I was gently wondering whether any of you know of a non-proprietary understeer spreadsheet?
I found one that seems quite nice, sadly it is rather algorithmically challenged. (it is wrong).
https://projects.skill-lync.com/projects/Final-Project-83167
Posted Jan 22, 2020, 9:14 AM UTC
Like so:Cornering Compliance Budget
Yes, that's the thing. I'm happy to do follow this up over at eng-tips if you like.
Link to the relevant eng-tips forum thread to save everyone else a minute of googling if anything exciting happens over yonder
https://www.eng-tips.com/viewthread.cfm?qid=463747
Greg: could you provide any insight on some easy ways a FSAE team could get relevant cornering compliance data WITHOUT time on a K&C machine. One method I've seen mentioned is using a ratchet strap and a scale between wheels for I assume FY/MZ steer/camber compliance coefficients. It would be nice to see more practical compliance testing beyond the frame torsional rigidity tests, but I don't know where to start without going above my pay grade in this area. I have seen this video of a K&C senior design project https://youtu.be/08psY2ILeOo--is their approach on the right track?
Also does anyone know of a textbook (not RCVD) that has a lot of FSAE suspension/VD relevant info on compliance--measurement, integration into simulation, etc.. I've found Dixon's susp. geo and computation has a small chapter (9), but not that much meat.
FEA would be one approach. The advantage of that is that you are going to get the frame stiffness effects, which can be considerable on a steel spaceframe. The disadvantages are legion.
The problem with trying to do physical compliance testing without a rig is that everything moves (it does with a rig as well). But as you suggest, bracing the car against itself solves the problem.
So - there are three important compliances
toe/MZ
toe/FY at the contact patch
camber/FY at the contact patch
So use laser pointers to measure angles, I like your idea of straps, but if you use struts to get compression as well why not build a frame that clamps onto the hub and allows you to pass 4 struts across the car, load cell and winching mechanism (turnbuckle?) in each.
and kinematics
roll steer and camber - jack the wheel up one side and down the other. compare springs on and springs off, sta bar on and off.
I should add I've never done this, it will take a bit of debugging.
UofM has had its cars on the GM K&C machines (1 axle at a time because of the short wheelbase). A modern wheel alignment machine with digital toe and camber read-outs can easily be a platform for generating useful K&C data. Keep in mind that the FSAE cars don't roll a lot, the tires have relatively low camber stiffness, and their toe (Mz) compliances are astronomical due to sloppy gear mountings, soft U-Joints at high angles, pasta chassis tubes and tierods at angles that excite all kinds of jellow flavors. This is often the reason the cars are understeering (besides locked up diffs) in spite of the rearward weight distribution coupled with relatively stiff tires. They loosen up quickly when the tire Mz goes South, though, and the speen-outz get noticed. Same is true for cars with high octane badges. They have relatively large amounts of compliance steer and roll steer at one end to solve a bunch of problems, but these 'patches' wear out when the tires see large g-levels. [ I posted a Cornering Compliance Summary generator on the Eng-Tips forum for Greg. "Have a Nize Day !" -- John Bon Jovi....
If it's already a nice spreadsheet, why not just fix the issues and use it? Would it require a huge amount of rework to make it functional?
If it had named variables I'd do it straight away. But we have spaghetti code in that sheet, I was hoping to find that all the FSAE teams had neat little spreadsheets/matlab scripts for this, or at least bicycle models.
A lot of them do, but I believe that teams typically keep things like these to themselves, but not even for competitiveness related reasons. While finding and sharing these tools is great for professionals who already understand the concepts, part of the learning process you'll find in FSAE is building an understanding of the vehicle dynamics to the point where you can create these tools yourself to demonstrate your understanding of the concepts. Employers love that kind of stuff. For this reason, students often prefer to develop these tools themselves. For students, sharing these tools with others kind of feels like allowing them to cheat off your test.
The most fruitful discussions in this subreddit typically have more to do with the concepts of Vehicle Dynamics rather than the tools/methods used to analyze them. People on this sub are not inclined to be very helpful when when students come in asking others to solve their problems for them. However, they try to be really helpful when students come in attempting to get a better understanding of the concepts so they can solve their problems on their own.
Fair call. I'd forgotten that side of it- for example most of the analysis that needs to be done could be a bicycle model or Tune to Win. So everyone out there who is struggling with ADAMS (for example) is basically going for a level of modelling that is not justified by the data, or, really, the competition. But for your future careers, having that awful experience under your belt is hopefully, if not gold, at least bronze.
When and if you get a chance, ask Claude why his own personal car has so much toe steer and roll steer ! Record his answer for posterior reflection ... 911Call 911
Interesting that all 3 examples manage to get twice as much FCC as RCC. I've used that as a rule of thumb in the past, and often failed to meet it. Interesting to compare the approaches in those examples, the Commodore and Corvette are all about front axle compliance with far more SAT steer than I'm used to, whereas the 911 is more of a mixture of rollsteer and compliance.
Using SAT steer adds an interesting non linearity as you approach peak grip - your SAT fades away, so presumably the understeer decreases. Hmmm.
Greg: Which forum would you like to continue with this? As I mentioned, this one has too much chit chat, regulation spew, and too many car-tunes. But, the CC recipe is usually the result of what the weights and weight distribution is, tire size(s) that can deliver realistic BOTH front and rear tire CC elements, suspension architecture chosen by styling and manufacturing, and competitive assessment. Then its up to the transient handling and steering gain goal seeking routines to fetch the unique CC solution for response time, system damping, steering sensitivity, spare tire use, max GVW acceptability and tire availability [Some tire sizes can carry the weights but can't deliver the differential CAF/CAR duo needed. Hence, you fight Styling to get the tire size than CAN produce the performance goals PFC [and fits in the wheelhouse] and fill in the rest with realistic K&C parameters to take care of load cases, spare tire operation, aftermarket baloney skins, foreseeable misuse (low tire pressure and worn tread and tire mismatches. Of course, steering feel gets into this nowdays, so there are now some tabu no-nos in the budget. If you want to have happy campers at high speed (over 160 kph) rear roll understeer is out and a roll axis must be chosen to make sure the roll frequency NEVER comes near the yaw velocity natural frequency. Otherwise, you will have very bad dynamics and very happy lawyers. Another big player in this is the yaw radius of gyration [k^2/ab0. This mostly means powertrain architecture and here we see where big motors with lousy transmission fixations make the tire, understeer and damping solutions work but 'feel' like chitt. See you on the other side...
Greg: Do you want to know the deg/g components of front & rear cornering compliances [hence 'understeer], given axle weights, tire Fy, Mz, Mx, roll steer and camber coefficients, lateral force and aligning moment steer and camber coefficients from K&C data, etc? If so, I can post the formulas here or out on the Eng-Tips forum. My stuff goes all the way back to Tom Bundorf & Ron Leffert (Both of whom I worked for at GM).
I'd prefer the Eng-Tips deal because I have figured out how to post pix, etc. AND this place is sort of the Saturday Night Live/Comedy Central version of FSAE entertainment. (Oh my, princess,say it ain't so).
Well talking about it like that does nothing to help legitimize it. Every other FSAE related discussion forum is either dead or toxic.
I just want to let both of y’all know that we really appreciate having your experience around here.