Determining correct axle length

[Andrew Robertson]

Hi All,

At the stage of sorting the axles on my McLaren M1C rep - which uses 930CVs at both ends, a 930 transaxle, and suspension of period design. Maximum axle length is at shock-limited full droop, minimum is at ride height and is about 12mm shorter.

Measuring the CV assembly centred and at zero angle the CV inner section that receives the axle is 4mm thinner (2 each side) than the CV outer main body, meaning that the theoretical circlip to circlip length on the axle is about 4mm shorter than the measured flange to flange distance. I'm assuming the CV centre stays central to the outer as axle angle is introduced.

I'm not sure how much extra length is customary to provide over and above this theoretical value, but it would be nice if 10mm were acceptable as that would accommodate all likely suspension adjustment and the slight left side to right side variance I have. This would mean nearly an inch of float at ride hight - there is enough depth for that but unsure if it's excessive.

I'm also planning to have my axles made up from a set of heavy duty porsche axles I have that are too long - I helped a friend with a cobra build and he had a jag IRS narrowed by cutting and shutting the halfshafts - Circle Track Engineering in Christchurch did the job and the result looked great and has been reliable. The axles I have neck down to 26mm and the finished result will be about 365mm long - is a cut and shut procedure valid from a strength and fatigue perspective?

Keen on your advice on both determing length and axle mods.

Cheers, Andrew Robertson, Wellington NZ

 

[Terry Oxandale]

The axle's inner race can move (float) about 10mm in regards to the outer race, which is 20mm total for each axle. Mine would only move about 12mm from full droop to full compression, BUT, my static ride height is not with the axles horizontal. What became more of an issue was the fact that I could not drop the transaxle down far enough in the chassis (inverted 930) to have the axles horizontal. This meant that minimum axle length was instead at about about 35mm of compression on the suspension. So what I did was lengthen the distance between the spiral clips by about 6mm. This allowed me to go to full droop without risking the balls binding up (hate it when that happens) in the cage. For those situations where the axle is even more inclined upward toward the case at static ride height, the need to push the inner races even further apart must be carefully weighed against more probable binding if not measured correctly.

 

[Dimi Terleckyj]

Provided you have at least 5 mm of float at either end of the suspension travel you should be fine.

The only other consideration is the axle's angles so they dont bind in the cages.

Dimi

 

[flatchat(Chris)]

Here's some clues for standard length options :-
www.dansperformanceparts.com/buggy/trans/buggytran1.htm

The distance between the flanges (gearbox and wheel) is what the axle length O/A is, the CVs do the rest -- is what I've figured anyways

 

[Terry Oxandale]

I agree with Chris for as much as I know about this. Chris' method is what I used to get me close enough to double and triple check my final lengths. His flange to flange dimension is what I used for total length of entire shaft, which in my measurements allows for a minimal amouint of material behind the circlip (or spiral lock), plus the thickness of the clip itself (or groove). Both of these account for roughly 5 to 6 mm per side, which then places the inner race of the CV about midway into the outer race, and allows enough float without bottoming out of the axle's ends against the flanges.

 

[BruceA]

Dont forget to add a wee bit more for when the engine and gearbox move around on the mounts. (assuming you have rubber mountings).

 

[Fred W Ballinger]

Depending how much you need to shorthen the shafts by, you might want to consider doing what I thought was a good idea I saw somewhere:

When you cut the shafts, leave say a 2" overlap in the length
Machine half the thickness of the shafts away at the ends to be joined for the lenght of the overlap (create a "D" shape)
Machine a say 3" long sleeve with an ID that is a snug fit over the shaft OD.
Slide the shafts into the sleeve so that the two "D" shapes mate and form a positive location.
Weld the shafts to the sleeve at the ends and with puddle welds through the sleeve
Check the complete assembly for straightness

Regards

Fred W B

 

[flatchat(Chris)]

Nahhh! it will twist and sheer eventally --from experience.