Quote:
Originally Posted by drob23
I'll throw you a curveball and just go super theory route - The suspension system can be approximately modeled with mass damper systems, with the main purpose being to attenuate any "disturbance" from the road. Be it jittery road noise or big road dips. A perfect suspension system would make the car feel like it's floating on air.
The suspension can be approximated as 2 spring/mass systems in series - the pneumatic tire makes one spring, and the suspension the second spring. My background is in EE, where some interesting math theory can be used to say any force input over time (we'll call the road noise a "disturbance" force input from now on), can be approximated as the superposition of sinusoidal signals (see Fourier Transform). There are nice tools that then show how the suspension reacts to these "disturbance" signals as a function of *frequency*, usually described as a gain (amplification or attenuation). So if you were to ride over a periodic dipping road, then the amount of up/down movement you would feel would depend on the *frequency* of the dip (assuming the magnitude of the bump is fixed). Point is, the suspension will work really well for some frequencies, but maybe not so well for others. What I'm talking about is also a strong approximation, you can read full on research papers going into suspension design here.
In practice, there is a lot of tuning that is done to set everything from springs, dampers, suspension geometry, and the most disliked item by chassis engineering ---- big wheels.
To specifically answer your original question, I'd argue that both assertions were correct. The lighter wheels (less un-sprung mass) will allow the tires to easily travel up/down when hitting bigger dips, since now they have less inertia restricting suspension travel. On the other hand, lighter wheels will have a more negative affect with little high frequency jittery type road noise, since suspension travel will be easier.
When thinking of stuff like this, I just take things to the limit, assume that the wheels are 5000 lbs...you'd be great until you hit a pot hole, likewise with feathers on the wheel you'd feel every bump. So my opinion is you'll have less floatyness but jittery rock like bumps felt. But I will concede that these types of questions can require answers that are much more complex than one could dream. It might be that everything is fine until you hit a bump at 50 MPH for example. I think the most direct route to road comfort is a large tire sidewall, and non-stiff springs. Just my .02 
Edit - reread what you wrote (which was long but so was mine) and I think I just repeated your second conclusion. After re-reading the tirerack statement (which I'm guessing they actually tested), I think the situation is if driving at high speed and you have a single pot hole, the larger wheel will resist the suspension travel and result in the other side of the hole hitting the tire closer to the contact patch (not high up). I think this points out that it's hard to generalize much of anything on a car to "this makes the car ride better in all situations."
Interestingly enough (nerd alert), the single pot hole actually shares more in common with high frequency road noise then with a repeated low frequency series of dips. Fourier transform of a square pulse will have a lot of high frequency spectral content, which in this case would be amplified by the lighter wheels. |
I'm still trying to fully understand what you wrote
It's been a long time since I learnt FT (and all the other transforms you learn in signal processing) and its all out of my memory now
Quote:
Originally Posted by grimlock
This is for the first impact only, a heavier wheel that is off the road and exerts a force on the body of the car naturally with move the car more (harder to damp).
But if the wheel stays in contact with the road effectively its mass should not matter, the dampening done is on the body of the car wrt to the fixed wheels/road.
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What you say makes sense if you assume that the wheels directly touch the ground. In that case the wheels and the ground form one effective body of infinite weight. However, bring tires into the picture and the equation is something like this:
EARTH --> Spring(tire) --> Wheel weight --> Spring (suspension) --> Car weight
It is going to be an interesting theoretical exercise to figure out how (and if) changes in the wheel weight will effect motion in the car.
At least once person in this thread(ronin) has confirmed from real life experience that bumps decreased after moving to a lighter wheel (but then again that could have been due to the non-RFTs too).