Spektrum Telemetry "Roll Out" Issue

[BlackWolf]

I tested out the telemetry unit in my Jammin CRT today. The speed in MPH was way off.

QUESTION: When figuring out the roll out, do you use the car's INTERNAL gear ratio (3.3) or the FINAL ratio (14.5). I have the sensor mounted to pick up off the spur gear. Why would I use the FINAL ratio to figure the roll out if I'm using the spur gear as the reference; the ratio changes with different clutch bells. When I use the manual's technique, I get an incorrect speed (150+MPH). Why would I need to use the final ratio to calculate roll out if I have the sensor on the spur gear?

Maybe you could shed some light on this: Internal ratio (diff/crown gear): 3.30 Pinion ratio (clutch bell/spur): 4.43 = final ratio: 14.52.
Wheel diameter is 5.5 so 17.27 is the circumference. If I use a roll out from the final ratio, I get 5.2. If I use roll out from final ratio, I get about 1.2.

When I use 1.2 as the roll out, the speed seems closer to correct (still off though) but: I'm using the spur as pickup, not the clutch bell. Clutch bell size shouldn't make a difference if I'm taking the reading off the spur gear... Please throw me a bone here. Either this thing is not calibrated at the factory, or I'm doing something totally wrong.

Dave

[vtl1180ny]

Ok... You've got me confused...

Do you have the sensor picking up the RPM between the Diffs and the spur gear?

If you do the only figures you need are tire diameter and final drive ratio.

So Tire diameter divided by FDR is how many revolutions the magnet will make...

Try 3.898

I have both the Nomadio and Eagle Tree and both go about things differently, which I'm sure the Spektrum does also....

[BlackWolf]

Ok... You've got me confused...

Do you have the sensor picking up the RPM between the Diffs and the spur gear?

If you do the only figures you need are tire diameter and final drive ratio.

So Tire diameter divided by FDR is how many revolutions the magnet will make...

Try 3.898

I have both the Nomadio and Eagle Tree and both go about things differently, which I'm sure the Spektrum does also....


Why would I use the FINAL drive ratio? That figures in the clutch-bell for that solution. I'm reading off the SPUR-GEAR.

http://home.comcast.net/~aruguete/sensor_on_spur.jpg

Anyways, I'm going to get a radar gun from somebody and see how accurate the spektrum telemetry MPH feature is. I wish you could adjust roll-out to the 1/100ths (like 1.34 instead of 1.3). We'll see. Thanks for the input vtl1180ny. It's much appreciated.

[vtl1180ny]

Ok... You have to take into consideration any gearing between the Magnets/Sensor and tires...

I'll give you a few examples (although the math won't be accurate)

I have a sensor mounted on the rear axel of my 12th scale, since there are no gears in between my only concern is the actually Tire diameter and Rollout.

On 2 of my TC's I have the magnets on the pinion cups on the front diff case. So, to figure out that I need my Rollout and then I divide my Rollout with the Final Drive ratio since there are gears inbetween the magnets and the wheels.

So, if my 12th scale had a rollout of 30mm for every revolution of the magnets the car will move 30mm...

On my TC I have a 30mm rollout but a gear reduction of 1.5:1 in the diff case I need to divide the rollout by the FDR... 30/1.5 is 20mm... So for every revolution of the magnets my TC will roll 20mm....

When mounting a speed sensor never mount if Before any 2 speed assembly. You will only be able to set a number for 1 of the 2 speeds.

Nomadio actually gives me the option to get a lot more accurate than my Eagle Tree recorder ever did...

For my eagle tree it would only figure out speed based upon 1 revolution of the magnet, Nomadio lets you multiple both the Rollout # and Magnet Rev's number by a higher number which is great if you end up with a rollout of like 30.7mm... So I multiply my rollout and Rev's by a number of like 500. My figures would be 15000 for Rollout and 500 for magnet revolutions....

I hope this makes sense to you.... I'm not at my computer with my actual figures stored on to give you exactly what I use...

[ducati777]

I tested out the telemetry unit in my Jammin CRT today. The speed in MPH was way off.
Dave

Your right about measuring off the spur gear takes the clutchbell size out of it. Thus you're only worried about the ratio in the diffs and the tire size.

Next up, it appears you drilled a hole in the spur for the sensor to read? Is this an optical sensor? If so I see the spur has other holes in it... are they interfering?

[vtl1180ny]

It's not Optical, he drilled a hole to fit a Magnet in the spur gear...

[Ball Racing]

Jammin truggy spur gears are drilled from factory, and the spektrum came with a "sticker" to put there,
do you think he has a hole right into the mesh of the gears? come one :)

[rsilvers]

The manual says to measure how many inches the car moves forward for each sensor pulse. That is all it wants to know.

The easiest way to check it is with a hand-held GPS on it.

I am having trouble getting mine to work, and I assume it is picking up false reflections from the screws on my spur gear.