How to determine safe limits for sidehill driving?

[RoyJ]

Hi RoyJ,

Your a mirror of my way of thing and practices. Back seat removed, platform over foot well with heavy items down low and RTT on top. My Cherokee's had the sway bars in the garage with under 50 mils on them. I have since got a JKU and done the same with it. It is the Rubicon and has the electric sway bar disconnect so it is still on the vehicle.

Common sense dose not come in a bottle, but a box and you eat it like dry cereal one piece at a time!

Great minds think alike!

 

[rayra]

"just about all OEM vehicles can hit 1.0g of cornering (steady state) without rolling"


I'm going to have to take exception with this as it was a big damned deal when a factory corvette was finally able to do this while maintaining traction, in the mid 80s. Things haven't improved that much since then and I highly doubt something like my '02 Suburban would even make a significant portion of lateral 1G force without going over. Don't conflate the cornering performance of higher-end sports cars with phat traction tires on good pavement with the center of gravity and tilt angle of modern trucks / SUVs in a hillside traverse. Totally different things.



eta here's a motortrend article from a decade later and eventhe top sports cares were making only .8G
http://www.motortrend.com/roadtests/112_9705_track_testing_event/surface_maximum_lateral_g.html

 

[oscar the grouch]

never wheel alone and never wheel outside your experience level. If you have to ask what your tip over point is then you need more wheel time. join a local club and learn from the experienced guys.

 

[Root Moose]

Some great information in this thread.

One thing that I don't see mentioned a lot is how soft coil springs, dampers, disconnnected sway bars and low tire pressures affect how easily a truck can tip over, especially when the truck is a RTI ramp champ.

Once the body is mostly decoupled from the frame it can "flop" around side to side at will as it climbs over obstacles. This leads to more pucker factor as the CG changes while this is happening. A bad bounce can lead to a bad day.

 

[Happy Joe]

Personally I am a fan of pucker factor determination of wheeling limits (it has kept the greasy side down for me); with the exception that some neophytes do not seem to be able to get in touch with it. I agree that experience is the cure.
It matters little what the actual number is, IMO, if you are looking at a gage when you should be wheeling/paying attention to the vehicle & trail; you are a candidate for a flop/roll.
Go slow (wheeling is mostly about slow), fast normally means increased chance of damage, those situations where wheel speed is advantageous (mud, snow some powder sand are usually pretty level (go straight up or straight down dunes).
Avoid extreme/excessive lifts; they are a sign of poor engineering, IMO; low CG (Center of Gravity) with droop limited by straps/chains is the way to go, IMO.
Disconnection of anti-sway bars can normally give a little extra wheel travel; although if the vehicle has lockers or electronic traction control it is often not worth while/necessary on merely difficult terrain.
Shocks (dampers) should be adjusted to the spring rates for best performance on or off road (excessive stiffness should be avoided as much as excessive softness, IMO).
Avoid the ram an slam method of wheeling; if you can't get through without excessive throttle your vehicle (and possibly driving technique) needs improvement.
Avoid raising the vehicle CG caused by excessive loading (especially in roof racks).
Enjoy!

 

[pappawheely]

I learned about weight distribution racing autocross. Using the steering, throttle and brakes can either save you or make you crash. it's not just about speed.

 

[WUzombies]

. There is no fixed value,

Sure there is, you start by calculating the longitudinal center of mass, then the lateral center of mass (they are typically in the same spot although they may not be), using the longitudinal to verify the lateral value. Then you can take that value and calculate the static roll threshold. Taking that value you can compare it to the dynamic roll threshold with respect to the roll center height of the suspension. There have been some SAE papers on the subject and it's discussed in the Northwestern manual (Fricke). Typically these calculations are used to determine g load for the trip point in rollover collision events, but they can be applied here to determine a measurable value. Getting there requires a lot of measurements, 4-accurate scales and a little bit of math.

Or do it like thousands of others and build the personal experience slowly to get a feel for it.:smiley_drive:

 

[RoyJ]

"just about all OEM vehicles can hit 1.0g of cornering (steady state) without rolling"


I'm going to have to take exception with this as it was a big damned deal when a factory corvette was finally able to do this while maintaining traction, in the mid 80s. Things haven't improved that much since then and I highly doubt something like my '02 Suburban would even make a significant portion of lateral 1G force without going over. Don't conflate the cornering performance of higher-end sports cars with phat traction tires on good pavement with the center of gravity and tilt angle of modern trucks / SUVs in a hillside traverse. Totally different things.



eta here's a motortrend article from a decade later and eventhe top sports cares were making only .8G
http://www.motortrend.com/roadtests/112_9705_track_testing_event/surface_maximum_lateral_g.html

You misunderstood me.

I didn't say all vehicles are capable of hitting 1.0g performance wise, not even close (I'm a Z06 owner btw, and know what 1g+ feels like).

What I'm saying is, for most STOCK vehicles, including light trucks, IF they can hit 1g (imaginary skidpad with warm slick tires), they can do so without toppling over. LIFTED vehicles can rarely achieve this.

