# Calculate % clutch slip

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Hi all,

I have a question about % clutch slip calculation. I want to calculate my % clutch slip using my ECU log file.

I’m logging Wheel speed (on output shaft) and also Engine speed (RPM).

Is it possible to manually calculate % clutch slip? I thought export data to excel and do the calculation.

I don’t have a spare DI. Thanks,

Dolfo

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Hi Dolfo,

Yes, this will be possible. You are going to need to know the engine speed, driven wheel speed, selected gear ratio, differential ratio and wheel circumference to determine the gearbox input shaft speed. Here is an example with some assumed values:

• Driven wheel speed: 100 km/h
• Selected gear ratio: 0.93
• Differential ratio: 3.875
• Wheel circumference: 2.041m   (circumference = π * wheel diameter => 3.14 * 0.65m => 2.041m)

First we need to determine the rpm of the driven wheel:

100 km/h => 1.667 km/m => 1667m/m

wheel speed/wheel circumference = wheel RPM:

1667/2.041 = 816.76 RPM.

The wheel RPM then gets multiplied by the differential ratio to give the output shaft speed of the gearbox:

816.76*3.875 = 3164.95 RPM

The output shaft speed then gets multiplied by the selected gear ratio to give the input shaft speed of the gearbox:

3164.95*0.93 = 2943.40 RPM

You can then compare the engine speed to the calculated input shaft speed to determine your clutch slip. If your engine was doing 3100 rpm, and your calculated input shaft speed was 2943.40 rpm, then your clutch slip % would be:

Clutch slip = ((Engine Speed - Input shaft Speed) / (Input shaft Speed)) *100)

Clutch slip = ((3100 - 2943.40) / (2943.40) )*100

Clutch slip = (156.6 / 2943.40) *100

Clutch slip = 5.32%

Scott

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 bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbHi Scott,Thanks for the explanation!! I forgot the say it was for a motorcycle clutch slip calculation. What I don’t understand is the: Selected gear ratio and the Differential ratio.The rest of the calculation is clear. Attached is a gearing worksheet of my motorcycle. Maybe you can tell me what I need to use for selected gear ratio and differential ratio for my calculations. Is selected gear ratio the ratio per gear? And differential ratio (17/43)?Wheel circumference: 1.356m   (circumference = π * wheel diameter => 3.14 * 0.43m => 1.356m)First we need to determine the rpm of the driven wheel:100 km/h => 1.667 km/m => 1667m/mwheel speed/wheel circumference = wheel RPM:1667/1.356 = 1229.24 RPM.The wheel RPM then gets multiplied by the differential ratio to give the output shaft speed of the gearbox:1229.24*?? = ?? RPMThe output shaft speed then gets multiplied by the selected gear ratio to give the input shaft speed of the gearbox:??*?? = ?? RPMSo far I came with my calculations.Thanks,Dolfo

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Hi Dolfo,

I didn't realise you were on a bike. Your Front/Rear sprocket ratio (2.529) can be used for your differential ratio. The ratio for the selected gear depends on which gear you have selected in the log, for example if you were in 6th gear you would use 1.269.

Scott

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Hi Scott,

Does this look right to you? I see clutch slip % goes into minus (negative) slip what is the explanation for this?

Wheel speed has a delay before it will start reading. What can I do to get it to read as soon as the bike starts moving forward? This causes a big number of clutch slip % at the start of the run.

Thanks,

Dolfo

 Section Time Engine Speed Gear Speed -DI2 Speed Back wheel Output Shaft Speed of the Gearbox Input Shaft Speed of the Gearbox Clutch Slip % 4.5 6967 0 0.3 4 9.328 25.09 27666.19 4.52 7102 0 0.3 4 9.328 25.09 28204.22 4.54 7290 0 0.3 4 9.328 25.09 28953.47 4.56 7491 0 0.3 4 9.328 25.09 29754.53 8.26 9488 3 148.3 1823 4611.022 7903.29 20.05 8.28 9664 3 149 1832 4632.786 7940.60 21.70 8.3 9541 3 150.1 1845 4666.988 7999.22 19.27 8.32 9630 3 150.8 1854 4688.753 8036.52 19.83 8.34 9674 3 151.5 1862 4710.518 8073.83 19.82 8.36 9684 3 151.5 1862 4710.518 8073.83 19.94 8.38 9827 3 153 1881 4757.156 8153.77 20.52 8.4 9801 3 153.3 1884 4766.484 8169.75 19.97 8.42 9827 3 154 1893 4788.249 8207.06 19.74 8.44 10026 3 155.1 1907 4822.451 8265.68 21.30

