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Old 07-20-2021, 01:23 AM   #1 (permalink)
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Ambulance Aero

Hello All.

Question: Do you think the larger cab on a medium duty ambulance will give better aero than a light duty ambulance?

Below is Freightliner M2 106 next to a Ram 4500 with the same size box. I know box trucks and Class C RVs have horrible aerodynamic due to the large flat profile on the front of the box. The MD ambulances have much less of the box exposed and I'm thinking that might be why I'm seeing similar mileage claims for medium duty ambulances as for light duty ones.



The back-story:

As some may know my wife and I are looking to travel full time in about 2 years. Our current campervan is too small and we are looking for something bigger. At first I was looking at high roof Transits or a box truck.

Then I got turned on to ambulances which are basically a box truck with a MUCH better built box on them. All aluminum construction with lots of lockable storage already built in. The only down side is most are van based and after 2 decades of helping my father wrench on Econoline vans I have no desire to own a Type III ambulance built on a cutaway van chassis.

There are Type I ambulances built on truck chassis. Those have some benefits like easy suspension upgrades and more room to work under the hood. (Although if a turbo goes out the cab has to come off!) The big downside is that Type I trucks built on a F250 - F450 chassis rarely have a walk-through to the cab. That is a must have for us

Then I starting thinking about Medium Duty Ambulances. At first I though they would be WAY to big but the box is the same size. The major difference is the size of the cab....

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Old 07-20-2021, 02:50 AM   #2 (permalink)
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Holding a straight edge up to the screen suggest that the hood height is the same but the box on the left is taller. Does it have a higher floor level? I'd guess the medium duty has the advantage aero-wise.

Not having a walk-through sounds like a solvable problem.
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Old 07-20-2021, 09:18 AM   #3 (permalink)
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The finnish way to build them:
https://www.profilevehicles.com/en/s...rez-ambulance/
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Old 07-20-2021, 09:48 AM   #4 (permalink)
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Quote:
Originally Posted by freebeard View Post
Holding a straight edge up to the screen suggest that the hood height is the same but the box on the left is taller. Does it have a higher floor level? I'd guess the medium duty has the advantage aero-wise.
Yes, a MD has a higher floor height and cab height. The higher cab and lower front of the hood makes for better forward sight-lines and smaller blindsports in front of the vehicle. (The tall long hood on the GMC I recently rented made for HORRIBLE sight-lines)

Top of the frame rails to the top of the cab:

47 inches - F450
58 inches - E450
68 inches - M2 106

I suspect this is why most pickup based ambulances do not have a walk-though. It would be more like a crawl through.

Quote:
Originally Posted by freebeard View Post
Not having a walk-through sounds like a solvable problem.
Yes, given enough effort just about anything can be done. However, ambulances have a LOT of wiring. Type I ambulances without a pass through put their wiring at the front of the box under the window. So to make a pass through I would need to relocate all of this




Quote:
Originally Posted by Vekke View Post
We call that a Type II ambulance. Just a high top van with medical gear added. If I was going to go with a van there would be no reason to buy an ambulance - I would just buy a high-top Transit van.

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Old 07-20-2021, 09:52 AM   #5 (permalink)
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The walkthrough is a local option. Class C camper cabs have it. Why not just use a box truck? Them 16 ft Isuzu things get flogged to death and keep on trucking.
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Old 07-20-2021, 01:25 PM   #6 (permalink)
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Quote:
Originally Posted by Piotrsko View Post
The walkthrough is a local option. Class C camper cabs have it.
Yes, Class C RVs build on truck chassis have a walkthrough. They also have a living space overhead that ties into the cab that make it easier to make a large vertical opening.

