Custom Cab apparatus question?

[gvfc19]

Hello, I have a question about custom cab apparatus. Our area is rural and almost all of our surrounding companies have commercial cab fire trucks. However, through fema grants, two of our neighboring companies have received new custom cab trucks. I have driven both new units. One is a 2007 Spartan MetroStar engine/tanker with a Smeal body (1700gal, single axle), the other is a 2010 Spartan MetroStar engine/tanker with a Smeal body (3500gal, tandem axle). The question is, why do both units "hop" so bad when the throttle is applied? The front end just hops up and down and then your foot starts to hop on the throttle and it gets worse. The only way to stop this is to completely floor it all the time, which is slightly hazardous on icy/wet roads. Just wondering if all custom cabs do this, or is it a case of improper weight on the front axle? I drive commercial trucks of all different types everyday and have never encountered this. Thanks for the input.

[FWDbuff]

I have never experienced this, and I have driven a boatload of custom-cab chassis in my volunteer and career existence. Almost sounds like an imbalance that is being magnified when lots of torque is applied too quickly. Has anyone contacted the Mfr's of the rigs for input?

[BoxAlarm187]

I'm with FWD on this. Having delivered lots of custom cab apparatus for various manufacturer's, I've never experienced this, or ridden in a rig that experienced it. Are the guys who are routinely driving these rigs experiencing the same thing, and if so, have they done anything to try to remedy it?

[gvfc19]

Thanks for the input. I pretty much figured that it was an balance issue, but am not that familiar with custom cab trucks. Yes, all the drivers are having the same problem. I am not sure if either department has contacted the mfg about it, but with that being said, I am going to try to nudge the chief's (both are my brother-in-laws), into calling them. Thanks

[RFDACM02]

Are you sure that it isn't your foot that "hops" first? The new electronic throttles seem to be far more sensitive and often when a driver lightly depresses the pedal his/her foot might pulse a little, the electronic throttle does exactly what it's told and the truck "hops". We had a few incidents of this with our new Spartan chassis while driver's got used to it.

[retofdac]

Noticed this quite a bit with ALF custom chassis. It was easily overcome by making sure your heel rested on the floor of the cab so your foot was not totally on the pedal. What seems to occur is the chassis bounced once, but because of the pedal sensitivity your foot kept bouncing on the pedal and the chassis kept bouncing. Keeping the heel firmly on the cab floor while pressing the pedal pretty much eliminates the problem.

[3260e]

I agree with "reto". I've found that if you seat is to high and your heel is hanging (just your toes on the pedal), you can get alot of "bounce". Try lowering your seat and getting your heel on the floor.

[Sleeper6]

We have a ladder that does this. It does seem to be a trait of the drive by wire systems these days. The timing of the throttle response and the chassis bounce sometimes come into alignment and you get a situation where you are unintentionally applying and releasing the throttle at exactly the right time to feed the bounce in the chassis. It will progressively get worse until you floor it of let off completely to stop the throttle movement.

We have found aside from keeping your heel on the floor like others have mentioned that applying some pressure between your boot and the doghouse causes enough friction to help eliminate the bounce.

[txgp17]

As others have said, it's the driver's foot causing this.

My F-250 does it, but that's because I had the ECU flashed with a more aggresive fuel injection program. Having a seat that's more "above" the pedal than "behind" it increases the likelyhood of this occuring. Air ride seats make it worse. If it's an air ride seat, inflate it fully. If that's too high, deflat it fully. If that's too low, raise and lower back down on a block of wood that's the right height.