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Hi Dolfo,

I just went through one line of your chart (8.26) and I end up with the same clutch slip % as you. If the clutch slip is negative then the input shaft speed must be more than the engine speed. This could happen if the rear wheel is driving the input shaft while the clutch has disconnected the engine from the input shaft, and the throttle is closed so engine speed falls lower than input shaft speed.

I expect that the speed delay is because the signal coming from the speed sensor is either too low in frequency or amplitude at the point the bike starts to move forward. How many teeth does the speed sensor see? If you have access to an oscilloscope you could try and measure the amplitude of the speed signal at slow speeds.

Scott.

Edited by Scott
fixing my mistake
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Scott,

You’re correct about the negative clutch slip. I can see it in my data. I was off the throttle from 100% to 40% and lower. That’s when I get the negative clutch slip.

The speed sensor see 4 teeth. I will try to get access to an oscilloscope so I can measure the amplitude of the speed signal. How can I make this signal stronger? Maybe get the sensor closer to the pickup?

Thanks,

Dolfo

 Section Time TP (Main) Engine Speed Gear Speed -DI2 Speed Back wheel Output Shaft Speed of the Gearbox Input Shaft Speed of the Gearbox Clutch Slip % 6.74 44.9 8491 1 114.1 1403 3547.657 9543.20 -11.03 6.76 31.1 8281 1 111.6 1372 3469.926 9334.10 -11.28 6.78 22.6 7603 1 104.7 1287 3255.387 8756.99 -13.18 6.8 18.7 7332 1 104.7 1287 3255.387 8756.99 -16.27 6.82 20.8 7687 1 99.3 1221 3087.488 8305.34 -7.45 6.84 24.9 8465 1 101.1 1243 3143.454 8455.89 0.11 6.86 29.4 8256 1 105.4 1296 3277.152 8815.54 -6.35 6.88 32.8 8074 1 107.6 1323 3345.556 8999.54 -10.28 6.9 40.9 8191 1 105.8 1301 3289.589 8848.99 -7.44 6.92 54.8 8256 1 107.2 1318 3333.119 8966.09 -7.92

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Hi Dolfo,

Yes, getting the sensor closer to the teeth is the best way, another option is to change the sensor for a different sensor that has a higher output. This would take some experimentation.

Scott.

____________________________________________________________________________________________________________________________________________________________________________________________

One thing to watch for, we recommend a maximum frequency input of 500 Hz to our normal digital input channels. With 4 teeth and an input shaft speed of 9543 rpm you are at 636Hz. There are a couple of ways to reduce the frequency:

• Use a frequency divider circuit.
• Reduce the number of teeth. You could possibly go to two teeth.

The problem with these solutions is that they reduce the pulses at low speeds, so this could make the problem you have at low speeds worse.

Scott

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• 3 months later...

Hi Scott,

I decided to make my life a bit easier and bought myself a new fury ecu. Now I want to configure the parameters so I can log clutch/TC slip % of my motorcycle. My problem is I don’t have a speed sensor on the input shaft to measure the frequency. I have a speed sensor on my output shaft. Is it possible to measure clutch/TC slip % with my current setup? It’s impossible to put a speed sensor on the input shaft.

Thanks,

Dolfo

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Hi Dolfo,

Good work on the new ECU  The problem with using the output shaft instead of the input shaft is that the ratio between the input and output shaft changes depending on the gear you have selected. You could set the clutch/TC slip % to work for a particular gear.

Scott

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• 4 weeks later...

Hi Scott,

Maybe you can put this idea on the wish list for the engineers to take a look at?

Some ECU's have the option to put the gear ratios to calculate clutch slip using a speed senor on the output shaft.

Thanks,

Dolfo

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Hi Dolfo,

I'm happy to ask the engineers to consider this. The ECU already has a section where the gear ratio of each gear can be entered, so possibly they could build on this.

Scott

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