Pulling out of a steep ramp with 2 WD truck

[2kwik4u]

I agree that deflation increases contact area, critical to improve traction on a slippery surface like a boat ramp. I don't follow the relation between contact pressure and PSI. Add some weight to the truck bed to increase the contact pressure and lower your PSI by 1/3-2/3

Contact pressure between tire and pavement is directly related to footprint. Less pressure in the tire increases footprint and decreases contact pressure. Contact pressure is the same units a air pressure. Pounds per square inch. A vehicle has a given weight on an axle, if you increase surface area you decrease contact pressure.

For example, if you have 2,000lbf on an axle, and you have 2 tires on that axle each tire carries 1,000lbf. Say each tire has a contact patch of 1in^2. Each tire has a contact pressure of 1,000psi. NOW, deflate each tire by 50% and get an increase of 25% in contact patch. You now have 1,000lbf per 1.25in^2 or 800psi.

We are taught in general physics that friction force (traction in this case) is defined as a factor times the normal force (1,000lbf). F = k*N. Now, that k factor is a general catch all that takes mechanical keying, surface interactions like electron bonding and adhesion, as well as other factors into account. It's far more complex of an interaction than just a simple multiplier. However, in general, as you lower contact pressure, you lower that k value.

This makes sense both intuitively and by example. We know if we push our hand harder against a surface it's more difficult to move than if you push less. You can develop the same mathematics here except you are varying force instead of contact patch. Likewise, we know from drag racing that given a particular compound of tire, the wider you go the less grip you have to accelerate. 60ft times in fast accelerating vehicles decrease when adding width to the tires (assuming you keep similar compounds).

Moral of the story here is that additional contact patch rarely results in better traction if all other variables are held constant. In practice, airing down will likely decrease traction on a slippery ramp.

 

[GTBRMC]

Contact pressure between tire and pavement is directly related to footprint. Less pressure in the tire increases footprint and decreases contact pressure. Contact pressure is the same units a air pressure. Pounds per square inch. A vehicle has a given weight on an axle, if you increase surface area you decrease contact pressure.

For example, if you have 2,000lbf on an axle, and you have 2 tires on that axle each tire carries 1,000lbf. Say each tire has a contact patch of 1in^2. Each tire has a contact pressure of 1,000psi. NOW, deflate each tire by 50% and get an increase of 25% in contact patch. You now have 1,000lbf per 1.25in^2 or 800psi.

We are taught in general physics that friction force (traction in this case) is defined as a factor times the normal force (1,000lbf). F = k*N. Now, that k factor is a general catch all that takes mechanical keying, surface interactions like electron bonding and adhesion, as well as other factors into account. It's far more complex of an interaction than just a simple multiplier. However, in general, as you lower contact pressure, you lower that k value.

This makes sense both intuitively and by example. We know if we push our hand harder against a surface it's more difficult to move than if you push less. You can develop the same mathematics here except you are varying force instead of contact patch. Likewise, we know from drag racing that given a particular compound of tire, the wider you go the less grip you have to accelerate. 60ft times in fast accelerating vehicles decrease when adding width to the tires (assuming you keep similar compounds).

Moral of the story here is that additional contact patch rarely results in better traction if all other variables are held constant. In practice, airing down will likely decrease traction on a slippery ramp.

You make some thought provoking points about the physics involved here. Per your comment about the coefficient of friction indicating traction is related to both normal force (why load sand in RWD pickup bed increases traction) as well as contact patch size (one reason dragsters have very wide rear tires), the difference between static and kinetic coefficient of friction is also significant.

Static coefficient of friction is almost always > than kinetic coefficient of friction. This explains why spinning wheels at ramp (or stuck in mud or snow) is usually a bad idea vs slowly adding applied torque to just before point of spinning.

Also worth noting is the fact that friction is an interaction between TWO surfaces, not just one. So, the tire material, contact patch, normal load, and tread design matters… a lot. But ramp material also matters, sometimes a lot. Variables to consider include wet vs dry, bare vs marine growth (usually some kind of green slime), bare vs sand-covered (grains of sand can act like little ball bearings), asphalt vs concrete vs gravel vs sand base material, etc.

The grade angle of the ramp also matters (steeper ramp = lower normal force due to delta in trigonometry or shorter vertical component vector) but a shallower ramp is more likely to force you to launch with rear axle deeper in the water.

AWD or 4WD reduces significance of all of the above. But AWD or 4WD comes at both an up front and an ongoing operating cost penalty. Worth noting, I have seen many Bronco’s Guru-type videos of trucks pulled down the boat ramp into the water, but I have never seen a properly-sized AWD or 4WD tow vehicle pulled into the drink.

 

[greg10]

Great discussion 2k and g8! Maybe we can one day find a study and develop models that accurately match a slimy, angled boat ramp with a boat trailer attached.

