Direct Acting Spring Damper

[Z]

Bob,

Yes, I did notice that!

(Other Teams might take GFR's Michigan-2015 results as reasonable evidence that while DASDs might not, in themselves, make the car much faster, they certainly DO NOT SLOW YOU DOWN!)

BUT (!!!) ... I am not sure I even want to contemplate this ..... but ... was it the DASDs that cost you 45 points in Design Event???
~~~o0o~~~

Ritwik,

DASD Spring-Rate and Damper-Rate calcs are done the same way as with Rocker-Activated-SDs. But you must take account of the MR correctly. Which means MR must be used "right-way-up" (which depends on how you define it), and it is squared.

Some rough rules to get you in the right ballpark...

1. A DASD acting "vertically" above the wheelprint has MR = 1.
So SR and DR calcs can be done without consideration of MR. Call these the default, or "Wheel-", Spring-Rate and Damper-Rate.

2. A DASD acting at ~45 degrees to a ~horizontal Control-Arm (eg. wishbone) has MR = ~0.7, and MR-squared = ~0.5.
So in this case you use a SR and DR that is TWICE the default Wheel-Rates. Given that in your earlier post (calc'd with MR = 1) you had selected damper-valving between the softest and second-softest available DRs, means that in this MR = ~0.7 case you ONLY have to go up to about the middle-stiffness valving, from that graph. Also, buying a Spring-Damper with stroke of only ~40 mm is enough to give the mandated 51+ mm suspension travel, so a potentially lighter damper can be used.

3. A DASD acting at ~30 degrees to a ~horizontal CA has MR = ~0.5, and MR-squared = ~0.25.
So in this case you use SR and DR that is ~4 x the default Wheel-Rates. From your earlier post's graph, you would need the stiffest, or second-stiffest, damper valving. But this suspension would then also be good for ~130 mm (5"+) of suspension travel, so good enough for Baja+! Countless real cars over the years, both production and racing, have had MR = ~0.5. So NOTHING UNUSUAL here.

4. The car company best known for the most comfortable ride, and excellent grip on rough roads, and hence "really good suspension", is Citroen (with the DS and 2CV being the standouts). On these cars the typical MR = ~0.3, or less. Draw your own conclusions.

5. Worst case, on smooth-track racing such as FS/FSAE, "...any suspension will work, if you don't let it...".
So, worst case, stiffen your DASDs up as much as you want, and still OK. But I strongly recommend keeping the "main" SRs and DRs at the softer end of the range, and then make good use of your BUMP RUBBERS for the worst cases! These are squishy-rubber- (or better, foamy-polyurethane-) doughnuts-around-damper-rod. The dampers should bottom-out on these at least once per lap, at the worst road-bump, car-wobble, whatever.

6. The big decision here is really a structural one, related to the overall car/frame design. Pick a chassis mounting point for the top of the DASD that best suits the overall frame design, NOT because of some obscure "spring/damper equations". Roughly speaking, the DASD should have its lower-end pointing at the wheelprint, and its upper-end mounted at an already strong frame-node, and pointing roughly towards the car's CG. This way the DASD takes a large part of the worst-case wheel forces, making life a little easier for the other control-arms.

7. Finally, KEEP IT SIMPLE (but well executed)!!!

Z

(PS. Ritwik, Bob said that "...multiple shock pickups ... could be coupled with exchangeable [rod] extensions...". My guess is that GFR didn't think this extra complication was necessary to win AutoX and Enduro. It wasn't...)

[Kevin Hayward]

Bob,

ECU guys beat you to it, but with maybe a little less speed The guys really like the simple approach to cars and we chuffed to see GFR go down the same road.

I agree that rebuilding the dampers is pretty easy. Feasible to do it track side in a half an hour or so. ECU keeps two full sets for this reason. Although two full sets of the Penskes still costs less than one set of the Ohlins.

Like Z, I am also curious as to how it was received by the judges. Our team was criticized in Oz for not having enough adjustments.

Car did 3.8s Accel, 2nd in skidpad by almost nothing, 2nd fastest car in Autocross, and quickest car in Endurance. Big dynamic improvement over the previous car that had more damper adjustments. However with the DASDs the students had to learn a lot more about rebuilding.

Kev

[Mitchell]

Bob,

ECU guys beat you to it, but with maybe a little less speed The guys really like the simple approach to cars and we chuffed to see GFR go down the same road.

