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Thanks to @TurnipJF and @ThunderDragonCy I now know how to calculate FDR. In fact after being told how to do it I found the same info in the instruction manual for my Asterion. Now I know how to work it out the next question is why do I want to acheive a specific FDR. For example the TT02-S that I am going to build is a road going 4wd. It will be fitted with either 13.5T or 10.5T brushless motor and with the gearing that I have purchased it will acheive an FDR of around 5.5. Thats all understood but what I dont get is Why? Does that give me maximum acceleration, top speed or what? I am also planning putting a 13.5T brushless motor in my Asterion. In stock form using a 06 module 21T pinion it achieves an FDR of 8.42. As mentioned it has the same internal gear ratio of 2.6 as the TT02. If I jump to a 29T pinion I can acheive an FDR of 6.10. Would that be more suitable for the type of motor that I'm planning and frankly why?. The Asterion is more likely to be used on loose surfaces so does that need to be factored in? In simple terms what outcome does a higher/lower FDR achieve? I have also put a brushless motor in my Blitzer should I be changing the pinion to get better performance, I believe there are only 2 choices? Sorry if these questions seem a bit basic but knowing how to something is one thing know the whys and wherefores is some thing else:unsure:

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I like to think of it as bicycle gears.    

If you are going down on a smoothly paved hill, you can use taller gears (like FDR 6).  You can get faster top speed without sacrificing too much acceleration.  If you are going UP a rough hill?  You can't even get the bicycle going with the same fast gear selection.  You'd want something like FDR of 8 at the cost of top speed.  Obviously most RC cars can't change gears, and it has to go up and down.  One ratio you choose has more to do with the motor selection and the terrain.   

Brushless motors have greater torque.  If the silver can was doing well with FDR 7, you can do 6 with 13.5t.  It depends your preference, terrain and temperature.  If it struggles to accelerate on high grip surface (but gets better top speed after a few seconds) that means the gear ratio is too tall.  If it was 5.5, you might want to change it to 6 or 7 to make it accelerate faster (but shaves off the top speed--there is no free lunch).  You can see the same effect when you put big tires.  I haven't seen big tires running with FDR of 5.5.  That could burn out the motor.  Big tires would want FDR 8, 9 or 10.  

If it bolts out quickly on high traction area with FDR 9 but top speed is disappointing, you might want to lower the ratio to 8 or 7 or 6.  It would bolt out with less gusto, but the speed at the top-end would increase.  

Loose surface has low traction.  Tires are not locked onto the surface.  Taller gearing won't overheat the motor as much.  You might feel disappointed with sluggish start of FDR 5.5 on high grip surface, but if the surface has low traction, you might be okay with 5.5 because the tires will spool without much grip anyway.  (Just like getting out of snow or mud with 1:1 cars. You can get going on 2nd gear. But if you did the same on dry road, you are likely to kill the engine.)   

So... FDR 5.5 is for on-roaders only.  Off-roaders generally tend to be at around 8 to 10.  If you are going to put bigger tires on a buggy with FDR 7, make it FDR 8, 9 or 10.  Conversely, if you make an off roader with FDR10 into an on-road car by putting on smaller tires, make it FDR 6 or so.  Brushless motors are stronger, so you could afford to go from FDR 7 to 6.  In the end, it's your preference.  If you are shooting for land speed, you'd never want FDR 10.  But if you want to quickly dash about left and right, without reaching top speed, you might want FDR 10.  

 

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FDR is only useful when discussing vehicles with same sized tyres.

If you want to compare cars with different sized tyres, convert to "Rollout" (= FDR x circumference of tyre)

 

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To add to @Juggular explanation, the FDR will also depend on the motor. Motors have an optimal point where they operate best (don't even start on adjusting the timing). Usually this will be a range of say 3.5 - 4.5 FDR (for a touring car in 21.5T blinky). This may require a change of spur gear and pinion to get this range (but thats fine on a modern Touring Car as its easy to swap them out). You would go with 3.5 for a track with long straights and not much infield. 4.5 for a small tight track where acceleration is key. Probably end up somewhere in between for most tracks. You can adjust on the day depending on how hot your motor is getting (3.5 may mean the motor is working too hard and overheat it) how fast your competitors are (you find that you keep up in the infield but get blown away on the straights, then lower FDR) etc.

Brushless motors can run crazy low FDR compared to brushed motors. Most Tamiya buggies would be suited to 8.5T - 13.5T motors (Boomerang for example, biggest pinion would suit a 10.5T motor). I run FDR 5.5 in my 4wd race buggy with s 17.5T motor, and am probably going to go lower as my new motor is very torquey. Basically in this car its heat that determines it, a 17.5T motor always accelerates well so its going for the max speed.

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I was shown this afew years ago to use as a rule of thumb

2019-05-03_01-13-55

In practice i have found that these recommendations are top end and you often end up using a lower gear. For instance, in 17.5 touring car it recommends 3.5, but even on a large outdoor track everyone at my club uses 4-4.5. I run 4.4. I run 6.7 for a 13.5 in my Thunder Dragon 4wd buggy, which is a little lower than 7 in the table and the motor gets hot after grass running, but is generally ok if i don't have loads of timing on it. 