 

[2kwik4u]

You make some thought provoking points about the physics involved here. Per your comment about the coefficient of friction indicating traction is related to both normal force (why load sand in RWD pickup bed increases traction) as well as contact patch size (one reason dragsters have very wide rear tires), the difference between static and kinetic coefficient of friction is also significant.

Static coefficient of friction is almost always > than kinetic coefficient of friction. This explains why spinning wheels at ramp (or stuck in mud or snow) is usually a bad idea vs slowly adding applied torque to just before point of spinning.

Also worth noting is the fact that friction is an interaction between TWO surfaces, not just one. So, the tire material, contact patch, normal load, and tread design matters… a lot. But ramp material also matters, sometimes a lot. Variables to consider include wet vs dry, bare vs marine growth (usually some kind of green slime), bare vs sand-covered (grains of sand can act like little ball bearings), asphalt vs concrete vs gravel vs sand base material, etc.

The grade angle of the ramp also matters (steeper ramp = lower normal force due to delta in trigonometry or shorter vertical component vector) but a shallower ramp is more likely to force you to launch with rear axle deeper in the water.

AWD or 4WD reduces significance of all of the above. But AWD or 4WD comes at both an up front and an ongoing operating cost penalty. Worth noting, I have seen many Bronco’s Guru-type videos of trucks pulled down the boat ramp into the water, but I have never seen a properly-sized AWD or 4WD tow vehicle pulled into the drink.

Yea, you're right on point. The #1 driver here is tire compound and surface prep (or lack thereof). organic growth and some "hard" tires will yield really poor results. Prepped/Grooved broom finish concrete with good tires will give excellent results. Each location will have it's own set of conditions. This is likely where we see the large variation in experiences from users here. Not many of us that cover the full gamut of available ramps.

The friction coefficient is a simple and easy way to communicate how two surfaces interact as it relates the amount of pull required to the amount of downward force present. Being that it's a simplification of greater interactions at play it leaves a lot to be desired when doing a detailed analysis. You're also spot on with the geometry being unhelpful on steeper ramps and opposing that is the depth at which you have to launch on a shallow ramp.

SOOOO many variables here.

Interesting thing about dragsters and wider tires is the VHT additive that is sprayed on the track to help increase friction coefficient, as well as the heat that is generated during the burnout process to get the tires good and "sticky". So it's not so much the width as it is the compound and the rigorous process beforehand that generate the sidewall curling traction.....years ago I was neck deep in the Syclone/Typhoon crowd. Those trucks, with some simple mods, can pull 1.8-1.9 60ft times on all-season tires. A popular upgrade was to move to Corvette wheel setups with LARGE 315 tires in the rear and 275's in the front. Factory it was fitted with 235's all around. The reduction in sidewall, and increase in width made for a stiffer tire overall, and without changing the compound (sticking with the all season range), most guys saw 60ft times drop into the 2.0-2.1 range. The lack of contact pressure was the culprit there. Rotating mass had an effect as well, but typically not at those slow rotation rates.

Final point for me......4WD/AWD is cheap insurance from my seat. When I had selectable 4wd, I rarely used it at the local ramps. Most of the ramps I frequent are perfectly acceptable with a 2WD vehicle. But there is that once a year event where I need it, and it almost instantly justifies the upfront and maintenance expense in convenience alone. Boy Scout motto right "Be Prepared", and the penalty to be prepared in this case is relatively low in comparison to the other costs in trailer boating.

Happy Sunday all!

 

[Naut Krazy]

I have 4WD and never use it. Good tires > 4WD. I see a lot of people gun it out of the ramp too. Go slow and let the torque build up.

Neat idea though - I don't think I would have thought of it.

I use my 2WD Avalanche in low gear. Hold brake and gas then slowly let off brake as truck EASES forward. Works for me.

 

[adrianp89]

Watched a guy do the OPs method yesterday. Was pretty annoying/disrespectful as they pretty much were revving the boat onto the trailer for 5 minutes then kept revving until he pulled out of ramp. Nothing like churning up a wake at the ramp.

 

[GTBRMC]

Watched a guy do the OPs method yesterday. Was pretty annoying/disrespectful as they pretty much were revving the boat onto the trailer for 5 minutes then kept revving until he pulled out of ramp. Nothing like churning up a wake at the ramp.

Many locations frown on and some do not allow power loading of any type, often due to concerns about prop wash erosion adjacent to the ramp.

That plus revving for several minutes is obnoxious.

 

[adrianp89]

Many locations frown on and some do not allow power loading of any type, often due to concerns about prop wash erosion adjacent to the ramp.

That plus revving for several minutes is obnoxious.

This ramp has signs not to do so. Doesn't stop anyone. Honestly I do it too but I rev for about 3 seconds to get me up the trailer. If I don't get it far enough to where I want, I just winch it... I won't try again.

