Kinetic Research wrote:
BB69 wrote:
I think it will be interesting to see. My only thought is that one of the main things you are not simulating is the tire slip angle. The real world grip of a tire is very dependent on slip angle, and I believe you have to have the tires rolling for that. You also won't be seeing any of the longitudinal forces associated with drive torque or rolling resistance. I don't know how those forces change the contact pattern, but they do.
Ken
Thanks for the input Ken, not being a suspension or tire dynamicist I bet I am guilty of over simplifying. Slip angle, drive torque and rolling resistance are all critical to overall handling.
https://www.youtube.com/watch?v=W8UiE7yvO_MMy rig is based around my understanding of the traction circle, hence is only valid for that micro instant were the car has neither drive nor braking vectors, but is purely side loaded - which would be the maximum point for cornering force. Its at this point where the car will understeer, oversteer or be neutral - any further control inputs like gas, brakes or steering will affect this balance point. I want to show some of your videos because they have influenced my thinking over the years (thanks for posting them, would love to see more!)
In the video (Picking through Grattan traffic)
https://www.youtube.com/watch?v=5C0OgpvWuxc&t=9s the steering rarely turns more than 100 degrees off centerline. I don't know what ratio of steering wheel to wheel angle is in your setup, the Mclaren F1 runs 18.5 degrees steering to 1 degree wheel rate, so in that case the front end would only turn 5.5 degrees. The front wheel could be turned in the static testing to simulate this influence, then all geometry and contact patch pressures would also be replicated.
Front slip; the steering has to be turned to take a corner, and this affects the contact patch due to factors such as tire width and section, camber, caster, kingpin inclination, ackerman, tire vertical load, steering angle etc.
Rear slip: My rear end will be setup very close to yours - Wide back tires appear to mostly be deflecting;
https://www.youtube.com/watch?v=2HEQDpLfncQ Another way to look at the testing rig is if the load was applied purely from the front, it would simulate the maximum braking grip due to load transfer and geometry changes in the suspension (for discussion please ignore the critical effect of the disc friction vs temperature). The loading can only act through the COM to be "accurate".
First hurdle is to build my car first, the test track is 1.5 hrs drive away so I was thinking of ways to reduce my setup time and cost but its best done (and is more fun) on the track like everyone else does.
Its a pity the server apocolypse of 2019 wiped out the photos in your track monster build log - were you going to build a mark 2?
cheers,
Marcus.
I'm no expert for sure, but that micro instant you are talking about is purely theoretical in my experience. The goal of maximizing the friction circle means you always have a longitudinal portion of acceleration. Also, under and over steer is a dynamic event. What I mean, is that your car will transition from one to the other rapidly. It's this transition you should really care about, not some instantaneous static point that is over before you can do anything about it. More than anything, you want a car that does the same thing lap after lap so you can learn what to do. If it understeers on entry and then rapidly changes to oversteer with power, you can learn to drive that way. If it understeers one lap, and then oversteers the next, you will always struggle to control the car and your lap times will suffer. Most importantly, you won't have confidence driving the car.
I have more Excel sheets than I can count, trying to calculate what the car is going to do. They are all pretty much useless for me when I'm at the track. There are a couple reasons for that, and I will explain those so you can decide if you will have the same issue.
1. Tires. You are talking about trying to tune the car around some admittedly small handling window. Are you willing and able to use tires in the exact same condition every time you make a change? I'm not. I use scrub tires from pretty much wherever I can get them. They are different sizes, brands, and mfg dates. This alone means I will never reach the ultimate in handling because I am constantly changing one of the biggest factors. If you go spend some time with the guys consistently winning races, you will see they are adamant about reducing variables when testing and racing. This means they go through a lot of tires. They will test a set for only a few laps, and then they will put on a new set for the next round of changes so they can eliminate the tires from the equation. This is anything but locost.
2. Attention to detail. There are lots of other factors that change the handling of the car, and I don't have the time or drive to control them every event or session. For instance, are you going to have the car aligned before every event? Small changes in toe or camber from a bushing moving or wearing will affect the car. Are you adjusting your tire pressures for track conditions every run? How are you making sure your differential bias is changing (big factor for handling assuming you are using a limited slip of some kind)?
In the end, wondering these things is very interesting for me. I'm working on a braking spreadsheet now to optimize my system. Once you have the car built and running though, you will realize you learn so much more for every lap turned compared to reading/calculating. So, build the rig in your spare time, but don't let it slow down the car build.
As for me, I'm always thinking about rev 2. I need a new garage first. In the meantime, I'm working on some custom rear control arms; inboard rear coilovers; power steering; ABS; and revisions to the overall brake system sizing. I always try to post more videos. I'm pretty sure I have all the photos from my build still, so ask if there is something you want to see.
Keep at it.
Ken