Has anyone tried using a differential?

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Slistack   10 mW

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Re: Has anyone tried using a differential?

Post by Slistack » Oct 05 2014 4:42pm

you can use it on either truck the beauty is you just flip two of the phases and it becomes fwd or rwd. I think that there is a difference between the two. Fwd sort of pulls you along and if you don't have as much weight in the back then if you accelerate to quickly then it skids out. Rwd is fine too. I think its more visually appealing to have rear wheel drive so that when you hold the board you hold the side without the motor. These little differentials cant take a ton of power either I would say 1100w for a 1/8 scale one. I haven't dealt with a bigger one cause i think its unnecessary. They way i have my design setup you can pretty much use any wheels you want. Now granted you will go slower/faster when you change wheels, but it is designed for around 90mm wheels. And what you are talking about is just silly the job if the differential is to spread the power where it is needed. So if you are turning it allows power to still be put through the axles and have one wheel turn faster than the other. I think its a wicked simple idea, I mean just about everyone in the world uses it every day in their car. There is a reason why cars don't use chain drive and belt drives anymore, so why should electric long boards be different. Also there is no reason to shift because electric motors are so good at giving their peak HP at any speed. Gasoline engines need to shift because they cant supply 500hp at 800 rpm. That is the beauty of the electric motor. I don't think you need a more powerful motor too, the motor I am using now is a 6354 and it almost has too much torque for me.

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Re: Has anyone tried using a differential?

Post by onloop » Oct 05 2014 7:26pm

@pediglide not sure how your skateboard works, but normally when you make a turn on a skateboard both outside wheels (front & back) need to rotate more than the inside wheels. The inside wheels both travel less distance then the outside wheels.
Slistack wrote:and the differential is more efficient then a belt drive, probably like 10% or more.
how do you know this? or are you guessing... some study or data would be good to back this up...

@slistack have you tested this diff with weight of human passenger?.. how much does a 1/8 scale buggy weigh? I can only assume this diff is built to carry a certain load.... probably similar to that of a 1/8 scale buggy.

maybe look into some 1/5 scale stuff....

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Re: Has anyone tried using a differential?

Post by LEVer » Oct 05 2014 7:53pm

onloop, I have the impression that when I initiate a turn the inside rear wheel will travel faster than the outside wheel (or the outside rear wheel will travel slower than the inside wheel).
Then, during the turn, both rear wheels will rotate at the same time.
At exiting the turn, the outside wheel will turn faster than the inside wheel and thus eventually both wheels will parallel the board.

Am I wrong? When you turn left, doesn't the rear truck turn to the right, making the inside wheel go faster than the outside wheel?

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Re: Has anyone tried using a differential?

Post by LEVer » Oct 05 2014 10:08pm

Heres some data:

http://quadskating.com/skates/roller-skate-trucks.htm

Do you guys have your rear trucks reversed and they are turning the same side your front trucks are turning?

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Re: Has anyone tried using a differential?

Post by dirkdiggler » Oct 05 2014 10:24pm

The baja board uses a diff. It's been done before, but it does make things more complex. Baja designed a whole setup from scratch. It would solve some issues using a diff, but it does create other problems. How are you going to add this to a truck system or are you making your own trucks with it? Looks to also add a bunch of weight with what you've got so far. Need to trim some of that weight. I'm curious as to the efficiency - doesn't seem to be more efficient than a belt system as you say, but I'm not a mech E. I believe cars don't have other options - imagine everyone leaning in the vehicle to one side so it would turn. Or ever try and drive a fixed axle? Doesn't work so well unless you want to drift on your way to the store. I would think an electric car with a motor on each wheel, would be the most energy efficient design. Pretty much what eboards are currently doing. Definitely not trying to discourage you though. Baja made a great board.
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onloop   10 kW

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Re: Has anyone tried using a differential?

Post by onloop » Oct 05 2014 11:15pm

I don't think baja use a diff. They have four motors, one for each wheel.....

