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Jonny Eff

Dt03 Truggy Gearing

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Hey guys,

Hope someone can point me in the right direction.

I've truggied my Racing Fighter which is stock, except for metal bearings, and is running nimh battery. I pretty much exclusively run on grass.

First test run and it felt quicker than usual and I loved the handling. No surprises there.

The battery lasted maybe 15-20% shorter time. Again as expected.

Checked it over after playing on the postal racing circuit, which is short and lots of corners and throttle control.  Motor was hotter than usual, as we're the battery and the ESC. Again, nothing surprising as I was expecting that too.

So my question is how to improve it? I can't change the spur, and all I can do is change the pinion to 19t which will increase the rpm and exasperate the problem.  So I need to change the motor.

Now, I've scoured t'internet and looked at various stuff, all of which tells me about gearing, motors, kv etc. But I can't find anywhere to calculate the effect on the rpm with altered wheel size.

I know, from this forum, that the best solution is a 13t brushless with 2s lipo. It's an option, I'm building a tt02 which will be lipo so I have all the relevant stuff.

But what I'd like to know is HOW to make calculations so I can do it myself and compare different motors etc. I like to work things out and understand, rather than just be given an answer which works.

Sorry for the long post. Cheers.

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I have a DT03T too and short answer is you can't do anything about the gearing. 17t is the way to go as it's the shortest gearing possible. I think someome here put a 16t on which worked ok. If you want to try that get a steel 16t 32dp pinion from rw racing. 

This chart is my go to and super useful

2019-05-03_01-13-55

Its for brushless motors only, and it's a little on the tall side so maybe add 0.5 to each suggestion. The gearing with 17t pinion on the DT03 is 9.22. As you can see for 2wd truck that puts it between 10.5 and 13.5. In my experience (i have run 13.5 and have a 10.5 in mine right now) 13.5 is where its at for these. My 4000kv 10.5 is really too fast for the chassis, and if honest, for safety even in a large public park and i have to run zero timing on the esc or it gets really hot. I used to have a 13.5/3300kv motor and that was spot on. I need to go back to that really. 

FYI for your on road car, i race 17.5 brushless touring car, and neither of my cars is geared taller than 4.2, despite the suggested gearing being 3.5 on the chart. 

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Thanks Cy. That's really helpful, I think you've helped kick my brain into gear.

It's to do with the transmission ratio isn't it? As the wheel size increases, it reduces the transmission ratio, which in turn reduces the gear ratio as if a smaller spur or larger pinion was used?

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36 minutes ago, ThunderDragonCy said:

I think someome here put a 16t on which worked ok.

I've ran 16, 17 ,19 and 20t.

The 16T ran, but it only runs on around half the teeth height. So although it ran ,I wouldn't trust it to run high torque applications, and would expect a significant increase in teeth wear.

I'd go for a 10bl120 brushless esc, and see what motor you could find on eBay, brushless generally run cooler and have more torque.

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20 minutes ago, Jonny Eff said:

Thanks Cy. That's really helpful, I think you've helped kick my brain into gear.

It's to do with the transmission ratio isn't it? As the wheel size increases, it reduces the transmission ratio, which in turn reduces the gear ratio as if a smaller spur or larger pinion was used?

Exactly that. Bigger wheels effectively gear up the whole system, so you have to gear down the transmission to compensate. 

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15 hours ago, ThunderDragonCy said:

Exactly that. Bigger wheels effectively gear up the whole system, so you have to gear down the transmission to compensate. 

Seems I've gone full circle again, as I already knew that at the start! 😂

My process seems to be 

Read about gearing and roll out.

Scribble sums.

Get the same result and realise I've done the same thing, but in a different way.

Get annoyed with myself.

 

I'm still at a loss as to how to calculate how larger tyres will affect the motor. Everything I find is based on gearing and assumes we'll retain the motor and change either the spur or pinion. Very frustrating for me.

I guess the simple answer is that the charts, like the one you supplied above, were probably designed by trial and error, changing things and checking temps.

I suppose at this stage I'll run with people's experience. Not that I'm adverse to that, I just wanted to learn how to make the calculations myself.

So 13t brushless is the way forward? 

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I think the thibg with fearing is that it's not just the tyre diameter that's the issue. The larger tyres and wheels are also heavier, which requires for more torque to move, which is the thing that draws amps and generates heat. If yku want to work it purely on tyre size though, that's fairly straightforward. 

A regular buggy tyre is 88mm diameter for the sake of argument. Say Truck tyres are 105mm. The circumference of the tyres determines what distance the vehicle travelfor each revolution of the motor. Circumference is Pi x d. Pi is approx 3.142. So for the buggy 3.142*88 = 276mm. For the truck 3.142*105 = 330mm. So for every revolution of the wheel the buggy travels 276mm, the truck 330mm. So by swapping truck wheels onto your DT03 but keeping the internal gearing the same it travels 330/276 = 1.19 = 19% further for every tutn of the motor. Just to account for wheel size (ignoring weight effects for now) you therefore need a 19% lower FDR. So, if your FDR is 7 with buggy wheels, you ideally need to make 7 * 1.19 = 8.37. According the the chart, for a 13.5 motor it says go from 7 to 8.5 gearing between buggy and truck, which is about right based on this calculation. 

Hope that helps 

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4 hours ago, ThunderDragonCy said:

I think the thibg with fearing is that it's not just the tyre diameter that's the issue. The larger tyres and wheels are also heavier, which requires for more torque to move, which is the thing that draws amps and generates heat. If yku want to work it purely on tyre size though, that's fairly straightforward. 

A regular buggy tyre is 88mm diameter for the sake of argument. Say Truck tyres are 105mm. The circumference of the tyres determines what distance the vehicle travelfor each revolution of the motor. Circumference is Pi x d. Pi is approx 3.142. So for the buggy 3.142*88 = 276mm. For the truck 3.142*105 = 330mm. So for every revolution of the wheel the buggy travels 276mm, the truck 330mm. So by swapping truck wheels onto your DT03 but keeping the internal gearing the same it travels 330/276 = 1.19 = 19% further for every tutn of the motor. Just to account for wheel size (ignoring weight effects for now) you therefore need a 19% lower FDR. So, if your FDR is 7 with buggy wheels, you ideally need to make 7 * 1.19 = 8.37. According the the chart, for a 13.5 motor it says go from 7 to 8.5 gearing between buggy and truck, which is about right based on this calculation. 

Hope that helps 

Aha. You can tell I'm tired and brain not running properly at the moment.

Percentage comparisons makes sense. Nice one, appreciate it.

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