I agree that rebuilding the dampers is pretty easy. Feasible to do it track side in a half an hour or so. ECU keeps two full sets for this reason. Although two full sets of the Penskes still costs less than one set of the Ohlins.

Like Z, I am also curious as to how it was received by the judges. Our team was criticized in Oz for not having enough adjustments.

Car did 3.8s Accel, 2nd in skidpad by almost nothing, 2nd fastest car in Autocross, and quickest car in Endurance. Big dynamic improvement over the previous car that had more damper adjustments. However with the DASDs the students had to learn a lot more about rebuilding.

Kev

Kev,

Do you use a known shim for particular rates or just go up/down in shim thickness as required? We have been using the 7800s for over 5 years, new shocks are out of the question ($$). This is the first year they will be direct acting and I was hoping to avoid multiple trips to the 3rd party shock dyno.

Mitch

[Kevin Hayward]

Mitch,

I couldn't tell you what the guys ended up putting in the car, or even too much of how they chose the final settings. I agree that 3rd party dynos can be a pain. However, we have a small damper dyno in the workshop, which makes things a lot easier. A heck of a lot of testing and shim stacks can be tried in a solid day or two of testing. The Penskes are a really good solid little damper. My choice would be with them over the Ohlins twin-tube jobs just about any day of the week. The Ohlins are good, but fundamentally the oil has a much more torturous path on the latter.

When at UWA as a student we put together our own damper dyno. Work was mainly done by one of the 3rd years. It was a bit cooler than the Roehrig at ECU because it was a hydraulic controlled job that we could program all sorts of inputs. Put together with stuff lying around the uni.

For these little 1/4 midget shocks you could put together a pretty good dyno pretty easily. Don't really need to bother with a scotch yoke mechanism. Just hook the damper to a crank attached to a speed controlled motor. Hook the other end to a load cell. Linear pots are pretty cheap now. DAQ through your ECU, a little USB DAQ device, or any logger you might use already. I bet an arduino could do a heck of a lot. All in a sturdy frame of big steel sections. If you are really tight do the speed control through a belt drive with a fixed speed motor. Most of the time the guys just test the damper at one motor frequency. While it helps to alter the frequency to investigate inertia effects you can go a long way through development without bothering. If you cant afford a load cell then make your own with a few strain gauges and an amplifier.

Don't need to move it too quickly. The Roehrig we have hits 284mm/s at the stroke we run, but that is significantly higher than where the damper spends most of its time on the car. Temp is also not too critical, as most fluids are quite good with density and viscosity stability in the temp range you will see, and the dampers don't tend to get too hot on the cars. Monitor it though if you have a spare tyre temp sensor. Even better just use a simple thermocouple and dont even worry about emisitivity.

Kev

[ritwikdas18]

Z Thank you very much for your response to this thread , but my question(silly) still remains unanswered.
Q1.The chassis shock pickup point has to be on a node , so in order to have multiple shock pick-up points won't I need to have Multiple Tabs/Clevis on the member of the chassis.Its very unlikely that shims can provide adjustability of more than 0.1 change in MR . So what exactly does it mean?
Q2. I also didn't quite get exhangeable rod extensions since when you fix the chassis pick up point and point on the LCA the motion ratio is fixed. So how will exhangeable rod extensions help in varying motion ratio?
I am also attaching an image of our current car's chassis design with front and rear damper pick up point and also monash's Direct acting Setup.
Chassis.jpg
13626_942937229091936_455007072995637602_n.jpg1621961_705424822812106_1688038843_n.jpg10150815_750203488334239_7521423623452457624_n.jpg10441251_810596775628243_4692776369804080663_n.jpg

[Swiftus]

...thus very different wheel rates if spring rates are around the same values...

So why not get some different springs?

[ritwikdas18]

Yes , thats what we would do.

[Mitchell]

Mitch,

I couldn't tell yo

...

emp range you will see, and the dampers don't tend to get too hot on the cars. Monitor it though if you have a spare tyre temp sensor. Even better just use a simple thermocouple and dont even worry about emisitivity.

