Newbie asks: departure angle vs. breakover angle

[moroza]

Hi all!
I'm a mechanic, BMW enthusiast, a newbie with 4x4's, and slowly beginning the build of an expedition vehicle out of an '81 Toyota. I'm trying to keep it as compact and lightweight as possible, but it's apparent to me that the 7 feet between cab and bumper are not enough. 8 feet would be good, but I don't know where to put the extra foot: between the axles by lengthening the wheelbase (Ford springs flipped backwards), or have it dangle over the rear? The former gives me the benefits of a longer wheelbase (ride, stability on hills) but reduces breakover angle. The latter saves me the drawbacks of a longer wheelbase (less maneuverability, more stress on frame) but reduces departure angle. The wheelbase question, per se, I can work out based on experience as a mechanic and driver, but I don't have much experience with the importance of those angles. Those who do, can you give me your $.02 on the tradeoff between departure and breakaway, for typical expedition vehicle use (that is, not rock-crawling, dune-racing...)?

(I'm not sure in which subforum to post this question, or build/question threads about the camper box itself. Suggestions?)

 

[brianjwilson]

I've having a hard time understanding what you're trying to do. Are you trying to build a longer bed? Or are you going to build a permanent camper back on the frame?

I'm just thinking out loud but...
If you're building a camper on the frame, think more about weight distribution. I think this will have much more effect on ride and handling. You'll want to add some weight to the front axle for stability, and this means that the camper of gravity (not just halfway down the side) should be ahead of the rear axle. The wheel base doesn't always need to be stretched if you're keeping the heavy items forward in the camper. One technique I've seen a lot is to angle the rear of the camper up away from the frame. In other words making the camper a foot or sooner than the frame, and angling it up at 45 degree angle so the departure angle is unaffected. If the center of gravity works out that's a good option.
If you still feel like the frame needs to be longer, you'll just have to decide if you're comfortable dragging the rear end around. I've had a handful of longer wheel base rigs with lower clearance but decent approach and departure angles, and I just learned that I had to angle over obstacles sometimes. It's a little more tricky (for me) to keep from dragging the rear than it is to keep from high centering. But it depends on the terrain too. Might want to just go half way by adding 12" of frame and moving the rear axle 6" back...?

 

[moroza]

I've having a hard time understanding what you're trying to do. Are you trying to build a longer bed? Or are you going to build a permanent camper back on the frame?

The stock bed is 7 feet long. I want to build a permanent camper on the back of the frame, replacing the bed, and I'd like it 8 feet long.

I'm just thinking out loud but...
If you're building a camper on the frame, think more about weight distribution. I think this will have much more effect on ride and handling. You'll want to add some weight to the front axle for stability, and this means that the camper of gravity (not just halfway down the side) should be ahead of the rear axle.
The wheel base doesn't always need to be stretched if you're keeping the heavy items forward in the camper.

I don't want to get too sidetracked about a discussion of weight distribution, because that's a separate matter and I have it figured out well beyond what you've mentioned here. I'm not stretching the wheelbase for weight reasons, I'm doing it for volume reasons (and trying to minimize width and height mean that I'm increasing length, which has to go somewhere).

One technique I've seen a lot is to angle the rear of the camper up away from the frame. In other words making the camper a foot or sooner than the frame, and angling it up at 45 degree angle so the departure angle is unaffected. If the center of gravity works out that's a good option.

I've seen this on some commercially-made 4x4 motorhomes. I may end up doing it regardless of which layout I choose, but it's more compelling with a stock wheelbase and longer tail, for two reasons. One, I have to worry about departure angle more with that setup. Two, I would like to have the camper floor be flat until the rear, past the frame rails, where it would drop down a bit to form a "shelf" for storing the densest items (batteries, water), in order to keep them lower; based on experience with track cars and farm tractors, having extra rear weight is a price well worth paying for a lower center of gravity.

