What other factors determine the number of rear axles?

[hinesfire]

Besides the obvious weight factor, are there other factors to consider whe deciding if a tender/tanker needs multiple sets of axles? We are thinking about a 1800-2200 gallon truck, can folks help suggest other issues that dictate the need or advantage of multiple axles?
Thanks

[fireinfo10]

For given tank size a tandem rear will typically give a shorter WB and thus a shorter turning radius (WB for a tandem is from the CL of the front axle to midpoint of the two rears). But you end up with less usable space for compartments between the two. Fire Service typically choses to overweight rear axles and more compartment space.

You note that for similar weight classification, the fire service 2000gal tanker is a single rear while a truck the military would buy has tandem rear. DOD spends the extra $ for a reason.

[Fyrtrks]

Personal opinion here:

1 Up to 2000 gallons single rear axle is fine. 2200 I would say is pushing my comfort level.

2 If you are going to 2200 why not go to 2500 and make your tandems worth the money you are spending.

3 The advantage of a tandem is one word "braking".

Yes you will loose compartment space, however it is a tanker used to haul water not equipment. Yes I know tankers have to haul equipment but if it is that much of a concern then you need a bigger unit.

[islandfire03]

What all above have said.

The addition of the tandem rear gives you more suspension to handle the weight and to give better stability to that load. Add in the power divider with the tandem and you get better traction control on the drive and the additional set of brakes to help stop the load.

I reviewed a spec for a tanker recently that they were pinching pounds to stay under GVW on an IH 7400. They were down to putting aluminum wheels on the truck and choosing equipment items by weight in order to have enough tare left to allow a fat driver.

This dept. also has length and height issues to fit in the fire station.

If your getting that close to GVW on paper , It's time to move up and get a tandem.

[Command6]

Personal opinion here:

1 Up to 2000 gallons single rear axle is fine. 2200 I would say is pushing my comfort level.

2 If you are going to 2200 why not go to 2500 and make your tandems worth the money you are spending.

3 The advantage of a tandem is one word "braking".

Yes you will loose compartment space, however it is a tanker used to haul water not equipment. Yes I know tankers have to haul equipment but if it is that much of a concern then you need a bigger unit.

Yea, what he said. Except my comfort level is more around 2k. As far as braking, I also agree with Fyrtrks; you have to figure on stopping that 15k-20k of H2O, along with everything else.

C6

[Command6]

What all above have said.

The addition of the tandem rear gives you more suspension to handle the weight and to give better stability to that load.

If your getting that close to GVW on paper , It's time to move up and get a tandem.

Another thing for consideration; you face a 10 percent penalty on your ISO grading points for overweight apparatus.

C6

[Rescue101]

For given tank size a tandem rear will typically give a shorter WB and thus a shorter turning radius (WB for a tandem is from the CL of the front axle to midpoint of the two rears). But you end up with less usable space for compartments between the two. Fire Service typically choses to overweight rear axles and more compartment space.

You note that for similar weight classification, the fire service 2000gal tanker is a single rear while a truck the military would buy has tandem rear. DOD spends the extra $ for a reason.

Yup,PRIMARILY Off road performance. T.C.

[Frmboybuck]

we have 2, 2000 gallon, single axle Midwest tankers and are very happy with them. With that said, I wouldn't go much over that on a single axle

[Pa6050]

Single axle will get around much better on the hard road they turn much better than a tandem. Our old tanker was a single then we bought a tandem and we all miss the single axle. We have alot of winding mountain roads and long diveways the single axles just handle those roads sooo much better. If you have alot of flat strait roads then not as big of an issue i guess.

[fireinfo10]

Single axle will get around much better on the hard road they turn much better than a tandem. Our old tanker was a single then we bought a tandem and we all miss the single axle. We have alot of winding mountain roads and long diveways the single axles just handle those roads sooo much better. If you have alot of flat strait roads then not as big of an issue i guess.

That's just intimidated drivers (which is certainly a valid consideration). "3 axles it's a "big truck" 2 axles and it's just a big pickup".

If you're hauling a big load on mountain roads you need more tires and brakes not less. See military trucks (true eEven if your not going cross country).

Turning radius is about wheelbase and cramp angle.

Not mentioned is that on paved roads a tandem may shorten rear tire lift due t scrubbing of the tires. Less distance between front axle and rear axle reduces risk of high center/dragging.


Both our tankers are 6x4 (2500gal tanker and 3000gal tanker/pumper).

[HuntPA]

We have a 1500gpm / 2000 gallon on an International WorkStar. Neighbor has a 1250 gpm / 1800 gallon on a Freightliner. The turning radius is not even close. We can turn inside them every time. Like it was said before, turning radius is all about wheelbase and cramp angle. Ours is 8" shorter wheelbase and 2 degree tighter cramp angle.

We are also able to run on soft roads and driveways without any problems where they tend to get stuck. Even going down the road, our truck is a lot more stable than the single. When we had our truck built, the manufacturer told us weight would be between 42,000 and 44,000 when fully loaded. We opted for a 60,000 GVW vehicle with a 20,000 pound front axle. We don't want to be that department that has a vehicle out of service all of the time for suspension and drivetrain problems.

