ZZMT crawling - clod stall effect

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ZZMT crawling - clod stall effect

Post by guerd87 »

hey pimps, ive been on my desk working for the last hour, doing some drawings etc etc and i finnally just decided to write all of this down to see what you guys think, or if you can understand where im coming from

when we look at the large scale crawlers, some of them that have seperate front and back diffs, we get something called clod stall (mainly because the clodbusters were the first ones to start this) which lets the back wheels slip and the front pull up, not causing the usuall backflip on hills

after doing some research i found that the front and back motors are just powered from the same speed controller, and its the diffs (mainly the dogbones) that do the slipping when geared low enough. there was a thread a while ago where clint was actually planning on using 2 x seperate mini rc servos for the seperate speed controllers, but this was pretty expensive

im jsut chasing down some opinions on the crawling capability of 2 x front half ZZMT's driven off a center gearbox mounted inside, for eg, my tube chassis. do you think it will crawl better? worse?

the only 4WS ZZMT around here that ive seen is HirotoR34's and i havnt seen that thing crawl..?

im going to be trying this out once i finally get back into my domain and get my cars/tools back and hope for the best. this will actually be setup like a modern car

Tube chassis holding gearbox, electrics, lipo, winch etc etc
under neath will be the 2 front halves, connected in the centre to the gearbox
i am planning on using some old xmod shocks springs to keep it sitting nice and high above the 'diffs' but still allow flex

anywho, enough of my rambling, i hope you guys can understand what i just wrote :\ if anyone wants my sketches i just finished of the whole idea let me know and ill put them up

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Post by frizzen »

Before we get too far into this, post up your drawings!!!

"Clod Stall" works because you're running 2 motors on 2 COMPLETELY SEPERATED axles off 1 speed control. It doesn't really work at all when you connect the front and rear together, unless MAYBE your rear has some type of a slipper clutch that is set up perfectly for the conditions that day.
The rear wheels don't slip, the motor stalls out.
A differential that does the slipping (especially at the DOGBONES) would be pretty useless for even forward motion on flat ground. When you get an extreme climb angle, the weight shifts more to the aft wheels so they get more grip and will do a wheelie-endo unless something gives, like the motor stall. The front wheels have weight unloaded so less traction, but when you just have the rear wheels stalled out and only holding it up instead of driving, the fronts can usually pull the truck on up over the obsticle.

http://bitpimps.lixlink.com/pages/phpFo ... axle+stall

Dual front ends will definately help your crawler, but be aware that the stock drive inputs are offset. HOWEVER, being driven off the center drive it's going to have it's ability for forward motion limited to about the same as the stock setup ZZ-M complete with the wheelie-endo, and it won't perform like a baby-clod.

Anyway, sorry if this seems like I'm being mean, it sounds like a cool project! Let's see some pics already!!! :D
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Post by SHAUN »

When you see a 1:1 crawler have only its front wheels spin, that is because they use a "dig" set up, this allows the driver to only send power to the front wheels.
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Post by redrustbucket »

If someone gets this to work it is going to be sick for sure. Like frizzen said the best way is with completely separate motors and drivetrains. It's tricky to set up for sure but not impossible. I guess it depends how crazy you get with it.

A stick type set up would seem to be the easier way to go because you may be able to get away without running links. Links are difficult to set up for the proper geometry and a major pain in the ass at this scale. When building larger scale crawlers I can bend and weld up a chassis in a few hours and spend weeks or months setting up links, steering and suspension.

Probably just do something similar to the way you sleeved your driveshaft but on top of the axle assemblies. You could add a small spur gear just behind the pinion and then some kind of reduction to it. Keeping your drive motors and battery as low as possible will help you lower your center of gravity which is the main thing that will cause your crawler to roll, more so than too much traction in the rear. Too low will hinder your ground clearance and your ability to clear the ledges that are flipping you over backwards anyway.

It would be something of a balancing act. btw. c.o.g. and clearance. Keeping as much of the weight as you can on the axles will help your stability immensely. In 1:1 terms its called un-sprung weight. The more unsprung weight you have the more stable your crawler will be. Thats part of why sticks work so well. They are virtually all un-sprung weight. All the electronics, batteries and motors are on the axle on a stick.

Fullsize crawlers get over this hurdle by running suspension set ups with large amounts of droop ( down travel ) and very little up travel. This allows them to have a very low center of gravity and lots of flex. By running a smooth belly pan underneath and enough un-sprung weight they can clear large ledges by simply letting the axles drag the chassis over the ledge.

On a stick batteries ( probably the heaviest single component) can be run under the knuckles keeping the weight low and outboard. This lowers the center of gravity and increases your side to side stability which allows your crawler to stay upright in extreme off camber situations. Unfortunately I don't know of any cells small enough to fit under a zzmt knuckle otherwise it would work great. :-? Locating your lipo low will help a lot. Plus with your extended wheelbase you can probably locate your board lower as well.

The best way to run a clod in my opinion is with a 4 channel radio. This way you have independent control of the front and rear motors and independent f/r steering. I have been wondering if the board and transmitter from a micro helicopter would work for this but I don't know if they are 4 channel or not.

Another simple way to slip the rear would be to modify the rear axle and wheels so the wheels will spin on the axles. Then add sort of an adjustable slipper clutch to each wheel by adding a disc attached to the axle and a disc to the back of the wheel. You could adjust it by adding a spring and tightening or loosening the lugnuts. They would slip independently of each other though and this could be a detriment or a plus. It's just an idea but it may be a simple solution.

Post up your drawings I'd def. like to see what you're planning, or do it the way I do and just start hacking shit up and make it work. :-o
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Post by guerd87 »

well going on by what has been said i dont relaly think my idea will work, which is why i posted it up initially, to see what everyones opinions are

i have decided to scrap that idea...and start the rev 0.2

the way this is going to work is attaching mini gearbox's to the back of 2 x front half MT's, going back to clints ideas of converting mini servos into speed controllers

im going to do the first one on the cheap, dual PCB'd( 1 each freq.) and modify 2 controllers into 1 controller which will control the whole unit

dual motors powered seperatly and true 4ws thanks to dual PCB's! this rig will be very dodgy i guess first off for the testing, but since i have a few spares around now i can pretty much use alot of parts if i need to

if the idea works great and turns into a awsome crawler, i will consider upgrading the halves to servo speed controllers

building a crawler with dual motors and 4ws is easy, getting the remotes apart and into 1 complete unit could be a haslle, but hopefully ill get around that :-o

i am currently jotting down my new ideas and i will pose them up when i finish them 8-)
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