Flatshift on Gravel

Gents,

I am having difficulty getting the flatshift to work on gravel in a rally car. It works great on tarmac but as soon as we are on gravel I’m getting about 33% success.

The car is an 04 Subaru STI with Kaps Sequential, Vipec v88, and an easyshift giving closed loop.

I suspect the engine cut isn’t enough to get the gear change on gravel but wanted to get some direction before giving it another try. Thanks!

Gravel actually creates quite a few problems with gear cut control, particularly in the lower gears. The reason we see problems is because often in the lower gears there will be some amount of wheel slip present. When the cut is initiated the torque is removed from the wheels and hence the slip is eliminated. This results in the loading remaining on the drive side of the dogs in the gearbox and it's hard to initiate the change. There is no real solution sadly as a longer cut will help but you'll have a very slow shift. Even in the realms of WRC this is an issue and you'll find a workable solution is to just use the clutch lightly to aid the upshift. You don't need to back off the throttle like a conventional shift as the cut will still take care of this but punching the clutch has the effect of unloading the dogs and initiating the shift.

Thanks for the reply Andre. I wonder if has something to do with the design of the gearbox. Last year I raced a new STI that had a PPG sequential and M150 that shifted like butter on gravel. Comparing settings between the M150 and Vipec and the only thing I spotted different was the Motec reduced ignition retard the higher the upshift while the Vipec is static. Rocket Rally out of Canada did a lot of work with PPG refining that box so perhaps that's the difference. I'll play around a bit more otherwise do as you say with some clutch assistance.

From what I understand, reducing the back cut on the dogs can be really helpful with the shifts on gravel but I haven't experimented with this myself. Retard or cut are both really trying to reduce torque to unload the dogs but this won't be overly effective if there's a moderate slip % when the shift is initiated. Personally I like to use a moderate cut to get the shift initiated and then couple this with retard to smooth the torque reintroduction.

Quick update. Had a test day a few weeks back to work on tuning. Dialed in settings where Flatshift is working perfectly from 3-4-5-6. I've always clutched 1-2 anyway to be nice to the gearbox so I just carried that with 2-3. Made it about halfway through testing until I tore out the wedge collar from the throw out bearing. I suspect I was pretty aggressive with my clutch stabs which led to the failure. I ordered an ACT Monoloc collar that I'm going to fit to hopefully reduce the chance of that failure happening again (and try to be a bit smoother on the clutch pedal).

Next time I take the box apart I'm going to look at reducing the back cut and see if I can get 2-3 without use of the clutch.

Bear in mind that reducing the back cut will require precision grinding so that the torque is still evenly distributed between the number of dogs, but I'm sure you knew that.

Yup, thanks Chris! Good info for anyone that reads this thread.