LRM Transmission question

[orangejulius]

First off, awesome forum. Looks like I found the place I have been looking for ( I have several rc10 cars, as well as 3 jrx variants) Even though was sitting in an elementary school classroom when these machines hit the market, I still have huge respect for the roots of this hobby. They just don’t make things like they once did.
On to the question:
I have one jrxt set up as a monster truck. Modern silver can 15t, imex road hog tires, and very mild gearing. The car was fresh built with NOS and used parts to this configuration (everything checked out at time of build). I was only able to get about 3hrs of run time out of the setup (mild off road in the yard, mostly parking lot road courses, no bashing, motor well within operating temps, proper dif setting) A striped diff gear ended the fun. Do you think I am putting too much stress on the trans with these tires (heavy, rotating mass, large dia)? Trans looked good otherwise, bearings were tight, idlers and shaft gear were fine. Anything else I can check out prior to running? Any upgrade options? I had a heck of a time finding a dif gear, and would hate to grenade this one as well.

Thanks for the help,

Chris



(please excuse the poor quality picture)

[jwscab]

I would think you would be fine running the truck mildly, but I know that the center diff gear on the LRM is kind of the weak point, I have had my share of melted ones.

If you are using a modern 15t and running for long periods of time, I suppose the gear will take a beating, especially with the heavy tires. One thing that will help tremendously would be to fit a slipper clutch on the trans. This way in situations where the gear would strip, the slipper instead would function to prevent that from happening.

I'm not sure what is still available for the LRM these days, someone here on the forum running one should be able to point you in the right direction.

[kaiser]

i remember melting diff gears back in the day, but with my current jrx2 i have had no issues.

granted my x2 is a 17.5 powered vintage racer so it lives a pretty easy life, only see's track time a couple of times a year.

[orangejulius]

Thanks for the comments! Jwscab, a slipper is a very good idea. What setups are still available for the LRM (or can e retrofitted without much hassle) ?

[jwscab]

Losi made a slipper setup for the LRM trans, as well as the hyrdadrive, it was actually both systems in one, and you could run hydradrive and slipper, or just the slipper. I'd suggest start looking on ebay, and vintagelosi.com for other info, such as manuals and part numbers.

otherwise, as I said, someone who knows this stuff better than I should give you the info. my losi knowledge is very rusty.

[mytimac]

When you say stripped, what part of the gear are you talking about. The teeth on the gear, the center hole of the gear, or the little diff balls made a large donut of the gear?

If teeth, then probably put to much stress on the teeth from jumps or slamming from reverse into forward or vise versa.(trying to do a wheelie or spin outs) This would mean, stop doing crazy stunts with the throttle at wide open or get a slipper setup to reduce strain on gear.

The original design was not great for the a3006, not terrible, but not perfect. It has no bearing for the center of the gear. This would cause lots of wear and friction in the center of the gear. They fairly quickly offered a bearing version of the same gear a3020.

If you made a big donut from the small diff ball area, that is too loose of differential and you need to tighten it more probably.

[orangejulius]

Mytimac, thanks for the input. I in fact striped the differential gear (this would be the actual teeth on the outside of the gear). I have jumped this truck from time to time. Nothing extreme by any means, but I am thinking you hit the nail on the head. If I had to venture a guess, I would say the weight and increased diameter of the tires (think leverage) puts a larger shock on the drive train over stock sized components. I have never done quick forward / reverse maneuvers with this rig; forward only ESC installed. But I can certainly see how this would cause damage in a slipper-less setup.

So with that said, is it safe to say I can stop looking at the trans for reasons that would increase gear mesh distance? I was unsure if there was a common flaw with the trans that contributes to this issue or not. Looks like a slipper is a good solution; any suggestions?

[mytimac]

Check the top gear on the input shaft. Is it nice and smooth, not chewed up? Bad bearings that all the parts rotate inside the transmission on? If a bearing got chewed up and introduced wobble to the top shaft or the whole differential assembly that would cause the issue as well. Check all your sealed bearings. I have had them just fall apart inside.

Are all the areas that the bearings rest in the transmission housing in good shape?

[Lonestar]

The LRM was also in the LXT Truck

Did you run the ball-bearing equipped a3020 or the plain a3006?

Paul

[orangejulius]

I have the a3020 installed in this rig, (a3006 in the other runner, stock size tires, same running conditions, no issues). Top shaft, gear, and other parts check out. Looking into slipper options now.

[highwayracer]

Check the top gear on the input shaft. Is it nice and smooth, not chewed up? Bad bearings that all the parts rotate inside the transmission on? If a bearing got chewed up and introduced wobble to the top shaft or the whole differential assembly that would cause the issue as well. Check all your sealed bearings. I have had them just fall apart inside.

Are all the areas that the bearings rest in the transmission housing in good shape?

This good advice...and wanted to add:

The gearbox will wear on these cars and the place that holds the the top shaft bearing will begin to "oval". Once this happens, the spur / pinion gear mesh will need to be corrected (brought closer). As this is adjusted over time, it will pull the diff gear away from the top gear. Eventually, the teeth will wear and eventually fail. This has happened to all of my gearboxes...and part of my maintenance would be to change the gearbox halves every few weeks.

Now, when I rebuild a LRM gearbox, that's the first thing I check.

hope this helps....

Regards,
highwayracer