I don't mind rear weight, anyway. My daily driver is a BMW wagon that's about 52% rear-heavy when empty. The sedan equivalents, otherwise identical, are about 49% rear-heavy and I can feel a difference, preferring the wagon's sharper steering and better corner-exit grip. The latter is a non-issue with this truck camper, of course, but for off-road, rear weight bias seems safer for a different reason: if I'm driving along and suddenly drop a front wheel into a hole I didn't see, rear weight keeps the whole rig upright and I can back out. Front weight bias plunges that corner into the hole while the rear is lifted off the ground. This happened to me once in an empty 4x4 pickup, and my first impulse was to get some buddies to sit on the tailgate so that 3 wheels could be back on the ground for me to reverse out of there.

If you still feel like the frame needs to be longer, you'll just have to decide if you're comfortable dragging the rear end around. I've had a handful of longer wheel base rigs with lower clearance but decent approach and departure angles, and I just learned that I had to angle over obstacles sometimes. It's a little more tricky (for me) to keep from dragging the rear than it is to keep from high centering. But it depends on the terrain too. Might want to just go half way by adding 12" of frame and moving the rear axle 6" back...?

This is the kind of info I want to hear. What kind of terrain have you seen? I'm guessing that in general, it's easier to avoid high-centering than dragging the rear, but I don't have much experience with this kind of stuff.

 

[goodtimes]

I don't want to get too sidetracked about a discussion of weight distribution, because that's a separate matter and I have it figured out well beyond what you've mentioned here. I'm not stretching the wheelbase for weight reasons, I'm doing it for volume reasons (and trying to minimize width and height mean that I'm increasing length, which has to go somewhere).

Actually - this is a critical consideration. If you add that extra foot to the back of the truck, when you build your camper, it will disproportionally effect the rear suspension. Just ask anyone who puts an 8' long camper in a short bed (6') truck. It acts as a big lever, adding weight to the rear suspension and taking it off of the front. It works - but the handling & ride isn't nearly as good as it could be. When I rebuilt my camper, I moved as much weight up front as I could (30 gallons of water, batteries, etc), and it still overloads the rear springs on my 3/4 ton Dodge. Your plan to put batteries, water, etc., on the very tail end will only increase this effect.

As to the risk of running out of break-over angle - that all depends on where you are going & how the numbers settle out when you get your truck loaded down. With a stock height Dodge (extended cab, short bed) & a long bed FWC (Grandby), I haven't rubbed the belly a single time. On the other hand, I have drug the rear bumper more than once.

 

[1leglance]

Howdy Moroza and Welcome...
Please take this with the kindness it is intended - You are a self declared newbie to the 4wd/overland world. So make sure the first thing you do is remove from your mind everything you have learned about vehicles that corner well, accelerate well, and overall "handle" well.
You are building an overlanding vehicle. You will go the speed limit or slower often times as your camper/brick will get bad enough gas mileage without using the skinny pedal.
You will be off camber on trials, rocking side to side, climbing/descending grades and such at a slow speed but the effects will be magnified by the hgt/wgt of your rig.

Listen carefully please to the advice the guys with truck campers give you. Many of them have either done it wrong before or learned from others how to do it right.

The wgt should go as far forward/center of your rig as possible, low is good but forward/center is better.
Angle the rear if you are worried about departure angle. The Euro guys have lots of great setups where an angled rear section folds down to be steps and such.
The mounting system has been discussed endlessly here and there are some pro-builders with excellent knowledge to draw on.

Build it for free in Google Sketch up, post your thoughts and work it out wasting no more than electrons instead of money.

 

[ExpoMike]

Having a club cab, long bed full size Dodge Ram with a 8' Four Wheel Camper on the back, the extra length in the wheelbase has not caused any problem versus dragging the hitch/bumper, which I have done many times.

Also, the longer wheelbase will greatly increase the ride comfort, especially on expansion joint style freeways and washboard dirt roads.