This is just another way of saying they can reach a static 45 degree slope without flipping. Since Tan(45) = 1.0

In other words, traveling a 45 degree side slope and performing a sustained 1.0g turn exerts the same force vector at a vehicle's COG. If this force vector, extended from the COG, does not extend to the outside of the car's track width, the vehicle does not roll.

 

[RoyJ]

BTW, in case anyone didn't know, to correlate the rollover threshold of lateral g-force to angle of tilt, simply take the arcTan aka inverse tan of the angle of tilt.

Example, a sedan that can (theoretically) corner 1.5g before rolling, would have a side slope threshold of: tan^-1(1.5) = 56.3 degrees

A semi tanker than flips at 0.4g have a slope limit of: tan^-1(0.4) = 21.8 degrees

Likewise, if you know a car's COG and track width, you can approximate the theoretical limits by arctan(track width/2/COG). My Vette has a COG of 18" and rear track of 62.5", so:

tan^-1(62.5/2/18) = 60.0 deg or 1.73g

Again, all in theory, assuming infinite friction and flat surface. In reality, aerodynamic downforce and track bank angle can drastically increase that number to over 2.5g.

 

[rayra]

Sure there is, you start by calculating the longitudinal center of mass, then the lateral center of mass (they are typically in the same spot although they may not be), using the longitudinal to verify the lateral value. Then you can take that value and calculate the static roll threshold. Taking that value you can compare it to the dynamic roll threshold with respect to the roll center height of the suspension. There have been some SAE papers on the subject and it's discussed in the Northwestern manual (Fricke). Typically these calculations are used to determine g load for the trip point in rollover collision events, but they can be applied here to determine a measurable value. Getting there requires a lot of measurements, 4-accurate scales and a little bit of math.

Or do it like thousands of others and build the personal experience slowly to get a feel for it.:smiley_drive:

Balderdash. You are talking about a static tipping point. linear forces. You're the only one doing so. The rest of us are talking about a traverse on dirt, with all its surface variations, traction, obstructions, speed etc. Incalculables. Hence all the talk about 'feel' and staying well clear of those mechanical limits. Too fast, you jounce, radically altering your CG and over you go. To slow and you slide sideways, a tire catches traction or hits in immovable object and over you go. You have to get a feel for it.

 

[rayra]

You misunderstood me.

I didn't say all vehicles are capable of hitting 1.0g performance wise, not even close (I'm a Z06 owner btw, and know what 1g+ feels like).

What I'm saying is, for most STOCK vehicles, including light trucks, IF they can hit 1g (imaginary skidpad with warm slick tires), they can do so without toppling over. LIFTED vehicles can rarely achieve this.

This is just another way of saying they can reach a static 45 degree slope without flipping. Since Tan(45) = 1.0

In other words, traveling a 45 degree side slope and performing a sustained 1.0g turn exerts the same force vector at a vehicle's COG. If this force vector, extended from the COG, does not extend to the outside of the car's track width, the vehicle does not roll.

No, I don't agree. You're attempting to equate two wildly different things on the basis of nothing but a G measurement. A track car / truck, with big sticky tires, making a hard turn under a buttload of power. It's a lot different from crawling across an off-camber dirt slope in a vehicle with a lot higher CG and a rough loose ground surface. You can attempt to reduce it to nothing but a comparative G measurement but it's a totally different exercise with a lot of different factors involved. And consequences.

 

[Hilldweller]

Balderdash. You are talking about a static tipping point. linear forces. You're the only one doing so. The rest of us are talking about a traverse on dirt, with all its surface variations, traction, obstructions, speed etc. Incalculables. Hence all the talk about 'feel' and staying well clear of those mechanical limits. Too fast, you jounce, radically altering your CG and over you go. To slow and you slide sideways, a tire catches traction or hits in immovable object and over you go. You have to get a feel for it.

Silly.
There are no variables offroad. Offcamber hills are laboratory environments...
[/cheeky.wink.off]

 

[RoyJ]

No, I don't agree. You're attempting to equate two wildly different things on the basis of nothing but a G measurement. A track car / truck, with big sticky tires, making a hard turn under a buttload of power. It's a lot different from crawling across an off-camber dirt slope in a vehicle with a lot higher CG and a rough loose ground surface. You can attempt to reduce it to nothing but a comparative G measurement but it's a totally different exercise with a lot of different factors involved. And consequences.

You're focusing too much on the details of how 1.0g PERFORMANCE SPECs are achieved.

Let's make this simple:

In a vacuum of space with earth's gravity, on a flat plane, a block of mass is experiencing a 1g lateral acceleration (9.81 m/s^2), exerted at its COG.

If that block does not tip, I can then state the same block can be tipped at 45 degrees, without tipping.

Better?

 

[Root Moose]

Free body diagrams, kids, free body diagrams.

 

[mowerman]

I'd like to apologise for reviving a slightly elderly thread but I wanted to post this vid to show how a simple and innocuous event can become a roll over due to a slightly poor line and slightly too much speed

http://youtu.be/VHOHEQRVnpE

Nothing to do with me by the way.
I always think speed is the enemy on cross slopes, but I also struggled with body lean until I fitted new rear springs.