Pivot Turn / Stall Help

I have modified a mower from a John Deere SST15 that may have not been the best choice. I was given this donor from a neighbor for free, so the price was right. The SST models were produced for only a couple years and I know why they went away quickly. The motor being mounted on the middle/front of the chassis over the front wheels causes some major weight on them. They also used a hack job using a steering wheel to zero turn. I dumped the entire drive system out of it and put electric motors on it. I am having issues when I come to a stop and try to make a pivot turn. The front wheels do not want to swivel and the mower just sits in place with the motors pegged to stall current. I have 2 350w 24v motors on back and a 2x60 RoboClaw. These motors came off a bariatric wheel chair and are beasts.

If I switch to manual mode from auto and bump forward just a tiny bit to get the front tires to start the swivel, then it turns very easily. Is there a setting or possibly a code change that could make a pivot turn bump forward a bit before executing. Trying to turn from stand still with wheels pointing forward is no go without getting them rolling from the weight on them. This was more of a prototype to find what works and doesn’t work and plan to build a chassis over the winter, but would like to make this work for testing.

I have a 26gal sprayer I mounted to the back of the chassis (cantilever) and filled with water to try to get some of the weight off the front tires for testing purposes and it helped a lot, but did not fix the turns completely. I also don’t want to be mowing with that much weight. This will be a dual purpose mow/spray rig when done.

Any suggestions on what I could change to get the mower moving a bit to start a turn?

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Hi @nichord,

If it is just a matter of requiring more power be sent to the motors to overcome some kind of friction then I think inceasing the PID controller’s I-gain could help. So maybe try increasing the speed controller’s I-term (ATC_SPEED_I) and/or the turn rate controller’s I-term (ATC_STR_RAT_I).

I will take a look at the I parameters again and see if bumping them up might help. I will double check in Manual mode first though. I think it is something that does it in manual mode and cannot move by just left or right stick alone. I know if that is the case, then tuning will not fix the issues. I had wet grass previously and would just spin the tires. Now it is has dried out and stalls. You can hear the frame and mounts creaking when it does this from the stress the drive motors are putting on it. It is only an issue when the front wheels are straight ahead and needs to turn. The length of the mower frame combined with the narrowness of wheels up front and a heavy 15hp motor sitting toward front wheels just seems to be an engineering no-no.

Any small movement forward or backward then turning allows the front casters to swivel then turns as expected. All of the steering params on chart look really good when the mower is moving. I will see if I can shoot a video and post it. The transmission/hydro out of this thing was very heavy and now I know why. Poor design for sure. Was just hoping there was some kind of pop I could give it like a quick movement forward before pivot made.

Going to try to disable pivots and see how it drives as well. Would really like pivot turns though.

@rmackay9 Thanks for the follow up. Here is a quick video of the issue. This happens in manual mode, so no way to tune it out. I do realized this is an issue with either underpower or design. I am going to go with the latter as if I give it more power, then tire spin becomes an issue on grass vs it actually overcoming the design issue. I was more questioning on how to make a “rolling pivot” I guess. I have some coding background and could branch the code and try to fix. I thought it would be worth asking if there was a way that you guys could think of. Sometimes it is better to go to the source. If you say this is going to be a very hard fix, then I know I am done. If it were something where you have a suggestion on where I should look and what might be done. Was using this as a test platform to figure out what works and doesn’t for my custom build, and I have found an issue with too much weight being on the front casters of zero turn mowers :slight_smile:

YouTube video demonstrating issue:
Pivot Turn Issue


I put a big servo on the pivot/castor rear wheel of my rover, because leaving it free turning did all kinds of funny things to the steering/tuning of the rover. Depending on the surface, the wheel would turn easier or harder, so the rover needed a jumpstart to get turning. When it turned on grass alright, it would start oscillating on asphalt.
The castor also always tried to roll downhill, so the tuning was never right for all situations. Even slight inclines made the rover lose control when going downhill or parallel. The active castor turns 180° to make pivot turns possible. I do not know if you want to experiment further with this mower, but it was the only solution I coukd find for the castor problems.

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Interesting set of issues. If you want to stay with those drives and keep your weight down, I’d forget pivot turns. Regular rolling turns should work well with tuning. I have a “tug” pulling a trail mower so I can’t use pivot turns and it works well. 100+ hrs to date and using it weekly. More air in the front tires will help a bit but not solve the problem. Just not the best design (CG wise) for this application. Good luck!

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I turned off pivot turns and things are so much better. Had to slow down my turns a bit, but finally able to make this thing work enough to convince the wifey it is worth it. Of course it is 100 degrees here now and grass will not be needing cut until we get some rain. Hopefully can get some good tuning in before fall and work on a custom chassis over winter. Thanks for the input and suggesting to turn the pivot turns off. I was at that point, but helped push me to just do it. Terrible design from JD. This was one of those steering wheel zero turns that never worked very well to start.

Sharp learning curve seems like on every step of this project, but makes you appreciate all the work everyone has done to allow us to play!

Congrats on making it work!