 

[seanpistol]

put a flatbed on it

 

[JamesW]

I know mine is a SWB but even with small enough overhangs they catch when going up hills. But i'd say i've only ever bellied out a handful of times.

But that being said i've rarely gotten stuck on my overhangs,whereas when I do belly out it nearly always results in me needing pulled out

 

[moroza]

Sounds like people's experience points quite clearly towards longer wheelbase & shorter overhang, though an angled overhang might make the whole thing moot. Back to the drawing board for me...

Problem with an angled overhang is the lack of bumper. Do I need one at all? Other than as a step and tow hitch, I have no experience using my bumpers in any kind of vehicle; never been rear-ended or run into anything. Seems a lot of campers get away without them.

Howdy Moroza and Welcome...
Please take this with the kindness it is intended - You are a self declared newbie to the 4wd/overland world. So make sure the first thing you do is remove from your mind everything you have learned about vehicles that corner well, accelerate well, and overall "handle" well. You are building an overlanding vehicle. You will go the speed limit or slower often times as your camper/brick will get bad enough gas mileage without using the skinny pedal.
You will be off camber on trials, rocking side to side, climbing/descending grades and such at a slow speed but the effects will be magnified by the hgt/wgt of your rig.

Thanks for the welcome, and 10-4 on forgetting about vehicles that can handle. While I haven't done an overland camper per se, I have learned a thing or two from vehicles that weigh something like 6 tons and whose rear "suspension" is the fat on your gluteus maximus - farm tractors. Tall, tippy, 4wd tractors tilling near-virgin soil with some big overgrown holes at 2.2kph, including at night. If the front left wheel falls off a cliff, the vehicle's axis of rotation is between the front right and rear left. If the center of gravity is on the not-cliff side of that axis, you stay upright and can back out of there. Am I missing something? Or is the assumption that I'm going to be at least a little rear-heavy no matter what, and shouldn't take it too far?

[/quote]
The wgt should go as far forward/center of your rig as possible, low is good but forward/center is better.[/quote]

Assuming a long enough wheelbase and rear suspension rated for the load, what's the big problem with rear-heavy offroaders? I'm by no means dismissing your experience - it's what I'm here to learn from - but tipping sideways seems a much likelier scenario than endovering backwards, no? I'm not going to be going fast enough for Beetle/Corvair/911/MR2-style snap oversteer to happen very often. I should also point out that my truck is narrower than most here, but sits just as high off the ground.

Build it for free in Google Sketch up, post your thoughts and work it out wasting no more than electrons instead of money.

I've been doing exactly this for over a year now. I'd attach the .skp file but I haven't updated it to reflect that I plan to build it out of structural aluminum rather than 2x4's. I even ran time-intensive calculations using a spreadsheet, of weight distribution assuming a worst-case "bulked-out" scenario. The purpose of those calculations was to decide on cargo space layout (big closet behind the cab, or at the way back), with the longer wheelbase assumed as a given. With the forward closet, I estimated 58% rear weight and 39* traverse angle; with the rear closet, 64% rear weight and 38* traverse.

Speaking of numbers, here's what I measured on my truck with the bed removed and no cargo. I'm going to replace the front springs because they're flat (the truck's 32 years old) and will keep working on the rears until it sits at original height. In other words, I don't see them getting worse once the vehicle is loaded.
approach angle: 59
departure angle with bumper: 33
departure angle without bumper, to frame rail: 54
departure angle without bumper, to end of bed: 42 (approx.)
breakover angle: 23

estimated breakover angle with extra 280mm to rear wheel: 21
estimated departure angle with extra 280mm at bed level, stock wheelbase, no bumper: 39
same but with bumper: 26

A monstrous "Cruise America" RV just rolled by and reminded me to mention that what I'm building might be considered a full size class smaller than what most people here have: 8 feet long, 4 feet high (just enough to sit up, not stand up; pop-top will be a future project), 5 feet wide. Sleeping arrangements will be front to back and that's primarily why I want it longer than the stock bed.