Say you do a complete 360 degree circle on your stateboard. The inside wheels have less distance to travel to complete the circle..... so the circumference of the path/circle the inside wheels travel is less then the circumference of the circle which the outside wheels travel. The outside wheels need to travel further to complete the circle. Therefore they must travel at a greater speed then the inside wheels if they are to make the distance in the same time.

If you rear truck is on backwards you cannot turn.

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Re: Has anyone tried using a differential?

Post by LEVer » Oct 05 2014 11:22pm

Thats true for the front truck, but is it true for the rear truck?

If the rear outside wheel travels faster than the inside wheel, wouldn't the rear wheels go back to being parallel with the board and then eventually turn the other way, ending up on the same side your front wheels turning?

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Re: Has anyone tried using a differential?

Post by Slistack » Oct 05 2014 11:43pm

I understand what you are all saying. Look at it from not a electric longboard, but as a car or something. If you turn the outer wheel turns faster than the inner wheel. We can agree on this. It doesn't matter which way it is facing or fwd or rwd. It could be more efficient to have two motors that were direct drive to the wheels IF and only if your electronics are sophisticated enough to handle that. The job of a motor controller is to keep a motor at a certain rpm. as soon as you turn one wants to turn faster than the other so they fight each other. And yes I pretty much make a whole rear truck. You are also correct the differential cannot take your weight that's why its floating. it pretty much receives no load, the load bearings take all the weight off that's why I can use such a tiny thing. I mean the external gear is only 1.5 inches in diameter. As for efficiency it could be 5% more or better, depends on the belt system. Also each wheels spins independently so I don't know why you say its a fixed axle, I don't know if you guys are grabbing the concept fully. Here is a video to describe. As for weight it doesn't add much at all besides the motor, because you would have to use the bearings and such if you had a regular truck. I have been using another version of this board for over a year now and it work perfectly. I would say that the motor size i have on it would have more trouble with a 200lbs person but I think it would be fine. I didn't really want to show my newer design but I guess I will. Here is a side by side from version 1 to version 2. I will NOT explain how I built it because I want to make it into a kickstarter. So please do not ask me whats inside the magic box. What you see is what you get right now. I will tell you this in order to make it you need a CNC mil a very accurate lathe good to +-.0005. Otherwise it will vibrate itself apart and die. I am very proud of what I have done, even though belt drives work, they kinda stink. If you want to order one from me I am perfectly happy making you one without electronics and a board and battery and stuff. It is just plug and play.


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Re: Has anyone tried using a differential?

Post by psychotiller » Oct 05 2014 11:49pm

Pediglide wrote:Thats true for the front truck, but is it true for the rear truck?

If the rear outside wheel travels faster than the inside wheel, wouldn't the rear wheels go back to being parallel with the board and then eventually turn the other way, ending up on the same side your front wheels turning?

If you are using matching trucks, in a turn, the outside wheels follow the circumference of a larger circle. Inside and outside wheels will complete the circles they started.
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onloop   10 kW

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Re: Has anyone tried using a differential?

Post by onloop » Oct 06 2014 12:08am

Slistack wrote:You are also correct the differential cannot take your weight that's why its floating. it pretty much receives no load, the load bearings take all the weight off that's why I can use such a tiny thing.
Im talking about a torque load from the motor.... If the wheels can't turn for any reason (too much load) Then the pinion gear will receive all the load from the motor, so depending on how strong the pinion gear is - it either breaks or your motor burns, or the wheels start to turn..

the weakest link will always break.... so the weakest link should be cheap & easy to replace....

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Re: Has anyone tried using a differential?

Post by LEVer » Oct 06 2014 12:15am

Ok, I've put more thought into this and yes you guys are right about the outside wheel traveling faster than the inside wheel.

But, I think I have to specify driven/powered wheel vs non-driven/powered wheel. And I may have solved my own problem, lol.

In 2FWD, its the outside wheel that needs to be driven/powered and will spin faster than the inside wheel.

