Hi! Does anybody here have a direct experience in comparing speed from GPS vs speed sensor mounted on axle?
Andy, funny you should ask. I called AIM a week and a half ago because I wanted to utilize a collar and sensor on my rear axle so that I could calibrate my gear position with the kart on the stand. The rep told me that it “won’t work because the 5s 2T will default to the GPS for speed”. I pointed out their documentation states you can choose frequency of a speed sensor data read . I asked for clarification and haven’t received an email or call back. I do not think he was correct due to the axle collars with magnets they sell. Hopefully someone else can chime in for your question.
uhm, that’s weird for sure! Either plugging in directly or via expansion module, the 5s should allow speed to be picked up by the sensor…mmh maybe I’ll be your guinea pig…I’m doing some testing on KZ, in my case comparing some data around different primary, gearbox and final gearing and need precise speed readings in relation to engine RPM to test some stuff, and I think the GPS speed readings are throwing me off.
In short, you have engine RPMs, then based on primary/clutch gearing you transfer rotation to gearbox, then based on that gearing you transfer to secondary/driveshaft, then final gearing from sprockets you transfer to axle then based on tire circumference I should get exactly the speed I should be reading at that RPM as there is no other variable at that point. I’m testing some variants of primary and gearbox combinations, but speed is off vs what GPS tells me, by a fairly constant ratio through the combos, so I suspect GPS has some delay or error of sorts in measuring speed
Then again taking speed from an axle sensor doesn’t compensate for tire growth at higher speed tracks. Most likely the GPS will show a slightly higher speed.
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Try calculate the gear ratio with a calculator. I use this one, but search the one of your engine model. Hope it helps!
TM K9A, K9B, K9C, KZ10, KZ10B Shifter Kart Gearing Calculator.xls (37 KB)
I assume the tire diameter input parameter is in mm…?
No. The tire diameter is in cm.
Thanks for sending the file!
I think that excel file shows the circumference in inches…a new tire is about 87cm in circumference (not diameter), or 34.25 inches which is what that excel model shows.
My model and that model (using TM gearing and same final gearing/rpm) have a difference of 0.1mph so I think we are good and I’ve checked it also aligns with the homologation fiche.
Say max speed is 91.2mph +/- 0.1 per model. My mychron clocks 84 mph, at same RPM. If anything, tire stretch should make you go faster, so it’s even more off.
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Do kart tires expand in diameter at high speed?
Yes, if you rev it on the stand you can visually see the tire expand.
I dont think so, the kart is heavy. Also, if it expands,I don’t think it’s considerable, it will expand at most 1mm
Years ago and I really mean years ago, when I ran enduros someone snapped a photo of my kart at the end of straight, probably hitting 115 MPH. The tires looked like they grew at least 5% to 10%.
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That tracks with GPS data I have in memory from Daytona. 5% growth around 100mph.
At 100mph, that 5% increase in diameter is responsible for 4.8 mph, pretty crazy to think about
As in the expansion of the tire results in higher top speed? Or slower?
If the tire gets bigger, the surface area increases. So it would need a bit more distance to go 1 revolution.
Does that make it slower or faster?
Larger tire diameter (and thus circumference) effectively gives you a taller (lower numerically) gear ratio.
For the purposes of a speed sensor on rear axle, if you tell the data logger your tire circumference is X cm but at the end of a straight it has really grown to Y, then the speed sensor will under-report your actual speed.
GPS speed would not be impacted by this, although it has its own accuracy issues.
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Ah so there’s no actual change to speed, it’s just like a slightly different gear? More/less top end etc
Is this significant enough that one would need to adjust gearing for tire growth over the course of a race?
If present, it certainly impacts the effective gear ratio and thus the top speed you achieve.
However, you are effectively “blind” to it from a gearing perspective unless you change something about the tires or wheels. It’s basically a constant factor in what gear ratio achieves the best performance for a given track for your kart/driving/etc whether you know it is happening or not.
Change wheel widths and you change tire diameter/circumference. Swap tires for a different construction and you may get more/less growth at any given speed (or start out with a slightly different circumference at rest). Make any of these changes and you may well have to change gear ratio to compensate.
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I did a direct comparison of wheel speed and GPS speed in 2021 to check this exact thing. I’ll have to dig up the Excel sheet, but my conclusion was “the better 20 Hz GPS System on the 5S is really really close”
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The answer is yes, no and it depends. Like you said, it basically changes the gear. That may, or may not make the road speed faster depending on if the engine can pull that gear to the higher speed.
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