Hi guys,
sorry for being absent - had a business trip to Japan, major race meet and a chest infection thrown in to the last 3 weeks!
The sheet was a calculation based on empirical data of flow through real (ie not zero length) orifices from laminar to turbelent ranges. Our pistons and typical speeds fit within the range of conditions used to create the empirical model, so I believe it is mostly valid, however as I mention on the sheet it does not include the effect of fluid aeration (which can affect viscosity and oil compressibility) which is obviously critical in an emulsion shock.
The calculations can be shown as a F-V curve, and ideally for correlation of the model we would overlay that with Scotts data for a few different piston / oil types. The non linearities are far more evident at higher velocities, so until the tests are done at these higher speeds its hard to fully correlate the model. Once that is done, we will overlay the results on my original graph and see if it was worth the time and effort it took to find the research paper and create the sheet
Scott and I have exchanged several emails and he has been kind enough to share the raw data with me and there is a lot of additional data contained there which we are beginning to fully understand and be able to explain. As always data throws up more questions than answers in the beginning.
As for damping coefficient - some care needs to be taken when calculating the 'ideal' coefficient for a vehicle suspension. Most text books talk about a single degree of freedom (1DOF) model (1 mass, 1 spring, 1 damper) and for this case its easy. But cars have several degrees of freedom - there is the mass of the suspension (unsprung) and the mass of the chassis (sprung) as well as the coilover unit and the tyre (which is actually a spring / damper). There is a different ideal damping ratio for the sprung mass and unsprung mass, not to mention pitch and roll modes. As an example, if you keep increasing damping with a 1 DOF system, it will become more and more damped. However with a car, if you make the damper too stiff, you reach a point where the suspension stops working and the tyre becomes the majority of the suspension - and as the tyre is relatively undamped, the car as a system will have less damping (this is a big issue in F1 cars). There is a balance and it heavily depends on the tyre / insert performance.
Sorry for the long response.
Ray
Originally Posted by
icecyc1
Ray Munday's plot is a calculation plot, not dyno data. With that said, it's not too bad from a relative perspective. It definitely shows the relation between your piston options and viscosity and how they can affect low speed or impact damping. I think it would be better if Ray speaks for his plot himself, but I think it is a well designed plot. Maybe he'd like to recreate that plot with actual data at some point?? :-) As with any calculations, many assumptions and simplifications usually need to be made, but it doesn't mean they are wrong. As I like to say, a model or test should only be as complex as it needs to be to give you the information you need in order for you to make the "correct" decision.
I have just made an impact tester, so I'm starting to get some higher velocity data. I'll say it doesn't look quite like I was expecting, but I think it can be justified. I need to review it more before I publish it. There is a lot of interesting little details that can be seen in the plots that a more experienced eye than mine can see (thanks ray). I'm still learning, but I hope to pass along those details as well. I think a lot of folks will be interested in what goes on inside their shocks, there seems to be a lot more than what initially meets the eye.
My next major publication will be a piston hole and number study at all three velocity levels. It will definitely take me a few weeks to gather, analyze and report that data, but it will be coming. I'm just as excited to see this data as you all are.
Originally Posted by
Riv2SC10
I've been doing some more figgerin'. Ray Munday has posted a damping comparison for several 12mm and 10mm pistons in his thread in the Aussie section. The graph shows the damping force for the different pistons with varying viscosities at two different piston speeds (50mm/s and 2000 mm/s). Obviously that's just a short step away from calulating the damping coefficient (N/mm/s, or N-s/mm), and for the same piston and oil, the results are much different between the two speeds. This appears to indicate that the damping coefficient of the shock is not linear with speed. Hopefully, Scott is able to generate some data at higher speeds to confirm or deny this. I believe the graph Ray has posted is based on calculations rather than actual dyno data, but I guess I'm not sure (maybe he can chime in on that).
Plus, when looking at Scott's F-V graphs, there appears to be a bit of non-linearity (exponential???) displayed on the rebound (downstroke) of the shock, particularly in the Viscosity Effects graph. Perhaps that would also be showing up on the upstroke (compression) side as well if it weren't for the "spring affect" from the air in the oil. The lines are much cleaner on the down stroke, and you can see some non-linearity on that side.
The flip side is that this non-linearity doesn't show up as much in other comparison data Scott has presented (Emulsion vs Bladder, etc.). In fact, the Bladder and Foam graph appears to be very linear. So maybe there isn't anything to my interpretation of the graphs. I'm curious, though, to see how this shakes out if Scott is able to generate some higher piston speed data. The data provided by Ray appears to show non-linearity of damping coefficient with speed, and based on simple flow through orifice type calculations and whatnot, there should be some sort of exponential component to the damping with increasing to piston speed. But maybe at this scale the typical orifice flow models are no good??? I'd believe Scotts dyno data over a calculation, personally. It's hard to argue with good data, but easy to overlook something when creating a calculation.
Thoughts?