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Old 08-26-2020, 01:51 PM   #21
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Note on previous post: The 900 figure leaves some load for four passengers and a bit of stuff. The number VW advises without WD after considering the 270 lbs shifted from the front axle puts the rear axle near capacity also after including passengers and some stuff.

DCPAS, your intuition does not fail you. The idea is indeed to get the most rubber on the ground to get maximum grip but here is the caveat; the rubber pattern must be aligned in the desired vehicle direction. So when you want to accelerate straight with maximum traction, like at a drag race or when you're stuck in the mud or sand, 8 or 12 lbs under is great! Things get complicated when cornering. The lateral forces on the tire along with sidewall flex and tread sheer will rotate and distort the tire footprint plus allow for a bit of lateral slip so the effective direction of the tire foot print is not the same as the direction of the overall tire. The greater the later force, the greater the flex and the higher the slip angle. Wider tires and taller wheels always help quite a bit as your intuition might tell you. Without changing the wheels and tires, increasing the tire stiffness but not so much as to greatly reduce static tire footprint is the best way to increase tire cornering stiffness and thus cornering performance. So, if the vehicle has oversteer issues, higher pressure in the rear and lower pressure in the front will make a dramatic difference.
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Old 08-26-2020, 02:07 PM   #22
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Without changing the wheels and tires, increasing the tire stiffness but not so much as to greatly reduce static tire footprint is the best way to increase tire cornering stiffness and thus cornering performance. So, if the vehicle has oversteer issues, higher pressure in the rear and lower pressure in the front will make a dramatic difference.
This is what I fail to understand. In my experience (admittedly racetrack, not trailering), adding "stiffness" through inflation to a tire reduces cornering performance. Reducing pressure in the front tires and adding pressure in the rear tires should increase oversteer, not decrease it. Or more likely, reduce the inherent understeer built into road vehicles.
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Old 08-26-2020, 03:28 PM   #23
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The principles are all the same but race cars, their relative mass vs wheel and tire geometry, the tires, speeds and most importantly the high lateral forces are all a different animal from passenger, trailer and truck tires and so the trade-off points are so different, much of what I described does not apply so well. My understanding is that they are most limited by rubber to road grip as we have engineered most or all of the tire distortion issues out of the equation. i mean how long does a racing tire last anyway? 200 miles? Thus they behave more like, well, racing tires where road grip dominates. Maybe someone else can talk about race tires more than I.
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Old 08-26-2020, 03:40 PM   #24
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My understanding is that they are most limited by rubber to road grip as we have engineered most or all of the tire distortion issues out of the equation. i mean how long does a racing tire last anyway? 200 miles? Thus they behave more like, well, racing tires where rad grip dominates. Maybe someone else can talk about race tires more than I.
I have raced in a lot of endurance series where you have to use road tires. Aggressive ones, but still a closer example to our tow vehicle tires than pure race tires. And it's the same. Within limits, more air pressure results in less grip. So when it comes to that lower pressure front and lower pressure back recommendation for avoidance maneuvers; I can't see it. On the other hand, I can see very good reasons for higher pressure in the rear tires in load carrying ability and temperature. Yet my owner's manual does not say anything about increasing pressures when towing.
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Old 08-26-2020, 04:09 PM   #25
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Hmm, the only thing I can say is when towing and hauling and even for passenger vehicle, but not so much performance vehicles, the relative heavy mass and inertial moments dominate. I suppose I should have emphasized the vehicle differences (light relative weight, nothing on the vehicle that is not required) more than the tire differences. Again I recognize there is a tipping point with respect to inertial moments (mass) and I just haven't spent a lot of time dealing with vehicles that are relatively light compared to tire grip, it's a bias I suppose, a blind spot. I can assure you though, when hauling, the concept Colyn, Andy Thompson (CanAm) and I describe applies.

I just don't have much experience with racing, other than going straight. This was eye opening for me, a reminder about something I should have known if you will. Thanks for that.
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Old 08-26-2020, 04:24 PM   #26
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Correction: I wrote lower pressure front and lower pressure back is what I did not understand. I meant lower pressure front and higher pressure back.

