New tires, and managing oversteer.

Big Rig Dynamics 203, or here’s some things for y’all to ponder that’ll help you better understand why your rig handles as it does.

If you drive the same truck all the time, you become very familiar with how it handles. And nothing changes that more than an axle or two of new tires. Why? Because new tires and their deep tread are much more prone to tread deformation under side loads, especially higher side loads (greater cornering g’s.) For many of you, you’ll never corner fast enough to notice, but those of us who can take advantage of the improved handling of stable, low center of gravity loads can really see the effects of this.

Other than rolling straight down the road, the actual direction a tire is moving is never quite aligned with the direction the tire is rolling. The rotation will be slightly inside the actual direction of movement, this is slip angle. On the front axle, that’s understeer, a push that requires more steering than it theoretically should to generate a turn. But on the drive and trailer it leads to oversteer, and an awareness of it will take away the uncertain feeling that the truck isn’t doing exactly what you asked it to do.

Let’s ignore the trailer a moment, and think about what’s happening at the drives as a tractor trailer corners. The driver initiates the corner, the truck loads the tires as the cornering forces increase… and the rear wants to oversteer towards the outside of the corner, requiring a correction by the driver if they weren’t prepared for that to happen, as this results in the truck turning tighter than expected. You’ve all seen it: the truck that wiggles around corners because the driver is constantly making corrections which end up being over-corrections.

The cause of this is the tread squirm of the new tires, which deform more and more as the cornering loads increases, increasing the slip angle of tire, and allowing the rear to step out a bit, which tightens the arc the truck will travel. So the driver straightens the steers a bit to correct this, which lowers the cornering forces and therefore the slip angles and now the truck isn’t turning as well as expected. And so goes the endless cycle of chasing the truck and wiggling around the turn.

So what’s the answer? Learning your truck, essentially. You just have to know that oversteer is coming, and adjust your initial steering input to account for it. If you do it correctly, you’ll enter the corner with a slightly shallower turn than you would expect, and as the side loads increase, the tire slip allows the truck to tighten the turn to the expected radius. That’s hard to get perfect, so the trick is to use a 2 stage turn-in when cornering. You start the turn, let the drives load up, and then make a very small correction once you sense how much steering you need. This keeps the drives loaded and avoids the wiggle. It’s not really two actions, it’s more of a slight delay in applying your final steering: TURN, turn, hold. With practice you’ll learn to feel the slip start with your backside and make your final minor correction just as the truck settles into the turn. This effect fades as your tires wear, and it’s hardly noticeable once they’re worn to near the limits..

Now the same thing happens back at the trailer, but you really don’t feel it up front. What happens is your trailer runs much wider than you anticipated, and can even step out enough to leave your lane, even though your truck does not. The way I’ve always dealt with it is when I make my final correction, I’ll bring the truck much closer to the inside line than one would normally do a lower speeds. (We’re talking about cornering near the posted curve speeds here, not something recommended for the inexperienced!)

To visualize whats going on, picture a tractor trailer cornering with only the steer tires turned into the corner, and the trailer straight behind the truck. The drives and trailer are both oversteering towards the outside of the corner, effectively rotating the entire rig to follow the steers around the corner. This is entirely possible, although I don’t push that hard loaded as a general rule. It is still funky to glance at the mirror and realize the trailer axles are the traveling further towards the outside of the corner than the drives, the middle of the trailer is often closer to the inside edge of the lane than the tandem.

Now, at more typical speeds, the oversteering of the trailer is only a very minor issue, just something to keep in mind on tight curves so that you don’t end up with the trailer out of your lane or even just too close to the lane edges. It also should be considered as you are exiting corners, as if you’re not aware of it, the trailer can snap across lane a foot or so as the tread squirm unwinds. (This really shows on the carhauler, with all the weight we carry behind the tandem.) You have to initiate your turn exit a tad earlier, and stay a bit closer to the inside of the corner, and the whole truck will drift slightly to the outside of the turn as you’re straightening the steers. If you watch in the mirrors when you do it correctly, the truck and trailer are very nearly in a straight line yet still moving towards the outside of the turn. It’s basically a drift, just not the lurid slide normally associated with that term, and more obvious as cornering loads increase.

Hopefully this drops a few more pieces in place for those of you wanting to better understand the dynamics involved in hustling along in big truck. This isn’t for newer drivers, as it takes a good while to become one with the truck enough that you need to concern yourself with such things. Always, ALWAYS, know your limits, and stay well away from that edge, as things go bad in hurry on the backside of your limit curves. It’s a gentle slope up, and a cliff on the way down the other side! And none of this really applies if you’re pulling anything other low center of gravity loads/equipment, the safe cornering speeds for me are a recipe for a rollover with a high CG load.

~Well I was rollin’ down the road in some cold blue steel…

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