Why are the u-joints smaller on the front driveshaft?

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I thought the 80 used a 50/50 torque split unless there was slippage - how come the front u-joints are so much smaller than the rears? Shouldn't they all be the same size if they are transmitting the same amount of torque?

I am 3/4 of the way through replacing all my u-joints. As I progress it gets a little easier, but I think I would have to do 20 or 30 to get comfortable with the job. I bought a 12 Ton hydraulic press from my local Harbor Freight store for $135, and I would hate to try the job without it.

http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=33497

Thanks.

John Davies
96 LX450
 

fzj

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I've always kinda wondered why that is too, and it's not just the u-joints, alot of full time four-wheel drive vehicles seem to have all smaller components up front, diffs, u-joints etc.... It would seem to me a system is only as strong as it's weekest point!
 
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A press?? Use a hammer, a few sockets and block of wood...that's the man's way!! :D

My guess is the rear driveshaft being stronger is just how vehicles are mostly used. Normal 4wheeling, towing, etc, all loads the rear more than the front. So even though the torque might be 50/50 front and rear the front possibly has a better chance to spin than the rear, so the rear's components would need to be stronger. Even wheeling hard this is definetly true, going up a steep hill, etc the rearend takes more weight and therefore is doing a higher share of the work.

Yes the system is only as weak as the weakest link, if everything is being used the same, if the frontend doesn't get as much weight, even though it's smaller components, it could actually be stronger than the rear, because the rear has a harder task at times. If you flip roles here and backup a steep hill so the frontend takes the harder task then you can easily break things. I've seen many, many axles broken when people were trying to backup steep parts, it seems the frontend on most vehicles isn't quite ready for the added weight and such that comes from backing up trails.

Also if you lock your center diff then all that 50/50 stuff goes away, you can twist either driveshaft without affecting the other one.
 
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Often a manufacturer will use parts that have another application that's heavier. In the 80s case, the rear DS and axle may be used under a medium duty RWD truck/bus/mining vehicle so it has to be tough enough to take 100% of the engine power in an extreme application. The assembly is also likely used in part time 80s (or at least engineered for a part time 80) where again 100% of the engine power will go to the rear most of the time.

Then the front axle/DS will be designed to handle full time use under an 80 where it will at most see 50% engine power on a continuous basis, low range 100% engine power only infrequently when locked and used heavily offroad. So, while it can easily handle the 100% low range, this is not as tough as what the rear shaft/axle components were designed for.

You'll also see this on almost every other mfrs 4WD vehicles.

DougM
 

cruiserdan

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The rear drive shaft u-joints are the same as used on both ends of 62's, 60's 55's 40's etc. The fronts are as used on both ends of 4WD mini trucks.

As many of our regulars know, the front diff in the 80 is smaller than the rear and is similar in size (8 inch ring) to the mini truck. In addition the front is a reverse rotation drive with a top pinion.

My read is that Mr T sized the front drive components accordingly and decided that it was good enough.

U-joint failures, of any kind, are rare in an 80 and as such I feel that Toyota knew what they were doing when the u-joints were selected.

Front diff failures are quite infrequent as well as are birfield failures.
 
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Thanks for the replies. I figured there was a good reason for the differences, but i didn't know what it was ;)

BTW, I am done with the u-joint replacement and my driveline clunk is gone ;)

John
 
R

robbie

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Take a look at a 100 series, the u-joints are the same both front and rear. and guess which u-joint it is? Why it is the mini truck size ones and it is reversed. With the slip yoke at the differential. Now go figure that one out. It must be that toyota has over sized stuff for a long time and with newer materials coming along, they can make smaller stuff do the same as the bigger stuff. Who says bigger is better? robbie
 
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Are the front and rear geared the same? If the front shaft turns faster it wouldn't need to be as stout. Same goes for 100 series diffs.
 

wngrog

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robbie said:
Take a look at a 100 series, the u-joints are the same both front and rear. and guess which u-joint it is? Why it is the mini truck size ones and it is reversed. With the slip yoke at the differential. Now go figure that one out. It must be that toyota has over sized stuff for a long time and with newer materials coming along, they can make smaller stuff do the same as the bigger stuff. Who says bigger is better? robbie
Totally different. The 80 series has the reverse cut 8" 3rd in it because it runs a rear tie rod setup. It is there for clearance issues.

The 100 has an IFS and there are no clearance issues so they went back to the large Ujoint in the front.

When they designed the 80, they did not have a plan to replace it with the 100 in the design stage...come on??
 

wngrog

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scottm said:
Are the front and rear geared the same? If the front shaft turns faster it wouldn't need to be as stout. Same goes for 100 series diffs.

Unless you run a different size tire front to rear, the gearing must be identical or nearly identical.
 
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robbie

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Nolan they went to the smaller u-joint both front and rear drive shafts on the 100 series. You may not of read my post right. they did not go back to the big u-joint.
The gearing is the same on the front and rear of the 80 series. The 100 series gearing is the same also front and rear, but went to 4.10 when the 5 speed auto came in, instead of the 4.30 gears on the early 100's.
Come on what nolan, I do not understand what you are trying to say.You may want to stick to what you know, makeing chey, ford, toyota monster trucks. later robbie
 
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