Anyway, thanks for your comments, Brian.
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Old 08-27-2020, 11:47 PM   #27
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Brian: Both you and Collyn agree on the inflation adjustment so there must be something to it, but I am not following the logic. To my understanding, generally lower pressures will result in more tire grip. More compliance surface meeting the road. However, the tire will be able to carry less load and will run hotter with lower pressure. From a towing perspective I can see good reason to increase rear tire pressure in order to accommodate the added load, but I cannot figure out how it helps in an emergency handling situation. What am I missing?
This is an issue that is far more complex than it appears, and (from feedback from my many published articles and books) is generally misunderstood.

A tyre develops a so-called contact patch that has a partially frictional and partly molecular grip. When steered by the tow vehicle's front wheels, these patches distort - in turn causing the vehicle be steered as the driver desires. The amount by which this occurs is a function of both loading and tyre pressure.

They also develop a 'steering effect' if subjected to a side force (such as a wind gust or the trailer swaying).

It is vital the amount by which they do so is less at the tow vehicle's rear tyres - as that will cause that vehicle to oversteer. If that happens, the rig may jack-knife.

As a generalisation it is thus necessary to run the tow vehicle's rear tyres at about 7-10 psi higher than at the front.

This article of mine explains all ( https://rvbooks.com.au/caravan-and-t...icle-dynamics/ . It does, I fear use metric numeration - this not an issue for Americans involved in physics and engineering - but the gist of what happens is still clear.

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(A possibly interesting matter is that Australian time is one day ahead of the USA. It 4 pm Friday 28 August). We claim no other issues re Trumpslyvania!
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Old 08-27-2020, 11:59 PM   #28
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I have raced in a lot of endurance series where you have to use road tires. Aggressive ones, but still a closer example to our tow vehicle tires than pure race tires. And it's the same. Within limits, more air pressure results in less grip. So when it comes to that lower pressure front and lower pressure back recommendation for avoidance maneuvers; I can't see it. On the other hand, I can see very good reasons for higher pressure in the rear tires in load carrying ability and temperature. Yet my owner's manual does not say anything about increasing pressures when towing.
I think you have misread my post/s re this. I advise to raise the tow vehicle rear pressure by 50-70 kPa (7-10 psi).

I suspect the reason why not all vehicle manuals cover this is that only a tiny per-cent are used towing what Americans call travel trailers. We (in Australia), and most other countries refer to them as 'caravans'.
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Old 08-28-2020, 11:02 AM   #29
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This article of mine explains all ( https://rvbooks.com.au/caravan-and-t...icle-dynamics/ .
Thank you for the link, Collyn. Where does this statement come from? "A tyreís cornering power decreases with load and increases with tyre pressure." A link for you: https://www.windingroad.com/articles...ire-pressures/

I truly am confused. In road racing, load on a tire increases grip. Weight transfer and managing slip angles is pretty much the core of the exercise. More weight on a tire gives more grip. Less weight gives less grip. Ask anyone who has spun a 911 in a turn because they lifted throttle. Or why do not think they put wings creating downforce on race cars? It's for increased grip in corners; it slows them on the straights because of the extra drag.

As to tire pressure, there is a tire pressure for optimum grip and for optimum turn in. See the link. But you will also see that at some point increased inflation will reduce grip. In fact, race cars use quite low inflation pressures to keep the tire surface compliant in order to enhance its contact with the tarmac.

Increased rear pressures make sense to me if you want more load carrying capability and reduced temperature at speed. More grip? I don't see how unless the tire cannot handle the load put on it in the first place. If you follow your manufacturer's inflation recommendations in a vehicle capable of towing, that should never be.
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Old 08-28-2020, 11:19 AM   #30
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the goal of WD is to balance the load across the TV and AS so that each wheel set has 1/3 of the total weight
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Old 08-28-2020, 12:00 PM   #31
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DCPAS, as Collyn indicated the dynamics of cornering is complex with many counteracting variables and effects. As you noted, vertical load increase the frictional component so when tire to surface grip is the overriding factor as is most often the case in racing more load is helpful. When we are dealing in the realm where grip is not limiting, footprint distortion is overriding and even here higher vertical loads increase cornering stiffness. Thus aeodynamic wings which increase verticle load but don't add mass are helpful for both grip and cornering stiffness. However if the load increase is achieved by shifting vehicle mass, lateral force requirements generally increase to a greater extent than does cornering stiffness and the net result is more distortion and greater slip angles on the tires experiencing more mass. Again, this applies when frictional and molecular grip exceeds the required cornering force. Paradoxically, the 911 example is a case of rapidly changing cornering stiffness and not initially a tire grip issue. It supports my description, as the engine torque increases the torque based load on the rear tires but in a corner if the driver lets off on the throttle, the torque load subsides, rear cornering stiffness drops and the vehicle oversteers and then spins out.

