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Discussion and questions related to the course Launch Control
The Webeenar was as slick as ever, but, as I have AWD I was horrified to find out that I can only run off ground speed,
What were they thinking . . . What can I do . . . . Why did'nt I get the G4+ . . . . .Is there any way out of this nightmare.
I'm not sure exactly what you are referencing here?
The M1 only references ground speed however if you've gone through the worked example you will see where I set up the system to respond exactly the same as a basic 2 step launch control system. On the face of it I'm not sure that would be much different to how I'd set up the G4+ except the G4+ gives you the option of triggering the launch control system off a digital input.
If I've missed the mark on your question I'm happy to elaborate. Let me know :)
I got that you can set the Motec as a 2 step, as you say "the next best thing"
I'm just not clear on the "trigger switch" function, and if it is of such little consequence to the operation of the system
why dose anyone bother to use one.
If i want "proper" launch control on an AWD
A. with the ability enable and dis-enable the system,
B. select from at least 2 presets (wet/dry)
C. operate from the driver seat,
whats my option?
Can you clarify what you mean by 'trigger switch" Are you referencing the M1 here?
On the M1 to achieve launch control as you want to run it I would do the following:
1. Fit a multi position rotary switch and configure it as Driver Rotary Switch 1 (or any other number you're not already using)
2. Set this switch as the Index under 'Launch Switch' and set the mapping so that the Launch system is OFF in the 'Default' and 'Zero' positions. Set it to 'On' in position One and Two. This will mean that launch control is disabled when the switch is in position zero but on in position one and two.
3. Set the Launch Aim Switch Index to 'Driver Rotary Switch 1'
4. Set your desired launch rpm for position one and two on the engine speed table to suit your dry and wet setting.
I hope that helps.
The trouble with having AWD launch control is that it is difficult to get a consistent reference speed for the ECU to operate on, as it is possible for all 4 wheels to be rotating at the same speed, and the vehicle not moving, in this situation, the ECU will not detect the loss of traction. GPS has too much lag to respond correctly, and if you are only using wheel speeds, how does the ECU know which wheels are slipping, and which have grip?
There are a number of ways that can be used (not currently in the M1) that will provide a pseudo AWD launch control, that rely on either accelerometers (they need to be very accurately calibrated for this to function correctly) or rate of change calculations, where the ECU has calibration table for engine speed or wheel speed that says that the fastest that the speed can change is x/sec, and if the rate of change is greater than this, then control the engines torque output to get the speed change back under the set curve. Niether of these options give proper feedback to run a full closed loop launch control system.
If you want to have a rolling start launch, then a number of the high power R35's use the pit speed limiter to do this.
When you try to do launch control, the aim is to match the engine RPM limit to a known ground speed. As the ground speed goes up, the rpm limit follows. On a 4wd system, there is no way to determine an accurate and reliable ground speed. Some people say to use the slowest of the 4 wheel speeds, but as you would know, that only works up to the point where the car is capable of spinning all four wheels. You an use a combination of long G and wheel speeds, but that also has its own issues including accuracy and speed of G readings as well as the angle of the launch area (up or down hill).
You could use an open loop accel rate of change of wheel speeds, against a table of allowed rate of change, or a timer based RPM limit, but both are a compromise.
Given all of this what do people think will actually work as a launch control system for 4wd cars?
That's all, good stuff. Thanks for the Launch Aim Switch plan Andre
The M1 also has the switch option for the A & B rev limits, so I might use that as well.
Jamie on my old 4WD drag car I used the rpm rise rate function in the M800 to control or limit wheel spin in 1st gear and it was remarkably effective - Actually I've implemented that on every 4WD drag car I've been involved in tuning. This is a quick and easy feature to tune hence it doesn't make it hard to configure - you don't need a mechanical engineering degree to understand it or set it up.
Adding the option of putting this rate of change against another axis (either time or wheel speed) would allow a more thorough option for tuning for those who wanted to take it to the next level. That would allow a dumbed down version and a more complete option in the same function.
I understand the limitations of GPS however I'm interested to know if anyone has tried implementing the Race Grade 20 Hz GPS (or similar 20 Hz GPS) in a custom package for launch control? I feel like it has potential and I'm interested to hear how it pans out in the real world.
The GPS lag is far too long to be useful for this sort of thing. The attached image shows a launch with raw GPS values at 10hz with a GPS that I believe is at least as good as the racegrade units.
The I suspect that the RPM rate of change (or wheel speed rate of change) method is going to be the best compromise, but I thought it was worth asking if anyone out there had some great ideas on how it could be done better...
Thanks for confirming the GPS lag Jamie. Pretty clear from that logging that it would be next to useless.
I know GPS is pretty useful from taking a AIM sports datalogging training, they use alot of algorithms to predict G forces and acceleration based on GPS data. but in this case when the wheels can spin but car is not moving wont tell you much. Maybe its useful to see how long the car spins before it actually starts to accelerate. Could this be used to tell if the launch is overloading the tires? just throwing some ideas out there.