9398 - Lego Technic 4x4 Crawler

[z3_2drive]

My plans are to hook up two l motors in the middle of a chassis, have a four link setup for both axles, and have servo directly on axle, but its only worth it if 2 l motors have more torque than 1 xl motor. Do they? and does this idea sound possible?

[DLuders]

...if 2 l motors have more torque than 1 xl motor. Do they?

STALLED TORQUE of Power Functions motors (from Philo's "LEGO 9V Technic Motors compared characteristics" webpage):

L-Motor: 18 N.cm [Newton-centimeters]

XL Motor: 40 N.cm

2 x L-Motors = 2 x 18 N.cm = 36 N.cm (less than the 40 N.cm of a Power Functions XL motor).

Edited July 8, 2012 by DLuders

[z3_2drive]

STALLED TORQUE of Power Functions motors (from Philo's "LEGO 9V Technic Motors compared characteristics" webpage):

L-Motor: 18 N.cm [Newton-centimeters]

XL Motor: 40 N.cm

2 x L-Motors = 2 x 18 N.m = 36 N.m (less than the 40 N.m of a Power Functions XL motor).

Thanks! Well, I guess I'll just use the xl, unless I give l motors more gearing. I forgot they ARE faster but xl with less gearing is simpler-so many pros and cons, I'll just do both!

[Jetro]

STALLED TORQUE of Power Functions motors (from Philo's "LEGO 9V Technic Motors compared characteristics" webpage):

L-Motor: 18 N.cm [Newton-centimeters]

XL Motor: 40 N.cm

2 x L-Motors = 2 x 18 N.cm = 36 N.cm (less than the 40 N.cm of a Power Functions XL motor).

However, if you look at mechanical power at 9V you get

L-Motor: 1.85W

XL-Motor: 2.21W

2 x L-Motors = 2 x 1.85W = 3.7W (considerably more than one XL-Motor

[efferman]

My plans are to hook up two l motors in the middle of a chassis, have a four link setup for both axles, and have servo directly on axle, but its only worth it if 2 l motors have more torque than 1 xl motor. Do they? and does this idea sound possible?

you will imho dont have an advantage when both motors drive both axles. during my test i have seen that it works better when both axles has its own motor.

btw, my "no diff axle for 9398" instruction is finished

[Sariel]

I finally found out that my Crawler came with a faulty battery box. The same batteries with another battery box give it much more power. My apologies for misjudging this set. I have never come across problems with the battery box before.

[efferman]

very interresting. have you an idea what is the problem? maybe not every batterie was connected to the cuircuit?

[Sariel]

very interresting. have you an idea what is the problem? maybe not every batterie was connected to the cuircuit?

Yeah, I'm not really into electrics but it looks like the power was coming from 5 batteries instead 6.

[davidmull]

I finally found out that my Crawler came with a faulty battery box. The same batteries with another battery box give it much more power. My apologies for misjudging this set. I have never come across problems with the battery box before.

That's great u found the problem, I was thinking ur crawler was very poor when I seen ur video.How did u figure out the problem?

Edited July 8, 2012 by davidmull

[Sariel]

That's great u found the problem, I was thinking ur crawler was very poor when I seen ur video.How did u figure out the problem?

I have noticed that my L motors perform much better when powered from 8878 battery than when powered from a battery box. So I tried swapping Crawler's battery box for another one, and it worked.

[DLuders]

I have noticed that my L motors perform much better when powered from 8878 battery than when powered from a battery box.

Perhaps the lighter weight of the 8878 Power Functions Rechargeable Battery Box offsets the lower output voltage (7.4V). Maybe the 9398 4x4 Crawler is built TOO ROBUSTLY (to handle rolls and abuse by kids), so if one could LIGHTEN the chassis or body shell, Crawling performance would increase.

Edited July 8, 2012 by DLuders

[Sariel]

Perhaps the lighter weight of the 8878 Power Functions Rechargeable Battery Box offsets the lower output voltage (7.4V). Maybe the 9398 4x4 Crawler is built TOO ROBUSTLY (to handle rolls and abuse by kids), so if one could LIGHTEN the chassis or body shell, Crawling performance would increase.

Less weight = worse grip. No, the performance with another battery box is just as good as with 8878. It was apparently just one box what was faulty. I'll be uploading an outdoor Crawler's video in a moment.

Here it is. Besides improved performance, this video demonstrates why differentials are so bad:

Edited July 8, 2012 by Sariel

[DLuders]

@ Sariel: That's a nice "torture test"! To prevent the 9398's stock tires from spinning inside of the Lego 44772 wheels, one could use CA (Cyanoacrylate Adhesive, or "Superglue") along the rims -- that's what RC Monster Trucks use. Do you think that the top-heavy 9398 could benefit from WIDER AXLES, to prevent it from tipping over?

Edited July 8, 2012 by DLuders

[Sariel]

@ Sariel: That's a nice "torture test"! To prevent the 9398's stock tires from spinning inside of the Lego 44772 wheels, one could use CA (Cyanoacrylate Adhesive, or "Superglue") along the rims -- that's what RC Monster Trucks use. Do you think that the top-heavy 9398 could benefit from WIDER AXLES, to prevent it from tipping over?

It sure could. A quick way to make it more stable is to use the Power Puller wheels, which are wider and heavier, so they lower its center of gravity. But they also eat more torque than any other wheels.