In 2RWD, its the inside wheel that needs to be driven/powered but the outside wheel will still spin faster, so as you guys said, to complete the circle.
This does not make outside rear wheel more powerful and does not counter the normal turning of the rear truck and therefore not allow the rear truck to end up turning on the same side the front truck is turning.

Does that make sense?

If this is true, then this differential would be more efficient in front.
An awesome setup would be two motors at the rear driving the rear wheels independently and one motor with this differential system in the front.

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Re: Has anyone tried using a differential?

Post by onloop » Oct 06 2014 12:24am

it won't matter if DIFF is front or back, the efficiency will not change. the diff simply allows power to be distributed to the wheels that need it. say in a turn. the outside wheels need to spin faster, so the diff allows this to occur.

anyway this is the best video EVER regarding the diff!

ANYWAY I THINK YOUR PROTOTYPE LOOKS GREAT! GOOD LUCK


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Re: Has anyone tried using a differential?

Post by Slistack » Oct 06 2014 1:27am

Thank you for posting that video I was going to but I posted already and i was like w/e people might understand. THE MOST efficient is to have one of everything, one speed controller one motor one battery. Every time you add a component you lose efficiency because everything isn't perfect. And the spur gear I found out is not the weakest link. There is a little pin on one of the inside gears that wears out. Guess what the easiest thing to do is buy another diff and plop it in. instead of messing with things its a 20 dollar fix. But if you use a current limited controller I think that the battery would wear out before the drive mechanism. I really do think this is the best most hassle free solution for electric longboards. The only problem that I want to fix are these. If you have any advice I would love to hear it. I really need it I am not a longboarder what so ever. I am a engineer haha.

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Re: Has anyone tried using a differential?

Post by LEVer » Oct 06 2014 2:02am

That is a video about a vehicle with front wheels steering and fixed rear wheels. If you make the rear wheels steer and with diff, it will end up like this:
Liebherr_LTM1160-2_B04.jpg
Crab Steering
Liebherr_LTM1160-2_B04.jpg (53.91 KiB) Viewed 1319 times
You can fight it and lean more, but it won't be efficient.
SteeringModes.gif
SteeringModes.gif (2.36 KiB) Viewed 1319 times
In fact, if you lean too much, you run the risk of lifting the outside wheels and losing traction.
Now if it has LSD, then I will shut up because obviously you're a mechanical genius.

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Re: Has anyone tried using a differential?

Post by Slistack » Oct 06 2014 7:03am

you are correct you will lose traction but its the same as if you didn't have an e board. I don't really know what you are saying anymore so.

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Re: Has anyone tried using a differential?

Post by Barndom » Oct 06 2014 10:16am

It looks like the motor is facing inwards, is it problem with clearing the board? Thinking a outward facing motor under a kicktail have better clearance, but the pivot have to be closest to ground.

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Re: Has anyone tried using a differential?

Post by LEVer » Oct 06 2014 10:20am

Hey I like your differential idea, I just think it needs to be put in the front. In fact I'm thinking of implementing it on one of my boards in which has the worst turn radius:
20140223_113743.jpg
20140223_113743.jpg (159.92 KiB) Viewed 1290 times
I hope you can make one for this size of wheel.

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Re: Has anyone tried using a differential?

Post by psychotiller » Oct 06 2014 10:52am

Pediglide wrote:That is a video about a vehicle with front wheels steering and fixed rear wheels. If you make the rear wheels steer and with diff, it will end up like this:

Pedi, The only way crab steer could happen is if one of your trucks was mounted backwards. Nothing else is even a factor. If your trucks are mounted correctly it couldn't physically happen. It doesn't matter which wheels are driven or what combo of wheels are driven because our wheels don't turn on individual spindles. They turn in pairs from the center of our axles.
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Re: Has anyone tried using a differential?

Post by LEVer » Oct 06 2014 11:02am

Yes, I understand what you're saying, but guys I'm not saying putting the open differential on the rear will not work. It will, just not efficient because the outside rear wheel would want to go turn the rear truck back. You can fight it but its not efficient.
The best way is to put the power on the rear wheel with the most traction, in this case its the inside wheel. Its still a differential, but with LSD/Positraction.

