Outer Limits 4x4 Board

Hi I am building a 89 sierra for my son and it has 31 inch tyres on it and I would like to know if 5.1:1 diff ratios would be to low or would they be ok for road use.

Thankyou Darryl

They'll be pretty low - expect revs to be up around 4000rpm for cruising with the 31's on - that's about 500rpm higher than stock.

It's OK, but not ideal.

Off road, they're not much chop - crawl runs about 41.1 - nowhere near enough for steep work. (OK for mud though)

Reduction transfer gears are pretty cheap these days. They'd be a better bet if you want to do off road work with it.

5.12's have their place, but if you want to correct road revs, try 4.6 gears from a three speed auto vitara.

Steve.

Most ppl ive seen with 5.1 diffs break pinions and crownwheels in real extreem stuff, go the transfer gears cheaper and stronger.
If you want to get real extreem then get a brace ring or cradel for the transfer, several ppl make them or you can buy a calmini off the shelf, makes them very strong

You may well be right, I have seen a 5.12 break, but it was a very old diff that had been badly set up at some stage and had a poor wear pattern on it. (I've seen some tall gears like 3.7's break too though)

Additionally, I have a hunch thrust on the pinion bearings increases with the higher ratios. This compounds the problem if crush sleeves collapsing, loosening bearings, accelerating wear (and seal failure) and leading to failure.

I've seen LOTS of sierra diffs with loose pinions. I have had my front diff setup with a solid spacer to try an eliminate this problem. Fingers crossed!

I reckon it's important to get sierra diffs setup by a professional. Consider it an investment.

Steve.

go the 5.1's...you'll love em. and steve u have it arse about, you decrease load on the pinion bearings as you go up numerically.

My reasoning was the smaller pinions have smaller contact surfaces on the ring gear and the angle of the helical cut is larger, so it stands to reason they thrust more.

Steve.

they are under no-where near as much load [ input power and torque ]to turn say a 31 inch tyre, so the thrust forces are reduced. the thrust forces on a 3.9 with 31's is way more, and combined with a reduction t/case is way more again. the angle of the cut is larger so they can get more tooth contact. they may have made the pinion smaller, but have maintained as much contact area as possible.

Just in case anyone wants to work out the thrust - here's some links to TIMKENS gear thrust data...

index of terms : http://pdf.directindustry.com/pdf/timke ... -_19.html#

Hypoid Formula : http://pdf.directindustry.com/pdf/timke ... 9-_21.html

Gave me a headache so I'll leave it to the numerically inclined.

back to the question, it will be numming at 4300 rmp @ 1ookmh. bit too high for my likings.and while the 4.6 will be running at 3800rpm @100kmh, good luck finding 2 front centres a wrecker. be cheaper to buy transfer gears
jason

5.1's 4057 rpm.gen 3 hopper 3723 rpm.

how are you working out your rpm? i'm getting 4310rpm for .865 5th gear, 1.45 wt transfer, 5.12 diff 31'' tyre @ 62mph
http://www.rocky-road.com/calculator.html
jason

transfer is 1.409. my chart is done at 60mph.

what model sierras have 5.12:1 diffs

5.1's are from vits