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Ive got my tt02-s set at 5.9 with a 64t spur and 28t pinion. With the bearings flushed of grease and lubed with bearing oil and a smooth shimmed drivetrain the car is a rocket with the torque tuned motor and coming off the track after 5 min at about 110f

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Generally speaking you will want to run the lowest possible ratio without overheating the motor on any typical run. If your getting to hot or thermaling out your esc you need to move up the ratios a bit till you find a happy medium. Every motor is different you cannot say that one 13.5t  is equal to another 13.5t in terms of its rpm and or its gearing requirement. In many circumstances this can be because of differences in timing, but often the winding or motor structure does vary. There can be as much as 500kv difference between manufacturers. 
 

of course the other limiting factor is wheels and tires, the outside diameter of the tire will alter your required gearing. Additionally drivetrain load and weight will again effect your gearing choice. I generally just start either with the kit stock gearing or the recommended gearing “from the motor manufacturer”. From there I gauge performance, if it’s lacking I check engine temps, if things are really cool I’m way under geared, dropping the gear ratio until I find a balance of heat management, run time and overall speed and torque production. 
 

a undergeared brushless motor will not produce more torque than a correctly geared motor, this is because the torque of a brushless motor is load based, it does not produce torque without load. Of course you can overload the motor at which point maximum torque is lost. There is a balance point. And this is the fine line we run especially in those high turn race classes where we pushing final drives as low or lower than 3.0.  
 

it is likely in many cases we run less than optimal gearing for whatever brushless motor we own, either because the car doesn’t have the options, we don’t have on hand the right gears, we don’t care, or we don’t know.   
 

A really good comparison of why your FDR matters, i have on hand a number of different brushless motors. The most stark contrast I’ve come up against was I have onisiki 3200kv 13.5T motor in one my on TT02’s, I ran it on the stock gearing (8.04:1) and it was beastily fast but run quite hot, and would overheat my TBLE-02S with a fan, it didn’t last long maybe 3-4 runs then went into cogging badword and stopped working properly, so I had to replace it. 

Not wanting another onisiki (the 2nd that had failed me) I picked up the rather boring looking but slightly more expensive hobbywing quicrun 3656 13.5T. Wacked it in the car... man this sux, slow, gutless, I’m thinking Whats that all about then?. Picked up the manual and read it (who does that? unless something goes wrong right?) at that point I realised this motor was 2700kv compared to the onisiki’s 3200kv. Looking at the gearing it dawned on me, hobbywing says 4.0:1 to 4.7:1, so I swapped the closest I had on hand  which gave me 5.74:1, and the car was transformed, I had all the power and speed from the onisiki, but without the overheating esc and longer run times (so there was a bit more to be had with even lower gearing). 
 

it’s pretty simple in the end, watch your temps and you can figure out the gearing relatively easily. 

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On 1/20/2020 at 11:14 AM, ThunderDragonCy said:

I was shown this afew years ago to use as a rule of thumb

2019-05-03_01-13-55

In practice i have found that these recommendations are top end and you often end up using a lower gear. For instance, in 17.5 touring car it recommends 3.5, but even on a large outdoor track everyone at my club uses 4-4.5. I run 4.4. I run 6.7 for a 13.5 in my Thunder Dragon 4wd buggy, which is a little lower than 7 in the table and the motor gets hot after grass running, but is generally ok if i don't have loads of timing on it. 

Of course depends all on the weight of the vehicle. On my F103LM, I use a 4.5T mod motor with a gearing of 1.87FDR 

according to calculations top speed should be 190km/h

but on my t1fk05 the internal is 1.7 + 87/26 so a total of 5,04FDR with a 10,5T brushless.

 

 

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can we continue this thread on a second grade level please.:unsure: some of us need to have this broken down without the math so we get it then get more in depth, just sayin. like taller vs top speed vs acceleration, vs spur tooth vs pinion tooth vs final gear drive vs Whats that all about then?.....  i will be the sacrificial lamb. i am not stupid, but i need to learn this. i cant learn it from an advanced calculus third year seminar. 

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Right, let's give this a go. My apologies if I simplify things too much - just trying to be clear, not condescending.

So, let's start with two imaginary cogwheels. Forget the rest of the car for now. Just imagine them mounted on shafts in a simple frame or something that keeps them meshed.

Now let's take a closer look at our imaginary cogwheels. One of them looks bigger than the other. If we count the teeth, we find that the smaller one has 10 teeth and the bigger one has 20 teeth.

Now imagine that we turn the cogwheels by hand. We notice that each time we turn the smaller one through one full revolution, the bigger one only turns half a revolution. In order to make the big one turn one full revolution, we need to turn the smaller one through two full revolutions. So we have a 2 to 1 ratio. If the small cogwheel was connected to a motor and the bigger one to the axle of the car, like on a typical 1/10 F1 chassis such as the F104 for example, you would have a final drive ratio (FDR) of 2.

