[HELP] Motors for Steering a Crawler

[dtoerien]

Hello. I am relatively new to LEGO and very new to Technic. I am particularly interested in the challenges presented by building rock crawlers. My first build was JJ2's 4x4 Super Cralwer V1, which was very rewarding. SevenStuds then kindly directed me to Yoraish's Little Boy Rock Crawler as an instructive build, which it certainly has been. Having just completed it, I have a number of questions that I have not been able to satisfactorily work out for myself and am hoping to benefit from community wisdom.

(I am using a rechargeable battery and a V2 IR receiver, along with the original 2 XL motors for driving and the M motor for steering. I am also using very cheap tyres that approximate Pro-Line Flat Iron 1.9" tyres as a test before forking out for the real ones.)

1. I understand, I think, that you might choose to use a M motor (or a L motor?) for steering if (a) you don't have a Servo motor, or (b) have space constraints and/or (c.) require greater torque. Are there any other circumstances where you would use a M or L motor for steering over a Servo motor?
   
2. I tried using a Servo motor instead of the M motor and it does not turn the wheels at all, although it attempts to. I assume that this means that it does not have enough torque? In Sariel's book, he mentions that the Servo motor has "huge torque" and so "won't be easily stalled" (p. 169). The M motor, however, easily turns the wheels. ​Does this mean that the M motor has significantly more torque than the Servo motor (Sariel's book doesn't appear to give the torque for the Servo motor, or if it does I missed it) If so, are there any circumstances where you would/could use a Servo motor for steering a crawler and how would you decide this other than trial and error?
   
3. Yoraish's
   has the Little Boy doing some pretty extreme crawling (at least it seems so to me) with little difficulty. I find it difficult to tackle a pillow in my lounge without it sounding like something is going to break while steering and have, in fact, managed to pop a wheel off (I experienced the same difficulties with the Super Crawler, which JJ2 kindly helped me to address). I gather that the M motor just keeps turning until you stop (but that unlike the Servo motor does not return to centre when released); this sounds distressing and seems like it could cause actual damage. Is there any way to overcome this? I ask because I and my son - 2 and 1/2 - find it difficult to concentrate on negotiating obstacles due to concerns about damaging something and/or bits popping off. Perhaps it requires greater skill than I/we have at steering? Sorry if I have not asked this particular question well.
   

Many thanks,

Darryl and Reuben

P.S. I think/hope I have done the images correctly.

P.P.S. Any other recommendations for refining this build would also be appreciated (DLuders was very helpful in devloping the initial thread - many thanks).

[tomek9210]

If you have installed Servo in the place of M motor and left the gearing as they are now with M motor, then it barely could move the wheels because of the gearing. Servo can turn left or right 45 degrees only, so 12 tooth gear barely turned the 36 tooth gear.

I prefer using M motor for steering because it is more precise and I don't like the return to center feature.

[Zerobricks]

Hi there

How does the current steering setup work for you? IMO it should have enoigh torque to steer the wheels, BUT a lot depends on the way you made steering hubs. The closer you bring the wheel to the steering pint, the less power you will need to steer and in the end a servo might be enough. Can you show more detailed pictures of the hubs?

[dtoerien]

Hi there

How does the current steering setup work for you? IMO it should have enoigh torque to steer the wheels, BUT a lot depends on the way you made steering hubs. The closer you bring the wheel to the steering pint, the less power you will need to steer and in the end a servo might be enough. Can you show more detailed pictures of the hubs?

Hello and thank you. FYI, I was very interested in building your crawler (karma.lxf) but didn't have a V1 IR receiver.

See below for the hubs (top and bottom view). The steering works really well, it's just that it over-steers very quickly; i.e., the motor keeps trying to turn but there is nothing left for it to turn. Perhaps the steering requires more finesse on my part?

[Zerobricks]

I think you can bring the wheel closer to the steering point by eat least a stud. Do you run out of the gear rack?

[dtoerien]

If you have installed Servo in the place of M motor and left the gearing as they are now with M motor, then it barely could move the wheels because of the gearing. Servo can turn left or right 45 degrees only, so 12 tooth gear barely turned the 36 tooth gear.

I prefer using M motor for steering because it is more precise and I don't like the return to center feature.

Thank you. Unfortunately the issue of gearing is another area that currently way beyond me. I had suspected as much, but it will take a little work for me to figure out the next step.

[Zerobricks]

Also why did you use only 12:20 gear ratio in the hubs? Increase it to 8:24 so the U joint wont suffer as much.

