Fleet Tacoma Bucking/Shuddering/Bouncing

[PackRat]

Hmm....I wonder if the porpoise problem for SOME guys' rigs is due to the suspension having either too much weight on one axle and not enough on the other...or replaced shocks that don't respond the same way on the front as the rear? Once you get that oscillation started with the front axle (not overloaded) hitting an expansion joint and bouncing UP and down and the rear axle not responding in the same way due to different shock or and overloaded suspension your truck starts to act like it is going over waves.

So maybe...being under the GAWR for the front axle not only lets the lighter load on the front suspension rebound HIGHER, being over the GAWR for the rear does NOT let the rear suspension rebound as high and sets up that roller-coaster effect. Since some guys indicate simply slowing down changes all that, this could be the issue here.

Solution: Buy "enough" truck you don't need a couple thousand bucks worth of add-on suspension to sleep at night, just keep the GAWR on the rear axle CLOSE or UNDER its rating and then if you want to mess with the ride characteristics you can choose shocks that stiffen the ride with the camper loaded and live with the ride when it isn't.

Maybe I have it wrong, but hanging a couple feet of camper behind where the tailgate closes on an 8 ft PU that is a 250/2500 or even a 350 probably won't matter much if the load itself is a) not ridiculous, b) not heavier at the rear, or c) way over the rear GAWR.

I think anyone who experiences these problems should try dismounting the camper, filling the truck with fuel and weighing both axles individually and then with both axles on the scale. Then repeat the scenario with the camper filled up with everything you take and all water, food, personal items, passengers accounted for....

The results may point to your truck being overloaded on the rear axle to the point that ANY....ANY issue with the road surface will start the porpoise effect. I know crappy road surfaces make for a lousy ride for all loaded up camper/truck combos, but if your rig is not set up right and weights distributed and GAWRs as recommended, you are asking for problems.

I know we all tend to overload the rear axle GAWR by 5% to 7% but if you are 10% plus you are ripe for problems....if you have to readjust the headlights if the camper is dismounted or mounted, your truck is telling you something is outta whack. The question is; do you want to throw money at the symptom or realize what the problem is and deal with it?

 

[rando]

I don't think this issue is specific to being over the rear GAWR. There is one section of concrete road near me that several of my vehicles have had issues on (not terrible, but definitely a resonance), typically longer wheel base vehicles. As others have pointed out it is really just an issue of resonance - when the frequency you are perturbing your suspension at (which = speed * distance between joints) aligns with a resonant frequency of your suspension system, you end up with this resonant bouncing. It gets really bad when the resonant wavelength is twice your wheelbase, as then the front and back are 180 degrees out of phase and the front goes down while the back goes up leading to major porpoising. I drove my flatbed with no camper on this same section of road and had much worse resonant bouncing than I have now with camper on. It would also happen at higher speeds (therefore higher frequency) without the camper on than now with the camper on, which tells me that adding the camper weight moved my resonant frequency lower.

 

[ntsqd]

Which comes back around to the damping not being able to handle that range of input. Shocks don't have to be stiffer to deal with any specific input frequency, they just need to be tuned to work there too.

 

[rando]

Damping will certainly help reduce the amplitude of the oscillation, but it won't eliminate the problem as it doesn't change the natural frequency of the system.

A very similar situation can be found with washboard dirt roads. In this case the wavelength of the 'bumps' are much shorter so the speeds we are talking about are much slower. Most people find that there is a certain speed on washboard roads (which is usually pretty close to the sensible speed for the road) where the vibration is terrible - but if you can go faster or slower than the typical speed, you get much less bounce because you have moved the driving frequency away from the natural or resonant frequency of your suspension.

The problem with concrete highways is that you don't really get to choose the speed you drive, so you can't get away from the resonance. You can try to move the resonant frequency of your suspension which is ~ (k/m)^.5, but that will likely lead to other handling issues, so good shocks to damp the resonance are probably the best option.

 

[Squatch]

Rando you are right on the money!

 

[ntsqd]

Maybe I don't understand "Natural Frequency" correctly, but to me if you change the damping function of a system you've altered its natural frequency.

 

[rando]

I think you are right and that resonant frequency is a better model for this system than natural frequency. The resonant frequency is the frequency that you get the largest response from when you drive a system, whereas the natural frequency is the frequency that a system tends to in the absence of a driving frequency. Natural frequency doesn't really change with damping, but damping does have a small effect on resonant frequency:
https://en.wikipedia.org/wiki/Resonance

The primary effect of the shock absorbers is to decrease the amplitude of the oscillation, which will help regardless of the resonant frequency.