 

[goodtimes]

Or is the assumption that I'm going to be at least a little rear-heavy no matter what, and shouldn't take it too far?

Correct - assume you're going to be biased to the rear, no matter what. The further you go, the harder it will be to get the components that will keep both the front and rear suspension working together.

Assuming a long enough wheelbase and rear suspension rated for the load, what's the big problem with rear-heavy offroaders?

The problem is more a matter of getting the front and rear suspension to work together with the added weight - and the bigger the bias, the bigger the hassle.

Not too many people (myself included) go through the time and expense of tuning the spring rate and shock valving on both ends of a vehicle (remember, proper ride height does not equal proper spring rate). We typically work with what is readily available - which means we work with springs and shock valving that were produced for what someone else thinks that the masses use. Since the masses don't use truck campers, the suspension manufacturers don't make much for those who do.

You didn't mention if you were planning on having your camper permanently mounted. If you do, it will be easier to make the suspension work, since you only have to tune it for one load. Most of us remove our campers when we aren't using them, so we see huge swings in the load on the rear suspension. Get springs heavy enough to support the load, and you'll be looking for a good chiropractor when ever you don't have the camper on. Keep the stock springs and you'll be beating the bump stops to death every time you do have it on. Get smart and go the air bag route & you're a lot closer, but you still don't have the sway control - unless you up-size the anti-sway bar. Of course, then you're too stiff when you don't have a load. It's a big balancing act.

By putting as much of the camper weight up front, you spread the weight increase out over 4 springs instead of two. It makes it easier to balance everything, and puts the weight at each corner closer to what the suspension manufacturers have available.

 

[brianjwilson]

I'm not going to back into a lot of what I already said, but having a lot of weight behind the axle will create several issues. Weight distribution really is everything and wheel base should be secondary in my opinion. But center of gravity should be correct for the chassis by design of the camper if that makes sense.

You're talking about building a camper on the back, independent of the cab (even if you try to tie it together it is more or less independent). Riding down washboard roads, bad highways, expansion joints etc it creates an upset in the chassis as the two units (cab and camper) move in opposite direction and create bucking or porpoising as the frame flexes. If the majority of the weight is forward of the axle it tends to move together more. The chassis and suspension is designed to carry most of the weight in the middle, rather than behind the axle.

Also as you center the weight behind the axle, you're unloading the front axle. Since we're not talking about cars that sit low to the ground with firm suspensions, stiff sway bars etc running around on the street, you can throw out that knowledge as it works a lot different. It's different in a car where weight is mostly between the wheels, low to the ground versus stacking weight on the back, behind the axle.
Using some random numbers, say you have a 120" wheel base. You place a 1000lb box 12" behind the axle. Now you've removed 100lbs from the front axle. That now gets added to the rear axle, along with the 1000lbs of the box. Now you're down 100lbs from the front and up 1100lbs on the rear. Is the axle up to the task of taking on even more weight? Unloading your front axle is a bad thing and negatively affects steering and handling. With the weight up higher you have to think about weight transfer as well. Accelerating is going to move the weight even further to the rear. This is the same issue when climbing hills or obstacles. Trying to climb up an obstacle while unloading the front axle substantially will make it difficult to steer and near impossible to get any traction to the front tires.

You're idea about dropping a front tire into something and being able to back up isn't something to worry about really. Handling should be placed well ahead of this consideration. As I already said (and others are also telling you), I've drug bumpers or hitches several times but almost never hung up in the center. It's just something that's easier to judge and prevent.

Feel free to check out posts on sites like RV.net in the truck camper sections, and read up on the importance of center of gravity. A very strong frame can handle some weight, a little behind the axle but generally you'll hear a lot of talk about porpoising.


Here's a picture of my truck and camper that I sold last spring...