I agree with your comments on the trade off in tire pressure and road grip. Again though in passenger vehicles and load carrying vehicles at highway speeds on dry pavement, tire to road surface grip is not limiting, instead it is tire slip (footprint distortion).
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Old 08-28-2020, 12:33 PM   #32
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Again though in passenger vehicles and load carrying vehicles at highway speeds on dry pavement, tire to road surface grip is not limiting, instead it is tire slip (footprint distortion).
Brian: I do not have a sufficient technical background to fully understand what you are saying on stiffness versus surface grip, but I can tell you that tire slip is the limiting factor on the track. While a bit car dependent, maintaining a slip angle between 6 and 8 degrees is generally considered optimal. To put it more simply, every corner should have a small slide involved. The same thing happens on the road. Once the slip angle is too great, you are spinning or, in trailering, jackknifing. But some slip angle gives the most grip. If you increase inflation it seems to me that you will have reduced the amount of slip angle the tire will tolerate, which I submit reduces the grip before you spin / jackknife.

Collyn's presumption on which he based is rear tire inflation recommendation is that "A tyreís cornering power decreases with load and increases with tyre pressure." Generally speaking, both statements are wrong, but perhaps in this towing environment they make some sense. I am just trying to understand how.
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Old 08-28-2020, 12:44 PM   #33
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Paradoxically, the 911 example is a case of rapidly changing cornering stiffness and not initially a tire grip issue. It supports my description, as the engine torque increases the torque based load on the rear tires but in a corner if the driver lets off on the throttle, the torque load subsides, rear cornering stiffness drops and the vehicle oversteers and then spins out.
I forgot to address this. You may be correct, but 911 pilots generally describe this as resulting from lower rear grip owing to deceleration from the lift. Less weight on the rear and more on the front. Happens to all cars, but the rear weight bias of a 911 makes it "special."
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Old 08-28-2020, 12:58 PM   #34
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“Collyn's presumption on which he based is rear tire inflation recommendation is that "A tyre’s cornering power decreases with load and increases with tyre pressure." Generally speaking, both statements are wrong, but perhaps in this towing environment they make some sense. I am just trying to understand how.”

A tires cornering stiffness(grip) does increase with load but it does not increase directly proportional to the load, it’s always less. If you increase tire load by 20% cornering stiffness might only increase by 15%. With that extra load cornering stiffness is now reduced. This what happens when tongue load is applied to the TV it reduces the TV cornering power. This reduces the TV understeer.
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Old 08-28-2020, 01:38 PM   #35
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A tires cornering stiffness(grip) does increase with load but it does not increase directly proportional to the load, itís always less. If you increase tire load by 20% cornering stiffness might only increase by 15%. With that extra load cornering stiffness is now reduced. This what happens when tongue load is applied to the TV it reduces the TV cornering power. This reduces the TV understeer.
Profxd: If load increases grip, but at a lower rate than the amount of load applied, isn't grip still increased?

I get the idea that nearing loss of control in a turn the trailer is going to push the rear to the outside of the turn and threaten to exceed the amount of grip available resulting in oversteer. We should all want to have the most grip available from the rear tires. My question is how increased rear tire pressures get us there.
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Old 08-28-2020, 04:32 PM   #36
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Profxd: If load increases grip, but at a lower rate than the amount of load applied, isn't grip still increased?

I get the idea that nearing loss of control in a turn the trailer is going to push the rear to the outside of the turn and threaten to exceed the amount of grip available resulting in oversteer. We should all want to have the most grip available from the rear tires. My question is how increased rear tire pressures get us there.
Yes grip does increase but the load also increases the lateral forces acting on the tires. So as far as cornering power goes it is reduced because grip and load are not proportional. As Brian described above by using a wing/spoiler you can increase grip without adding a bunch of weight as long as you have the velocity. FYI a dual rear wheel pickup will have grip thatís directly proportional to load even with its extra mass while within its tire load rating unlike a single rear wheel vehicle.
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Old 08-28-2020, 04:43 PM   #37
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“My question is how increased rear tire pressures get us there.”

