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Old 02-02-2017, 04:00 PM   #1
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Appropriate weight behind rear wheels?

What is a good way to calculate the appropriate weight limit for tanks behind the rear wheels on a conventional school bus? I am looking at installing two 93 gallon tanks behind the rear wheels but above the frame in a raised floor. This would be in a 10-12 window conventional bus with 8-10 feet behind the rear wheel wells.

I want to make sure I am not rear loading too much weight. It looks like it would be about 1800 lbs of potential liquid weight at 93 gallons (more square tank) or 1400 lbs at 80 gallons (more flat tank).

I also wonder how this might affect handling in adverse conditions such as snow or ice? Would having this additional weight in the rear make the bus more likely to go out of control in a rear slide situation?


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For those following the discussion in my other thread about a raised floor, I am expecting a minimal amount of need for center of bus routing because plumbing stops at the rear wheels which is the bathroom. This makes me lean toward the use of wider flatter tanks to distribute the weight of the water.

I should still be able to maintain room between the tanks that is sufficient for any pass through electrical and valves isolated at the front of the tank along side the wheel well interiors.

I do worry about there being greater distance for "waves" to form. I am guessing I will need to find a tank with baffles in it?

Thanks for the help!
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Old 02-02-2017, 06:21 PM   #2
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A lot depends on where the rest of the weight falls. If balanced by something close to that amount further forward you should be OK but if there is too much too far back...yes...that could be trouble. Most RV's & coaches have their tanks between the axles. And you will definitely want plenty of baffling in any large fluids tank. That much moving mass can create a heck of a lot of force.
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Old 02-02-2017, 06:39 PM   #3
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i don't think any conventional bus would mind that kind of weight behind the tires, or really anywhere. there are some carhaulers on here that out weigh your water tanks.

180 gallons is a bunch of water, and you'll need a waste tank just as big too.

i shared a scale receipt on the forum a year or so ago. you can see the weight on my finished conversion, and the axle limits from the tag leave me more than 8000lbs of capacity on my rear axle.

you can see that thread here:
http://www.skoolie.net/forums/f13/we...ing-14296.html

good luck
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Old 02-02-2017, 06:57 PM   #4
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My tc1000 has a weight of 15100 lbs with nothing in it. Front axle is 9100, rear is 6k. With this knowledge, I want to put as much weight as possible, as far back as possible to even things out a little. The difficulty for me is that the rear of my bus is the 'garage', and has a fold up bed, and holds mortorcylces. But mortorcylces are not too heavy. If I were you, I'd start by finding a scale at a truck stop.
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Old 02-02-2017, 07:25 PM   #5
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Is that two whatever gallon freshwater tanks? Or a freshwater and grey water?
I understand wanting to leave home to wherever with a full tank of water that you know is good.
But your grey tank won't have anything much in it until the fresh tank has its contents between the two. So only one tank will be slam full, or one will 70% while one is 30% or the grey tank will be full while the freshwater is empty?
If I am following you correctly?
For me I centered my fresh water behind the axle in between the frame rails. Everything grey water related in house is on one side and the only brown water is to the opposite side with the underside tanks corresponding.
Haul freshwater in and haul the whatever out does and doesn't really work for my tank orientation because I will have more grey water(kitchen sink,bath sink,shower)than black water(toilet) so if I want to haul without dumping I might/could look a little lop sided to one side?
But after doing the things I have done with my 86 b-700 I don't think it will even care.
Never had the tanks full one way or the other but I have had several other things in there.
Just to clarify my personal truck has been down for awhile so my bus is my personal vehicle. It still don't move much cause I work 6-days a week out of a GPS monitored company truck. My bus takes me, my two sons, and three other boys to Boy Scouts every Monday night and camp outs every month.
Even in my interior finishing stages I have shoved 3 canoes and two kayaks in the back door plus young men and there gear for a survival camp out on the water.
You should see some of the stuff they bring but even though it's useless I let them carry/drag it around with them until they figure out its not needed much less worth the extra effort?
There was a point in there pertaining to this thread ?
Water is 8' pounds per gallon ?
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Old 02-02-2017, 07:31 PM   #6
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Originally Posted by SDR76 View Post
My tc1000 has a weight of 15100 lbs with nothing in it. Front axle is 9100, rear is 6k. With this knowledge, I want to put as much weight as possible, as far back as possible to even things out a little. The difficulty for me is that the rear of my bus is the 'garage', and has a fold up bed, and holds mortorcylces. But mortorcylces are not too heavy. If I were you, I'd start by finding a scale at a truck stop.
IS 15k your GROSS?
My 40 footer only weighs 17.9k. its gross is 33k.
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Old 02-02-2017, 07:40 PM   #7
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Originally Posted by Tango View Post
A lot depends on where the rest of the weight falls. If balanced by something close to that amount further forward you should be OK but if there is too much too far back...yes...that could be trouble. Most RV's & coaches have their tanks between the axles. And you will definitely want plenty of baffling in any large fluids tank. That much moving mass can create a heck of a lot of force.
I am planning to fill each of the tanks with these style of baffles - https://www.fs.fed.us/eng/pubs/html/.../00511302.html - They displace about 1-2% of the storage capacity, but effectively eliminate all wave motion in the tank.

