MC70 Sixth Gear | Join Date: Oct 2012 Location: Monument, CO Posts: 8,044 |
MC70
Sixth Gear
Quote:
Originally Posted by Injured Again
If you've driven your Miata hard, then you would know that chassis stiffness is not a strong point. Chassis stiffness is critical to good handling, and is a primary reason why car manufacturers are looking to increase rigidity year after year and then widely tout that. Locating and maintaining suspension geometry starts with the chassis.
There is no structural rust in my Miata. I've driven other Miatas and they all are equally flexy. If you continue to believe that I have structurally degrading rust on my car, I will give you ten to one odds and bet you $10000, as well as pay for someone to disassemble my car. If there is structural rust, you can have the $10k to use as you wish. If there isn't, you put the $1000 to a charity of my choice. Easy bet! Take it!
And about that flexy top. You can't judge a structure in an unconstrained state when it is used in a constrained manner. You can bend a soft piece of metal into a bow shape and step on it, and it'll easily flatten. Constrain the two ends of that piece of metal and it will support your weight. It is entirely possible that the method of constraining the top provides a stiffening effect to the chassis. If nothing else, a significant shock to the chassis that produces a large bending moment will be dampened by the mass of the hardtop, so dynamic stiffness may decrease more than static stiffness.
Miatas are flexy for a coupe, but pretty stiff for a convertible (since they were designed as convertibles). Over the last couple of decades, convertibles have gotten stiffer, but the the Miata really isn't a slouch in this department; compare it to an RX7, for example, and the world becomes much more clear in this regard.
You posted photos of HOLES rusted through your windshield frame. I'll just let that stand on its own.
And your argument for stiffness in a bow doesn't apply here; and the example you use isn't how the top (or the chassis) is ever flexed (unless you're picking it up and dropping it on the hard top). The energy is applied to one corner at a time of the hardtop, so it would be more akin to moving one end of that bowed metal up and down; how much resistance is it offering in that regard? It's like saying a shock tower brace that is a three piece construction helps with front end stability; my guess is that you believe it would. The trouble is the same with both arguments: when you deflect one end, while there may be a dampning effect, there is no increase in structural rigidity as there is no cross-bracing with an inflexible object. As the rear driver's side moves upward, the top, at the rear passenger side also moves upward. There is no stiffening between the left hand and right had side of the car across the rear of the car. While the top takes a tiny degree of effort to flex, diagonally, it is almost non-existent and the rear will rise and fall independently of the location of the opposite diagonal. This can be measured by measuring the force required to deflect the top, off of the car, from one diagonal to the other. My guess is less than 5lb to deflect it 3".
Now here's where the NVH becomes important. Due to the material used, as energy is applied to the top (let's use the same method above, as energy is applied to the rear corner) it is dispersed across the hard top. The fiberglass is stressed most at the the point of origin and does a good job of spreading that energy, fairly evenly, across the top. That means that the energy is transferred to multiple points on the car in a smaller value than the original input. This is how NVH is reduced, but adds no rigidity to the chassis.
And this brings us right back to your car and its compromised rigidity. If you're getting any sort of structural rigidity improvements from having a hard top on your car, your chassis is severely compromised.
As a secondary note, the suspension will always work better with a stiffer chassis, however a stiffer chassis is not the end-all-be-all of a good ride or great handling. The suspension itself is a huge component. It has been proven, repeatedly, that an ultra-stiff suspension causes slower lap times; it's a great source of fun on Top Gear when the Stig is doing back-to-back comparisons of cars with the sport mode turned on, then turned off. A well tuned suspension is capable of absorbing minor road irregularities and dissipating large impacts to the maximum extent possible prior to the impact energy overcoming the reaction dissipation capabilities of the shock and transferring the surplus energy into the chassis. It is at this last stage where chassis rigidity becomes important to ride quality. Handling is always affected by chassis stiffness, but can also be compromised if the act of stiffening causes the chassis to become much heavier, in which case you require stiffer suspension and succumb to inertia to a much greater extent. This is why light cars are always preferred.
The Miata is a fairly stiff, fairly light car. They've gotten stiffer and not much heavier over the years. The suspension has gotten a little firmer, however travel has increased to allow for more reaction capabilities.
Again, this is all elementary physics. Think of the hard top as a motorcycle helmet. It isn't designed to make your head harder, it's designed to dissipate energy. That's exactly what the hard top does.
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