Just curious. What do those of you with offroad experience think is an adequate departure angle? The vehicle I am considering has a 22 degree stock angle but with the tires and mods I plan it should be near 30 degrees. Is this enough to get me through places like Utah and Colorado trails? With more intensive mods and shortening the camper I could probably add 5 degrees but it would be alot of work. One option is to move the rear axle back, lengthening the wheelbase. So is a greater departure angle worth a slightly larger turning radius.
What your departure angle and is enough?
- Thread starter milo12
- Start date
charlieaarons
Explorer
Departure angle and ground clearance (under diffs and under transfer case) are very important parameters. Departure angle is really analogous to ground clearance under lowest point behind rear axle. The more the better but I wouldn't re-engineer the vehicle for that.
Stephen Stewart discussed this in his excellent dissertation on expedition vehicles and felt that departure angle was at least as important as ground clearance.
Maximizing tire diameter within reason (keeping axle gearing and tire clearance in mind) is perhaps the best way to start the process; sounds like you've done that.
Regarding wheelbase vs. d.a. vs turning diameter: When I purchased my U500 I had the option of lengthening the chassis up to 1000 mm (39"). It could have increased d.a. quite a bit and also increased rear tipover angle; but it would have decreased center breakover angle, increased turning diameter by 25% and cost $20,000. So I didn't do it.
I think 30 degrees is good enough for most situations. Expedition trucks are not designed to be competitive rock crawlers. What is the vehicle you have in mind, and what specific modifications are planned?
Charlie
Stephen Stewart discussed this in his excellent dissertation on expedition vehicles and felt that departure angle was at least as important as ground clearance.
Maximizing tire diameter within reason (keeping axle gearing and tire clearance in mind) is perhaps the best way to start the process; sounds like you've done that.
Regarding wheelbase vs. d.a. vs turning diameter: When I purchased my U500 I had the option of lengthening the chassis up to 1000 mm (39"). It could have increased d.a. quite a bit and also increased rear tipover angle; but it would have decreased center breakover angle, increased turning diameter by 25% and cost $20,000. So I didn't do it.
I think 30 degrees is good enough for most situations. Expedition trucks are not designed to be competitive rock crawlers. What is the vehicle you have in mind, and what specific modifications are planned?
Charlie
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theMec
Adventurer
I think 30 deg is fine for most off pavement travel - if you scout ahead a bit. My old stock FJ60 never had problems and my truck only had problems once (approach angle not departure angle). Obviously it depends where you intend on going.
I find it interesting that departure angle is mentioned frequently on expo and other sites but rarely approach or break over angle. For instance, my truck has less approach angle than departure angle. Many vehicles I've seen on this site don't have a particularly good approach angle or breakover angle. Any of these, along w/ poor ground clearance, could get you banged up - the truck that is
I find it interesting that departure angle is mentioned frequently on expo and other sites but rarely approach or break over angle. For instance, my truck has less approach angle than departure angle. Many vehicles I've seen on this site don't have a particularly good approach angle or breakover angle. Any of these, along w/ poor ground clearance, could get you banged up - the truck that is
Wow I really messed up the title to this thread.:drool:
The truck is a Dodge 5500. Planned mods are 37 inch tires plus I will ditch the rear leaves and 4 link it with coils and airbags. I do not plan on lifting it. I will cut as required around the front fenders. The airbags will allow me to raise the rear about 5-6 inches if I high center it or need a little more departure angle. But the primary reason for the airbags is to recapture the load capacity lost by going to coils springs. It will be a daily driver so ride quality is very important.
With the 37 inch tires approach angle is 45 degrees and departure is 30 degrees. There is about 20 inches of daylight under the truck so breakover angle should be good.
A future mod will be to replace the transfer case with a 4 speed Atlas unit. One thing I need to decide is which ratios to go with.
The truck is a Dodge 5500. Planned mods are 37 inch tires plus I will ditch the rear leaves and 4 link it with coils and airbags. I do not plan on lifting it. I will cut as required around the front fenders. The airbags will allow me to raise the rear about 5-6 inches if I high center it or need a little more departure angle. But the primary reason for the airbags is to recapture the load capacity lost by going to coils springs. It will be a daily driver so ride quality is very important.
With the 37 inch tires approach angle is 45 degrees and departure is 30 degrees. There is about 20 inches of daylight under the truck so breakover angle should be good.
A future mod will be to replace the transfer case with a 4 speed Atlas unit. One thing I need to decide is which ratios to go with.
That seems like a huge truck to be over worried about departure angles!
I have found with wagons that you end up dragging the rear down ledges anyway, regardless of departure angle. Just have a heavy duty rear bumper that will stand up to being set gently on the lip of the ledge and then dragged off. I still can't see a truck that big in Moab or anywhere else it would matter.