I am considering the implications of moving them forward in front of the wheel wells - but I would be concerned that is too much too far forward. It sounds like from the other posts in this thread, that 1800 lbs should not be an issue. There will only be another 500-750 lbs of stuff beyond the wheels. The kids bunk beds and some storage.
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Old 02-02-2017, 07:41 PM   #8
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Originally Posted by turf View Post
i don't think any conventional bus would mind that kind of weight behind the tires, or really anywhere. there are some carhaulers on here that out weigh your water tanks.

180 gallons is a bunch of water, and you'll need a waste tank just as big too.

i shared a scale receipt on the forum a year or so ago. you can see the weight on my finished conversion, and the axle limits from the tag leave me more than 8000lbs of capacity on my rear axle.

you can see that thread here:
http://www.skoolie.net/forums/f13/we...ing-14296.html

good luck
I should have clarified in my post that one tank is fresh and one is grey water. We will not have a black water tank, and instead will have a composting toilet. No nasty cleanouts to deal with. Put a bag over the toilet base, dump, done. LOL
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Old 02-02-2017, 07:48 PM   #9
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Originally Posted by Jolly Roger View Post
Is that two whatever gallon freshwater tanks? Or a freshwater and grey water?
I understand wanting to leave home to wherever with a full tank of water that you know is good.
But your grey tank won't have anything much in it until the fresh tank has its contents between the two. So only one tank will be slam full, or one will 70% while one is 30% or the grey tank will be full while the freshwater is empty?
If I am following you correctly?
For me I centered my fresh water behind the axle in between the frame rails. Everything grey water related in house is on one side and the only brown water is to the opposite side with the underside tanks corresponding.
Haul freshwater in and haul the whatever out does and doesn't really work for my tank orientation because I will have more grey water(kitchen sink,bath sink,shower)than black water(toilet) so if I want to haul without dumping I might/could look a little lop sided to one side?
But after doing the things I have done with my 86 b-700 I don't think it will even care.
Never had the tanks full one way or the other but I have had several other things in there.
Just to clarify my personal truck has been down for awhile so my bus is my personal vehicle. It still don't move much cause I work 6-days a week out of a GPS monitored company truck. My bus takes me, my two sons, and three other boys to Boy Scouts every Monday night and camp outs every month.
Even in my interior finishing stages I have shoved 3 canoes and two kayaks in the back door plus young men and there gear for a survival camp out on the water.
You should see some of the stuff they bring but even though it's useless I let them carry/drag it around with them until they figure out its not needed much less worth the extra effort?
There was a point in there pertaining to this thread ?
Water is 8' pounds per gallon ?
You make a good point here about the fresh / gray water ratio. I can probably get away with a much smaller tank there.

What I may do is install smaller grey water tanks below each (say 30 gallons each) the toilet and the shower, and then run a large fresh water across the width of the bus with the outlet in the center going down the middle of the bus. I only anticipate keeping the freshwater tank half full most of the time as we will be in place for a minimum of 2 weeks with hook ups in most situations.
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Old 02-02-2017, 08:25 PM   #10
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i'm confused.

both your fresh tank and your waste(grey) tank will be full at the same time?

you don't do that. you fill your fresh tank and drain it into your grey. then you dump grey (icky) legally, and refill fresh. disposing the poo in a bag is your choice and an extra step.

having your fresh and waste tanks both full would be fool hardy.
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Old 02-02-2017, 08:31 PM   #11
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IS 15k your GROSS?
My 40 footer only weighs 17.9k. its gross is 33k.
15k is the actual weight of the bus. I'm good to go up to 33k. How I licensed it, they said keep it under 26k. But my point of the post is that my actual weight on each axle is very disproportionate with the front being heavy. Mind you, FE bus, with the engine in front of the front axle, coupled with a short wheelbase, leads to a very heavy front...
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Old 02-02-2017, 08:34 PM   #12
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I should have clarified in my post that one tank is fresh and one is grey water. We will not have a black water tank, and instead will have a composting toilet. No nasty cleanouts to deal with. Put a bag over the toilet base, dump, done. LOL
There are no real "composting toilets". And yes, cleaning out the dry toilet can be VERY nasty.
A certain time of the month for ladies can be a very gnarly time to use a dry toilet.
Just my 2 cents. Everyone else loves paying a grand for a plastic litter box with a vent.

