I just put a set of 4.11s in my ride... A buddy of mine with a 79 chevy put a 96 LT1 in his and some 4.11s, and after a while he came up with a bent driveshaft, credit given to the shaft not being balanced well enough to handle the extra rpms that lower gears produce. Will I need a new driveshaft as well? I've heard these newer ones are balanced better but I dont know how much better. Theres no speed limiter on this pickup, and sometimes I like to find a looooong stretch and let it eat up the road.... How much will the stock shaft take?

 

never heard of anyone having a problem with there stock driveshaft.

 

The driveshafts rpm's are not effected by the gears you select to go with.
The engine and tranny only effect the driveshaft speed. With a lower(bigger numbered) gear ratio the engine and tranny are able to turn your wheels and tires with less strain.
If anything that will increase your tranny, u-joints, and driveshafts life expectancy. At the same time it puts a bit more wear on the engine due to higher rpm's it will be turning. Also extra stress is placed on the axles in your differencial.
Your actually increasing your torque & hp ratio at the rear wheels with 4.11's over 3.42's or higher (smaller numbered) ratio gears.
Your friends driveshaft probably bent due to age, bad u-joints, or the engines increased HP & torque it produces.
If you do the gears, go a head and replace all of the u-joints on your driveshaft with heavy duty or higher quality ones for a safety precaution.
The stock driveshaft will handle the hp & tq your engine is putting out.
Even S/C, turbo, and n02 guys are still using their stock driveshafts.
Hope this helps.

Jim

 

It's not so much his HP that will kill DS' (it obviously will if you R running high enough, but usually it is a U-jont that will fail under high HP/TQ scenarios)...it's the speed of the DS. I agree that the gear doesn't directly effect the DS rpm, but it does indirectly. If you go with 4.11's you will obviously be raising your normal engine rpm at any given speed. Do the math...DS failures usually occur at high speeds...just ask the many guys on here who have lost DS' on a dyno. If you have 4.11's your driveshaft will have to spin that much more to make the tire turn one rotation. 3.73's will make the DS turn less times to get the tire to turn one rotation. Sooooo you need to know the critical speed of one's driveshaft to "guess" when it will fail or bend. The permanent bend comes from spinning so fast it causes the shaft to replicate the harmonics...like a sound wave. Higher gear #'s(4.11) will cause the shaft to spin faster at the same speeds as a lower gear #'s(3.73).

just as example...say with the 3.73's at 120MPH you are at 5000RPM(engine) and in OD. I belive the gear ratio is .75. So 5000 rpms divided by .75 is 6666 rpm's for the driveshaft.

now the 4.11's at 120MPH your are at say 5600rpm...divide that by .75 and the driveshaft speed is 7466 rpm's.
Now figure out what your driveshaft's critical speed is.

http://www.wallaceracing.com/driveshaftspeed.htm

I changed my single shaft to a 2 piece...thought you guys would appreciate this pdf file. Lots of really good tecnhincal info in it. http://www2.dana.com/pdf/J3311-1-HVTSS.PDF

 

i twisted mine in 2 pieces

 

if you get your stock one balenced you should be okay for a while (get new u-joints too if they are still the stock ones)... you get all this done for around 60-70 bucks at a good driveline place (that price is for balencing the shaft, and front and rear u-joints w/ the grease fittings)

 

The thing is, if your rev limiter is set at 6,200, then the driveshaft will not be spinning any faster than it would in a vehicle if it had 4.10's or 3.42's. at the same RPM. The rear gears will be the only thing changing the amount revolutions the axles spin at a given rpm. The driveshaft will spin as fast as the tranny & engine will spin it.

Jim

 

sigh...you R right the shaft will spin the same speed with 4.11's or 3.73's at 5000rpm, lol after all 5000rpm is 5000rpm. I am just pointing out what I thought he was asking...speed. The engine and trans and DS will always do the same thing even with a rear end change. Just not at the same speeds anymore. If he likes to go hummin down the back road at 120mph and he normal did this at 5000rpms. He does need to realize that shaft is gonna spin faster now...at 120mph he will be at a higher rpm. Hell if the limiter is at 6200 and he could somehow have enough *** to get the truck to get to 6100 rpms in OD...that shaft is spinning at 12200rpms. That's dangerously close to destruction. I am sure he will be fine...just chiming in with how you figure out how fast your shaft is spinning. From experience of replacing shafts. If it's the one piece 5" shaft that is about 72" long...it's critical speed is some where around 12K.

 

Nobody on this board can pull 6200 rpm in OD (60e,65e,70e) with 28" tires on the street with 4.10 gears in their street truck. That would give 180 mph.

i went from 3.08's to 4.10's and the driveshaft was spinning MUCH quicker at 100 mph than it did with the 3.08s.
5000 ds/rpm with the 4.10's vs. 3712 ds/rpm with the 3.08's

Pinion angle, driveshaft rpm, shaft length and shaft diameter become critical at high speed, esp. with high low rear gear ratios.

I wouldnt pull my engine up to rev limit (5800 rpm) in OD, that would give me 8285 shaft/rpm, I guarentee something would let go.

I have had a trans output shaft shear off and throw the front of the shaft at 4300 shaft rpm and that was an experience I'd soon not repeat.

i see why driveshaft loops are such a good idea.

peace
Hog

 

Good gosh, you guys are talking about the OD gear?
That's not even a sporty gear, but besides the point.
When you changed to 4.10's your driveshaft's speed still stays the same at a given RPM in OD. No matter what gears you may have installed. At that point your maximum MPH may not be able to be achieved due to the engine hitting the rev limiter before you get to a certain high speed MPH.
Hell, less stress is put on the driveshaft with lower gears.
To actually some this all up, the guy that originally posted this thread doesn't really have enough power output to hurt the factory driveshaft at this point.
So....he will be fine with the factory driveshaft for now.

Jim