I was wondering that i have a a 02' F-150 Flareside 4.6 automatic and i have it drop about a 3/4 drop and running on 20"s, is it save to change the gears from 3.55 to 4.10s?

 

I saw your other posts and I've compiled some basic information for you from a drivetrain expert.

"Driveline vibrations can be caused by several problems. Worn universal joints or a driveline that is out of balance are often the problem, but driveline angle can cause a balanced driveline with good U-joints to vibrate. If the U-joints are bad, they can cause several different noises from squeaking, to clunking, to grinding, to vibrations. If the driveline is out of balance, it will vibrate with a steady pitch that increases as the vehicle speed increases. If the pinion shaft is out of alignment and not parallel to the transmission yoke, the difference in the angles between the front and back U-joints can cause the driveline to vibrate. If the vibration is due to improper angles, it will create a cyclic sound that increases and decreases in intensity. An out-of-alignment problem can also be identified by the change in the noise when accelerating or decelerating. As the pinion yoke torques up from acceleration or down from deceleration, the rear U-joint angle changes and causes the vibration to change. I have had more than a few customers come back to our service shop after a gear ratio change to complain that we caused a new vibration in their vehicle. In actuality, we inadvertently caused the vibration by increasing the driveline speed. When changing to a lower gear ratio (numerically higher) the driveline speed increases proportionally to the change in the ratio. For example, when changing the ratio from 3.08 to 4.56 there is a 48% change in the ratio and a 48% increase in driveline speed. This means that a vibration that would have been noticeable at about 90 mph is now noticeable at 60 mph. This higher driveline speed makes the vibration more noticeable because while it previously existed at 90 mph, it may have gone unnoticed due to the amount of vehicle vibrations it had to overcome in order to be noticed. If the customer installed taller tires and raised the vehicle before changing the gear ratio, the vibration is probably caused by the lift. This vibration caused by the lift and wrong driveline angles would not be noticeable until the new ratio was installed, since the tall tires and stock ratio kept the driveline speeds too slow for the vibrations to be felt. Vibrations due to wrong angles are usually easy to identify because they are harmonic or cyclic. This means that the vibration varies in pitch or intensity even when traveling at a steady speed. The results is a "whir ... whir ... whir" type of noise. The solution to wrong angles is easy in theory, but not always easy in practice. There have been many good technical papers written on this subject, so I won't go into detail now. I will however cover a few of the basics. In order for a two U-joint driveline to be free of vibration it must be in balance, and the angle of the front U-joint must be the same as, or very close to, the angle of the rear U-joint. If there is a noticeable difference in the angles, there will be a vibration. If it is not possible to match the front and rear angles, a constant-velocity joint can be used at the transfer case end with no angle in the U-joint on the differential end. Although it is expensive, and only works if the pinion is not offset side-to-side from the output shaft, this is the only cure in many lift situations. I have seen vehicles use a constant-velocity joint at each end of the driveline. This is more expensive but will fix any angle problem. Another solution is to rotate the differential pinion angle up or down to match the front angle. This is usually not easy and can be accomplished with wedges, by moving the spring pads, or by rotating the axles tubes in relation to the differential center section. For some vehicles there are aftermarket kits available for lowering the transfer case or installing adjustable length control arms. Although these may seem like extreme measures, they are sometimes the only way to eliminate or lessen vibrations. Note: If the vehicle has a differential pinion offset to the side from the center of the output shaft on the transmission or transfer case, the angles due to this offset will not cause vibration. This is because the pinion shaft and the output shaft are still parallel with respect to these side-to-side angles. I have seen some lifted vehicles that vibrate no matter how much care is taken to balance the driveline and match the angles. It is one of the things that we sometimes have to live with in order to have a lifted vehicle."

 

thanx for responding back,

i also replace the U-joints with some heavy duty ones and still did it, but i have it in the shop and now i am probably going to install a bigger driveshaft because its about 1" or1.5" too short from the York to go into the transmission. Everything else looks fine but i think my driveshaft is too short. And thats where i get my vibration and there is where i cause the problem to my transmission failure when going to higher speeds. Even though i dont want to change the gear ratio but i want to last more than 100mph on the freeway with no problem at all. Is that possible with my tranny of a 4.6L 02" F-150? it was 3.55s before. Please help!!!!

 

You're Welcome ! Yes, it's fine to run 4.10s and many have done this without problems. I installed 4.10s in my 2000 F-150 which originally came with 3.55s. They worked perfectly at every speed. It would bury the speedometer and cruise all day long at 70mph. How did your driveshaft shrink? Was your driveshaft indexed each time it was removed? Have you measured your u-joint angles? Have you checked your transmission extension house bearing? Have you put your driveshaft on a balancer? Was damage caused during the u-joint installation? Did your drop kit include rear axle shims? Have you received a second opinion by a qualified driveline shop?

 

well i did all that except to check the bearings of the transmission, how do u do that? it has shims for the suspension and all is balanced with the u-joint too. Angles are straight and thats what my dirveline shop told me and does not know what to do next? because you know i keep having my transmission damaged from the connection from the driveshaft piece, why is that???
what is your opinion Max?

thanx

 

The "slip yoke" is the part of the driveshaft assembly that connects to your transmission. The interior of the yoke slips in and out along a splined output shaft and the exterior of the yoke slips in and out on a bearing behind the seal. To function properly, the exterior yoke and the bearing must provide a smooth surface of proper dimension. The rudimentary way to measure gross function would be to lay under the yoke/transmission intersection, grap the yoke/u-joint area, and force it up and down in a bench pressing position. The next step is to remove the driveshaft assembly and examine the yoke surface where it rides in and out on the bearing. The seal can be removed and the bearing can be examined as well. If it was in poor condition, you would probably be able to run your fingernail down the yoke on the bearing wear pattern and feel some ridges/rough areas.