 

[SCSCOTT]

Just my 2 cents...... A 2WD (RWD) SUV will typically have more weight over the drive axle on a boat ramp than a 2WD pick-up in the same size class. ie Full sized pickup vs full sized SUV ect.... Especially with newer Composite and Aluminum pickup beds. IMHO a 2WD RWD SUV will have less issues, in theory, at the boat launch. Granted I feel a 4WD/AWD would be best, Just in case (have it not need it ect)........ 99% of the time I do not need a 4WD in the Charleston SC area. Even if it snows here, don't laugh, we got 7 inches of snow Jan 2018. I grew up in Southern MI and I can drive any vehicle in snow.

Ramp conditions are key. Like many stated, too many variables to say any given vehicle in any situation will or will not work. The ramp I launch at is grooved (estimate 1/2 deep angled grooves), rocky cement. Ramp angles are such that the water just barely touches the tires when launching and I do not go in as deep to retrieve, maybe a foot from touching the tires, I do not power on or off. I have a full sized 2WD SUV (2008 Toyota Sequoia 6000 lbs) on 35"x12.5" A/T tires. Knock on wood, I have never slipped a tire retrieving my rather light weight FSH 195, a heavier boat might be a different story. I deal with tides and every 12ish+ hours a good chunk of the ramp is under water and then exposed. I ease into the throttle like I have an egg between my foot and the pedal I do not want to break. It works for me.

The ones I have seen slip tires are pickups with factory sized highway tires and larger boats. Maybe it is worn tires that contribute to the issue? Driver error? I would venture to guess these same pickups would reduce their chance of slipping with a good quality A/T Tire with the given ramp conditions.

Just my opinions based on my observations.

 

[gthorson]

Late add on to this thread but I didn't see this strategy recommended. For me slipping happens not in the water or in the dry, but in the middle where it is wet but not enough to wash the concrete clean. If the ramp is wide enough, most are double or more, get some momentum going while the boat weight is offset by the water and the concrete is washed clean, but don't pull straight up the ramp. Go diagonal and you will gain the equivalent of a mechanical advantage against the load. My 2WD van sometimes requires this strategy.

 

[FSH 210 Sport]

I agree with @2kwik4u statement of better to have it and not need it and thinner tires provide more traction on slippery surfaces. There’s no better feeling that when you get unexpectedly stuck in 2WD than turning the knob on the dash to 4WD and driving away, before auto locking hubs I have crawled out the drivers window slithered across the hood and reached down to lock the hubs before using the 4WD shift lever in the cab to engage 4WD. Every vehicle I’ve ever owned has been a part time 4WD truck. 99.5% of the time I’m in 2WD, but that .5% of the time I’m in 4WD it’s absolutely necessary. Now that I’m in the black hills the 4WD drive time is much greater than .5%. Maintenance on a part time 4WD is minimal, change the transfer case oil every 50,000 miles and give the steering knuckle universal joints a few shots of grease once a season and that’s it.

I was taught early on the best snow tires back in the day were the Owl 78 series snow tires, very similar to these tires on this ultra cool old Willys that I see at Pactola once in a while.






If the reader has trouble understanding the difference between a narrower tires’ ability to apply more force than a wide one, get your foot stepped on by a 100# pound woman’s spike heeled pump, and the same woman stepping on your foot wearing a running shoe. On ice that spike heel will have way more grip than the running shoe.

My local ramp at Pactola never requires 4WD, nor does Sheridan or Orman, Angostura is a different story, if it’s wet then I can usually get by with locking the rear differential, sometimes just locking the rear diff isn’t enough and 4WD is required. The ramp by the Dam at Lake Oahe is not that steep but there is no way you are getting up that ramp in 2WD locking diff or not. Just walking on that ramp when it’s wet requires you to pay attention or risk ending up on your butt. The ramp at Lake Powell is similar to Oahe and didn’t require the locking diff or 4WD.

I hadn‘t considered wheel chocks for use at the ramp, for me, most of the time that’s not feasible. What I do after backing the boat in the water is to shift to neutral with my foot still on the brake pedal and apply the parking brake, then slowly let off the brake pedal and make sure the parking brake is holding the load then shift into Park. This serves two purposes, making sure the parking brake is holding the load, and keeping any bind off of the transmission parking pawl. I did the same when I parked a manual transmission truck before turning off the engine and putting the transmission back in gear. I believe all owners manuals for cars and trucks say to apply the parking brake when parking the vehicle, relying on the pawl to hold the vehicle is a really bad idea.

 

[Jim_in_Delaware]

I’ll just throw out that 2WD and spinning rear tires can be negated to some degree by putting weight directly over the rear axle. Before 4WD became popular, it wasn’t uncommon to see folks keep a partial load of firewood in the bed of their truck in the winter. When I had my old 2WD trucks, I would often keep a couple of hundred pounds of stone or block over the axle. Not as good as 4WD but it got me home many winter nights in light snow.

Jim