I say just build one if going the dry route. They aren't complicated, or at least they don't have to be.
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Old 02-02-2017, 08:35 PM   #13
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i'm confused.

both your fresh tank and your waste(grey) tank will be full at the same time?

you don't do that. you fill your fresh tank and drain it into your grey. then you dump grey (icky) legally, and refill fresh. disposing the poo in a bag is your choice and an extra step.

having your fresh and waste tanks both full would be fool hardy.
Of course I do not plan to have both full at the same time on a regular basis, lol. I simply want to plan for the scenario where that might happen. I can see running out of fresh water or getting low, being able to fill up but not empty the grey water at the same time. I am planning everything based on maximum loaded value if possible. It is quite likely that we will be towing a small cargo trailer for vending at some point, so I have to plan for that as well.
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Old 02-02-2017, 08:39 PM   #14
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There are no real "composting toilets". And yes, cleaning out the dry toilet can be VERY nasty.
Certain times of the month for ladies can be a very gnarly time to use a dry toilet.
Just my 2 cents. Everyone else loves paying a grand for a plastic litter box with a vent.
LOL, yes, not properly managed, they can be nasty. The biggest issue we have had with the one at our old cabin was that the kids would forget to drop a handful of coconut husk in after they did their business. If you do this regularly with each disposal, and crank the handle, they work great.

I have seen bucket toilets and other contraptions, but they all had issues with smell. The composting toilet with the crank built in has worked well for us. Dealing with poo is never ever fun! Black water tanks just make me sick thinking about them, lol. I had to clean out a stoppage in a black water tank one time, will never ever one as a result lol.
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Old 02-02-2017, 10:02 PM   #15
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Old 02-02-2017, 10:11 PM   #16
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i'm confused.

both your fresh tank and your waste(grey) tank will be full at the same time?

you don't do that. you fill your fresh tank and drain it into your grey. then you dump grey (icky) legally, and refill fresh. disposing the poo in a bag is your choice and an extra step.

having your fresh and waste tanks both full would be fool hardy.
If you do anything that comes close to "full time" on the road you WILL find yourself in a spot from time to time where you have opportunity to fill your empty water tank and no place to dump your full waste tank.

Typically you would not have them both full but it does happen.
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Old 02-03-2017, 02:21 AM   #17
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I think it would be fine with the tanks back there. They're right at floor level, so they're fairly close to the center of gravity, and compared to the rest of the bus, they're not too heavy.

Worst case, you figure that when you're going to be on the road, and you really only want each tank half full.
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Old 02-03-2017, 03:07 AM   #18
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With an empty weight of a little over 15K and advised to not go over 26K that gives you a tare of around 11K. That is a lot of weight in built ins, junk and plunder, and people.

As you do your build out you will need to watch your weight so that it not only is balanced somewhat front to rear but side to side as well.

Most conventional buses are too light in the tail and are terrible in the ice and snow. Safety is not the only reason why fuel tanks have been moved between the frame rails behind the rear axle on front engine buses. I know some Type 'D' FE buses had ballast panels mounted to the frame behind the rear axles in order to not overload the front axle all the time. I know one school district that had a really short Blue Bird All American FE they used for special needs transportation. They finally built some compartments behind the rear axle so they could load close to a ton of bagged sand into the bus. Without the ballast whenever it even rained they couldn't get the rear brakes from locking up on even the slightest down grade. And forget about getting traction going up hill when it was slick. With the addition of a ton of ballast the bus not only rode a whole lot smoother but it was one of their best buses to drive when the roads got slick.

As to the amount of weight you are anticipating adding to the back of the bus, if you had five rows of seats from just in front of the axle to the back of the bus, the calculated adult passenger weight would be 5 rows of 4 adults each weighing 150 lbs. which equals 3,000 lbs. or 5 rows of 6 children each weighing 75 lbs. which equals 2250 lbs.

I think your plan would actually make your bus ride smoother and add traction ability on slick surfaces. I also think that as long as the tanks won't slosh around it would improve all handling characteristics.

Good luck and happy trails.
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Old 02-03-2017, 08:52 PM   #19
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Is there a technical reason why you can't use vertically mounted tanks? Just thinking that it would help reduce slosh and lower centre of gravity. Make it easy to install a sight glass as well.

Boats use a common sump for grey water and just pump it to the tank.

If you use a heat transfer tank style heater for hot water. That could be placed just in front of the rear axle to balance the weight directly on the axle.
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Old 02-06-2017, 10:41 AM   #20
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Is there a technical reason why you can't use vertically mounted tanks? Just thinking that it would help reduce slosh and lower centre of gravity. Make it easy to install a sight glass as well.

Boats use a common sump for grey water and just pump it to the tank.

If you use a heat transfer tank style heater for hot water. That could be placed just in front of the rear axle to balance the weight directly on the axle.
How would vertical tanks lower the COG?
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