Driving question - soft conditions

[Toyfool]

I was driving out to shoot an old stone building in the desert south of I-40, west of Albuquerque, yesterday afternoon and I came across an unsettling response from my trusted truck.

I've had this truck for 14 years now but recently decided to start "fixing it". It now has a modest lift from Toytec and bigger tires, 285/70/R16 Cooper Discoverer STMaxx's. Roads were generally in decent shape so I didn't bother to air down.

The issue was then when I would hit soft sandy patches of something resembling bull dust I could feel the rear starting to come around like I was driving in snow. I have not experienced this sensation in dirt before. Can anyone say whether this is a function of the tires and lift or was it more likely just due to me driving too fast for the conditions? Is the fix possibly more weight in the back? I did not experience this sensations while driving my wife's 4Runner out to John's Canyon below Muley Point in much worse conditions.

And to complete the story, here's the house I was going for. I had come across this house about 5 years ago when heading out to explore a cool spot called Petaca Pinta (https://www.blm.gov/visit/petaca-pinta-wsa) and finally made time to get back out there to get some shots.

And in profile, with a volcanic core common to this area, and a much bigger one (Mt. Taylor) in the distance.

Some art work on the east wall.

And the view out the window looking north.

 

[craigR]

Nice looking Tundra, and fantastic pics!

Were you using 4wd? Anytime I'm on dirt, sand, rocks, whatever, I use 4wd. The handling difference vs 2wd on loose road surfaces in my T4R is night and day. It's not that those road conditions require 4wd to traverse, but rather a matter of the safest method of doing so.

 

[The other Sean]

Sounds just like how my Old 2WD Ford ranger with a LSD would be at speed when it was snowing. Anything over about 40 MPH and the back end always felt "light" and wanted to walk out. Does your truck have a LSD? Were you in 2wd?

 

[Toyfool]

I was in 2wd when I felt the rear end shift out slightly. Letting off the gas cured it. Pushing the little button that makes the green 4wd light come on in the dash worked well too. Given the conditions - mostly hard packed grated clay surface with the odd soft spot - I had been in 2wd.

My question really comes from the fact that I don't recall this happening before I started messing with the suspension and tire size. I felt it in the snow and even slid backwards off the road into a snow bank once (that was scary), but never before on dirt. And after 176k miles with this truck I have an expectation of how it should react. Perhaps I was just going faster in more comfort due to the suspension and tires.

 

[Toyfool]

Nice looking Tundra, and fantastic pics!

Thanks for the compliment. I like shooting but I've been pretty lazy for a while. It was good to get out and do this and the light is fantastic in the desert southwest this time of year. If you care to check out more of my photog work here's a thread from over on Advrider which was about the last time I did anything useful with a camera: http://advrider.com/index.php?threads/in-search-of-muley-point-the-hdr-version.927770/

 

[The other Sean]

I was in 2wd when I felt the rear end shift out slightly. Letting off the gas cured it. Pushing the little button that makes the green 4wd light come on in the dash worked well too. Given the conditions - mostly hard packed grated clay surface with the odd soft spot - I had been in 2wd.

My question really comes from the fact that I don't recall this happening before I started messing with the suspension and tire size. I felt it in the snow and even slid backwards off the road into a snow bank once (that was scary), but never before on dirt. And after 176k miles with this truck I have an expectation of how it should react. Perhaps I was just going faster in more comfort due to the suspension and tires.

I've found I end up going faster than I realize on some gravel roads in my truck. I have no sway bars and it's springs and shocks are nice and compliant. A soft surface might mean you don't feel like you are going very fast.

 

[Sparky]

I'd probably check my tire pressures to make sure they're not higher than whatever your normal is, some mechanics and service centers have a tendency to top tires off without informing customers.

I'd also suggest looking into the load rating of your current tires compared to what you had in the past. I'm pretty sure my ST Maxx are load E which is quite stiff. I know higher load tires have stiffer carcasses, but how that relates to lateral traction in turns I'm not sure. Seems like the treads should be doing their job regardless but with stiffer sidewalls perhaps they could break traction sooner (less conformation).. I don't know but it's something to consider. For your given vehicle load you may want to run a slightly lower pressure to compensate for the stiffer sidewalls.

I'm on my 3rd set of ST Maxx, they've provided me a fantastic compromise between street and trail performance. They're stiff but there's a certain assurance that comes with having those thick sidewalls in the rocks.

 

[Toyfool]

I am still searching for the optimum tire pressure. Sticker in the door says 32 psi and that was fine when I was running Michelin LTX's but I have heard that with E rated tires you need to run something higher due to the stiffer side walls. (That doesn't really make sense to me - maybe someone can confirm or refute this point?). For now I have settled on 38 psi as it "feels" comfortable on pavement. I have my deflators set at close to 18 psi and that ride is really nice when driving off-road.

As for how to find the optimal tire pressure, the most convincing discussion I have heard so far is to find the pressure that gives you less than 5% variation from cold to hot. I have not bothered to go through this exercise yet so perhaps I should. Once you start messing with your suspension and tire choice I am not sure you can still go by the info printed on the sticker. Sure it's a guideline but is it still accurate?

 

[Sparky]

Don't quote me on me on this, but I believe the higher load rated tires require more pressure when loaded near or at their capacity, however if someone has Load-E for instance on an empty pickup they can safely run a lower pressure because the tires aren't deforming as much. Deformation, from either too low of pressure or load, is what causes heat to build up which is the safety and longevity issue. Can anyone confirm this? I can call tire rack or something tomorrow to see what they say.

 

[TnWalrus]

I am about as novice as you can get, but I have found this gentleman to be an outstanding source of info.

 

[BigDogKona]

Don't quote me on me on this, but I believe the higher load rated tires require more pressure when loaded near or at their capacity, however if someone has Load-E for instance on an empty pickup they can safely run a lower pressure because the tires aren't deforming as much. Deformation, from either too low of pressure or load, is what causes heat to build up which is the safety and longevity issue. Can anyone confirm this? I can call tire rack or something tomorrow to see what they say.

This is why 2007-2010 Dodge Ram HD trucks had an “empty” button on the dash... it would let you run lower pressure in the tires without having the annoying warning light on all the time. It’s very common to run lower pressure than the PSI rating on the sidewall. That rating is the max PSI and will afford the highest load carrying capacity. Running at max PSI on dirt is detrimental to traction and handling. That said, under-inflation will lead to tire failure... so as the last post said, you need to make sure the tires are at the right pressure for your rig and conditions.


Sent from my iPhone using OB Talk

 

[Zargon]

In my experience I have had the rear-end step out on my rover (full time 4wd keep in mind) when running at street pressures and higher speeds on nice dirt roads if I hit some soft stuff. The first time it happened it caught me off-guard until I figured out why it was stepping out, I figured out that when driving on the road I would find the best speed for smoothing out all the corrugations and bumps without looking at my spedo, and whenever I ran street pressure the "perfect speed" usually ended up being higher, and when that's combined with the lower grip from aired up tires it led to my truck stepping out if i hit a soft patch while taking a gradual corner. For my rig I still have plenty of control whenever this happens.

Also as a side note I'm sure you already know, but just remember that while letting off the gas in a 2wd usually helps with over-steer, it can actually induce over-steer in a 4wd!