Here is Josh, maker of Gnarboards, talking about his 4wd setup:

http://visforvoltage.org/forum/10352-4w ... -questions

"another interesting thing is how I am running 4 motors off of just one controller. Some people would advise against that since you have a chance of frying your motors. But here is the benefit to running 4 (Brushed DC) motors (In parallel) off of just one controller: You get the ultimate traction. It's like having a positraction system on your vehicle and here is why: When 1 of the 4 tires loses traction, it's (motor) Amperage demand drops and the controller stops feeding it lots of amps and instead feeds the other 3 motors however many amps they are demanding. The end result is "amps only when you need 'em". Now here is where it could be bad: If for some reason you lose traction on 3 of the 4 tires, and the 1 tire is stuck in a rut or something, then that 1 motor could "eat itself to death" by demanding too many amps, and the controller would have no problem supplying the extra amps since the other 3 motors aren't using them.
If you were to run 4 motors in series w/ the motor controller (and battery-pack) you would get the opposite effect. When 1 of the 4 tires would leave the ground it would become the only tire spinning and the other 3 tires would lose all power. (like an open differential).
So thats why I chose to just run a 24V system and 4 Brushed DC motors in parallel w/ a single Brushed DC motor controller.
I remember people telling me that this isn't an option for Brushless motors because they would eventually become out of sync with each other and the controller would have no way of syncing multiple motors. I do wonder though:
Can you run multiple sensorless brushless motors in parallel with a single brushless motor controller?"

Basically, he is saying you need to put an open differential in the front, because the outside wheel needs power; and an LSD type differential (Positraction) in the rear because the rear trucks turns oppositely compared to the front truck.
If you put an open differential in the rear, it will power the wheel with the least traction, the outside wheel because you are leaning on the inside wheel. Therefore it will not be efficient, and you will just get wheelspin, same as getting stuck in mud or snow.
I know he's talking about electronic differential and that he opted for a LSD-type differential on all wheels, but this is because I think he just wanted to use one controller.

I know its impossible to end up having a crab steering because you're leaning on the board, I'm just saying its the same principle.
Just remember, its not a car with front wheels steering and fixed rear wheels. Its a board with skate trucks that turn to opposite sides. Therefore you need a differential that works on opposite directions too.

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Re: Has anyone tried using a differential?

Post by LEVer » Oct 06 2014 11:23am

Also the front and rear skate trucks by themselves is a form of differential.
Golfboard uses one motor per axle to drive a both wheels on a live axle without a differential, guess how they achieve turning...because the trucks are turning.
If you guys notice, this turning is a big issue for us that use mountainboard trucks and pneumatic wheels. If you have narrow skate trucks, putting an open differential on the rear will be less noticeable

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Re: Has anyone tried using a differential?

Post by psychotiller » Oct 06 2014 11:25am

It just doesnt add up...The rear truck is just a mirror image of the front truck. It's not like a quark...It doesn't change because you look at it :shock:
There aren't any differentiating factors (pun intended). We center our weight over our boards. Equal pressure is applied to each wheel. It's still a skateboard. Not a monster truck. Even the gnar board...The wheels don't steer independently. Non-issue.
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Re: Has anyone tried using a differential?

Post by rs4race » Oct 06 2014 11:38am

You guys are severely over thinking this. It doesn't matter if you put it front or back, and it's not going to affect efficiency that much. The open diff is just going to allow the two wheels (inner and outer) to spin at different speeds. Since bevel gears generally have an efficiency range of 93-99% only 1-7% of the power your applying can be seen as resisting the differential motion. Since this is speed dependent, and the width of the trucks vs the turning circle radius is quite small I would expect the differential motion to be very little. This means the power loss of the differential should be very minimal, and thus any straightening torque to be minimal as well.

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Re: Has anyone tried using a differential?

