All Wheel Drive experts chime in please.

[Irving]

Outlaw:
Different gear ratios will grenade the t-case, but shouldn't effect the diffs themselves.

This is the way that I understand it.

[Outlaw1]

Outlaw:
Different gear ratios will grenade the t-case, but shouldn't effect the diffs themselves.

I have limited experience with Scubbys, but I know they don't have a traditional T-case. They have longitudinally mounted engine/trans and a center diff that transmits the power to the rear. Sorry if I'm stating the obvious.

Having the drivetrain in a bind with mismatched tires and a locked center differential will affect the whole drivetrain, from one end to the other. IMO, when someone says they 'smoked' the fluid, or 'melted' the bushings, it doesn't mean it got hot enough for the fluid to catch on fire, or actually completely melt away the soft rubber bushings. They might have just been deformed a little and the fluid just smelled. Regardless, you wouldn't know until you drained it. Also, the mounts for the rear diff are on the rear cover, so if the ring and pinion got hot it transferred it right into the mounts.

Heat + drivetrain in a bind = tweaked bushings.

[DFBrews]

I have limited experience with Scubbys, but I know they don't have a traditional T-case. They have longitudinally mounted engine/trans and a center diff that transmits the power to the rear. Sorry if I'm stating the obvious.

Having the drivetrain in a bind with mismatched tires and a locked center differential will affect the whole drivetrain, from one end to the other. IMO, when someone says they 'smoked' the fluid, or 'melted' the bushings, it doesn't mean it got hot enough for the fluid to catch on fire, or actually completely melt away the soft rubber bushings. They might have just been deformed a little and the fluid just smelled. Regardless, you wouldn't know until you drained it. Also, the mounts for the rear diff are on the rear cover, so if the ring and pinion got hot it transferred it right into the mounts.

Heat + drivetrain in a bind = tweaked bushings.

Here is my guess this is based on big trucks so i may be completely off. there are 4 driven wheel ends and normally power is provided to a front wheel end and than the opposite wheel end on the rear. Like a criss cross pattern. there are spiders that allow the axles on the same diff to turn at different speeds when going around corners etc. when you put the smaller tire on the front it made the spiders in the rear diff work really hard trying to provide equal power to the rear to compensate. which made it get hot and start melting stuff and burning fluid.

when i dyno a semi i have to lock in the power divider so both axles get equal power. if i do not BAD things happen

[Outlaw1]

Here is my guess this is based on big trucks so i may be completely off. there are 4 driven wheel ends and normally power is provided to a front wheel end and than the opposite wheel end on the rear. Like a criss cross pattern. there are spiders that allow the axles on the same diff to turn at different speeds when going around corners etc. when you put the smaller tire on the front it made the spiders in the rear diff work really hard trying to provide equal power to the rear to compensate. which made it get hot and start melting stuff and burning fluid.

when i dyno a semi i have to lock in the power divider so both axles get equal power. if i do not BAD things happen

In this case the rear spiders would be take a beating, but they're not having to work the rear wheel tire/tire bias while going in a straight line. The front would have the spider gears working, but they are not in an LSD, so it's not THAT bad on the spiders.

Because of the short tire up front, the front tires are trying to spin slower than the rears with the 50/50 split. The drive side of the ring and pinion in the rear diff was in a serious bind (as well as the coast side of the front diff & the center diff).

[blackford76]

DFBrews gets a cookie!
You have to remember on a Subaru it isn't just a mechanical system, the computer lost it's mind with the odd sized tire and seriously over worked the rear diff trying to compensate. That is why they wanted to add the fuse to disable the awd system.
The whole time the small wheel was on the ground, the computer sensed that one wheel was losing grip, so it sent power to the rear, it uses the abs wheel speed sensors to tell which wheel is "in trouble".

[DFBrews]

DFBrews gets a cookie!
You have to remember on a Subaru it isn't just a mechanical system, the computer lost it's mind with the odd sized tire and seriously over worked the rear diff trying to compensate. That is why they wanted to add the fuse to disable the awd system.
The whole time the small wheel was on the ground, the computer sensed that one wheel was losing grip, so it sent power to the rear, it uses the abs wheel speed sensors to tell which wheel is "in trouble".

Thanks Rob when the hell are we going shooting again


and outlaw no AWD is a true 50/50 split in power... ever

[blackford76]

Thanks Rob when the hell are we going shooting again

I dunno, I have weekends off now, but work nights. We're gonna have to figure something out, I've got 2 new 1911's I haven't shot yet.

[Hoosier]

Subaru's computer is used to figure out which wheel needs to have power sent to it dynamically so that when you start to slide it will transfer power away from wheels that are slipping and to the ones that are gripping. Perhaps it kept sensing that all the wheel except the right front were slipping (turning slower) and as a consequence kept trying to send power to the back?

I didn't know that about the Subaru and it's undersized spare. I've owned one for almost 12 years now.

H.

[rockhound]

you can cause the same issues in a standard rig also.

the spider gears in a differential are not meant to be used for extended periods of time.

you turn the corner the work, you go in a reasonable straight line the see no use.

if you are running different size tires on even a normal axle you can overheat the system and fry it. one end of the differential will be turning at a slower pace than other causing the differential to work at normal highway speeds causing it to overheat.

if you are doing 2500 rpms going down the highway and the ratios of your tires is off by say 5% the axle is then differential is turning at 125 rpms for the next 60 miles. under normal circumstance the spider gears would never see those kinds of rpms and if the did it would be for seconds at a time.

[MED]

Once you put a binding situation into the driveline on AWD, past the allowed slipping point, the weakest link will break. If the front wheels are turning/slipping respectively faster than the rear wheels on a locked AWD system, force will be applied to the rear drive. The drive shaft to the rear drive wants to go faster than the wheels are traveling thus putting pressure on the diff.

Anyhow, this is my guess….