This was barely within (or just outside of) acceptable range of center of gravity. The original plan was a smaller pop-up, shorter in length but I made a hasty decision that this would work ok.
The manufacturer center of gravity was estimated at 28" from the front wall, empty with no options. The center of the axle was only 24" from the front of the bed, leaving the estimated center of gravity 4" behind the axle. I added a battery in the front of the floor, tried to keep the water full (in front also) and carried very little. Clothes and stuff went on the bed (cabover) when traveling to try to keep weight forward. The camper itself was light (1300lbs) and the f150 frame is very rigid. I added air bags and a strong rear sway bar and the camper handled surprisingly well.
But due to the center of gravity sitting right on it behind the rear axle, it would occasionally porpoise a small amount. What was the most noticeable was a quick acceleration from a stop. You could feel the weight shift back quickly and unload the front axle pretty badly. I believe if it weren't for the very ridig frame I would have had a bad porpoising problem. I saw lots of toyotas with pop-ups (with good center of gravity) porpoise terribly on the same roads due to weaker frames, you could watching the cab/bed move independently. All in all it wasn't terrible but it was far from ideal. Keeping that weight forward would have made a huge difference. I ended up selling the whole setup but it was because it was more than we needed and way too big to play with off road. Hence the Jeep now and future Ursa Minor camper top.

Anyway I understand you're building a smaller and lighter box but the dynamics are still the same. If I were building another camper setup, weight distribution would be priority number one. I'd be aiming to keep at least 60% of the weight forward of the axle. I would plan the camper to keep the weight as far forward and low as possible, then put the axle back as far as I needed. The rear corners can be angled up or you can design a rear bumper with sliders incorporated that allows you to drag the rear over obstacles if needed.

 

[86tuning]

That 81 with a normal cab should have a 104" wb if I remember right. Later 84 trucks with the extra cab had an extra 8" of cab and wb to make it 112". If you're running 33" tires you should be fine with a 112" wb, even at stock or close to stock height.

I had an 84 pickup with 104" wb that I slightly stretched to 105.5" wb to fit 33" tires without a lift. I did this by moving the front axle forwards to clear the kick panel area of the truck. Never ever got the belly hung up after installing a Budbuilt transmission crossmember. I did tag the rear bumper often enough to bob the bed 8" and install a new high clearance bumper-crossmember up higher at the frame height.

For your buildup I'd do a high clearance trans crossmember and then not worry too much about belly clearance. Just put armour or skids on everything plus a winch in the nose and don't worry about it.

Not an over lander but quite a capable 4wheeler.

Fwiw my fzj80 has the same 112" wheelbase but is on 35" tires. It did occasionally get hung up when crawling over nasty rock gardens when it was at stock height. With a simple 2" lift I now have enough belly clearance that I don't really worry too much.

 

[charlieaarons]

IMHO the most important parameters are departure angle and weight distribution. Breakover angle is secondary and is mitigated by big tires and adequate suspension. Please read pertinent portions of:
http://www.xor.org.uk/silkroute/equipment/choosevan.htm

If I had to build my camper truck all over again, I would lengthen the wheelbase from 3900mm to 4500mm.

Charlie

 

[GilesExpo]

Weight first, then angles

If you are loading a vehicle down with a lot of weight, you will have to be able to carry that weight somewhere before you even think about off roading. Focus on suspension and weight distribution (which will get you somewhere on a street), then focus on angles which will apply off road. Don't build until you have considered both factors and how much time you will spend off road. You said you won't be rock crawling, so breakover might not be as important as your suspension here. If you were rock crawling more, then angles would hold more weight in your dilemma. Think about how much time you will spend relying on your breakover angles (not too often I hope) and how much time you will rely on your suspension and ride quality (all of the time, forever). This ought to help you prioritize a bit. Departure angles can be large, but not be important with some good skids. Sliding off of something is much easier than getting un-high centered.

 

[moroza]

Not too many people (myself included) go through the time and expense of tuning the spring rate and shock valving on both ends of a vehicle (remember, proper ride height does not equal proper spring rate). We typically work with what is readily available - which means we work with springs and shock valving that were produced for what someone else thinks that the masses use. Since the masses don't use truck campers, the suspension manufacturers don't make much for those who do.

