by paynebrody on Thu, 01 Nov 2018 5:02 +0000
Just sharing my learnings. I just installed the Iron Man 'add a leaf' which lifted the rear end of my single cab kun26 slightly, along with a 35mm Lovells lift at the front. I started getting a resonant driveline related vibration at approx 5km/h, i.e. just taking off or just coming to a stop. After a week of faffing about and reading, this is what I found:
- If the input and output flanges (i.e. the trans case output flange and the diff pinion input flange) on a two piece drive shaft are not parallel, then the output flange will have a non constant velocity. I.e. its rotational velocity will be sinusoidal (i.e. it will pulse faster and slower). If the input and output flanges are parallel, and the yokes are correctly phased, then this sin wave velocity profile is cancelled out between the input and output yokes, resulting in a constant rotational output flange velocity
- If the output flange rotational velocity is sinusoidal (or pulsing), then even a well balanced driveshaft may vibrate, hence balancing a driveshaft probably wont totally fix this issue
- Shimming the centre bearing, in theory, will not alleviate this issue either as the parallelism of the input an output flanges will not be affected by the collinearity of the two driveshafts, i.e. even if you make the two drive shafts share the same rotational axis, the input and output flanges will still have a relative angle between them
- Adding leaf spring wedges or angled shims adjusts the pinion flange angle, or rear axle castor, relative to the trans case output flange and is the only (easy) way to make the in/out flanges parallel (you could also adjust the resting position of the trans case but this is much harder than adding leaf shims)
- ALL THAT BEING SAID, I did mess around with packing out the centre bearing because I was going on a trip and didn't have time to order or buy any wedges. I also did measure the relative angle between my input and output flanges and because the angle was as close to 0 as I could measure (i.e as close to parallel), I thought I might be able to get away without wedges (though it is possible the measurement was out by say 1deg, and even this small discrepancy will have an effect apparently)
- I used washers to play around with different centre bearing offsets, starting by packing the bearing down by approx 5mm, then 10mm, then 15mm, 20mm etc
- Only after I added 25mm worth of washers did I notice a significant reduction in vibration, i.e. almost gone. This created a slight angle between the two shafts. There is still the faintest vibration but enough to live with
- Not a good idea to use washers as they will prob allow the bolts to loosen over time so I machined some aluminium spacers of the same dimensions to fit permanently
Not sure what the moral is. I guess check the in/out flanges are parallel first. If they are pretty close, then maybe packing the centre bearing will help but you will need at least 20-25mm spacers. If your in/out flanges are off by 1-5 deg, you will likely need to add angled leaf shims as neither packing the centre bearing nor balancing the driveshafts will help as these are not the root cause/s of the issue.
Last edited by
paynebrody on Thu, 01 Nov 2018 5:09 +0000, edited 1 time in total.