The force that the gear transfers is calculated from the equation T=F*radius; the final ratio has nothing to do with this. In fact if you change to higher mod same teeth count (increasing diameter), the force transferred decreases, even if you have the same ratio.
In the 200A 175kv mod1 17T case its
10.9Nm=F*0.0085m
F=10.9Nm/0.0085m=1282N
Thats a lot of force to be transferred by a single teeth.
This is assuming the motor has 100% efficiency and that 10.9Nm is actually 10.9Nm of course. I won’t even try predicting efficiency at such high torque but chances are its not anywhere near 100%.
But physics says that at ~11Nm torque, the 17T mod1 pinion is also destined to fail, but instead of from undesirable wear characteristics, it’s from fatigue accumulation this time.
Actually chatgpt is pretty decent for mechanical engineering calculations if quoted well so here’s what it spit out
TLDR we are past the 25-30% strength of the material where steel has 10^8-10^9 cycles, hence fatigue accumulation is quite quick and only 10^5-10^6 cycles are expected at max load, for the calculated stress ratio which is over 1.
Note that here for cycle life it assumes that full acceleration at 10.9Nm shaft torque is the only valid state of the skateboard, in practice it lasted me around 2000km of riding.
The key to sizing the diameter of the smallest pinion for “infinite” cycle life is to look at shaft torque and stay under ~25% material strength. For ~11Nm torque, I’d need about 27mm diameter to stay under the stress limit (mod1 27T / mod1.5 18T)
650kv at 375A is around half the shaft torque so gives a stress ratio well below 1. So actually math says for this build mod1 17T is strong enough to not fatigue early.