Relation inner gear ratio and drivetrain resistance

[Tamiyastef 1966]

As I remember correctly (it was 1989, so a long time ago) the drivetrain of my Turbo Optima Mid felt smoother than the drivetrain of my nephews Avante. I always thought this had to do with the fact my Mid used 18 bearings in total (only 6 in the drivetrain) and the Avante used 22 in total (10 in the drivetrain).

I recently saw someone on Youtube stating the inner gear ratio of the Super Avante is 2,6 and this got me thinking. I calculated the inner gear ratio of the Turbo Optima rerelease to be 3,79. So theoretically using the same spur and pinion on both cars (for example 60 spur and 20 pinion to make the calculations easy) you get a higher gear ratio on the Turbo Optima (60/20 x 3,79 = 11,37) and a lower gear ratio on the Super Avante (60/20 x 2,6 =7,8). So the Optima will have better acceleration and the Avante higher top speed.

I think this also indicates the drag or resistance created in the Super Avante's drivetrain is higher than in the drivetrain of the Turbo Optima. This would mean if you would match the gear ratio's on both cars (by running different spurs and pinions) the Turbo Optima would have better acceleration and a higher top speed than the Super Avante running the same motor. Is anyone with me or am I completely mistaken? 

[M 800STD 586]

Or could this be due to belt being more efficient than shaft?

[Juggular 4964]

I think it's a bit more than that.  

For example, you can have pinion 10, spur 20, spur to diff is 1:1.5.  Gear ratio is 2.5, but it's a bit too tall to be smooth. What if the spur to diff is 10:20, would it be less smooth because the FDR is 4?  What if you have a bunch of 1:1 gears like Konghead? What if you stretch the chassis to include 10 more 1:1 gears?  Even if you lower the gear ratio, It's not going to accelerate smoothly because it's got so many gears.  

But you are onto something.  To reduce internal gear's resistance, I replace the lubrication of bearings with something lighter.  

Let's say each rotation of a bearing can add 0.01g of friction.  The motor can turn 12,000 rpm.  That'll add 120g of resistance.  That's as heavy as a stack of 20 quarter coins per minute.  If the gear ratio with spur is 1:3.5, then the spur gear turns 3400 times per minute. Only 34g of resistance.  If the spur to diff is 1:2, then it only turns 1700 rpm, only 17g of resistance.  This is why you could find a worn out pinion gear, but you'll never find diff's ring gear worn out.  It's seeing only 1/8th of the action the pinion is seeing.  I pick out 2 fastest rolling bearings and assign them closest to the motor.  

If you need 22 bearings, there is no need to spend $220 for full ceramic bearings.  You just need 2 of them, closest to the motor where they will see the most rotation, and the rest can be ordinary steel bearings... Actually, you don't even need ceramic bearings at all.  You can just take off shields and apply tiny bit of Teflon grease. If the gearbox is sealed, no need to cover the bearings either. For 90% less price, you can approach 90% of ceramic bearing's efficiency. But it involves 90% more hassle. (there is no free lunch!)

The other thing is the gap between teeth. Gears slide on each other's surfaces. If you look at below GIF for minutes like I do, first, you'll get dizzy. Second, you'll see that the wear is not uniform between pushing and pushed tooth.  By the time the contact point moves half way on the pushing tooth, it has gone 2/3 on the pushed tooth. For the other half on the pushing tooth, only 1/3rd is moved on the pushed tooth.

For 10 meters of tire rolling, there could be 1 meter of gears skidding on each other, if you count all the gears. You can almost double that for 4x4, triple it for Konghead. For Dynahead, for 10 meters of tire rolling, gears could be skidding 10 meters. (37 gears, as opposed to 3 gears of the Lunchbox)... Talk about drivetrain resistance!  The more gears you have, the more important the grease becomes. (I use Teflon). 

Fine tooth gears are smoother.  Big tooth gears have more gaps. (some of it is engineering too) Slops in several gears can also add up.  If there are more gaps between dog bones and drive cups, they could add up.  So, Kyosho might be using finer gears and there might be less gap in dog bones/driver cups (belts don't have gaps like metal parts do).  

But it's not hopeless for Tamiya drivers.  If you want, you can re-lubricate bearings. I choose the fastest turning ones for the gearbox. You could use Teflon grease for gears. I don't have any Kyoso to compare, but doing those two things can make things a lot smoother.  

 

[OoALEJOoO 3127]

8 hours ago, Juggular said:

Let's say each rotation of a bearing can add 0.01g of friction.  The motor can turn 12,000 rpm.  That'll add 120g of resistance.  That's as heavy as a stack of 20 quarter coins per minute.  If the gear ratio with spur is 1:3.5, then the spur gear turns 3400 times per minute. Only 34g of resistance.  If the spur to diff is 1:2, then it only turns 1700 rpm, only 17g of resistance.

I think (resistance) friction in bearings works differently and is normally measured in torque as opposed of force. The resistance indeed goes up with speed but you can't really stack the force linearly for each revolution as you describe. Typical bearing behavior is as follows:

Good application design implies choosing bearing that allows its duty cycle to perform where the friction vs. shaft speed is mostly flat (left side of the curve). You don't want to operate at a speed where the friction shoots upward.

I hope I didn't sound too nerdy

[OoALEJOoO 3127]

A bit of follow-up. Tamiya's plastic bearings are of the journal bearing type, which typically offer the highest friction and vibration, particularly once they start wearing out. Upgrading to ball bearings dramatically reduces the amount of friction and wear.

As @Juggular points out, you really want to have your best ball bearing at the location of highest shaft speed (i.e. closest to the motor).