Bigger tires, more lift and a tucked in bumper all will help with the issue.
I have found with wagons that you end up dragging the rear down ledges anyway, regardless of departure angle. Just have a heavy duty rear bumper that will stand up to being set gently on the lip of the ledge and then dragged off. I still can't see a truck that big in Moab or anywhere else it would matter.
Bigger tires, more lift and a tucked in bumper all will help with the issue.
charlieaarons
Explorer
milo12 said:
I believe the GVW rating on the Atlas is only 11000 lb. Are you going to use single 37s? (The highest load rated tires in that size range are Michelin 325/85R16 XMLs = 5070 lb) If so you will lose rear axle load rating. If so, why 5500 rather than 3500 or even 2500?
Charlie
Atlas is located right up the road from where I live so I went up and asked them about my application. They said it will be no problem at my expected gross weight of 11,000 to 11,500 lbs. They said the published 11,000 lb figure is conservative as they expect people to race at that weight. I will be driving very slowly so I will not be abusing the drivetrain.
There are multiple reasons I chose the 5500.
1. There is no size difference between a Dodge 1500, 2500, 3500, 4500 and 5500.
2. The suspension components on the 4500/5500 are at least 50% stouter than the 2500/3500. Stop by a dealer and look at the front end components. You will be amazed at the size differences, with the 4500/5500 Dodge has finally built a truck the way it should be built. I own a 2500 and they are known to have weak front suspension. In fact there is a profitable industry built around supplying upgraded Dodge diesel front end suspension components. The front suspension on the 2500/3500 diesel trucks is very inadequate as it is the same as the gas version and cannot handle the extra 500 lbs. Anyone that runs them on rough roads or with large tires has found they wear out very quickly.
3. The frame on a 4500/5500 is many times stonger than a 2500/3500. In fact the 4500/5500 frame is over 50% thicker and a higher grade of steel.
4. The rear differential is a stout Spicer 110 with limited slip. After speaking with rock crawiling buddies they recommend I go with the lowest gearing I can get and the strongest rear axle I can get. The 13,500 lb rated axle should easily handle the super compound low I will have with the Atlas 4 speed transfer case and my 11,000 lb weight.
I agree a full size truck is a large vehicle to go off road. That is exactly why I am trying to figure out what I need to do to maximize its capabilities. I have taken my truck on many trails that some think are only for jeeps. I have usually been OK. The most limiting factor is turning diameter. It is a real pain in the *** to find myself at the end of a narrow trail and no way to turn around. But unless I want 4 wheel steering that is something I will have to deal with.
The tire I am considering is a Continental MPT 81. If I read the technical data correctly it is good for 6779 lbs at 95 psi. My expected axle weights are as follows; front - 5300 lbs, rear - 6000 lbs. So I am easily within that weight rating. In fact I am within the Toyo MT ratings but I doubt the Toyo is as tough as the Continetal.
I really appreciate everyones input as I work on this project.
There are multiple reasons I chose the 5500.
1. There is no size difference between a Dodge 1500, 2500, 3500, 4500 and 5500.
2. The suspension components on the 4500/5500 are at least 50% stouter than the 2500/3500. Stop by a dealer and look at the front end components. You will be amazed at the size differences, with the 4500/5500 Dodge has finally built a truck the way it should be built. I own a 2500 and they are known to have weak front suspension. In fact there is a profitable industry built around supplying upgraded Dodge diesel front end suspension components. The front suspension on the 2500/3500 diesel trucks is very inadequate as it is the same as the gas version and cannot handle the extra 500 lbs. Anyone that runs them on rough roads or with large tires has found they wear out very quickly.
3. The frame on a 4500/5500 is many times stonger than a 2500/3500. In fact the 4500/5500 frame is over 50% thicker and a higher grade of steel.
4. The rear differential is a stout Spicer 110 with limited slip. After speaking with rock crawiling buddies they recommend I go with the lowest gearing I can get and the strongest rear axle I can get. The 13,500 lb rated axle should easily handle the super compound low I will have with the Atlas 4 speed transfer case and my 11,000 lb weight.
I agree a full size truck is a large vehicle to go off road. That is exactly why I am trying to figure out what I need to do to maximize its capabilities. I have taken my truck on many trails that some think are only for jeeps. I have usually been OK. The most limiting factor is turning diameter. It is a real pain in the *** to find myself at the end of a narrow trail and no way to turn around. But unless I want 4 wheel steering that is something I will have to deal with.
The tire I am considering is a Continental MPT 81. If I read the technical data correctly it is good for 6779 lbs at 95 psi. My expected axle weights are as follows; front - 5300 lbs, rear - 6000 lbs. So I am easily within that weight rating. In fact I am within the Toyo MT ratings but I doubt the Toyo is as tough as the Continetal.
I really appreciate everyones input as I work on this project.
