Wheel Slip / Tire Speeds
(and calibration for Motec/TC or similar free programable ECU's )
May 12/2009 - Wheel Slip vs. Wheelie Indication.
One of the most important Analysis Features for Traction Control, how to separate Wheel-Slip from a Wheelie:
A worn Tyre - Slip Detection
2 Minutes on track
Just an excerpt of a lot of logged values, what I want to show here is the Wheel-Slip, calculated by :
Wheel-Speeds front and rear Wheel ( with a accuracy of 40 samples / Impulses per wheel-turn )
and in conjunction with Throttle Pos
A precondition to get accurate Wheel-Slip Values is : you have calibrated both Tire-circumferences correctly.
To calibrate Tire-Speeds close to zero difference in reading you need to "roll" straight ahead at maybe 60-70 mp/h without Acceleration and declutched, so there's no load on the Wheels from no Side, a couple of seconds is enough, then you can see by data recording how far the speeds differ, and it just needs a simple rule of three to set the right calibration Factors into your ECU or logging device.
So - just rolling straight ahead on track with no load on the Tyres does normally not happen, that is why you see close to zero slip very seldom.
A next thing then - the rear tyre is more and more getting a smaller circumference than the front tyre the higher the lean angle is, this leads into some kind of shown slip ( up to a certain percentage , which differs a bit from the type of the Tire ), which is indeed no slip - it's just the measured difference between Tyre circumferences.
But above a certain percentage we can talk about real Slip, then the measured Tyre speed rear does not display actual the speed of the Bike anymore, means - the Tyre is spinning. But there's a 2nd Factor : when the fork is lifting the ground, the front tyre speed falls rapidly ( check the Picture ) and you also receive high Slip values . You can use this behavior to prevent undesired wheeling out of a corner with a good working TC.
A negative Slip you get at hard downshifting at higher speeds, then the rear Tire speed is decreased for a short moment ( check the Picture ).
I use no Slipper clutch, because I have some kind of "Air-valve" ( moreless same functionality as the 07/600RR and I think the 1098 has ), which is programmed to open at a certain level of negative Wheel Slip ( with some Variations depending on gear, speed, etc. ).
At hard adjusted Slipper clutches or no installed slipper clutch and/or road transmissions with wide spread gear ratios you can see negative wheel Slip up to -30%.
The higher the resolution for one turn of the wheel is , so the more samples per turn, the sooner you're able to react at undesired Circumstances ( which everyone can set individually ).
I.E. if you have 40 Impulses per wheel-turn and a speed of maybe 120km/h ( roughly 70mp/h ) , then - considered a rear wheel circumference of 2 Meters:
120km/h = 120.000 m/h = 33.33 m/s
That is 16,66 Wheel turns per second
16,6 WT/s x 40 Impulses = 666 Impulses/s ( from Sensor )
1 Second/666 Impulses = 0.0015 S , so 1.5 ms
Considered that a fast calculating ECU is able to manage that data-stream , and needs ~ 3 samples to calculate a undesired rate of wheel-spinning the ECU can react within a Time Frame of maybe 6-10ms - should be by far faster than any rider can manually respond by just closing the throttle a bit.
A bit more abstract here
Red is first deviation of throttle Pos ( so %/s)
Light blue TC working
Blue (4th row) Slip