Some reference for the kit-bashers & scratch builders out there:
I would really like to see someone (if not me) build this semi-tractor!
p.s. On the semi-tractor the third axle also steers.
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These are great pics, but not really related to the topic posted in, so I gave it its own topic.
One correction though, the third axle on the truck (last) is not steerable.
More info and pics here:
Refuelling UK Armed Forces ~ Oshkosh Tankers Pt1 | Joint Forces News
Refueling British Armed Forces ~ Oshkosh Tankers Pt2 | Joint Forces News
The truck is based on the Oshkosh USMC Mk23 MTVR. It could be converted using the Trumpeter kit of the same. I did a build review of the MTVR a few years ago on the old site. It is a very nice kit.
https://archive.armorama.com/review/9242/index.htm
Not doubting you Gino but apparently some of the tractors do have third axle steering. Given the Oshkosh TAK-4 axle design; adding or removing third wheel steering at the factory is not a bid deal.
I admit the two examples below are intended for the US Military, whereas all my examples earlier are of the vehicle in British Service so maybe the Brits decided to leave it off as unnecessary or cost prohibitive.
Yes, the Brit design went w/fixed rear axles.
The Mk31 Tractor that the USMC eventually adopted actually has all-wheel steering, meaning both rear axles turn.
From the Oshkosh site:
"The Oshkosh MTVR Mk.31 is powered by the Caterpillar C-12 11.9-liter turbocharged diesel engine. Engine was uprated from 425 to 445 hp. It is mated to an Allison 7-speed automatic transmission. Vehicle has a full-time all-wheel drive. It also has an independent suspension and is fitted with a central tyre inflation system. Tyre pressure is adjusted from the drivers seat depending on terrain conditions and payload. Also it has an all-wheel steer.
You can see both rear wheels turned below (and in your pic of just the Mk31 Tractor above).
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As you may already may know Oshkosh also built an experimental 8x8 version with at least three out of the four axles steering:
It is easy to do on the model as well. I talk about articulating the front axle, and that the others could be done as well, in my build review above.
Point of order Mister Chairman . . .
Gino, You may be correct. However at the risk of being proved wrong; I do know you are quoting the Oshkosh document directly but I think the technical copywriter may have gotten a bit carried away with saying all wheel drive and all wheel steering
I just don’t see any point in the number two axle being steerable unless the truck can also do that fancy lateral, side-slip, lane change maneuver as well.
During a turn the truck chassis would simply pivot around the fixed #2 axle and that would be the vehicle’s center of rotation for the turn,
Same with the 8x8. I believe the #2 axle is fixed.
Oshkosh would have to figure out the rotational math here, because the rear wheel steering has to turn at a slightly different angle than the front axle because the rear is closer to the center point of rotation than the front axle. (FYI - that cross on this drawing DOES NOT indicate the center of turning rotation.) The center of turning rotation would be directly over the #2 axle. (IMHO)
If you look at the pic in my post, axle #2 is definitely turned out as well. If it were not, the wheel would be tucked in line behind the mud guard.
You may certainly be correct but the angle of steer there would be so small that I am not willing to say for certain that the #2 axle is actually being deflected in your photo.
Yes, you COULD easily make the #2 axle steerable but that would only serve to move the center of turning rotation about 18" forward of the #2 axle. This WOULD increase overall maneuverability only slightly while increasing mechanical complexity quite a bit. It would be more a blatant statement of “engineering arrogance” to do this, rather than a real performance improvement.
The engineers would perhaps be doing this experimentally simply to prove they could, as I don’t see it as offering any real performance advantages.
I agree with you on the why. I don’t see why you would need any of the rear wheels steerable on such a short vehicle as a bobbed tractor truck, but they seem to be.
Not disagreeing just holding off for more information in the future.
I thought I had already fought this “turning” battle when building my 8x8. On that one I am fairly positive that my research indicated only the #1 – #3 & #4 axles were steerable.
With a regular semi-tractor you get a lot of lateral (side to side) tire wear. The center of turning rotation is halfway between the two rear axles of the bogie. This reduces tire life, increases turning resistance and even uses a little more fuel.
With a steerable #3 axle the center of turning rotation is AT the #2 axle. For all this you earn SOME increased vehicle maneuverability, a reduced vehicle turning radius, reduced wear and tear on the tires and even save a little fuel.
Is the added cost and complexity worth it? I suspect not.
Apparently the Brits did not think so as they bought front axle steering ONLY.
p.s. Oshkosh did offer all wheel computerized steering on their heavy 3 axle fire trucks with it offering greater maneuverability in tight urban streets. However there where some problems (I don’t know what they were.) and it is my understanding that the rear steering on these trucks has since been locked out