[superguy282]

Originally Posted by Sam800

In a FWD car, when one presses down the accelerator, the back of car pushes down a bit. When same is done in RWD car, the front lifts up a bit.

The job of dampers in suspension is to settle down the vibrations and oscillations of spring.
Now if dampers are starting to get worn out, they will start becoming softer (less effective) or in other case, harder than normal.

We know the coil spring is softest when pressed initially and gets harder progressively (more pressure is required farther down you press).

You drive a long MPV, when you drive in 4th, the torque provided by engine is higher than 5th gear, so is engine braking. The suspension will have 1-2mm of lift or compression depending on wether you accelerate or decelerate, hence the change in handling and settled ride.

Just get your suspensions checked. Any abnormal feel in car probably means there’s some fault somewhere. 75k kms on stock suspension is enough for a MPV. Suspension on my Indigo lasted 55-60k kms only

I think this is it.

From what I can gather, the engine torque will definitely vary, here's how:

Power = Torque x RPM

For a car of a certain weight to travel at a certain speed, the Power required to overcome all the resistive forces will be the same. And hence as I vary gears, the RPM changes and so does the Torque. However, the acceleration should be 0 as the speed is constant, but that may not be the case as after all, I do not have cruise control and my foot isn't that precise thus varying the acceleration. This is what I think atleast. This has turned quite interesting

[Sam800]

Originally Posted by superguy282

I think this is it.

Sir, I will still suggest you to check the suspensions because you're experiencing something which you never did before. Suspensions wear out with time and kilometres..

I am not an expert, i still hope it helped.

[Jeroen]

Originally Posted by superguy282

I think this is it.

No it is not. Trust me. You are so far off, I find it hard to explain.

Jeroen

[KKapadia]

Classic indicator of failing rear shocks. A Honda at that age and mileage definitely past its prime. I would replace all 4. You will feel the difference once all 4 are new.

[Jeroen]

Originally Posted by KKapadia

Classic indicator of failing rear shocks. A Honda at that age and mileage definitely past its prime. I would replace all 4. You will feel the difference once all 4 are new.

Could you please explain how worn shocks are the cause of this particular issue?
Thanks

Jeroen

[jomson13]

Originally Posted by superguy282

However we observed something strange with the handling. Let's say the car is cruising at 80kmph, and comes across road undulations, naturally the car would bounce around a bit before settling down. Whilst driving we observed that at the same speed, the car behaves better and settled quickly at 4th gear rather than 5th. In 5th the car would bounce around for a significantly longer time than at 4th even after the road evened out. The only factor changing between the two is engine rpm which for 5th is 2100 and for 4th is 2900/3000.

I can't explain the correlation between the bouciness and the engine rpm but here's my two cents:
I have a BRV and recently did a Pune - Bangalore trip. On the return leg the tyre pressures were higher (~36psi), uneven (since I topped up from a non digital tyre shop) and the boot was fully loaded. I experienced quite a bit of bounciness as you described but it was not related to the engine rpm in any way.
Drove again yesterday (with an empty boot) and the car is much better.

[megazoid]

Originally Posted by superguy282

The only factor changing between the two is engine rpm which for 5th is 2100 and for 4th is 2900/3000.

One other thing varying across these observations are the undulations themselves. Perhaps they themselves were the reason for the car settling down quickly or bobbing more. I am assuming that you were moving forward all the time and not testing it on the same stretch in different gears. Just guessing.

[arijitkanrar]

My suspicion is on worn engine mounts.
Please allow me to post my observations.

Originally Posted by superguy282

I think this is it.

From what I can gather, the engine torque will definitely vary, here's how:

Power = Torque x RPM

For a car of a certain weight to travel at a certain speed, the Power required to overcome all the resistive forces will be the same. And hence as I vary gears, the RPM changes and so does the Torque. However, the acceleration should be 0 as the speed is constant, but that may not be the case as after all, I do not have cruise control and my foot isn't that precise thus varying the acceleration. This is what I think atleast. This has turned quite interesting

It's the other way around.
The torque remains the same. The power being generated varies.

Visualize it this way. If the speed is constant, the forces of acceleration and deceleration (-ve acceleration) are in balance. There may not be any net acceleration but the forces are present and are in balance which is why the speed remains the same.

The force of acceleration is the torque (torque is rotative force) being produced by the engine which is then increased or decreased by the gearing).
The forces of deceleration are multiple (rolling resistance of the wheels, air resistance etc).
When the decelerating forces are counteracted by the combined torque output of the engine and gearbox the speed remains same.

Now lets bring the gearbox in the discussion.
So a lower gear produces a higher torque at a lower RPM and vice versa.

Now if take your equation of Power = Torque x RPM and keep the torque constant (as established above for a given constant speed) and the RPM higher for a lower gear and lower for a higher gear we can see the difference in power being generated.
Lets consider a hypothetical scenario. Suppose for the given speed (any speed), the torque required to maintain that speed is 150Nm.
Now lets say in 4th gear, the engine spins at 2900RPM to maintain this speed and in 5th gear it spins at 2100RPM (as observed by the OP)
The power being generated in these gears is

4th gear: 150Nm x 2500RPM = ~61HP
5th gear: 150Nm x 2100RPM = ~44HP

So as you can see the power varies. Power is a calculated unit and its a measure of the amount of work being done. Since work requires energy, you can get a indirect sense of amount of work done by looking at your fuel consumption (fuel is stored energy).

You'll notice that driving in higher gear will reduce the fuel consumption. The above calculation is consistent with this observation.
Higer gear = lower power = lesser work being done = lesser energy required = lower fuel consumption.


Now, power is not a physical entity and it cannot be measured. Torque and RPM are actual physical entities that can be measured.
Only these actual physical entities will contribute to the suspension behaviour given everything else stays the same.
We have established that the torque remains the same. So whats different? the RPM!

Which leads me to think that the difference in RPM is somehow causing the suspension behaviour observed.
One way this could be explained is that the lower RPM is bringing the engine vibrations closer to the natural fequency of the vehicle (chassis + load). The closer you are to a body's natural frequency, the higher the amplitutde of vibrations (more bounciness). This is a simplified explanation. Its more to do with lesser damping when closer to the natural frequency but lets keep it it this way.
This is why the bounciness is more in 5th gear (lower RPM) that at 4th as described by the OP.

Originally Posted by superguy282

...

Whilst driving we observed that at the same speed, the car behaves better and settled quickly at 4th gear rather than 5th. In 5th the car would bounce around for a significantly longer time than at 4th even after the road evened out. The only factor changing between the two is engine rpm which for 5th is 2100 and for 4th is 2900/3000.

...

One way the vibrations get transferred to the vehicle chassis is through the engine mounts.
Given the age and mileage of the vehicle, I think it'll be better to get the engine mounts checked and replaced if needed. With age, the mounts become harder and the engine vibrations end up getting transferred to the chassis more. This could be the cause of the bounciness.

Please let me know if I've made a mistake in the way I've approached.

[Jeroen]

Originally Posted by arijitkanrar

Now, power is not a physical entity and it cannot be measured.

In physics, power is defined as the physical quantity, which is equal to the work completed by a body per unit time. It describes the amount of energy that is converted or transferred in the unit time. In the standard international system, it is measured in watts, which is equal to Joule per second.

Engine power, torque and RPM the way you describe, are not relevant for this issue.

Jeroen

[CARSFASCINATEME]

@suoerguy282 did you get it checked? What were the initial reports?