Originally Posted by
fredygump
I'm glad you updated this thread. I wouldn't have known it existed if you hadn't! Just the other day I was musing that maybe I should build a shock dyno.
But what I really want to do is a little different than what you've done. I want to build a dyno that acts against the complete suspension of the car so I can better understand how much damping is best. And maybe also figuring out a good way to pick spring rates?
I'm debating the utility of testing. I'm not sure how useful it will be? I thought instead of measuring forces, to instead use high speed video to analyze the behavior of the entire suspension system.
I'm sure that tires and foams, and to a certain extent the wheels themselves, play a significant role in the suspension, but I don't know how they contribute to the suspension performance.
My first thought was to attach a chassis to a frame and place a platform under a wheel that moves up and down to simulate bumps. The test can be without tires, to see how fast the suspension can move while keeping in contact with the platform. And the test can be done with tires and foams, to see if the foams significantly change the results. And finally, if the platform has rollers of some variety, the variable of the wheel's rotation at various speeds could be added.
Another variable is how much energy is absorbed by the suspension and how much is transferred to the chassis. That would be possible to test, but again, I'm not sure how much could be learned from doing this.
I get excited thinking about how cool it would be, but then I think about the time, effort, and cost, and then I wonder if it would change anything or really help.
I'll tell you from my experience, it is indeed a lot of time, effort and cost. I did it mostly to satisfy my own curiosity based on the lack of available information, and I figured while I'm at it, I might as well share it hopefully boosting some thoughts about shock performance in the community.
Regarding your idea, I definitely feel it's interesting, but I do believe that it would be significantly more complex than a simple dyno. The dyno part was actually quite easy, it just took finding the right instrumentation and putting it together. The impact though, as simple as it seems, was MUCH more difficult to get repeatable and meaningful data. I'm still not totally certain in how best to define impact, but the results I am finding I'm able to reproduce, so I feel comfortable enough with it. But I still want to find a better, more "normalized" way of defining it. My big challenge after this is to figure out a cheap and simple way every racer can determine what their actual damping coefficient is without using expensive fancy equipment. I believe I can do that.
There is definitely an influence in tire/foam/wheel compliance in a suspension. To keep things simple in suspension calculations, the spring rate of the tire assembly is often assumed to be infinitely stiff, which we know is not true. It might be possible to measure the effective spring rate of the wheel assembly, and it could then be modeled into a suspension system. The difficult part though is it is unlikely that spring rate is linear (like ALL of our shock coils). This means, the rate depends on the amount of deflection. This makes modeling significantly more complex because now the actual position of the suspension must be known as well. Not stopping at just the wheels, you would also want to consider the flexibility of the suspension arms, and the chassis. You can see this is starting to get out of control. Building a full chassis dyno could help you figure out some of that, but you'd have to put in significant research into how they do them for full scale cars, and see what is required or relevant. I don't want to throw water on your fire, but in the end, it may not be worth the effort.
With that said, with basic geometry measurements of your suspension, and mass of your components, you should be able to get 90% there to predict the best spring rate combination for your car. But, due to the unaccounted for flexible components, you'll have to test or drive to tweak the last bit for the perfect suspension. One idea that's used in the autocross world is to outfit your suspension with linear displacement transducers, and monitor your suspension travel for a few laps. You will notice how active your suspension is, and how skewed from ideal it is. Then, you should be able to dial the damping/spring rates to align it to balance the data, and theoretically, that should be your optimum spring/damping for your car's mass.