RC Crawler Differential Survey

[AutoCrawler]

Originally Posted by Olle P

This diff is locked by default, so if a wheel slips more power is transfered to the wheel that has grip, and both wheels keep spinning at the same speed.
To unlock it an external force must add speed to one wheel only. (Reduced traction doesn't force a wheel to spin!)...

Looks like you mean an automatic locker. See here...

Locking differential - Wikipedia, the free encyclopedia

[Calderwood]

Have you ever looked at Tamiya high lift axles?

[AutoCrawler]

Originally Posted by nigels_world

Have you ever looked at Tamiya high lift axles?

Looks like they just have an open diff but I couldn't find a good description so they might be lockable.

Did come across this....

All Axle Dimension / Width / Gear Ratios - Scale 4x4 R/C Forums

which is a little off topic but it's interesting to see the various axles that are or have been available.

[wings_of_fire]

Originally Posted by Olle P

I think the last sentence I wrote says it all: To disengage the lock some external force must try to make one, but not both, wheels run faster than powered by the drive shaft.

A driven wheel that slips doesn't try to outrun the driving axle, it just puts up less resistance.
The ground friction of the outer wheel in a turn does try to make it run faster than the inner wheel and driving axle.
The external force added causes the lock to disengage, and as soon as both wheels spin at the same speed again the lock is reengaged.

Originally Posted by Olle P

Definitely not!

This diff is locked by default, so if a wheel slips more power is transfered to the wheel that has grip, and both wheels keep spinning at the same speed.
To unlock it an external force must add speed to one wheel only. (Reduced traction doesn't force a wheel to spin!)

What's good about it is that when there's grip it behaves as if open, but when the grip is lost on one or both wheels it stays locked.

I will try to put your explanation in a graphical form.

I am considering a situation where a vehicle is running straight.Then it takes a right turn and after the turn is completed, starts moving straight again. I have divided this whole process in a time frame of couple of seconds. I will try to explain what happens at every second of time.

I might need some further explanation from you.






I am still thinking further......

[wings_of_fire]

Seems like photobucket has reduced the image size... does anyone have a solution to that
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Got images resized...

[wings_of_fire]

I haven`t changed the drive shaft rpm. We are considering the drive shaft rpm of 150 to be stable in all situations.
To keep the drive shaft rpm stable, the rotational power coming from the traction wheel cannot go through the crown gear to the drive shaft increasing its speed over 150 rpm. So this energy should have been transferred to the slip wheel through the differential pinion gears. But then the slip wheel rpm will increase............
So what exactly happens? The image under is just a thought and should not be considered right until proven. I also have not added the vectors for the traction wheel.

[Olle P]

Originally Posted by AutoCrawler

Looks like you mean an automatic locker. ...

An automatic unlocker, yes. That's what I wrote in post #12.

WoF: The inner wheel will run at slightly less than 50 rpm during the turn.

[Calderwood]

Originally Posted by AutoCrawler

Looks like they just have an open diff but I couldn't find a good description so they might be lockable.

Did come across this....

All Axle Dimension / Width / Gear Ratios - Scale 4x4 R/C Forums

which is a little off topic but it's interesting to see the various axles that are or have been available.

The high lift question was directed at wings since It's a lockable diff and our brothers from across the pond make Tamiya diffs remotely lockable.


And ps wings... You should go over your math again.

[wings_of_fire]

Originally Posted by nigels_world

The high lift question was directed at wings since It's a lockable diff and our brothers from across the pond make Tamiya diffs remotely lockable.


And ps wings... You should go over your math again.

No I had no idea about the Tamiya diffs.

And regarding the math thing, did you say that because one of the vector is pointing towards the differential when it should be actually pointing towards the wheel?

[Calderwood]

Originally Posted by wings_of_fire

No I had no idea about the Tamiya diffs.

And regarding the math thing, did you say that because one of the vector is pointing towards the differential when it should be actually pointing towards the wheel?

No, I'm talking about the rpm changes in the rear wheels. From your first picture we can determine that we have a 3:1 ring and pinion ratio. So for example in your second picture if you are turning at an angle that makes the outside tire turn at 55 rpm then we can determine that the inside tire would be rotating at 45 rpm, not 50 as indicated.

[Exocaged]

I have had a few venom creepers. Take their design but run 12 pins into 12 holes through a steel diff case and into a side gear and you would be good. Being able to open the rear up while using dig would be wonderful. The problem with the original venom design is too few engaging pins, they only used 3. They bent, elongated the 3 holes in the plastic diff case and never stayed locked. Mine all got JB welded. Make it manual and/or servo actuated.

[AutoCrawler]

Originally Posted by Olle P

An automatic unlocker, yes. That's what I wrote in post #12.

You did its true but it just seemed part of your description then. Now its clear. This sentence from the wikipedia link I posted helped.

"They (automatic (un)lockers) will never allow either wheel to spin slower than the differential carrier or axle as a whole, but will permit a wheel to be over-driven faster than the carrier speed"

And here's a u choob on the Eaton NoSpin "Detroit Locker" as an example

Detroit Locker - YouTube

[wings_of_fire]

Originally Posted by Exocaged

I have had a few venom creepers. Take their design but run 12 pins into 12 holes through a steel diff case and into a side gear and you would be good. Being able to open the rear up while using dig would be wonderful. The problem with the original venom design is too few engaging pins, they only used 3. They bent, elongated the 3 holes in the plastic diff case and never stayed locked. Mine all got JB welded. Make it manual and/or servo actuated.

First of all there has to be a syncromesh mechanism in a differential lock unlock system. With out it, there will be no alignment in the dog clutch. Increasing the number of pins will not solve the issue completely, but it will increase the shear area of the total pins making it stronger. Anyway that design is by venom, hence I won`t have it in mine, but I have a better one in the development stage.