[Rescue101]

That's just intimidated drivers (which is certainly a valid consideration). "3 axles it's a "big truck" 2 axles and it's just a big pickup".

If you're hauling a big load on mountain roads you need more tires and brakes not less. See military trucks (true eEven if your not going cross country).

Turning radius is about wheelbase and cramp angle.

Not mentioned is that on paved roads a tandem may shorten rear tire lift due t scrubbing of the tires. Less distance between front axle and rear axle reduces risk of high center/dragging.


Both our tankers are 6x4 (2500gal tanker and 3000gal tanker/pumper).

EXCEPT: Short tandem axle trucks have a tendency to "plow"(go straight)on sand/loose gravel/Ice/Snow when trying to take tight turns. Longer WB tandems do too,but not as bad. An extra axle on brakes means your brakes run cooler and last longer. Generally more stable too. T.C.

[FFWALT]

By now you should fully understand that braking potential is way more important than power to get the load moving. One of my pet peeve's is that everyone is worried about being able to pull the load, they don't consider being able to stop it. This of course is a key item on a tanker.

This has been alluded to but I want to make sure those that read this post understand. I'll use HuntPA's numbers to explain and it will be over simplified but bear with me.

You have a truck that will weight 42,000 pounds. If you use tandem rear axles the weight is spread over 10 points of contact (8 tires in the rear, 2 in the front). Each tire transfers 4,200 pounds to the ground. If you have a single rear you have 6 points of contact which equates to each tire transferring 7,000 pounds to the ground. Which truck do you think will do better on soft ground? This will also affect braking because each tire is responsible to stop a smaller load with tandems vs. single rear if all things are equal. As I said, this is over simplified because the formula I used requires absolute balance of weight between front and rear axles but it should explain the concept.

Something else to consider is that if the suspension is not under max load 24/7 it will give you a longer service life than one that is loaded to the max 24/7. Not a big deal sitting in the station but it makes a big difference over the service life of the vehicle as you respond, operate and return from calls.

Long story short, if there is ever a question go with the extra axle.

Good luck,
Walt

[hinesfire]

By now you should fully understand that braking potential is way more important than power to get the load moving. ......................
Long story short, if there is ever a question go with the extra axle.
Good luck,
Walt

nicely described. Thanks all.
Does an extra axle have to add length to the apparatus?

[firepundit]

nicely described. Thanks all.
Does an extra axle have to add length to the apparatus?

No. However, you will loose some compartment space. On the plus side, if you maintain the same overall length and use a tandem, you will decrease turning radius while enhancing braking. In other words, a much safer vehicle.

At this point it really needs to be said that the majority of accidents with these larger vehicles is turnover from leaving the roadway and trying to return. Even the best engineered tanker will flip over if the driver doesn't know how to safely return to the pavement.

[DeputyChiefGonzo]

No. However, you will loose some compartment space. On the plus side, if you maintain the same overall length and use a tandem, you will decrease turning radius while enhancing braking. In other words, a much safer vehicle.

At this point it really needs to be said that the majority of accidents with these larger vehicles is turnover from leaving the roadway and trying to return. Even the best engineered tanker will flip over if the driver doesn't know how to safely return to the pavement.

Requiring those who drive tankers (or tenders, for those who feel the need to be fire service politically correct) to have a CDL and proper driver training would also help in preventing these kinds of accidents.

My own opinion... anyone driving fire apparatus should be required to have a CDL. Just my 10 cents worth.. Deputy Chiefs have to pay a little more!

[fireinfo10]

I think equally important is to discuss is the rear suspension.

In my opinion spend a bit more for a Hendrickson (or Chalmbers). Seems to me to have better combination of loaded/empty stability/no sagging when loaded 99% of the time. I think 2nd choice would be air. Spring way behind those.

If buying a new chassis get the roll stability package. They work.

[Rescue101]

I think equally important is to discuss is the rear suspension.

In my opinion spend a bit more for a Hendrickson (or Chalmbers). Seems to me to have better combination of loaded/empty stability/no sagging when loaded 99% of the time. I think 2nd choice would be air. Spring way behind those.

If buying a new chassis get the roll stability package. They work.

You may want to be more specfic. Chalmers ride like schit unloaded. Hendrickson SPRING suspension is the most RELIABLE over the long haul and has the best off asphalt performance. T.C.

[firepundit]

You may want to be more specfic. Chalmers ride like schit unloaded. Hendrickson SPRING suspension is the most RELIABLE over the long haul and has the best off asphalt performance. T.C.

The military was always fond of Hendrickson suspensions. That is probably why HME specs always promote the "military wrapper" where two springs wrap around the hinge pin instead of just one. It definitely is a strong, reliable setup.

As much as technology has enhanced our lives, I personally try to avoid over-complication with absolute basics. Axles resting on rubber bags have their place it just seems that a vehicle that stays primarily loaded 24/7 would be better off on springs. But, I can remember when positive crankcase ventilation was a novelty so I might be over-reacting.

As far as Chalmers goes, they certainly do like to stay loaded. Driving a cab only with a Chalmers rear is kind of like riding a kangaroo. Haven't driven one with an electronic throttle but I am not sure I could keep my heel on the floorboard to prevent wheelies.