Diff Install lower mount space, 4mm either side!

[CtrMint]

Hi,

I've been working on getting the new diff into the 420R this afternoon, but I can't get away from a feeling something isn't right with the carrier. 

I've placed the diff into the car using a rod through the top mount and then installed the lower bolts with just enough of the bolt through the chassis and into the carrier.  Doing this allows me to test fit the positioning of the diff carrier and measure the spacing etc.

What's troubling me is the spacing on the lower bolts.  If I place the carrier in the middle at the bottom I'm looking at 4mm spacing on both sides.  That seems pretty large too me.  Certainly not the spacing I had on my original diff carrier.

Looking at the structure of the carrier I can't see that the lower arms could be bent in during shipping etc. Nevertheless I'm left wondering why the gap is so large this time around.  Also If I measure the width of the carriers at the bottom mount, there's a different of about 3mm.

Any thoughts, should I just space it and carry on?  

Thanks

[Beagler]

 Choice is,

swap carrier for original or shim new one equidistant as required. 

[CtrMint]

Yeah, I appreciate that, but wondered if such a gap was normal.  My old carrier was certainly closer.

[ScottR400D]

Nothing's 'normal' with a Caterham. 

What you're suggesting is that there could be 2-3mm difference in the dimensions of the diff carrier. I would say that's quite likely. 

From memory I had about 2mm one side and 3 the other but the replacement diff's carrier was different, though I can't remember what that was. It never occurred to me that it wasn't to be expected! 

 

[CtrMint]

I've end up swapping the carrier over, the spacing on the bottom mounts was just wrong, I'm sure they don't look parallel too. There's about a 3mm difference between the original carrier lower bolt spacing.  I'm really starting to second guess the install process and where the reference point is when making sure the carrier is equidistant. 

Looking at the older build manual it says'

Measure from either side of the differential carrier to the outboard side of the chassis (outboard of rear ARB mount – 1” BOX) to centralise. During this process there will be a need to pull the bottom bolts out and use shim washers (item 21 in pack 30Z1067A) inserted in between the metalastic bush and carrier as necessary.

So the diff low bolt mounts should be centralized against the ARB mount?  

I have to say I've not done that previously, I always aligned it within the chassis mount, as per the Ikea manual illustration.  But regardless, if there is such a variance in the carriers as shown by the two I have, where is the reference point?  Or am I over thinking things here?  

Surely the only accuracy comes from the diff itself, shouldn't I be measuring from the drive flange, basically the casing?

 

 

[CtrMint]

I don't suppose anyone has any guidance on this, thanks

[Dave7vx]

PM sent 

 

[Wrightpayne]

Ring Caterham.

[ECR]

The bottom line is that the faces of your wheel hubs need to be equidistant from the centre line of the vehicle which is established as the point on the chassis equidistant from your suspension pick up points on the chassis

[ScottR400D]

#9 The position of the hub faces will be set by the location of the De Dion and its associated components. The diff could still be off centre. 

I took lots of measurements from the chassis rails to various points on the diff. I also made sure each side hub a the same distance outside of the chassis rails. 

I doubt it's too critical to have the diff exactly central but it's good to be close, for prop alignment; assuming the 'box is central and ignoring the fact that the prop isn't aligned properly in the vertical plane anyway! 

[aerobod - near CYYC]

The diff should be able to be centred within 1mm or so, the key test after centreing is to pull the driveshafts out a few mm against the CV spring, then gently release it to spring back into position. The spring tension should be even side-to-side, the seal shoulder on the driveshaft should be even and centred in the seal and the driveshaft should return to position to feel solid against the end of the hollow shaft next to the bearing face.

The free lateral movement of the driveshaft to allow it to always stay in full contact against the diff side shaft is key to avoiding oil leaks in my opinion, based on my experience of 35,000km with no leaks and the oil up to the "old" recommendation of the bottom of the fill hole.

[CtrMint]

Thanks all.

Comments have been really helpful.  I keep swinging from viewing the topic as me being overly fussy/paranoid to thinking it doesn't matter that much.  I guess ultimately I want to repeat the install and reduce any undue wear on the Titan, possibly noise, but that's wishful thinking.  There's also the satisfaction of doing the job precisely and correct.

I'm pretty much happy on how to approach the top bolt. I'm going make up a jig with an adjustable combination square and measure the diff using that to the box section.

That just leaves me with the front lower mounts.  I appreciate from the comments the prop might not be straight, but I'd like to try and get the diff as straight as possible.  I'm going to try and find the center line through the cage and onto the prop, face plate.  I'll use the center of the top bolt, taking a 90dgree square down the center etc.

I'll then mark the plate at 12 noon and 6pm.   I figure if I rotate the marks to 3pm and 9pm and measure to the same spot on either lower arm I'll identify the offset in the cage with respect to the plate.  Assuming the mounts in the chassis are central I can then space according to the measurement offset in each arm with reference to the prop face plate.  I will of course measure the thickness of the mounting on the lower arm, just in case there's a variance there.

I guess some will be thinking, but why... I know, but I just want to get it bang on.

[Geoff Brown]

You can usually get the centreing for the diff & DD tube within half a mil which is satisfactory when building a Seven. Any finer tolerance I think may not be possible.

Alternatively just utilise the largest percussive adjustment device available 

[CtrMint]

finally worked out how to calculate the spacing based on the center line of the diff and chassis.  I appreciate I can't account for an offset in the gearbox, or even chassis, but I think that's a good compromise.  Measurements shown are from original carrier.

 

 

[CtrMint]

I've made a pdf template if anyone else wants to do a similar sort of calculation.

Diff Diagram blank.pdf

[David Brown]

Don't think the rest of the car is manufactured to tighter than a 2-3mm tolerance. 

[gaxor]

There is a YouTube video in the Caterham Cave series showing him fitting a Diff unit to a 420R using the Diff cradle

[CtrMint]

Yeah I've seen it.  I've tried the technique with the diff out of the cradle but wasn't a fan, it didn't make the spacing easier.

[CtrMint]

Yeah I've seen it.  I've tried the technique with the diff out of the cradle but wasn't a fan, it didn't make the spacing easier.