Post by psychotiller » Oct 06 2014 12:03pm

Slistack wrote:Thank you for posting that video I was going to but I posted already and i was like w/e people might understand. THE MOST efficient is to have one of everything, one speed controller one motor one battery. Every time you add a component you lose efficiency because everything isn't perfect. And the spur gear I found out is not the weakest link. There is a little pin on one of the inside gears that wears out. Guess what the easiest thing to do is buy another diff and plop it in. instead of messing with things its a 20 dollar fix. But if you use a current limited controller I think that the battery would wear out before the drive mechanism. I really do think this is the best most hassle free solution for electric longboards. The only problem that I want to fix are these. If you have any advice I would love to hear it. I really need it I am not a longboarder what so ever. I am a engineer haha.

Wheels, They need to be skinny about 1 inch wide 90mm to 100mm in diameter
Speed controller, Current limited to 35a and brake with minimal noise
Hand controller, Control current limiting, cruise control, brake, battery health,
Charger, 10s onboard 100w charger with battery managment
Trucks?
Truck bushings?
Custom carbon board
Custom wood board
permanent grip on board
LED lights for night
Underglow?
Smart phone controlled, bad idea I think I dropped my phone once already.
Rubber tires, smaller than 120mm I have only find one so far
Long board short board?
Wheels, They need to be skinny about 1 inch wide 90mm to 100mm in diameter Try these: http://www.ebay.com/itm/KRYPTONICS-FORM ... 5b055e4a79

Hand controller, Control current limiting, cruise control, brake, battery health. You should use the Wiimote and Kama controller
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Underglow? Waste of resources...
Smart phone controlled, bad idea I think I dropped my phone once already. Expensive accident waiting to happen.
Rubber tires, smaller than 120mm I have only find one so far. Maybe try these? http://www.jacksboardhouse.com/Steadfas ... /ofrod.htm
Long board short board? Longboards are inherently way more stable.
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Re: Has anyone tried using a differential?

Post by LEVer » Oct 06 2014 12:09pm

psychotiller wrote:.... Equal pressure is applied to each wheel...
Not during a turn. More pressure on the inside wheel than outside. Good for an open differential on the front truck, bad for an open differential on the rear truck.
rs4race wrote:..The open diff is just going to allow the two wheels (inner and outer) to spin at different speeds...
Yes, but the open differential will give more power to the wheel with least resistance. In the rear truck, that will be outside wheel during a turn, which will make the rear truck behave like the front truck, thus the crab steering analogy.

You guys may not be experiencing it because you're using narrow skate trucks, shorter wheelbase and smaller urethane wheels.
I apologize for being hard-headed but I have experienced and still is experiencing it now. I have done what Josh have done and felt the difference.
Right now with my 2RWD, i have the motors connected in parallel (positraction) and when I turn and put "a lot" of pressure on the rear inside wheel, I feel it speeding up and getting more powerful, making me turn faster.
If I connect the two motors in series (open differential), it still turns, but on a longer arc, and does not give that oversteer feel.
I know this is super-easy to do on narrow trucks, short-wheelbase and slippy wheels, but believe me, its so hard to achieve with mtb trucks, a Carveboard and grippy rubber tires...on tarmac.
Again, I apologize for wasting you guys time.

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Re: Has anyone tried using a differential?

Post by LEVer » Oct 06 2014 12:58pm

I know I'm wasting more of you guys time, but I should have posted this before. Its so simple.

Just think of the one motor on the rear, if you turn and the motor is driving is the rear inside wheel, it doesn't matter how much lean, you will always have traction.

Now if you turn the other way, and the motor is the driving the rear outside wheel, there is a possibility that if you lean too much, you will lift the rear outside wheel and lose traction. <-----That is the same effect of an open differential on the rear truck.

You can counter it, try to avoid it from happening, tighten the hell your trucks, not care about the affect on efficiency or something, but still...its limited compared to when you have the power on the inside rear wheel.

Zboard users complain about this all the time, and they use narrower trucks than mine.

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