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4wd launch control strategy in dirt or sand

Launch Control

Relevant Module: Worked Examples > MoTeC M1 > Ground Speed Launch Control Tuning

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Discussion and questions related to the course Launch Control

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Have been watching all I can on launch control strategies and just haven’t seen anything for unstable surface drag racing like sand or dirt. I have seen some data recently that would suggest maximum tire speed displacing sand (or dirt) isn’t launching harder. Since GPS isn’t accurate enough, and we are spinning all 4 tires, do you have any idea’s we could try to achieve maximum acceleration? Unfortunately a dirt track can change drastically from damp dirt that hooks well to a freshly watered track with significantly less traction. I’ve been contemplating something that ties rate of rpm increase to G force, so if G force isn’t showing an aggressive launch, but rpm is increasing rapidly a map that pulls timing and or limits rate of RPM change. Am I over thinking this? MoTeC M150 with MoTeC C127 dash with logging enabled and a MoTeC GPS are on the machine as well as wheel speed sensors, transmission input RPMS’s and more to work with.

Good question! It's something i've noticed on dirt circle track racing that the normal driving style is to keep the tyres spinning, yet when a drive may have had to back off slightly and then get back on the throttle (s)he will often hook up MUCH better and drive right past an opponent!

Not something I've looked at, but I expect it's something you're going to need to experiment with, as there are so many variables to consider.

If you make detailed notes of the ground conditions - the sort of surface it is, how wet/dry it is, if you're doing an early or late run, etc, with the tyre pressures you're running, the tyre type and tread wear, etc, then you should be able to spot trends.

You can also pay careful attention to other competitors, and see how they line up, how they get off the line, etc, to see if there are any trends you can pick up on.

Assuming you are using GPR firmware you would likely want to use Race Time Vehicle Speed with Race Time Mode set to Launch.

The Vehicle Speed Estimate channel will be set to the values in the Race Time Vehicle Speed table, Vehicle Speed Estimate is available as an axis on the Launch Engine Speed table.

These can both be linked to multi position driver switches so that you can vary the launch engine speed limit and effectively the maximum allowable engine speed relative to time after the launch


I get the feeling what you're hoping for is a comprehensive setup that will optimize mid pass, which is a challenge in a 4 wheel drive situation. Can you clarify if it is in fact 4 wheel drive or all wheel drive, and if all wheel drive what the torque split and lockup situation is, and some info on the transmission?

First, thank-you for these idea's, we're installing an 8 position keypad, and I will certainly save some buttons for adjusting launch engine speed. Appreciate the thoughts on track and vehicle data to record and how to visualize a comparison on passes down the track also. I've got a lot to learn on the data tracking capabilities of the M150 and C127 Race dash.

Mike, more details on the vehicle; A Honda Talon Side-by-Side, this is 4wd with Honda's i4wd which will bias either front brake to drive power away from the slipping tire. The rear axle is locked there is no differential, the front axle is locked to the back with an electrically operated clutch, there is no differential between the front and rear axles. It is a 6 speed Dual Clutch transmission.

Control of this transmission is a steep learning curve for me, but where I also see the most potential.

In our racing pre-MoTeC the car did pretty well but we're really depending on the driver a lot and I'm hoping to automate more of it to be more consistent. Ramping in torque at launch and yes, mid-pass traction are certainly 2 area's that I'd like to get better control of. It is my understanding the GPS speed data isn't accurate enough to do a lot with, so just searching for other ways to accomplish some of this.

Wheel Speed sensors versus GPS. The Yellow trace is the GPS signal.

GPS Versus Wheel Speeds by Stephen Dean, on Flickr

0.4 seconds of lag between the wheel speeds and GPS.


GPS Time lag by Stephen Dean, on Flickr

Sounds good, I think time based is going to be your friend along with driver adjustable launch RPM targets, not just for staging, but throughout the launch.

Just to confirm, the center sounds like an e locker, not an electronic clutch pack that you can pulse to vary lockup?

The center could be pulsed, to date I am unaware of anyone trying it to get an understanding of life cycle for the part if it were allowed to slip. The clutch resides right at the input pinion for the front differential.

Gotcha. If you wanted to get into adjusting it, I'd measure the current to the coil at full diff lock and see how high it is. If it's within the range of a half bridge you have available, and your package has diff control or something you can repurpose as diff control, that's one option.

Another option is if it's 10 amps or less, I use a device which accepts a ground PWM out and converts it into coil control for electronic diffs, and then you could 3d map it with your dash if your current ECU package doesn't give you access to a control strategy that suits this.

One anomaly jumps out on a quick look - the rear diff is supposed to be locked, but there is a significant variation between the left and right rear wheels, which would be impossible if that were the case - are the traces incorrectly marked, or am I mis-reading it?

Hi Gord,

Are you talking about the data captures above? If so, they are from my car, not Tim's.

How is it going Tim?

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