Bouncing Ladder

[LtRichard]

We have a 2002 American Lafrance rear mount ladder. It is a 110 ft 750 lb tip load while flowing. The issue we have is in driving the the truck on the road between the speeds of 25 and 40 mph.For no apparent reason the truck will start bouncing to the point where you have to stop the truck.Is there anybody out there with this type of vehicle that is or has eperienced this problem.

[npfd801]

Is it on one particular stretch of road? I seem to recall a nearby department having an issue with their 105 foot quints on a newer stretch of roadway, where the expansion joints coupled with a specific speed caused just the right combination to damn near shake the truck apart.

Isn't this called harmonics when everything comes together to find the right cycle of vibration/bouncing that a pattern basically develops? Surely some physics geek can chime in.

I don't think its a specific issue that would be particular to an ALF, but I also don't know how to remedy the situation, especially if it happens at different times on different roads.

[LtRichard]

it does not matter on the stretch of road your are on.

[JR2451]

This phenomenon occurs quite often on newer apparatus. If I recall correctly, it has something to do with the throttle. The throttles on these computer controlled engines are "fly by wire." Under the throttle pedal is progressive switch -- the more you push, the greater the voltage on the circuit... which means more fuel to the engine.

Anyways, these throttle pedals are really sensitive. Often, the slight bounce of the suspension (even if you can't feel it), will start a harmonic feedback on the throttle pedal. It will get progressively worst unless you lift off the throttle.

[BFDENG51CAPT]

We have a 2001 HME/Smeal engine that does the same thing. Haven't really found a solution other than to make sure you keep the bottom of your foot on the floor of the cab only using the top of your foot to push the pedal. Letting off the gas usually seems to help as well

[fireman4949]

Several of our E-ones do that too. Keeping your foot gently braced against the dog house while on the throttle helps eliminate it.




Kevin

[BVFD1983]

the more you push, the greater the voltage on the circuit... which means more fuel to the engine.

Actually it means more air, which in turn the computer must match the increased air with more fuel.

[RFDACM02]

I've seen this problem on pick up trucks with agressive tread tires. They call it a "wheel hop" somehow caused by one tire's tread not running inline. Maybe this can happen on larger vehicles too? I know it shakes the living cr*p out of you and makes you stop quick! Everything is better once you stop and start again.

[Chauffeur6]

We have the same thing with our 1990 75' rearmount. The accelerator pedal is so loose that as soon as you bounce up and down on it once or twice (the air seat doesn't help!), rhythm takes over and not only does the truck start to bounce, but you get the engine surging as well. It's pretty entertaining actually, and the only way to really stop it is to either let off the pedal completely, or just mash down on it (depending on the road/traffic conditions, obviously). Oddly enough, it's the only truck that it happens with. Not even its fraternal twin pumper does it.

Of course it could also be a legit problem with the wheels or tires being unbalanced or out of round slightly, especially if the throttle pedal is pretty tight and you know it's not your foot causing it. Best to get it checked out.

[5FVFC3]

We had a similar problem with a 72 engine and it turned out to be flat spots on the tires from all the weight and from not being moved much due to a rural area. We replaced the old bias ply tires with radials and and worked for many years however, it is starting to due the hop between 45 and 55 mph again.

[SBrooks]

If it's not the accellerator pedal harmonic, which is possible in most of the fire trucks I've ever driven, take it to a scale and have the curb weight measured at each axle. You might have too little weight on the front axle, especially if you have a relatively short wheelbase. Quite a few trucks need a stack of steel plates mounted in one of their bumpers.

[jfTL41]

What is bouncing, front or rear? Is it an air ride suspension? Either way take the rig off line, as you have admitted in a national forum that this problem exists and is potentially unsafe. Take it off the road and to until it is diagnosed and corrected let it sit.

[Fyrtrks]

It sounds like it is throttle harmonics. I delivered twin Saulsbury's in 1990 that had this problem. The front of the apparatus would jump off the ground like Carlos Mencia's low rider. The chassis manufacturer replaced the foot throttle with a remote cable opereated one. The problem went away. I would suggest you look at the foot throttle and see if it has wires going in or not. You may want to invest in the remote throttle. I can't say for sure this will fix the problem but it sounds as if it will.

[canuck1]

I've had it happen many times, in many different trucks. The second you notice it happening, lift your foot off the pedal, and it goes away...

Electronic engine / you bouncing in the seat / your foot bouncing on the pedal.

Try anchoring your heel on the ground, and it will lessen the frequency of occurence .

[metrobob]

I have experienced this on several newer and older chassis. Different brands and chassis styles as well. Short of mfg. involvement, try the following;

a) While driving in or around town, select 3rd or 4th gear and leave it there.

b) We used an old tennis ball wedged under the acelerator to help dampen the pedal action.

c) The foot stradling the floor and pedal also works well.

[DD8968]

You may also want to check your tire pressures

[firepiper1]

[QUOTE=LtRichard]We have a 2002 American Lafrance rear mount ladder. /QUOTE]


We have the same problem with a '92 LTI 75' and a neighboring dept had the same problem with their LTI's, to the point where they almost sent the trucks back. I have driven many different brands over the years and never had the problem with any Spartan chassis rigs.

[IronsMan53]

[QUOTE=firepiper1]

We have a 2002 American Lafrance rear mount ladder. /QUOTE]


We have the same problem with a '92 LTI 75' and a neighboring dept had the same problem with their LTI's, to the point where they almost sent the trucks back. I have driven many different brands over the years and never had the problem with any Spartan chassis rigs.

It happens to Spartans too. We have 3 towers and 2 quints on Spartan chassis with Cummins ISM engines and Allison Automatic Transmissions. The towers have done it with me a few times. I am with the rest saying its throttle harmonics. It only happened to me when the rig is travelling in the 30MPH range with lower engine RPM's. I feel that range is a sensitive to any throttle inputs and once any fluctuation starts, it gets violent quickly. I have learned to either drive slightly faster or slower than that range, or manually shift the transmission into a gear that gives me higher RPM while driving at that speed.

[Chief1FF]

We have two trucks with this problem.

1. 1991 Spartan w/Detroit 8V92
2. 2003 ALF w/Detroit 60 Series.

In my almost twenty years in the fire service and my employment I have driven many different manufacturer's trucks and the one thing they all had in common was the Detroit engine. It was explained to us as a problem with the throttle pedeal. (I think that's what I remember) I have been told if you buy a Detroit engine you will inherit this problem. Even with this problem (which can be overcome by a GOOD driver.) I will always spec a Detroit over a Cummings and Cat.

[donethat]

We had an aerial ladder with one of the early electric throttles doing this. The manufactuer said it was the combination of the air ride seat and the electric throttle and the ride frequency of the aerial ladder. At certain speeds the chauffer would get bouncing in the seat and his foot would be bouncing on the throttle and the harmonic of the aerial would get worse.
The manufactuer put on a air operated remote throttle and problem went away. The air throttle dampened out the harmonics.
Now every thing is cool.