[efferman]

oh works much better than mine at stock. hm it seems that my batterybox has the same problem like yours, or did you power it with non recheargable batteries (9V) instead accus (7,2V)

[DLuders]

@ Sariel: Do you think there is a way of incorporating your "3-Studs Wide Differential Lock" into the 9398's drivetrain? I don't yet own the set, so I can't try it myself. The Lego 9398 and 8297 models are very similar, and you wrote:

"An idea on how to lock the new, 3 studs wide differential in a compact, reliable way.

The 8297 set came with a new, 3 studs wide differential, which is already a third generation of Lego differentials. It’s narrower than the previous ones, and much easier to transfer the drive to, but lacks any specific means to lock it (at least at the time of the 8297 set’s release). The solution shown here is the most compact locking mechanism I was able to develop so far. It relies on the transmission driving ring, which is a rare part but very easy to engage/disengage. It’s so compact that the driving ring actually touches the differential case, but the resulting friction is minimal thanks to its smooth surface. The 16-teeth and 20-teeth gears are well meshed and capable of handling a considerable torque, while their ratio reduces the torque transferred to the transmission driving ring by 20%. I suppose the driving ring and the differential have similar torque resistance, but the driving ring is a much more expensive loss."

Edited July 8, 2012 by DLuders

[efferman]

there is definitely no space. only with a wider axle it would be possible

[Sariel]

hm it seems that my batterybox has the same problem like yours, or did you power it with non recheargable batteries (9V) instead accus (7,2V)

It has the LEGO 8878 battery, running at 7.4V.

@ Sariel: Do you think there is a way of incorporating your differential lock into the 9398's drivetrain? I don't yet own the set, so I can't try it myself....

Not without a major redesign of the whole model. The axle's structure is very compact, with no space to really add anything, and you would need another IR receiver + a motor to lock & unlock the differentials. I honestly think you'll be better off by removing the differentials completely and inserting knob wheels instead - this will keep all wheels under power at all times, while impairing the handling - but it's one or the other, really.

[davidmull]

oh works much better than mine at stock. hm it seems that my batterybox has the same problem like yours, or did you power it with non recheargable batteries (9V) instead accus (7,2V)

Guys is it the standard batterie box that's the problem? That seems 2 of ye have same problem! Or is it ur batteries?

Less weight = worse grip. No, the performance with another battery box is just as good as with 8878. It was apparently just one box what was faulty. I'll be uploading an outdoor Crawler's video in a moment.

Here it is. Besides improved performance, this video demonstrates why differentials are so bad:

I have too say I can't see what people are expecting from this set,it works great in the video and can climb most obstacle s u put it up on,I'm very impressed with that video even if the differentials is a small issue,u seem to have great power now too :) sariel u must be more impressed now? :)

Edited July 8, 2012 by davidmull

[z3_2drive]

you will imho dont have an advantage when both motors drive both axles. during my test i have seen that it works better when both axles has its own motor.

btw, my "no diff axle for 9398" instruction is finished

Thank you for the instructions, I will definitely use this when I get the 9398, but I will still see if I can mount the servo directly on the axle of my new crawler:

[5150 Lego]

Less weight = worse grip. No, the performance with another battery box is just as good as with 8878. It was apparently just one box what was faulty. I'll be uploading an outdoor Crawler's video in a moment.

Here it is. Besides improved performance, this video demonstrates why differentials are so bad:

I don't see a problem. Yes, you got stuck, but by taking a different line you seemed to be able to get over every obstacle you attempted.

I'd like to see someone fully lock the diffs and do a comparison on a tight course. Also curious if the CV's could handle the stress of locked or eliminated diffs.

[Zerobricks]

I don't see a problem. Yes, you got stuck, but by taking a different line you seemed to be able to get over every obstacle you attempted.

I'd like to see someone fully lock the diffs and do a comparison on a tight course. Also curious if the CV's could handle the stress of locked or eliminated diffs.

I dont quite agree less weight means less traction. The traction coefficient is same, but the tyres are less loaded and therebye contact patch is smaller with a lighter vehicle. And yes diffs suck offroad, its like you always have 2wd instead of 4wd.

[mrklaw]

How about adding another IR receiver and driving each axle motor independently? Then on a difficult obstacle you could effectively ''brake' the front or rear and drag it round to help overcome poor turning circle if you lock the diffs?

[Sariel]

Yes, you got stuck, but by taking a different line you seemed to be able to get over every obstacle you attempted.

Not every. Also, by taking a different line I need a lot of time and a lot of space for each obstacle. Differentials suck big time.

its like you always have 2wd instead of 4wd.

And you only have power on the wheels that do nothing at the moment - like, rotate in mid-air.

How about adding another IR receiver and driving each axle motor independently? Then on a difficult obstacle you could effectively ''brake' the front or rear and drag it round to help overcome poor turning circle if you lock the diffs?

I don't think you can get enough power to drag anything round if you don't have enough power to drive through an obstacle in the first place.

you will imho dont have an advantage when both motors drive both axles. during my test i have seen that it works better when both axles has its own motor.

I strongly disagree. When both motors drive both axles, then every single axle can receive the total power of two motors. With separate motors, you can't have more than single motor's power per axle. Consider a simple situation: one of your axles gets stuck with both wheel in the air, as it happens with small ground clearance sometimes. The motor on this axle does nothing then, and the other motor on the other axle is your only hope. So you depend on the power of a single motor, even though you have two. I believe I've seen over a hundred TrTr vehicles and a drivetrain that connects motors together always works better, simply because motors can help each other.

Edited July 8, 2012 by Sariel

[davidmull]

Did someone say the Lego says do not use 8878 batterie pack?if so why?