Driveline Angle

[JMD]

I know this has been talked to death, but I wanted to double check my findings.

I have a Blueprint 302 block with FFR supplied Energy poly engine and tranny mounts with the TKX and IRS. I initially used three 1/4" shims supplied by Forte on top of the transmission mount to lift the tail but had a noticeable vibration. I added two thick hardened washers on each side, raising it another 5/16" (total of 1 1/16") and the vibration was better, but still noticeable and increasing with speed. My driveshaft went from 2.5 degree up angle to 2.3 degree down angle with this adjustment. I removed one of the washers (now at a hair over 0.9" total shim) and it is a lot better. I'm not sure it's completely gone, but much better. I'm not quite sure what 'normal' vibrations for this car are yet.

My down angle on the engine is 2.6 degrees, my driveshaft is at 0 degrees, and my diff has a down angle of 0.3 degrees (all in relation to the frame). Total angle of diff in relation to driveshaft is 2.3 degrees but I think that's as good as I can get it without changing motor mounts. I wonder whether there's room to add a washer in front of the top diff mounts to increase the diff down angle...that would probably put me within the desirable 2 degree specification. I will take a look tomorrow to see if that's a possibility.

I did some searching to see what angles were working for others, but there's a lot of confusion about it and the terms up angle and down angle are often mixed up when referring to the diff. I would love to know other angle people are seeing with their FFR supplied IRS differential.

Here is a pic of where it sits now:

IMG_4187.jpg

[CraigS]

In you pic I can see that the trans angles down toward the diff. But I am not sure of the diff. Does it point up slightly toward the trans? I ask because you either have a 2.9deg diff trans to diff or 2.3deg difference. Ideally trans and diff shafts should be parallel. They don't have to be exactly in line w/ each other but should be parallel. I would have expected an improvement w/ more shims under the trans since that would have brought them closer to parallel. Another thought. You mention the vibration increasing w/ speed. So that may not be driveline angle related at all. Usually a driveline angle caused vibration will be felt between 25 and 50mph and will be worse w/ more throttle. In other words you could go through that speed range at light throttle and never feel a thing. Maybe you actually have a wheel/tire balance problem?

[rich grsc]

100% agree with Craig. The 2* angle difference is for solid axle cars and is consider "ideal", the truth is drive lines can safely operate a twice that. Your difference isn't the cause of the vibration, unless the drive shaft itself is out of balance. The angle is ok and safe, just not "ideal" or perfect

[JMD]

The trans and diff are both at a down angle, so the difference should be 2.3 degrees. I did have the tires balanced, but wouldn't hurt to have them double checked. Changing the driveline angle did seem to have an effect on the vibration so I'm thinking it's related to that. I've heard of driveshafts not being balanced properly from the factory, but there seems an almost 0 probability of that being the problem. But I'll keep in in mind after eliminating all other variables.