Some questions, in no particular order:
With the way you've cut the chassis to get the lift, how do you plan on rebuilding the ribs?
Have you taken into account the single plane of stress caused by making a straight cut?
Is aluminum sheet a metal of convenience, or is there a specific reason for it?
How are you planning on fastening the skin together, especially if it's different metals that can have galvanic reaction with each other?
What specific things (other than being able to play basketball - ha!) inside the bus do you desire that high of a lift?
Do you plan on a lot of driving, or mostly leave it parked from one place to the next?
Ok, observations, again no particular order:
I think unless you've got a specific use for that height, my seat-of-pants engineering guess thinks that you're too high for that chassis. The school bus chassis is designed to be TALL - from the ground to the floor. This way it lifts the passengers above most collision points with the passenger compartment with normal vehicles by being tall on the frame. So, the primary engineered goal of a school bus is safety, not convenience.
Many other large/tall coaches are a lattice space frame with a low floor between the wheels. This affords the ability to go down before you go up. Even still, the tall coaches are pretty light on top - fiberglass roof, not a lot of structure except glass and the steel frame structure. A school bus with solid steel sheeting across the roof and frame spars spaced every 16" on average is pretty heavy.
Pushing that mass up higher will have a much greater effect when weight transfer occurs as you're driving. If you're planning on short infrequent drives, I don't think the height will be a big deal - it'll survive but I think it would be a white knuckle ride when there's sidewind with that large of a vehicle. The combination of the sail effect plus the top mass of a dynamic vehicle would be interesting.
I calculated an 18" raise the BB 40' AARE that I have as giving an 12-14% (~10% tolerance) increase in lateral (side to side) mass transfer over the factory height. As a reference, this effectively translated into 1" of extra "heel" as measured from the highest point of the vehicle to the outside of a 100' radius while traveling at 15 mph.
This was calculated attempting to take into account things such as spring rate capacity of the suspension and damping of shocks. I would love to instrument a vehicle and test this.
This translates into varying amounts of side forces depending on a turn radius * vehicle speed over time.
A way to think of this process:
If you have a given constant for lateral acceleration (N) for a given rate of change over a given time, this will translate to a real world force on the chassis and tires.
Adjusting the moment arm will cause (N) to increase by some amount, based on the amount of mass moment arm you've raised your vehicle.
This could cause things like unintended oscillations at speed when changing lanes, or unacceptable lean characteristics in slower speed maneuvering. The calcs I ended up with for the above BB AARE 18" lift were acceptable tolerance - no more than if all the kids were standing instead of sitting the bus while underway vs. sitting down.
If I were in your shoes I'd be trying to figure out what potential real change you have by lifting an additional three feet will do to the dynamics.
As for the metal choice for the sides, I'd really suggest keeping like-kind where you can. I know Vlad used aluminum sheeting on skirt and pop-outs, but to be honest, the areas where he was adding the sheeting was not as structural as the steel framework he first built, then attached the skin to. The sheeting that gets re-riveted between the body hoops and overlapping other sheets will transfer a lot of stresses to the other hoops on the vehicle.
I'm not so sure that aluminum sheet would be the best choice, simply because it doesn't have as much "fudge factor" as steel does. In an aircraft, things are engineered to a significant degree, and operating conditions are logged and controlled to a high degree.
Ground vehicles not so much. A grumman olson panel van with aluminum sides is very much overbuilt, but also has like-kind materials throughout the vehicle body. I'm not saying they're the same, and I'm not saying aluminum won't work, but I am saying that the longevity of the solution may not be acceptable.
This is all based on a perception that a bus will be driven a lot or without undue concern, and like I mentioned earlier - if it's just going to move from site to site and sit most of the time, I don't really see anything wrong with your approach.
You should be placing high mass items such as waste water, fresh water, and batteries in such a way that when your construction is complete, you have loaded the axles properly. If you have a 13.5k capacity front axle and a 22.5k capacity rear, I'd be wanting to ensure that I've biased my weight towards the rear a little bit, and give a corresponding percentage of capacity headroom for each. This should ensure deviations from the static loading weight (empty vs full tanks, people, cargo, etc) are never overloading one of the axles in operation.
Take my advice with a grain of salt, it's free, etc. I've only built one bus in my life so far. I have however built a number of large offroad trucks, and deal with weird thorny math issues and simulation of these sorts of things as a hobby.
Don't use aluminum - get some electroplated sheet steel. Don't go higher than 24" of roof lift.
Originally Posted by zoomy
Hadn't really taken moment arm in to considerate but it is a pusher so the wheels are set back pretty far, I would have to think the axis angle would be similer to a norman height front engine.
Do you have any thoughts on that, suspention, or weight placement. My photo bucket is giving me greif so pics arnt up to date but I'm about to skin and water tank placement will be in the next month. I'm not sure if I want it closer to the front or the rear. I have 10' of under storage to play with and at 100 gallons thats 800lbs I have to favor the front with, but that could cause more stress by hanging 800 lbs from the middle of the chassis