Pinion Angle
09-07-2009, 07:33 PM
#1
4th Gear Member
Thread Starter
Join Date: Jul 2008
Location: texas
Posts: 1,175
Pinion Angle
Working on a 68 mustang coupe, running a 351w based motor,backed by a tko600, thats been milled on topfor the 68, from American Driveline. The rearend is a factory 8inch. The car developes a buzz (pulsing) vibration at around 90mph with 2.78 rear gears, and at 70 with 4.11 rear gears.
Weve taken degree measurements from the valve cover, starter, trans yoke, driveshaft and rear yoke. What were running into is the valve cover,starter, and trans measure out at -2.5*, the DS measures out -2.5*, and the rear is 0*.
Trans_______ -2.5
Drivesahft ___-2.5
Rearend______0
We would like to get the trans up to 0* giving the rear and trans te same degree with a negatice degree onthe DS, but the trans is all the way to the trans tunnel.(well as close to hitting itas it could be). Need some help r infoon what to do. Should we try shimming the rear up towards the trans 2.5*, giving the trans and rear a -2.5 and the DS somewere around a 1*or so? any info will help greatly.
Also bee thinking going to a 9inch, is ittrue that the 9"pinion sits lower. Is so would it helpto set the pinion angle.
Weve taken degree measurements from the valve cover, starter, trans yoke, driveshaft and rear yoke. What were running into is the valve cover,starter, and trans measure out at -2.5*, the DS measures out -2.5*, and the rear is 0*.
Trans_______ -2.5
Drivesahft ___-2.5
Rearend______0
We would like to get the trans up to 0* giving the rear and trans te same degree with a negatice degree onthe DS, but the trans is all the way to the trans tunnel.(well as close to hitting itas it could be). Need some help r infoon what to do. Should we try shimming the rear up towards the trans 2.5*, giving the trans and rear a -2.5 and the DS somewere around a 1*or so? any info will help greatly.
Also bee thinking going to a 9inch, is ittrue that the 9"pinion sits lower. Is so would it helpto set the pinion angle.
09-07-2009, 07:37 PM
#2
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Join Date: Dec 2005
Location: Hills of California
Posts: 5,208
In theory setting the rear end to +2.5 would be the answer. Since that is a very cheep and easy possible fix I would try that first. Simply buy a set of 2.5 deg wedges and slide em in. They call this parallel... Setting the engine line parallel with the pinion line.
It might just work.
-Gun
It might just work.
-Gun
09-07-2009, 08:32 PM
#6
Moderator
Join Date: Dec 2005
Location: Hills of California
Posts: 5,208
yeah if you find the answer let me know I have been actively pursuing that same issue for over a year now. I think in my case its a DS balance issue that doesn't show up until the DS exceeds 5000 RPM which is easy to do with 3:80 rearend.
DO get the shims and DO try both of the above suggestions if the first one you try doesnt help or makes things worse.
Good Luck
-Gun
DO get the shims and DO try both of the above suggestions if the first one you try doesnt help or makes things worse.
Good Luck
-Gun
09-07-2009, 08:56 PM
#7
6th Gear Member
Join Date: Apr 2007
Location: California
Posts: 10,468
2+2 beat me to it. Get the wedge spacers and shim the rear axle housing, then shim the trans mount if you need to. Proper pinion angle actually varies from vehicle to vehicle depending on application and suspension types etc. Generally -2 is considered a good starting point, leaf springs sometimes need more like -3, but again, it will depend on how stiff the suspension components are and how much power you have.
09-07-2009, 09:35 PM
#8
3rd Gear Member
Join Date: Aug 2009
Location: Michigan
Posts: 516
You need to have the pinoin angle at -2 to -5 degrees. When you step on the gas, the rear end will try to rotate back. If you start out at zero, and it rotates back then you end up with what is commonly called a broken-back alignment. It will wear out U-joints and cause a vibration, and will make the tailshaft bushing wear prematurey as well.
On a race car we set it at between -5 and -7, but we have a lot more suspension movement, and probably more HP too. Going a little too far to the negative will not hurt in a street car at all. I'd get as close as I can to -5 with the shims.
On a race car we set it at between -5 and -7, but we have a lot more suspension movement, and probably more HP too. Going a little too far to the negative will not hurt in a street car at all. I'd get as close as I can to -5 with the shims.
09-08-2009, 05:05 AM
#9
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Join Date: Oct 2006
Location: Cork, Ireland
Posts: 6,417
ya. on hard accelleration the (rear axle) pinion will rotate downwards, since my pinion is already too low (according to the proper geometry above), the vibration in my car is way worse when I accellerate in 3rd at 80 than when I cruise at 80.
Gonna shim it as well this week or next week. Have a 2.5 and a 4.5 shim. I'll probably even need the 4.5 degree one from what I measured.
If you change the axle to a 9" you'll have to re-measure again
And yes, as all the others say: driveshaft angle doesn't matter. You have to have the same angle at tailshaft against the pinion. just the other way around.
Another thing to look out for is that the tailshaft is centered in the car. not pointing to the right or left unless the yoke is offset
Gonna shim it as well this week or next week. Have a 2.5 and a 4.5 shim. I'll probably even need the 4.5 degree one from what I measured.
If you change the axle to a 9" you'll have to re-measure again
And yes, as all the others say: driveshaft angle doesn't matter. You have to have the same angle at tailshaft against the pinion. just the other way around.
Another thing to look out for is that the tailshaft is centered in the car. not pointing to the right or left unless the yoke is offset
09-08-2009, 07:20 AM
#10
5th Gear Member
Join Date: Sep 2006
Location: Australia
Posts: 2,266
what are you running for a drive shaft and what revs is the vibe? does it go away if you increase the revs?
i cant do the math but there's a relationship between driveshaft length and diameter and the rotation speed. at some point the shaft will start to bow in the middle (harmonics) and that causes a vibe. but the kicker is that it can happen at exactly half the revs too. so you tend to build a drive shaft suited to more revs than the engine can do but then you can sometimes have an issue within the range. example...your engine is good for say 6000. You use a drive shaft that's good for 8000. between 3900 and 4100 you may experience a vibe.
assume you've checked wheel balance and axle runout? 8 inch axles are prone to twisting under decent torque with modern stickier tires.
good luck
i cant do the math but there's a relationship between driveshaft length and diameter and the rotation speed. at some point the shaft will start to bow in the middle (harmonics) and that causes a vibe. but the kicker is that it can happen at exactly half the revs too. so you tend to build a drive shaft suited to more revs than the engine can do but then you can sometimes have an issue within the range. example...your engine is good for say 6000. You use a drive shaft that's good for 8000. between 3900 and 4100 you may experience a vibe.
assume you've checked wheel balance and axle runout? 8 inch axles are prone to twisting under decent torque with modern stickier tires.
good luck