This is very demanding on the motor, so if we want the motor to have an easier time of it, we can make the spur gear bigger, say give it 40 teeth instead of 20. Now the pinion gear connected to the motor needs to turn 4 times to turn the spur gear once. You would now have a FDR of 4.

We could have achieved the same thing by making the pinion gear smaller, giving it 5 teeth instead of 10. It would still need to turn 4 times in order to turn the 20 tooth spur through one full revolution. The FDR is still 4.

Now as we know, all but the simplest chassis have more than just 2 gears. This complicates matters a bit, because they are typically not all the same size. Diff ring gears are bigger than diff input gears for example. So say your diff ring gear had 20 teeth and your diff input gear had 10 teeth. The input gear needs to make two revolutions in order for the ring gear to rotate once. This again gives a 2 to 1 ratio.

Now imagine a standard shaft drive touring chassis such as a TT-02 for example, where the spur gear spins along with the propshaft and diff input gears. Imagine this has been fitted with our imaginary cogwheels from earlier, with the smaller 10-tooth one connected to the motor as a pinion and the bigger 20-tooth one connected to the propshaft as the spur. Turning the pinion once makes the spur turn half a revolution, and this half revolution makes the diffs turn a quarter of a revolution. The pinion needs to turn through 4 full revolutions for the diffs to turn through 1 revolution. Thus, even though the spur to pinion ratio is 2 to 1, and the diff ring gear to diff input gear ratio is also 2 to 1, the FDR is again 4.

This is a very simple example, using nice easy numbers to calculate, but few chassis designs make it this straightforward. There may be multiple gears of different sizes, belts, pulleys, etc. Thus we usually find a table in the manual, telling us what spur and pinion combinations result in different FDRs. We also often have tables in the motor manuals telling us what FDRs are recommended for different motor ratings in different vehicle types. So we look up the FDR we need for a given motor in a given vehicle, say 5.5 in a 13.5t touring car for example, then we look up the spur and pinion combo that gives us a FDR as close to this as possible, say a 25t pinion and 55t spur for example.

The higher the FDR, the easier it is on the motor, so the car will accelerate quicker but have a lower top speed, current draw will be less so temperatures will be lower and runtimes will be longer. Conversely, a lower FDR will be harder on the motor but the car will have a higher (theoretical) top speed. Acceleration will be more gradual, current draw will be greater so temperatures will be higher and runtimes shorter. (I say theoretical top speed, as it is possible to over-gear a car to the point that the motor doesn't have enough torque to do anything at all except possibly melt.)

Therefore, if we are unsure what FDR we need or what FDR will result from a given pinion and spur combo, it is better to start with a big spur and small pinion, and work from there, as this will give a high FDR, which has no negative consequences apart from a slower-than-optimal car. Starting at the other end of the spectrum isn't advisable, as this can result in overloaded/overheated motors, ESCs and batteries.

I hope this all makes sense and that I haven't made any glaring errors. Please feel free to correct me if I have!

 

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On 5/16/2020 at 5:20 AM, TurnipJF said:

This is a very simple example, using nice easy numbers to calculate, but few chassis designs make it this straightforward. There may be multiple gears of different sizes, belts, pulleys, etc. Thus we usually find a table in the manual, telling us what spur and pinion combinations result in different FDRs. We also often have tables in the motor manuals telling us what FDRs are recommended for different motor ratings in different vehicle types. So we look up the FDR we need for a given motor in a given vehicle, say 5.5 in a 13.5t touring car for example, then we look up the spur and pinion combo that gives us a FDR as close to this as possible, say a 25t pinion and 55t spur for example.

The higher the FDR, the easier it is on the motor, so the car will accelerate quicker but have a lower top speed, current draw will be less so temperatures will be lower and runtimes will be longer. Conversely, a lower FDR will be harder on the motor but the car will have a higher (theoretical) top speed. Acceleration will be more gradual, current draw will be greater so temperatures will be higher and runtimes shorter. (I say theoretical top speed, as it is possible to over-gear a car to the point that the motor doesn't have enough torque to do anything at all except possibly melt.)

Therefore, if we are unsure what FDR we need or what FDR will result from a given pinion and spur combo, it is better to start with a big spur and small pinion, and work from there, as this will give a high FDR, which has no negative consequences apart from a slower-than-optimal car. Starting at the other end of the spectrum isn't advisable, as this can result in overloaded/overheated motors, ESCs and batteries.

I hope this all makes sense and that I haven't made any glaring errors. Please feel free to correct me if I have!

 

That has worked for my tiny non-engineering brain, thanks!

It also helped explain why the FDRs shown in the Super Astute manual are completely different to those shown in the DF03 slipper clutch manual, despite both using an 82T spur and .05 mod pinions. As you explain it, it's not just the spur and pinion playing a part, there are other factors in the gears of those two chassis that create the final FDRs

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