# Thread: Drive Ratio in Traxxas Link App - How to Calculate it, what does it mean???

1. ## Drive Ratio in Traxxas Link App - How to Calculate it, what does it mean???

Hi, I run a heavily modified Stampede 4x4 (has come a long way since its first purchase in Sept 2012) and use the Traxxas TQI remote with Iphone and Traxxas link software for telemetry. In the Traxxas link software, you can change your spur and pinion gear sizes, tyres, esc and motor types. There is also an option to change your drive ratio which is set at 2.846 by default. My question is what is this drive ratio number and what does it mean? I've moved from the stock 11t / 54 spur to an 18t/54 spur with the switch over to the Castle 2650kv motor and Mamba Monster 2 Watrerproof ESC. The spur is the stock 32p (TRA3956) 54T. The pinion is the stock Castle 18T (MOD8) pinion, which I may move to a 20T once I start running the new combo (still putting it together).

Dividing 54 by 18 = 3.0 but this is not the number that needs to be input into this drive ratio setting. The default 11t when divided by 54 gives you 4.91 which clearly shows the value of 2.846 is something else. Does anyone know what this value is?

If you look at the gearing chart in the Traxxas manual, it doesn't refer to this "Drive Ratio" figure but the combo of spur and pinions. The stock 11t/54 spur gives you a value of 13.97 and an 18t/54 spur gives you a 8.54 value.

All this still doesn't explain where the 2.846 drive ration comes from??? I'm sure there is a simple explanation to this.

Extra info which is probably useless. If you divide 13.97 / 2.846 you get 4.9086. If you then divide the value of 8.54 from the Traxxas chart by 4.9086 you get 1.739. So my question still remains on whether the "Drive Ratio" in the link application should change from 2.846 as a result of changing to an 18T pinion.

Hopefully haven't confused everyone...

2. So are there any Traxxas tech staff on these forums and if so can they help answer my question please

3. The 2.846 is the gear ratio for the diffs, and doesnt change with your pinion/spur gearing. Kinda like a transmission. You will always multiply this by the pinion/spur ratio.

54 ÷ 11 = 4.91 x 2.846 (drive ratio) = 13.97 (overall drive ratio)

I've never really payed attention to all those settings because I don't run anything stock. My plans are to calculate speed via the GPS Module when it is available for sale.

Thats the way I look at it. If i'm wrong let me know. I didnt really explain it that well, hopefully not confused you more.

4. how do they calculate speed on the tqi from just engine rpm when the trans has 2 speeds ? or is it not really accurate speed on the tqi without GPS ?

5. I also want to know now the speed is calculated. I have a stock set up, was doing 31 ish, then moved to an 18T pinion from the 13T stock Slash 4x4 and now I'm at 34, I'd have thought the change in pinion would have resulted in a more significant change in top speed. I change the pinion set up in the link app, but maybe I need to do something else different.

Paul B

6. can we get some solid answers on this please

7. Originally Posted by boydsc331
how do they calculate speed on the tqi from just engine rpm when the trans has 2 speeds ? or is it not really accurate speed on the tqi without GPS ?
I am wondering how this works with the 2 speed tranny as well?

8. Anyone ? ........................ Bueller ????

9. Frye.............Frye..............

10. Its an estimate of speed based on RPM using stock set up. On the summit, it only works on high gear and not low gear.

When one selects the model, the default spur, pinion, drive ratio, tires, battery, and ESC are put into the Model Setup. While things can be changed, there is a limited number of tires sizes that can be put in based on what Traxxas sells.

To get real speed, one would need to know tire circumference and rotations per minute. In the absence of that, one can only estimate it based on approximations using knowledge of pinion and spur gear as well as axle ratio (which may be part of the drive ratio calculation)

Slash drive ratio 2.7, Summit drive ratio 5.2 for high gear, don't know what it is for low gear, ERBE 5.2

This is really good only for RPM and for showing bystanders a speed, but if one really needs to know the speed, one really just needs to get a GPS that records max speed and mount it onboard during speed runs.

11. Here is where I go for easy gear ratios, and I saved it on my phone so I can change them and look them up if need be but I have the ones I used almost in memory now. Love the link though and it saved me from going bonkers...Bueller...Bueller...anyone...anyone...;-)

http://www.robinsonracing.com/gearratio.html

12. Hey Paul, what battery are you running, and how fast does it look like it's going with the 18? Mine hits 74 mph on 3s Lipos with 18-54 gearing. L8R, Michael

13. Some notes below on Final Drive Ratio calculation and a link.

Calculating speed based on RPM telemetry is not accurate due to all sorts of factors, including artificially elevated speeds when slipper is loose allowing motor to really rev to high rpms. The speed display is mostly for show. If you want truly measure speed, put a GPS in (i use my iphone well padded but would be cheaper to use a small gps like a garmin fortrex 301 or 401).

Selection of tires and motors is limited. Tire ballooning affects rollout (how far the truck will go per turn of the pinion)

More importantly, calculating the final drive ratio is just as important but cryptic.

Final Drive ratio = gear ratio x transmission ratio
gear ratio = spur / pinion
transmission ratio = differential gear / input gear

http://www.google.com/url?sa=t&rct=j...58187178,d.aWM

14. Originally Posted by Dementor
Some notes below on Final Drive Ratio calculation and a link.

Calculating speed based on RPM telemetry is not accurate due to all sorts of factors, including artificially elevated speeds when slipper is loose allowing motor to really rev to high rpms. The speed display is mostly for show. If you want truly measure speed, put a GPS in (i use my iphone well padded but would be cheaper to use a small gps like a garmin fortrex 301 or 401).

Selection of tires and motors is limited. Tire ballooning affects rollout (how far the truck will go per turn of the pinion)

More importantly, calculating the final drive ratio is just as important but cryptic.

Final Drive ratio = gear ratio x transmission ratio
gear ratio = spur / pinion
transmission ratio = differential gear / input gear

http://www.google.com/url?sa=t&rct=j...58187178,d.aWM
Agree with your post. I've just ordered two of these GPS speed measuring loggers for both my Stampedes so I can get some comparison measurements.

http://www.skyrc.com/index.php?route...&product_id=43

Don't know how good they are but they are super light and are made for RC enthusiasts. They also capture everything you need such as average speed, top speed, etc. Will see how they go.

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