Newbie help - tyres and lift

[V]

21 hours ago, TimC said:

What tyre pressure is best to run off road if you want to air down is a very good question. I bet V or PDB know the answer .

 

I think everyone just has to find what works best for them and their Jeep. There are too many variables to find a perfect formula for all situations. I've run 33x9.50 and 33x12.50 on the same Jeep over the same obstacles and found that the 9.50 worked better with almost street pressure, the 12.50 needed to be down below 20psi. Each of these tyres gave the Jeep a completely different feel with no other modifications.

 

The bigger the tyre volume the more you have to experiment with. On 35" radials my XJ runs 26psi on the street, and 16-24psi off road. My deflators are set for 16psi and I will stop to air air-up again if I think I will benefit from doing so. I am comfortable going down to 16psi for rocks without bead locks on radials. For green laning I will run 22-24 psi. My biggest problem running low pressures is my diffs getting hung up.

 

I would be wary airing down too low on some modern tyres that are close to stock size for two reasons:

 

1) Is the tyre a road optimised off road tyre? Does it have a crowned section for street use? You don't want the outer tread blocks coming into contact with tarmac too often with this type of tyre. They are designed to run on paved highways with the central 50% of the tyre in contact with the tarmac. If they run too low a pressure on the street, they will be noisier and can prematurely wear out the outer tread blocks. This is something new to me in off road tyre design that I have only been aware of for a year or so. It is possible to spot from looking at the tread particularly if the central blocks have ramped lugs and the outers have square shoulders. The old chalk test cannot be used on crowned tyres, they need to be aired up to find the tarmac sweet spot so you can then determine the pressure that airing down begins to be a benefit offroad.

 

2) Tyres that are lower than 70 series will have lower profile sidewalls than the 80+ series that the traditional Jeeping advice is based on. The gains from airing down are going to be in a much narrower band with low profile tyres. There will be a trade off between improved grip from airing down and tyre reliability. Keeping the tyre bead sealed on the rim and not suffering sidewall damage would be my main concerns.
 

If your new off road tyres are close in size to the factory originals, start off with the factory recommended pressures and experiment by dropping a few psi at a time and noticing what difference it makes to ride comfort, steering, noise, and tyre temperature. It's OK to stay safe and go with the factory recommendation. The guy that's spectating telling you to air down isn't going to buy you a new tyre if you damage a sidewall after taking his advice. It's your choice. Just be aware that airing down is a tool you can use for extra traction when you need it. It's not magic, so don't expect miracles. Some times a push from a bystander is just as effective.

 

Please bear in mind that the lowest profile off road tyre I have used was a 65 series and that was on my Chrysler Grand Voyager. You can see from my avatar how much rubber to wheel ratio I prefer on my Jeep. One more thing - Tyre shine is a good indicator for seeing the effects of what the sidewalls are doing. It gets scrubbed off sidewalls and can help you decide if your pressures may have been too low.

[Caroline12]

Thank you V, there is a lot there to take in and think about, to be careful with what you do, so not to damage your tyres.

my tyres 225/65 R17 have ramped lugs, so basically be careful, don’t go to low, a little and try to see how it feels. 
 

I appreciate your response, very informative.

 

caroline 

[V]

You may find that other Renegade owners are thinking "Why lower the tyre pressure?". Their experience may prove that there is little practical advantage to gain by doing so.

 

The traditional advice for lowering tyre pressures comes from long ago, perhaps pre-WW2 before tyres had a quoted width to section height ratio. Amongst 20th century Jeepers, this advice typically applies to a tyre with a sidewall section height of 6.3" (160mm) or more. That's based on my assumption from the old days when a Wrangler was shod with 215/75R15 tyres. With your 225/65 tyres, and a rough estimate of sidewall size being 65% of 225mm = 146mm, your tyres are close enough to be worth trying it out.

 

[Caroline12]

Thank you, I suppose it also depends on what tyre you have, tyres perform differently and at different pressures on different vehicles. 
my understanding comes from tyre pressure and mountain biking, lower pressure to aid with loose gravelly surface, greater surface area for traction. Spinning out wheels on loose gravel up hill with leg power is not fun.

I appreciate sand and loose gravel you need more footprint, a 2.3-2.4” tyre width.

For mud and bikes, a thinner tyre 1.8-2” to go through the mud, to cut it. Large surface area tyres sit on the mud, then you spin out and go splat in the mud. 
I won’t be changing my tyres every 5 minutes, but having the correct pressures for the conditions, makes the difference of getting up that hill or through that area, where normal road pressures would struggle more. 

Edited July 13, 2021 by Caroline12
Forgot a figure

[V]

10 hours ago, Caroline12 said:

I appreciate sand and loose gravel you need more footprint, a 2.3-2.4” tyre width.

For mud and bikes, a thinner tyre 1.8-2” to go through the mud, to cut it. Large surface area tyres sit on the mud, then you spin out and go splat in the mud.

 

You've got it nailed! It's exactly the same for a 4x4.

 

However, you will only get a bigger footprint airing down a 4x4 tyre if it has sufficient sidewall to create that footprint.

 

Another thing to be aware of is that if your wheels have 146mm between the wheel rim and the road at street pressures, and say 125mm-100mm aired down. Your wheel rims and tyre beads could be more vulnerable to the debris that you are driving in.

 

When my first Jeep was almost stock with OEM size mud terrains I often found broken twigs wedged between the tyre bead and rim after forest driving. I was running 235/75R15 back then with 6.9" sidewalls. Worth remembering if you think you have a puncture. It could just be a serious tyre burp cause by a stick breaking the bead seal.

[Caroline12]

Good advice there, thanks V I think probably max down to 125mm, 20mm of drop is quite a lot, that would be a drop of 1/7 th. I’ll have to give it a go and see what it’s like. Thanks