I should point out that I do in fact have the time, ability, facilities, motivation, and budget (at least for non-fancy hardware) to do exactly this, as well as any other tuning and fabrication that may be required. This project has the full weight and significance of a gearhead's Project, with a capital P; I'm not just throwing together whatever's readily available. At least not for the vehicle side of the project.

You didn't mention if you were planning on having your camper permanently mounted.

Yep, permanently. I'm cutting a hole in the back of the cab, building a 5-sided insulated box to call the "cabin", and coming up with somekinda heavyduty flexible seal between the cabin and the cab.

My plan was to put it on these 1655-pound Ford springs I've got (stock Yotas are 1100# and that's payload, not including the substantial empty weight difference between a F250 and a Hilux), load it with all the stuff I intend to carry, put each axle on a scale, and only then buy shocks and possibly spring parts to match.

You're talking about building a camper on the back, independent of the cab (even if you try to tie it together it is more or less independent). Riding down washboard roads, bad highways, expansion joints etc it creates an upset in the chassis as the two units (cab and camper) move in opposite direction and create bucking or porpoising as the frame flexes. If the majority of the weight is forward of the axle it tends to move together more. The chassis and suspension is designed to carry most of the weight in the middle, rather than behind the axle.

I hear you about the porpoising; that's something I've experienced in convertibles and I don't care for it. I calculated the CG of the rear-closet cabin with longer wheelbase to be somewhat ahead of the rear axle, but not by much.

Here's a picture of my truck and camper that I sold last spring...

... I saw lots of toyotas with pop-ups (with good center of gravity) porpoise terribly on the same roads due to weaker frames, you could watching the cab/bed move independently. All in all it wasn't terrible but it was far from ideal. Keeping that weight forward would have made a huge difference.

I should point out that mine's a singlecab (and a very small cab at that) and I am tuning up the rear suspension to handle it. Toyota axles are strong enough, but you're right, the frames are lighter and flimsier than domestic iron. One similar to mine cracked the frame in two by hitting a dip with some good speed. That was with a lot of weight and admittedly very rough use, but the wheelbase was stock like mine.

US-market Toyota Hilux/Pickup wheelbases:
79-83 had short-frames and long-frames. All were singlecabs and the difference is in the bed: 6' or 7'. Shorts are about 103", longs 2850mm (~111")
84-88 had short-frames, long-frames, and a couple of different extra-long frames. Short frames were 6' bed singlecabs and 4Runners, also about 103". Long frames were either 7' bed singlecabs or 6' bed extra-cabs, about 112". All extra-longs (116, 122, 132", IIRC...) are 2wd-only and mostly chassis/cab trucks with 1-ton dually rear axles.
89-95 had something similar going on, same 103" for the shorts, except the long wheelbase was about 122" (extracab shortbeds, and a few rare singlecab longbeds).

For your buildup I'd do a high clearance trans crossmember and then not worry too much about belly clearance. Just put armour or skids on everything plus a winch in the nose and don't worry about it.

Not to get sidetracked by technicalities of the chassis, but I think the stock trans crossmember is fully boxed for a reason; I can't imagine the Budbuilt one 1. being half as durable if taking an impact, 2. stiffening the frame nearly as well.

Cool truck you've got! I see you're of the tall/skinny school of thought on off-road tires?

IMHO the most important parameters are departure angle and weight distribution. Breakover angle is secondary and is mitigated by big tires and adequate suspension. Please read pertinent portions of:
http://www.xor.org.uk/silkroute/equipment/choosevan.htm

That is a great link, thanks! Reading it is keeping me busy for a while. It's reassuring that while I've never done quite this before, all of the sub-topics it mentions have been given some good attention in my own planning files. This thing's been a paper project for nearly two years. I've had the chassis to build on since last March, and all I've done is rebuild the front axle and take the bed off to do measurements.