on many ocations I have seen cars that had bottomed out on their diffs in a deep rut get out and lower the tyre pressure when they should have been pumping them up!
I run a light Suzuki and my pressure ranges from 0-18 psi depending on the terain, the speed, the load and the desired comfort level (both bum comfort & steering comfort).
Peter.

[HeathGQ]

Another angle, I am suprised no-one has touched on,
is, the lower the tyre pressure the lower the diff clearance,
on many ocations I have seen cars that had bottomed out on their diffs in a deep rut get out and lower the tyre pressure when they should have been pumping them up!
I run a light Suzuki and my pressure ranges from 0-18 psi depending on the terain, the speed, the load and the desired comfort level (both bum comfort & steering comfort).
Peter.

agree with this too. Notice it in the GQ, as the car works better with the little 32's running at higher pressures to keep diff clearances up.

Also, AussieCJ7, agree with you. tyre bagging is not necessarily width ways, but long ways. This is why skinnier, H/T or A/T tyres work better in the sands.