Robthebrit
Explorer
what sized MPT81's?
Rob
Rob
There is discussion over in the full size domestic forum about Fords. Similar issues are up for discussion. There is a cool pic of a Dodge with portals and huge tires.:rockon:
http://www.expeditionportal.com/forum/showthread.php?t=11738&page=2
I have always thought portals would be the ticket so I spoke with Portaltek about putting their portals on a rig the size I am building. They claim they are strong enough. I'm a little worried about putting the torque of a diesel that has been reduced 10:1 through a ford 9 inch ring and pinion. Also even as important iis they want $20,000 for a set.
The tires I am considering are the 335/80R20. i have to run 20 inch rims because the Dodge 5500 has 15.5 inch diameter brakes.:jumping:
http://www.expeditionportal.com/forum/showthread.php?t=11738&page=2
I have always thought portals would be the ticket so I spoke with Portaltek about putting their portals on a rig the size I am building. They claim they are strong enough. I'm a little worried about putting the torque of a diesel that has been reduced 10:1 through a ford 9 inch ring and pinion. Also even as important iis they want $20,000 for a set.
The tires I am considering are the 335/80R20. i have to run 20 inch rims because the Dodge 5500 has 15.5 inch diameter brakes.:jumping:
Robthebrit
Explorer
My 421 unimog has 335/80r20 MPT81s and reading directly from the sidewall it says 147K (load range M) 6760 pounds at 90psi. Measuring across them is about 41 inches. My 416 has their bigger brother (365/80r20) reading from the side walls: 152K (load range N) and 7860 pounds at 85psi (these are about 44-45 inches across). I run both sets at 35-40psi when on the road as as low as 10 when offroad, they seem to stay on the beads very well and I have never pulled one off.
You must remember these are 100km/h max tires. If you want to drive at freeway speeds these are not for you - they will fail. If you blow one of these at 90psi doing 70+mph it will take parts of the truck with it - don't forget you drop 10 inches too! If its a front wheel you are in trouble.
The good thing about a mog being slow is you never feel like you are out of control. I can hit 62mph which is exactly 100km/h so I don't have to worry about it. The camper is faster I never go above 55-60.
Rob
You must remember these are 100km/h max tires. If you want to drive at freeway speeds these are not for you - they will fail. If you blow one of these at 90psi doing 70+mph it will take parts of the truck with it - don't forget you drop 10 inches too! If its a front wheel you are in trouble.
The good thing about a mog being slow is you never feel like you are out of control. I can hit 62mph which is exactly 100km/h so I don't have to worry about it. The camper is faster I never go above 55-60.
Rob
Thanks for the great info Rob. Maybe you can clear up some confusion I have regarding the size of the tires. From the technical manual Continental states the following;
Normal dimension in service - 1076mm (42.36 in)
Continental tire dimension - 1032mm (40.63 in)
Static radius - 480mm (18.90 in radius, 37.80 in diameter)
Circumference - 3120mm (39.10 in)
OK so how the heck can the same tire have 4 different diameters? The only one that seems to make sense is Static radius. I assume this means the dimension from the center to the ground when loaded.
I appreciate your warning about blowing a tire, been there done that, not fun. I have no problem keeping my highway speed to 100kph. I plan on using the Continentals only for off road trips, for daily driving I'll use Toyos as I will be about 2500 lbs lighter.
Normal dimension in service - 1076mm (42.36 in)
Continental tire dimension - 1032mm (40.63 in)
Static radius - 480mm (18.90 in radius, 37.80 in diameter)
Circumference - 3120mm (39.10 in)
OK so how the heck can the same tire have 4 different diameters? The only one that seems to make sense is Static radius. I assume this means the dimension from the center to the ground when loaded.
I appreciate your warning about blowing a tire, been there done that, not fun. I have no problem keeping my highway speed to 100kph. I plan on using the Continentals only for off road trips, for daily driving I'll use Toyos as I will be about 2500 lbs lighter.
Robthebrit
Explorer
I don't know how they get all the different dimensions. I can take a guess but thats not going to help. Measuring across the tire while inflated and on the truck is somewhere between 40-41 inches - these tires are brand new. Its difficult to be more exact because there is slight curve off at the edge of the treads and the tread pattern itself is about an inch deep.
Rob
Rob
charlieaarons
Explorer
milo12 said:
If your rear axle is really going to be that light or near that (expedition rigs gain weight rapidly) you might consider Michelin 335/80R20 141K. It is only rated at 5780 lb @ 62 psi but at 65 mph. A 3mph difference may not be much but it's 3 mph. "Pushing" it by adding 5-10 psi at <75% load will get you up to 70 (definitely not recommended....). Does anyone know: does anyone make tire pressure remote sending units that give an exact psi reading allowing one to calculate tire temperature?
Charlie
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