Pinion Angle ?

[Schmo]

I've noticed a slight vibe when accelerating at freeway speed and have found that the vibes are much more noticable when the Jeep is not weighed down with a full tank of gas, tools, hilift, etc. This brings me to my question . . . I've read that if the vibe is only during acceleration, then the pinion angle needs to be lowered. But in my head (probably the problem), I imagine that the angle of the driveshaft would increase when the Jeep is not weighed down which would theoretically lower the pinion angle (in relation to the driveshaft) . . . right?:dunno: What am I missing here? When I realized that the Jeep felt better weighed down, I thought by lengthening the control arms to pitch the angle of the pinion up, I'd get it closer to the angle it is when it's weighed down. But then I read that acceleration vibes require lowering the pinion angle and I am left scratching my head.

[Shadly1]

Load it up and check the pinion angle. See how much it changed. Actually, you might want to check it unloaded. You might be settling already.

[Schmo]

I'm confused as to whether I need to lengthen or shorten the control arms. From what I've read, an acceleration vibe requires the shortening of the arms (lower pinion angle) whereas I'd assume that by lengthing the arms, I'd be able to get the angle closer to what it is when the Jeep is weighed down (which I think would be raising the pinion angle). :dunno:

[Shadly1]

Weight makes the pinion higher. Sagging and settling make the pinion higher. Acceleration makes the pinion higher. You set your pinion a little low to compensate for this. If you want the pinion lower, shorten the UCAs. If you are accelerating with a full load, you may be passing centerline on your pinion angle. Don't do anything without measuring the angle first. I just saw a magnetic angle finder at Harbor Freight on sale. Couldn't have been more than $5. Just keep in mind that if you set it properly unloaded, it won't behave right loaded and vice versa. I'd set it at the weight you use it most and tough it out for less or more weight. You'll never get it perfect for all situations.

[Schmo]

Weight makes the pinion higher. Sagging and settling make the pinion higher. Acceleration makes the pinion higher. You set your pinion a little low to compensate for this. If you want the pinion lower, shorten the UCAs. If you are accelerating with a full load, you may be passing centerline on your pinion angle. Don't do anything without measuring the angle first. I just saw a magnetic angle finder at Harbor Freight on sale. Couldn't have been more than $5. Just keep in mind that if you set it properly unloaded, it won't behave right loaded and vice versa. I'd set it at the weight you use it most and tough it out for less or more weight. You'll never get it perfect for all situations.

Hey Shadly,

Can you shed a bit of light on this for me? I understand how accelerating can make the pinion angle higher as the force would twist the axle up, but I don't understand how the sagging, settling, and weighing down would make the pinion angle higher. I figured that the settling, sagging, and weighing down would lower the T-Case . . . therefore decreasing the angle of the DS . . . which would in essence create a similiar affect as increasing the angle of the Pinion. If this is the case, then wouldn't I need to increase the pinion angle to create the same angle when the Jeep is not weighed down? I'm assuming that the pinion angle is not changing with the extra weight/sag . . . is this where I'm getting hung up? When I first had the UCA's adjusted, I measured the angles and with a full tank of gas and highlift, the pinion was right about 1-2* lower than the DS so I thought it would be perfect, especially after setting a bit.

[Shadly1]

Sagging, settling, weight and acceleration have the same effect on pinion angle... It decreases the angle in relation to the DS. If you set your pinion angle 2* low, you will lose that 2* when you factor in sagging, settling, weight or acceleration and you will approach 0*. If you exceed centerline(0*) by more than a degree or two, you will get vibes on acceleration. Those vibes will be more intense with more weight. Make sure this is the case though by checking the angle. You can shorten the UCA to give you your angle back but if you overdo it, you will have vibes when you are cruising on the freeway unloaded.

I love geometry

[Schmo]

Sagging, settling, weight and acceleration have the same effect on pinion angle... It decreases the angle in relation to the DS. If you set your pinion angle 2* low, you will lose that 2* when you factor in sagging, settling, weight or acceleration and you will approach 0*. If you exceed centerline(0*) by more than a degree or two, you will get vibes on acceleration. Those vibes will be more intense with more weight. Make sure this is the case though by checking the angle. You can shorten the UCA to give you your angle back but if you overdo it, you will have vibes when you are cruising on the freeway unloaded.

I love geometry

This is why I'm so confused . . . if the extra weight/sagging adds to the problem, why does the Jeep feel better when loaded up? Shouldn't the vibe be worse then when I'm loaded up and accelerate?

I liked algebra . . . geometry not so much

[Shadly1]

This is why I'm so confused . . . if the extra weight/sagging adds to the problem, why does the Jeep feel better when loaded up? Shouldn't the vibe be worse then when I'm loaded up and accelerate?

I liked algebra . . . geometry not so much

Oh, I misunderstood then. If it feels better loaded and accelerating, then you are ideally set for a loaded jeep. When you unload it, you may have greater than 2*. All this is hard to tell without having it right in front of me. It may not be the CV shaft at all, you know? Brand new knobby tires have the same effect. Vibes when it's unloaded and coasting, no vibes when it's loaded and accelerating. All you can really do with that CV shaft is check it every so often with an angle finder to make sure it's in spec. If your sure it's the shaft, try having it balanced locally.

[Schmo]

More head scratching

So I stopped by Harbor Frieght and picked up a floor jack, jack stands, and the angle finder (3 bucks) so I don't have to keep borrowing them from Tomb Raider 940 (thanks bro!).

Here are the measurements . . . I'll try again tomorrow with more light and see if I can get the weight on the DS out of the way.

The DS is at 18.5*

Here's where it gets weird:

If I measure like Tom Woods tells me to like in the pic below, I get 16.5* which is on the high side of what I'm shooting for (1*-2* under). But when I measure the flat parts on the bottom of the diff opposite the cover, I come up with 18.5* which would seem to explain the acceleration vibe and mean I need to shorten the UCAs. I'm not measuring the bottom part of the diff right by the U joint . . . it seems to be anlged more than the rest of the diff . . . I was measuring farther back. Any idea why my diff measurements aren't matching?

[Schmo]

Found another supposedly more accurate way to measure the pinion angle so back under the Jeep I went. I measured loaded and unloaded with about a half tank of gas. The drive shaft angle is about 1/2* lower loaded and the pinion angle is within 1/4* either way. The pinion is between 1*-1.5* lower than the drive shaft in both situations so I'm leaving it alone and rather than messing with it, will just keep my tools and highlift loaded. I'd feel like a schmuck anyway if something came loose or something and I didn't have my tools. I've come to grips with being a schmo but am trying to avoid being a schmuck