Quote:
Originally Posted by Piotrsko View Post
Why not just use a box truck?
Basically because an ambulance is a box truck with a better box.
I started off looking at box trucks. Specifically, something like a Penske 12 foot built on a van chassis. I was drawn to the rectangular shape that make upfitting the interior WAY easier than trying to match the interior profile of a van. (Lesson learned from building our first campervan)



A typical box truck has a flimsy wooden box on a flatbed covered with either fiberglass or aluminum. They are prone to leaks and rotting of the wooden core. I also doubt most would survive long if subjected to a steady diet of rough forest service roads. Most box trucks are also made to back up to shipping dock 48 inches off the ground so the box is too high. (Bad for Aero, bad for getting in and out of the box, bad for overhead clearance)

Ambulance boxes must be crash tested and certified. They are made of welded aluminum tubing and some have the seams of the exterior aluminum welded. Most are insulated from the factor. They also come with a bunch of external storage compartments already made that are lockable and watertight.

Ambulances have:
  • Rear Heat and A/C
  • Shore power
  • 110V power with a quality invertor designed to work with sensitive medical equipment
  • Dual alternators
  • House batteries (Usually 2-4)
  • Air Ride (Which can also be used to level the box when parked)
  • Onboard air compressor to air the tires up and down
  • Certified seats with seatbelts to allow people to legally ride in the back.
  • Lots of the wiring already done (and individual wires printed with the circuit info)

Then there is rust or lack of rust. A M2 ambulance has an aluminum cab with an aluminum box.


Quote:
Originally Posted by Piotrsko View Post
Them 16 ft Isuzu things get flogged to death and keep on trucking.
They do. They also have cramped cabs with the passengers sitting right on top of the front axle. I’ve spent some time on expeditors forums and the general consensus is that cab-overs great for maneuvering around town but miserable to drive long distances on the highway. The requirement for the cab to swing up for service also makes doing a walk-through to the box harder. I haven’t seen any with a low entry box.



Back to aerodynamics.

I would likely do an air deflector on the top of the cab (factory option so easier to find / install than some custom fabrication)

Medium Duty side profile



I would likely do an air deflector on the top of the cab (factory option so easier to find / install than some custom fabrication)


Pickup side profile

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Old 07-20-2021, 02:31 PM   #7 (permalink)
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What's the difference in rated fuel economy?
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Old 07-20-2021, 03:09 PM   #8 (permalink)
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If the medium duty has a passthrough cab, then there's no comparison. They're both a wreck, aerodynamically, but a nicely curved deflector looks easier on the medium duty cab. Also less side overhang

Passthrough cab over all.
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Old 07-20-2021, 03:27 PM   #9 (permalink)
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I mean, it's the squared off rear-end that's the biggest problem; and probably not easily solvable while maintaining the ease of access.
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Old 07-20-2021, 05:03 PM   #10 (permalink)
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Quote:
Originally Posted by redpoint5 View Post
What's the difference in rated fuel economy?
There is no rated fuel economy for vehicles over 8500 lb GVWR so all I have to go on is various forum posts from different websites. I’ve seen claims anywhere from 6 mpg to 18 mpg for ambulances in general. As I said in the OP - I’m seeing very similar numbers claimed for medium duty vs lighter duty vehicle.

I’d be happy with 15 mpg at 60 to 65 mph. That is more for personal satisfaction than economics. At 20K miles a year and $3.50 a gallon the difference between 15 mpg and 10 mpg is $2333 per year or $195 a month.


Quote:
Originally Posted by samwichse View Post
If the medium duty has a passthrough cab, then there's no comparison. They're both a wreck, aerodynamically, but a nicely curved deflector looks easier on the medium duty cab. Also less side overhang.
They are a wreck but much better than this:

Some friends of ours have an RV like this with the Ford V10 and get 6 mpg at 60 mph

Quote:
Originally Posted by redpoint5 View Post
I mean, it's the squared off rear-end that's the biggest problem; and probably not easily solvable while maintaining the ease of access.
The shipping industry has that covered. One of the benefits of going with a square vehicle instead of something curved like a van.


The back doors won’t be used to get in and out of the vehicle. Ambulances have a side access door on the right side. I plan to have a bed across the back of the vehicle with a “garage” storage below.


I jumped in a M2 106 at lunch and the sightlines are night and day different compared to a full-size truck. Nice roomy cab too. That is another check in favor of a Medium Duty.

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