I will prefer electronically actuated
Thanks for outlining the technical problem and sugession

[wings_of_fire]

Originally Posted by AutoCrawler

"They (automatic (un)lockers) will never allow either wheel to spin slower than the differential carrier or axle as a whole, but will permit a wheel to be over-driven faster than the carrier speed"

Would that not mean that if the drive shaft rpm is kept constant at 150 through out the situation, then the automatic locker won`t allow the slip wheel rpm to fall below 50 ?

And I was confused about the word u choob in your earlier post.....

[AutoCrawler]

Originally Posted by wings_of_fire

Would that not mean that if the drive shaft rpm is kept constant at 150 through out the situation, then the automatic locker won`t allow the slip wheel rpm to fall below 50 ?

And I was confused about the word u choob in your earlier post.....

Yes.

When turning, the outside wheel wants to go faster and is allowed to but the inside wheel retains the carrier speed. This is why you get more tire wear and why you can lose control on ice. The inner wheel can break traction since it's forced to rotate possibly faster than it can without braking traction. It is also why you get under-steer when this is used on front axle. The inner wheel is forced to rotate faster than it should to follow the turn and so retains traction going forward resisting the turn.

The numbers will depend on the axle ratio but assuming 3 to 1 then the axle is turning at 150 and doesn't change then the inner will never fall below 50. The outer will rotate at 50 + until the turn is complete and then it will return to 50.

[Exocaged]

Originally Posted by wings_of_fire

First of all there has to be a syncromesh mechanism in a differential lock unlock system. With out it, there will be no alignment in the dog clutch. Increasing the number of pins will not solve the issue completely, but it will increase the shear area of the total pins making it stronger. Anyway that design is by venom, hence I won`t have it in mine, but I have a better one in the development stage.

I will prefer electronically actuated
Thanks for outlining the technical problem and sugession

with 12 holes on the small side gear alignment was never really an issue, brute strength was. You are right about the sheer strength being much higher. We need that. If you look at our motors and weight in comparison to real vehicles, some of us have 1000+hp in these things. I will be interested to see how you get a synchro-mesh system in one of these axes and get it to live. It's going to be tight. If by chance you accomplish this and it proves strong. I need 4 of them.

On a side note, you could do a face spline design and get more then 12 splines in there and rather then engagement pins it would be an engagement plate. with matching splines of course.

[Olle P]

Originally Posted by AutoCrawler

... "They (automatic (un)lockers) will never allow either wheel to spin slower than the differential carrier...

I think that depends on individual design, of which there are a couple.

The one I'm referring to does most definitely allow the inner wheel in a turn to run slower than the diff carrier, after the outer wheel begins to spin faster than the carrier and thus open the lock.
While the lock is open it's acting like a regular (open) differential.

[Calderwood]

Originally Posted by Olle P

I think that depends on individual design, of which there are a couple.

The one I'm referring to does most definitely allow the inner wheel in a turn to run slower than the diff carrier, after the outer wheel begins to spin faster than the carrier and thus open the lock.
While the lock is open it's acting like a regular (open) differential.

I didn't realize at first ether but he is referring to Detroit style diffs. A design that will never fit in a 1:10 scale diff and have any strength. I, like you, figured we would be talking about the simpler designs.

[pritch]

I think this would be a cool feature to have, depending on the vehicle.

My x-crawler has a top speed of about 3mph thanks to gearing and 55t motors, so a working diff is irrelevant.

But take the TLT I've just bought as a semi-scaler - I'm keeping that as a cross-over vehicle. I guess you could say it's a small imitation of something like the Traxxas Summit, a truck that can speed around and crawl a bit too, and that's not an easy thing to pull off well. Having diffs that can lock and unlock is kinda essential, as with real vehicles that have to do the same thing. Traxxas' solution is cool, but the Summit is a pricey truck and not everyone considers their system a complete success.

Being electronic means that much more to go wrong, especially at smaller scales. In an ideal world I would like a mechanical action on the diff itself. You could then open front, rear or both diffs etc. and with no need to worry about doing that on a radio. I wouldn't be that bothered about losing the ability to do it on the fly. Tamiya used to make XB models with a two-speed transmission selector on the axle itself - something like that.

Good luck with the project.

[AutoCrawler]

Originally Posted by Olle P

I think that depends on individual design, of which there are a couple.

The one I'm referring to does most definitely allow the inner wheel in a turn to run slower than the diff carrier, after the outer wheel begins to spin faster than the carrier and thus open the lock.
While the lock is open it's acting like a regular (open) differential.

I have only come across two mechanical "automatic" unlocking differentials - (detroit and lunchbox) and both work with one axle locked to the carrier as part of their operation. I could not find any reference to the Volvo 903. Do you have an reference to the design you mean? It's not important. Just interested.

Originally Posted by nigels_world

I didn't realize at first ether but he is referring to Detroit style diffs. A design that will never fit in a 1:10 scale diff and have any strength. I, like you, figured we would be talking about the simpler designs.

Not sure who the "he" is you are referring to. I suspect WingsOfFire, the originator of this thread, has something specific in mind that he hasn't shared. The rest is just chat about differentials trying to understand what we mean.

Actually, cost and benefit aside the Detroit Locker is compact and elegant enough that it might scale down quite well.

Detroit Locker - YouTube

I don't think the same could be said for the "lunchbox/Lock Right/Aussie" style since it relies on a single central pin to split some dog toothed gears which wear fast at 1:1 scale.

Lock Right Locker - Part 1: How it works [HD] - YouTube

Project-JK.com - Jeep JK Wrangler Resource » Dynatrac Presents - Lockers 101