Increasing tire pressure stiffness the tire carcass. Collyn describes using higher pressure in the rear tires vs the front to control overall vehicle handling. This helps maintain a margin of Understeer while towing or heavily loaded. This is reason you see most 3/4 ton trucks with a front to rear pressure offset vs the same pressure front and rear for 1/2 ton and other vehicles. The down side to this pressure offset ride comfort.
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Old 08-28-2020, 04:59 PM   #38
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ďMy question is how increased rear tire pressures get us there.Ē

Increasing tire pressure stiffness the tire carcass. Collyn describes using higher pressure in the rear tires vs the front to control overall vehicle handling.
That is true only is Collyn's statement that increased pressure increases grip. To my understanding, within normal operating ranges, it does the opposite. Stiffness does not, in and of itself, increase grip. Suppleness that comes with lower pressure does.
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Old 08-28-2020, 07:33 PM   #39
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That is true only is Collyn's statement that increased pressure increases grip. To my understanding, within normal operating ranges, it does the opposite. Stiffness does not, in and of itself, increase grip. Suppleness that comes with lower pressure does.
As you have already mentioned in a previous post there is a optimum tire pressure for a given condition. Too much pressure will cause in loss of contact surface area and grip but too little and it canít generate enough force while cornering causing loss of grip. A lower pressure may be fine for straight line traction but a tire needs ample stiffness to maintain grip while cornering and a higher pressure will do that.
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Old 08-29-2020, 07:29 AM   #40
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Profxd: If load increases grip, but at a lower rate than the amount of load applied, isn't grip still increased?

I get the idea that nearing loss of control in a turn the trailer is going to push the rear to the outside of the turn and threaten to exceed the amount of grip available resulting in oversteer. We should all want to have the most grip available from the rear tires. My question is how increased rear tire pressures get us there.
You are 100% correct that increasing tire pressure above what is required to support the load, generally will reduce tire footprint and grip. All that you describe in racing where the tires and vehicles are tuned to near neutral steering and grip is king I agree with. I don't have much direct experience and I generally don't think about that situation. As I mentioned, it's a bias I carry, but your description matches absolutely with theoretical physics of the situation. On the 911 I do have direct experience. A friend and his uncle both drove 911's and I directly experienced the oversteer with those "back heavy" cars a few times. It initiates as a tire distortion oversteer event but rapidly transitions into a tire grip issues as the turn radius deteriorates.

Collyn's statement wasn't so much incorrect as it was imprecise. His statement about tire pressure increasing tire "sidewall" stiffness is correct but it would be imprecise to claim it increases grip to the road surface, more like it increases "net effective grip" in the desired direction of travel. Increasing tire pressure to a point within the range of tire design will reduce footprint distortion and thus increase tire "cornering" stiffness. It will also reduce footprint area and thus grip as you correctly noted so there is a trade off. Now Collyn also indicated that decreasing load improves cornering performance and again he was simply imprecise or rather incomplete. When the increase load is due to a shift or an increase in vehicle mass, his statement is correct. When the increased load is projected from a torque moment like weight distribution, or an aerodynamic force from wings or skirts and air dams, well he failed to address this allowing his statement to be misinterpreted. You and profxd have helpfully corrected it.

So this gets back to your base question. I see how and why you are questioning this advice, I mean oversteer doesn't generally completely become uncontrollable and proceed to jackknife until tire grip fails so I can understand your concern and can see why you would be reluctant to give up some tire grip. I can see you argue this point even more effectively in the rain or snow.

So how can we rationalize applying Collyn's and my advice? Remember oversteer occurs in sharp corners at excessive speed, in emergency maneuvers and as the late component of an amplifying sway event. The rationale I am applying is the ounce of prevention worth a pound of cure principle. I do this because the inertia of the trailer is driving the situation, where lateral forces multiply quickly once ball angle gets above 2.5-3% (25-30 mph effective corners) and surface grip becomes an issue, it's way to late for the driver to recover using handling principles, so tire grip is mostly academic. At this point the only hope is to accelerate the vehicle while applying hard braking on just the trailer. That will quickly bring ball angle and yaw angles back into a range where steering response is effective and hopefully before the setup hits an obstacle.
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