# Wheel Speed Sensors

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I have read lots of other threads on wheel speed sensors and them having too many teeth / too high a frequency.

Help files says:

"Typical values for speed calibration numbers range from about 170 to 280.  The maximum frequency the digital input can accept is 500Hz"

In my logs I can see 1600Hz before the Fury gives up.

Rear: 50 tooth, 650mm diameter = calibration 2441

Front: 50 tooth, 490mm diameter = calibration 3248

That's a LONG way from the 170-280 typical.

No, the Xtreme DI's are limited to 500Hz.  common ABS rings with more than about 20 teeth will be too many for these.  If you dont need ABS then you can grind some teeth off to make them work.

So I think that's what I'm up to, I don't need ABS but would like the wheel speeds to keep an eye on tyre slip (was getting up to 20% last year)

What's the math on removing teeth?

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if you have 50 teeth and calculate a calbration number of ~2400, but ~240 is more in the ballpark, cut off 9 out of every 10 teeth and you will drop your frequency to 1/10th what it is now.

You can "test" this but logging via GPS etc what speed you are actually doing when the frequency on the DI hits 500Hz or some other easily divisible number.

Lets say at 500Hz you are doing 30kph, but your car is expected to max out at ~250kph. again dividing by 10 will give you a max of 300kph at the recommended 500Hz, so again you would cut off 9 teeth and leave every 10th.

The key this is the teeth MUST be evenly spaced 50 doesnt divide into too many things evenly so you probably only have a choice of 1/5 or 1/10. If you had 48 teeth you would have a lot more options.

If you are handy with electronics you can build a frequency divider with about \$5 of parts from jaycar. 1x for each wheel. The important bit is a 4024 chip - this will let you take a frequency and output a lower frequency anywhere from 1/2 to I think 1/128th

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I was also curious why I logged 1600Hz on a channel that was meant to have a 500Hz limit, wondering if I can rely on the 1600Hz as the "real" limit?

Your math helps, thanks.  I get it now. 1, 5, 10 or 25 tooth are my options with these ABS discs.

At 111 meters a second with a tyre circumference of 2060mm the wheel is doing 54 revs per second and 2700Hz on the sensor with 50 teeth.

With 10 teeth its 540Hz and 5 teeth 270Hz but I doubt I could keep the integrity of the disc with just 5 or 10 teeth on it.  1 tooth would be more do able.

Frequency divider sounds more like the right option but I doubt I'll get it done in time.  I think it falls into the "future project" list now.

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I've tested something similar myself and from memory got about 1300 or 1400Hz reading on a DI before it just started reading 0. I'm assuming the 500Hz recommendation allows for a bit of error (ie doesnt blow up if you hit 501Hz), or maybe was the real limit on some older ECU models and the new ones are a bit more refined and can read up to a bit over 1000Hz. I wouldn't want to rely on it being accurate at frequencies above 500Hz.

If you can move the sensor away from the teeth a bit, and the teeth are thick enough you may be able to drill into just 1 (or 5) of them and press in a metal dowel pin that sits a few mm higher, then set the sensor at the right distance so it only reads the teeth with dowel pins in them. I've got no idea what thickness metal etc your're working with to know if this is a good idea though.

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Very clever left-field idea of moving the sensor out from the disc, that might be do able.

Gives me something to think about, thanks CJ

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The max frequency that the DI's can do is related to how busy the processor is.  So our quoted 500Hz limit is worst case.  A V8 sequential, direct spark with individual cyl fuel and ignition trims will get a lot less frequency than say a basic 4 cyl with distributor set up.

25teeth would probably work for you reliably by the sounds of it.

Here's a cheap divide by 8 device that I have used before:  https://www.vems.com/vr-to-hall.html

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Good one, that clears-up the frequency difference between the help file and the logs.

Considering my front wheel wasn't moving when I was on the dyno, the logged 1600Hz on the rear will have less available cycles if the front wheel is also logging.  I think I'm back to one tooth or 5/10 if I can get small bolts into the slots and offset the pickup.

Thanks

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Took the ABS disc off (errr, and the brake disc).  Put two "bolt, nut, nut" in every 10th hole with loctite.

Added 3 custom washers to the sensor, and used a longer bolt.

Spun by hand and it reads 10kmh which was about what I could spin by hand on the full 50 teeth so it looks like it's going to work okay.

Front looks little trickier but I'll do the same.

Thanks for the help fellas.

Andy.

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