By erikNC on Tuesday, November 20, 2001 - 08:15 pm: Edit

Hi everyone,

I am seeking around some information and naturaly calling for knowledge and experience here again :-)

I have been often told that, with a rear diff locker engaged, if you are on a trail and the trail is leaning, some people report that lockers cause the rear of the vehicle to "crab" sideways toward the downside of the trail. Some over say you would roll over to the roof...

The thing I dont get is the physical reason why a locked rear diff makes the vehicule "crab" sideways toward the downside of the trail, more than with an open diff ?

Any explanation for that behaviour ?

Morevover, could someone tell me how does a Truetrac react in such a situation compared to a locked axle and an open one ?

Thank you all for your contribution.

Best regards.

Erik
South Pacific Disco

By PerroneFord on Tuesday, November 20, 2001 - 08:57 pm: Edit

Easy... take an ice cream cone and roll it along a table. What happens?

With a locker, it FORCES both ends of the axle to move at the same rate, just like the two ends of a cone move at the same rate. When that happens, something has to turn. The steeper the camber (of the trail or the cone), the stronger the pull for the turn.

-P

By Kyle on Tuesday, November 20, 2001 - 09:29 pm: Edit

A locked truck will crab down hill but only if its spinning. You get locked to prevent spinning.

Kyle

By Slider on Tuesday, November 20, 2001 - 10:31 pm: Edit

Does it have anthing to do with weight transfer (or off center CG) to the down hill side of the truck...and an aired down truck's tires would be more squished on that side, making a smaller diameter wheel...and thus making the larger diameter (up hill side) tire want to cover more ground...pushing the truck downhill even more ???

Does this make sense...I don't have enough locker experience...

The cone thing doesn't make sense to me unless you had two diff. tire diameters on one axle???

By kyle on Tuesday, November 20, 2001 - 10:34 pm: Edit

I am not sure where Perrone was going with that either..

Kyle

By Mike B. on Tuesday, November 20, 2001 - 10:35 pm: Edit

The weight of the vehicle on the downhill side effectively makes the circumference of the downhill tire smaller. The uphill tire diameter becomes larger (no weight on it). This provides a similar effect to the ice cream cone illustration. It does not matter if your are spinning or not.

Of course, the more you spin, the greater the effect. Also, the wieght of the vehicle, the size of the tires, the amount you air down, the type of tires, and the terrain all play a roll in the amount of crabbing you will feel.

Thanks,
Mike B.

By PerroneFord on Tuesday, November 20, 2001 - 11:09 pm: Edit

Thanks Mike! I TOLD you that you were smarter than me! I knew what I wanted to say but you articulated it better than me.

Sorry for any confusion.

-P

By Kyle on Tuesday, November 20, 2001 - 11:11 pm: Edit

Ahhh , but I run around 60 PSI all the time. I guess thats why I aint having it....

Kyle

By PerroneFord on Tuesday, November 20, 2001 - 11:27 pm: Edit

60!?

You running a Greyhound on the trails?

-P

By Chris Merritt (Smokinbro) on Tuesday, November 20, 2001 - 11:30 pm: Edit

I don't think an aired-down tire changes circumference significantly. inner tubes, when inflated, or over inflated increase in circumference as a result of the rubber streching. Tires on the other hand are belted, which severely limits their ability to increase circumference (unless you are running a top fuel dragster with wrinkle walls).

air down a tire and you only increase the footprint and as a result reduce ground loading per square inch( total load does not change)

the end result though is that the circumference of the tire does not change, the tire carcass just flexes more, produces more heat etc.

I think crabbing is just a combination of the tires breaking loose from the trail surface, reducing friction and letting gravity take over.

my 2 cents

By Kyle on Wednesday, November 21, 2001 - 12:39 am: Edit

yes Chris , I agree there. I see what Perrone is saying and in theory he is right. But how long would that section have to be to get some decent tire slippage going ?
Perrone,
I aint in to the whole airing down thing. I run the tires where they are supposed to be run and I dont have to stop to fix a broken bead or any other dumb shit caused by under inflation..

Kyle

By Milan on Wednesday, November 21, 2001 - 12:53 am: Edit

Well, airing down DOES reduce the static loaded radius of the tire. Circumference of the tire will change also unless all of the "bulging" due to airing down were length-wise as opposed to width-wise. But it's not.

Other factors will play a role too, I'm sure. The up-side is compressed, so the downside wheelbase is shorter than on the upside essentially turning the fornt downhill a bit and the rear uphill a bit making your rover ride on a curve with it's apex on the high side (i.e. aiming down the vehicle down the hill).

this would be true of almost any suspension design. However, I'm sure it has more to do with simple gravity and traction being lost somwhere. Imagine a log, of even diameter throught its length, lying on a side hill with one end pointing up and another down the hill. It will be ok if you don't move it. Now start rolling it alongside the hill. I bet it will slip and tend to go down.

Not everything has to be over analyzed, you know.

By Milan on Wednesday, November 21, 2001 - 12:55 am: Edit

Hey Kyle, if you ran your tires where they're supposed to be at, you'd run around 34 on the street and around 8 off-road. And no, I'm not looking for another discussion about tire pressures...:D :D :D Man we need more emoticons.

By erikNC on Wednesday, November 21, 2001 - 12:07 am: Edit

hi guys,

so many thoughts !

That is interesting.

Ok for the theory, but what about experience. How do Rear truetrac owner go thru off camber trails ?

How does this one react compared to a locked diff or an open diff in this situation ?

Ok, it is clear now, I am wondering about the + / - of truetrac compared to an ARB... other than the way they mechanically work

I mean, how does a truetrac behave in situations were it is better to have an open diff (I mean an unlocked ARB there) ?

Thanks to everyone here.

Erik.

By Chris Merritt (Smokinbro) on Wednesday, November 21, 2001 - 03:42 am: Edit

Kyle
yes Chris , I agree there. I see what Perrone is saying and in theory he is right. But how long would that section have to be to get some decent tire slippage going ?

hard to say, all tires have some variation in circumference when they leave the factory (i don't know whatthe accepatble variance is so don't ask) so perrone's suggestion that it an ice cream cone would be in effect at all times.

I think you'd need a really significant difference in tire circumference to cause the slippage.

Try this example.

Snowy day, with lots of white stuff on the ground.
Most cars and trucks can move forwards in the snow even on off-camber roads. However, punch the gas a little, you get tire spin and then gravity (plus momentum et al) draws the read end of the vehicule down. Same thing as crabbing.

By RVR OVR (Tom) on Wednesday, November 21, 2001 - 03:50 am: Edit

Chris is right. I have crab-walked sideways down hills unlocked. As soon as you loose traction, you loose it. Period. Maybe it is a bit worse with a Detroit, but come on, if that is its only flaw, think of all the places where it will otherwise save your hiney.

Tom

By Milan on Wednesday, November 21, 2001 - 12:45 pm: Edit

The True Trac will slip less than the Detroit especially if there is some traction to be had. However, how much less depends on conditions. It is more agressive than an open diff, so if it's slippery, it will slip. However, I feel it is way better than an open diff (and as somebody mentioned, if it's slick you loose it no matter what you have) for almost all conditions and I'd take either (TT or DL) over the stock stuff any day. I know this does not specifically answer your question much but you'd have to try them to see.

I'm a little biased towards the TT because I like them a lot but I don't rock crawl, so they get me around quite well. And the times I wish I had full lockers they just keep things interesting for me.