Kev

Thanks for the great info Kev.

[Z]

Ritwik,

... my question(silly) still remains unanswered...
Q1. ... multiple shock pick-up points ...?

"Multiple shock pick-up points" are NOT NECESSARY. Repeat, they are an UNNECESSARY COMPLICATION!

Find ONE suitable "shock pick-up point" (per corner), with MR anywhere between 0.5 and 1.0, that best suits the chassis structure.

Then calculate suitable spring-rates and damper-rates. Be honest, and call these your "first best guess". Buy said springs and damper-shim-kits. ALSO buy 10-30% SOFTER springs/damper-shims, and 10-30% STIFFER springs/damper-shims, in case your "first best guess" was not close enough. Now you have THREE suspension set-ups for each corner -> soft, medium, and hard.

Next do lots of testing. Importantly, DO NOT buy any other springs/shims. Just work with what you have got (but see below *). Adjust tyre-pressures, and do more testing. Adjust toe-angles, and do more testing. Adjust camber-angles, and do more testing. Learn how to drive fast with the spring-dampers you have (ie. either S/M/H).

Maybe, just maybe ... next year you might buy some more springs or damper-shims. Other things are much more important. Weight-distribution! Aero! These might require new spring-dampers, but that is next year...
~o0o~

Q2. ... how will exhangeable rod extensions help in varying motion ratio?

They DO NOT. But exchangeable, or length-adjustable, rod extensions are good for easy adjustment of ride-height relative to damper-stroke. So USEFUL. The conventional threaded adjustment similar to the ends of many push/pull-rods is a good enough way to do it.
~o0o~

* To repeat what I said earlier, IMO the easiest way to improve the "suspension related" performance of FS/FSAE type cars is to have fairly soft springing, but back this up with appropriate BUMP-RUBBERS. These might appear to be stupid-low-tech, but they make a big difference!

The biggest performance improvements come from better weight-distribution (less total mass, and lower and more rearward CG), stickier tyres, and MORE AERO! Soft springs with damping-ratio = ~0.5 -> 1 x "critical" keep everything well-planted, and the bump-rubbers take care of the occasional excesses (ie. a big-bump, or too-much-body-roll-from-sudden-turn-in).

Z

[ritwikdas18]

OK Thanks for all the valuable information Z. I need a bit of help in my ride and roll rates calculations which I have been doing over the past few days. This is the excel sheet that I am using for my calcs

https://docs.google.com/spreadsheets...it?usp=sharing
My procedure was in the following way
Static Deflection->Ride Frequency,Ride Rate->Wheel Rate(Taking into Account Tire Rate)->Front Spring Rate,Rear Spring Rate,Front & Rear Roll Rate ->Roll Gradient -> Front & Rear Lateral Weight Transfer-> Dynamic Front & Rear Wheel Travel -> Total Front & Rear Wheel Travel -> Check whether you are bottoming the car or not.Check whether I have the desired roll gradient or not-> If not change Static Deflection and go on.
1.Also I had a constraint that the chassis was already designed(This should not have happened.I know.) . So my front and rear motion ratios were fixed to around 0.766 and 0.5 respectively.
2. Also another mistake was the chassis ground clearance is around 2.5 inches (Very high I know). It should have been ~1.5 inches since the minimum usable travel is only 1 inch.So any suggestions to damage control this mistake in ride rate calcs or any other way?
3.I don't quite know how to to decide my Lateral Load Transfer Ratio.I know that I can control the under/oversteer of my car by doing so, but how do I arrive at a ballpark number with rough calcs according to my requirements.
4. I have also not yet decided on the roll gradient of my car, but will do so in a few days.
5. Lastly For calculation of roll rates Optimum G neglects Tire Rates and uses wheel rates in their formula. I think this should be Ride Rates . Am I wrong?
6.Any iteration can be done on the excel sheet by changing the static wheel deflection and everything else changes accordingly.I have also mentioned all formulas used while calculation and I assure that I understood all of them by the classical mechanical methods before using them.
Z I must thank you that although it may seem trivial to choose static wheel deflection or ride frequency first since they are interrelated but I personally can make more tangible sense out of static wheel deflection than ride frequency.