[PunkTacoNYC]

I have tried, in many different ways, to replace my super geared down (worm gear to 8 tooth, 12 tooth to 40 tooth) M motor with a servo in a setup like this. It seems simpler and more efficient but the problem is that the PF servo requires energy - a LOT of energy - to keep in its current position. So too often when I was turning and driving at the same time, the Servo would hog up all the power and, on top of the two XLs at full capacity, the bb would shut down. I'll try to use the servo soon but for now, geared down M and L motors on a gear rack are your best bet as they are rigid and (with a worm gear) lock in place when not turning.

[dtoerien]

I think you can bring the wheel closer to the steering point by eat least a stud. Do you run out of the gear rack?

Yes, i run out of gear rack very quickly, which is why it sounds like something is shredding or about to pop off. If the gear rack were longer, which I was thinking of doing, would't that create the same problem?

[Zerobricks]

Making gear rack longer can cause two things:

- making the gear slip if the surrounding bracing is not sturdy enough

- stalling the motor if the surrounding bracing is strong enough.

Running out of gear rack may sound loud, but at least you dont stall the motor or slip gears.

[dtoerien]

Making gear rack longer can cause two things:

- making the gear slip if the surrounding bracing is not sturdy enough

- stalling the motor if the surrounding bracing is strong enough.

Running out of gear rack may sound loud, but at least you dont stall the motor or slip gears.

Thanks, that is what I was starting to conclude. Your other advice is beyond me at the moment, so I will need to work through it more carefully.

I have tried, in many different ways, to replace my super geared down (worm gear to 8 tooth, 12 tooth to 40 tooth) M motor with a servo in a setup like this. It seems simpler and more efficient but the problem is that the PF servo requires energy - a LOT of energy - to keep in its current position. So too often when I was turning and driving at the same time, the Servo would hog up all the power and, on top of the two XLs at full capacity, the bb would shut down. I'll try to use the servo soon but for now, geared down M and L motors on a gear rack are your best bet as they are rigid and (with a worm gear) lock in place when not turning.

Thanks. Your link informs me that hotlinking is not allowed. I gathered from another topic that a worm gear might feature, but am not sure what this means.

[dtoerien]

Many thanks for the helpful advice. I'm beginning to appreciate the difficulties of dealing with this in the axle.

Edited May 2, 2016 by dtoerien

[JJ2]

1. I understand, I think, that you might choose to use a M motor (or a L motor?) for steering if (a) you don't have a Servo motor, or (b) have space constraints and/or (c.) require greater torque. Are there any other circumstances where you would use a M or L motor for steering over a Servo motor?
   
2. I tried using a Servo motor instead of the M motor and it does not turn the wheels at all, although it attempts to. I assume that this means that it does not have enough torque? In Sariel's book, he mentions that the Servo motor has "huge torque" and so "won't be easily stalled" (p. 169). The M motor, however, easily turns the wheels. ​Does this mean that the M motor has significantly more torque than the Servo motor (Sariel's book doesn't appear to give the torque for the Servo motor, or if it does I missed it) If so, are there any circumstances where you would/could use a Servo motor for steering a crawler and how would you decide this other than trial and error?
   
3. Yoraish's
   has the Little Boy doing some pretty extreme crawling (at least it seems so to me) with little difficulty. I find it difficult to tackle a pillow in my lounge without it sounding like something is going to break while steering and have, in fact, managed to pop a wheel off (I experienced the same difficulties with the Super Crawler, which JJ2 kindly helped me to address). I gather that the M motor just keeps turning until you stop (but that unlike the Servo motor does not return to centre when released); this sounds distressing and seems like it could cause actual damage. Is there any way to overcome this? I ask because I and my son - 2 and 1/2 - find it difficult to concentrate on negotiating obstacles due to concerns about damaging something and/or bits popping off. Perhaps it requires greater skill than I/we have at steering? Sorry if I have not asked this particular question well.
   

1. The main reason people use M-L motors for steering is the proportional of the steering instead of just one way right and visa versa.

2. The M-motor does not have more torque than the Servo motor but when geared down 3:1 it has good torque and if it stalls it stays in the same spot instead of returning to the center. Using a servo in crawlers is usually for smaller builds that are not very heavy because of the small turning angle and no way to gear it down.

3. Another factor to maybe the steering and drive problem is the 3rd party tires you are using, since they offer much more grip and the steering pivot is far outside the wheel they offer a lot of resistance against the steering motor so maybe try it using 9398's wheels and make the M-motor gearing 8-40 instead of 12-36.

Hope this helped

[SevenStuds]

Hi Darryl, have you gone through the solutions on the axle collection thread?

http://www.eurobricks.com/forum/index.php?showtopic=92099

Since you have a servo, I'm sure there will be a solution using it. Perhaps one where you don't need to gear it down.

When testing the steering lock, try first by hand, without a motor or wheels and with the classis in the air. That way you can get a good sense of the friction and rotation angles required to turn the wheels.

Good luck!