Axle ratings...

SlimJim Z71

LawnSite Senior Member
Okay... on GM pickups with IFS, how exactly is the axle weight rating calculated? The actual weight is not really on the axle, more so on the torsion bars and control arms. How do they get their figures? I know that brakes, springs (torsion bars), axle, and control arms are all part of the final figure. Theoretically, if you upgrade to 3/4-ton torsion bars on a 1/2-ton, you'd probably be better off. I've heard of guys doing this, but I was wondering exactly how much the truck would benefit from that.

-Tim
 

DaveO

LawnSite Member
Location
N. Dighton MA.
Tim,

I believe the weight rating is determined by several factors. Thses would include bearing capacity, brakes, spring, and tire/rim capacity. Whichever is the weakest would determine the rating.

Dave
 

Chuck Smith

LawnSite Senior Member
Location
Nutley, NJ
Dave is correct. There is also the gears in the differential to consider. If the resistance at the wheels is too much, the pinion gear will knock teeth off the ring gear. Once a tooth gets knocked off, many more will follow!!

This is evident to me, by a friend who overloaded his 1/2 ton Silverado all the time. He went through 3 GM 10 bolt rear axles in 100,000 miles. He constantly carried a 3000#+ "cube" of bricks, or concrete blocks.

Another time, he had me put a new hitch on his truck so he could tow a bobcat on a trailer with a pintle hook. Talk about scarry! He left the rental yard, got about a mile up the road, and there was a long steep hill. About 1/2 way up the hill, his clutch started slipping until he started going back down the hill, while in 2nd gear!

He had to call the rental yard to come get their machine and deliver it to the job site for him.

Ironically, he had better luck with the front end. He ran a Western 7.5' Pro Plow on the truck for 2 seasons. The suspension was bottomed out 90% of the time, but the front end held up ok.

He finally wised up and bought a truck that suits his needs better. A 1999 F-350 Superduty PSD. That truck has handled all he could dish out with ease.

I think the term GAWR is a little out dated, as far as IFS trucks go. Yes there is still an axle in the front, but it handles weight differently than a straight axle.

Straight axles are sometimes "trussed" to help them handle added stress. This is to help prevent the axle tubes from getting tweaked or cracking.

For instance, my 77 Chevy K/20 has a GAWR for the rear, of 5,500# respectively. I know for a fact, that the axle itself is rated to carry 7,000# max.

The same axle is in my 80 GMC K/25, and it's GAWR for the rear is 3,980#. The difference? Brake drums and shoes, and the number of leaves in the springs. Also the tire sizes.

Typically, on a front IFS system, the weakest parts are the drag link, center link, pitman arm, and idler arms. They will wear out first along with bushings.

When I put 35" tires on my 77 Chevy, the first thing I noticed is the drag link began to wear out fast. Then again, no faster than my 80 GMC during a winter with a lot of plowing events.

~Chuck
 

DaveO

LawnSite Member
Location
N. Dighton MA.
Chuck,

I know of 3 guys who blew their 10 bolts last year. All were around 1990's, two blazers, one pickup. None of them worked their trucks. But all had oversize tires on them.

I actually saw one of the Blazers blow his. Had 33's on it. He pulled out into traffic, got on it. The rear tires chirped...then BANG, clunk, clunk, clunk. Took out the ring/pinion gear, like you said. Has a heavier setup in it now. I didn't realize they were THAT weak.

Dave
 

TLS

LawnSite Fanatic
Slim Jim,

The 1/2 ton and 3/4 & 1 tons had different front Ring gears(pumpkin cases too) 8 1/4" and 9 1/4" They also have larger brakes and bearings. The frame is beefier and yes the torsion bars are different. They used a lot of different rate and indexed bars depending on what was ordered (1/2, 3/4, 1T , extended cab, crew cab, reg cab, short and long beds, Z71, towing package, snow plow prep, etc.) I think that you may be on the right track with replacing your torsion bars. If I can remember correctly, the Z71's used a spongier, softer bar, that was more forgiving for off roading. Order a pair from a 3/4 T with a HD front end.

 

Chuck Smith

LawnSite Senior Member
Location
Nutley, NJ
Dave,

The 10 bolt has been weak from day one. I have no idea why GM started using the 10 bolt in place of the 12 bolt older models had. I had 2 friends also, who blew 10 bolts, and both had stock tires on the trucks with less than 10,000 miles on the odometers. One was an 85 K/5 Blazer, the other was an 86 K/10 pick up. Interestingly, a third friend bought a new 87 K/20, and blew the 14 bolt rear in less than 5,000 miles. All three of these guys had stock tires, and babied their trucks since they were still new.

Now, just try and find a 12 bolt rear axle for one of these trucks, talk about hard to get! We called all around here after my friend blew his second rear in that 93 Silverado. None to be found. I tried to talk him into a 14 bolt, but he didn't want different size tires and rims on the rear.


~Chuck
 

DaveO

LawnSite Member
Location
N. Dighton MA.
Chuck,

Your right. We looked for a 12 bolt to replace the 10. Forget it. Even a good used 10 bolt was hard to find, and expensive too. Like $900... We ended up replacing it with a setup made by Richmond Gear if I remember right. A limited slip unit.

Dave
 

cowboy

LawnSite Member
Location
Renton, WA
Well I won't say GM 10 Bolts are the beefiest axle, but I will say my 81 C10 has 331,000 and has been used for towing, tree business, etc, and has never been touched

but I have friends with Dana 60's that blew just over 100,000 mi.

of course my truck has a 250 I6 but it is a good motor,

anyone ever seen a late 70's Chevy or GMC with the 292 I6? I have been looking for one for a while and haven't been able to find one, this is what I would like for plowing, if not a 250 I6 but they are hard to find in 4x4's.

anyway, my two cents :)

Adam
 

diggerman

LawnSite Senior Member
Location
Iowa
Whoa there Cowboy,talk like that is treason around here.Your likely to be banned,if you don't know the truck you are talking about could never plow snow.There just is'nt enough horse power and it's far to old.So please try not to bring it up again you are really disturbing the 400hp new 1 ton truck gods.
 

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