2WD Trucks and Wet Traction - a losing battle?
- Thread starter The Critic
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The article doesn't say what the credentials the author, Rick Cotta, has. You can find mine by following the link to my website.
Nor does it say who he may have talked to in preparing the article, but he has repeated a commonly held misconception. I'll try to explain why this misconception is wrong - but first the obligatory: I am not a Rubber Chemist, but I have worked along side some talented ones, and I've picked up a few things by doing so.
First, the rubber in a tire is not as stiff as the steel and fabric is. But even more interesting is that the stiffness of a tire is mostly about inflation pressure. So it doesn't matter how stiff (or hard) the tread rubber compound is, the dominating factor to deflection is the inflation pressure.
I know, it seems counterintuitive that a stiffer tread compound has little effect on rolling resistance, but when you factor in how little the stiffness of rubber has compared to the inflation pressure, it starts to make sense.
What that means is that even the stiffest tread rubber compound has minimal effect on the deflection - and by extension, the rolling resistance. What does have an effect is what is called hysteresis. One way to look at hysteresis is that it is "internal friction". So think about bending a stiff piece of rubber versus a soft piece of rubber the same amount: Which would consume the most energy? The stiff one, because the very thing that makes it stiff, also causes more internal friction.
So if a rubber compound that is soft has less internal friction than a stiff rubber compound , why are grippy tires soft? Or why aren't low rolling resistant tread compounds, which are also soft, not grippy?
It has to do with tear resistance.
A soft tread rubber compound penetrates the macrotexture of the pavement, but in order to get more grip, the rubber has to have a higher tear strength - which low RR compounds do not.
So there is a 3 way triangle between RR, traction, and treadwear. Good treadwear compounds are stiffer and do not penetrate the macrotexture of the pavement as much, so they wear better. Rubber compounds with good hysteresis are soft and penetrate the macrotexture, but take less force to tear off, so they have less grip. Rubber compounds with good grip have good tear strength and are soft so they penetrate the macrotexture of the pavement.
I hope that explains things.
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How does taking less force to tear off equate to less grip?
Also how does a softer tire with less internal friction improve efficiency? You want the tire to bend less, not more. That's exactly why you'd want higher PSI of trying to maximize fuel economy. A more pliable tire would be more like lowering the pressure in the tire, no?
Even with skateboards harder durometer wheels roll faster than softer durometer wheels. I don't really see how softer anything would improve rolling resistance in a car tire.
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Sounds like the cheap thing to do is to toss some weight back there and try again. I wouldn’t think 6/32’s is too thin leaving a standing stop though, makes me think these tires have gotten old.
Dumb question: have you done a brake test yet? I mean, if it’s having traction issues taking off, maybe they aren’t braking as well either. That may sway your opinion on best path here.
Dumb question: have you done a brake test yet? I mean, if it’s having traction issues taking off, maybe they aren’t braking as well either. That may sway your opinion on best path here.
In order to develop grip, the tire has to slip. The more slip, the more grip - up to a point. So a tread rubber compound that has more tear resistance can develop more grip.
If the rubber is bending the same amount, the rubber compound with less internal friction consumes less energy, and that makes it more efficient.
Yes, you'd like the tire to bend less for better efficiency - and that's why more inflation pressure results in better efficiency. But the bending is mostly the function of inflation pressure. For practical purposes, the tire deflects the same at the same inflation pressure regardless of how stiff the tread rubber is.
Don't forget, skateboard wheels aren't inflated. If they were they would react differently. In fact, most of the time, tires are designed with rubber compounds that only have the 2 dimensional aspect - high grip vs high wear resistance - and the amount of energy it takes to roll is about the same for both. It's only when we look at fuel economy as an objective that we encounter the problem of grip vs wear vs RR.
As an example of how this works, if I change inflation pressure 4 psi, I can change the RR about 5%. If I change tread compound, I can get up to 30% with no change in pressure. The biggest contributor to RR is the tread rubber compound's hysteresis.
You need to read my webpages on Fuel Economy:
Barry's Tire Tech: Rolling Resistance and Fuel Economy
Barry's Tire Tech: Followup on Rolling Resistance and Fuel Economy
Pay particular attention to the graphs supplied by Smithers to the California Energy Commission for the same size tire. Those 2 graphs show up to a 60% difference best to worse for the same size and same inflation pressure, but Smithers included different kinds of tires and that exaggerates the kind of differences you'd get if you stayed with the same kind of tire and varied only the tread compound.
Update to this post: I noticed that the webpage has incorrectly displayed the plus-or-minus sign. I'll have to fix that once I figure out how to do that. Just be aware that the plus-or-minus 25% isn't displayed properly.
My dads pickup came with those tires and they had terrible wet traction. A new set of cooper AT3 solved that
OVERKILL
$100 Site Donor 2021
As I'm sure you recall, we ditched the Goodyear tires on our 1500 because they were downright scary when wet. It doesn't sound like yours are anywhere near that bad but the Conti's were a massive upgrade in the wet traction department.
A set of Michelin defenders will go a LONG way. A couple bags of sand will solve the remainder.
I don’t have a RWD truck, but I had similar issues with a RWD sedan. I could feel the rear axle kick out or the car breaks traction accelerating from a stop in the rain. It was bad with Costco-special BFG Touring T/As. Somewhat better with Conti’s ExtremeContact, the best tires I had for that car were Michelin Pilot Exalto A/S.
Nick1994
$100 site donor 2024
I'd say if you're concerned about stopping or cornering on wet pavement, and it's wet often enough to also be a concern, replace them.
But if it's just slipping on a takeoff, let 'em spin.
But if it's just slipping on a takeoff, let 'em spin.
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@CapriRacer if wider tires offer lower RR, then why do hybrids and EVs come with skinny tires, while supercars come with wide tires?
I'm thinking this doesn't necessarily have a lot to do with hydroplaning, where I recall typically tire shops recommended having more tread on the rears. But then again tires age and who knows what happens to wet traction as they wear down.
Good question. I see you've been reading my webpages on RR.
Skinny tires means skinny wheels, which weigh less. Skinny tires also mean skinny fenders, which also weigh less.
Super Cars? They need big diameter brakes. That means wide, low profile tires - which also happen to have a larger contact patch = more grip.
So skinny tires = less vehicle weight and wider tires = more vehicle performance.
But the real reason I make a big deal out of "wider is better" is that many people would mistakenly buy skinny tires with less load carrying capacity, and those are more likely to fail. So by pointing out that the data says wider tires have better RR, they are more likely to buy larger load carrying capacity tires, which are less likely to fail.
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I was thinking of wet traction and not necessarily hydroplaning. Like after it rains but there’s plenty of time for water to have drained but it’s still wet. I’ve been in those conditions where I had really poor traction with worn tires. But I think part of it was that the tires had poor wet traction to begin with, they were really old - like more than 10 years. I was in San Francisco during a rainy day but the rain had stopped. I don’t believe oily pavement released by the rain was an issue. I was on a steep hill where I had to stop several times and there were a few times that I thought that I wouldn’t be able to move forward and even though of using the parking brake to modulate like it was a manual transmission. I just kept on spinning the front tires.
edyvw
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Most people focus on hydroplaning. However, that doesn’t mean tire is good in rain if it is good in hydroplaning resistance.
I had Kumho Ecsta LX tires that were excellent in hydroplaning resistance, but praying worked better than pressing brake if one wanted to make sudden stop in rainy conditions. I could actually throw rear end on FWD vehicle in wet conditions with that POS tire.
