8 3/4 thoughts!

Anonymous · Post by **Anonymous** » Sun Sep 23, 07 12:52 pm

After stripping a few rear ends recently for repair I got to thinking on the strength auguement of the 8 3/4

If a locker or any type of LSD is fitted then 489 every time

But in an open case i'm not so sure the pinion size makes much difference except in certain circumstances.

Take a street driven car with an open 8 3/4, very easy to spin one wheel out usually on a slow corner, inside wheel spinning flat out, outside wheel barely turning, the only HP this wheel is getting is exactly the same as the friction between the inner wheel and the tarmac.

Now think about the actual diff gears, 1 side gear is virtually stopped, the other and the 2 planetaries, carrier, are spinning flat out around the slower side gear, the weak link here is the cross shaft and planetaries, the planetaries will pick up on the cross shaft, snap the scroll pin and trash the carrier, the only way pinion size will matter in this instance is if the spinning wheel finds good traction before the diff gives up.

Seen 3 489 diffs up to now with the scroll pin shattered and the carrier worn badly.

Point of this is if you have an open diff be careful spinning out 1 wheel whatever the pinion size as planetaries aren't designed to spin like that for too long

MilesnMiles · Post by **MilesnMiles** » Sun Sep 23, 07 12:55 pm

Will, 'spinning one wheel'...as in burnouts with an open diff?

Anonymous · Post by **Anonymous** » Sun Sep 23, 07 1:06 pm

not really Miles, the example above is the worst case, slow corner on the street.

It comes down to the speed difference between the 2 tyres, both relatively equal then there should be no problem.

Don't forget if 1 is stopped the other is doing exactly what it says on the speedo, the side gears are no more than 3" in diameter (maybe smaller) so if the speedo says 20mph go figure the speed of the planetary gears, I'm not clever enough to figure that out but lets just say DAMN fast

Anonymous · Post by **Anonymous** » Sun Sep 23, 07 1:23 pm

dead 489 cross shaft

Anonymous · Post by **Anonymous** » Sun Sep 23, 07 1:26 pm

diff gears

yes it's a 9 1/4 but it's the same really

imagine 1 side gear stopped and the rest rotating around it at the MPH it says on the speedo

Kev · Post by **Kev** » Sun Sep 23, 07 4:49 pm

I always wince at the burnout comp when one leggers are giving it the berries. After a while the other wheel sometimes joins in, no doubt where the planetaries have seized up!!!

Anonymous · Post by **Anonymous** » Sun Sep 23, 07 5:10 pm

the other wheel joining in is relative to the traction on the spinning wheel, as the temp of the tyre increases it will be more sticky so more resistance to the spin so the same amount of HP is sent to the slow wheel.

As the slow tyre spins it heats up so it gets more traction, can go either way, they either equal out and everything is good to go OR theres still a huge difference in speed

Matt · Post by **Matt** » Sun Sep 23, 07 5:51 pm

If one roadwheel is stopped, the other roadwheel is doing twice what it says on the speedo. Planet pinions are rotating at same rpm as diff casing (provided sun & planet pinions are same diameters)
Whatever , it's still a lot to ask of a plain bearing.

Anonymous · Post by **Anonymous** » Sun Sep 23, 07 6:17 pm

I disagree with that matt

crown wheel and pinion still has the same ratio, that doesn't change

it's just the carrier and every thing in it rotates around the slower side gear

Anonymous · Post by **Anonymous** » Sun Sep 23, 07 6:48 pm

Interesting topic Neil.

Matt · Post by **Matt** » Sun Sep 23, 07 8:32 pm

Hi Neil,

C&P ratio doesn't change and carrier is still rotating at the same speed . But the axles are not fixed to the carrier.

If one roadwheel is stopped, the other roadwheel rotates at twice original speed.

Dave999 · Post by **Dave999** » Sun Sep 23, 07 10:54 pm

so does that mean with a single spinner you are more likely to do the planetary gears in or snap the splined end of an axle rather than rip the teeth off grown wheel.
dave

Anonymous · Post by **Anonymous** » Mon Sep 24, 07 7:49 am

Dave
I think which bit fails is down to what happens with the spinning wheel,

if it carries on spinning at whatever speed then theres a good chance the planetaries will sieze or pick up on the cross shaft, there's no bearing between the 2 it's just steel to steel, also they must be forced outwards into the carrier body while all this is happening so the thrust washers take a beating too.

Now if the spinning wheel suddenly finds good traction then anything could break.
If both wheels have traction then it's back to C&P and shafts

It would be interesting to find out how many people have broken a pinion, what pinion it was, and whether it just snapped, or snapped because some other peice of scrapnel stopped the whole centre section dead.

Matt, I understand the theory, but in practice i've never seen a spinning wheel doing twice what the speedo says, I've had to recover/move HGVs, Loading Shovels and dump trucks up to 100ton, axle failures are common, they have planetaries at the wheels also, so when these fail it locks the wheel dead, trick is remove the wheel, chain the axle up that side and you can move the thing on the other wheel, a loading shovel is 4x4 so say a front hub is smashed, the 2 axles are driven at the same speed (no diff in the transfer case) the other front hub doesn't rotate any faster than the back 2, this is just what i've seen

Dave-R · Post by **Dave-R** » Mon Sep 24, 07 9:38 am

Certainly when you are turning one wheel by hand to count the ratio against the driveshaft it throws the count off by x2. So yes. I agree with Matt. If one wheel is not turning at all the other goes twice as fast for the same driveshaft speed (ie speedo reading).

We used to take advantage of that to get our Ford Escorts to spin a wheel turning left from junctions back when I was a naughty lad.

Dave999 · Post by **Dave999** » Mon Sep 24, 07 2:58 pm

Wil wrote:Dave
I think which bit fails is down to what happens with the spinning wheel,

if it carries on spinning at whatever speed then theres a good chance the planetaries will sieze or pick up on the cross shaft, there's no bearing between the 2 it's just steel to steel, also they must be forced outwards into the carrier body while all this is happening so the thrust washers take a beating too.

so 4 gears on a cross are better than 2 on a single shaft then.

are any mopar diffs 4 planetary gear diffs?

sorry i'm on a bit off a diff frenzy at the mo. been reading stuff. making plans that probably won't come to fruition this decade

packages arriving in the post etc

dave