1. #1
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    Default Tandem vs. single rear axles & Turning Radius

    1. Does an apparatus with a tandem rear axle turn differently (smaller/larger turning radius) than a single rear axle truck with the same wheelbase* (assuming all other factors are the same)?

    2. What is the cramp angle of your typical commercial chassis of the 90's or 00's? How does this compare to the 45 degree cramp angles being offered by most custom chassis manufacturers as far as turning radius goes (eg, how much larger will the turning radius of a commercial chassis be compared to a custom, assuming all other factors to be equal?

    3: What are all the factors that affect turning radius of an apparatus, and how importand are each of them (eg, wheelbase, cramp angle, axle weight rating, tire size, etc)


    *Knowing that the wheelbase of a tandem is measured to the center of the two rear axles, and the wheelbase of a single rear is measured to the center of the rear axle.
    Last edited by FFEMT545; 03-02-2004 at 09:00 PM.

  2. #2
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    Default wheelbases and turning radius

    1. Turning Radius Single Vs. Tandem- Call a manufacturer and they can tell you the exact numbers with a specific design. BUT, they will ask many questions! You will need to have a fairly narrowed down set of overall configuration parameters prepared for them. As it can be dramatically effected by overhang dimensions and auxiliary components. There will also be turning radius dimension of "wall to wall" and "curb to curb". The more dramatic impact is what is known as “wheel scrub” with the long wheel travel of the tandems and worsens with shorter wheelbase tandem designs. This is one to work on with your list of manufacturers being considered. All have the capabilities to help, if they are a legitimate company.

    2. Cramp Angle- Each manufacturer has their "claims to fame" and many play "me too", so be careful. Most custom chassis's I am familiar with, boast 45-degree cramp angles, but watch out! Some times they are not the same on the left and right, due to steering gear mounts, etc, but the manufacturer uses the higher number in the sales pitch. Many times manufacturers use axle stops to prevent it from turning to the full 45 degrees to prevent collision of components that are frame mounted on the chassis. Each manufacturer will vary from my experience. As far as commercials are concerned, they offer higher cramp angles, with some to 50 degrees on the models I am familiar with (with standard equipment), improving turning radius. While cramp angle improves turning radius, the commercials inherently have longer wheelbases due to their cab design, verses the custom engine forward tilt cabs. Some pumper configurations with 4 door commercial cabs, result in outlandishly long wheelbases! A good example is a 4 door Commercial, top mount, 1000 gallon pumper, averages a wheelbase around 250" from what I see in most manufacturers literature. Little do these poor unsuspecting people realize, that this is a wheelbase commonly found on many of the nations 100' ladder platforms and then they want to drive them around rural roads without restriction! Most customs offer the mentioned 45-degree cramp angle, but can be impacted by your third question. DO NOT let me forget that there are some "select" Texans who love their long wheelbases! OMG! I saw a refinery unit that had a 300" wheelbase.....but then again, "everything" is big in Texas I am told! God bless you boys and to each his own! No pun intended guys! LOL!

    3: Effects of Turning Radius- Big axles, "typically", more impact. DO NOT take that comment wrong, as the heavy axles are reliable, they just change the name of the game in many instances. Better to have the right axle than one that is marginal, you will pay later! I have personally learned this the hard way! GVWR is limited by the weakest link component in the chain of axle related components. The combination of axle, spring, and tires measure this rated GVWR and then other components are impacted by this, which affect brake sizes/types, steering gears, steering linkages, etc. Other “things” such as front suctions/inlets, large diameter discharges to the front bumper and ancillary equipment can impact the finished cramp angle. Front axles including and over 18,000# can be impacted with the required larger 9-13” wheels to maintain GVW, verses the smaller 8.25” wheels on the axles up to 14,600#. Again, all of this is driven by your overall design parameters, the mission of the vehicle you’re designing and the particular manufacturer you’re working with. You really need to narrow down your design and then get some educated sales people involved, which may require factory assistance, from my experience.

    Sorry I cannot shed more "specific" light on your question, as I do not know your overall design parameters.

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    Default I failed to mention.....

    Sorry, but I truly should have mentioned something else... DO NOT be afraid of tandem axles! There are efficient and safer than a marginal or max'd out single axle rear end. The benefit of double the braking and greatly improved stability are a wonderful thing. Wheels can be adjusted for your configuration to accomodate turning radius's that are equal to that of a single axle, but again, the configuration of the vehicle needs to be narrowed down. Once that is done, the manufacturer can do a detailed weight analysis of your particular design once they know what you are looking for. It can be general to start with such as: 2000 water, 1250 pump, double high side compts, 1000' of 5", 4- 1 3/4 preconnect 200' each etc. Then a target can be determined. Don;t be afraid to ask them to do this, i do over and over again, as it is critical on all apparatus today and be sure to thing about future growth and expansion when do ing so. Calculate more equipment than NFPA tells you, as it (in my opinion) is grossly understated by NFPA. Just do not be afraid of tandems. The only thing that is bad, is that sometimes the 100" of wheel travel back there intimidates people that are not accustomed to driving tandems. They will get over it after driving a few times and gain the benefit of handling and great braking characteristics. Good luck! Fish

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    Default

    Ok, I need a solid yes or no answer: does a tandem rear axle truck turn in the same radius as a single rear with the same wheelbase, with all other factors being the same?

    The rest can stay up for debate.

  5. #5
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    Default I am a man of few words!

    FFEMT,

    I find it VERY hard to write one sentence....LOL....YES a tandem can turn the same curb to curb dimension of the single axle!...........BUT BUT BUT there is much more to the story.........but frankly and forced to provide a "solid" answer as you have asked, YES, it can be done...Narrow up your design parameters and get some experts involved...I am not being sarcastic, but it is similar to asking if the Titantic can float? Sure it can float, BUT are there any icebergs out there? If I can help, once you narrow the overall desires, I would love to try help and give more info to you including steering radius charts which I have access too through several manufacturers. Feel free to mail me at firefish1488@yahoo.com and lets see what we can do.

    Good luck and good fishing.

    Fish

  6. #6
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    Default

    If you have a 200" wheelbase single axle and a 200" wheelbase tandem axle, then turning radius is essentially the same.

    However, some manufacturers use an FL front axle and depentent on your application, you may be able to use a 20,000 lb front vs. a 21,500 lb front axle. The 20,000 lb has a better cramp angle than a 21,500 in that application.

    Some use the MFS axle and it does not make a difference.

    So, if front axle is same and you just change the rear, you will have the same turning radius. But like stated earlier, stability and twice as much rubber on the ground for braking is a good thing. You need to decide if you are willing to safrice compartment space for better performance. Or are you just placing so much on the truck that a single rear is not feasible.

    Don't get wrapped up a manufacturer trying to sell you a 33,000 lb single rear in place of a 31,000 either. The 33,000 rear forces you into a 24" rim, tube type tire and jacks the rear up 3" as the tire is 6" taller than the standard 22.5 tires.

    Also, as stated, get the left and right turning radius figures. There is usually a difference and a good manufacturer will tell you this while the bad sales guy will give you the best radius.

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